category: literaturenote citekey: folksonintroduction2014 title: Introduction authors: "Folkson, R." year: 2014 date: 2014-00-00 2014 doi: 10.1533/9780857097422.1 url: "https://doi.org/10.1533/9780857097422.1" zotero_key: 29C6EKNV zotero_storage: V6BNV9G5 collections: transpordi_planeerimine folder: 001_artiklid firstAuthor: "Folkson, R."

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Alternative fuels and advanced vehicle technologies for improved environmental performance

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Woodhead Publishing Series in Energy: Number 57

Alternative fuels and advanced vehicle technologies for improved environmental performance

Towards zero carbon transportation

Edited by Richard Folkson

Woodhead Publishing is an imprint of Elsevier 80 High Street, Sawston, Cambridge, CB22 3HJ, UK 225 Wyman Street, Waltham, MA 02451, USA Langford Lane, Kidlington, OX5 1GB, UK

Exceptions to the above:

Chapters 2 and 3: © 2014 R. J. Pearson and J. W. G. Turner. Chapter 4 was prepared by US Government employees; it is therefore in the public domain and cannot be copyrighted. Published by Woodhead Publishing Limited.

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	Contributor contact details	xv
	Woodhead Publishing Series in Energy	xix
1 I	ntroduction R. Folkson, University of Hertfordshire, UK	1
1.1	Introduction	1
1.2	Technology roadmaps to deliver low carbon targets	2
1.3	Vehicle technology contributions to low carbon targets	2
1.4	Powertrain technology contributions to low carbon targets	5
1.5	Regulatory requirements and consumer trends	7
1.6	Traffic management factors	8
1.7	Global manufacturing and consumer trends	8
1.8	Commercial vehicles and buses	9
1.9	Electrification of transport technology	10
1.10	Current and future trends	10
1.11	Affordability and consumer appeal	12
1.12	Long-term vision: solar energy/hydrogen economy	13
1.13	Conclusion	14
1.14	Sources of further information and advice	14
1.15	Acknowledgements	14
1.16	References and further reading	15
Part I	Alternative fuels, advanced additives and oils to improve environmental performance of vehicles	17
2 T	he role of alternative and renewable liquid fuels in environmentally sustainable transport R. J. Pearson and J. W. G. Turner, University of Bath, UK	19
2.1	Introduction: competing fuels and energy carriers	19
2.2	Market penetration of biodiesel	26
2.3	Market penetration of alcohol fuels	27

vi	Contents
2.4	Future provision of alternative liquid fuels: the biomass limit	36
2.5	Beyond the biomass limit: sustainable organic fuel for transport (SOFT)	37
2.6	Renewable fuels within an integrated renewable energy system	40
2.7	Conclusions	43
2.8	Acknowledgements	44
2.9	References	45
2.10	Appendix: abbreviations	50
3	Using alternative and renewable liquid fuels to improve the environmental performance of internal combustion engines: key challenges and blending
	technologies R. J. Pearson, and J. W. G. Turner, University of Bath, UK	52
3.1	Introduction	52
3.2	The use of biodiesel in internal combustion engines: fatty acid methyl esters (FAMEs) and hydrogenated vegetable
	oil (HVO)	53
3.3 3.4	Alcohol fuels: physico-chemical properties Alcohol fuels for spark-ignition engines: effects on	55
	performance and efficiency	65
3.5	Alcohol fuels for spark-ignition engines: pollutant
	emissions, deposits and lubricant dilution	69
3.6 3.7	Alcohol fuels for compression-ignition engines Vehicle and blending technologies for alternative liquid	72
	fuels: flexible-fuel vehicles	73
3.8	Vehicle and blending technologies for alternative liquid fuels: ethanol-gasoline and methanol-gasoline bi-fuel
	vehicles	77
3.9	Vehicle and blending technologies for alternative liquid fuels: tri-flex-fuel vehicles and iso-stoichiometric ternary
	blends	78
3.10	Conclusions	82
3.11	Acknowledgements	82
3.12	References	82
3.13	Appendix: abbreviations	88
4	Alternative and renewable gaseous fuels to improve

vehicle environmental performance 90

M. Mintz, J. Han and A. Burnham, Argonne National Laboratory,

4.1 Introduction 90

USA

	Contents	vii
4.2	Fossil natural gas	91
4.3	Fossil natural gas production, transmission and distribution	92
4.4	Natural gas engines and vehicles	95
4.5	Biomethane/biogas	98
4.6	Biogas production, distribution and storage	101
4.7	Liquid petroleum gas (LPG)	106
4.8	LPG production, distribution, storage and use in vehicles	106
4.9	Hydrogen	108
4.10	Hydrogen production, distribution, storage and use in
	vehicles	109
4.11	Life-cycle analysis of alternative gaseous fuels	111
4.12	Future trends	113
4.13	References	114

5 E	lectricity and hydrogen as energy vectors for
	transportation vehicles	117
	J. W. Sh ield and K. B. Martin, formerly of Missouri University eff
	of Science and Technology, USA and R. Folkson, University of
	Hertfordshire, UK
5.1	Introduction	117
5.2	Overview of hydrogen production	119
5.3	Overview of electricity production	127
5.4	Hydrogen storage and transportation	129
5.5	Conclusions	134
5.6	References and further reading	135

6	Advanced engine oils to improve the performance
	of modern internal combustion engines	138
	K. Howard, Lubrizol Ltd, UK
6.1	Introduction	138
6.2	The role of the lubricant in a modern internal combustion
	engine	139
6.3	The composition of a typical modern engine lubricant	143
6.4	Diesel engine lubricant challenges	145
6.5	Gasoline engine lubricant challenges	147
6.6	Industry and original equipment manufacturer (OEM)
	specifications for engine oils	151
6.7	Lubricating modern engines in developing markets	152
6.8	Future engine oil evolution	157
6.9	Conclusions	162
6.10	Acknowledgements	163
6.11	Sources of further information and advice	163
6.12	References	163

7	Advanced fuel additives for modern internal
	combustion engines J. Bennett , Afton Chemical Limited, UK	165
7.1	Introduction	165
7.2	Additive types and their impact on conventional and
	advanced fuels	166
7.3	Impacts of additives on combustion characteristics	175
7.4	Diesel performance and deposit control additives	178
7.5	Gasoline performance and deposit control additives	184
7.6	Conclusions and future trends	192
7.7	Sources of further information and advice	192
7.8	References	193
Part II	Improving engine and vehicle design	195
8 I	nternal combustion engine cycles and concepts J. D. Naber and J. E. Joh nson, Michigan Technological University, USA	197
8.1	Introduction	197
8.2	Ideal engine operation cycles	198
8.3	Alternative engine operating cycles	206
8.4	Comparison of engine cycle performance	211
8.5	Advantages and limitations of internal combustion
	engines	217
8.6	Conclusions and future trends	219
8.7	Sources of further information and advice	221
8.8	References	221
9 I	mproving the environmental performance of heavy duty vehicles and engines: key issues and system
	design approaches	225
	Q. Xin and C. F. Pinzon, Navistar, Inc., USA
9.1	Introduction: classifying engine and vehicle types	225
9.2	The use of alternative fuels to improve environmental
	performance	228
9.3	Electric, hydraulic, and flywheel hybrid powertrains for
	improved fuel economy	234
9.4	Vehicle emissions and fuel economy regulations	237
9.5	Improving vehicle design to meet environmental
9.6	regulations Improving engine design to meet environmental	242
	regulations	244

	Contents	ix
9.7	Developments in light-duty diesel engine technologies	248
9.8	A system design approach to address challenges in
	advanced engine and vehicle technologies	251
9.9	Summary of next-generation technologies for heavy-duty vehicles	264
9.10	References	271
9.11	Appendix: units and unit conversion	277
10 I	mproving the environmental performance of heavy
	duty vehicles and engines: particular technologies Q. Xin and C. F. Pinzon, Navistar Inc., USA	279
10.1	Introduction	279
10.2	Fuel injection systems and engine performance	280
10.3	Conventional combustion technologies and engine performance	281
10.4	Advanced low-temperature combustion systems	285
10.5	Engine air flow and turbocharging systems	288
10.6	Engine downsizing, down-speeding, and down-breathing	294
10.7	Mechanical and electrical supercharging systems for
	improved emissions control and performance	312
10.8	Turbocompounding to improve engine performance	313
10.9	Exhaust gas recirculation (EGR) systems	326
10.10	Improving conventional valvetrains and the use of
	variable valve actuation (VVA)	327
10.11	Heavy-duty diesel engine cooling and thermal
	management systems	338
10.12	Aftertreatment technologies for emissions control	339
10.13	Waste heat recovery (WHR) systems	347
10.14	Engine mechanical friction reduction technologies	350
10.15	Electronic controls and on-board diagnostic (OBD)
	systems to optimize engine performance	352
10.16	Development of natural gas engines	355
10.17	Future trends	356
10.18	References	357
10.19	Appendix: units and unit conversion	367
11	Advanced and conventional internal combustion
	engine materials	370
	L. L. Myagkov, Bauman Moscow State Technical University,
	Russia, K. Mah ov, Northumbria University, UK, N. D. kam Ch ainov, Bauman Moscow State Technical University, Russia and
	I. Makh ova, Northumbria University, UK kam
11.1	Introduction	370

x	Contents

11.2	Advanced internal combustion (IC) engine materials: compact graphite iron (CGI)	371
11.3	Graphite/carbon and carbon/carbon fibre-reinforced
	polymer composites (CFRPs)	373
11.4	Advanced polymers: polyamides for manufacturing intake manifolds	375
11.5	Advanced alloys and ceramics for manufacturing valves
	and other components	377
11.6	Materials for particular components in IC engines	381
11.7	References	391
12	Advanced transmission technologies to improve
	vehicle performance	393
	S. N. Doğan, G. Henning, T. Gödecke, M. Somm er, K. Fronius, M. Kroh n, J. Kiesel and J. Dorfschm id, Daimler AG, Germany
12.1	Introduction	393
12.2	Manual transmission: six-speed front-wheel-drive SG6-
	310	395
12.3	Dual-clutch transmission: seven-speed front-wheel-drive
	7G-DCT	396
12.4	Automatic transmission: seven-speed 7G-Tronic Plus	399
12.5	Continuously variable transmission: front-wheel-drive
	CVT AUTOTRONIC	411
12.6	P2 hybrid transmission	418
12.7	Two-mode hybrid transmission advanced hybrid system
	cars (AHS-C)	422
12.8	Automated commercial vehicle transmission: sixteen
	speed G260-16	431
12.9	References	432
13	Sustainable design and manufacture of lightweight
	vehicle structures	433
	G. S. Daeh n, The Ohio State University, USA
13.1	Introduction	433
13.2	The value of mass reduction	435
13.3	General challenges and opportunities	443
13.4	Possible architectures of the next-generation vehicle	446
13.5	Specific lightweighting technologies	452
13.6	Future trends	456
13.7	Acknowledgements	459
13.8	References	459

		xi
14 I	mproving vehicle rolling resistance and aerodynamics M. Juh ala, Aalto University, Finland	462
14.1	Introduction	462
14.2	Overview of vehicle aerodynamics	464
14.3	Rolling resistance in vehicles	466
14.4	Advanced vehicle design for drag reduction	468
14.5	Advanced tire design and materials	471
14.6	Conclusions and future trends	474
14.7	Sources of further information and advice	474
14.8	References	475
15	Mechanical and electrical flywheel hybrid
	technology to store energy in vehicles	476
	K. R. Pullen and A. Dh and, City University London, UK
15.1	Introduction	476
15.2	The development of flywheel technology	479
15.3	Types and properties of flywheels	480
15.4	Transmissions for flywheels	486
15.5	Performance evaluation of flywheel hybrid vehicles	493
15.6	Technical challenges in flywheel development	501
15.7	Conclusions and future trends	501
15.8	References	502
16	Hydraulic and pneumatic hybrid powertrains for improved fuel economy in vehicles Z. Filipi, Clemson University, USA	505
16.1	Introduction	505
16.2	Hydraulic hybrid principle of operation and system
	architectures	507
16.3	Hydraulic component design and modeling	511
16.4	Integrated hydraulic hybrid vehicle simulation	520
16.5	Design and control of hydraulic hybrid powertrains	522
16.6	Examples of practical applications	532
16.7	Pneumatic hybrids	535
16.8	References	538
17 I	ntegration and performance of regenerative braking and energy recovery technologies in vehicles P. Tawadros and N. Zh ang, University of Technology, Sydney, Australia and A. Borett i, RMIT University, Australia	541

17.1 Introduction 541

xii	Contents
17.2	Types and properties of regenerative braking and energy
	recovery	542
17.3	Hybridisation with energy recovery: design and
17.4	performance issues Design integration and operational optimisation	547 550
17.5	Advantages and limitations of regenerative braking	557
17.6	Conclusions and future trends	559
17.7	Sources of further information and advice	562
17.8	References	562
Part III	Electric/hybrid vehicle technologies	565
18	Hybrid drive train technologies for vehicles T. Hofm an, Eindhoven University of Technology, The Netherlands	567
18.1	Introduction	567
18.2	Hybrid vehicle configurations and classification	569
18.3	The challenges of hybrid vehicle design	572
18.4	Solutions to the design problem	577
18.5	Conclusion	579
18.6	References	579
19	Battery technology for CO2 reduction N. M. Joh nson, Ricardo, Inc., USA	582
19.1	Introduction	582
19.2	CO2 reduction opportunities of using batteries	583
19.3	Battery functionality and chemistries for vehicle
	applications	598
19.4	Lithium ion cells	610
19.5	High voltage battery pack design	616
19.6	Battery management systems	621
19.7	Future trends	625
19.8	Conclusions	628
19.9	Sources of further information and advice	628
19.10	References and further reading	630
20 C	onventional fuel/hybrid electric vehicles M. Eh sani, Texas A&M University, USA	632
20.1	Introduction	632
20.2	Basic components of a hybrid electric vehicle system	633

20.3 Architectures of hybrid electric drive trains 633 20.4 Series hybrid electric drive trains (electric coupling) 636 20.5 Parallel hybrid electric drive trains (mechanical coupling) 641

	Contents	xiii
20.6	Series-parallel hybrid electric drive trains (electric and
	mechanical coupling) and plug-in hybrids	646
20.7	Control and performance	647
20.8	Future trends	653
20.9	References	654
21 P	ure electric vehicles K. T. Ch au, The University of Hong Kong, People's Republic of China	655
21.1	Introduction	655
21.2	System configurations	656
21.3	Electric propulsion	661
21.4	Energy storage and management	666
21.5	Charging infrastructure	673
21.6	Vehicle-to-grid (V2G) technology	677
21.7	Benefits and limitations of EVs	680
21.8	Conclusions and future trends	682
21.9	Acknowledgements	683
21.10 21.11	Sources of further information and advice References	683 683
22 F	uel-cell (hydrogen) electric hybrid vehicles B. G. Pollet, University of the Western Cape, South Africa, I. Staff ell, Imperial College, UK, J. L. Sh ang, University of Birmingham, UK and V. Molkov, University of Ulster, UK	685
22.1	Introduction	685
22.2	Energy storage devices (ESDs) for the transport sector	686
22.3	Batteries	687
22.4	Hydrogen and fuel cells	693
22.5	Electrochemical capacitors (ECs)	704
22.6	Current status of low-carbon vehicle technologies	705
22.7	Battery electric vehicles (BEVs)	708
22.8	Fuel cell electric vehicles (FCEVs)	709
22.9	Technical prospects and barriers	714
22.10	Improving the safety of hydrogen-powered vehicles	719
22.11	Conclusions	730
22.12	Acknowledgements	731
22.13	References	731
22.14	Appendix: abbreviations	734

Index 736

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(* = main contact)

Editor and Chapter 1

R. Folkson Royal Academy of Engineering Visiting Professor for Innovation and Design University of Hertfordshire Hatfield AL10 9AB, UK

E-mail: rfolkson@aol.com

Chapters 2 and 3

Bath BA2 7AY, UK

R. J. Pearson* and J. W. G. Turner Department of Mechanical Engineering University of Bath Claverton Down

E-mail: thepearsons7ar@tiscali.co.uk; jwgt21@bath.ac.uk

Chapter 4

M. Mintz*, J. Han and A. Burnham Argonne National Laboratory 9700 S. Cass Avenue Argonne, IL 60439, USA

E-mail: mmintz@anl.gov

Chapter 5

J. W. Sheffield and K. B. Martin Formerly of Missouri University of Science and Technology 1870 Miner Circle Rolla, MO 65409, USA

R. Folkson* Royal Academy of Engineering Visiting Professor for Innovation and Design University of Hertfordshire Hatfield AL10 9AB, UK

E-mail: rfolkson@aol.com

Chapter 6

K. Howard Lubrizol Ltd The Knowle Nether Lane Hazelwood DE56 4AN, UK

E-mail: keith.howard@lubrizol.com

Chapter 7

J. Bennett Afton Chemical Limited London Road Bracknell RG12 2UW, UK

E-mail: John.Bennett@AftonChemical. com

Chapter 8

J. D. Naber* and J. E. Johnson Mechanical Engineering – Engineering Mechanics Department Michigan Technological University 815 R.L. Smith Building 1400 Townsend Drive Houghton, MI 49931, USA

E-mail: jnaber@mtu.edu; jenesbit@mtu. edu

Chapters 9 and 10

Q. Xin* and C. F. Pinzon CAE Analysis Navistar, Inc. 2601 Navistar Drive Lisle, IL 60532, USA

E-mail: Harry.Xin@navistar.com; QianfanHarryXin@yahoo.com; Christian.Pinzon@navistar.com

Chapter 11

L. L. Myagkov Department of Piston Engines Bauman Moscow State Technical University Russia

K. Mahkamov* and I. Makhkamova Faculty of Engineering and Environment Northumbria University Ellison Place Newcastle upon Tyne NE1 8ST, UK

E-mail: khamid.mahkamov@ northumbria.ac.uk

N. D. Chainov Department of Piston Engines Bauman Moscow State Technical University Russia

Chapter 12

S. N. Doğan*, G. Henning, T. Gödecke, M. Sommer, K. Fronius, M. Krohn, J. Kiesel and J. Dorfschmid Daimler AG HPC D670, Mercedesstr. 137 70327 Stuttgart, Germany

E-mail: suereyya_nejat.dogan@daimler. com

Chapter 13

G. S. Daehn Department of Materials Science and Engineering The Ohio State University 2041 College Road-477 Watts Hall Columbus, OH 43210, USA

E-mail: daehn.1@osu.edu

Chapter 14

M. Juhala Department of Engineering Design and Production Aalto University Puumiehenkuja 5A Espoo Otaniemi P.O. Box 14300 00076 Aalto, Finland

E-mail: matti.juhala@aalto.fi

Chapter 15

K. R. Pullen* and A. Dhand School of Engineering and Mathematical Sciences City University London Northampton Square London EC1V 0HB, UK

E-mail: k.pullen@city.ac.uk

Chapter 16

Z. Filipi Clemson University International Center for Automotive Research 4 Research Drive, Room 337 Greenville, SC 29607-5257, USA

E-mail: zfilipi@clemson.edu

Chapter 17

P. Tawadros and N. Zhang Faculty of Engineering and IT University of Technology, Sydney 15 Broadway Ultimo NSW 2204, Australia

E-mail: peter.tawadros@uts.edu.au; nong.zhang@uts.edu.au

A. Boretti* RMIT University School of Aerospace, Mechanical and Manufacturing Engineering Bundoora VIC 3083, Australia

E-mail: a_boretti@yahoo.com

Chapter 18

T. Hofman Department of Mechanical Engineering Eindhoven University of Technology Den Dolech 2 5600 MB Eindhoven, The Netherlands

E-mail: t.hofman@tue.nl

Chapter 19

N. M. Johnson Ricardo Inc. Detroit Technology Campus 40000 Ricardo Drive Van Buren Township, MI 48111 USA

E-mail: neil.m.johnson@att.net

Chapter 20

M. Ehsani Power Electronics and Motor Drives Laboratory Department of Electrical and Computer Engineering Texas A&M University College Station, TX 77843, USA

E-mail: ehsani@ece.tamu.edu

Chapter 21

K. T. Chau

International Research Centre for Electric Vehicles

Department of Electrical and Electronic Engineering The University of Hong Kong Pokfulam Road Hong Kong, People's Republic of China

E-mail: ktchau@eee.hku.hk

Chapter 22

B. G. Pollet* HySA Systems Competence Centre SAIAMC Innovation Centre Faculty of Natural Sciences University of the Western Cape Robert Sobukwe Road Private Bag X17 Bellville 7535, Cape Town, South Africa

E-mail: bgpollet@hysasystems.org

I. Staffell Imperial College Business School Imperial College London London SW7 2AZ, UK

J. L. Shang PEMFC Research Group Centre for Hydrogen and Fuel Cell Research School of Chemical Engineering University of Birmingham Edgbaston Birmingham B15 2TT, UK

V. Molkov HySAFER Centre University of Ulster Newtownabbey BT37 0NL, UK

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Edited by Jas Devgun

61 Handbook of Process Integration (PI): Minimisation of energy and water use, waste and emissions Edited by Jiří J. Klemeš

62 Coal power plant materials and life assessment Edited by Ahmed Shibli

63 Advances in hydrogen production, storage and distribution Edited by Ahmed Basile and Adolfo Iulianelli
64 Handbook of small modular nuclear reactors Edited by Mario D. Carelli and Dan T. Ingersoll
65 Superconductors in the power grid: Materials and applications Edited by Christopher Rey
66 Advances in thermal energy storage systems: Methods and applications

Edited by Luisa F. Cabeza

67 Advances in batteries for medium and large-scale energy storage: Types and applications Edited by Chris Menictas, Maria Skyllas-Kazacos and Lim Tuti Mariana

68 Palladium membrane technology for hydrogen production, carbon capture and other applications

Edited by Aggelos Doukelis, Kyriakos Panopoulos, Antonios Koumanakos and Emmanouil Kakaras

69 Gasification for synthetic fuel production: Fundamentals, processes and applications Edited by Rafael Luque and James Speight

70 Renewable heating and cooling: Technologies and applications Edited by Gerhard Stryi-Hipp
71 Environmental remediation and restoration of contaminated nuclear and NORM sites

Edited by Leo Van Velzen

R. Folkson, University of Hertfordshire, UK

DOI: 10.1533/9780857097422.1

Abstract: There are very many different technology alternatives available to deliver sustainable vehicles with low carbon emissions. This chapter reviews available methods and the use of technology road-mapping to plan for future adoption by manufacturers. Vehicle technology contributions to carbon reductions are considered together with the role of powertrains in the challenge. Regulatory requirements and consumer trends including the role of traffic management are discussed. There is a review of global manufacturing and mass production issues affecting scale. Commercial vehicle and bus issues are compared with car technology. The increasing role of electrification is discussed including sources of electricity. Current sales statistics and volumes are reviewed with forecasts for the future including a longer term vision.

Key words: low carbon emissions, technology road-mapping, powertrain, regulations, traffic management, global manufacturing, electrification, sales forecasts.

1.1 Introduction

There are very many different alternative technologies available for manufacturer to deliver sustainable vehicles with low carbon emissions. This chapter will review all of the major alternatives available, which will be described in greater detail in subsequent chapters. This section is intended as an overview of all the alternatives, how these are likely to be adopted by manufacturers and consumers and the longer term vision for where the industry is likely to be going.

Technologies that are considered to deliver sustainable vehicles with low emissions include the following:

∑ internal combustion engines including downsizing and new combustion technology
∑ hybrids
∑ battery electric drives
∑ flywheel stored energy
∑ bio-fuels
∑ synthetic fuels
∑ gaseous fuels
∑ hydrogen
∑ fuel cells.

Each technology will be discussed in detail including vehicle and powertrain effects, affordability and consumer acceptance issues.

1.2 Technology roadmaps to deliver low carbon targets

Manufacturers and government agencies use technology roadmaps to assist in identifying major areas for the development of new technologies. An example of a current roadmap was developed by the Automotive Council UK in 2010 and can be reviewed at Automotive Council UK (2010).

The principle areas of development identified in the roadmap were:

∑ internal combustion engines
∑ electric machines and power electronics
∑ lightweight vehicle and powertrain structures
∑ intelligent transport systems
∑ energy storage and energy management.

The benefit of roadmapping is that it provides guidance to manufacturers and government agencies on major areas that are considered priorities for future research and investment projects. The roadmap contains the vision statement that 'Individual manufacturers will prioritise certain technologies to fit with brand values, but OEMs share a common view of a high level technology roadmap'. All of the technologies shown in the roadmap are reviewed in later chapters of this book. The principal technologies in the roadmap are shown in Fig. 1.1. Technologies to reduce carbon emissions can be split into two major areas – vehicle technology and powertrain technology – which are discussed in the next two sections.

1.3 Vehicle technology contributions to low carbon targets

Vehicle technologies are those associated with all areas of the vehicle other than the propulsion system (or powertrain). These are generally focused on reducing energy losses that result from moving the vehicle along a road and through the air. The major aspects of vehicle technology that offer potential energy savings are:

∑ weight reduction
∑ aerodynamic drag reduction
∑ rolling resistance and friction improvements.

Weight reduction is a key element of reduced energy simply due to Newton's law:

1.1 Technology roadmap (Source: Automotive Council UK, based on an independent report 'New Automotive Innovation and Growth Team' (NAIGT); http://www.automotivecouncil.co.uk/wp-content/ uploads/2010/12/Tech-Road-Maps-RD-Capability-Final.pdf).

Force = Mass ¥ Acceleration

The higher the mass, the more force, and therefore energy, is required to accelerate the vehicle. Mass also plays an important role in friction as it is directly proportional to the rolling resistance of the vehicle. So weight is the enemy and all measures that can reduce it are beneficial for energy use efficiency.

Manufacturers are therefore increasingly using lighter weight materials that include:

∑ high strength steels
∑ aluminium and magnesium alloys
∑ engineering plastics
∑ fibre reinforced plastics including carbon fibre.

High strength and alloy steels such as boron steel are used to reduce material thickness but retain strength in highly stressed components such as door pillars in the area of side impact protection. These are frequently joined to lower strength steel for less stressed areas by utilising laser welded blanks prior to forming or, more recently, through the application of tailor rolled sections that provide steel sheet which is thicker in high stressed areas and thinner where lower stresses are experienced (Fig. 1.2).

Aluminium is increasingly used in premium vehicles such as the Jaguar XJ and XK models, Audi A8 and high performance sports cars such as

1.2 Tailor rolled boron steel B-pillar.

Ferrari and Aston Martin. Manufacturers use several different methods for weight reduction in aluminium and magnesium structures with different joining methods using adhesives or self-piercing rivets, sheet metal forms, extrusions and die-castings. The weight savings offered by aluminium and magnesium are likely to see increasing applications for parts such as closure panels (bonnet/hood, boot lid/trunk and doors) in mass market vehicles as fuel prices rise and legislative pressures force manufacturers to achieve ever improving fuel economy, although thinner gauge high strength steel will continue to be competitive for many applications.

Plastics and composites will continue to be increasingly adopted for weight reduction and major manufacturers such as Ford are investigating how to reduce the cost of carbon composites for high volume applications. The challenge with all weight reduction technologies is to balance the affordability of the new technology, which is generally more expensive than mild steel, against the energy savings over the life of the vehicle and the total cost to the consumer.

Aerodynamic drag reduction is concerned with minimising the forces associated with air resistance as vehicles push air aside during motion. This is characterised by the equation:

$$F = C_{\rm D} \times A$$

where F = force, *C*D = drag coefficient and A = frontal area of vehicle.

Manufacturers apply much detailed design effort to minimising the drag coefficient by ensuring laminar air flow over the body and reduction of discontinuities which disturb the air flow such as openings, gaps and sharp edges. Actions include lowering frontal body structure to create an air dam, rear spoilers and smooth underbody to achieve laminar flow. There is an increasing use of active aerodynamic aids such as deployable spoilers, reduced suspension height at high speeds and active radiator shutters which close the flow of air when cooling is not required. An example of such an active shutter is found on the 2010 Ford Focus models (Plate I between pages 392 and 393).

Rolling resistance can be reduced by attention to friction and parasitic losses, some times called hotel load, which consume energy without contributing to vehicle motion. All manufacturers are developing low rolling resistance tyres, reduced drag from brake rotors and bearings, reduced viscosity lubricants and low power absorption ancillary drives. Electric power assisted steering that does not absorb energy when driving straight ahead, smart alternator and charging systems, electric air conditioning pumps and water pumps are all used to reduce the engine load and improve efficiency. Further research is underway to recover waste heat and kinetic energy from exhaust and braking systems. All these technologies can contribute to fuel efficiency, but require careful analysis to ensure that the lifetime costs produce a net benefit to the consumer.

1.4 Powertrain technology contributions to low carbon targets

Considering all vehicle technology contributions to low carbon targets, it is estimated that roughly 10% of the total reduction necessary to achieve longer term goals can be achieved from vehicle technology improvements. The remaining 90% of the target will be achieved by powertrain technology. Powertrain technology covers all aspects of the propulsion system that propel the vehicle. These cover a wide range of technology and include all of the following (Fig. 1.3):

∑ internal combustion engines petrol/gasoline
∑ internal combustion engines diesel
∑ hybrid powertrain micro (start stop)
∑ mild hybrid limited battery size
∑ full hybrid series
∑ full hybrid parallel
∑ combined series/parallel hybrid
∑ pure electric vehicles battery/super capacitor
∑ plug in electric/hybrid
∑ hydrogen fuel cell
∑ hydrogen internal combustion engine
∑ compressed gas LPG/CNG
∑ bio-fuels internal combustion engine/hybrid

1.3 Powertrain technology timeline based on a D-class car@ 8.5 km/L (~268 g/km).

∑ flywheel hybrid mechanical/electrical
∑ compressed air motor
∑ hydraulic/pneumatic hybrid
∑ plus any combinations of the above.

All of these technical solutions are discussed in later chapters of this book. There are a number of important considerations when selecting powertrain solutions to achieve carbon reduction targets. Firstly, affordability for the consumer is critical and the total costs will be very dependent on the customer usage. A low mileage urban user will have a very different profile to a high mileage high speed drive requirement with significant energy consumption issues around total cost over the life of the vehicle. Secondly, there needs to be consideration of the life cycle analysis of the total resources used in the construction, use and recycling of the vehicle at the end of the vehicle life. A battery electric car that requires a second battery to be installed at say 10 years after initial use needs to reflect the cost and resource issues over the life of the car. Similarly, maintenance needs to be considered for all systems which may add positive lifetime benefits such as reduced brake wear for energy recovery systems on hybrids or flywheel equipped vehicles.

Powertrain technology will account for the vast majority of the improvements necessary to deliver low carbon targets, but it will be important to also educate consumers to make the right choice for their operating needs. There will also need to be significant driver education and infrastructure changes to ensure that the full benefits of all these technologies can be achieved.

1.5 Regulatory requirements and consumer trends

Vehicle manufacturing is one of the most highly regulated industries in the world and is required to meet legislation around the world that controls all aspects of the product and its manufacture. These include emissions, safety, environmental standards and differing tax regimes to influence consumer purchase decisions.

Emissions regulations were first introduced in the 1970s to control exhaust pollutants. The regulated gases measured during prescribed tests control NOx (oxides of nitrogen), CO (carbon monoxide), HC (unburned hydrocarbons) and PM (particulate matter representing carbon particles or soot). Carbon dioxide is also measured correlating to fuel consumption during the test. Measurements are made in gm/km during a defined drive cycle specified in the regulations. This is usually the NEDC (New European Drive Cycle) for European regulations or the NHTSA (National Highway Transportation and Safety Administration) for North American requirements or CARB (California Air Resources Board). Regulatory requirements are frequently revised and emissions levels are typically halved at each iteration. Exhaust emissions of pollutants have now been reduced by around 99% from unregulated levels prior to legislation.

Safety regulations have introduced standards for energy absorption during front, side and rear impacts which have progressively required added structures and air bag systems to meet the requirements with consequent increases in vehicle weight. This has resulted in increased vehicle size and mass with detrimental effects on vehicle fuel economy and carbon dioxide emissions.

Consumer trends have led to increased diversification of vehicle types with more niche vehicles. There has been a particular trend to increased popularity of SUVs (sports utility vehicles) and MPVs (multi-purpose vehicles) or people carriers which are generally heavier, taller and larger than conventional cars, with a detrimental effect on fuel economy (carbon dioxide emissions). There is a general trend in all vehicles for the fitment of more comfort and convenience features such as air conditioning, central locking, electric windows and in car entertainment systems which have a detrimental effect on fuel economy. Consumers also expect continued improvements in vehicle and engine performance which frequently off-sets improvements in fuel economy. Although fuel economy is an important factor in customer buying priorities, it generally comes lower in the ranking than styling and design, safety and convenience features.

1.6 Traffic management factors

One of the most important factors affecting vehicle fuel economy is congestion. A vehicle with an internal combustion engine that is stationary in a traffic jam with the engine running is producing an infinite amount of carbon dioxide per km when expressed in gm/km because the emissions are divided by zero (km) as the vehicle is not moving. Traffic management and congestion avoidance can therefore make a major contribution to reduced carbon dioxide emissions. Intelligent transport systems (ITS) will increasingly allow vehicles to interface both with each other and the built environment to reduce the time spent in congestion and to improve road safety. These systems can actively manage vehicle speeds and routing with capability such as external speed control, adaptive cruise control, active safety systems for crash avoidance and lane keeping warnings.

1.7 Global manufacturing and consumer trends

Global car production results in the manufacture of around 50 million vehicles per year, comprising roughly 15 million in Europe, 15 million in North America and the remainder from China and India. China is now the largest manufacturing country in the world accounting for production of more vehicles than any other region. There has been a significant rationalisation in the number of vehicle manufacturers over the 100 years since the industry started in Europe and North America, resulting in a relatively small number of very large multi-national companies such as Ford, General Motors, Toyota and Volkswagen dominating the production volumes with many millions of vehicles produced worldwide. There is a much greater number of companies producing vehicles in China and India, but this is likely to face similar rationalisation into larger groups as the industry matures.

There is a general consumer trend towards a higher mix of downsized fuel-efficient vehicles, but hybrids and electric vehicles currently account for a very small percentage of total vehicle sales on a global basis, despite significant growth in this sector in recent years. Over a period of ten years Toyota has produced 4 million hybrid vehicles, which is a relatively small percentage of worldwide production, but the Toyota Prius hybrid has now established itself as a high volume mass market product becoming one of the world's best-selling single nameplates by 2012. However, alternative fuelled and advanced powertrain vehicles will need to manufactured in much larger numbers before they start to make a significant contribution to reducing carbon dioxide emissions on a global scale. Conventional internal combustion engines account for the vast majority of total vehicles manufactured and a relatively small percentage improvement in fuel economy for a vehicle produced in many millions such as the Volkswagen Golf or Ford Focus will have a much greater effect on global emissions reduction than the contribution from the smaller number of hybrids made currently.

1.8 Commercial vehicles and buses

Commercial vehicles and buses will share similar technology to that used for carbon reductions on cars, but also present some unique challenges. Heavy trucks used for long distance haulage are already highly optimised for fuel efficiency and duty cycle. They employ peak power for climbing gradients and would not benefit from hybrid or electric drive systems for long distance haulage. Battery electric solutions significantly reduce the available payload that can be carried efficiently on commercial vehicles. Improved aerodynamics, weight reduction and rolling resistance improvements will all be employed to improve fuel efficiency, but the main drivers will be commercial efficiency rather than regulatory pressures.

Buses used in urban duty cycles can be greatly improved by many types of hybrid technology due to energy recovery from braking particularly during the continuous stop-start operation. There are many trials and operational examples of electric hybrids using both series and parallel technology and reduced size internal combustion engines. Mechanical and electrical flywheel systems also offer major efficiency opportunities. The main challenge with all commercial vehicle and bus technologies will be the business equation to recover the additional costs of the advanced technology from fuel usage during the operating life of the vehicles, combined with the need for high levels of reliability.

1.9 Electrification of transport technology

All vehicle types will see an increasing level of electric propulsion technology over the coming years ranging from relatively low cost micro hybrid stopstart systems, through mild hybrids that use battery technology to improve operating efficiency through to high cost full and plug-in hybrids that utilise mains electricity to make a contribution to operating efficiency. These vehicles will need to offer consumers a rational business proposition that offsets the higher purchase costs against the reduced fuel consumption over the life of the vehicle. Increasing vehicle volumes will also reduce the costs of batteries, motors and electrical systems as economies of scale enable lower component costs.

A major difficulty for manufacturers will be the transition from low volume high cost solutions to the much lower cost high volume solutions necessary to make a major impact on global emissions. Some of this change will be driven by regulatory pressures that penalise manufacturers who fail to meet legislated fuel economy targets. It is likely that there will be large markets for pure electric cars like the Nissan Leaf for those consumers who are primarily short-distance urban drivers (Fig. 1.4) and series hybrid cars such as the Vauxhall Ampera (Fig. 1.5) or Chevrolet Volt for consumers needing long-distance capability. The mix and balance of battery size against range and cost will be dictated by consumer demands as they become more familiar with this type of product.

1.10 Current and future trends

Hybrid and electric vehicle designs are now starting to emerge as mainstream alternatives to internal combustion petrol (gasoline) and diesel vehicles. However, the current product range available is limited and total sales volumes are a small proportion of global markets. Technology solutions are relatively expensive and many hybrids and particularly electric vehicles are significantly more expensive than conventional vehicles. Most electric cars are at least double the cost of competitive conventional cars with additional concerns relating to residual values and battery life. There will need to be significant reductions in purchase costs before alternative fuel and electric vehicles make an appreciable contribution to total vehicle sales. This is something of a vicious circle as costs will not reduce until volumes increase and sales volumes will remain low until costs come down.

1.4 Nissan Leaf EV.

1.5 Vauxhall Ampera plug-in hybrid.

Market analysts and vehicle manufacturers have consistently predicted significant growth in electric vehicle and hybrid vehicles for many years, but in general actual performance has failed to meet market predictions. In 2002, JD Powers, Booz-Allen and others were predicting that hybrid cars would account for 3% of global sales by 2010 (Fig. 1.6). This would equate to a worldwide market of around 1.2 million cars equivalent to the total production of a company the size of BMW. This is a worthwhile market segment for manufacturers such as Toyota to pursue, but actual sales of hybrids has been slower than predicted.

1.6 Hybrid market demand prediction. Source: JD Power, Booz-Allen, Internal data.

This trend in optimistic performance for electric car sales can be traced back to the earliest days of the automotive industry. There were more battery electric cars operating on the streets of Detroit in 1904, dominated by a car called The Brown Electric car, than internal combustion engine cars. Once manufacturers had invented and introduced the clutch and starter motor to make it easier to get cars moving, they rapidly replaced electric cars. Ever since that time electric car advocates have predicted that the electric car will replace internal combustion engines, but if one looks at any point in history since 1904 it has always been projected that it will take 10 years for electric drives to take over in the mass market. It is probably still true today that it will take another 10 years for electric cars to be significant in the mass market and this 10 year projection has been termed 'The Folkson Constant'.

This trend is only likely to change if engineers make a technological and economic breakthrough that greatly improves the affordability of battery, motor and control technologies. Conventional internal combustion cars will continue to dominate the mass market for many years into the future, but with an increasing level of electrification to improve efficiency, advanced combustion systems to reduce consumption and downsized engines to reduce weight and improve specific power output.

1.11 Affordability and consumer appeal

Car purchase normally represents the second largest consumer expenditure after buying a property for accommodation. It is therefore one of the most significant decisions facing people during their lives and something that is affected by a large number of different factors.

In general the most important factors in deciding on a vehicle purchase are prioritised in the following order:

∑ styling and design
∑ performance
∑ comfort and convenience
∑ safety
∑ purchase cost.

Fuel efficiency and environmental performance are important to some consumers but do not generally achieve a higher priority in the purchase decision than the aforementioned features, particularly for mass market products.

Hybrids and electric cars are generally significantly more expensive than conventional petrol or diesel cars and consumers have a fairly short time horizon for the period in which they expect the operating cost savings to be recovered to repay the higher purchase price. Few consumers are willing to recover the higher cost in longer than three years. Apart from the financial aspects, consumers expect alternative fuel and advanced technology cars to fully meet their other expectations for styling, performance, safety, comfort and convenience features. Cars need to remain appealing if they are to secure the large amount of financial commitment required to make the purchase. Consumers will need to be convinced of other major advantages before they would be willing to accept any compromise to the product such as operating range, the time to refuel or residual values compared with normal expectations.

1.12 Long-term vision: solar energy/hydrogen economy

Sufficient solar energy falls on the surface of the earth in one hour to provide all of man's energy needs for a year. The challenge is how to efficiently capture solar energy to meet our needs. All fossil fuels, wind and wave energy are effectively derived from solar energy and the effect of the sun on our planet. Current methods for capturing solar energy directly such as photo-voltaic cells are relatively inefficient compared to natural processes that capture sunlight like photosynthesis. In the long term the best opportunities will come from fundamental breakthroughs in chemistry and physics that will allow efficient recovery of solar energy to provide the majority of our energy needs. One potential route to decarbonisation of energy would be to convert solar energy into hydrogen by splitting water and using the hydrogen as an energy vector to power vehicles.

Current electrolysis methods for producing hydrogen from water are relatively inefficient due the limitations of anode and cathode chemistry and alternative methods such as natural gas reforming using steam do not use low carbon processes as carbon dioxide is a natural by-product from combustion used to produce steam. Hydrogen also lacks energy density compared to fossil fuels and so would require storage and efficiency improvements to serve as a transport fuel, probably restricted to road transport because of storage limitations. However, the long-term visionary solution may well utilise solar energy to produce plentiful, low cost hydrogen to power a carbon-free transport system. This would require major construction of a large-scale infrastructure that would itself result in carbon emissions during the building phase, but the overall system could result in a significant reduction in manmade emissions over the long term.

1.13 Conclusion

There is no single solution that will meet all consumer needs for transport energy, but rather there will be multiple technologies developed to meet diverse consumer and usage needs across different transport sectors. These are likely to evolve over a significant period of time gradually reducing the demand for fossil fuels in road transport and potentially replacing them before supplies are exhausted. Subsequent chapters will cover in more detail all of the potential technologies currently available.

Former US Vice-President Al Gore said in his 2006 film An Inconvenient Truth about the potential threat of global warming that 'There is no silver bullet, but rather silver buck shot' to solve the issue. Meaning that there is no single technology to fix the problem but many different solutions are needed. This is certainly true and it is important that legislators do not try to 'back winners' and select technical solutions that may not provide the optimum solution. Ultimately it will be engineers providing a host of different alternatives to the market that will enable consumers to choose the most cost effective, efficient and desirable solutions that they decide to purchase.

1.14 Sources of further information and advice

An Inconvenient Truth: a global warning, Paramount Films 2007. Bosch Automotive Handbook, 7th Edition, Wiley 2007.

Institution of Mechanical Engineers – Transport Theme Report, Low Carbon Vehicles – Driving the UK's Transport Revolution, IMechE publication 2008.

1.15 Acknowledgements

The author would like to acknowledge help from the UK Institution of Mechanical Engineers and Ford Motor Company Ltd.

1.16 References and further reading

Automotive Council, UK (2010), Technology Roadmap, http://www.automotivecouncil. co.uk/wp-content/uploads/2010/12/Tech-Road-Maps-RD-Capability-Final.pdf
EC (2009), European Parliament and Council Regulation (EC) No 443/2009, Setting emission performance standards for new passenger cars as part of the Community's integrated approach to reduce CO2 emissions from light-duty vehicles.
National Highway Traffic Safety Administration (2010), Light-Duty Vehicle Greenhouse Gas Emission Standards and Corporate Average Fuel Economy Standards; Final Rule, May 2010.

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Part I

Alternative fuels, advanced additives and oils to improve environmental performance of vehicles

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The role of alternative and renewable liquid fuels in environmentally sustainable transport

R. J. Pearson and J. W. G. Turner, University of Bath, UK

DOI: 10.1533/9780857097422.1.19

Abstract: Renewable liquid fuels provide an opportunity for nations to increase energy independency or reduce greenhouse gas emissions by supplying energy-dense fuels which are miscible with petroleum gasoline and diesel. This chapter shows how liquid fuels are the most pragmatic way to increase the renewable energy content in the transport sector, enabling an evolution, rather than a revolution of both vehicle and fuel technology. The historical development and present market penetration of these fuels is discussed, together with aspects of current and future legislation which may incentivise or limit their use. An integrated system is proposed which combines biomass with fuels made from re-cycled CO2 as a means of storing off-peak renewable energy.

Key words: biodiesel, butanol, ethanol, flexible-fuel, FAME, methanol, ternary blends.

2.1 Introduction: competing fuels and energy carriers

The drive to reduce dependency on fossil fuels over recent years has focused attention on the use of alternative fuels for transport, particularly in the USA, where the use of biofuels blended into fossil fuels has been adopted. When they are made from feed stocks which satisfy appropriate sustainability criteria and do not give rise to the emission of significant levels of greenhouse gases in their cultivation, biofuels can help to alleviate concerns regarding energy security and climate change within the road transport sector which, globally, is over 90% dependent on oil (IEA, 2010). Through their miscibility with conventional gasoline and diesel fuels, bioethanol and biodiesel have been introduced into the fuel pool in significant quantities. The use of these alternative fuels has been possible without a quantum change in either the transport energy distribution infrastructure or the technology and, therefore, cost of the vehicles in which they are used. In the respect that they offer an evolutionary rather than a revolutionary transition, the adoption of liquid alternative fuels could serve as a more stable pathway than electrification or the use of molecular hydrogen in order to address issues of climate change and security of energy supply for transport.

This section will give a high-level justification of the assertion made above, based on comparing fundamental properties of the fuels and energy carriers and examining the implications on vehicle costs. The current market penetration of alternative liquid fuels will also be discussed, focusing on significant geographical regions. Details of vehicle technologies specific to the use of particular renewable liquid fuels are discussed in Chapter 3.

2.1.1 On-board energy density

Renewable liquid fuels provide, at low additional vehicle cost, the on-board energy storage levels required by vehicles for personal transport which are not dedicated solely to regular short-range routes. This is not the case for some other competing technologies. The very low net gravimetric and volumetric energy densities (including the mass/volume of the fuel/energy containment system) of current technology batteries are shown for lead-acid, nickel-metal hydride (NiMH), and lithium ion chemistries in Fig. 2.1. To match the range of a conventional gasoline-fuelled vehicle with a 50-litre fuel tank would require a useable battery capacity of approximately a 100 kWh, accounting for the greater tank-to-wheel (TTW) efficiency of an electric vehicle. The mass of a fuel tank containing 50 litres of gasoline would be about 46 kg;

2.1 Net system volumetric and gravimetric energy densities for various on-board energy carriers (based on lower heating values).

that of a 100 kWh lithium ion battery would be in the range 700–900 kg, depending on the technology and the permissible depth of discharge.

As energy carriers, batteries are fundamentally limited by the requirement to carry the oxidant in addition to the reductant (analogous to oxygen in the air, and the fuel, respectively, in a combustion reaction). It is interesting to note that an ICE-powered vehicle would have to transport about 545 kg of air to combust 50 litres of gasoline if it had to carry its oxidant as well as its fuel, taking the mass of fuel and oxidant to almost 600 kg. Batteries are also constrained by the electrical potential available from the elements used in the construction of the cells. Advances in metal–air batteries, where oxygen from the ambient air is drawn through a porous cathode, have recently been made using ionic liquid electrolytes, but these technologies are presently in the early stages of laboratory testing (Rodgers and Voth, 2007). Such advances might clearly reduce the mass disadvantage of batteries.

Hydrogen is also fundamentally limited as an energy carrier in that it is the least dense element in the Periodic Table. Figure 2.1, which includes the system package volumes and masses, shows that, while the net on-board energy density of hydrogen comfortably exceeds that of current batteries, it is still very low compared with liquid fuels. Because of the extreme physical conditions required to package hydrogen the bulky system volume becomes a high percentage of the net volumetric energy content (Amaseder and Krainz, 2006; Eberle, 2006). The packaging problems are exacerbated by the constraints on the tank shapes imposed by pressure vessel design considerations and the requirement to minimise heat ingress in cryogenic systems. Pressure vessels and cryogenic tanks are extremely heavy: a 700 bar system for automotive use, holding 4.6 kg of hydrogen (the energy equivalent to 17.5 litres of gasoline) is quoted by Eberle (2006) as weighing 95 kg, whilst cryogenic systems can weigh around 170 kg and contain only 9 kg of hydrogen (the energy equivalent of about 34 litres of gasoline). In contrast a tank for a liquid hydrocarbon fuel system may weigh around 10 kg.

While physical metal hydride storage systemsfor hydrogen achieve similar volumetric energy density to a 700 bar gaseous system, the gravimetric energy content is comparable with lithium-ion batteries (Bossel, 2006). Chemical metal hydrides can achieve superior volumetric hydrogen storage density to 700 bar gas storage or liquid hydrogen but their gravimetric energy density is significantly worse, being in the region of 1–3 MJ/kg (Eberle et al., 2006; Casten et al., 2000). Many of the metals used in hydride systems (e.g. lanthanum, titanium, manganese, nickel, zirconium) are expensive and, while some lower-cost materials (e.g. magnesium-based compounds) also offer higher gravimetric densities, they may have high heats of formation and require high temperatures (>200°C), and therefore energy levels, to release the hydrogen (Casten et al., 2000).

If mechanical and electrical losses are also considered, the total energy used for compression of hydrogen to an 800 bar supply pressure may reach around 15% of the higher heating value (HHV) of the hydrogen undergoing the process (Bossel, 2006). With a liquid hydrogen approach the energy efficiency of liquefaction plants is strongly dependent on size. For a largescale plant about 40% of the higher heating value is consumed in the liquefaction process. For small-scale systems the energy consumed in the liquefaction can approach or exceed the energy content of the fuel (Bossel, 2006). The effect of boil-off losses during distribution and re-fuelling can lead to unacceptable loss of hydrogen (von Helmolt and Eberle, 2007).

Clearly since alcohols are partially oxidised they do not have the energy density of liquid hydrocarbon fuels but they are significantly better than current technology batteries and hydrogen storage systems, as shown for ethanol and methanol in Fig. 2.1. The fact that they are liquids means that vehicles can be fully re-fuelled in two or three minutes and the shape of the tanks containing them can be more easily adapted to available vehicle package space with simple vapour recovery systems and without the additional requirements for cooling systems required by electrochemical storage systems.

2.1.2 Vehicle cost

Key to enabling the widespread availability of sustainable transport is to provide solutions which customers can afford to purchase. Renewable liquid fuels satisfy this criterion by enabling the evolution of vehicles and fuel distribution infrastructures which are broadly compatible with current technologies. Figure 2.2 compares the vehicle bill of material costs for a variety of alternative fuel vehicles with a conventional vehicle powered by an internal combustion engine (ICE). A fixed 'glider' (vehicle rolling chassis, including the body) cost is assumed for all options (Pearson et al., 2012). For the battery electric vehicle (BEV) options a minimum state-ofcharge (SoC) of 15% has been assumed; for the extended range electric vehicle (EREV) and proton exchange membrane fuel cell (PEM FC) options a minimum SoC of 35% has been assumed. The battery cost assumed, for volume production levels, is \$750/kWh (slightly better than current prices). An EREV can be thought of as a plug-in hybrid electric vehicle (PHEV) with a significant electric-only range and with the engine taking the role of an electrical generator operating largely independently of vehicle speed. For EREVs the strategy is often to try to size the battery so that a large portion of the distance travelled by the vehicle can be done in EV mode.

It is clear that, for a range-equivalent vehicle, the cost of the battery makes the BEV unaffordable to most customers. Reducing the vehicle range to 150 km from 550 km brings the costs down to a more accessible level but this significantly range-compromised vehicle is still about 2.5 times more expensive

2.2 Cost comparison of alternative energy vehicles. Assumed vehicle range (unless stated) = 550 km; battery cost = \$750/kWh.

than a conventional vehicle with a much higher utility level. This presents the customer with a very large negative price-performance differential. The EREV option, which enables lower storage capacity batteries to be used but requires both an electric motor and a fuel converter/generator (ICE assumed in this case), has a similar cost premium but is not encumbered by range compromise.

A significant portion of the high cost premium of a hydrogen-fuelled vehicle, whether using fuel cells or ICEs (the latter vehicle powertrain variant is not shown in Fig. 2.2), is the cost of the hydrogen storage system. Estimates for the volume manufacture of a carbon-fibre wound pressure vessel required for 700 bar hydrogen storage range from the proposed target of 72000 (US\$3000) (Eberle, 2006) to 710 000 (US\$15 000) (Jackson, 2006) for systems capable of storing around 10 kg of hydrogen. In Fig. 2.2 a cost of 75000 (US\$7500) has been assumed for the hydrogen storage cost of the PEM FC vehicle. Cryogenic storage systems, which store hydrogen in its liquid state at –253°C, are significantly more complex and therefore expensive; they also have to manage boil-off losses effectively. In Fig. 2.2 the total energy storage system costs for the PEM FC option includes both the hydrogen storage costs and those of a 14 kWh battery (the same as the EREV) which is necessary to manage the operating locus of the fuel cell in order exploit its theoretically high efficiency levels. The costs of hydrogen fuel cell vehicles are very sensitive to the assumed cost per unit power output of the fuel cell itself (included in the 'power plant' costs in Fig. 2.2). Leaving aside the potential vehicle cost at volume production levels of between 3 and 6.5 times that of a conventional vehicle, the expense and basic incompatibility of the fuel-distribution infrastructure is the biggest obstacle to the adoption of hydrogen as an energy vector in transport. Mintz et al. (2002) estimated the cost of providing a hydrogen infrastructure in the USA capable of re-fuelling 100 million fuel cell vehicles (40% of the US light duty vehicle fleet) as up to \$650 ¥ 10⁹

2.1.3 Environmental benefits

The debate regarding the environmental benefits of renewable fuels is complex and controversial. For a given feedstock and fuel the specific production process, fertiliser used, transport and distribution, and, importantly, land-use change (direct and indirect) must be considered. In addition to well-to-wheel, or life cycle greenhouse gas (GHG) emissions, parameters such as 'carbon pay-back' times have been calculated to quantify the time period over which a biofuel must be produced in order to off-set the negative GHG impact of cultivating land which was formerly a carbon sink in the natural ecosystem. Whilst many studies have discussed these effects (RFA, 2008; Fargione et al., 2008; Searchinger et al., 2008; Bringezu et al., 2009; Zinoviev et al. 2010) they are not within the scope of the present work. However, the biofuels introduced into the EU as a result of the Renewable Energy Directive (EC, 2009a) are governed by sustainability criteria with a view to transport energy suppliers reducing the life cycle GHG emissions of their fuel by at least 6% at the end of 2020 – there are also interim 2% and 4% targets to be met by the end of 2014 and 2017 respectively (EC, 2009c). Article 7b of the Fuel Quality Directive states that the GHG reductions for biofuels sold in the EU must be at least 35% (currently), rising to 50% in 2017, and 60% in 2018 for biofuels produced in installations in which production started after 1 January 2017 (EC, 2009c). Criteria for calculating these GHG benefits have been developed and default values for various fuels and pathways are defined in the Directive (EC, 2009c).

In addition to meeting supply targets the US Renewable Fuel Standard (EPA, 2010a) also has requirements that fuels meet GHG emissions thresholds for compliance with each of four types of renewable fuel categories. California has its own initiative, the Low Carbon Fuel Standard (ARB, 2012), calling for a reduction in the carbon intensity of the transportation fuel pool used in the state of 10% by 2020. Figure 2.3 has been developed in order to indicate the well-to-wheel CO2 emissions of vehicles with a range of tank-to-wheel CO2 emissions as a function of the carbon intensity (g CO2/MJ) of the fuel being used. The vertical lines in figures (a) and (b) represent ethanol at E20 and E85 levels, respectively, which meets the EU GHG reduction targets

2.3 Well-to-wheel CO2 emissions (g CO2/km) as a function of fuel well-to-tank carbon intensity (g CO2/MJ) and vehicle tank-to-wheel CO2 emissions. (a) using E20; (b) using E85. Carbon intensity of default fossil fuel is that given in EC (2009c) of 83.8 g CO2/MJ. No TTW CO2 benefits due to the use of ethanol are assumed.

for biofuels mentioned above. It can be seen that using low-carbon-intensity renewable fuels at high concentration levels enables low well-to-wheel of operating efficiencies. These GHG emissions are similar to (E20), or substantially lower than (E85), the well-to-wheel emissions of an electric vehicle operating on electricity generated at the EU average carbon intensity (Pearson and Turner, 2012).

It should be noted that one of the motivations for the use of methanol in the fleet trials held in California in the 1980s and 1990s was the improvement it made to local air quality relative to using gasoline in the engines of that time. These issues, together with that of methanol as a potential safety and environmental hazard are discussed by Pearson and Turner (2012) and reported by Machiele (1987; 1990) and Brusstar et al. (2008).

2.2 Market penetration of biodiesel

In the previous section it has been established that renewable liquid fuels are a potentially pragmatic route to de-carbonising transport because they provide evolutionary transition mechanisms for both the vehicle technology and fuel distribution infrastructure. These fuels are already making a significant contribution to transport energy supply. Figure 2.4 shows that, globally, ethanol was the largest contributor to alternative road transport fuel in 2009 with consumption of 38.7 Mtoe/a representing 29% of the alternative road transport energy supply but only 2.3% of the total global fuel consumption for this sector of 1701 Mtoe/a (IEA, 2010). Only natural gas is close to

2.4 Alternative road transport fuels as a fraction of global total alternative fuel supply. Based on data in IEA (2010).

ethanol in terms of energy supply as an alternative road transport fuel. The 13 Mtoe/a of biodiesel production gives it almost 10% of alternative road transport energy supply, but represents only 0.7% of total road transport fuel consumption (IEA, 2010).

The European Union (EU) dominated the production of biodiesel in 2009 with 54% of production, followed by the USA and Brazil with 13% and 9%, respectively (IEA, 2010). About 75% of EU biodiesel is made from rape seed feed stock, with 13% coming from soy bean, and 8% from palm oil (Haer, 2011). China and India produced 2% and 1% of the world's biodiesel, respectively. The main market for biodiesel consumption is Europe, in particular Germany, which introduced the use of 'pure' biodiesel (B100) from rapeseed feedstock grown on fallow land in the 1990s. Tax relief on this fuel meant that initially the retail price was 5–15% lower than fossil diesel on an energy equivalent basis and some OEMs decreed that their existing light duty vehicles were compatible with B100 (Kramer and Anderson, 2012). These factors gave rise to 1900 fuel stations offering biodiesel by 2006, with about 70% of sales being of B100. The imposition of taxes by the German government in 2008 in order to recoup lost revenue transformed the market to one where most biodiesel sales originated from blending with fossil diesel.

Simultaneously, vehicle manufacturers raised compatibility concerns regarding the use of B100 with modern common rail fuel injection systems and particulate filters and their regeneration strategies. Additionally, concerns regarding oil dilution and degradation, deposit formation, and materials compatibility limited the blend concentration of biodiesel for use in all vehicles in the EU to 7% by volume (B7) (EC, 2010). Cooper (2011) has shown that the world cereal and grain production is far greater than that of vegetable oil. This ameliorates the threat of production of bioethanol on food prices relative to that of biodiesel. In this chapter the focus will henceforth be primarily on the production and use of alcohol fuels from biological and synthetic techniques, although some of the technology discussed regarding the onward synthesis of hydrocarbons has relevance to alternative diesel fuels.

2.3 Market penetration of alcohol fuels

The potential of ethanol as a fuel in the internal combustion engine has been recognised for over 100 years (White, 1907) and its high octane index meant that it was initially used as a knock inhibitor in gasoline until tetraethyl lead came to dominate the market. The US produced 56% of the ethanol consumed by the road transport sector in 2009, with Brazil producing 33% and the EU 4% (IEA 2010).

2.3.1 Brazil

Historically, Brazil has the world's most mature market for bioethanol in road transport fuels. Although ethanol-gasoline blending was taking place in Brazil on a significant scale in the 1930s, it was the OPEC oil crisis of 1973 which prompted the large-scale introduction of ethanol made from sugar cane as part of a national alcohol programme ('ProAlcool') in 1975 (Soccol et al., 2005). The evolution of alcohol-fuelled vehicles in Brazil is well summarised by Kramer and Anderson (2012). The extent of the penetration of ethanol in the Brazilian fuel pool is such that it is not possible to purchase gasoline which does not contain bioethanol. The level of ethanol in gasoline (to form 'gasohol') is currently allowed to vary between 18–25% by volume, depending on the state of the global sugar market. Hydrous ethanol is also sold, consisting of at least 94.5% by volume ethanol with the balance being a permitted mix of components consisting of water (mainly), hydrocarbons, and other alcohols.

After initially blending ethanol with gasoline at around 20% by volume, dedicated E100 vehicles were introduced in 1979 and, by 1985, these vehicles represented 80% of light-duty vehicle production, assisted by ethanol prices which were sufficiently lower than the gasoline price to easily off-set the volumetric energy density differential. Subsequent fluctuations in the global sugar market led to gasoline being cheaper than ethanol and the demand for dedicated E100 vehicles virtually disappeared by the mid 1990s. The introduction of flexible-fuel vehicles (FFVs), capable of using anything from gasohol to E100 (actually hydrous ethanol about E94 with 6% water by volume), around 2002 rejuvenated ethanol sales and enabled customers to exploit the price and tax advantages of E100 over gasoline whilst protecting them from the volatility of the sugar price by enabling operation on lower ethanol blends when desirable.

2.3.2 USA

In the US the oil crisis of the 1970s and the drive to improve air quality in states such as California gave rise to an interest in methanol. Abundant availability of indigenous feed stocks (i.e., coal and natural gas) and low production costs drove the introduction of gasoline-M85 FFVs (the first 'modern' FFVs), capable of operating on any mixture ranging from conventional gasoline to a blend of 85% by volume methanol/15% gasoline. A national M85 standard (ASTM D5797 – covering mixtures containing between 70% and 85% methanol in gasoline) was put in place. Political factors, combined with a drop in the oil price and a methanol shortage brought about by the sale of methanol stocks to make methyl-tert-butyl ether (MTBE) for use as an oxygenate in reformulated gasoline, required by the Clean Air Act Amendments (EPA, 2012), led to the reduction of interest in methanol in the late 1990s and the rise of bioethanol production. Nichols (2003) gives a compelling account of the methanol trials in the US up to this time. The DuPont Waiver (Owen and Coley, 1995) permits blends of up to 5% by volume methanol with a minimum of 2.5% by volume co-solvent alcohols having a carbon number of 4 or lower (ethanol, propanol, butanol, and/or gasoline-grade tertiary butyl alcohol) as long as the total oxygen content does not exceed 3.7% by mass (see Chapter 3).

The role of ethanol as an oxygenate source in reformulated gasoline began to rise in the early 2000s when the use of MTBE as an oxygenate additive was phased out, as shown in Fig. 2.5 (a). Ethanol consumption has risen strongly since 2005 due to the federal policies which encouraged its use, reaching over 7.3 billion gallons gasoline-equivalent (gge) in 2009 and representing over 90% of US alternative fuel energy demand in 2009. The rapid growth in consumption has been matched by the rapid growth in production, as shown in Fig. 2.6 which reveals that the US is now by far the largest producer of ethanol, its output having risen to 62% of the total of 22.4 billion gallons produced globally in 2011. Together, the US and Brazil accounted for 87% of global production in that year.

Figure 2.5 (b), focuses on the lower demand fuels in the US market; they are all dwarfed by the quantities of ethanol used as an oxygenate component (see Fig. 2.5(a)). The rise, decline, and recent huge jump in biodiesel consumption can be seen, together with the gradual fall of liquid petroleum gas (LPG) consumption. Natural gas consumption (compressed (C)NG and liquefied (L) NG in Fig. 2.5) has shown a steady rise, and it might be postulated that the current low cost of natural gas resulting from the exploitation of shale gas reserves could lead to further significant increases. An alternative scenario is the conversion of natural gas to methanol for transport use (Turner et al., 2012). It is also clear that electricity and hydrogen have no significant demand up to 2009, comprising 0.06% and less than 0.002% of US alternative fuel energy demand in 2009, respectively. Electricity and hydrogen both appear as negligible on this scale, although in 2010 the electricity use was 4.847 million gge, compared with 0.152 million gge for hydrogen.

The original Renewable Fuels Standard (RFS) (EPA, 2007) was created under the Energy Policy Act of 2005 and required 7.5 billion US gallons of renewable fuel to be blended into gasoline by 2012. As a result of the Energy Independence and Security Act of 2007 (US Congress, 2007) the Renewable Fuel standard was expanded (RFS2) (EPA, 2010a) to include diesel as well as gasoline, and mandated an increase of renewable fuel from 9 billion gallons in 2008 to 36 billion gallons in 2022, it established new categories of renewable fuels, setting separate targets for volume and GHG reduction for each fuel. Figure 2.7 shows the ramp up in fuel volumes required to meet the RSF2 stipulation. The target for 'biomass-based diesel' is a minimum of

2.5 Alternative fuel consumption (normalised to gasoline-equivalent US gallons (1 US gallon = 3.785 litres) in the US road transport sector (a) 2003–2009 and (b) 2003–2010/11. Based on data in Davis et al. (2011) and US DoE (2012). ('EtOH as ox' and 'MTBE as ox' indicate the use of ethanol and MTBE as oxygenates in gasoline fuel.)

1 billion gallons from 2012–2022 (with the exact target to be set by future rulemaking) whilst that for 'cellulosic biofuel' was set to rise from 0.5 billion gallons in 2012 to 16 billion gallons in 2022, most of which is expected to

2.6 Evolution of ethanol production by country since 2007. Volume in US gallons of ethanol. Based on data from US DoE (2012).

2.7 Renewable fuel volume requirements for RFS2 (based on data in EPA 2010a). Note: any fuel which meets the requirement for cellulosic biofuel or biomass-based diesel is also valid for meeting the advanced biofuel requirement.

be in the form of cellulosic ethanol (EPA, 2010a). In total 21 billion gallons of 'advanced biofuels' (including cellulosic biofuels) are required in 2022, leaving 15 billion gallons to be supplied by first-generation fuels, mostly in the form of corn ethanol. Many problems have been encountered meeting the targets for cellulosic biofuels and the 2012 target has recently been revised downward to 8.65 million gallons – this will represent less than 0.006% of US fuel usage that year (more precisely, the figure of 0.006% represents cellulosic biofuels as a fraction of non-renewable gasoline and diesel use), compared with 9.23% overall for renewable fuels, comprised mostly of 'corn ethanol' (EPA, 2011a).

Anderson et al. (2012) provides an excellent account of the introduction of ethanol into the gasoline fuel pool in the US, including details of the changes in the gasoline blend stock, i.e. the blend-stock for oxygenate blending (BOB), in to which the ethanol is blended. Between 2000 and 2010 US ethanol consumption grew from 1.6 billion gallons/year to 13 billion gallons/year – the latter figure represents a hypothetical nationwide uniform ethanol-gasoline blend level of almost 10% by volume (Anderson et al., 2012). In fact virtually all the ethanol used in US transport is used in the form of E10 (a mixture of 10% by volume of ethanol in 'gasoline') which has been available in the US since the 1980s but is now widespread due to the political and environmental developments described above.

A 'blend wall' has arisen due to all the low-level ethanol-gasoline blends, often referred to as 'gasohol', being close to the maximum ethanol content (10% by volume) which vehicle manufacturers will allow for a user to remain within their warranty provision. In order to address this issue the EPA has granted two partial waivers (EPA, 2010b; EPA, 2011b) which allow but do not mandate the introduction of E15 for use in light-duty vehicles of model year 2001 and later. The approval process is inherently slow since it involves extensive testing and is open to challenge by vehicle manufacturers.

Anderson et al. (2012) examines various scenarios for ethanol introduction, including the most optimistic, where the RFS2 targets are met. This latter scenario would lead to notional uniform ethanol blend levels of E24 by 2022 and of E29 by 2035 (assuming a 2% annual growth post-2022). In the absence of significant growth in E85 sales this could not be implemented with new waiver approvals since these levels of ethanol are well beyond the tolerance capability level of current conventional vehicles. New vehicle and engine specifications would be required, together with the maintenance of a protection grade fuel (E10 or E15) for existing vehicles. A discussion of octane targets for these blends is also included.

Flexible-fuel (or flex-fuel) vehicles (FFVs) are capable of using ethanol in concentration levels of up to 85% (E85) by volume. However, despite the registration of over 9 million such cars at the end of 2011, incentivised by the rating of FFVs under CAFE legislation (EPA, 2010c), representing 4% of the US LDV fleet (Anderson et al., 2012), only 1% of the total ethanol use has been in the form of E85 sales, as shown in Fig. 2.5 (a) and (b). This has led to a reappraisal of the Alternative Motor Fuels Act (AMFA) credits given to FFVs in future CAFE and EPA fuel economy and emissions legislation (EPA, 2011c). The fuel economy rating of an FFV is calculated by taking the harmonic mean of the fuel economy on gasoline (or diesel) and the fuel economy of the alternative fuel divided by 0.15. Thus, a vehicle which achieves 25 miles/US gall. using gasoline and 15 miles/US gall. using E85 would be rated having a fuel economy of (2/((1/25)+(0.15/15)) = 40 miles/US gall. This assumes that the FFV operates on the alternative fuel 50% of the time. US Congress has extended the FFV incentive (called the 'dual-fuelled' vehicle incentive) to model year (MY) 2019 but has provided for its phase-out between MY 2015 and MY 2019 by gradually reducing the allowed limit of the maximum fleet fuel economy increase for a manufacturer due to this credit from 1.2 miles/gall. (MYs 1993–2014) to 0 miles/gall. after MY 2019 in 0.2 miles/gall. decrements over that period (EPA, 2011c). FFV credits for MY 2020 and beyond will reflect the 'real-world' percentage of usage of the alternative fuels (EPA, 2011c).

Ethanol in high-concentration form, E85 (now defined by ASTM D5798 as 51%–83% by volume ethanol in gasoline), has suffered both from limited availability and uncompetitive pricing on an energy basis in the US. There are fewer E85 pumps in the US than there are EV charging stations (about 2500 versus about 6750 in 2012). The requirement for FFVs to run on any concentration of ethanol in gasoline, from 0%–85%, also means that the vehicles are generally not configured to be capable of exploiting the high octane numbers of the higher level ethanol blends – this prevents such vehicles being able to off-set the reduction in volumetric energy content of the fuel by increasing the thermal efficiency of the engine. Conversely, if FFVs were to be optimised to have high compression ratios in order to reduce their volumetric fuel consumption on E85 (as discussed in Chapter 3), they would give an unattractive increase in fuel consumption when operating on fuel with lower ethanol concentration such as E10 if the octane level of the BOB is maintained (Anderson et al., 2012). Although this may be viewed as a mechanism to incentivise the FFV customer to use E85, the proposition is only reasonable if there are sufficient fuel stations offering the fuel.

2.3.3 EU

Whilst the European gasoline specification EN228 allows up to 3% methanol in gasoline, there has never been any specification for high concentration of methanol in Europe analogous to the ASTM 5797 standard. In the EU, by the end of 2020, the Renewable Energy Directive (RED) and the Fuel Quality Directive (FQD) [EC, 2009a; EC, 2009c] together require that 10% of transport energy is supplied in renewable form and that the overall GHG intensity of fuels should be reduced by 6%. With diesel penetration at approximately 50% across the EU it is possible that, due to lack of sufficient supplies of sustainable vegetable oils for biodiesel manufacture and some issues of achieving emission compliance of modern vehicles using more than 7% by volume of biodiesel, the RED and FQD targets may need to be met by supplying base fuel in the form of E20, when the lower volumetric energy density of ethanol is considered (Cooper, 2011). However, recent attempts to introduce E10 into the German market did not go well due to some customer confusion (Kramer and Anderson, 2012).

In contrast to the CAFE regulations which make it attractive to manufacture FFVs in the US, the fiscal penalties for GHG emissions from cars sold in the EU is based only on tailpipe CO2 emissions. This gives vehicle manufacturers no incentive to spend even the small extra amount required in order to produce an FFV (c. 7100/vehicle). It has recently been suggested by a major transport energy supplier (Cooper, 2011) that attributing some CO2 benefit to manufacturers will provide a more compelling reason for OEMs to make flex-fuel vehicles and thus produce a greater outlet for ethanol as an automotive fuel.

Despite the lack of apparent incentive, manufacturers such as Saab, Ford, Volvo, Renault, and VW have introduced FFVs into their vehicle range. The EU vehicle tailpipe CO2 penalty system does, however, presently allow a 5% reduction in tailpipe CO2 to be claimed for any flex-fuel vehicle that an OEM sells, provided one-third of the fuel stations in the country in which the vehicle is sold has at least one E85 refuelling pump (EC, 2009b). In Sweden there has been a co-ordinated activity to install E85 pumps so that by 2009 50% of the network was covered (Bergström et al., 2007), rising to 59% in 2011 (Kramer and Anderson, 2012), and in 2008 22% of all new car sales were FFVs (Kramer and Anderson, 2012), driven by government fuel tax relief (so that E85 was energetically cheaper than gasoline) and vehicle use initiatives. Kramer and Anderson (2012) show that this fuel tax benefit has been variable since 2005. They also show that the drop in FFV sales which occurred in 2009 coincided with a period when E85 had a cost disadvantage of 30% relative to gasoline.

The benefits of liquid fuels compared with their gaseous counterparts are again highlighted by the 2006 Swedish legislation requiring that all fuel stations above a certain size offer at least one alternative fuel: most stations covered by the law installed E85 pumps and storage tanks which, at 740 000–45 000, offered a ten-fold lower installation cost compared with biogas storage and dispensing equipment (Kramer and Anderson, 2012).

In the rest of Europe legislation and incentives for high concentration ethanol use are largely absent and thus fuel pump availability for E85 and FFV sales remain sparse. For Germany in 2010 FFVs represented 0.05% of the 2.9 million new vehicle sales (Kramer and Anderson, 2012). It is, however, perhaps worth noting that for a vehicle at the 2011 EU average of 135.7 g CO2/km, and at the highest proposed fine rate in 2015 of 795/ (gCO2/km), this represents a saving to the manufacturer of 7541 per car (with the benefit limited to 5.7 g CO2/km in this instance since the target would be achieved), which the authors contend is significantly greater than the additional costs of producing a vehicle which is E85 flex-fuel capable (Turner et al., 2012).

2.3.4 China

Ethanol blends in gasoline have been used in five Chinese provinces since 2004; however, the use of methanol is favoured in order to avoid conflicts with food demand. Whilst China produced only 3% of the ethanol used in road transport globally in 2009 (US DoE, 2012), it has dominated the production and use of methanol in this sector. The consumption of methanol in China, mainly in the form of M15, is around 3 million tonnes per annum (Niu and Shi, 2011) and is motivated by China's large coal reserve which offers the potential of greater energy independence. Processes to convert coal to ethanol are also being investigated (Pang, 2011).

Fleet trials using methanol have been run in China since the early 1990s, overlapping with the Californian fleet trials described in Section 2.1.3. Up to 26 provinces and municipalities have independently sanctioned the blending of methanol into gasoline for some time, including M5, M10, M15, M25, M30, M45, M50, M85 and M100 grades. The stronghold of such activity is the coal-producing area of Shanxi province. A national standard for M85 was implemented on 1 December 2009 (NSPRC, 2009) and several indigenous vehicle manufacturers produce M85-gasoline flexible-fuel vehicles. A national standard for M15 blends is also being developed. Private vehicles, taxis, and buses are using methanol-gasoline blends across the range described above (Li et al., 2008). The attractiveness of methanol to fuel suppliers is its low cost – it is often significantly cheaper than gasoline on a cost per unit energy basis (up to 30% (Iosefa, 2011) when sourced from fossil-based feed stocks). Unfortunately this can lead to poorly controlled 'splash blending' in to gasoline which may then be used in non-compatible vehicles.

High sulphur and ash content coal tends to be used for methanol production since low-sulphur coal is preferred for power generation. The proven Chinese reserves of this class of coal are 230 billion tons, ten per cent of which could produce 6.4 billion tons of methanol (Niu and Shi, 2011). Although the use of methanol is also ostensibly driven by security of supply, which has also led to the recent construction of several large methanol production plants, lower cost methanol from natural gas-fed plants is being imported.

Methanol made from coal has a carbon intensity of approximately twice

that of gasoline, whereas natural gas-sourced methanol has a similar carbon intensity to gasoline (Bandi and Specht, 2006). Significant reductions are claimed for the coal process by using 'polygeneration' technology where secondary co-products such as acetic acid or sponge iron are made, or by adding coke oven gas after the coal gasification stage (Niu and Shi, 2011).

2.4 Future provision of alternative liquid fuels: the biomass limit

The rise in global oil demand over the next 25 years is likely to originate entirely from the transport sectors of emerging economies as increasing prosperity drives greater levels of personal mobility and freight. Both China and India already have consumption-to-reserves ratios of oil which are similar to the current EU and the US levels. By 2035 China will become the largest importer of oil. The financial implications of this lack of self-sufficiency are dramatic: at an average oil price of \$100/barrel the EU and the US each transfer \$350 billion (every year) outside their borders in order to meet their oil demand (IEA, 2011a). By 2035 a gross capacity increase equal to twice the current total production of the middle-eastern OPEC counties is required simply to negate declining production of current fields (IEA, 2011a).

The development of renewable liquid transport fuels which are not feed stock constrained will enable the continued provision of full-range affordable vehicles and will mitigate, and potentially eliminate, this wealth transfer and exposure to price volatility. Here the provision of liquid and gaseous fuels which are synthesised from CO2, water, and renewable energy are discussed. These carbon-neutral fuels may be regarded as 'electrofuels' since they are effectively repositories for renewable energy which is nearly always initially converted into electrical form. Their gaseous counterparts also provide a convenient mechanism for bulk storage and recovery of off-peak renewable energy. This function is a pre-requisite for the eventual replacement of fossil fuels. An energy system integrating the transport, heat, and power sectors is shown in which biomass is an integral part. Without the provision of synthetic carbon-neutral liquid fuels biofuels will appear as merely palliatives in the processes of alleviating issues regarding security of energy supply and reducing transport CO2 emissions.

The world-wide sustainable potential of biogenic wastes and residues has been estimated at approximately 50/EJ year (1 EJ = 1¥1018 J). Estimates of the global sustainable potential of energy crops have a huge spread, ranging from 30 EJ to 120 EJ/year, depending mainly on the assumptions made regarding food security and retaining biodiversity. These combined values put the total potential sustainable bioenergy supply in 2050 between 80–170 EJ/year (Pearson and Turner, 2012). Current global energy use is about 500 EJ/year thus the mid-point value of the total sustainable bioenergy supply is around one quarter of this and less than one tenth of the projected global energy use in 2050 (WBGU, 2008). The global transport energy demand in 2007 was about 100 EJ (EIA, 2010) and, (extrapolating the EIA value of 143 EJ for liquid fuels in 2035 (EC 2011)), is projected to grow to about 170 EJ in 2050. Assuming that half the available sustainable biomass feed stock was available for biofuel production at a conversion efficiency of 50% (Bandi et al., 1995) limits the substitution potential of biofuels to about 20% of the 2050 transport energy demand. The IEA make the slightly more optimistic prediction that 32 EJ of biofuels will be used globally in 2050, providing 27% of transport fuel (IEA, 2011b). For individual countries the biomass potential could be significantly higher or lower, depending on their population densities and sustainable agricultural potential. A further limiting factor is the requirement to feed the increasing global population, with its shift toward westernised diets demanding much greater amounts of land and water (UN, 2006). These issues may constrain biofuel production to the use of the wastes and residues quantified above.

2.5 Beyond the biomass limit: sustainable organic fuels for transport (SOFT)

In this section the concept of sustainable organic (meaning carbon containing) fuels for transport (SOFT) is introduced as a means of circumventing the biomass limit to the penetration of biofuels in transport. The concept is proposed as a long-term solution to supplying carbon-neutral liquid fuels which could eventually supply the bulk of transport energy demand beyond the 20%–30% which can be sustainably supplied by biofuels. These fuels, like biofuels, would be miscible with current petroleum-based fuels so that an evolutionary transition from one organic liquid fuel to another could occur. Whilst Section 2.1 described the problems of using molecular hydrogen as a transport fuel, here it is proposed to 'package' the hydrogen in a more convenient manner by combining it with re-cycled CO2 to synthesise energydense liquid fuel. In this way hydrogen is used in the fuel rather than as the fuel. Since the concept is based on the reduction of CO2 and water to synthesise the fuel and its subsequent use results in oxidation returning the fuel to these components, the process is not feedstock-limited. Importantly, if all processes are powered with carbon-free energy and the CO2 used to make the fuel is captured directly from the atmosphere, then the combustion of this fuel would result in zero net increase in the atmospheric CO2 concentration.

Methanol, with its H/C ratio of 4, is particularly well suited to the process, being the simplest organic hydrogen carrier which is liquid at normal ambient conditions. Gasoline, diesel, and kerosene can also be synthesised as dropin fuels at a higher energy penalty and using more complex plant than that required for methanol production (Pearson and Turner, 2012). The concept of synthesising fuel from water and re-cycled CO2 was first proposed in the 1970s by Steinberg (1977) and there have been many other proposals in the meantime (Bandi et al., 1995; Stucki et al., 1995; Weimer et al., 1996; Olah et al., 2009, 2011; Jensen et al., 2007; Pearson and Turner, 2007, 2009; 2012; Littau, 2008; Jiang et al., 2010; Graves et al., 2011).

2.5.1 Recycling CO2

The most familiar manifestation of renewable liquid fuels is in the form of biofuels which recycle CO2 by extracting it from the atmosphere as part of the photosynthesis process. This simultaneously reduces H2O using chlorophyll to absorb the energy in sunlight and transform it into chemical energy in the form of carbohydrates in the resulting biomass material. Synthetic carbonneutral liquid fuels mimic this process. The energy input to the process can either be provided directly in the form of sunlight (to produce 'solar fuels') or may be off-peak renewable energy (see Section 2.6). The reduction of CO2 to produce hydrocarbons has a direct analogy to the reduction of nitrogen to produce ammonia. In both cases energy is supplied to the process to convert feed stocks to a higher level of chemical availability (Edwards, 2012).

By combining hydrogen with CO2 it can be chemically liquefied into a high energy density hydrocarbon fuel. Clearly, if the captured CO2 stems from the combustion of fossil energy resources this approach is not renewable and will still result in an increase in atmospheric CO2 concentration. Rather than a re-cycling process it amounts to CO2 re-use and offers the potential of a notional reduction in emissions of approximately 50% (Graves et al., 2011). In a closed-cycle fuel production process ideally there is no net release of non-renewable CO2. The hydrogen generation process is likely to be via the electrolysis of water and this represents by far the greatest energy input to the process. For this reason the fuels produced in this way may be referred to as 'electrofuels' as they are essentially vectors for the storage and distribution of electricity generated from renewable energy. When the feed stocks are water and CO2 from the atmosphere the fuel production and use cycle is materially closed and therefore sustainable. Such a cycle offers security of feed stock supply on a par with that of the 'hydrogen economy' since the time scale for mixing of CO2 in the atmosphere is sufficiently short to ensure a homogeneous distribution. With access to sufficient water and renewable energy, the process has the potential to provide fuel from indigenous resources for most nations. As the oil price escalates the provision of carbon neutral liquid fuels can ultimately be financed by the elimination of the wealth transfer involved in the purchase of oil.

CO2 is, with water, the end product of any combustion process involving materials containing carbon and hydrogen. Further reactions to form carbonates are exothermic processes. The capture of CO2 in inorganic carbonates and other media is a burgeoning area of research and some of the literature is described in detail in Graves et al. (2011) and Pearson et al. (2012).

Whereas the adoption of battery electric or hydrogen fuel cell vehicles requires paradigm shifts in the costs of the vehicles themselves or their fuel distribution infrastructure, or both (Pearson et al., 2012), the development of carbon–neutral liquid fuels enables a contiguous transition to sustainable transport. Drop-in fuels such as gasoline, diesel, and kerosene can be produced from CO2 (via CO) and H2 via Fischer–Tropsch (FT) synthesis but the simplest and most efficient liquid fuel to make is methanol (Pearson and Turner, 2012). Indeed the option to make gasoline is retained even if methanol is produced initially since the former can be made via the Exxon–Mobil methanol-togasoline process. In addition to being the simplest fuel to synthesise from CO2 and water feed stocks, methanol provides much greater biomass feed stock diversity since it can be made from anything which is (or ever was) a plant.

2.5.2 Fuel synthesis

Once hydrogen and CO2 are available the simplest and most direct route to producing a high quality liquid fuel is the catalytic hydrogenation of CO2 to methanol via the reaction

$$CO_2 + 3H_2 \rightarrow CH_3OH + H2O \quad \Delta H{298}^0 = -49.9 \text{ kJ/(mol. methanol)}$$ [2.1]

During the production of methanol via direct hydrogenation of CO2, by far the largest component of the process energy requirement is the hydrogen production (Pearson and Turner, 2012; Pearson et al., 2012). This is true of any electrofuel using hydrogen as an intermediate or final energy carrier. Assuming an electrolyser efficiency of 80% and a CO2 extraction energy of 250 kJ/mol. CO2 (representing about a 10% rational thermodynamic efficiency relative to the minimum thermodynamic work requirement of 20 kJ/mol.CO2) gives a higher heating value (HHV) 'electricity-to-liquid' efficiency of about 45%, including multi-pass synthesis of the methanol and re-compression of the unconverted reactants (Pearson and Turner, 2012; Pearson et al., 2012). In the late 1990s Specht et al. (1998a, 1998b) measured total-process CO2 capture energy levels of 430 kJ/mol. in a demonstration plant using an electrodialysis process to recover the absorbed CO2, representing a rational efficiency of less than 5%. Despite this low CO2 capture and concentration efficiency the measured overall fuel production efficiency was 38%.

Without policy intervention the intermittent use of alkaline electrolysers, due to their limited current densities, is likely to be too expensive (Graves et al., 2011) to produce fuel under present market economics. Improvements on this technology are at an advanced state of development (Graves et al., 2011; Ganley, 2009) and other promising technologies are emerging. Graves et al. (2011) describes the use of high temperature co-electrolysis of CO2 and H2O giving close to 100% electricity-to-syngas efficiency for use in conventional FT reactors. This ultra-efficient high temperature electrolysis process using solid oxide cells combined with a claimed CO2 capture energy (from atmospheric air) as low as 50 kJ/mol. (Lackner, 2009) leads to a prediction of an electricity-to-liquid efficiency of 70% (HHV basis). With a constant power supply this high overall efficiency enables the production of synthetic gasoline at \$2/gallon (\$0.53/litre) using electricity available at around \$0.03/kWh (Graves et al., 2011). Doty et al. (2010) states that off-peak wind energy in areas of high wind penetration in the US averaged \$0.0164/kWh in 2009 and the lowest six hours of the day averaged \$0.0071/ kWh. With more pessimistic values for the cost of CO2 capture such as the \$1000/tonne quoted by House et al. (2011), the gasoline cost component due to the supply of the carbon feedstock alone might be as high as \$7.5/ gallon (about 71.30/litre). With 20% electrolyser capacity the cost of fuel synthesis could be as high as \$4/gallon at \$0.03/kWh electricity (higher current density electrolysers could reduce this to \$2.2/gallon) (Graves et al., 2011). For perspective, the total cost of \$11.5/gallon is around 72.05/litre. Currently gasoline retail costs in the EU range from 71.14/ litre to 71.67/litre including duties and taxes (which can be as high as 70.9/litre).

In a reconfigured market which bases fuel duty and taxation on nonrenewable life cycle carbon intensity, a fuel made from air-extracted CO2 and water might be commercially attractive in the medium term, i.e. in advance of the point where sequestration of air-captured CO2 becomes economically feasible. Recycling of the CO2, rather than sequestrating it after it has been removed from the atmosphere, cannot result in any net GHG reduction. Its inclusion in a closed carbon cycle to make transport fuels, however, can potentially have the effect of rendering carbon-neutral the fastest growing GHG emissions sector as well as providing a spur to the development of air capture technology for CO2 which may subsequently be used for sequestration purposes.

2.6 Renewable fuels within an integrated renewable energy system

Renewable energy sources, such as those based on wind and solar power, are limited in their ability to meet current and future energy demands. The constraints are imposed not by the resource potential, which for wind and solar is many times current demand, but by the intermittent nature of supply. Thus it is presently necessary to provide a large capacity for base-load power generation and back-up from fossil fuels. To escape this conundrum and enable the eventual provision of all energy from renewable resources requires mechanisms for large-scale storage. Biomass is a form of large-scale storage of solar energy but, whilst it may be part of a sustainable system, it cannot underpin it. The contribution of biomass is likely to be limited because of its low energy sequestration rate per unit land area and the large demands in feeding the burgeoning world population.

In areas of high wind penetration in the US, prime on-shore wind energy is claimed to be cost competitive with coal, with a levelised cost of energy (LCOE) as low as \$45/MWh (Doty et al., 2010). The global annual growth rate of over 20% which wind energy has sustained over the past 15 years is likely to stall in the future due to the difficulties of dealing with off-peak generation. Large-scale energy storage solutions are the key to unlocking this problem. The options are limited by the requirements of scale, as evidenced by the fact that underground pumped storage is being advocated (Pickard et al., 2009).

One possibility for large-scale energy storage is to use off-peak renewable energy to synthesise chemical energy carriers. Chemical energy storage systems, based on the conversion of renewable energy into a gaseous or liquid energy carrier, enable the stored energy to be either re-used for power generation or transferred to other energy sectors such as transport, where the de-carbonisation issue is more problematic, and there is an ever-present demand to supply a high-value energy carrier. In the case of liquid fuels the demand to fuel the vehicles is already in place. The general approach is to store renewable energy first in hydrogen via the electrolysis of water but then, for an additional small energy penalty, this hydrogen is reacted with CO2 to form an infrastructure-compatible hydrocarbon energy carrier, such as the carbon-neutral liquid fuels described Section 2.5.

In addition to the ready synthesis of methanol from CO2 and hydrogen, methane can also be made from the same feed stocks using the Sabatier process. It then has the advantage that it can be stored in the gas grid, which, for most developed countries, far exceeds the capacity of existing renewable energy storage media (e.g. pumped hydro) or proposed systems such as large flywheels or redox fuel cells. Sterner (2009), Specht et al. (2009) and Breyer et al. (2011) describe such a concept, where the synthesised methane is stored and readily retrievable to smooth out the supply of renewable energy through conversion back to electrical energy via combustion in conventional power stations. This process is given the name 'renewable power methane' (RPM), and its operation within a renewable energy system based on wind, solar, and biomass has been modelled over a period of one week on a one hour resolution based on a winter load demand. The renewable-power-tomethane efficiency is predicted to be 48% (Sterner, 2009; Specht et al., 2009) using measured energy values for capture and concentration of CO2

from air of 430 kJ/mol. which, as discussed in Section 2.5.2, is a realistic value achievable with current cost-effective 'off-the-shelf' technology.

Because the limit on the amount of renewable energy that can be stored would now be significantly beyond any expected day-to-day variation in energy utilisation, the upper limit on the level of renewables in which it is economically attractive to invest is effectively raised. While the 'round-trip' losses in converting renewable electricity to methane and back to electricity are significant and would amount to an expected process efficiency of about 20% (electricity-to-electricity), the entire process is practical, achievable and most importantly, represents an evolutionary path to a fully sustainable energy economy.

The production of renewable electricity and RPM for power generation reserve (and its related use in the heat sector, where it would displace fossil natural gas) can also be integrated with the production of liquid fuels, in the form of both biofuels and electrofuels, for use directly in transport (Pearson et al., 2012). A schematic representation of such a system combining the power, heat and transport sectors is shown in Fig. 2.8, where the renewable liquid fuels are represented by methanol (CH3OH), ethanol (C2H5OH), and hydrocarbon fuels (n(-CH2-)). The latter can be synthesised from methanol using such processes as the methanol-to-gasoline (MTG) process. In this system, where it can be sustainably produced, biomass is able to contribute to the transport sector via the production of ethanol (or biodiesel), or the energy sector via direct combustion for power generation, as shown in Fig. 2.8. Clearly, the production of SOFT liquid fuels also offers an additional energy

2.8 Integrated power, heat, and transport system featuring large-scale energy storage capacity. The system combines renewable methane and liquid fuels together with renewable energy and biomass.

storage option, and this route gives a similar efficiency to the 'round-trip' efficiency of producing renewable methane, but allows the use of significantly cheaper vehicles and only limited changes to the transport fuel infrastructure. The direct production of liquid energy carriers such as methanol also opens up the possibility to exploit so-called 'stranded energy' in regions of high wind or solar availability but little local demand for energy such as deserts or with off-shore wind farms in remote locations. This obviates the need to build long-distance power grids or pipe lines to distribute the energy.

2.7 Conclusions

The production of low-carbon and, ultimately, carbon-neutral liquid fuels is the most pragmatic way in which to either de-carbonise transport or increase its sustainability and thus increase the security of energy supply to the transport sector. This route also allows the continued provision of globally compatible, affordable transport via the retention of low-cost internal combustion engines supplied both on and off the vehicle by low-cost liquid fuel systems. These powertrain systems have high power densities, energy storage densities, and low embedded manufacturing and materials extraction energies. They also have considerable potential for further efficiency improvement, especially using highly boosted small ('downsized') engines exploiting the superior qualities of alcohol fuels.

Low level biofuel blends, particularly in the form of E10, are already displacing significant quantities of gasoline in countries such as the USA. Ethanol can provide fuels with high resistance to knock which are synergistic with the trend towards downsized pressure-charged spark-ignition engines. Without significant changes to the fuel taxation system the low volumetric energy density of ethanol is likely to limit the market penetration of high concentration blends such as E85. Biodiesel penetration is limited by the reluctance of manufacturers to warrant their vehicles for use with higher levels of fatty acid methyl ester (FAME). These factors, together with the limitations on vegetable oil feed stocks, may lead to higher levels of ethanol in gasoline in order to satisfy energy security, renewability, or GHG reduction targets. Butanol and methanol may also have a part to play.

The replacement of fossil fuels with carbon-neutral liquid fuels would not significantly compromise vehicle range due to the relative ease of packaging liquid storage tanks and their low component costs. Low-carbon-number alcohols can be used for personal mobility and light-duty applications, and synthetic hydrocarbons for applications where maximum energy density is crucial (such as aviation). All transport energy can be supplied, using biofuels up to the biomass limit, and beyond it using carbon-neutral liquid fuels made using renewable energy, water, and CO2 re-cycled from the atmosphere – this approach uses hydrogen in the fuel rather than using it as the fuel. The role of biofuels in this transitional route and end-game would prevent them being regarded as a dead-end by vehicle manufacturers and politicians alike, thus ensuring their continued production and development. The potential to synthesise methanol from anthropogenically re-cycled CO2 and water in large quantities 'rules in' the use of some of the limited amount of truly sustainable biomass as a transport fuel, rather than ruling it out because full amounts cannot be supplied. In addition to minimising the environmental impact of the rapid growth of transport-related CO2 emissions, the use of atmospheric CO2 and water as feed stocks for renewable energy carriers offers potential freedom from dependency on imported oil and a concomitant reduction in associated financial transfers.

The use of CO2 recycled from the atmosphere may impose a short-term cost and energy penalty over flue-gas captured CO2 but directs the necessarily large investment toward leveraging sustainable resources. Financing the provision of renewables rather than extending the use of 'depletables' avoids the economic lock-in caused by large capital investments required in applying carbon capture and storage technology to fossil-fuelled power generation. The resulting long-term commitment to purchasing feed stocks such as coal in competition with countries which have superior capital reserves exacerbates concerns of energy security. The recycling of CO2 from the atmosphere to make fuels does not produce the reduction in CO2 concentration which is possible from air capture and sequestration. However, making a high value-added product such as transport fuel from the extracted CO2 has the potential to accelerate the commercialisation of air capture technology and its cost reduction for later use in sequestration schemes. Recycling CO2 by transforming it into a sustainable consumer product provides a more progressive concept for investors than the sequestration or 'land-fill' option. By enabling the neutralisation of transport GHG emissions the greater economic viability of carbon-neural fuels may be the most pragmatic route to climate stability and security of energy supply.

2.8 Acknowledgements

The authors wish to thank the following people for the benefit of discussions with them during the compilation of this work: Arthur Bell (SASOL), Martin Davy (University of Oxford), Eelco Dekker (BioMCN), Peter Edwards (University of Oxford), Matt Eisaman (Brookhaven National Laboratories), Stefan de Goede (SASOL), Ben Iosefa (Methanex), Karl Littau (Stanford University), Leon diMarco, Richard Stone (University of Oxford), Andre Swartz (SASOL), Gordon Taylor (GT-Systems), Sebastian Verhelst (University of Ghent), Chris Woolard (University of Cape Town), and Paul Wuebben (CRI).

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2.10 Appendix: abbreviations

AFR air:fuel ratio

ASTM American Society for Testing and Materials

ATDC after top-dead-centre

BXX blend of XX% by volume of biodiesel (FAME) in diesel

BEV battery electric vehicle

BMEP brake mean effective pressure BOB blend-stock for oxygenate blending

BTL biomass-to-liquids

CAFE corporate average fuel economy

CNG compressed natural gas

CTL coal-to-liquids DI direct (fuel) injection

EREV extended-range electric vehicle

EXX blend of XX% by volume of ethanol in gasoline

EU European Union

FAME fatty acid methyl ester FFV flexible-fuel vehicle

gge gallons gasoline equivalent

GHG greenhouse gas GTL gas-to-liquids HC hydrocarbon

HFCEV hydrogen fuel cell electric vehicle

HHV higher heating value

ICE internal combustion engine

ICEV internal combustion engine vehicle IMEP indicated mean effective pressure

LCOE levelised cost of energy LDV light-duty vehicle LHV lower heating value LNG liquid natural gas LPG liquid petroleum gas MTBE methyl-tert butyl ether MTG methanol-to-gasoline

Mtoe million tonnes of oil equivalent

Mtoe/a million tonnes of oil equivalent per annum

MXX blend of XX% by volume of methanol in gasoline

MY model year

NiMH nickel metal hydride NMEP net mean effective pressure OEM original equipment manufacturer

OPEC Organization of Petroleum Exporting Countries

PEM FC proton exchange membrane fuel cell

PHEV plug-in hybrid electric vehicle

RFS renewable fuel standard RON research octane number

SI spark-ignition TTW tank-to-wheel US United States WTT well-to-tank WTW well-to-wheel

Using alternative and renewable liquid fuels to improve the environmental performance of internal combustion engines: key challenges and blending technologies

R. J. Pearson and J. W. G. Turner, University of Bath, UK

DOI: 10.1533/9780857097422.1.52

Abstract: This chapter focuses on the use of biofuels in spark-ignition and compression-ignition engines. The use of biodiesel in the form of FAME and HVO is discussed briefly before centring on the use of use of the low carbon number alcohols, methanol and ethanol; the use of butanol is also discussed. The physico-chemical properties of these pure fuels are described and the variation of these properties as the alcohols are blended with gasoline is quantified. The chapter also reviews the technologies which enable the operation of flexible-fuel vehicles, alcohol-gasoline bi-fuel vehicles, and tri-flex fuel vehicles. The concept of iso-stoichiometric ternary blends is introduced as a means of providing a more fungible platform for renewable liquid fuels.

Key words: bi-fuel, biodiesel, butanol, ethanol, flexible-fuel, FAME, HVO, methanol, ternary blends, tri-flex-fuel.

3.1 Introduction

The previous chapter provided an overview of key issues relating to the use of renewable liquid fuels in internal combustion engines, current market penetration and possible future provision of these fuels within an integrated renewable energy system. Building on this foundation, this chapter briefly reviews the use of biodiesel in compression-ignition engines before focusing on the use of alcohol fuels in both spark-ignition and compression ignition engines. The effects of using these fuels and blends of them in gasoline on the performance, efficiency, and emissions levels produced is discussed. It also reviews the technologies required for the operation of vehicles on high concentration alcohol fuels, including flexible-fuel, ethanol-gasoline and methanol-gasoline bi-fuel and tri-flex fuel vehicles. The concept of iso-stoichiometric ternary blends is also introduced which enables mixtures of different alcohols to be blended with gasoline in order to match the stoichiometric air–fuel ratio and volumetric energy density of a nominal single alcohol/gasoline blend. This, together with close matching of other physicochemical properties, enables these blends to be considered as having the potential to provide a more fungible platform for renewable liquid fuels.

3.2 The use of biodiesel in internal combustion engines: fatty acid methyl esters (FAMEs) and hydrogenated vegetable oil (HVO)

Whilst alcohol fuels present the fuel blender, additive supplier, and vehicle manufacturer with a tightly defined blend component having consistent properties, the chemical composition of biodiesel formed by trans-esterification of seed-oils or animal fats to form fatty acid methyl esters (FAMEs) is dependent on the original feed stock source and the esterification process. This results in wide variation in FAME composition.

The cetane number, which measures the auto-ignitability of fuels for compression-ignition engines, varies over wide ranges for FAME components from the same feed stock origin. The cetane number of FAME is dependent on the distribution of fatty acids in the original oil or fat from which it was produced. Higher cetane numbers are caused by higher saturation levels in the fatty acid molecules and longer carbon chains (Geller and Goodrum, 2004). Bamgboyne and Hansen (2008) report cetane numbers for biodiesel fuels from a wide range of feed stocks. The cetane number of biodiesel derived from soya bean oil (soya bean methyl ester, or SME) has been found to vary between 45 (lower auto-ignitability) and 60 (higher auto-ignitability) whilst that of rapeseed methyl ester (RME) can vary between 48 and 61.2. Palm oil methyl ester (POME) has been measured to have cetane numbers between 59 and 70. This compares with a typical cetane number of a premium European diesel of around 60. The minimum cetane number required by conventional diesel fuel specifications in the EU (EN 590) and the US (ASTM D975) is 51 and 40, respectively.

The fact that FAME molecules are esters and are therefore oxygenated means that they will have a lower gravimetric energy density than petroleum diesel fuels. However, the large size of the molecules (in the range C12 to C22) means that the impact of the two oxygen atoms which comprise the ester functional group is much lower than that of the oxygen atom contained within the methanol (C1) or ethanol (C2) molecules. The degree of this energetic deficit is also ameliorated by the fact that the density of FAME is higher than that of petroleum diesel due to increased chain length and the presence of carbon–carbon double bonds. Thus, whilst the gravimetric energy density of FAME can be perhaps 12 per cent lower than a mineral diesel such as US No. 2 diesel fuel, its increased density can reduce the difference in volumetric energy content to about 7%.

The wide variations in the FAME composition and its consequent variable interaction with the base diesel in a blend can have markedly different effects on low temperature vehicle operability, with the fuel pour point and cold filter plugging point changing significantly (Saito et al., 2008). The fuel's oxidation stability (McCormick et al., 2006; Miyata et al., 2004), its compatibility with the vehicle fuel injection equipment, its propensity to form deposits (Caprotti et al., 2007), and the effects of fuel dilution on the engine lubricant (Thornton, 2009) also vary significantly with the composition of the FAME. Bespoke additives are required for specific blend compositions, making the task of ensuring fuel compliance with the vehicle fleet a complex task. The issues are well summarised by Richards et al. (2007). In contrast hydrogenated vegetable oils (HVOs) and biomass-to-liquids (BTL) fuels have compositions which are much closer to their petroleum diesel counterparts, however both types of fuel are significantly more energetically intensive to produce than FAME-based fuels and therefore more expensive. The ability of FAME-based biodiesel to lubricate fuel pumps and fuel injectors (its 'lubricity') is superior to that of petroleum diesel; however, the lubricity of HVO is lower, requiring lubricity additives.

As with all fuels, adherence to rigorous quality standards is necessary for increased penetration of biodiesel into the market. Standards exist in the EU and US for biodiesel quality. The EU standard, EN 14214, controls the quality of FAME used either as a fuel itself or as a blending component in diesel fuel. This biodiesel standard specifies the minimum ester content (96.5% by mass) and maximum methanol (used in the production process), glyceride, and glycerol content. Density and viscosity ranges are specified, and the minimum cetane number, set at 51, is identical to that of petroleum diesel fuel (a discussion of the relevance of the specification parameters is given by Ferrari et al. (2011)).

The ASTM D975 standard for conventional diesel fuel allows biodiesel concentration of up to 5% by volume. Such blends are approved for safe operation in any compression-ignition engine designed to be operated on petroleum diesel (US DOE, 2012). Blends of up to 20% biodiesel in 80% petroleum diesel (B20), controlled by ASTM D7467 (B6-B20), are the largest outlet for biodiesel in the United States and provide a good compromise in balancing cost, emissions, cold-weather performance, and materials compatibility. Operation on B20 and lower-level blends does not in principle require engine modification however not all diesel engine manufacturers warrant their products for use with such blends. ASTM D6751 regulates the specification of B100 for use as both a fuel and a blending component in petroleum diesel.

The use of FAME as a fuel can cause increases in nitric oxide emissions but generally results in significant reductions in other toxic emissions. Gardiner et al. (2011) used FAME from feed stock sources which included tallow, jatropha, and rape methyl ester in a 3.0 litre V6 diesel engine with an EU V emissions calibration. They found that the emissions, measured over the New European Drive Cycle, were a function of the feed stock type from which the FAME was made. For B10 blends the increase in NOx emissions over petroleum diesel varied between 8% for FAME from tallow, to 20% for FAME from rape seed – the same fuels in B30 blends caused NOx levels to increase by about 13% and 22%, respectively. The NOx emissions changes from jatropha use were intermediate between the two. Particulate emissions were reduced in the range 1–2% for rape seed FAME and by 20% for tallow FAME using B10 blends. The B30 tallow blend reduced particulates by almost 50%. For hydrocarbon emissions the spread was much lower: the use of FAME resulted in a decrease ranging from about 12% to 17% for all the B10 blends and 23% to 28% for the B30 blends. For all emissions, the changes resulting from the use of a B60 tallow FAME were of a similar order (slightly higher) to the those found for the most sensitive B30 fuel. The increases in fuel consumption found were in the range 1% or less for the B10 blends, 2% for the B30 blends, and 7% for the B60 blend tested. It was found that the increased NOx emissions can be negated by small adjustments in the EGR rate used which has a minor impact on the particulate emissions (which are substantially reduced in any case).

Whilst there can be significant emissions benefits from the use of biodiesel it seems that, in the form of FAME, there is an industry-wide desire to limit its use to relatively low-level blends. The quality of conventional diesel fuel in the EU is controlled by the EN 590 standard which currently limits FAME content to 5% by volume (B5). An extension of the FAME limit to 7% (B7) is proposed, with no limit to levels of HVO or 'diesel-like hydrocarbons made from biomass using the Fischer-Tropsch process' (EC, 2009). The varying properties of FAME as a function of its feed stock gives rise to conservatism on the part of the vehicle manufacturers to warrant their vehicles for use with higher blends and may be an obstacle to achieving compliance with the RED and FQD. Additional bio-content may be required from HVO or BTL fuels or from a disproportionate increase in the use of ethanol in gasoline blends.

3.3 Alcohol fuels: physico-chemical properties

The principal alcohols which have been used or are being considered for use as either a component in fuel blends with gasoline or as the dominant fuel blend constituent themselves are methanol, ethanol, and butanol. Methanol (CH3OH) and ethanol (C2H5OH) have one and two carbon atoms, respectively, and no isomers, whilst butanol (C4H9OH) has four carbon atoms and four structural isomers. The higher-order alcohols, that is, those with more than two carbon atoms (primarily propanol, butanol, and pentanol (amyl alcohol)), formed in the fermentation process are sometimes known as fusel alcohols.

As the number of carbon atoms increases the influence of the functional hydroxyl (OH) group on the physico-chemical properties of the molecule diminishes. The presence of the OH group in place of one of the hydrogen atoms of an alkane induces significant polarity in the molecule due to the two lone pairs of electrons present on the oxygen atom (Pearson and Turner, 2012). The concentration of negative charge around the oxygen atom produces a net positive charge on the rest of the alcohol molecule, which, in particular, is focused around the hydrogen atom attached to the oxygen atom of the hydroxyl group. This intra-molecular polarity generates strong inter-molecular forces, known as hydrogen bonds. These forces give the lowcarbon-number alcohols higher boiling points and enthalpies of vaporisation than would be expected for non-polar compounds of similar molecular mass. As in FAME-based biodiesel however, the presence of the oxygen atom reduces the energy density of the fuel. Its impact is more pronounced the smaller the molecule so that methanol has less than half the gravimetric energy density of gasoline (generally a mixture of C4 to C8 hydrocarbons) and about 40% of that of methane, the corresponding C1 alkane. More detail on the properties of alcohols as fuels can be found in Pearson and Turner (2012).

3.3.1 Volumetric energy density and stoichiometry

The impact of alcohol concentration on the volumetric energy density of gasoline blends with ethanol, methanol, and butanol is shown in Fig. 3.1. For 10% alcohol concentration by volume in gasoline, the reduction in volumetric energy density relative to the gasoline is 1.5%, 3.3%, and 5%, for butanol, ethanol, and methanol, respectively, whereas for 85% alcohol concentration the respective reductions in energy density are 12.8%, 28%, and 42.5% for these alcohols. With vehicles which use engines not optimised for use with alcohol fuels (which is the current situation, even with FFVs) the consequence of this reduced volumetric energy content is approximately proportional increase in volumetric fuel consumption for operating cycles which are not sufficiently aggressive to exploit the higher octane numbers of the resulting fuel blends. A consequence of this is that it is difficult to achieve significant market penetration of fuels with high alcohol concentrations with a volumetric-based fuel taxation system, rather than one based on the energetic content of the fuel, or the non-renewable carbon component (Turner and Pearson, 2008).

As a corollary to the presence of the oxygen atom in alcohol fuels having an impact on reducing the volumetric energy content which reduces as the length of the carbon chain increases, Fig. 3.2 shows how the stoichiometric

3.1 Volumetric energy density variation of alcohol-gasoline blends.

air–fuel ratios of gasoline-butanol mixtures is closer to that of gasoline than those of gasoline-ethanol and gasoline-methanol mixtures. This is one of the reasons that gasoline standards have oxygenate limits. The impact of the alcohol concentration on the oxygen mass fraction is shown in Fig. 3.3. The European EN228 standard for gasoline currently limits the mass fraction of oxygenates in the fuel to 2.7% – this limit corresponds to about 7% by volume of ethanol in the blend (E7) and 11% by volume of butanol (Bu11). Methanol is limited as a specific component to 3% by volume. The increase in the oxygen level to 3.7% specified in the Fuel Quality Directive (EC, 2009) allows E10 and Bu15 blends. Methanol content is again limited to 3% by volume. For comparison, a 3.7% oxygenate limit allows approximately 22% MTBE or ETBE by volume in gasoline – these fuels can be made from methanol and ethanol, respectively, with a process energy overhead.

3.3.2 Vapour pressure

Vapour pressure is an important property of automotive fuels, influencing the start quality of an engine in cold ambient temperatures (if the vapour pressure is too low), and affecting the evaporative emissions from the vehicle (adversely if the vapour pressure is too high). Gasoline specifications stipulate allowable vapour pressures dependent on the season, geographical location, and alcohol content – these are extensively reviewed by Andersen et al. (2010). Figure 3.3 shows how vapour pressure (calculated as Reid vapour pressures – or RVP) of methanol, ethanol, and iso-butanol vary as a function of the volumetric concentration of the alcohol in a blend with a reference

3.2 Stoichiometric air–fuel ratio of alcohol-gasoline blends (MF = mass function).

gasoline fuel (RF-02-03 (Turner et al., 2012b; Pearson et al., 2014)). Both ethanol and methanol cause a peak in measured RVP at concentration levels between 5%–10% by volume, increasing the value relative to the base gasoline by about 8 kPa and 23 kPa respectively. This requires an adjustment in the vapour pressure of the blend-stock for oxygenate blending (BOB) in order for the blend to remain below normal specified limits. The EU Fuel Quality Directive (EC, 2009) allows a derogation from the maximum summer vapour pressure for low level ethanol blends.

Andersen et al. (2010) found that, although tert-butanol (2-methylpropan-2-ol – which is a solid below 25 °C) causes a slight increase in RVP at 5% volumetric concentration, the three other butanol isomers (n (or 1) –butanol (butan-1-ol), 2-butanol (butan-2-ol), and iso-butanol (2-methyl-1-propanol) all caused a monotonic reduction in the RVP level of blends with gasoline. Figure 3.3 predicts this behaviour for iso-butanol. Andersen et al. (2010) also discuss the possibility of using 'dual-alcohol' blends with gasoline where 1-butanol is used in combination with ethanol and methanol in order to limit the vapour pressure rise of adding the latter two alcohols in low concentrations. In these blends the need to use the higher octane isomer isobutanol was obviated by the high octane values of ethanol and methanol.

Figure 3.3 shows that, at high concentrations, the vapour pressure of the methanol and ethanol blends reduces below that of the gasoline blend-stock: this occurs at about 80% and 45% by volume for methanol and ethanol, respectively, with the gasoline used in this case. At high concentrations this can cause low temperature cold-start difficulties in unmodified engines but this issue is easily over-come in flexible-fuel vehicles (FFVs), particularly with direct fuel injection technology by injecting into the hot compressed air found close to top-dead-centre. Siewart and Groff (1987) have achieved cold start at –29 °C using charge stratification and late fuel injection. Kapus et al. (2007) and Marriott et al. (2008) have proposed high pressure late injections, using several split injections to further augment the quality of the start. For port-fuel-injected engines measures such as heating the fuel rail can enable acceptable cold-start performance down to –25 °C. Bergström

3.3 Calculated variation of vapour pressure of methanol, ethanol, and iso-butanol with alcohol volume fraction. For validation data see refs (Turner et al., 2012b; Pearson et al., 2014). The base gasoline has an RVP of 65 kPa. Courtesy M. Davy, Loughborough University.

et al. (2007a) report acceptable cold starts down to –25 °C in the absence of additional technology on an engine with port-injection (PI) of fuel using a Swedish winter grade bioethanol (E75 with a Reid vapour pressure of 50). Hadler et al. (2011) discuss multiple injection strategies and the effects of fuel rail pressure on the low-temperature (–28 °C) start quality of a downsized boosted engine used in a production FFV. Nakata et al. (2006) show that methanol is easier to start from cold than ethanol, and discuss various methods to improve the situation significantly. In the US the ASTM D5798 standard for E85 can allow the ethanol concentration to be as low as 51% by volume ethanol – this provides a significantly easier blend for low-temperature starting.

The non-ideal behaviour of alcohol/gasoline mixtures (i.e. not obeying Raoult's law) is thought to be caused by hydrogen bonding where four alcohol molecules link together to form a 'quasi-super-molecule' known as a cyclic tetramer (Furey, 1985). In the case of methanol, these cyclic tetramers have an effective molecular mass of 128 (four times that of an individual molecule) rendering the vapour pressure of the pure methanol relatively low compared with that of the gasoline blend-stock. The hydrogen bonds are progressively weakened and become less extensive when the alcohol is mixed with increasing quantities of gasoline, which acts as a non-polar solvent, making them behave as low molecular mass components (32 in the case of methanol) which increase the vapour pressure of the mixture. Turner et al. (2012b) and Pearson et al. (2012b) show vapour pressure data for the iso-stoichiometric blends of gasoline, ethanol, and methanol discussed in Section 3.9.1.

3.3.3 Octane numbers

Table 3.1 shows that low-carbon number alcohols have a number of physico-chemical characteristics which are synergistic with modern engine designs, and in particular highly pressure-charge downsized engines. In particular, the high research octane numbers (RONs) of methanol, ethanol, and iso-butanol are facilitators for improved combustion phasing and lower component protection over-fuelling (to control exhaust gas temperatures). The motor octane numbers (MONs) of these fuels are proportionally less high and thus their sensitivity values (RON-MON) are very high relative to gasoline; nevertheless, these fuels would not be considered as having a low MON relative to most gasoline sold throughout the world.

Because of the significant differences in the properties of methanol and ethanol relative to most of the significant gasoline fuel components, suggestions have been made regarding the modification of the ASTM RON and MON test procedures (ASTM D2699 and ASTM D2700, respectively) in order to increase the robustness of the results. Fitting a variable fuel jet needle and

Table 3.1 Properties of 95 RON gasoline, methanol, ethanol, and iso-butanol

LHV = lower heating value *h*fg = ethalpy ('heat') of vaporisation.

Fu l e	L H V ( M J / ( kg fu l ) ) e	hfg ( k J / ( kg fu l ) ) e	De i ty ns N T P t a ( kg / l )	R O N / M O N	Bo l in g in 1 t a t p o ba ( °C ) r	S ic h. to A F R ( ) (– ) m as s	S ic h. A F R to ( le ) (– ) m o

Gasoline 95 43.2 350 0.74 95.3/85.9 37–167 14.31 47,12 3.02 24.5 72.85 Methanol 20.09 1100 0.79 108.7/88.6 65 6.44 7.14 3.12 170.9 68.44 Ethanol 26.95 925 0.79 108.6/89.7 79 8.86 14.29 3.01 103.3 70.98 Iso-butanol 33.08 585 0.81 106.3/90.4 118 11.14 28.57 2.97 52.5 71.90 1Note that the properties of '95 RON gasoline' vary considerably depending on local refinery streams and other blend components.

LHV (MJ/(kg stoich. air))

*^h*fg (kJ/(kg stoich. air)) CO2emission (g/MJ)

an additional heating device were recommended by Hunwartzen (1982) on behalf of the German Institute of Standardization (DIN) in order to overcome the lower volumetric energy content of alcohol fuels and their high heats of vaporization. Results shown by Anderson et al. (2012b) indicate that using the latest GE Energy CFR engine and control equipment specification and following a robust warm-up procedure obviates the requirements for the modifications suggested by Hunwartzen (1982).

Moran and Taylor (1995) posited that, because the intake manifold temperature is not controlled during the RON test, the high heat of vaporisation of the low-carbon number alcohols would cause a physical cooling effect which would have a direct bearing on the knock-limited compression ratio. In contrast, the MON test uses a heater to try to control the intake manifold temperature to 149°C which is well above the respective boiling points of these alcohols and is thus not affected by this physical cooling effect. Yates et al. (2010) point out that, whilst the hypothesis is reasonable, it ignores the fundamental differences in the autoignition chemistry of lowcarbon number alcohol and paraffinitic hydrocarbon fuels. They show that when fuels with two-stage autoignition characteristics, such as the primary reference paraffins, are blended with fuels such as methanol and ethanol, which have single-stage autoignition chemistry, the cool flame temperature rise is progressively reduced in proportion to the alcohol concentration.

The octane numbers for blends of gasoline with methanol and ethanol vary nonlinearly with volumetric concentration, with disproportionately large increases in octane number resulting from the addition of relatively small amounts of alcohol as shown in Fig. 3.4. It can be seen that a large proportion of the octane benefit of ethanol is achieved at 20% volume fraction, with the majority of the benefit gained with E50 blends. Anderson et al. (2012b) investigated the variation of octane numbers with gasoline blendstock octane number and found that the greatest relative RON increases resulting from ethanol addition were obtained with the blendstock with the lowest RON as shown in Fig. 3.4.

The nonlinear response of RON to ethanol volume fraction means that volumetric 'blending octane numbers' are of limited value for predicting the octane number of alcohol-gasoline blends. Anderson et al. (2010) showed that octane numbers of methanol- and ethanol-gasoline blends vary approximately linearly with the mole fraction of alcohol in the blends. Subsequently Anderson et al. (2012b) refined this approach for ethanol, using a range of gasoline blend stocks, by adding an additional term containing a single scaling parameter, x, which is determined as the measured octane number of a 50:50 molar blend of the gasoline and alcohol so that the RON of any ethanol-gasoline blend is given by

$$N{\text{RON}} = (1 - x{\text{e}})N{\text{RON}{\text{g}}} + x{\text{e}}N{\text{RON}{\text{e}}} + \xi x{\text{e}}(1 - x{\text{e}})(N{\text{RON}{\text{e}}} - N{\text{RON}_{\text{g}}})$$ [3.1]

3.4 Variation of RON with ethanol concentration for various gasoline blendstocks. Based on data in Anderson et al. (2012b).

where *x*e and *N*RONe are the mole fraction and octane number, respectively, of ethanol, and *N*RONg is the octane number of the gasoline used in the blend. The MON of any ethanol-gasoline blend is given by an analogous equation. Clearly when x is zero a simple linear relationship based on mole fraction exists.

The disproportionate increase in octane numbers obtained with relatively low concentrations of the low-carbon number alcohols is more easily understood by plotting the relationship between volume fraction and mole fraction for several different alcohols mixed with a nominal gasoline fuel treated as a single component using appropriately averaged properties as shown in Fig. 3.5. For mixtures of components where there is a large difference in the ratio of component density to its molar mass there will be a large difference in values of concentration expressed in terms of mole fraction and volume fraction. Thus, since methanol and ethanol have low molecular masses relative to gasoline, and similar densities, there is considerable difference between the concentrations defined in terms of volume fraction and mole fraction. For ethanol a 10% volume fraction in gasoline equates to 23% ethanol concentration by mole fraction, i.e. 23% of the molecules in the mixture are ethanol molecules. The relative uplift in concentration numbers reduces for higher level blends: E20, E50, and E85 defined on a volumetric basis become, respectively E40, E73 and E94 on a molar basis. In the case of methanol, M10, M20, M50, and M85 defined on a volumetric basis become, respectively M30, M49, M79, and M96 on a molar basis. These differences in implied concentration between volume fraction and mole fraction are useful to bear in mind when assessing other mixture properties.

3.5 Mixtures of alcohols with gasoline: mole fraction of alcohol as a function of volume fraction.

The high heats of vaporisation of the low-carbon number alcohols (see Table 3.1) have led to a significant amount of work trying to resolve the chemical and physical effects of ethanol on the knock limit of an engine. Using both DI and PI fuel systems on a modern turbocharged SI engine, Kasseris and Heywood (2012a, 2012b) infer an effective octane number from the knock behaviour of the engine which they claim represents only the chemical anti-knock benefit of ethanol. As expected from the above discussion and the trends shown in Fig. 3.4, they show that this chemical octane benefit varies in a nonlinear manner, with there being little benefit from blends of over 40% by volume ethanol with the premium grade 97 RON gasoline blendstock which was used. In contrast, the evaporative cooling benefits of the blends leads to the 'evaporative octane number', inferred from the anti-knock response of the engine, rising monotonically up to the E85 blend.

Stein et al. (2012) used a modern single-cylinder research engine with DI and PI fuel systems, together with a 'upstream fuel injection system' (UI) in order to examine physical and chemical octane benefits of ethanolgasoline blends. The UI system, which used a high-pressure DI injector to finely atomise the fuel, was intended to give the fuel maximum chance to evaporate in the intake charge before entering the cylinder at the controlled air temperature of 52°C so that the chemical octane characteristics of the fuel could be examined in isolation. They show that for stoichiometric mixtures, the specific enthalpy of vaporisation of the ethanol-gasoline blends used varies between about 25 kJ/kg of air for E0 to 80 kJ/kg of air for E75. Hydrous ethanol (94.2% ethanol, 5.8% water) had an enthalpy of vaporisation of about 125 kJ/kg of air. Stein et al. (2012) show that the effect of chemical octane and the effect of charge-cooling due to fuel vaporisation are both significant for high ethanol blends with DI fuel systems and contribute to knock avoidance to a similar degree. For higher octane fuels of nominally the same RON it is shown that the higher ethanol blends allow a higher NMEP for a given combustion phasing (the crank angle position after topdead-centre corresponding to the point at which 50% of the fuel mass in the cylinder has been burned: CA50) or, conversely, the higher octane blends allow a more optimally phased combustion event for the same NMEP. This effect is more pronounced at higher loads. Stein et al. (2012) also showed that both denatured ethanol (97% ethanol, 1% water, 2% gasoline) and hydrous ethanol are extremely knock resistant, allowing CA50 values of about 15° ATDC at 25 bar NMEP at 14.0:1 compression ratio.

3.4 Alcohol fuels for spark-ignition engines: effects on performance and efficiency

3.4.1 Performance

Nakata et al. (2006) used a high compression ratio (13:1) naturally aspirated port-fuel injected spark-ignition engine and found that torque increased by 5% and 20% using E100 compared with the operation on 100 RON and 92 RON gasoline, respectively. The full improvements in torque due to being able to run MBT ignition timing were apparent for E50. Marriot et al. (2008) show significant performance benefits for E85 (RON measured as 107.7) compared with a 104 RON gasoline when used in a naturally aspirated directinjection gasoline engine. Peak torque generated by the engine increased by 5% and peak power by about 4% at the same enriched air–fuel ratio. An increase in volumetric efficiency of about 3% was measured at the peak torque operating point. Smaller but still significant performance benefits were available from operation at stoichiometric conditions when using E85 fuel. The majority of the combustion-related benefit in performance using E85 was determined to come from a reduction in the cumulative heat energy rejected to the engine coolant.

Korte et al. (2011) and OudeNijeweme et al. (2011) show significant synergies between the high octane properties of alcohol fuels and pressurecharged down-sized engines. Alcohol blends were made up using a 95 RON blendstock so that the E10 blend had a RON of 99 and the 16% by volume isobutanol blend (Bu16) had a RON of 98. Note that due to the use of a 95 RON gasoline blendstock the value of 99 RON for E10 is higher than that of a typical commercial E10 blend, even within Europe. Octane numbers of alcohol fuels with various blendstocks are discussed by Anderson et al. (2012a; 2012b) and shown in Fig. 3.4 for generic gasoline blend components.

Other blends used were E22, which matched the 102 RON of the high-octane

forecourt-specification gasoline included in the study, Bu68 (RON 104) and E85 (RON 106). Korte et al. (2011) conclude that all the high octane fuels used show a synergy with down-sizing under high-load conditions and that such fuels enable either higher levels of downsizing or increased compression ratios to be used, leading to additional CO2 improvements. In related work using substantially the same blends, Stansfield et al. (2012) showed that it is possible to use high-alcohol blends in an unmodified production vehicle which has not been sold as having flex-fuel capability. Here E85 gave appreciably the most power (approximately 20% more than 95 RON gasoline, with the other blends forming a fairly tight band lying between the two).

The engine used in the Tri-Flex-Fuel vehicle described in Section 3.9 was modified to realise the benefit of the charge cooling effect at full-load by introducing a portion of the fuel load upstream of the supercharger. The significant full-load thermal efficiency and performance benefits, facilitating the use of MBT spark-timing at nearly all full-load speed points using E85 fuel in this engine, have been described in detail elsewhere (Turner et al., 2007a; 2007b).

3.4.2 Efficiency

Using E100 a full-load thermal efficiency at 2800 rev/min. of 39.6% was reported by Nakata et al. (2006), compared with 37.9% and 31.7% using the high and low-octane gasoline, respectively. A thermal efficiency improvement of 3% was achieved using E100 over the 100 RON gasoline at the 2000 rev/min/2 bar brake mean effective pressure (BMEP) operating point, where the engine was far from the area where knock becomes a limiting factor. The efficiency benefit using ethanol in the part-load region was attributed to the lower heat losses due to the reduction in combustion temperatures. At high loads, when the engine is knock-limited, spark timing must be retarded and this leads to higher exhaust gas temperatures. Typically a pre-turbine temperature limit is imposed. High octane fuels enable more advanced combustion phasing which in turn requires lower levels of over-fuelling in order to remain within the temperature constraints.

Marriott et al. (2008) point out that the customary practice of using specific NOx emissions as a surrogate for indicating internal exhaust gas residual level is not accurate when comparing the effects of fuels with high ethanol contents due to the differences in adiabatic flame temperature and enthalpy of vaporisation. They used instead a model which considers the integrated curtain areas of the valve overlap period and the pressure differential across the engine to show that, at part load (away from areas of the operating map which are knock limited), and at a fixed trapped residual fraction, combustion stability with E85 fuel was better than that obtained using the baseline 95 RON gasoline fuel. This effect correlated well with the faster 0–10% mass fraction burned (MFB) period. The higher diluent tolerance resulted in reduced throttling of the engine when running on E85. Indicated engine efficiency was 5% higher when evaluated at the same combustion stability limit at 1300 rev/min/3.3 bar NMEP and the detailed analysis conducted attributed this to the reduced heat loss during combustion. The benefit of the increased exhaust residual gas tolerance of E85 relative to gasoline was found by Marriott et al. (2008) to increase with load to the extent that at 2200 rev/min/4.55 bar BMEP, the engine used was able to tolerate an additional 4.5% trapped residual gas fraction over the 95 RON baseline gasoline fuel. Up to 6% thermal efficiency benefit was found at other low speed/load points, with CO2 emissions being reduced by up to 11%, with the additional benefit above the efficiency gain being due to the low CO2 emissions per unit energy released by combustion of the alcohol fuel. Pitcher et al. (2012) have investigated ternary blends of gasoline, ethanol and methanol in a direct-injection optical engine and found that the behaviour of the fuel spray with different blends is such that operating an engine on any of them should create no issues. They also found that, for a fixed ignition angle, E85 had a faster burn rate than for gasoline, but that the latent heat may be a factor leading to a reduction in cylinder pressure during the cycle. In a production engine, this would be addressed by the flex-fuel calibration.

Bergström et al. (2007a; 2007b) found that, using a production turbocharged ethanol-gasoline flex-fuel engine with port-fuel injection, the high knock resistance and concomitant lower exhaust gas temperatures experienced when running on E85 allowed the fuel enrichment level at full-load to be reduced to the extent that, for the same limiting peak pressures as those tolerated using gasoline fuel, stoichiometric operation across the engine speed range is possible. Kapus et al. (2007) found that for identical engine performance the more favourable combustion phasing when operating on E85 at full load leads to less requirement for fuel enrichment giving a 24% improvement in efficiency compared with operation on 95 RON gasoline. Thermal efficiency improvements at full-load of over 35% relative to 95 RON gasoline have been found using E100 in a direct-injection, turbocharged spark-ignition engine (Brewster, 2007) operating at high BMEP levels.

Cairns et al. (2009) investigated the diluent tolerance of various gasoline, gasoline-ethanol, and gasoline-butanol blends in a four-cylinder internal combustion engine with an under-port direct-injection fuel system. It was found that the blends with low alcohol content (up to E22) allowed small increases in EGR tolerance whereas, for E85, the faster combustion rate in the 0–10% mass fraction burned period allowed a significant increase in EGR tolerance. Conversely, the use of alcohol blends did not improve excess air tolerance. At high loads EGR tolerance increased with increasing ethanol content. At 4000 rev/min. at a fixed combustion stability limit of 5% CoV of IMEP, EGR tolerance increased from 13.5% for 95 RON gasoline to 19% for E85. In addition to the higher octane levels of high-concentration alcohol blends enabling a more favourable combustion phasing, it is likely that this improvement in EGR tolerance is related to the significantly lower molar air-fuel ratio of low-carbon number alcohol fuels.

Korte et al. (2011) show detailed fuel flow rate maps for an engine with a peak BMEP of over 30 bar using the fuels described in Section 3.4.1. It was found that an increase in fuel flow was required with the alcohol fuels in order to obtain the same load for part-load operation. In order to protect engine components from over-heating 'over-fuelling' is applied. When the load was above 20 bar BMEP, operation on E22 (rated at 102 RON) required a lower fuel flow rate than when 95 RON gasoline was used. In order to protect engine components from over-heating 'over-fuelling' is applied. Around the peak power point, operation on E85 required approximately the same fuel flow rate as the 95 RON gasoline whilst the fuel consumption of E22 was about 20% lower. At the peak power point these fuel flow rates correspond to increases of thermal efficiency, relative to the 95 RON gasoline, of 35% and 38% for E22 and E85, respectively. The full-load torque curve of the engine could be achieved at stoichiometric air–fuel ratio using E85. Using the measured fuel maps in a vehicle simulation model Korte et al. (2011) show significant reductions of tailpipe CO2 emissions over 95 RON gasoline for 102 gasoline, Bu16, E10 E22, Bu68, and E85 blends for NEDC, Artemis, US06, and a bespoke high-load drive cycle. The magnitude of the benefits increased in the order of the listed fuels and in the order of the listed drive cycles, with E85 giving a reduction in tail pipe CO2 of approximately 8% on the NEDC and 10% on the bespoke high-load cycle. In a further paper reporting this work, OudeNijeweme et al. (2011) show that, at the 2000 rev/min/4 bar BMEP point, the engine running on Bu68 and E85 blends is operating at intake plenum pressures of 64 kPa and 61 kPa, respectively, compared with 77.5 kPa for operation on 95 RON gasoline. Thus, at this operating point, the use of E85 gave slightly lower efficiency than the 95 RON baseline gasoline fuel. No attempt, however, to exploit the increased diluent tolerance of E85 was made in order to recover the loss in efficiency.

3.4.3 The efficiency of dedicated alcohol engines

The greater dilution tolerance of methanol and ethanol was exploited by Brusstar et al. (2002) who converted a base 1.9 litre direct-injection turbocharged diesel engine to run on M100 and E100 by replacing the diesel injectors with spark plugs and fitting a low-pressure alcohol fuel injection system in the intake manifold. Running at the 19.5:1 compression ratio of the base diesel engine the PI methanol variant increased the peak brake thermal efficiency from 40% to 42% while parity with the diesel was achieved using ethanol. Cooled exhaust gas recirculation (EGR) enabled the engine to achieve closeto-MBT ignition timing at high loads, while high levels of EGR dilution were used to spread the high efficiency regions to extensive areas of the part-load operating map. Emissions of NOx, CO, and HC were extremely low operating on methanol using a conventional 'three-way' after treatment system. Particulate and aldehyde emissions were not measured due to earlier work (Bruetsch and Hellman, 1992) which had established the ability to control these to very low levels using a conventional oxidation catalyst. Low-carbon-number alcohols can, with good mixture preparation, also give low particulate emissions, particularly methanol which has no carbon–carbon bonds. Similar results were found by Brusstar and Gray (2007) using a 4.5 litre V6 diesel as a base engine. The dilution limits for methanol and ethanol were established for throttle-less operation and these also increase with the proportion of alcohol in the fuel so that, for M100, throttle-less operation from a BMEP of 16 bar down to 4 bar is possible. This was possible due to the lower molar air–fuel ratios and higher flame speeds of the fuels providing higher combustion tolerance to cooled EGR.

Vancoille et al. (2012) have confirmed the effectiveness of this approach for methanol, investigating the viability of throttle-less load control using EGR or excess air. Experiments performed on a single-cylinder engine showed that the EGR and excess air dilution tolerances of methanol are significantly higher than for gasoline. Using a turbocharged multi-cylinder engine with a 19.5:1 compression ratio, similar to that used by Brusstar et al. (2002), they found it was possible to use EGR levels of up to 30% by mass at a range of engine speeds and loads with limited cycle-to-cycle combustion variation as shown in Fig. 3.6. In this way throttle-less load control was possible down to 3 bar BMEP and a peak thermal efficiency of 42% was obtained. The engine operating maps from this work were used by Naganuma et al. (2012) to model the potential benefits of a dedicated methanol-fuelled vehicle. The predictions indicated that a vehicle using an engine optimised for operation on M100 would increase the average engine efficiency on the NEDC from 22.8% for operation on baseline gasoline to 26% for M100, reducing CO² emissions from 167 g/km to 132 g/km. Still larger benefits are predicted for other vehicles (Naganuma et al., 2012).

3.5 Alcohol fuels for spark-ignition engines: pollutant emissions, deposits and lubricant dilution

Bergström et al. (2007b) show that, using a vehicle with a port-fuel-injected LEV2 emissions-capable engine on the US FTP 75 cycle, NOx emissions levels are reduced for all the ethanol blends tested without secondary air injection (SAI). CO emissions levels were increased by up to 50% for the

3.6 EGR tolerance over a range of engine speeds and loads for methanol (M100) and gasoline. Courtesy S. Verhelst, University of Ghent.

highest concentration ethanol blends tested (E64 and E85) and unburned hydrocarbon emissions (uHC) were increased by about 75% using the E85 blend. They also show that when SAI is used the uHC and CO emissions are significantly reduced. For uHC emissions, the levels for E85 were reduced by more than a factor of three and became independent of ethanol concentration. Bergström et al. (2007a) show that a similar technology engine calibrated to EU IV emissions compliance using gasoline would also be compliant using E85. Turner et al. (2012c) report emissions results for an EU V vehicle with a boosted PFI engine, using a bespoke E85 blend and the iso-stoichiometric ternary blends of gasoline, ethanol, and methanol described in Section 3.9.1 While emissions analysers are known to have a lower response to oxygenated species than to pure hydrocarbons, the level of reduction in uHC shown by Turner et al. (2012c) for their blends is of the order that correcting using a ratio of 1.154 from the earlier work of OudeNijeweme et al. (2011) (measured by them for E85) still resulted in uHC emissions no worse than for gasoline.

Of the various types of uHC emissions, those of the aldehydes in particular are a potential concern, and especially those of formaldehyde. This is because in the metabolism of formaldehyde the crucial step is that from formic acid to carbon dioxide and water, which depends on folic acid and gives rise to a wide variation in fatal dose depending on the victim's age, body mass and, in the case of women, whether they are pregnant or not. With alcohol combustion, aldehydes are intermediate species of the oxidation of the fuel: methanol primarily yields formaldehyde and ethanol acetaldehyde, together with lower amounts of formaldehyde. Early work, presumably with older engine and emissions control technology than is commonly used today, suggested that generally some changes to catalyst formulation may be necessary to ensure long-term catalyst durability with methanol (Nichols et al., 1988), but this is not expected to be an issue with present-day technology: flex-fuel vehicles have been shown to be capable of meeting limits for formaldehyde when operated on E85 (West et al., 2007) and are expected to be able to do so for other alcohols such as n-butanol (Gingrich et al., 2009). It is known that aldehyde emissions can be successfully neutralised by the type of three-way catalysts typically used to control emissions of modern spark-ignition engines (Menrad et al., 1988; Wagner and Wyszyński, 1996; Shenghua et al., 2007). Gasoline may actually yield greater challenges with respect to aldehydes on legislated drive cycles in the future: Benson et al. (1995) reported acetaldehyde emissions being three times higher in a flex-fuel vehicle when operated on gasoline than E85 and, in another study, West et al. (2007) reported that gasoline had higher aldehyde emissions than E85 in a vehicle operated on the US06 drive cycle (which requires higher driving loads than is typically the case for other drive cycles). Furthermore some potential technologies to improve the knock limit of down-sized engines such as cooled EGR have been found to increase aldehyde emissions from gasoline (Gingrich et al., 2009). Pearson and Turner (2012) discuss the potential health effects arising from aldehyde emissions from the combustion of the alcohols as well as giving an overview of other safety aspects of the fuels.

Cairns et al. (2009) found, using a boosted DI engine, that E22 blends produced the highest levels of smoke emissions in their study of ethanol and butanol blends with gasoline. Negligible smoke emissions were found using E85 fuel. Additionally they found that fuel injector deposits were highest using an E10 blend whereas the use of E85 at part-load produced almost no deposits. Bergström et al. (2007a) found significant deposits accumulating on a port injector using E85 due to the presence of poly-isobutylene components in the gasoline.

Price et al. (2007) measured particulate matter (PM) number concentrations for alcohol-gasoline blends in a direct-injection engine. They found that the PM number was lowest for E85, with 95 RON gasoline, E30, and M30 blends giving similar PM number concentrations, and M85 giving the highest level. They suggested that with the high-alcohol blends the potential for PM to form at high load is due to the fact that the local AFR is above the saturation point of the mixture, leading to droplet burning which is only partially offset by the oxygen bound in the fuel. Later work by Chen et al. (2012) showed that under stoichiometric conditions high-ethanol-blend fuels gave higher PM emissions than low-blend ones when operated in a DI engine under homogeneous stoichiometric conditions. Under rich conditions the PM output from an E10 ethanol blend was much lower, however, and they suggested that this is what gives much lower PM from vehicles operating on alcohol blends when they are operated on transient drive cycles. This suggests that any stratification in the combustion chamber will have some effect on PM formation with high-ethanol-blend fuels. Conversely, Stansfield et al. (2012) reported data from vehicle tests showing that the two high-alcohol blends that they tested, E85 and Bu68, both failed the Euro V particulate mass limit, although it should be noted that the production vehicle that they were using had not been sold as being flex-fuel capable.

Bergström et al. (2007a; 2007b) and Hadler et al. (2011) describe the potential for high levels of oil dilution in the engine lubricant using ethanol, particularly if the engine is repeatedly cold-started in a cold climate and run for a short time. Bergström et al. (2007a) state that the build-up of fuel is not harmful as long as the engine is given time to reach normal operating conditions, preferably within ten cold start events.

3.6 Alcohol fuels for compression-ignition engines

The corollary of the low-carbon-number alcohols having high octane numbers is that they have very low cetane numbers (CNs). For methanol the number is so low that it can only be measured by extrapolation of test data using additives. A CN number of 3 for pure methanol and 2 for methanol with 10% water was found by Hagen (1977). Since CN is a measure of a fuel's autoignitibility pure methanol and ethanol are unsuitable for use in conventional compression-ignition engines; however, they can be used in conjunction with another fuel which is more auto-ignitable, or with an 'ignition improver' in engines modified to have increased compression ratios.

The Detroit Diesel Company (DDC) and MAN (Jackson et al., 1985) produced modified versions of their compression-ignition engines in the 1980s which ran on methanol fuel with 5% by volume 'ignition improver'. These engines ran in service in heavy-duty applications (Jackson et al., 1985; Wuebben et al., 1990). The MAN engine was a four-stroke engine using sparkassisted ignition whilst the DDC engines operated on the two-stroke cycle, controlling the scavenging and using glow plugs to assist ignition (Toepel et al., 1983; Urban et al., 1989). Urban et al. (1989) showed that the base DDC diesel engine was easily modified to run on ignition-improved methanol and could develop more power at the same level of particulate emissions. A naturally aspirated 9.9 litre 6-cylinder diesel engine was modified by Hikino and Suzuki (1989) to run on pure methanol in compression-ignition mode. The compression ratio was increased from 17.9:1 to 27:1 in order to achieve auto-ignition and used EGR to increase the intake charge temperature at low loads. Significant reduction in NOx emissions was achieved as a result of the combustion system employed. Both ethanol and methanol produce low levels of particulate emissions when used in compression ignition engines.

SEKAB have supplied renewable ethanol-based fuel designated E95 for use in compression-ignition engines (SEKAB, 2008) such as those produced by Scania. These engines have been used in ethanol-fuelled buses which operate in various Swedish cities and give 43% peak thermal efficiency compared with 44% for their diesel-fuelled counterparts, meeting Euro 5 emissions legislation. The 95% ethanol is mixed with 5% ignition (cetane) improver which is a polyethylene glycol derivative and the compression ratio of the engine is raised to 28:1. Ethanol-fuelled buses have also been demonstrated in a fleet trial in Brazil (Moreira et al., 2008), and similar engine technology has been extended to passenger cars with CI-engines (Best-europe, 2008).

3.7 Vehicle and blending technologies for alternative liquid fuels: flexible-fuel vehicles

Ethanol/gasoline flex-fuel vehicles have been in existence for many years, the first practical example being the Ford Model T which could operate on gasoline or ethanol (thought at that time to be an attractive feature for potential customers in rural areas). Kramer and Anderson (2012) discuss the market penetration of many forms of alternative fuels and the related vehicles on a worldwide basis, including ethanol, compressed natural gas (CNG) and liquefied petroleum gas (LPG). They discuss and define the different types of vehicle and show that some flexibility in the use of different fuels is important to vehicle purchasers for market penetration of both fuels and vehicles. The fact that when ethanol was developed as a transport fuel for Brazil, and that dedicated fuel vehicles were first used which subsequently restricted the uptake of ethanol, versus the acceleration of the market when true flex-fuel vehicles were offered, is cited by them as a clear example of the desirability of fuel flexibility on board the vehicle. It is interesting to note that in Brazil hydrous ethanol is offered as a fuel and can be used in flex-fuel vehicles which may also have a proportion of gasoline in the tank, easing some of the concern about phase separation (because the vehicles can be made sufficiently flexible in operation to overcome this, and agitation of the tank when the vehicle moves will also tend to keep the various components mixed together). While gasoline starting systems have been used to date on Brazilian 'total flex' vehicles, there is a move towards replacing these with heated injectors to permit cold starting even on hydrous E100. On this subject, it is interesting to note that in Brazil, even gasoline (or 'gasohol') contains 22–25% ethanol by volume, meaning that the minimum level of alcohol in fuel is equivalent to that which could be realised in the entire fuel pool in the US (or Europe) were all available ethanol eventually blended to be into it (Anderson et al., 2012a).

Nichols (2003) relates that the first ethanol flex-fuel vehicles were conversions of M85 methanol/gasoline flex-fuel vehicles, and implies that the first such ethanol-capable vehicles were effectively tri-flex-fuel between the two alcohols and gasoline. This subject will be returned to later, but this, and the existence of total flex vehicles in Brazil, amply illustrates the ease with which a vehicle can be made multi-fuel capable, provided the fuels are miscible in the tank and the engine management system capable of adjusting itself to assure smooth and clean operation on the fuels. The previously mentioned fact that hydrous E100 can be used in total-flex vehicles illustrates that the presence of water is not an inhibitor to vehicle operation (although it is accepted that the fuel system materials must be capable of resisting any combination of gasoline, ethanol, methanol and water, and that the presence of water in any such mixtures may be an issue for the fuel supply industry).

In order to ascertain the exact proportion of gasoline and ethanol in the fuel tank so that the engine management parameters can be set accordingly, generally Brazilian flex-fuel vehicles have used a so-called 'virtual' sensor to assist in adjusting the fuelling rate. Knowledge of the ratio between the two fuels is necessary because of the different volumetric heating values and stoichiometries of the various fuels supplied to the marketplace. More recently, physical alcohol sensors have become commonplace, especially in markets with challenging emissions and on-board diagnostics requirements. The following section discusses the different fuel sensing technologies and various background reasons for their deployment.

Virtual sensors are so called because during a re-fuelling event they utilise the fuel tank level sensor to obtain an approximate value for the volume of fuel added, and subsequently employ an algorithm to allow the EMS to calculate two approximate 'new' values of the stoichiometric AFR, assuming that the fuel added was either gasoline or E85 only. Because prior to key-off the EMS knew the value of stoichiometry that the vehicle was operating with then, since it has calculated two possible new values for stoichiometric AFR, on restart, it monitors the O2 sensors and looks for a perturbation in their signal. Once it has ascertained whether the AFR is swinging in the rich or lean direction it adjusts the operating parameters gradually until it locks on to the new value, and the vehicle has been 'conditioned'. Figure 3.7 shows a representation of a virtual sensor detection system. The initial swings to the new AFR can be quite rapid, but an extended period of conditioning is necessary because commercial E85 fuels can feature a reduced ethanol content of 51% and still be within the ASTM D5798 specification, and low ethanol contents have habitually been used in winter to facilitate easier starting and better low-temperature behaviour. Vehicles certified to Euro 4 level emissions standards in Europe employed such a virtual sensor, an obvious attraction of which is that since they require no additional hardware they are very cost-effective.

For diagnosis of emissions system performance and reliability under Euro 5 regulations there is a requirement to diagnose all sensors used by the EMS, meaning that there is a requirement for all flex-fuel vehicles certified to this

3.7 Representation of a virtual sensor system to detect the concentration of alcohol in the fuel system of a gasoline/alcohol flexfuel vehicle. Reproduced from Turner et al. (2012d)

emissions level to employ an additional sensor capable of directly measuring the concentration of alcohol in the fuel system. This is termed a 'physical' sensor. Physical sensors generally measure the electric permittivity (or the resistance) of the fuel. Since these values vary widely between non-polar pure hydrocarbons of the type generally comprising gasoline, and compounds with high polarity (such as the alcohols), they can be used to indicate the alcohol concentration directly. The first flex-fuel cars employed alcoholgasoline sensors because virtual sensors had not been developed by then. Using such a system the performance of the virtual sensor can be used to diagnose the performance of the physical one and vice-versa, satisfying the requirements for on board diagnostics. Figure 3.8 shows a representation of a fuel system employing a physical alcohol concentration sensor.

If there is water present in the mixture of gasoline and alcohol (as would be possible in Brazil if a total-flex vehicle is partly fuelled on hydrous E100 and gasohol, for example), then regardless of the alcohol concentration detection method employed, there needs to be sufficient adjustment possible to account for the volume of water in the fuel; such amounts of water will alter the ratio of volumetric LHV to stoichiometric air–fuel ratio (AFR).

3.8 Representation of a physical sensor system to detect the concentration of alcohol in the fuel system of a gasoline/alcohol flexfuel vehicle. Reproduced from Turner et al. (2012d)

However, since the amount of water in hydrous ethanol is generally between 4.4 and 7.0% (the former being the azeotropic limit for water in ethanol) this is less than the typical adjustment range for modern EMS and so is not an issue.

Kramer and Anderson (2012) point out that in the US, flex-fuel vehicles have been widely introduced at no on-cost to the consumer, and from this it is reasonable to assume that the on-cost to the vehicle manufacturer, while not negligible, is small enough to be off-set against the benefits of producing the vehicle from a corporate average fuel economy (CAFE) standards point of view. In other markets such as Sweden the vehicles carry only a modest price penalty, which can quickly be recouped given favorable fuel prices. Bergström et al. (2007a) and Hadler et al. (2011) give a detailed account of the engine and fuel system design modifications required in order to produce a flex-fuel vehicle from a dedicated gasoline vehicle platform. In Brazil the vehicle production cost is slightly more because of the need for a small gasoline-fuelled starting system (itself mainly necessary since hydrous ethanol can be put into the primary tank of these vehicles). Future vehicles could adopt heated components in the fuel system to vaporise the fuel and, through the consequent ability to delete the gasoline-based starting system, reduce the price to closer to that of a simple gasoline vehicle. This would mean that the payback time for the customer would be concomitantly shorter (Kramer and Anderson, 2012). Note that beyond the use of a gasoline start system, Brazilian market vehicles still use flex-fuel technology; the next section will discuss vehicles which do not have the feature of flexible fuelling from a single tank.

3.8 Vehicle and blending technologies for alternative liquid fuels: ethanol-gasoline and methanol-gasoline bi-fuel vehicles

Following the definitions of Kramer and Anderson (2012) a bi-fuel vehicle has two independent fuel systems – one for each fuel. As such it is more expensive to produce and purchase than a flex-fuel vehicle, but advantages can be accrued from the targeted introduction of different fuels into the cycle. While Brazilian flex-fuel vehicles can be thought of as bi-fuel insofar as they use a separate gasoline tank for starting, which must be kept topped-up in addition to the main fuel tank, in some recently-proposed bi-fuel vehicles more is made of the advantages of each individual fuel.

In the concept proposed by Cohn et al. (2005) the main fuel used in a downsized engine would be gasoline. The engine would be modified to operate with higher boost pressures and/or compression ratio (i.e. a higher degree of downsizing and Otto cycle efficiency for fuel economy at part load), and alcohol would be directly injected to help suppress knock at high load. The alcohol would only be used when necessary. The gasoline could be directly injected via its own in-chamber injector, but generally it is imagined that it would be introduced into the intake port via a low-pressure injection system. The use of gasoline also permits cold start at low ambient temperatures and would also contribute to particulate emissions compliance. Cohn et al. (2005) performed some calculations showing that significant improvements in overall fuel economy are possible, and their work was continued in a US DOE-funded programme using a production direct-injection engine as described by Stein et al. (2009), who demonstrate that approximately five volume units of gasoline can be replaced by one unit of ethanol. The earlier work of Cohn and co-workers similarly suggested that, despite its lower LHV, methanol could be used to replace ethanol at the same volumetric rate; this was because the higher enthalpy of vaporisation had a greater knocksuppression effect per volume unit of directly-injected alcohol. Blumberg et al. (2008) have shown that using this approach, it would be possible to use alcohol and gasoline bi-fuel engines in heavy duty trucks.

Bi-fuel operation of diesel engines using CNG as the main fuel but with pilot injection of diesel for ignition purposes is also possible. Such conversions can offer significant reductions in exhaust emissions. They would also permit operation on diesel should the CNG tank be emptied, helping to avoid range anxiety. However, future dedicated spark-ignition CNG engines are likely to be used as the infrastructure for vehicular use of natural gas develops, since the cost of the diesel injection equipment (and associated exhaust after treatment (EAT) and fuel lines and tanks) is very high compared to a spark ignition system and its simpler EAT. Such concepts do allow the large recent finds of natural gas to be introduced to road transport, and while it is not carbon-neutral, natural gas does bring a reduction in GHG emission of over 20% due to its high hydrogen-to-carbon ratio (Hadler et al., 2011). Furthermore, methane (the main constituent of natural gas) can be decarbonised using biomethane or e-methane (also known as renewable power methane (Specht et al., 2009; Breyer et al., 2011)), so that this route also provides a means of gradually reducing fossil CO2 emissions.

3.9 Vehicle and blending technologies for alternative liquid fuels: tri-flex-fuel vehicles and iso-stoichiometric ternary blends

As mentioned previously, it is possible to create vehicles capable of operating on any volumetric proportion of methanol, ethanol and gasoline. Pearson et al. (2009) report one such vehicle which used a physical alcohol sensor to infer the approximate bulk alcohol concentration to set the ignition timing and a wide-range oxygen sensor to provide arbitrary control of the injector pulse-width using the oxygen sensor. Emissions capability of this vehicle was demonstrated by operating it on different ratios of the three fuels.

If a vehicle has originally been developed to operate on high-blend methanol fuels and gasoline, it is relatively straightforward to modify it to accept ethanol as well, as discussed by Nichols (2003). However, if the vehicle has not been developed to function satisfactorily on high proportions of methanol in an alcohol-gasoline system, it is still possible to introduce it into the fuel pool as a major fuel component. This can be achieved through the adoption of targeted blends of gasoline, ethanol and methanol with specific ratios targeting the stoichiometric AFRs of any ethanol-gasoline binary mixture that a flex-fuel vehicle has been developed to operate on. The following section discusses this possibility.

3.9.1 Iso-stoichiometric ternary blends

Gasoline, ethanol and methanol are all miscible together (with or without cosolvents to avoid phase separation, which varies with temperature). Ternary, or three-component, blends of them can be configured to have the same target stoichiometric air–fuel ratios (AFRs) as any binary gasoline-ethanol blend. Work has been conducted to date concentrating on such 'GEM' ternary blends with a target stoichiometric AFR equivalent to that of E85, i.e. 9.7, but equally GEM blends equivalent to E10, E22, etc. could be arranged. For a fixed stoichiometric AFR the relationship between the three components is defined by linear volumetric relationships as shown for E85-equivalent stoichiometry in Fig. 3.9. Importantly, when configured in such a manner, all such iso-stoichiometric blends not only have near-identical volumetric LHV, but have been found to have practically the same octane numbers and

3.9 Relationship between blend proportions of gasoline, ethanol and methanol in iso-stoichiometric GEM ternary blends configured with a stoichiometric AFR of 9.7. Reproduced from Turner et al. 2012a.

extremely close enthalpies of vaporisation (to +/– 2%, based on the mass ratios in the blend). In Fig. 3.9 one can see that as the volume percentage of ethanol is reduced, so the rate of increase of the methanol proportion is faster than that of the gasoline proportion, because as one volume unit of ethanol is removed, a volume unit of the binary gasoline-methanol mixture with the same stoichiometric AFR as ethanol has to be used to replace it. The necessary volume ratio of gasoline: methanol to give the required 9:1 stoichiometic AFR is 32.7:67.3, as discussed by Turner et al. (2011).

Note that Fig. 3.9 clearly shows that E85 contains no methanol and that its binary equivalent for a gasoline and methanol mixture (where no ethanol is present) occurs at volume percentages of 44 and 56, respectively. This is the left-hand limit for this stoichiometry. It is interesting to note that the ratio where the proportion of gasoline and methanol are equal occurs at approximately 42.5 volume percent ethanol, which is coincidentally half the volume which would be present in E85.

Experimental results using four GEM blends in a production vehicle using the virtual sensor approach discussed previously showed that, provided a certain minimum level of ethanol was present, the blends were invisible to the on-board diagnostics (OBD) system of the vehicle's EMS (Turner et al., 2011). In this context, ethanol performs the function of a co-solvent for gasoline-methanol mixtures well. Later, the minimum level of ethanol concentration required in a GEM blend was further investigated in another car using a physical sensor and certified to a higher emissions standard, but in this case, no minimum requirement for ethanol was identified. The fact that iso-stoichiometric GEM blends function invisibly to a physical sensor-based EMS was expected from earlier tests on the prototype tri-flex-fuel vehicle already discussed (i.e. one in which it did not matter what proportion of the three possible fuels, gasoline, ethanol and/or methanol was used). In this vehicle binary ethanol- and methanol-gasoline blends of known volumetric ratios and varying composition were tested and the response from its physical sensor recorded. Fig. 3.10 reproduces this data and clearly shows that the more-polar methanol molecule produces a greater response from the sensor than the ethanol molecule, as might be expected. The data recorded for ethanolgasoline mixtures correlated well with the information published by the supplier of this sensor. Using the letter and number conventions described in Turner et al. (2011) and Turner et al. (2012b) to denote the volume proportions in percent of the three components, and noting that the two limiting ternary blend cases of G44 E0 M56 and G15 E85 M0 produced a sensor response of 134 Hz and 129 Hz respectively, the maximum difference is only of the order of 4% across all possible ternary blends at 9.7:1 stoichiometric AFR, in turn suggesting that a physical-sensor-based EMS should have no issues with control of the system.

In the series of tests utilising the physical sensor, the vehicle was repeatedly cooled to –20 °C and cold start tests were performed (Turner et al., 2012b). No issues were recorded using any blend containing methanol; in fact, the blend with ethanol only (i.e. no methanol) was the only one which would not start at this temperature, all of the others being found to start as well as gasoline. Therefore, it seems reasonable to deduce that when considering both virtual and physical sensor technology then there is a minimum ethanol concentration needed to ensure satisfactory operation of all of the vehicles in

3.10 Response of production alcohol fuel sensor to ethanol and methanol in binary mixtures with gasoline. Limiting cases for GEM ternary blends at 9.7:1 (i.e. E85-equivalent) are shown.

the fleet, and that from these tests this minimum level appears to be set by the virtual sensor technology at in the region of 10% by volume. It follows from this that the most extreme equivalent ternary blend which could be used in all vehicles without the addition of other co-solvents is G40 E10 M50.

From this initial work the basic functionality of existing production flex-fuel technology with ternary blends was shown. In concert with this, efficiency improvements on the order of 5% were shown to be possible for any of the E85-equivalent ternary blends tested, regardless of the fuel sensing technology used (Turner et al., 2011; Turner et al., 2012b). This has important implications for the amount of primary energy that would have to be supplied to manufacture electrofuels when using the approach described in Section 2.5 of Chapter 2 is considered. Alternatively, the low cost of methanol when it is made from natural gas could also mean that, per unit energy, such a blend fuel could be made cheaper than gasoline (Turner et al., 2011; 2012b), which means that there are potential benefits for most stakeholders in the private transportation arena.

Preliminary investigations into material compatibility have been reported by Turner et al. (2012c), together with results showing that the primary exhaust pollutants can be readily controlled; all E85-equivalent ternary blends essentially out-performing commercial 95 RON gasoline in this respect, although particulate and aldehyde emissions were not measured in these tests. Proposals for fleet trials of such E85-equivalent ternary blends in production cars have been put forward in the US (where the methanol would be made from the extensive natural gas reserves) and Sweden (where there is interest in using wood waste to make it). In the US this may be especially attractive since there are already 9 million E85/gasoline flex-fuel vehicles in use, a result of their improved performance in terms of gasoline usage under US CAFE regulations, and a suitable fleet trial structure could easily be arranged since government fleets are mandated to be flex-fuel with suitable pumps and tanks. In any location a progressive subsequent rollout by methanol percentage and region could allow introduction of methanol as a major transport fuel component contributing to meaningfully reduced greenhouse gas emissions, improved energy security and better air quality before a vehicle specification change to M100 has to be adopted (e.g. as proposed by the US Open Fuel Standard). This approach was discussed in Turner et al. (2012a).

Note that, should their use be beneficial with regard to the utilisation of all available feed stocks, it is possible to produce ternary blends of other alcohols with gasoline, or even quaternary (and higher) blends. Examples of these may be mixtures of gasoline, methanol and butanol with or without ethanol, respectively. A more detailed description of the physicochemical properties of some of these mixtures, with additional remarks on iso-stoichiometric hydrous GEM blends, is given by Pearson et al. (2014).

3.10 Conclusions

This chapter has reviewed the impact of alcohol fuels such as methanol, ethanol and butanol on the properties of gasoline blends and has described their impact on engine performance, thermal efficiency, and emissions. It has also discussed vehicle technologies which can be employed to support the use of fuel blends having a high concentration of alcohol such as the use of sensors to support flexible-fuel vehicle operation. Whereas there are greater challenges in ensuring consistent quality in FAME-based biodiesel fuel blend components, the adaptation of engines and vehicles to use high concntrations of alcohol fuel components can can be affected at low on-cost with attractive performance and emissions levels.

The concept of iso-stoichiometric ternary blends enables mixtures of methanol and ethanol to be blended with gasoline in order to match the stoichiometric air–fuel ratio and volumetric energy density of a nominal ethanol/gasoline (binary) blend. The additional ability of these ternary blends to maintain relatively constant octane numbers enables them to be considered as having the potential to be considered as drop-in replacements for binary alcohol/gasoline specifications and thus provide a more fungible platform for renewable liquid fuels.

3.11 Acknowledgements

The authors wish to thank the following people for the benefit of discussions with them during the compilation of this work: Arthur Bell (SASOL), Martin Davy (University of Oxford), Eelco Dekker (BioMCN), Peter Edwards (University of Oxford), Matt Eisaman (Brookhaven National Laboratories), Stefan de Goede (SASOL), Ben Iosefa (Methanex) Karl Littau (Stanford University), Leon diMarco, Richard Stone (University of Oxford), Andre Swartz (SASOL), Gordon Taylor (GT-Systems), Sebastian Verhelst (University of Ghent), Chris Woolard (University of Cape Town), Paul Wuebben (CRI).

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3.13 Appendix: abbreviations

AFR air–fuel ratio

ATDC after top-dead-centre

BXX blend of XX% by volume of biodiesel (FAME) in diesel

BMEP brake mean effective pressure BOB blend-stock for oxygenate blending

BTL biomass-to-liquids

BuXX blend of XX% by volume of butanol in gasoline

CA50 the crank angle position after top-dead-centre corresponding

to the point at which 50% of the fuel mass in the cylinder has

been burned.

CAFE corporate average fuel economy CFR Co-operative Fuels Research

CN cetane number CNG compressed natural gas CoV coefficient of variation DI direct (fuel) injection EAT exhaust after treatment ECU electronic control unit EGR exhaust gas recirculation

EMS engine management system ETBE ethyl-tert butyl ether

EXX blend of XX% by volume of ethanol in gasoline

EU European Union FAME fatty acid methyl ester FFV flexible-fuel vehicle

GEM gasoline-ethanol-methanol GHG greenhouse gas

HC hydrocarbon

HFCEV hydrogen fuel cell electric vehicle

HHV higher heating value

ICE internal combustion engine

ICEV internal combustion engine vehicle IMEP indicated mean effective pressure

LHV lower heating value LPG liquid petroleum gas

MBT minimum advance for best torque

MFB mass fraction burned MON motor octane number MTBE methyl-tert butyl ether

MXX blend of XX% by volume of methanol in gasoline

NEDC New European Drive Cycle NMEP net mean effective pressure

NO^x oxides of nitrogen OBD on-board diagnostics PI port (fuel) injection POME palm oil methyl ester RME rapeseed methyl ester RON research octane number SAI secondary air injection

SI spark-ignition SME soya bean methyl US United States

Alternative and renewable gaseous fuels to improve vehicle environmental performance

M. Mintz, J. Han and A. Burnham, Argonne National Laboratory, USA

DOI: 10.1533/9780857097422.1.90

Abstract: This chapter reviews the different types of gaseous fuel for vehicles, fuel production and distribution processes, engine/fuel systems and on-board storage options. This is followed by a discussion and comparison of life-cycle energy use and greenhouse gas (GHG) emissions. Finally, the advantages and limitations of each fuel are summarized along with a general discussion of future trends.

Key words: natural gas for vehicles, biomethane, liquid petroleum gas (LPG), life-cycle analysis.

4.1 Introduction

At ambient temperature and pressure, gaseous fuels are much less energy dense than liquid fuels. As a result, gaseous fueled vehicles can require a significantly larger volume of fuel to travel comparable distances. Since space and weight are at a premium in most vehicles, increased fuel storage is usually not feasible. Thus, fuel conditioning via compression or liquefaction is critical to the use of gaseous fuels in transportation vehicles. Gaseous transportation fuels include:

∑ fossil natural gas
∑ biomethane or renewable natural gas
∑ liquid petroleum gas (LPG)
∑ hydrogen.

Each of these fuels is discussed below. After a brief description of the fuel and its production and distribution processes (i.e., the supply chain), engine/ fuel systems and on-board storage options are discussed. This is followed by a discussion and comparison of lifecycle energy use and greenhouse gas (GHG) emissions and a summary of the advantages and limitations of each fuel.

Note: This chapter has been written by employees of Argonne National Laboratory, operated by UChicago Argonne LLC for the US Department of Energy under Contract No. CE-AC02-06CH11357.

4.2 Fossil natural gas

Like coal and petroleum, fossil natural gas is a hydrocarbon fuel produced from the decay of organic material over millions of years. Although most natural gas is used for power generation, as a source of heat for residential and industrial processes or as an industrial feedstock, it has long been used as a transportation fuel in select vehicles and markets. In the past decade, advances in exploration and production technologies have dramatically increased economically recoverable supplies of natural gas. Global production is growing not only from wells drilled in deep off-shore waters, but also from the ability to tap known but previously unrecoverable on-shore deposits in tight formations (e.g., sandstone, shale and carbonate), coal seams and complex geologic structures. In North America, vast new supplies have depressed prices, resulting in an uncoupling of oil and gas prices. While this uncoupling has not yet occurred in much of Europe and Asia (where shale gas has yet to be developed and long-term gas supply contracts remain in effect) it may soon become a worldwide phenomenon. Lower gas prices, in turn, have prompted a major shift from coal to natural gas for power generation as well as increased interest in natural gas (and propane which is often produced in conjunction with natural gas) as a vehicular fuel.

Historically, compressed natural gas (CNG) has been an important fuel in countries with plentiful supplies of natural gas and policies that encourage its use (e.g., Pakistan, Iran, Argentina, Brazil, India and Italy). In the future, it may become increasingly important in countries with extensive shale gas resources (Fig. 4.1). Natural gas has also occupied an important niche for its potential to reduce emissions from buses, taxis and other vehicles operating in congested urban environments around the world.

Until recently, however, switching from gasoline or diesel to natural gas has been an expensive proposition for most users, requiring engine and fuel system modifications which can reduce power and invalidate manufacturers' warrantees, specialized maintenance and fueling equipment with attendant needs for training and regulatory compliance, and operational challenges due to reduced vehicle range and performance. Thus, in the absence of government incentives, natural gas penetration into the transportation sector has been limited primarily to fleet vehicles (i.e., buses, light and medium-duty trucks and vans) that return to a base location each day.

Moreover, a new generation of heavy-duty natural gas engines is providing diesel-like performance, refueling infrastructure is being developed along many travel corridors, and sharply lower natural gas prices are enabling many North American fleets to recover their upfront investment in two years or less. The resulting surge in North American demand is creating a further push to expand natural gas fueling infrastructure, convert existing trucks to operate on natural gas, increase the availability of natural gas

4.1 Estimated technically recoverable shale gas reserves (tcm). Source EIA 2013a.

engines offered by original equipment manufacturers (OEMs), and export natural gas to higher priced markets. At the end of 2013, four terminals (at Sabine Pass and Lake Charles, LA, Freeport, TX and Cove Point, MD were authorized to export LNG from the US and export applications for several other terminals, originally developed to import LNG, were under review by the US Department of Energy (USDOE, 2013).

It is important to note that there is no standard specification for natural gas. Each 'customer', generally a pipeline, sets an acceptable range of properties based on their current operations. Table 4.1 lists selected properties of natural gas at the wellhead (associated with oil production, non-associated with oil, or from well condensate) and in North American pipeline systems. Similarly, there is no standard specification for CNG used in transportation vehicles, although engine manufacturers and some regulatory agencies do specify some properties (Table 4.1).

4.3 Fossil natural gas production, transmission and distribution

At the wellhead, raw natural gas includes various liquid and gaseous constituents (Table 4.1). Gas processing plants reduce the concentration of constituents that exceed the allowable range of typical pipeline gas. Several processes may be used, including membrane separation, absorption,

Table 4.1 Selected properties of natural gas at the wellhead, in pipeline systems and as CNG

P i U i t t t t t o p e o c o n s e n n r r y r u		W ( i d ) D ( i d ) t t t e a s s o c a e n o n- a s s o c a e r y l l t t n a r a g a s n a r a g a s u u		C d P i l i t o n e n s a e p e n e g a s l l e g a s w		C N G f o r h i l e c e s v
So ur ce			( Fo 2 0 0 4 ) ss ,	( Fo 2 0 0 4 ) ss ,	( Fo 2 0 0 4 ) ss ,	( Fo 2 0 0 4 ) ss ,	( A R B, 2 0 1 0 )
L H V:	ra ng e	3 B / f tu t	–	–	–	a 8 6 0– 1 0 4 0	–
C H4 :	in im m um	Vo l %	6 4. 4 8	9 1. 0 1	9 4. 8 7	7 5	8 8
C2 H6 :	im m ax um	Vo l %	9 8 1 1.	8 8 4.	2. 8 9	0 1	6
C3 H8 :	im m ax um	Vo l %	8. 7 5	1. 6 9	0. 9 2	5	a 3
C4 H1 : 0	im m ax um	% Vo l	3. 8 4	0. 6 6	0. 5 3	2	–
C5 H1 : 2	im m ax um	Vo l %	1. 3 4	0. 2 7	0. 1 5	b 0. 5	–
C6 H1 : 4	im m ax um	Vo l %	0. 3 7	0. 1 3	0. 0 5	–	0. 2 c
C O2 :	im m ax um	Vo l %	–	–	–	3– 4	–

N2 & inerts: maximum Vol % – – – 3–4 4.5 H2S: maximum Grains/100 scf – – – 0.25–1.0 – H2O: maximum lb/MMCF – – – 7 –

a Adjusted from source-reported higher heating value (950–1150) to lower heating value (LHV) (GREET, 2013).

b Propane or heavier.

c Pentane or heavier.

adsorption and distillation (see Section 4.6.1). Most gas processing occurs in close proximity to the wellhead.

Transmission pipelinesare the primary method of long distance transportation of natural gas. Distribution pipelines deliver gas to local customers. As of 2011, the US had 491 000 km of natural gas transmission pipelines with 210 local distribution systems supplying 690 bcm of natural gas (EIA, undated). In the same year, Europe had 187 342 km of natural gas pipelines and supplied 463 bcm of natural gas (ENTSOG, 2011). For long-distance transmission via pipeline, natural gas is typically pressurized to 70–100 bar with compression stations located every 150–300 km along the line. For local delivery, the gas is depressurized at city gates where it enters a system of low-pressure distribution pipelines (usually 2 bar or lower). A major issue in pipeline transmission and distribution (T&D) is potential leakage. Lelieveld et al. (2005) estimate Russian pipeline systems leak or vent 1.4% (a range of 1.0–2.5%) of gas throughput during T&D. For the US pipeline system, Burnham et al. (2011) estimate that 0.67% (a range of 0.29–1.05%) of the gas produced is leaked or vented during T&D. In the US, leakage and venting emissions have decreased significantly (13% between 1990 and 2009) with the adoption of technologies like plastic piping (USEPA, 2011a).

4.3.1 Distribution of compressed natural gas (CNG)

Most CNG fueling stations are connected to low pressure, local distribution pipelines. The inlet gas, typically 2 bar or less, is compressed to 250 bar or more for on-site buffer storage in standard pressure vessels. CNG is dispensed into vehicles at 200–250 bar from a bank of pressure vessels (and associated controls) in which gas is stored at low, medium and high pressures. Known as a cascade system, gas is drawn from different tanks depending on the pressure within the vehicle's tank (as it is fueled) and that of the cascade tanks. CNG fueling stations can be dedicated to dispensing only CNG or they can dispense both CNG and other motor fuels.

According to the US Department of Energy's (USDOE's) Alternative Fuels Data Center there are currently 1290 CNG stations in the US, 645 of which are open to the public (AFDC, 2013). While that number is growing at well over 10 percent per year, it is far short of the more than 120 000 stations that dispense gasoline and diesel fuel.

4.3.2 Distribution of liquefied natural gas (LNG)

As with other liquid fuels, once natural gas has been liquefied, the most practical long-distance transportation mode is bulk transport via ocean tanker. In a typical LNG supply chain, raw gas is transported by pipeline from the wellhead to a gas processing plant and then to liquefaction facilities that are typically located near export terminals. Once liquefied, the LNG is loaded onto ocean tankers for shipping to a destination port, where it is re-gasified and then transported to customers via existing gas pipelines. For delivery to LNG fueling stations, pipeline gas may be liquefied at smaller-scale liquefaction plants and transported in cryogenic tank trucks. Clean Energy Fuels Corporation recently announced plans to deploy two such plants using General Electric's liquefaction technology in order to supply truck stops with LNG along US travel corridors (AP, 2012; Kelly-Detwiler, 2012). Similar announcements have been made by Shell which plans to build small-scale liquefaction plants in Alberta and Ontario (Canada) as well as on the US Gulf coast (Shell, 2013).

Liquefaction typically is accomplished via either the propane pre-cooled mixed refrigerant (C3/MR) process or the single mixed refrigerant and expander processes (Brendeng and Hetland, 2004). C3/MR accounts for nearly 80% of large-scale liquefaction units and almost 90% of global capacity while single mixed refrigerant and expander processes account for the rest of largescale liquefaction and some small-scale liquefaction. Liquefaction is highly energy- and GHG-intensive, with typical plant efficiencies of 88–91%, and is a substantial burden for the LNG supply chain (Vink and Nagelvoort, 1998).

At present, there are only 45 publicly accessible LNG stations in the US (AFDC, 2013). However, several major fuel retailers have announced plans to develop LNG stations along major interstate highways and pipeline systems. Clean Energy is building 'America's Natural Gas Highway', a network of 150 LNG stations to be located at truck stops operated by Pilot/ Flying J (Clean Energy, undated). Similarly, Shell and Travel Centers of America are adding 200 LNG fueling lanes at 100 truck stops along major travel corridors (Shell, undated). Other companies developing LNG stations in the US and Canada include Blu LNG, Encana and Kwik Trip.

4.4 Natural gas engines and vehicles

Globally, there are approximately 15.2 million natural gas vehicles (NGVs) on the road today (IANGV, 2012). In terms of energy use, natural gas is the second most widely used alternative motor fuel (after LPG). With annual growth of approximately 25% from 2001 to 2011 (IANGV, 2012), transportation use of natural gas is increasing in both established and new markets. The former include Iran and Pakistan which currently have the largest fleets of NGVs (about 2.9 million vehicles each), followed by Argentina (1.9 million), Brazil (1.7 million), India (1.1 million), China (1.0 million) and Italy (800 thousand). The latter includes North America where the current NGV fleet of 121 650 vehicles is becoming increasingly concentrated in high-use sectors where natural gas offers clear economic benefits (EIA, 2012). With a price advantage of \$US 1.50–2.00 per energy-equivalent diesel gallon, major shifts in purchasing patterns for new garbage trucks, buses, high-horsepower offroad equipment and long-haul trucks are occurring. Today, 40 percent of new garbage trucks and 25 percent of new buses in the US run on natural gas (Motavalli, 2012).

Natural gas has a high octane rating (i.e. antiknock characteristic) which allows an increase in power in spark-ignition engines. However, natural gas occupies a larger volume in the engine cylinder than liquid fuels, reducing the number of oxygen molecules (as a share of air in the cylinder), which reduces power. The net effect on natural gas power versus gasoline is relatively neutral, if the natural gas engine is designed to take advantage of the fuel's properties.

NGVs can be dedicated to natural gas as a fuel source or they can be bi-fueled, running on either natural gas or gasoline, or natural gas or diesel. Because most natural gas engines are spark-ignited, the usual bi-fuel pairing is natural gas and gasoline. Natural gas engine technologies differ in the method used to ignite the fuel in the engine cylinders, the air–fuel ratio, the compression ratio, and the resulting performance and emissions capabilities. In order to meet the competing demands of engine performance, efficiency, and emissions various types of natural gas engine technologies have been developed. The main types available today include spark-ignition stoichiometric, spark-ignition lean burn, and compression-ignition dual-fuel (with diesel for pilot ignition).

4.4.1 Spark-ignition lean burn engines

Spark-ignited lean burn engines operate at an air–fuel ratio higher than stoichiometric, which results in lower combustion and exhaust temperatures and often requires the use of a turbocharger to make up for the loss in power density. The main advantage of this engine design is that it operates more efficiently. The main disadvantage is that it has not been able to meet increasingly tight emission standards for nitrogen oxides (NOx).

4.4.2 Spark-ignition stoichiometric engines

Spark-ignited stoichiometric engines operate near the optimum air–fuel ratio where ideal combustion occurs (i.e. complete combustion of hydrocarbons). These engines use three-way catalysts to reduce NOx, carbon monoxide (CO), and hydrocarbon (HC) emissions, which are ideal for their typical operation as these catalysts are only effective near stoichiometric conditions. These engines do not need additional after-treatment equipment such as selective catalytic reduction or particulate filters. The disadvantage of this engine type is that it sacrifices efficiency as compared to a lean burn engine.

4.4.3 Compression-ignition dual-fuel engines

Compression-ignition dual-fuel engines use diesel fuel for pilot ignition of the natural gas. There are two main types of dual-fuel systems, a portinjected system that has a relatively simpler design and allows for full performance on diesel only and a direct-injection system that can maximize the amount of natural gas substitution but has degraded performance on diesel only. Both of these systems achieve diesel-like performance with a compression-ignition engine, while using significant amounts of natural gas. The port-injection system can achieve 60–70% substitution of natural gas for diesel under typical driving conditions, while the direct-injection system can achieve 95% substitution. The disadvantage of this engine type is that it requires additional after-treatment for emissions control (i.e., selective catalytic reduction to meet NOx standards and diesel particulate filters to meet particulate standards).

4.4.4 Off-road vehicles

In addition to its use in highway vehicles, natural gas is penetrating off-road high horsepower applications like drilling rigs, oilfield service vessels and ferries, and generating considerable interest among railroads and shipping companies. For large fuel users not requiring dispersed fueling locations, natural gas offers a viable alternative to diesel fuel. In maritime applications where operators are considering switching to ultra-low sulfur diesel or installing new emissions control equipment in order to comply with 2015 and 2016 International Maritime Organization sulfur and NOx standards, replacing aging diesels with natural gas engines may be an attractive option.

4.4.5 On-board fuel storage

Because it is a gaseous fuel at atmospheric pressure and occupies a considerably larger volume per unit of energy than gasoline or diesel, natural gas is stored on board a vehicle as either a compressed gas or a cryogenic liquid. The storage requirements are much greater than for refined petroleum products, which increases vehicle weight and tends to reduce fuel economy. To become LNG, natural gas is cooled to –162 °C and filtered to remove impurities. LNG is stored in double-walled, vacuum-insulated tanks and is primarily used in heavy-duty trucks, providing increased range over CNG.

On board a vehicle, CNG is stored in a pressurized tank or cylinder at up to 250 bar. In the US Honda Civic CNG, the tank is mounted in the trunk and replaces the existing fuel tank. In more recently developed European light-duty front-wheel-drive passenger vehicles, the tanks are mounted under the back seat and luggage compartment, thereby providing more storage space than in the Civic CNG design. In US trucks, the tank is mounted on the frame, and in US buses, it is mounted on the roof. Although tanks can be made entirely from metal, they are typically composed of lighter-weight metal liners reinforced by a wrap of composite fiber material. Because of the lower energy density of natural gas as compared to gasoline or diesel, vehicle range is generally reduced.

4.5 Biomethane/biogas

Biomethane or renewable natural gas (RNG) is produced from the decomposition of organic matter such as manure, sewage, municipal waste, green waste, plant material, food waste and crops. In its raw state the fuel is typically referred to as biogas. When derived from municipal solid waste (MSW) in landfills it is commonly called landfill gas (LFG); when derived from anaerobic digestion (AD) of wastewater, animal manure or other organic waste it is commonly called AD gas or simply digester gas. Unlike fossil natural gas which is composed mostly of hydrocarbons — 70% or more methane (CH4) plus propane and butane — biogas generally contains somewhat less methane, a significant amount of carbon dioxide (CO2), and lesser amounts of water, nitrogen (N2), hydrogen (H2), carbon monoxide (CO) and a variety of contaminants (Table 4.2).

With minor treatment, biogas can be used to generate power and heat for onsite use and excess power can be exported to the grid. Alternatvely, it can be upgraded and purified into gaseous or liquid products, known as compressed biogas (CBG), Bio-CNG©, biomethane, RNG or simply 'high Btu natural gas'. Upgraded to a standard of purity comparable to that of natural gas in local distribution pipelines, the produced gas is equivalent to fossil natural gas.

Figure 4.2 shows typical supply chains for LFG and AD gas.* For simplicity, the figure does not show the additional step of collecting the LFG (via a network of wells, headers and blowers) or piping biogas from digesters to the upgrading unit. Similarly, biogas from digesters is piped to an upgrading unit. Once the gas has been collected, the current practice for disposing of LFG (see Fig. 4.2) is mainly flaring, which is an effective means for reducing emissions of volatile organic compounds (VOCs), air toxics and methane (a potent greenhouse gas which has 25 times the global warming potential of CO2), but at the cost of producing CO2

^* For LFG, an additional step is needed to collect the gas via a network of wells, headers and blowers and direct it to a processing facility. Similarly, biogas from digesters is piped to an upgrading unit.

Table 4.2 Selected properties and constituents of biogas, biomethane and fossil natural gas

| Property or
constituent | | Unit | European biogas | | Fossil natural gas | | | | |-----------------------------------------------|------------------|---------------------------|-----------------|-------------------|----------------------------------|-----------------------------------------|-----------------------------------------------|--| | | | | From
LFG | From
AD-gas | From N.
American
gas field | From
non-N.
American
gas field | Average
in US
distribution
pipelines | | | Source | | (Persson et al.,
2006) | | | (Segeler, 1965) | | | | | LHV: | average
range | Btu/ft3 | 406 | 584 | 1081
835–1336 | 1145
627–1717 | 1049
945–1121 | | | Density: | average
range | g/ft3 | 34.8 | 32.1 | 22.4
19.5–27.9 | 23.9
19.5–36.9 | 21.5
20.3–24.6 | | | CH4: | average
range | Vol % | 45
36–65 | 63
53–70 | 51.5
84.7–98.8 | 77.0
22.8–98.0 | 89.4
72.8–95.2 | | | H2 : | average | Vol % | 0–3 | 0 | – | – | – | | | CO2: | average
range | Vol % | 40
15–50 | 47
30–47 | 0.55
0–6.0 | 4.1
0–29.0 | 0.7
0–2.0 | | | N2: | average
range | Vol % | 15
5–40 | 0.2
– | 4.03
0–29.4 | 1.7
0–12.1 | 2.9
0–17.1 | | | O2: | average
range | Vol % | 1
0–5 | 0
– | 0.06
0–0.4 | 0.1
0–1.4 | 0.0
0–0.4 | | | H2S: | average
range | ppmv | <100
0–100 | <1000
0–10 000 | 100
0–3100 | 400
0–5200 | | | | NMOC:
average
ppmv
(as hexane) range | | | | 1100
0–6600 | 2000
0–17 000 | 400
0–1400 | | | | NH3 | | ppm | 5 | <100 | – | – | – | |

emissions.** Disposing of the organic feedstock for anaerobic digestion is less straightforward. At large wastewater treatment (WWT) plants, the biogas may be used to fuel combined heat and power generators; smaller WWT plants are more likely to flare the gas. At livestock facilities, location, established manure management procedures, climate, livestock species and diet all affect current practice. While increasing numbers of landfills and digesters are using biogas to generate heat, power and, in some cases, vehicle fuel, biogas recovery is not the norm in most places.

As shown in Table 4.2, biogas typically contains 36–65% CH4 and 15– 50% CO2. Because of its high methane content, biogas is itself a significant source of GHG emissions that may be released to the atmosphere. The US

^** Since CH4 has 25 times the global warming potential of CO2, oxidizing it to CO2 (via flaring) effectively reduces GHG emissions, even though it fails to capture the energy in the flared gas.

4.2 Biomethane supply chains (a) landfill gas pathways and (b) anaerobic digestion (AD) pathways.

Environmental Protection Agency (USEPA) estimates that in 2009 over 190 MMT of CO2e were emitted from landfills, animal manure and WWT facilities in the US (USEPA, 2011b), while another 98 MMT and 16 MMT, respectively, were avoided by biogas recovery projects (USEPA, 2011a; 2011b). By avoiding the release of CH4 and instead recovering and using the RNG in vehicles, very large reductions in GHG emissions can be realized relative to petroleum-based fuels.

4.6 Biogas production, distribution and storage

Biogas is formed by anaerobic digestion (AD), a chemical process in which several kinds of bacteria digest organic matter in the absence of oxygen. Steps include breaking down carbohydrates, proteins and fats (into fatty acids, alcohol, CO2, hydrogen, ammonia and sulfides), metabolizing these products (into acetic acid, hydrogen and CO2), and converting them into longer chain hydrocarbons (biogas). Upon completion of the AD process, conversion of biogas into RNG requires water removal, pretreatment to remove contaminants and trace organics, and CO2 separation or purification.

Figure 4.3 illustrates a generic digestion process. It includes pre-treatment, mixing, digestion, de-watering and post-treatment steps. Since raw biogas contains corrosive hydrogen compounds, low concentrations (parts per million) of non-methane organic compounds (NMOC), sand and other inorganic materials, a pre-purification step is required to remove them before further upgrading. Then, if the particle size is too large for the digester to handle (e.g., municipal solid waste), the waste is passed through a shredder and, if the solids content is too high, make-up water or recycled process water is added. Both biogas and digestate are produced in the digester. Digestate includes inert and digestible solids and water. Water is removed in a dewatering process (typically by centrifuge) which produces biosolids which can be disposed of in a landfill, used for animal bedding or post-treated and applied as an amendment to the soil. Since water from the dewatering process is rich in

4.3 Schematic of a generic anaerobic digestion process.

nitrogen (N), phosphorous (P) and potassium (K) nutrients, it can be used as liquid fertilizer or recycled.

Different waste materials yield different quantities of biogas. As shown in Fig. 4.4, fats and greases yield an order of magnitude more biogas than manure or potato waste.

4.6.1 Purification to biomethane

The H2S-treated and dehydrated biogas is further purified by removing CO² from the gas stream. Four types of technologies are widely employed:

1. absorption
2. adsorption
3. membrane separation
4. cryogenic distillation.

In order to enhance the quality of the fuel product, more than one technology may be used in series. Further details are explained below.

Absorption

In absorption processes, molecules in gas phase (e.g., H2S and CO2) are removed as they become liquid phase in solutions. The amine process, one of the most common absorption processes in the natural gas industry, uses aqueous solutions of various alkanolamines to remove H2S and CO2. In a typical process, a CO2- and H2S-rich gas passing upward through an absorber contacts a downward flowing amine solution that absorbs the CO2 and H2S.

4.4 Typical biogas production by feedstock (m³ /ton). Source BMELV (2005).

The purified gas flows through to the top of the absorber, while the aminerich gas is collected at the bottom where it is sent to a regenerator to produce lean amine. In some biogas recovery projects, water scrubbing (i.e., water is the absorbent) is used in place of an amine solution.

Adsorption

Adsorption separates CO2 from biogas by collecting CO2 on the surface of an adsorbent. Parameters affecting the collection include temperature, total pressure and partial pressure of the constituent gases. In the most common such process, pressure swing adsorption (PSA), the pressure is increased (since CO2 is adsorbed more easily than CH4 under high pressure) and the CH4-rich gas that is not adsorbed flows through the vessel. Once the adsorbent in the vessel is saturated, adsorbed CO2 is removed by reducing the pressure to ambient. A cycle of PSA adsorption and desorption is typically relatively short since common industrial practice is to use multiple vessels to maintain a constant feed flow and output gas product. PSA operates at relatively high pressure, with the pressure of the product gas ranging from approximately 7 to 15 bar (Sperling Hansen Associates, 2007).

Membrane separation

Since CO2 is more permeable than CH4, the two may be separated by selective permeation. When the biogas contacts the membrane, more CO2 than CH4 permeates, although quantities depend on the partial pressure difference across the membrane. CO2 concentration in the feed gas affects the optimum number of membrane stages (Spillman, 1995; Datta and Sen, 2006). Since two-stage and three-stage membrane systems are more economical (in dollars per normal m³ of product gas) than single-stage systems, a second membrane system or a secondary amine process is often used in landfill gas projects (Pomerantz and Turcotte, 2004).

Membrane separation is amenable to a wide range of process stream volumes, CO2 concentrations and product-gas specifications. It also tends to be more environmentally friendly than amine processes. Since membrane separation operates at relatively high pressure, ranging from 15 bar to 40 bar (Air Liquide, 2009; Pomerantz and Turcotte, 2004), a further benefit is reduction in compression requirements for CNG production or injection into the gas grid.

Cryogenic distillation

Cryogenic distillation uses the relatively higher boiling temperature of CO2 — as compared with CH4 — to separate it from biogas. As in the amine process, dry compressed gas enters the bottom of an absorber column, flows upward and contacts a downward-flowing solution which 'washes' out contaminants. When the contaminant-free gas is cooled by refrigeration at the top of the absorber, the CO2 in the gas is condensed and the clean gas exits at the top of the absorber column. If CO2 concentration is still higher than pipeline quality natural gas or transportation fuel, further treatment or post purification (for example using membrane separation) may be required (Cook et al., 2005).

Mattirolo (2012) has identified the biogas upgrading technologies used in 226 European projects in operation in 2012 (Table 4.3). While absorbent and adsorbent processes (chemical, water or amine scrubbing, PSA) are most common, no single technology appears to dominate this as-yet immature market.

4.6.2 Distribution of gaseous biomethane

Since biomethane is chemically equivalent to fossil natural gas, it can be distributed via the natural gas transmission and distribution system. However, a pipeline connection is not always available and (even where available) barriers can limit access (see Section 4.6). In some cases, distribution is unnecessary as the RNG is compressed to CNG and dispensed at a fueling station located at or in close proximity to the production site.

Note that mechanisms comparable to the feed-in tariffs commonly used to bring distributed sources of power to the electric grid do not exist for RNG in many jurisdictions. Even where policies are clear, as in the Renewable Energy Directive of the European Commission, the motivations of individual companies and jurisdictions do not always align and projects may not gain access to the pipeline system. Table 4.4 shows the number and distribution of European projects where biogas is or will be injected into natural gas

Table 4.3 Upgrading technologies in European
biomethane projects

Technology	Installations
Chemical scrubbing	63
Water scrubbing	55
Pressure swing adsorption (PSA)	45
Unspecified	30
Genosorb	11
Membranes	8
Amine scrubbing	7
Cryogenic	6
Total projects	226

Source: Mattirolo (2012).

	In operation	Under construction	Planned
Austria	4	2
France	2
Germany	37	14	27
Luxembourg		2
Netherlands	6
Norway	1
Sweden	8
Switzerland	9		1
United Kingdom			5
Total	67	18	33

Table 4.4 European biogas feed-in projects as of May 2010

Source: Seisler (2010).

pipeline systems. As with biogas production in general, Germany led in pipeline injection, accounting for 37 of the 67 operating projects and 78 of the 118 total existing or planned projects as of May 2010 (Clean Fuels Consulting, 2010). Barriers to pipeline injection of biomethane include pipeline operators' concerns about gas quality (particularly with respect to LFG), government reluctance to impose mandates (preferring instead to rely on incentives and other less prescriptive measures), and wide variations in tariffs.

4.6.3 Distribution of liquid biomethane

The technology for producing a liquid product from RNG is more challenging, requiring additional purification and liquefaction steps. Although technologies to accomplish these steps are well established at large fossil gas liquefaction plants, most have not been found to scale well to the flow rates and production volumes typical at landfills or digesters. The Altamont landfill, near Livermore in California, is the only currently operational project producing renewable LNG in the US. Its production fuels 300 refuse trucks. In the UK Gasrec recently opened its first filling station to dispense a proprietary blend of up to 25% liquid biomethane mixed with fossil LNG.

4.6.4 Bulk storage

Like fossil natural gas, RNG can be stored in bulk within the pipeline system itself (by temporarily increasing the pressure in the line to 'pack' more gas into the available volume), in underground caverns for seasonal needs, or in pressure vessels at refueling stations for short term needs. If liquefied, biomethane can be stored like fossil LNG.

4.7 Liquid petroleum gas (LPG)

Globally, liquid petroleum gas (LPG) is the most widely used alternative motor fuel. Outside the US, LPG is commonly referred to as autogas when used as an automotive fuel. Autogas is Europe's leading alternative fuel, with approximately 10 million vehicles using it (WLPGA, 2013a). European countries with the largest percentage of autogas use for passenger vehicles are Turkey (at 25%) and Poland (at 15%). In South Korea, the autogas vehicle fleet grew from 400 000 in 1997 to 2.3 million in 2010. In its naturally occurring state, propane is a three-carbon alkane gas (C3H8) that can be liquefied when subjected to moderate pressure. Liquefied propane has an energy density 270 times greater than gaseous propane, thereby making it practical and economical to store and transport as a liquid. Propane is a byproduct of natural gas production and the oil refining process.

Three grades of LPG may be used as a motor fuel in the US, with HD-5 being the most widely used. The classification using 'HDxx' refers to 'heavy-duty' LPG, while the number represents the maximum percentage of propylene. For optimal performance, the fuel should meet HD-5 requirements and contain at least 90% propane, no more than 5% propylene, and no more than 2.5% butane (or heavier hydrocarbons). LPG that meets the California Code of Regulations is commonly referred to as HD-10, although this is not an official specification. HD-10 contains a minimum of 85% propane, a maximum of 10% propylene, 5% butane, 2.5% butene, and 0.5% pentene (or heavier hydrocarbons), and has limited sulfur content (80 ppm vs 123 ppm for HD-5). The third type of LPG is called 'commercial-grade', which has no limit on propylene and can be used in forklift engines (Ross, 2006). Because of its high octane value and other properties (Table 4.5) amenable to sparkignited internal combustion engines, LPG is an attractive automotive fuel.

4.8 LPG production, distribution, storage and use in vehicles

Supplies of LPG come largely from natural gas processing, which accounts for about 60% of total worldwide production (WLPG, 2013b). From the wellhead, raw natural gas is typically piped to nearby gas processing plants. During processing, high value liquids (also called natural gas liquids or NGLs) such as propane and butane are separated to be sold, while impurities such as CO2 and H2S are removed from the natural gas. The NGLs are transported by pipeline or rail to a bulk terminal for storage until distribution via heavy duty truck (Wang, 1999). Oil refineries account for nearly all of the remaining production as propane is produced in the distillation of crude oil as well as in further refining steps such as hydrocracking which is designed to increase yields of more valuable petroleum products. The propane produced from

Property or Constituent		Unit	HD-5 LPG	HD-10 LPG
Source			(Rood Werpy 2010, GREET 2013)	(Rood Werpy 2010)
LHV:	average	Btu/gal	84 950a	–
C3H8:	minimum	%	90	85
C3H6:	maximum	%	5	10
C4H10:	maximum	%	2.5b	5
C4H8:	maximum	%	–	2.5
C5H10:	maximum	%	–	0.5c
Sulfur:	maximum	ppm	123	80

Table 4.5 Selected properties of liquefied petroleum gas

crude oil can be used internally at the refinery or transported for external sale in a supply chain similar to that of NGLs.

Shale gas development in North America is expanding the supply of propane and other NGLs as shale gas producers, in response to low natural gas prices, are developing plays rich in higher priced NGLs. Over the next five years, US NGL production is expected to increase by more than 40% (BENTEK, 2012). While the heating market's demand for propane will likely drop in response to continued improvements in energy efficiency and conversions to natural gas, geothermal and electric heat, there may be other opportunities to offset this drop. In addition to strong export demand, fuel distributors may be expected to aggressively promote LPG as a vehicle fuel and allow North American fleets to negotiate favorable long-term fuel prices.

4.8.1 LPG vehicles and fuel delivery systems

In light-duty vehicles, the LPG tank is typically mounted in the trunk; for medium- and heavy-duty vans and trucks, the tank is located under the body of the vehicle. LPG tanks add weight to a vehicle and can slightly increase fuel consumption. Since the energy content of LPG is about 75% that of gasoline, more fuel is needed to travel an equivalent distance, even in an optimized engine (Wang, 1999). To operate on LPG as either a dedicatedfuel or bi-fuel vehicle (i.e., switching between gasoline and LPG), only a few modifications must be made to the engine. Several fuel delivery options are available, such as heavy-duty trucks and pipeline.

4.8.2 Vapor pressure systems

Until recently LPG vehicles commonly used a vapor pressure system that is somewhat similar to a carburetion system, wherein the fuel is vaporized and

a Based on value from GREET Model.

b Butanes or heavier.

c Pentenes or heavier.

mixed with combustion air in the intake plenum of the engine. This reduces efficiency as more air, rather than fuel, enters the cylinder for combustion. A slight power loss occurs in conversion to a vapor pressure system, but power can still be optimized with vehicle modifications of such items as the air–fuel mixture and compression ratios. Cold start is not an issue with vapor pressure systems since the air–fuel mixture is gaseous. Today, carbureted systems remain in use but are mostly confined to non-road applications.

4.8.3 Liquid injection systems

A liquid injection system is now available to inject fuel directly into the cylinder, resulting in no mixing penalty because air is not diluted with the gaseous fuel in the intake manifold. Use of a direct injection system improves engine efficiency. In current US after-market conversions a closed-loop system is used. This system incorporates an electronic sensor that provides constant feedback to the fuel controller, allowing it to make adjustments to maintain the proper air–fuel ratio. A complete conversion system includes a fuel controller, pressure regulator valves, fuel injectors, electronics, fuel tank, and software.

4.9 Hydrogen

Often called the 'fuel of the future', hydrogen (H2) offers an appealing prospect — a limitless supply of carbon-free, zero-emission energy (at the point of use and, potentially, upstream in the supply chain as well). Although H2 can be burned in internal combustion engines, doing so does not provide the energy efficiency needed to offset its low energy density and obtain the vehicle range acceptable to most vehicle users.

For H2 to realize its potential, it must be paired with highly efficient fuel cell electric vehicles (FCEVs). Beyond the challenge of producing massmarket FCEVs to compete with conventional vehicles, this will require producing H2 in economic quantities, delivering and dispensing it to potential customers when and where they choose, and storing it on board a vehicle and in bulk quantities along the supply chain. Although this will require significant breakthroughs, governments and OEMs have embraced the challenge, launching active research and development efforts like Germany's H2 Mobility, Britain's UK H2 Mobility, Japan's Fuel Cell program (FCJ) and the United States' recently announced H2 USA partnership.

Unlike spark-ignition engines that can tolerate a range of fuels with attendant properties, FCEVs require high purity H2. Table 4.6, which was developed by the US DOE's Fuel Cell Technologies Office (USDOE, 2011), based on the fuel quality specification adopted by the Society of Automotive Engineers (SAE J2719) shows the strict limits for a range of compounds

Constituent	Limit	Unit	Minimum analytical detection limit
Water (H2O)	5	mmol/mol	0.12
Total hydrocarbons (HC)	2	mmol/mol	0.1
Oxygen (O2)	5	mmol/mol	1
Helium (He)	300	mmol/mol	100
Nitrogen, Argon (N2, Ar)	100	mmol/mol	5
Carbon dioxide (CO2)	2	mmol/mol	0.1
Carbon monoxide (CO)	0.2	mmol/mol	0.01
Total sulfur (S)	0.004	mmol/mol	0.000 02
Formaldehyde (HCHO)	0.01	mmol/mol	0.01
Formic acid (HCOOH)	0.2	mmol/mol	0.02
Ammonia (NH3)	0.1	mmol/mol	0.02
Total halogenates	0.05	mmol/mol	0.01
Particulate concentration	1	mg/kg	0.005

Table 4.6 Hydrogen fuel quality specifications

Source: USDOE (2011).

including water, hydrocarbons, O2, CO2, CO, S, formic acid, ammonia and formaldehyde. The latter is particularly problematic as the maximum allowable concentration is at the limit of current detection equipment.

4.10 Hydrogen production, distribution, storage and use in vehicles

Although the most common of all elements, H2 does not exist in nature, but is bound in water (H2O), hydrocarbons (CnHm), carbohydrates (Cm(H2O)n) and various other compounds. Electrolysis is the most common method used to split H2 from water. It can be powered by a variety of renewable or nonrenewable sources including sunlight, wind, nuclear power, etc., producing gaseous hydrogen GH2 at approximately 20 bar.

Reforming is the most common method of separating H2 from hydrocarbons. Catalytic reforming (of naptha) occurs on a large scale in oil refining to produce the captive or internally used hydrogen needed to increase the refinery's yield of higher-valued petroleum products. A somewhat different process, steam methane reforming (SMR), is used to produce most commercial-grade or merchant H2. The latter process can be represented as two reactions. The first is performed at temperatures of 700–1100 °C in the presence of a metal catalyst to yield CO and H2. The second reaction, a lower temperature gasshift, increases H2 yield. The reactions can be summarized as:

$$CH_4 + H_2O \rightleftharpoons CO + 3 H_2$$ [4.1]

$$CO + H_2O \rightarrow CO_2 + H_2$$ [4.2]

In addition to intentionally produced H2, various chemical processes produce H2 as a byproduct. Chief among these are the catalytic reforming process used by refineries to increase yields of light petroleum products, and the chlorine, sodium hydroxide, chlor-alkali, sodium chlorate, styrene and ethylene production processes (Suresh, 2004).

The produced H2 can be injected into a dedicated pipeline system or conditioned further to increase its energy density and improve the economics of its distribution. At present, only a few pipeline systems exist, most of which are near large H2 users, such as petroleum refineries, chemical plants and oil sands upgrading facilities. Thus, fuel conditioning is essential. Currently, the most economical method for long-distance H2 transport is via cryogenic tank truck because such a truck can transport 15 times more energy than a truck carrying GH2. However, liquefying H2 is energy- and GHG- intensive. In the US, most H2 liquefaction facilities are located near low cost renewable electricity supplies (e.g., hydro) in order to reduce the cost and GHG emissions penalty of liquefaction. Although cryogenic tanks of liquid hydrogen (LH2) can be moved by barge or rail, most long-distance transport is by truck, with hauls generally limited to 500 km or less. Once LH2 arrives at a H2 refueling station it is vaporized for gaseous dispensing onto the vehicle.

Trucks with attached trailers containing multiple tubes of compressed gas (called tube trailers) are typically used for distributing smaller quantities of GH2. Such units are typically dropped off at refueling stations, with fresh units exchanged for spent ones which are then returned to a central location to be refilled. Typical pressure for tube trailers is about 200 bar, although 350 bar units are permitted in some locations. 'Cryo-compressed' tube trailers capable of storing low temperature H2 at pressures of 500 bar or more (with an associated three-fold increase in the quantity of H2 that can be delivered in each load), are currently under development.

A major problem in the transmission, distribution and storage of LNG and LH2 is boil-off due to heat influx from the atmosphere. Boil-off is a function of tank properties as well as the duration of storage and transit. Typical daily boil-off rates of LNG and LH2 are 0.1% and 0.3%, respectively. Usually 80% of boil-off gas is recovered.

Instead of using a battery to store electricity, fuel cell electric vehicles (FCEVs) use fuel cells, electrochemical devices that produce electricity as needed from H2 stored on board the vehicle. Among various fuel cell technologies, proton exchange membrane (PEM) fuel cells are considered most suitable for vehicular application because of their rapid startup and response and high power density. The main benefits of FCEVs are their high fuel economy (potentially three times greater than that of typical gasoline internal combustion vehicles) and zero tail-pipe emissions. Currently, only a small number of FCEVs (such as the Honda Clarity and Mercedes-Benz F-Cell) are available to private individuals for evaluation. However, several manufacturers (e.g., Toyota, Nissan, Hyundai, Renault, Ford, BMW, etc.) have announced plans to launch FCEVs within the next few years. In order to improve the market acceptance of FCEVs, several challenges must be overcome, most notably the cost and reliability of the fuel cell powertrain itself, the infrastructure for providing H2 to FCEVs, and on-board storage of hydrogen.

4.11 Life-cycle analysis of alternative gaseous fuels

This section provides an example of a full life-cycle analysis (LCA) of alternative gaseous fuels. The example results are generated from GREET1_2012 rev.2 developed by Argonne National Laboratory with the support of several programs in the USDOE's Office of Energy Efficiency and Renewable Energy. The selected pathways are summarized in Fig. 4.5. They include: LPG from US domestic or overseas crude oil or natural gas;

4.5 Schematic supply chains of gas-based alternative fuel pathways.

CNG and LNG from domestic or overseas fossil natural gas or from RNG (e.g., LFG and AD gas from manure and WWTP); and gaseous and liquid H2 from domestic or overseas natural gas, RNG, coal, biomass or electricity (using either the US mix or the renewable mix to generate the power). For each pathway, the main process fuel is listed in parenthesis inside the boxes, gases are assumed to be compressed at refueling stations using the US average grid mix for the required electricity, and H2 production and liquefaction occur in central locations using energy provided by the respective feedstock. A carbon capture and sequestration (CCS) option is also considered for the coal-to-hydrogen pathway via gasification for H2 production.

A major issue in biomethane pathways is the quantity of CH4 emissions, a highly potent GHG. A significant amount of CH4 is emitted throughout the pathways, including venting and leakage from equipment utilized in AD and from biomethane production and storage, leakage during the storage and transport of waste and AD digestate, and venting during digester maintenance. The venting and leakage rates from equipment are much higher, ~2% estimated by Berglund and Börjesson (2006), than those for fossil natural gas upgrading facilities (0.15%), possibly due to differences in scale (Burnham et al., 2011). Venting and leakage depends on many variables and uncertainties, such as the AD technologies and biogas upgrading methods themselves, the methane production potentials of different waste and digestate, the storage and transport methods used and their duration, and the maintenance schedule. Additional research on CH4 leakage in biogas pathways is needed for a more comprehensive understanding of biogas-based pathways.

Figure 4.6 compares life cycle GHG emissions and petroleum consumption for selected fuel/vehicle pathways with those for gasoline internal combustion engine vehicles (ICEVs). Note that the x- and y- axes are the ratios of petroleum fuel use and GHG emissions for the selected pathways to those for gasoline ICEVs. The symbols indicate the fuel/vehicle combination and are grouped by feedstock.

The LCA results depend largely on feedstock. For example, for feedstock other than crude oil, petroleum fuel use is less than 5% that of gasoline ICEVs. This is especially evident for AD gas from wastewater treatment plants (WWTP) which show negative petroleum fuel use because of the large electricity co-product credit associated with these pathways. Also, overseas natural gas consumes a relatively larger amount of petroleum and generates more GHG emissions because of liquefaction and ocean tanker transportation.

In terms of GHG emissions, no significant benefits (as compared to gasoline ICEVs) occur when the feedstock is not renewable (e.g., crude oil, fossil natural gas and coal without CCS). With CCS, however, FCEVs using GH2 from coal gasification reduce GHG emissions as much as FCEVs using renewable fuels. Among renewable fuels, RNG provides significant

4.6 Ratios of petroleum fuel use and GHG emissions for various alternative gaseous fuel/vehicle pathways to those of gasoline ICEVs.

GHG emission reductions because of large credits from avoided emissions in the reference or 'current practice' cases. This is especially clear for AD gas pathways that have negative GHG emissions because their reference cases generate large amounts of CH4 emissions. GHG emissions by FCEVs using H2 from electrolysis depend on the generation mix. With the US average mix (largely from coal and natural gas), GHG emissions by FCEVs are much higher than those for gasoline ICEVs. With renewable electricity GHG emissions and petroleum consumption are almost nil.

Among fuels with the same feedstock, compressed gaseous fuels (CNG and GH2) are less petroleum- and GHG-intensive than liquefied fuels (LNG and LH2). The exceptions are when natural gas from overseas wells is used as LNG instead of being re-gasified and compressed, and when liquefaction uses renewable energy while compression uses fossil energy (such as RNG and H2 from biomass gasification or renewable electricity).

4.12 Future trends

Globally, LPG or autogas has been the primary gaseous fuel for vehicles. While CNG has captured a significant share of the fuel market in gas-rich countries with tax and pricing structures that incentivized its use in transportation, its appeal has been limited to buses, taxis and other urban fleets attracted by its environmental benefits (primarily reduced emissions of NMOC, CO, and price at the pump will remain well below that of gasoline or diesel for the foreseeable future (EIA, 2013b), natural gas is emerging as more than a niche fuel in many markets. Some countries with extensive shale gas resources are seeing rapid growth in the number of NGVs on their roads, in the refueling infrastructure to serve them, and in the product lines of manufacturers seeking to convert vehicles and stationery equipment to the fuel. By contrast, other markets are dominated by concerns over the environmental impact of the shale gas bounty. These concerns include groundwater pollution from the chemicals used in hydraulic fracturing, seismic activity from the fracturing process itself, and methane leakage during initial well development and subsequent production.

Biomethane or RNG is a less controversial alternative gaseous fuel and may well 'green' the natural gas grid. Although the technology for producing RNG from raw biogas is well established, few North American projects currently do so. Many more projects use the biogas to produce electricity. This is largely because the biogas upgrading process is often more expensive than the value of the RNG produced. In other words, since the energy value of RNG is worth less than that of a comparable unit of renewable electricity (i.e., the 'spark spread' between the two energy sources) production of renewable electricity tends to be favored over production of RNG. In Europe, the economics are more favorable, especially with government incentives.

4.13 References

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Electricity and hydrogen as energy vectors for transportation vehicles

J. W. Sheffield and K. B. Martin, formerly of Missouri University of Science and Technology, USA and R. Folkson, University of Hertfordshire, UK

DOI: 10.1533/9780857097422.1.117

Abstract: Electrification of road vehicles will play a key role in the development of lower carbon transport solutions and a longer term technology is likely to include the use of hydrogen as an energy vector. This chapter discusses industrial methods for the large-scale production and distribution of hydrogen together with the major sources of electricity production. The chemistry of hydrogen production methods is reviewed together with the challenges of storage, distribution and transport usage. Different sources of renewable electricity are discussed including wind, wave, nuclear and solar.

Key words: hydrogen, energy vectors, storage, distribution, transport, renewable, nuclear, wind, wave.

5.1 Introduction

According to the US Energy Information Administration (EIA, no date), global energy consumption will increase by 53% between 2008 and 2035, representing an average annual growth rate of 1.6%. In addition the IEO2011 projects that China and India together will consume 31% of the world's energy in 2035, up from 21% in 2008. These projected increases in global energy demand compounded with the economic and geopolitical repercussions from expected oil supply security issues and shortages has renewed interest in hydrogen as an energy carrier for sustainability. Concerns about the environment and the ability to integrate larger quantities of renewable energy into the electric grid have resulted in interest in the expanded usage of hydrogen as an energy storage medium. Within the transportation sector, global energy consumption includes energy consumed in moving people and goods by road, rail, air, water, and pipeline. Energy consumed in the road transport component includes light-duty vehicles, such as motorbikes, automobiles, sports utility vehicles, small trucks and vans, as well as heavyduty vehicles, such as large trucks used for moving freight and buses for passenger travel. Naturally economic growth and population changes are the two key factors that determine transportation sector energy demand. In

developing economies, increased economic activity leads to growing income per capita; and as standards of living rise, demand for personal transportation increases. Indeed, a primary factor in the projected increase of energy demand in the transportation sector is steadily rising demand for personal travel in countries of both the developing and mature economies. In the developing economies, with gains in urbanisation and personal incomes, demand for air travel and motorised personal vehicles increases.

Sustainable mobility remains the core strategy of transportation policy in developed regions such as Europe. The strategy of sustainable mobility includes various policy measures aimed at improving the quality and efficiency of transportation systems. The European Commission has adopted several major initiatives promoting use of clean and energy-efficient vehicles, including the Commission's Directive on the Promotion of Clean and Energy Efficient Road Transport Vehicles, European Green Car Initiative, and Strategy for Clean and Energy Efficient Vehicles, among others. As part of those initiatives, Europe has also introduced various tax incentives for purchases of hybrid and electric vehicles.

Governments of some developed countries have allocated significant funds to promote transportation electrification. Hence ambitious predictions for electric vehicle market penetration have been presented for the next decade. For example, the German government allocated \$715 million toward its goal of having one million electric vehicles on the roads by 2020 as part of its 'National Electro-Mobility Development Plan', and France plans to spend around \$2.1 billion with a target of having 2 million electric and hybrid cars on the roads by 2020. The United Kingdom also has provided incentives for electric cars with grants of up to \$8,000 for nine models from 1 January 2011 and thus reducing the cost of eligible cars by approximately 20%. Infrastructural improvements include electric vehicle recharging points on streets, parking facilities, and commercial/retail facilities.

Despite major government efforts to promote sales of electric vehicles (EVs) and plug-in hybrid electric vehicles (PHEVs) in OECD Europe, sales of the vehicles remain very low. In Spain, the government set a target of selling 2,000 electric vehicles with lithium ion batteries in 2009–2010 and 20,000 electric and hybrid vehicles in 2011. However, as of December 2010, only 98 EV passenger cars and 778 EVs of other types had been sold, with an average subsidy amounting to \$4,700 per vehicle. In Germany, of the 49.6 million cars in operation, only 22,300 are hybrid vehicles and 1,500 vehicles are all-electric, comprising less than one-tenth of one percent of the country's total passenger vehicle fleet. The main barriers to mass penetration of EVs and PHEVs are their relatively high prices and the lack of requisite charging infrastructure. Although sales of EVs and hybrids undoubtedly will increase in the future, mass adoption of the new technologies is likely to take many years.

5.2 Overview of hydrogen production

5.2.1 Steam methane reformation

Steam methane reformation involves four basic steps: desulphurisation, reformation, carbon monoxide conversion and purification. Two general processes are utilised to accomplish these steps. The first process involves natural gas, which is primarily methane (CH4), being catalytically treated with hydrogen to remove sulphur containing compounds. This treatment is accomplished by converting the sulphur compounds into hydrogen sulphide via catalytic hydrogenation reaction. The hydrogen sulphide is then removed via a scrubbing process in a ZnO bed following the reaction:

$$H_2S + ZnO \rightarrow ZnS + H_2O$$

When the natural gas feedstock contains high amounts of hydrocarbons greater than two carbon atoms a prereformer is utilised. The prereformer converts the higher carbon hydrocarbon chains into methane, carbon monoxide/dioxide and hydrogen. Higher hydrocarbons are more reactive than methane and tend to form coke within the main reformer, thus deactivating the catalyst particles if not removed prior to being fed to the main reformer. The use of a prereformer also reduces the steam to carbon ratio in the reformer increasing the overall efficiency of the process and increasing the ability of the production plant to accept a variety of feedstock compositions.

The methane rich stream is then fed to the main reformer where it is mixed with steam and passed over a catalyst such as nickel, producing carbon monoxide and hydrogen. Typically, the steam is fed in molar ratio of 2:1 in order to avoid coke formation:

$$CH_4 + H_2O \rightarrow CO + 3H_2$$

This is followed by a catalytic water–gas shift reaction to convert the carbon monoxide to hydrogen and carbon dioxide. Even though the reaction is slightly endothermic, increases in temperature result in the thermodynamic equilibrium favouring the left side of the reaction equation. Thus, a trade must take place between high flow rates with low conversion and low flow rates with high conversion. Thus, in order to achieve both high flow rates and high carbon monoxide conversion percentages, a series of high and low temperature reactors are used. This process is able to achieve 92% conversion of carbon monoxide into hydrogen (Spath and Mann, 2001):

$$CO + H_2O \rightarrow CO_2 + H_2$$

Finally, the hydrogen is purified, either using a gas separation unit or pressure swing adsorption (PSA). In a gas separation unit, the carbon dioxide is reacted with a solvent such as monoethanolamine, ammonia, methanol, water or potassium carbonate, reducing the concentration of carbon dioxide to about

100 ppm. The carbon dioxide is further reduced in a methanation reactor by reacting it with hydrogen in the presence of a catalyst to produce methane. Typically, modern production plants use the PSA process, which operates at 20 atm and utilises several molecular sieve adsorption beds in parallel to separate and purify the hydrogen gas.

Hydrogen production by steam methane reformation has several advantages. Steam methane reformation plants already exist and provide the majority of the hydrogen needed by industries in the United States and the rest of the world. The overall efficiency of hydrogen production is between 40% and 60% depending on the specific process (Di Profio et al., 2009). This is one of the highest overall efficiencies of any hydrogen production process. As such, the cost of hydrogen produced by steam methane reformation is the lowest of any process. The feedstock, natural gas, is widely available and easy to handle. In addition, a vast natural gas distribution infrastructure already exists in the United States.

There are, however, some challenges. The hydrogen produced has to be purified before it can be used in fuel cells. Existing facilities are operating at maximum capacity, which means new facilities would have to be built or existing ones upgraded to meet any additional demand. The environmental impact is the greatest challenge since steam methane reformation results in the emission of carbon dioxide. In order to address the carbon dioxide emissions produced many carbon capture and sequestration technologies have been developed. Large-scale reformation results in the emission of about 11 kg CO2/kg H2 and small-scale reformation results in the emission of about 9 kg CO2/kg H2 (Lemus and Martínez Duart, 2010). Carbon sequestration can remove most of the CO2 from the emissions (over 70%) (Lemus and Martínez Duart, 2010). An alternative to the direct capture and storage of carbon dioxide is the production of sodium carbonate which is achieved by adding sodium hydroxide to the reformation reaction (Saxena et al., 2011). The addition of the sodium hydroxide also reduces the process temperature and alleviates the need for catalysts. However, the use of these technologies increases the cost of the hydrogen. In addition, the effects on the environment are not clear and may depend on how the carbon is stored. The volatile price of natural gas certainly adds another issue to a complex problem.

A typical hydrogen production rate when using the steam methane reformation process at petroleum refineries and chemical plants is 1.5 million m3 /day (122 tonnes/day) (Spath and Mann, 2001). However, most of this hydrogen is used as feedstock for various chemical processes. For comparison, current hydrogen fuelling stations for vehicles have production rates of 2–200 kg/day (Fuel Cells 2000 (2011)), with Forsberg and Karlstrom (2007) determining that fuelling stations with on-site steam methane reformation experienced optimal production rates at 1100–1400 kg/day.

5.2.2 Coal gasification

Globally, there are approximately 200 years of recoverable coal reserves (BP, 2011). Coal is also a low cost fuel; however, it contains a high ratio of carbon to hydrogen, resulting in significant carbon dioxide emissions, which is a major concern for the future use of coal. However, due to the stability in supply and price, coal will likely remain as a significant feedstock in the energy supply portfolio into the future. Improvements have been made to decrease carbon dioxide emissions. However, to make a significant reduction in emissions, carbon dioxide generated from coal utilisation needs to be captured and sequestered. A study by Smolinski et al. (2011) demonstrated that lignite coal produced slightly higher hydrogen production concentrations (66–67 vol%) than hard coal (59–64 vol%) or biomass (59–62 vol%) while utilising a gasification process at 700 °C. However, hard coal produced lower carbon dioxide emissions (18–23 vol%) than lignite coal (22–24 vol%) or biomass (30–32 vol%). Biomass produced the lowest carbon monoxide content (7–9 vol%).

A coal gasification system consists of coal preparation, gasifier, water gas shift reactor, gas separation unit, and a sulphur recovery unit. The coal is either fed to the gasifier as a slurry or dry. Within the gasifier, the coal is turned into syngas via the following reactions:

$$2C + O_2 \rightarrow 2CO$$

$$C + H_2O \rightarrow H_2 + CO$$

$$C + 2H_2 \rightarrow CH_4$$

In addition, the plant may also utilise an air separation unit in order to feed pure oxygen to the gasifier. The use of oxygen in the gasifier increases the concentration of the carbon dioxide in the product stream from the gasifier. The higher concentration affords the opportunity to capture and sequester the carbon dioxide at a lower cost. Steam is also frequently employed as a gasification agent, however, carbon dioxide and hydrogen can also be used as gasifying agents. Prior to being fed to the water gas shift reactor, the syngas from the gasifier is cooled to remove ash particles. Once the solids are removed, the syngas is treated to remove sulphur before being fed to the water gas shift reactor. Within the water gas shift reactor, carbon monoxide produced within the gasifier reacts with water to form carbon dioxide and hydrogen. If the carbon dioxide is to be captured and stored, it must be collected at this point. In a similar fashion to steam methane reforming, the product stream is then either sent to a pressure swing adsorption unit or to gas separation and methanation units for purification.

There are three main groups of gasifiers based upon fuel and oxidant flow directions: entrained flow (co-current flow), fluidised bed (counter current flow), and moving bed (counter current flow). The entrained flow gasifier exhibits carbon conversion of greater than 95%. The use of oxygen instead of air to entrain the pulverised coal results in avoiding the need to separate the nitrogen from product hydrogen later in the process. A key point about this is the ability of this technology to accept all types of coal as a feedstock. However, as the coal must be pulverised into a fine powder, this type of gasifier is not suitable to for use with biomass unless it undergoes pyrolysis beforehand.

In a fluidised bed gasifier, coal is consistently feed from either above the bed or directly into the bed where it is mixed with partially gasified and fully gasified coal. The fluidising medium is usually air, although oxygen or steam is also used. The bed of coal is fluidised by flowing gas from the bottom of the gasifier, causing the pulverised coal particles to float within the bed. However, as the particles are gasified, the particles become light enough to be entrained out of the reactor. It is also important to maintain the temperature within the bed below the initial ash fusion temperature of the coal in order to avoid particle agglomeration. Fluidised bed gasifiers are most useful for fuels that form highly corrosive ash, such as biomass fuels, which damage the walls of slagging gasifiers.

In a moving bed gasifier, coal is fed from the top of the reactor with steam, oxygen, or air fed from the bottom of the reactor. However, unlike in a fluidised bed reactor the pulverised coal collects and creates a solid bed of particles. The by-product ash is collected and removed from the bottom of the reactor. Moving bed gasifiers typically have high methane content in the produced gas. However, moving bed gasifiers produce tars and oils that contain sulphur and ammonia which complicates the gas clean up. The complex composition of the product gas results in large capital investments to handle by-products, relatively high maintenance costs, and relatively small throughput compared to land usage.

Hydrogen production from coal is a promising near- to mid-term technology. The development of integrated gasification combined cycle (IGCC) power plants provides a large scale, dual production (electricity and hydrogen) capability pathway which could address early uncertainty with hydrogen demand. Coal is also the least expensive fossil fuel with the US containing the world's largest proven reserves. However, concerns about greenhouse gas emissions and the early state of carbon capture and sequestration methods have raised concerns about the expanded use of coal. Although early IGCC plants experienced very high capital cost, next generation IGCC plants are expected to have simplified designs reducing capital costs and higher thermal efficiencies. This is due in part by using oxygen and carbon dioxide to gasify the coal. The carbon dioxide is extracted from a closed gas turbine system that allows for the separation by compression and liquefaction instead of having a separate capture system.

5.2.3 Electrolysis

During an electrolysis process, water undergoes disassociation into hydrogen and oxygen when an electric current is applied. Electrolysers may be classified as either unipolar or bipolar. Unipolar electrolysers are simply systems which consist of a single tank that has the electrodes separated by a membrane or diaphragm. As the electrodes are connected in a parallel fashion, unipolar electrolysers operate under high current, low voltage conditions. Bipolar electrolysers contain individual electrodes separated by insulators that allow one side of a single electrode to act as cathode while the opposite side serves as the anode for the adjacent electrolysis cell. The electrodes in a bipolar electrolyser are wired in series resulting in a low current, high voltage device.

Electrolysers are also classified based upon production technology. Currently, there are three main types of electrolyser under development: alkaline, polymer electrolyte membrane, and solid oxide electrolyser cells. Alkaline electrolysers typically use a 30 wt% aqueous solution of potassium hydroxide as the electrolyte. A potassium hydroxide solution is used as it has high conductivity and the energy loss related to the oxygen evolution reaction is minimised. This solution is fed into the electrolyser along with the water feedstock. The operating temperature of an alkaline electrolyser is typically limited to 100 °C in order to limit alkali corrosion of electrodes. However, there has been research at operating at significantly higher temperatures (400 °C) (Ganley, 2009). Alkaline electrolysers typically use iron, nickel, and nickel alloy as materials for the electrodes and can either be unipolar or bipolar. A bipolar alkaline electrolyser may have more than 100 cells packaged together in a filter press arrangement to form a stack with each cell fed water in parallel. The electrolyser follows the following reaction kinetics:

Cathode: 2H+ + 2e– Æ H2

Anode: 2OH– Æ ½ O2 + H2O + 2e–

Overall: H2O Æ ½ O2 + H2

The second type of unit is a proton exchange membrane (PEM) electrolyser. Unlike in an alkaline electrolyser, a solid ion conducting membrane is used to separate the electrodes and transport protons from the anode to the cathode. In this unit, the electrolyte is a solid ion conducting membrane and the electrolyser is fed only pure water. This membrane has a polymer structure with sulphonic acid groups attached that allow for proton transfer. Electrodes are typically loaded with catalysts such as platinum and ruthenium. Efficiency of a PEM electrolyser can be increased by increasing the temperature. This is achieved because the chemical reaction rates at the electrodes are increased thus reducing the overpotential and amount of electrical energy required. However, the membrane must stay hydrated in order to function properly, which typically limits the operating temperature to below 100 °C. There is current research on high temperature PEM electrolysers which operate around 130 °C (Xu et al., 2011). PEM electrolysers have several advantages over alkaline electrolysers such as higher purity of product gases, lower power consumption, ecological effects and smaller overall dimensions of systems. However, water purity requirements are much higher for PEM electrolysers.

Solid oxide electrolyser cells (SOECs) have gained much attention to generate hydrogen from electricity as they operate at high temperatures (approximately 500–950 °C) which increases the reaction kinetics and reduces electrical energy requirements (Jin et al., 2011). SOECs use a solid ceramic material as the electrolyte that selectively transfers negatively charged oxygen ions to the anode. Unlike PEM and alkaline electrolysers, water is disassociated into hydrogen and oxygen ions at the cathode. Nuclear power systems and renewable energy sources, as well as, waste heat from high temperature industrial processes could provide the electricity and heat required for operation of SOECs (Stoots et al., 2010). SOECs can also operate in autothermal mode; this operation does not require a high temperature source for the electrolysis reaction but only one capable of vaporising sufficient water for the desired production rate. SOECs are further discussed in the following section.

In general, electrolysers provide a carbon-free pathway for hydrogen production when renewable energy is utilised. Recently, there has been a focus on a wind-hydrogen systems which utilise electrolysers (Martin and Grasman, 2009; Carton and Olabi, 2010). In addition, they are capable of being scaled to a variety of production rates leading to higher utilisation rates. However, challenges remain including reducing the capital costs of the equipment, improving the energy efficiency of the electrolyser, and production of larger systems if a centralised production system is selected. A method which could lead to higher overall system efficiencies is operating the electrolyser at higher pressures required for storage therefore lowering or avoiding separate compressor costs.

5.2.4 High-temperature conversion from nuclear energy

Hydrogen produced by nuclear power plants is very pure and suitable for use in hydrogen fuel cells, which require up to 99.999% purity. A 600 MW nuclear reactor would be able to produce 640,000 m³ /day of hydrogen along with 175 MW of power (Naterer et al., 2008). Hydrogen can be produced utilising nuclear power either by utilising solid oxide electrolyser cells (SOECs) and/or thermochemical conversion. Using generation IV reactors, creates additional benefits as they will have passive safety features, increased proliferation resistance, improved economics, and better management of radioactive waste. However, generation IV reactors are still only in the design phase and are not expected to be in operation before 2020. In particular, the very high temperature reactor (VHTR) is a generation IV reactor type currently being developed due to its potential for hydrogen production.

High-temperature electrolysis using nuclear energy is currently being developed by a number of countries including Canada, France and South Korea (Ryland et al., 2007; Yvon et al., 2009; Koh et al., 2010). Helium is the primary coolant and is used to drive a gas turbine to provide the electricity needed for electrolysis. In addition, some of the heat contained within the primary coolant is transferred using a heat exchanger to the water before electrolysis. The addition of this high-temperature process heat decreases the amount of energy that must be converted from heat (from the nuclear reactor) to electricity (from the generator) and then to chemical energy (in the hydrogen molecule), thereby raising the overall efficiency of the process. With operating temperatures up to 1,000 °C, the VHTR can be coupled with certain thermochemical processes to produce hydrogen with an efficiency of around 50% (Naterer et al., 2008).

Several thermochemical processes for hydrogen production have been developed based on the very high-temperature reactor (Vilim et al., 2004). Oak Ridge National Laboratory is currently pursuing the sulphur-iodine cycle for hydrogen production. Canada is developing the copper-chlorine cycle for hydrogen production with the goal of using it with future hightemperature gas-cooled reactors. It is estimated that the thermochemical splitting of water to produce hydrogen using the heat from generation IV nuclear power plants will be able to provide hydrogen at between 4.3 and 6.8 ¢/kWh by 2020 (Lemus and Martínez Duart, 2010). For comparison, it is estimated that for a 600 MW thermal capacity, direct Brayton cycle power plant with an outlet temperature of 900 °C, the cost of hydrogen will be about 9.69 ¢/kWh (O'Brien et al., 2010).

The copper chlorine cycle involves the following equations:

$$2 \text{ CuCl} + 2 \text{ HCl} \rightarrow 2 \text{ CuCl}_2 + \text{H}_2 \qquad (<100 \text{ °C})$$

$$2 \text{ CuCl}_2 + \text{H}_2\text{O} \rightarrow \text{CuO} \cdot \text{CuCl}_2 + 2 \text{ HCl} \qquad (400 \text{ °C})$$

$$2 \text{ CuO} \cdot \text{CuCl}_2 \rightarrow 4 \text{ CuCl} + \text{O}_2 \qquad (500 \text{ °C})$$

This cycle has the ability to use low-grade waste heat and requires a relatively low voltage. It requires temperatures only up to 500 °C, whereas certain other processes require temperatures up to 800 °C. In addition, the reactants are relatively common and react to completion without side reactions (Naterer et al., 2010).

The S-I cycle involves the following equations:

$$I_2 + SO_2 + 2 H_2O \rightarrow 2 HI + H_2SO_4$$ (120 °C)

$$2 \text{ HI} \rightarrow I_2 + H_2 \tag{450 °C}$$

$$2 \text{ H}_2\text{SO}_4 \rightarrow 2 \text{ SO}_2 + 2 \text{ H}_2\text{O} + \text{O}_2$$ (830 °C)

This cycle is a purely thermal process, which is suitable for large-scale hydrogen production. It has a high thermal efficiency of up to 50% and is an all-fluid process, which makes it easy to expand and to achieve continuous operation (Zhang et al., 2010).

There are some challenges that must be overcome in order for nuclear energy to provide the energy needed to produce large-scale quantities of hydrogen. The main drawback is that nuclear power plants are highly capital-intensive. In the United States, the construction of nuclear power plants generally takes four years from ground breaking to operation. This does not include the time needed by the Nuclear Regulatory Commission to license the plant, which can be up to two years and precedes any construction. The initial investment can range from \$5 billion to \$14 billion (based on budgets for nuclear power plants built after 2005) (WNA, 2011). The long-term storage of nuclear waste must also be addressed if nuclear energy is to become the main source of hydrogen production. The Nuclear Waste Policy Act of 1982 specifies that spent nuclear fuel is to be disposed of in deep geological repositories. However, no such repository is currently in operation in the United States and nuclear utilities are forced to store their spent nuclear fuel on-site.

5.2.5 By-product and industrial hydrogen

Hydrogen is an important raw material in the chemical, petroleum, metallurgical, pharmaceutical, electronic, and food industries with a total of 18MM tonnes produced in 2010 (Garvey, 2011). During the production of many industrial processes, hydrogen is also produced as a by-product. The total generation of by-product hydrogen is significant in the United States as 3.4 MM tonnes were produced in 2006 (EIA, no date). The largest consumers of hydrogen in the United States are oil refineries (4.08 MM tonnes), ammonia synthesis facilities (2.62 MM tonnes) and methanol production plants (0.39 MM tonnes) with a total of 7 MM tonnes consumed in 2003 (Suresh et al., 2004).

Although these industries are consumers of hydrogen, they may also serve as important locations of hydrogen supply during the introduction of hydrogen as an energy carrier. Petroleum refineries and commercial hydrogen production facilities could serve as a hydrogen source in California and Texas. For example, approximately 275 vehicles/day could be fuelled by using only 1% of the hydrogen produced at a typical petroleum refinery. In the Midwest, ammonia production facilities could serve as a hydrogen source for the region as it is produced as a feedstock. Petroleum refineries in New Jersey and Pennsylvania could serve the Northeast with hydrogen. Another important process is the chloralkali process which is used to produce chlorine, sodium hydroxide, and other chemicals. Approximately 70% of the United States chloralkali production facilities are in the Gulf region (Thornton, 2000). This industry produces about 0.35 MM tonnes of hydrogen every year (EIA, no date). The cost of recovered by-product hydrogen depends on the merchant, but it is generally less than the cost of hydrogen that is produced at a dedicated facility.

5.3 Overview of electricity production

Electricity is produced from a multitude of sources around the world including fossil fuels, nuclear, hydro, wind, wave and solar, but the overwhelming majority of global electricity is dominated by fossil fuels comprising coal, natural gas and oil. It is not the purpose of this section to explore the technical details of electricity production, but rather to examine the implications for the potential future electrification of transport. Whilst there are wide variations in national electricity generating practices varying from countries such as France with a high proportion of nuclear generation, or Norway that has 98% hydro-electricity, most countries use a significant proportion of fossil fuels in electricity production. China, one of the largest global producers uses coal for 66% of production and has now become the second largest global emitter of carbon dioxide after the United States.

The United Kingdom is fairly typical of electricity production in many industrialised countries with a fuel mix of coal (28.9%), natural gas (44.2%), nuclear (17.3%), renewables comprising wind, wave and tidal (7.9%) and other (1.7%) (2010 data – reference Chapter 21). Whilst the mix of fossil fuels will vary by country and hence the carbon dioxide emissions associated with electricity generation for use in electric vehicles will also vary, there are very few instances where electricity can be considered a zero emissions solution. Even in countries that use a high proportion of nuclear or hydroelectric power, it is necessary to also consider the embedded emissions from the construction of the generating facilities. All generating plant use significant amounts of cement in their construction and cement production is also a major global source of carbon dioxide emissions as the principal production technique is to drive the carbon dioxide out of calcium carbonate (chalk) to produce cement. Nuclear power stations, wind, wave, tidal and hydro schemes all depend on very large amounts of concrete that uses cement during construction and the embedded emissions should be considered when calculating the lifetime emissions of motor vehicles that use electric propulsion from grid generated electricity.

The current UK generating mix results in battery electric vehicles producing the equivalent of around 80gm/km of carbon dioxide when charged from the UK grid. Some other countries such as France with around 75% nuclear generation may be able to demonstrate much lower equivalent emissions. UK figures may actually result in a growth in carbon dioxide emissions as the existing older nuclear power stations are gradually decommissioned over the coming years, with no clear strategy for building next generation nuclear power facilities to replace them. The Fukushima nuclear accident and subsequent events in Japan have had a significant effect in reducing the enthusiasm of many governments, such as Germany, for building new nuclear facilities. The current UK trend is to build combined gas turbine (CGT) generating facilities, which are much cheaper and faster to construct than nuclear and use relatively low cost natural gas for production. These are very suitable for flexible generation to meet peak demands and have a relatively high thermodynamic efficiency that can approach 50%.

Nuclear power is more suited to providing base load electricity, as it is far less flexible in terms of increasing or decreasing output to match demand. Similarly wave power is more suited to continuous base load electricity production, although building in areas that have sufficient continuous wave energy represents a construction and maintenance challenge. Tidal power electricity generation can provide solutions that store the base load and intermittent energy delivered from nuclear, wind and wave power in large lagoons by pumping water using electricity generated during periods of low demand such as overnight and then releasing the water to supply electricity at times of peak loading during the day and early evening.

Wind power is favoured in many countries to provide an increasing share of generating capacity, particularly in the United Kingdom, Germany, France and Denmark which have areas well suited to wind energy generation. However, there are some drawbacks in terms of the ability to generate electricity at times to match demand and the intermittent nature of wind generation, which requires more flexible fossil fuel based solutions such as CGT to be used in parallel with these.

In the longer term it is possible that the widespread adoption of battery technology in vehicles may allow them to be used as an electricity power source to feed back into the grid at times of high demand when the vehicles are normally parked for charging. They can then be replenished in the middle of the night using base load capacity in conjunction with 'smart' charging regimes. This solution will require sufficient numbers of vehicles to have been produced to make a significant contribution to the overall grid balance, which may take several years to achieve based on current uptake of battery electric vehicle technology.

A further opportunity for battery technology will be presented by use of obsolete batteries that are beyond their useful life for vehicle propulsion, but are still suitable for storing lower levels of electricity for grid balancing or to accept intermittent charging such as from wind turbines. This could provide a viable end of life use for batteries that can also improve the economic proposition for consumers by improving vehicle residual values.

Longer term, technology may develop to produce micro generation facilities, potentially using hydrogen fuel cells to generate electricity and heat for local domestic and industrial use instead of reliance on national grids, but these will depend on improvements in the economic viability and fuel availability for such solutions. Solar energy may also be the eventual solution for providing the planet with plentiful low cost energy, but fundamental breakthroughs in chemistry to improve efficiency and reduce costs will be required before this becomes a reality that may take another 50 years to achieve.

5.4 Hydrogen storage and transportation

Regardless of whether production is centralised or decentralised, the hydrogen that is produced will require some form of storage. The choice of storage would depend on the type of application, the energy density needed, the quantity to be stored, the storage period, the capital and operating costs, local resources, and the safety of the method. Each of the several storage methods has its own advantages and challenges. They can be classified as large-scale or small-scale, chemical or physical, on-board or stationary.

5.4.1 Large-scale storage

In order to support a fully developed hydrogen economy, large scale storage facilities may be required in order to maintain adequate reserves between production facilities and small scale storage facilities.

Cryogenic

Cryogenic hydrogen has a density nearly twice that of compressed hydrogen at 70 MPa. Liquid hydrogen is stored in specially insulated cryogenic tanks under pressure, which have provisions for cooling, heating, and venting. Tank sizes can range from 1.5 m³ (100 kg) to 75.0 m³ (5,000 kg). However, liquefaction is an energy-intensive process. Estimates are about 12.5–15.0 kWh/kg for liquefaction compared to about 6.0 kWh/kg for compression to 70 MPa (Di Profio et al., 2009). Due to the energy-intensive nature of liquefaction, the CO2/kg of hydrogen stored is the highest for cryogenic storage (Di Profio et al., 2009).

Currently, there are approximately 450 large-scale liquid hydrogen storage sites in the United States spread across 41 states (EIA, no date). One of the largest tanks for storing liquid hydrogen is located at Cape Canaveral, Florida and has a capacity of 3,800 m³ (245 tonnes). If cryogenic storage of hydrogen is adopted, new storage facilities would have to be developed. The initial capital investment for a new storage facility is high due to the need for liquefaction equipment as well as storage. The operating costs are also high due to the energy-intensive liquefaction process. Larger plants with higher liquefaction capacities cost more initially, but the cost of hydrogen and the energy needed to liquefy hydrogen decreases per kilogram of hydrogen liquefied. Typical liquefaction capacities can range from 100 kg/h to 10,000 kg/h and typical on-site storage capacities can range from 115,000 kg to 900,000 kg. In the United States, the total liquefaction capacity is about 69,000 tonnes/day (EIA, no date). Cryogenic storage of hydrogen is economical at large production rates and at long storage periods.

Underground

Natural underground formations such as aquifers and depleted natural gas and man-made caverns can be used to store hydrogen. Aquifers are an underground layer of water-bearing permeable rock or sand trapped between layers of impermeable rock. Hydrogen can be compressed and injected to form a gas pocket that is contained underground by water below and by impermeable rock above. Hydrogen can also be stored in the porous rock found in natural gas fields in a similar fashion as that used in aquifers. The storage of gases underground has taken place since the early 1900s when natural gas began being stored in depleted oil fields. Man-made caverns such as salt caverns are created by pumping fresh water into a salt dome and dissolving the salt. The resulting cavern can then be used to store, among other things, hydrogen. The pressure depends on the amount of gas in the cavern and decreases as the gas is depleted. A surface lake connected to the bottom of the cavern can maintain a constant pressure equal to the head of the lake, as long as the gas and the rock do not dissolve in water. Underground mines with suitable geological features can be used in a similar way.

Currently, there are only two underground hydrogen storage sites in the United States. One of these facilities is Clemens Terminal in Texas with a capacity of approximately 30 million m³ . In comparison, there are approximately 400 underground storage sites utilised for natural gas with a total capacity of 102 billion m³ (EIA, no date). However in 2001, the total capacity for underground storage of natural gas in the United States was 238 billion m³ (Forsberg, 2005). Therefore, there is an opportunity to modify and utilise some existing natural gas storage for large scale hydrogen storage. The utility of many of these sites is also increased as they are located close to industrial and urban centers.

Countries such as France, Germany, Russia, and the United Kingdom have experience storing hydrogen as a mixture of gases in aquifers and salt caverns without gas losses or safety concerns. Once a site has been developed, most of the costs associated with its operation are from hydrogen compression, which is independent of the actual storage. Properly characterising the geology of the site in order to ensure a stable facility for storage would be a challenge in the development of new sites. The initial capital investment may also be a challenge. Based on natural gas storage experience, an underground facility capable of storing 238,000 tonnes of hydrogen would cost between \$200 million and \$400 million or between \$0.80/kg and \$1.60/kg of hydrogen (Forsberg, 2005). A more recent estimate places the cost at about \$1.20/kg, which makes underground storage the most economical option of currently available storage options (Lemus and Martínez Duart, 2010). However, storing hydrogen on such a large scale may not be needed for a long time.

5.4.2 Small-scale storage

A hydrogen economy will require small-scale storage to distribute and utilise hydrogen in various applications. Small-scale storage is characterised by relatively small amounts of hydrogen and relatively short storage periods. Vehicles that run on hydrogen, whether fuel cell or internal combustion, may require hydrogen storage at a fuelling station in addition to onboard storage. Residential and portable power applications such as home fuelling, stationary fuel cells and emergency backup power units may also require small hydrogen storage capabilities.

Compressed

Compressed hydrogen must be stored in specially designed tanks capable of withstanding the storage pressures, which can range from 17 MPa to 70 MPa. These tanks are usually made of steel. However, tanks made of carbon fiber lined with aluminum, steel, or specific polymers are used when weight is a consideration. When compressed, the density of hydrogen at 35.0 MPa is about 23 kg/m3 and at 70.0 MPa is about 38 kg/m³ . This leads to an energy density of 767 kWh/m³ (27 °C, 35 MPa).

The volume of the storage tank is the biggest challenge, since the density of compressed hydrogen is lower than that of liquid hydrogen. Compression of hydrogen is an energy-intensive process which increases the overall cost. Estimates are about 6.0 kWh/kg for compression to 70 MPa, which leads to the CO2/kg of hydrogen stored to be high (approximately 1.3 kg of CO2/kg of hydrogen) (Di Profio et al., 2009). However, compression only consumes a third of the energy that liquefaction does. In addition to the cost of compressing hydrogen, the cost of compressed storage tanks must also be taken into account. The cyclic loading of tanks, which tend to heat up as they are filled with compressed hydrogen, reduces tank life.

Compressed storage is mostly done above ground. However, underground storage is also possible, especially for fuelling stations since it decreases the

amount of land used. This reduces the chance of accidents since the storage tanks are isolated, but increases the difficulty associated with inspection and maintenance. There are approximately 600 small-scale compressed hydrogen storage sites in the United States (EIA, no date). Storage capacities range from 100 kg to 1,300 kg and storage pressures range from 1 MPa to 30 MPa Hydrogen fuelling stations in the United States have storage capacities which range from 10 kg to 150 kg and storage pressures range from 1 MPa to 54 MPa (Fuel Cells 2000 (2011)).

Cryogenic and cryo-compressed hydrogen

Stationary cryogenic storage is mostly done above ground with underground storage possible. Hydrogen vehicles in particular benefit from the greater energy density when compared to compressed hydrogen. Although boil off of hydrogen has been a problem in the past, recent advances in tank design show that venting only takes place during periods of infrequent use (Michel et al., 2006). However, the energy required to liquefy hydrogen consumes 30% of the chemical energy stored based upon the lower heating value (LHV) of 120 MJ per kg of hydrogen. Due to this and several other factors including losses during transfer and packaging of these systems into vehicles many automakers have elected to focus on other storage methods.

The cryo-compressed tank is an alternative storage which combines using high pressure at cryogenic temperatures. The method is flexible in fuelling source, using liquid, gas or cooled gas. Hydrogen stored as a supercritical fluid can exceed the density of liquid hydrogen resulting in vessels capable of containing 2–3 times more fuel than conventional compressed hydrogen vessels (Aceves et al., 2010).

Metal hydride

Metal hydrides are compounds of one or more metal cations (M+) and one or more hydride anions (H– ). When pressurised, most metals bind strongly with hydrogen, resulting in stable metal hydrides that can be used to store hydrogen conveniently on board vehicles. Examples of metal hydrides are LaNi5H6, MgH2, and NaAlH4. Metal hydrides can be liquids or powders that are usually stored in tanks at approximately 1 MPa. As the pressure is reduced or the temperature is increased (between 120 °C and 200 °C), hydrogen is released. The metal hydride can be recharged without the use of a high pressure compressed gas or cryogenic liquid. When designing efficient metal hydride systems, the critical material properties to manipulate are thermal conductivity, heat of reaction and activation energy (Jorgensen, 2011).

Metal hydride storage has a low risk of accidental leaks since the hydrogen is stored within the metal hydride crystal and requires energy to be released. In addition, the energy spent in storing hydrogen using metal hydrides is about half as much as that of compression (70 MPa) and about a sixth as much as that of liquefaction. Thus, the CO2/kg of hydrogen is the lowest of any storage method due to the low storage and release energies involved (Di Profio et al., 2009). In addition, metal hydride storage has an energy density (kWh/m³ ) that is about three times higher than compressed storage and cryogenic storage (Di Profio et al., 2009).

However, there are some challenges as well. Metal hydride storage has a relatively high energy density by volume, but a relatively low energy density by weight (kWh/kg). Typical values are 1% to 9% by weight (Zhang et al., 2010). This results in metal hydride storage tanks that are about four times heavier (250–300 kg) than gasoline tanks. Even with improvements in metal hydride technology, storage tanks are currently too heavy for use in passenger vehicles and are limited for use in other applications. The cost of metals such as lanthanum and lithium raises the overall cost of the storage systems to a level that limits its adoption to small scale applications. Early systems exhibited slow uptake rates further limiting appropriate applications. Concerning safety, metal hydrides tend to react violently with moist air making handling a challenge. Hydrogen purity is also a concern as impurities are detrimental to the performance of metal hydrides. The efficiency and reversibility of metal hydride storage systems over a large number of cycles and the thermal management of these systems are also proving to be challenge.

Surface adsorption

A relatively recent proposition for hydrogen storage is using surface adsorption. At cryogenic temperatures, hydrogen is effectively adsorbed on porous materials such as activated carbon and carbon nanotubes. As the temperature approaches room temperature, the weight density of stored hydrogen decreases. However, as the pressure increases, so does the weight density of stored hydrogen. In order to release hydrogen, some energy must be supplied. While the energy density by weight is similar to that of metal hydrides, the energy density by volume is six times lower. Weight densities of 5.5% to 19% have been reported (Assfour et al., 2011). In addition, a well-to-wheel analysis yielded a cost of 20.3 ¢/km for carbon nanotube storage (de Wit and Faaij, 2007).

5.4.3 Transportation

There are three main methods available to transport hydrogen:

trucks, trains, and barges carrying compressed or liquefied hydrogen

2. pipelines transporting compressed hydrogen and
3. chemical carriers such as hydrocarbons and hydrides that are reacted at the point of use.

Gaseous tube trailer and cryogenic liquid truck delivery play an important part in the distribution of hydrogen. Tube trailers are capable of delivering 300 kg at 18 MPa. Cyrogenic tanks capable of being delivered by semi truck operate at 1.1 MPa and are capable of transporting 3,200–4,500 kg of hydrogen. However, studies have shown that pipeline is the most cost effective and energy efficient method to transport hydrogen over long distances and large quantities (Leighty et al. 2006; Keith and Leighty, 2002). Currently, there are 1,100 kilometers of pipeline transportation in the United States. Hydrogen pipelines typically operate at pressures around 1–2 MPa, but operating pressures may reach 10 MPa (IEA, 2005). Sodium borohydride (NaBH4) and methanol (CH3OH) are leading chemical hydrogen carriers. These carriers could be used for mass transportation of hydrogen with the hydrogen released either before distribution or by the final customer. NaBH4 can be stored under ambient conditions as a stable aqueous solution without any safety precautions besides alkalinity concerns. The hydrogen release process entails bringing the solution into contact with a catalyst. The NaBH4 system does have the advantage that the carrier is liquid, however, the decomposition product is a slurry of NaBO2 which has to be recycled. Methanol is another potential chemical carrier of hydrogen. Methanol is a liquid at normal operating temperatures and pressures and can be handled in much the same way as gasoline. In general, this process entails central production of methanol which is then transported by conventional methods to end users where it is reformed into hydrogen.

5.5 Conclusions

A key driver towards energy independence is the cost reduction of hydrogen. This is particularly evident concerning the adoption of hydrogen as a transportation fuel. In order for hydrogen to be adopted for transportation use, it must be comparable to conventional fuels and technologies on a perkilometer basis. In support of this ultimate goal, many concepts such as combined heat and power (CHP) systems (Braun et al., 2006) and combined heat, hydrogen, and power (CHHP) systems have shown great promise at reducing the cost of hydrogen technologies. Hydrogen can be produced using diverse, domestic resources including natural gas and coal (with carbon sequestration); biomass; nuclear; and renewable energy technologies, such as wind, solar, geothermal, and hydro-electric power. The diversity of feedstocks lends itself to investigating whether selected production technologies may become regional.

Electricity production is likely to remain dependant on fossil fuels for many years in the future, but the development of more renewable sources combined with efficient production methods to produce low cost hydrogen may eventually lead to the formation of a hydrogen economy.

A second aspect of a future hydrogen economy, which is partially dependent on the production technology, is the distribution of production. Some technologies such coal gasification, thermochemical processes, and high temperature electrolysis lend themselves to a large scale, centralised production method. Steam methane reformation and electrolysis processes are scalable within a range allowing these technologies to operate either in a decentralised or centralised mode. Centralised production units typically are more energy efficient and if green house gases are by-products would afford the potential opportunity to sequester them. However, large-scale, efficient transportation methods would have to be developed in order to minimise additional transportation costs. In addition, centralised production plants would require large initial capital investments in a developing market. A decentralised approach would mean producing the hydrogen locally or on-site instead of a centralised facility, thereby alleviating the need for large-scale transport of hydrogen. Decentralised production also allows for a gradual investment in hydrogen infrastructure. However, small-scale hydrogen generation typically has a higher cost and if natural gas reformation is utilised would nearly alleviate any carbon capture possibility due to the high costs. In summary, the mass acceptance of hydrogen, including the infrastructure selection, is complex with multi-purpose and specialty applications showing the greatest opportunities for near term deployment and success.

5.6 References and further reading

Aceves, S., Espinosa-Loza, F., Ledesma-Orozco, E., Ross, T., Weisberg, A., Brunner, T. and Kircher, O. (2010). High-denisty automotive hydrogen storage with cryogenic capable pressure vessels. Int J Hydrogen Energy 35(3), 1219–1226.
Assfour, B., Leoni, S., Seifert, G. and Baburin, I. A. (2011). Packings of carbon nanotubes – new materials for hydrogen storage. Advanced Materials 23(10), 1237–1241.
Braun, R. J., Klein, S. A. and Reindl, D. T. (2006). Evaluation of system configurations for solid oxide fuel cell-based micro-combined heat and power generators in residential applications. Journal of Power Sources 158(2), 1290–1305.
BP (2011). Statistical Review of World Energy. British Petroleum. Retrieved from http:// www.bp.com/productlanding.do?categoryId=6929&contentId=7044622.
Carton, J. G. and Olabi, A. G. (2010). Wind/hydrogen hybrid systems: opportunity for Ireland's wind resource to provide sustainable energy supply. Energy 35(12), 4536–4544.
de Wit, M. P. and Faaij, A. P. C. (2007). Impact of hydrogen onboard storage technologies on the performance of hydrogen fuelled vehicles: a techno-economic well-to-wheel assessment. Int J Hydrogen Energy 32(18), 4859–4870.
Di Profio, P., Arca, S., Rossi, F. and Filipponi, M. (2009). Comparison of hydrogen hydrates with existing hydrogen storage technologies: energetic and economic evaluations. Int J Hydrogen Energy 34(22), 9173–9180.
EIA (Energy Information Administration) (no date). Retrieved from http://www.eia.gov/ forecasts/ieo/.
Eucar, C. (2004). Well-to-wheels analysis of future automotive fuels and power trains in the European context. Tank to Wheels Report. Joint Research Centre of the European Commission.
Forsberg, C. (2005). Futures of hydrogen produced using nuclear energy. Progress in Nuclear Energy 47(1–4), 484–495.
Forsberg, P. and Karlström, M. (2007). On optimal investment strategies for a hydrogen refueling station. Int J Hydrogen Energy 32(5), 647–660.
Fuel Cells 2000 (2011). Hydrogen Fueling Stations in the US Retrieved from http://www. fuelcells.org/info/charts/h2fuelingstations-US.pdf.
Ganley, J. (2009). High temperature and pressure alkaline electrolysis. Int J Hydrogen Energy 34(9), 3604–3611.
Garvey, M. (2011). The hydrogen report after losing some momentum in 2010, hydrogen markets pick up. CryoGas International.
IEA (2005). Prospects for hydrogen and fuel cells International Energy Agency. IEA Technolgy Analysis Series.
Jin, C., Yang, C. and Chen, F. (2011). Characteristics of hydrogen electrode in high temperature steam electrolysis process. J Electrochemical Society 158(10), B1217– 1223.
Jorgensen, S. (2011). Hydrogen storage tanks for vehicles: recent progress and current status. Current Opinion in Solid State and Materials Science 15(2), 39–43.
Keith, G. and Leighty, W. (2002). Transmitting 4,000 MW of New Windpower From North Dakota to Chicago: New HVDC Electric Lines or Hydrogen Pipeline. Retrieved from http://www.leightyfoundation.org/files/ND-Chicago-HVDC-H2pipeline.pdf.
Koh, J., Yoon, D. and Oh, C. H. (2010). Simple electrolyzer model development for high-temperature electrolysis system analysis using solid oxide electrolysis cell. J Nuclear Science and Technology 47(7), 599–607.
Leighty, W. C., Holloway, J., Merer, R., Somerday, B., San Marchi, C., Keith, G. and White, D. E. (2006). Compressorless hydrogen transmission pipelines deliver largescale stranded renewable energy at competitive cost. International Gas Union World Gas Conference Papers, 4, 1787–1811.
Lemus, R. G. and Martínez Duart, J. M. (2010). Updated hydrogen production costs and parities for conventional and renewable technologies. Int J Hydrogen Energy 35(9), 3929–3936.
Martin, K. B. and Grasman, S. E. (2009). An assessment of wind-hydrogen system for light duty vehicles. Int J Hydrogen Energy 34(16), 6581–6588.
Michel, F., Fieseler, H. and Allidieres, L. (2006). Liquid hydrogen technologies for mobile use. World Hydrogen Energy Conference, Vol. 16.
Naterer, G. F., Fowler, M., Cotton, J. and Gabriel, K. (2008). Synergistic roles of offpeak electrolysis and thermochemical production of hydrogen from nuclear energy in Canada. Int J Hydrogen Energy 33(23), 6849–6857.
Naterer, G. F., Suppiah, S., Stolberg, L., Lewis, M., Wang, Z., Daggupati, V., et al. (2010). Canada's program on nuclear hydrogen production and the thermochemical Cu-Cl cycle. Int J Hydrogen Energy 35(20), 10905–10926.
O'Brien, J. E., McKellar, M. G., Harvego, E. A. and Stoots, C. M. (2010). High-
temperature electrolysis for large-scale hydrogen and syngas production from nuclear energy – summary of system simulation and economic analyses. Int J Hydrogen Energy 35(10), 4808–4819.
Ryland, D. K., Li, H. and Sadhankar, R. R. (2007). Electrolytic hydrogen generation using CANDU nuclear reactors. Int J of Energy Research 31(12), 1142–1155.
Saxena, S., Kumar, S. and Drozd, V. (2011). A modified steam-methane-reformation reaction for hydrogen production. Int J Hydrogen Energy 36(7), 4366–4369.
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Spath, P. and Mann, M. (2001). Life Cycle Assessment of Hydrogen Production via Natural Gas Steam Reforming (NREL Publication No. TP-570-27637). Golden, CO.
Stoots, C., O'Brien, J., Condie, K. and Hartvigsen, J. (2010). High-temperature electrolysis for large-scale hydrogen production from nuclear energy – experimental investigations. Int J Hydrogen Energy 35(10), 4861–4870.
Suresh, B., Schlag, S. and Inogucji, Y. (2004). Chemical Economics Handbook Marketing Research Report. SRI Consulting.
Thornton, J. (2000). Pandora's Poison: Chlorine, Health, and a New Environmental Strategy. MIT Press.
Vilim, R. B., Feldman, E. E., Pointer, W. D. and Wei, T. Y. C. (2004). Generation IV Nuclear Energy System Initiative – Initial VHTR Accident Scenario Classification: Models and Data. Nuclear Engineering Division, Argonne National Laboratory, Argonne, IL.
WNA (2011). Nuclear Power in the USA. World Nuclear Association. Retrieved from http://www.world-nuclear.org/info/inf41.html.
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Zhang, P., Chen, S. Z., Wang, L. J., Yao, T. Y. and Xu, J. M. (2010). Study on a labscale hydrogen production by closed cycle thermo-chemical iodine-sulfur process. Int J Hydrogen Energy 35(19), 10166–10172.

Advanced engine oils to improve the performance of modern internal combustion engines

K. Howard, Lubrizol Ltd, UK

DOI: 10.1533/9780857097422.1.138

Abstract: High quality engine oils are essential for modern internal combustion engines. This chapter describes the role of engine oil and its composition. It then goes on to describe in detail the challenges faced and the way in which they are addressed. Future challenges from evolving fuel and material types are covered.

Key words: engine oils, additives, fuel economy, durability, emissions.

6.1 Introduction

There are many developing alternatives to the internal combustion engine, but the engine as we know it (or in a similar form) will be around for several decades at least. The internal combustion engine has come a long, long way as designers have sought to improve performance and efficiency. 'Survival of the most efficient' is a rule that drives the never-ending quest for improvement. A wide range of technologies have been applied to both diesel and gasoline engines, resulting in a complex and challenging environment for the modern engine lubricant.

Wander through the engine oil aisle at a car accessory shop and you could easily become baffled by the sheer range of choices available. However, each of these oils is aimed at a specific application and as long as the right oil for the application is chosen, modern oils are capable of protecting the engine under far more severe operating conditions than their predecessors.

This chapter first looks at the role of the lubricant in a modern engine, together with an overview of the composition of a typical engine oil. The varying challenges provided by the diesel and gasoline engine are examined, and an overview is given of the way in which original equipment manufacturers (OEMs) create specifications to ensure their engines are protected. Next the requirements for engine oils in developing markets are examined, including the issues related to varying fuel quality and types. Finally a look to the future is included, covering the challenges from biofuels and new engine materials.

6.2 The role of the lubricant in a modern internal combustion engine

The basic function of the engine lubricant can be divided into three fundamental parts, which are:

1. Safeguarding the durability of the engine.
2. Contributing to fuel economy improvements.
3. Helping to maintain a low level of emissions.

In this section each of these distinct requirements is examined and the way in which the lubricant fulfils the requirement is covered.

6.2.1 Safeguarding engine durability

The fundamental performance requirement for an engine oil is that of protection. This is a complex task but can be expressed simply as the need to keep the engine working as the designer intended. To achieve this objective the oil must perform a range of tasks. A more detailed discussion on specific durability requirements for gasoline and diesel engines can be found later in the chapter, but three very important common objectives are:

1. To prevent wear by keeping surfaces apart wherever possible via full film (hydrodynamic) lubrication or by protecting surfaces under boundary lubrication via the use of anti-wear additives.
2. To keep parts clean of deposits formed by combustion products or by degradation of the lubricant itself. Deposits may be in the form of hard carbon and varnish, commonly found in the piston ring area and which may cause sticking of the piston rings in their grooves. They may also be in the form of thick sludge deposits which can block oil filters and passageways.
3. To prevent corrosion. The oil may become acidic and corrosive as a result of its own degradation or via contamination from combustion gases and unburnt fuel. The stability of the base oils and the performance of the additives both play important roles in this area. Condensation of water within the crankcase can also be an issue. Water condensing on internal surfaces of the crankcase and other metal components can lead to significant corrosion. This is critical in factory fill fluids as engines are operated for a short period and then stand still in varying ambient temperatures while the new vehicle is shipped and parked awaiting delivery to the customer. For this reason factory fill specifications often have increased corrosion protection requirements.

6.2.2 Contributing to the fuel economy of the engine

OEMs will go to extraordinary lengths to enhance the efficiency of the internal combustion engine for reductions in fuel consumption and carbon dioxide (CO2) emissions. Incentive and penalty systems, such as the North American CAFE standards regulated by National Highway Traffic Safety Administration (NHTSA, 2010) or the European CO2 penalty system (covered in regulation EC 443/2009 (EC, 2009)) to encourage these improvements are strengthening year by year. The engine oil has its part to play in the quest for efficiency. It is obvious that keeping the engine clean and relatively wear-free will help to maintain efficiency, but the engine oil can offer a more direct contribution, which will be discussed in more detail in Section 6.8. Briefly, the oil's contribution can be split into three main areas, each of which is discussed below.

The first and most obvious way an oil formulation can increase efficiency is to reduce the viscosity of the fluid in order to minimise power lost to viscous drag. Of course this is balanced with the previously mentioned task of keeping surfaces apart to prevent wear. The key parameter is the high temperature, high shear, or HTHS, dynamic viscosity measured in milli-Pascal seconds or centipoise (cP). OEM specifications and those of industry bodies such as ACEA (Association des Constructeurs Européens d'Automobiles – the European automobile manufacturers association) set minimum HTHS values. The most common value used is 3.5 cP, but the drive for improved fuel efficiency has placed increased emphasis on lower HTHS grades down to 2.9 or 2.6 cP HTHS. The range of fuel economy benefits from reducing HTHS viscosity, compared to a 15W–40 reference oil, is shown in Fig. 6.1.

6.1 HTHS viscosity vs M111 test fuel economy.

This shows results in the industry standard M111 fuel economy test (CEC L-54-T-96), an engine dynamometer test simulating the new European drive cycle (NEDC).

A second way to improve fuel economy is to use friction modifier (FM) additives to make surfaces more 'slippery', particularly under boundary lubrication conditions where complete separation of surfaces is not achieved. These are generally long molecules with polar head groups which attach to the metal surface. The result is an easily sheared layer on the surface which reduces friction and improves fuel economy. The effect is illustrated in Fig. 6.2, and more discussion of advanced friction modifier technology can be found later when future oil evolution is discussed.

The third main way in which the oil helps to maintain engine efficiency is by handling oxidation and contaminants, which tend to thicken the oil. Oxidation is handled by the use of anti-oxidant additives and by the selection of good quality base oils having inherent resistance to oxidation. A contaminant of interest for oil thickening is soot from diesel combustion. This is handled by dispersants within the additive package. Dispersants attach themselves to small soot particles and repel adjacent ones, thereby preventing the agglomeration which leads to soot thickening. By handling oxidation and contaminants the oil retains a viscosity profile which provides

6.2 The action of friction modifiers in engine oils.

the required film strength for protection without excessive energy losses due to thickening.

Fuel economy retention over the drain interval of the oil is a requirement of ILSAC specifications (see Section 6.6) where the sequence VI fuel economy test (ASTM D 7589) takes into account the fuel economy benefit of the oil tested as both in its new state and after a period of running in a gasoline engine.

6.2.3 Helping to maintain a low level of emissions

This area is very relevant to the latest advanced low-emission engines. As with fuel economy, the engine oil's ability to prevent wear and deposits is the foundation of good emissions performance. However, a fundamental shift in the challenge to engine oils occurred in the first few years of the 21st century. The evolution of legislation regulating emissions resulted in the adoption of advanced aftertreatment such as particulate filters, NO^x adsorbers and selective catalytic reduction (SCR) systems. Many of these technologies are sensitive to elements normally found within key lubricant components. Restrictions on the levels of sulphated ash (the term 'sulphated' refers to the way lubricant ash content is measured), phosphorus and sulphur have created a new range of aftertreatment compatible oils, known as 'low SAPS' oils. The reasons for each of the limits imposed are:

∑ Sulphated ash is limited due to the use of particulate filters. These devices are extensively used in diesel engines (the term DPF for diesel particulate filter is used) but, as discussed later, may be used on some newer gasoline engines. They are extremely effective in reducing particulate matter, with filtration efficiencies usually exceeding 99 per cent. The fine matrix of a particulate filter is soon blocked by the captured carbonaceous matter, but this can be burned off by raising the exhaust temperature for a short period. The long-term and more permanent blockage of the filter is caused by ash, much of which comes from the inevitable combustion of a small amount of the engine oil. In time this may necessitate removal and cleaning or replacement of the filter. Figure 6.3 shows a DPF blocked with ash. Note that the blockage is of alternate channels as the filtration occurs when the exhaust gases flow through the walls between channels. Reducing the level of ash in the engine oil is a key contributor to lengthening the DPF service interval.
∑ Phosphorus is present within engine oils, for example within the anti-wear/anti-oxidant molecule zinc dialkyldithiophosphate (ZDDP), which is an extremely effective part of the additive package. However, phosphorus is known to adversely affect catalyst performance. In order to ensure long-term emissions compliance those engines fitted with

6.3 A section of an ash-blocked DPF.

advanced catalytic devices may require a lower phosphorus content, typically limited to 800–900 parts per million (ppm) or less for lower SAPS oils. It is also now recognised that some phosphorus containing molecules provide more volatile phosphorus than do others, meaning that more of the phosphorus finds its way into the exhaust via crankcase ventilation. The ILSAC GF-5 specification (see Section 6.6) contains a test to measure the volatility of phosphorus in the engine oil.

∑ Similarly sulphur has a negative effect on catalyst devices. Fuel sulphur levels have fallen in parallel with successive revisions of emissions legislation, and with these levels now as low as 10 ppm, the level of sulphur in the engine oil is a key issue for the catalyst. Engine oil sulphur comes from the base oils (especially less refined ones), anti-wear and detergent additives. Limits to sulphur content in lower SAPS oils are typically 0.2 per cent to 0.3 per cent.

6.3 The composition of a typical modern engine lubricant

Modern lubricants are extremely sophisticated fluids compared to their counterparts of only a decade ago. Typically, 70 to 80 per cent of a modern engine oil will be base stock, which may be mineral, synthetic or a mixture of the two. Base stocks are classified by API, the American Petroleum Institute, using five groups. Groups I to III are derived from crude oil and are divided by saturates, sulphur content and viscosity index. Group IV base stocks are synthetically produced polyalphaolefins and those in group V are any which do not fall into the other categories. In recent years, highly refined group III base stocks have become more widely used, and a wide range of high-performance, cost-effective lubricants now are available with these base stocks.

Improved additive technologies combined with good quality base stocks means that lower viscosities are feasible, even for severe applications. The 5W–30 grade, especially when based on group III base stocks, is currently a well-accepted, all-round performer and is supplied in high volumes throughout the industry, although there is an ongoing trend for viscosity reduction.

Some of the key components of the other 20 per cent of the finished fluid (in typical order of volume within the package) are as follows:

∑ Viscosity modifiers (VMs): These enable the lubricant to maintain a more stable viscosity across a wide temperature range. Modern VMs also are expected to outperform older types in terms of deposit-forming tendencies and also may have dispersant properties.
∑ Dispersants: These function to keep solid contaminants in suspension and thereby prevent the formation of sludge and other deposits. The control of thickening due to soot loading in diesel engines is an important function of dispersants.
∑ Detergents: These keep engine parts clean and also provide a reserve of basicity to neutralise harmful acids formed via 'blowby' of combustion gases and unburnt fuel into the crankcase and also by degradation of the oil itself.
∑ Anti-oxidants: As the name implies these increase the oxidation resistance of the lubricant, thereby reducing oxidative thickening and the formation of deposits. This is becoming particularly important for engines running on bio-fuels, which are inherently less oxidatively stable than their mineral counterparts and also for downsized engines with high power densities where oil temperatures are typically much higher.
∑ Anti-wear agents: Although a relatively small proportion of the additive package, anti-wear additives play a key role by forming thin, tenacious films on the surfaces of highly loaded parts to prevent metal-to-metal contact. Zinc dialkyldithiophosphate (ZDDP) is commonly used and has the added advantage of offering anti-oxidant properties.
∑ Friction modifiers (FMs): Generally added at low concentrations, they are surface active (see Fig. 6.2) and, as the name implies, help reduce friction. They are particularly important because there often is the need to maintain a higher viscosity whilst improving fuel economy.
∑ Pour point depressants: Additives used in small amounts to enable the oil to flow at very low temperatures.
∑ Corrosion inhibitors: These prevent engine corrosion caused by condensation and acidic combustion by-products. They are often used at higher levels in factory fill oils as there may be considerable time

between engine shipment from the factory and the vehicle entering service.

Formulating an additive package is a complex and very expensive process. Additives can interact with each other positively or negatively. The formulator's job is to maximise the positive interactions and minimise the negative ones to produce a finished fluid capable of meeting the demands of a today's high-power, low-emission engines. Chemical limits in more recent industry and OEM specifications, as discussed earlier, are an added challenge.

6.4 Diesel engine lubricant challenges

There are two key lubricant challenges posed by the modern passenger car diesel engine. Virtually all new diesel cars are fitted with DPFs and so most manufacturers specify lower SAPS lubricants, which restricts formulating as mentioned earlier. The other challenge comes from the nature of modern diesel fuel, and specifically the increasing biodiesel content. The combination of DPFs and biodiesel can be a deadly one for the engine oil.

DPFs need to burn off the accumulated soot (regenerate) periodically to prevent an excessive restriction on exhaust gas flow. In most cases regeneration occurs at an interval determined by algorithms based on operating conditions and by direct measurement of the pressure drop across the filter. Typical regeneration intervals may be several hundred kilometres, but this interval may be reduced dramatically under low temperature operation such as city driving. Under such operation the exhaust temperature is artificially raised periodically by a process known as post-injection.

Figure 6.4 shows the typical post-injection used to regenerate the DPF. The post injection phase involves a small additional fuel injection and occurs with the piston further down the combustion stroke, leading to higher exhaust temperatures. This will burn off the accumulated soot and leave the filter relatively clean and 'regenerated'. However, the inefficient combustion and cylinder wall-wetting of post-injection leads to a significant increase in fuel dilution of the engine oil and corresponding viscosity reduction.

The chart in Fig. 6.5 shows how a typical 5W–30 lubricant with a HTHS viscosity value slightly higher than 3.5cP can thin out to 3.0 or less with fuel dilution levels over 10 per cent. Such levels are now being encountered due to DPF regeneration strategies.

Biodiesel is much less volatile than mineral diesel. The latter will evaporate more readily, meaning that the biodiesel accumulates to a higher concentration in the oil than mineral diesel and that the bio-content of the fuel in the oil is much higher than that in the fuel tank. Biodiesel is also less stable in terms of oxidation (and the stability varies depending on the biodiesel source). This will inevitably degrade the lubricant. Figure 6.6

6.4 Schematic – fuel dilution due to post-injection.

6.5 Dynamic viscosity reduction due to fuel dilution.

compares engine oil degradation in a laboratory oxidation test from ultra low sulphur diesel (ULSD) and rapeseed methyl ester (RME – a commonly used biodiesel in Europe). It shows the increased severity of biodiesel on total acid number (TAN) and kinematic viscosity increase at 40 °C with a

6.6 Increased oil degradation from biodiesel dilution.

typical 5W–30 formulation. These increases can ultimately lead to issues with piston deposits, sludge and wear. This topic is well covered by Jones (2008).

From the points discussed it can be seen that universal DPF use combined with pressure to use increasing amounts of biodiesel will be a real challenge for the lubricants of modern diesel engines. The inclusion of biodiesel presents a two-fold challenge because the fuel is there in greater quantities and is less stable. Lubricant and additive technologies will continue to develop to improve engine protection, but this is one area where lubricant performance will really be challenged. Forthcoming emissions legislation and the need to make further improvements in fuel economy and CO2 emissions are posing further challenges for future diesel engine oils. These will be covered later in this chapter.

6.5 Gasoline engine lubrication challenges

Advances in gasoline engine technology are driving major advances in gasoline engine oil development. The key technology introduced in recent years is gasoline direct injection (GDI), which is frequently combined with turbocharging (TGDI) to provide a compact, fuel efficient yet powerful engine. The focus of this section is on the challenge posed by these engine types.

Figure 6.7 shows the power density comparison between typical port fuel injected (PFI) and TGDI options available on one OEM's range of vehicles. The power density of the latter can be seen to be far higher, often well over 100 PS (Pferdestarke – metric horsepower) per litre, and these numbers are increasing with each new engine range introduced. These changes are creating

6.7 Power density comparison, PFI to TGDI.

a more severe environment for the lubricant. Higher power density means an increase in cylinder pressures and temperatures, and larger forces acting on smaller bearings. Turbine temperatures in small turbocharged engines can reach well over 1000 °C, and there are cost and design pressures on the use of liquid turbocharger cooling.

The drive cycle obviously has an influence on the durability of the turbocharger. A mode of operation found to be particularly severe consists of high speed motorway/Autobahn driving with occasional stops. At high speed and power the turbocharger is working hard and gets very hot. Exhaust turbine temperatures may exceed 1000 °C. When the car stops the heat soak effect on the turbocharger provides an extreme test for the lubricant within it. Lubricant oxidation under such conditions can result in significant deposits on the turbocharger shaft and bearing area, eventually leading to seizure of the bearing. Figure 6.8 shows two examples highlighting the improvement possible with a higher quality lubricant.

If turbocharger deposits do not significantly harm the turbocharger itself they may still cause damage elsewhere in the engine. The photos in Fig. 6.9 show the deposits retrieved from the oil pickup pipe on a turbocharged gasoline engine. The small, hard particles are characteristic of the more severe oxidising environment in the turbocharger bearing area. They have been seen to block the oil pickup screen and to cause sludge deposits. As can be seen from the 'slug' of sludge found in the pickup pipe, these can severely restrict the supply of oil to engine's oil pump. Increases in sludge deposits in the 'traditional' areas such as the sump and cylinder head deck

6.8 Turbocharger shaft/bearing deposits, TGDI engine.

6.9 Oil pickup deposits before and after solvent wash.

A further illustration of the relative severity of TGDI engines can be seen when the viscosity of the oil is tracked over the course of a test on the engine dynamometer, as shown in Fig. 6.10. The test was run using the same fuel and oil for PFI and TGDI engines. The shape of the viscosity curve is characteristic. Initially some shearing takes place, and then the oil has a period of relatively stable viscosity. In the period between 160 and 220 hours the viscosity of oil in the PFI engine begins to take off as the oxidation really starts to take hold. At the end of the test, dismantling the engine reveals

6.10 Engine dynamometer tests on PFI and TGDI engines.

sludge and harder deposits in critical areas of the engine. The TGDI engine was run on a slightly different test cycle more suited to this engine type. However, the severity of the cycle is considered similar to that of the PFI engine. What is apparent is that the oxidation is much more aggressive and the viscosity take off occurs much earlier. Running for an equivalent distance on the road can have catastrophic effects on the condition of the oil, and therefore the engine. The situation can be significantly improved by using improved formulations including more oxidatively stable base stocks and more robust additive packages.

When fuel is injected into the intake port it has time to vaporise fully and does not tend to give significant fuel dilution. GDI, however, means that a fine fuel spray is injected at high pressure into the cylinder. Accurate control and fine atomisation are achieved, but there is inevitably an increased tendency for fuel spray to come into direct contact with cylinder walls, increasing the amount of fuel swept into the crankcase to dilute the oil. Deposits on the injectors can affect the precision of the spray pattern, further increasing the tendency for fuel to get into the oil. The obvious effect of this fuel dilution is the thinning of the oil, which is a serious concern given higher bearing loading resulting from higher torque and narrower journals. For this reason OEMs are often reluctant to reduce the viscosity of the engine oil, and the ambitions to improve fuel economy are compromised in order to protect the durability of the engine. The composition of the fuel, combined with the type of operation, has a significant effect on how much of it stays in the oil and what effect it has on the lubricant.

The challenges posed by these engines were summed up well by Daniel Kapp, Ford's director of powertrain research (Kapp, 2010). He addressed the North American Society of Tribologists and Lubrication Engineers in May 2010 and emphasised the challenges that these engines (called EcoBoostTM by Ford) pose for the lubricant. 'Lubricants can continue to play a very integral role, but maybe some of the challenges are a little bit different' he remarked. 'If we just look at EcoBoosting, we're certainly seeing higher operating temperatures and much higher specific loads. So imagine now very small engines operating at very high combustion temperatures.' He went on to mention issues such as high fuel dilution and high specific loads. In January 2012, Ford of Europe introduced a 1 litre EcoBoostTM engine which replaces PFI engines up to 1.6 litres. This is typical of the trend in this area. The maximum exhaust temperature is quoted as 1050 °C and the power output is over 120 PS. An article in the January 2012 edition of Automotive Engineer magazine (2012) gives further details. Other OEMs are following very similar strategies in the pursuit of reduced fuel consumption and CO2. It is clear that they will require a better quality of oil than their predecessors.

6.6 Industry and original equipment manufacturer (OEM) specifications for engine oils

There are many engine oil specifications covering a wide range of technologies in order to ensure that the right oil for the application is used. It is not the intention to go into the details of the specifications here. However, engine oil development is a specification-driven process and some understanding of the various specifications is helpful.

Formulators work to develop fluids capable of meeting a range of limits in industry and OEM specifications, and it is those formulations, providing maximum specification coverage in the most cost-effective manner, which go on to be the most successful products in the field. Most specifications used by OEMs across the world are either North American or European, with some limited use of Japanese specifications. All are a combination of chemical/physical limits with laboratory and engine tests covering the range of protection for cleanliness, wear and other requirements. Together the combination of tests and limits is intended to weed out inferior formulations, leaving only the best performers meeting the standards.

The North American market is dominated by specifications from the American Petroleum Institute. The structure is quite simple. Those for gasoline engine oils fall under API 'S' (service) categories, and those for heavy-duty diesel engines fall under the 'C' (commercial) categories. Each has a second letter designating the version of the category. Currently the latest versions are SN and CJ-4 respectively. The '4' represents 4-stroke diesel engine use, as some older categories were split between the requirements of 2 and 4 stroke diesels. API S oils may also meet the additional engine protection and fuel economy requirements of the International Lubricant Standardisation and Approval Committee (ILSAC), a joint effort of US and Japanese automobile manufacturers. ILSAC GF-5 is the latest version and was introduced in October 2010. (A useful guide can be found at the Lubrizol sponsored website for GF-5.) Oils meeting ILSAC requirements are identified by a 'starburst' certification mark on the can. There are no specific categories or tests covering North American passenger car diesel engine oils due to the very low penetration of diesels into the market.

The European system of specifications is far more complicated. The oil sequences published by ACEA are split into three classes. The 'A/B' class covers passenger car engine oils. Originally 'A' oils were for gasoline engines and 'B' oils were for diesels, but as the market penetration of diesels increased they were combined in 2004 to provide oils covering both engine types. Also in 2004 the 'C' class of oils was introduced to cover the increasing need for aftertreatment compatibility. 'C' oils have reduced SAPS levels (see Section 6.3.3) and are normally mandatory for cars fitted with particulate filters. Heavy duty diesel engine oils are covered by the 'E' sequences. Within each class there are four categories covering a range of requirements. So for passenger car use there are eight choices of oil category compared to two in North America.

The divergence of engine technologies in Europe has led OEMs to bring in their own specifications to protect their hardware. Generally, North American OEMs have been far more reluctant to do so (although in 2009 General Motors brought in their dexosTM specifications, covering the global needs of the various GM brands). All major European OEMs have their own specification system. They are usually based on ACEA categories with added tests and limits which suit individual OEM requirements. The challenge for oil and additive companies is to cover a range of OEM specifications with the same formulation. There are limits to this. Some categories are mutually exclusive and others stretch the formulation to the point where it is just not feasible to cover the desired combination. Wherever an engine oil is purchased, it is reassuring to know that an oil meeting the requirements of an established industry or OEM specification has been tested to the point where the vehicle owner need have very few concerns over its ability to protect the engine into which it is poured.

6.7 Lubricating modern engines in developing markets

The days when the latest car models were only available in fully developed markets are long gone. Consumer demand in developing markets is for sophisticated cars combining comfort and performance. This means that new engines using the most up-to-date hardware and control systems are required. The phenomenal growth in the demand for the latest cars in markets such as China has led the introduction of new engine types by joint-venture OEMs and more recently by their domestic counterparts, who have invested heavily in engine research and design.

Providing effective lubrication for the latest engines in these markets is not without problems. The power densities (and hence operating temperatures) of newer engines place major stresses on the engine oil, even when operated under ideal conditions. The major issue in many developing markets is the type and quality of fuel available. Major issues associated with varying fuel types and quality levels include:

∑ Sulphur levels
∑ Ethanol
∑ Methanol.

These are discussed in more detail below.

6.7.1 Sulphur levels

In Europe and North America levels of sulphur in fuel have fallen dramatically in parallel with successive reductions in emissions limits. Sulphur is known to poison oxidation catalysts and also directly contributes to tailpipe emissions. Reductions in fuel sulphur levels in developing countries have been much slower. For example, diesel fuel sulphur levels of over 1000 ppm can be found in markets such as China, compared to less than 10 ppm for the latest fuel quality in Europe. Efforts to implement legislation to reduce China's emissions to Euro 4 levels are being hampered by the ability to secure a consistent supply of low sulphur fuel.

The effects of sulphur on the engine's internal components are well known. Some sulphur from the fuel will inevitably find its way into the oil via the 'blowby' of combustion gases past the imperfect seal provided by the piston and rings. Once in the oil there are direct and indirect effects. The direct effect is a raise in the acidity of the oil with consequent acid corrosion issues, especially for soft bearing metals. The indirect effect is that the acids in the oil promote oxidation and oil thickening, with longer term (but no less serious) consequences for the hardware.

6.7.2 Ethanol

Ethanol is blended with gasoline around the world in concentrations ranging from 2 per cent (E2) to 85 per cent (E85) or used neat (E100). The US is the largest user of ethanol in fuel, with over 75 per cent of gasoline containing ethanol, mostly as E10. The key area for the use of ethanol in very high percentages is Brazil, due to government agricultural/energy policies aimed at maximising the utilisation of sugar cane, a very good ethanol feedstock. The majority of gasoline in Brazil contains between 24 and 26 per cent ethanol, though extensive use of 'wet' E100 (ethanol which contains 4.6 per cent water) in this market creates some major issues for the engine oil.

Ethanol can work its way into the sump and dilute the oil in the same way any fuel can. Also, the high hydrogen to carbon ratio of ethanol relative to gasoline means that combustion of ethanol forms much more water, some of which ends up in the engine oil. Ethanol is hygroscopic, so it can attract water in the fuel or oil. Neither water nor ethanol have good solubility properties in engine oil. If the engine works hard and the oil heats up sufficiently then they can boil off, but in cooler conditions one or both will remain in the oil, with two main risks. The first is the formation of 'white sludge', a creamy mayonnaise-like substance that can build up in such areas as the camshaft bearings and breather tubes with serious effects on engine operation. The second is separation of the oil from the water. If this goes on for long enough the risk is that neat water will be drawn into the engine's oil circuit with disastrous results.

Ethanol is corrosive and high ethanol blends have been known to attack the cylinder walls of aluminium alloy engines, and also the crankshafts of some engines. Degradation of ethanol in the engine oil creates ethanoic (acetic) acid, which attacks soft metals and increases the oxidation of the oil.

6.7.3 Methanol

This fuel is of particular relevance in China. Having little oil reserves of its own, but possessing massive coal reserves, China produces and uses more methanol in fuel than any other country in the world. Early attempts to operate production engines (as opposed to race engines with limited lifespan) on high methanol blends, e.g. M85, met with failure due to the extremely corrosive nature of methanol to aluminium and incompatibility with many gasket and seal materials. The challenges of methanol use at lower levels (M15 or less) are similar, but more severe than those of ethanol with respect to water pick up, solubility, corrosivity and acidity via the creation of methanoic (formic) acid.

Lower quality fuels tend to have higher levels of heavy, high boiling point fractions and when these find their way into the oil their low volatility means they are far less likely to boil off. Although these fractions may be initially present in small amounts in the oil, they accumulate over time to give significant fuel dilution levels. They are also prone to interact with the oil to degrade it. Although these fuels are rarely found in Europe and North America, they are specifically chosen as worst case fuels in tests to ensure the oil is capable of resisting 'black sludge'. The black sludge phenomenon is very rare in Europe and North America these days but these fuel types are present in developing markets and sludge occurs more frequently.

An increasing number of small, high efficiency engines using turbocharging and gasoline direct injection (TGDI) are being introduced into developing markets. These engines are more susceptible to fuel quality for two reasons. Firstly, the direct injection of gasoline produces an increased level of fuel dilution. Secondly, hot spots such as the turbocharger bearing area increase the rate of oxidation, particularly with lower fuel quality.

The photograph in Fig. 6.11 shows an oil pickup from a TGDI-engined car operated in China on a poor quality fuel (the fuel was chosen to represent the quality level found outside the major cities) and a basic quality oil. The pickup is blocked by sludge deposits. The pressure drop across the oil filter was measured and dramatically increased, then levelled off. This indicates opening of the bypass valve and the supply of unfiltered oil to the engine, a clearly undesirable situation.

Rapid urbanisation into 'megacities' is another feature of developing markets such as India and China. Mumbai and Shanghai each have over 20 million inhabitants. Traffic congestion in these cities is such that the average vehicle speeds are often no more than walking pace. Low speed, stop-start driving results in less efficient combustion and a reduction in oil temperature, meaning that less fuel is 'boiled off' and more of the harmful degradation products remain in the oil. While low speed traffic is not unique to developing countries, the combination with poor fuel quality exacerbates the challenge to the lubricant.

OEMs have not been slow to recognise the potential consequences of the latest engine technology combined with the poor fuel and driving conditions

6.11 Oil pickup from TGDI engined car operated in China on poor quality fuel.

found in many developing markets. Specifications have been modified to increase the robustness of oils in order to ensure they can cope with the challenges posed, and drain intervals are very short compared to those in developed markets. Oil and additive companies are responding with the engine oil technology required to protect modern engines in developing markets. Some of the solutions are discussed below.

A priority is a move to better quality base oils with enhanced oxidation stability. As discussed earlier, mineral base oils are classed as group I to group III depending on the type and level of refining. Group IV oils are fully synthetic (the term is sometimes used for group III oils but technically it should not). The base oil effect is shown in Fig. 6.12. The photographs show how the oxidation stability varies with basestock type, and give a rating out of 10 for deposit control. The plates are from a micro coking test which simulates the hot surfaces on the inside of the engine. Here the same oil formulation has been tested in typical examples of four basestock types. The group I oil can be seen to have produced significant deposits, and when considering the turbocharger and other very hot areas it may not be appropriate. At the other end of the spectrum, group IV oils, being fully synthetic, provide excellent performance but are expensive, especially for use in developing markets. There is a great deal of focus on group III base stocks as they combine very good performance in a cost effective package and have good availability with a number of suppliers across the globe. In the photos it can be seen that the performance is close to that of the group IV based formulation. These days group IV based full synthetic oils are limited to high performance niche applications, although inclusion of some group IV can provide low temperature viscosity advantages due to the improved viscosity index (a measure of viscosity change with respect to temperature).

6.12 Micro coking test results with varying base oil quality.

Improved anti-oxidant systems are needed to cope with the combination of high power density, turbocharging and direct injection when used with poorer quality fuels. Anti-oxidants are seen as a key focus area across all markets for more modern engines running at higher temperatures. The improved protection covers a range of problems as oxidation is the precursor to many undesirable phenomena such as oil thickening, increased oil acidity, piston deposits, and sludge formation.

Higher levels of alkalinity (known as total base number or TBN) are needed to combat the acidic combustion by-products accumulating in the oil, the acids from ethanol and methanol combustion, and the increased acidity of oxidised oil. TBN is measured by a process of titration and is expressed in milligrammes of potassium hydroxide per gramme of oil (mgKOH/g). TBN levels have typically been around 8 mgKOH/g for passenger car oils but recent concerns have seen OEMs demand levels of 10 or more.

It is clear that the combination of the latest engine technologies with the fuels and driving conditions found in developing markets is a major concern for all OEMs. Although the problems are not easy to solve, strong co-operation between OEMs, oil and additive companies is addressing the concerns and putting the right engine oils in place to ensure that the latest cars can continue to operate effectively and reliably in these challenging environments.

6.8 Future engine oil evolution

The engine oils of the future will continue to address the three perennial issues of fuel economy, emissions and durability. Legislation for air quality in Europe has been joined in 2012 by the CO2 penalty system for OEMs. Both low emissions and fuel consumption must be achieved without sacrificing durability. Unreliable cars will not sell, however clean and efficient they are. The engine oil has a vital contribution to make to in all three of these areas.

6.8.1 Future fuel economy challenges

The contribution to fuel economy of the engine oil was discussed earlier in this chapter. With the introduction in 2012 of CO2 penalties, OEMs are searching for any opportunity to improve fuel economy and CO2 figures. The engine oil can make a valuable contribution via reductions in viscosity and also by improvements in additive technology.

In addition to the lower viscosities which many OEMs are now using, much effort is being put into the development of new friction modifier (FM) technologies. Molybdenum dithiocarbamate (usually shortened to MoDTC) has been a commonly used FM but there are concerns over its contribution to deposits in turbochargers, which are a key strategy in CO2 reduction. Conventional organic friction modifiers offer an alternative, but have been shown to have a reduced effect at higher temperatures compared to molybdenum.

Ideally a modern friction modifier should not contribute to turbocharger deposits and should bring good friction reduction across a wide temperature range. Sutton et al. (2007) reported on the development of new organic FM technology demonstrating such performance. Figure 6.13 shows results in the widely used high frequency reciprocating rig (HFRR). Here the temperature is ramped from 40 to 160 °C over 85 minutes and the friction coefficient is monitored.

It can be seen that the friction coefficient of lubricant D stays low across the temperature range, giving performance better than MoDTC (lubricant B) without the drawbacks mentioned above. Figure 6.14 shows that these gains are reflected in the previously mentioned M111 test, where an improvement of 0.6 per cent over the equivalent MoDTC based FM can be seen.

It is clear that there are still many opportunities to enhance the direct contribution of engine lubricants to better fuel economy and lower CO2 levels. Viscosity optimisation, FM and additive package composition will all evolve with friction reduction as a high priority.

6.8.2 Future emissions challenges

The continuous drive for low emissions is ongoing globally. As the author is closely involved with European engine oil development the examples

6.13 Friction modifiers in the HFRR test.

6.14 Friction modifiers in the M111 test.

given are from this region, but there are similar and no less challenging requirements in other regions. In Europe engines are now being developed to meet Euro 6 emissions standards. There are very different limits for diesel and gasoline engines. For diesel engines the focus is on reducing emissions of oxides of nitrogen (NOx). There are different ways of achieving this, each of which will provide a different challenge to the engine oil. While designs and combustion strategies are still evolving it is difficult to predict the effects on the engine oil, and Euro 6 engine oil development is in an early stage.

Higher exhaust gas recirculation (EGR) rates are probable for Euro 6 diesels. These will increase the level of soot in the engine oil. Also, EGR gases are cooled, causing acidic combustion by-products to condense, and some of these condensates will find their way into the engine oil. OEMs are finding that some of their low SAPS specifications for DPF equipped diesels need to be updated in order to offer protection from soot-related wear and the degrading effects of condensed acids.

SCR systems are likely to be introduced on larger cars. SCR catalysts themselves seem to be quite robust with respect to the contaminants which may originate from the engine oil. However, high NOx conversion rates rely on control systems with very sensitive NOx sensors, and it is these areas where much attention is being focused.

For gasoline engines the main change from Euro 5 to 6 is the introduction of particulate mass and number limits for gasoline direct-injection (GDI) engines. This issue is a concern because GDI engines emit comparatively high levels of ultrafine particulates, which are increasingly being linked with deep penetration into the lungs and subsequent increased risk of lung cancer. A key question is whether these particulate emissions will necessitate the addition of a gasoline particulate filter (GPF) to remove them.

It may be that modified calibration of injection, ignition and other control unit maps will allow the limits in the legislation to be met without a GPF. However, pressure to reduce emissions may well result in one OEM taking an environmental lead by fitting GPFs to reduce particulate emissions further, and others will follow so as not to be labelled 'dirty'. This happened with DPFs. Many OEMs chose to fit them in order to appeal to more environmentally conscious customers, and others were forced to follow suit.

If GPFs are adopted there may be a need for lower SAPS engine oils to be used in order to prevent ash blockage and catalyst poisoning (the details of these oil oils were covered earlier in this chapter). Indeed, OEMs currently revising their specifications are 'hedging their bets' by building lower SAPS into their gasoline engine oil requirements. The European launch in early 2012 of the 1.0 litre EcoBoost™ engine by Ford was accompanied by a new oil specification which, although focussed on gasoline use, still has lower SAPS levels in order to future-proof it against potential GPF adoption. Robert Haigh of Ford Europe outlined the specification in his paper at the ICIS World Base Oils and Lubricants Conference in February 2012 (Haigh, 2012).

6.8.3 Future durability challenges

Engine development to meet emissions and fuel economy requirements is creating new durability challenges. Further challenges come from the need to use an increasing amount of biofuels from a wider range of sources. Downsizing of engines usually means that bearings are narrower and are therefore subject to increased stresses for the same load. Turbocharging and moving to direct injection further increases these stresses. The move to thinner oils for fuel economy is gaining momentum, with several OEMs developing new, even lower viscosity specifications down at dynamic viscosities of 2.6 cP or lower. Research is ongoing to identify the weak spots of new engines when using very thin oils. Additional additive technologies for enhanced wear protection will be required.

The use of biofuels and the consequences for engine oil durability were covered earlier. The long term future of biofuel use is uncertain. More concern is being expressed over the wider environmental impacts, taking into account such factors as fertiliser use and deforestation to make room for their cultivation. However, they will be around for some years to some and OEMs are putting additional measures in place to protect their engines. For example, ACEA are looking at three tests currently under development in 2012 targeting biodiesel related issues of piston deposits, sludge, oxidation and low temperature performance. These will be included in the 2012 update to the engine oil sequences, or later updates if test development is not complete in time.

Hybridisation and the increasing use of stop-start systems have not, as yet, given rise to major lubricant concerns, but they can create two issues for the engine oil. Firstly the increased percentage of running time spent under boundary lubrication conditions means that anti-wear chemistry may change, either in amount or type. Additionally cars which spend most of their time in an urban environment with frequent stops rarely get the engine oil very hot. Water and unburnt fuel can build up in the oil giving rise to increased oxidation, corrosion and the potential for mayonnaise-like 'white sludge' in cooler parts of the engine. The previously discussed hygroscopic nature of any ethanol in the fuel increases the risk of water in the oil. Additive chemistry can be optimised to improve corrosion resistance and to resist the formation of white sludge.

6.8.4 New materials

The continued drive for improved fuel efficiency is also changing the materials that engine oils interact with. Many aspects of the interaction between new materials and surfaces were covered by Neville et al. (2007). Material changes include nitride or tool steel for piston rings, copper and nickel based aluminium alloys in pistons (e.g. Al 336), silicon containing aluminium alloys (e.g. AA 390) for cylinder bores/liners. Traditional valve train metallurgies such as high chromium cast iron now include carburised steel and powder sintered alloys (Fe-Cr-Mo-C).

Beyond this selection of materials are a series of coatings which are used predominantly to improve the wear resistance or frictional properties of key components. These can be deposited by a range of techniques, such as chemical vapour deposition (CVD), physical vapour deposition (PVD) or plasma assisted CVD. Initially these coatings typically involved chromium or nickel, but now also include diamond like carbon (DLC) or molybdenum disulphide coatings. It is worth noting, however, that each coating can vary significantly. This is probably most varied with DLC coatings. The variations are there to change the physical properties such as the frictional or hardness of the DLC, and achieve this by changing the chemical properties such as carbon-carbon bonding type, the hydrogen content and the inclusion of heteroatoms. This change of the chemical nature of the coatings can have a significant impact on how lubricants interact with them, and how additive chemistry needs to be tailored to give optimal performance.

Not only is the chemical nature of the surfaces within the engine changing, but also the physical nature. New coating techniques such as Daimler's NanoslideTM technology enable thin coatings to be applied to the surface of cylinders or liners. These surface coatings not only allow for improved

durability, but also enable improved friction by changing the need for honing of the cylinder bore. The coatings enable small pockets of lubricant to be held and therefore allow for smoother finishes to the surface to allow for improved frictional properties and lower oil consumption.

Clearly as additive chemistry is tailored to interact with the operating surfaces of the engine, all of these changes will have impacts on the lubricant. As there is a rise in the use of new materials and surface coatings this is driving the way in which lubricants are formulated to deliver optimal performance. This ever increasing pace of material changes is forcing the need for new lubricants to be developed to enable the engines to be lubricated effectively. The diversity of new materials will further challenge the ability of oils to cover multiple OEM requirements.

6.9 Conclusions

The development of a high quality lubricant is a multi-dimensional challenge covering such aspects as fuel economy, wear protection, piston cleanliness, sludge prevention and control of oxidative thickening. Meeting the lubrication challenges of the latest engine designs is a monumental task. Faced with these challenges future lubricants are likely to have some key features, including:

∑ High quality base stocks with good thermal and oxidation stability. Group IV stocks for fully synthetic engine oils give very strong performance but group III stocks are more cost effective, give similar performance and have seen increasing use.
∑ Lower viscosities in many cases, in order to give a fuel economy boost; however, scope for viscosity reduction may be limited by higher power densities and the use of biofuels.
∑ Improved friction modifiers with good reduction in friction coefficient across a wide temperature range.
∑ Strong anti-oxidant performance to combat high engine temperatures and the degradation caused by biofuels.
∑ Possible division of gasoline and diesel specifications to allow optimisation for fuel economy.
∑ Low ash, phosphorus and sulphur to protect emissions reduction aftertreatment.
∑ Changes in chemistry for improved compatibility with new engine materials.

Lubricant and additive companies have invested a great deal of time, effort and money into the development of new engine lubricant technologies. These lubricants combine a significant contribution to reducing CO2 emissions with excellent protection for engine hardware. This has been achieved in parallel with major reductions in ash, sulphur and phosphorus to provide exhaust aftertreatment compatibility. Such lubricants will be a key enabler, allowing future internal combustion engines to give maximum performance and efficiency with minimum emissions.

6.10 Acknowledgements

The author would like to express his gratitude to Dr Mike Sutton and Dr Craig Jones of the Engine Oils Group at Lubrizol Limited for their assistance in preparing this chapter.

6.11 Sources of further information and advice

He, X., Williams, A., Christensen, E., Burton, J. et al. (2011) Biodiesel Impact on Engine Lubricant Dilution During Active Regeneration of Aftertreatment Systems SAE Int. J. Fuels Lubr. 4(2): 158–178, doi:10.4271/2011-01- 2396
Okuyama, Y., Shimokoji, D., et al. (2011). Study of Low-Viscosity Engine Oil on Fuel Economy and Engine Reliability SAE Technical paper series 2011-01-1247.
Taylor, R. I. and Coy, R. C. (2000). Improved fuel efficiency by lubricant design: a review. Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology 214.
General engine oil technology information: www.lubrizol.com/ EngineOilAdditives
ACEA European engine oil sequences: www.ACEA.be
API licensing system for North America: www.api.org/certificationprograms
ILSAC GF-5 information: www.GF-5.com

6.12 References

Automotive Engineer magazine (2012), January-February 2012 edition. By the Number, Ford 1.0 EcoBoost. http://ae-plus.com/magazine/
EC (2009) European Parliament and Council Regulation (EC) No 443/2009 : Setting emission performance standards for new passenger cars as part of the Community's integrated approach to reduce CO2 emissions from light-duty vehicles.
Haigh, R. (2012) 'Engine lubricant development for modern gasoline engines', ICIS World Base Oil and Lubricants Conference, London, 24th February.
Jones, C (2008) *'*Interactions Between Biofuels and Lubricants and Effects on Engine Oil Durability', SAE Biofuels, Specifications and Performance Symposium, Paris, July.
Kapp, Daniel (2010) 'Address to Society of Tribologists and Lubrication Engineers', Las Vegas, 17 May. Reported by Lube Report, http://www.imakenews.com/lng/e_ article001798580.cfm?x=b11,0,w
National Highway Traffic Safety Administration (2010), Light-Duty Vehicle Greenhouse Gas Emission Standards and Corporate Average Fuel Economy Standards; Final Rule, May 2010.
Neville, A., Morina, A., Haque, T. and Voong, M. (2007) 'Compatibility between tribological surfaces and lubricant additives – How friction and wear reduction can be controlled by surface/lube synergies, Tribology International 40, 1680–1695.
Sutton, M., Kocsis, J. A. and Nakagawa, I. (2007) 'Improved Friction Modifiers to Aid in Future Fuel Economy Targets', SAE 2007-01-4134.

Advanced fuel additives for modern internal combustion engines

J. Bennett, Afton Chemical Limited, UK

DOI: 10.1533/9780857097422.1.165

Abstract: Additives and the fuels they are used in continue to evolve. This chapter will discuss the various types of additive, what changes are occurring in market fuels, how these together impact on vehicles, and how changes in vehicles are affecting their appetite for additives.

Key words: additives, injector deposits, biofuels, direct injection, diesel, gasoline.

7.1 Introduction

Additives have a long history of use within automotive fuels. Overall they can be considered to fall into two categories: finished fuel additives and performance fuel additives. Finished fuel additives are generally used to help ensure the fuel is fit for purpose and in compliance with the relevant local standards, whilst performance fuel additives will help to improve the function of the engine and vehicle and are typically used by branded fuel retailers to provide product differentiation. The types and composition of additives is affected by the fuel that they are intended for. Changes to mineral oil derived fuels have been taking place, with a particular focus on sulphur removal and also on minimising components such as benzene.

Alternatives to conventional mineral oil derived fuels have at the same time been growing. A significant portion of these alternative fuels are derived from biomass, particularly by conversion into ethanol and biodiesel. The other main source of alternative fuels is by the conversion of other fossil fuels such as natural gas or coal into a liquid form. Whilst all of these fuels are capable of being used as single components or at high concentrations, they are most commonly used as extenders within conventional fuel blends. Doing this has the advantage of allowing them to be used within the normal fuel distribution system and be utilised in conventional vehicles, thereby maximising market uptake.

Ethanol blends typically incorporate up to 10% ethanol. This level has been widely used in the United States for many years, and has more recently been introduced in Europe and other countries. Discussions about increasing this to 15% and beyond are ongoing. Having a high indigenous capacity for production of ethanol (from cane sugar), Brazil has for many years used a higher blend known as gasohol with nominally 22% ethanol content. Even higher levels of ethanol are also used in various countries, either as E85 (a blend of 85% ethanol and gasoline) or E100, but in both of these cases, specially modified vehicles are necessary to use them.

Biodiesel (FAME) is widely used in Europe where up to 7% blends within normal retail fuels are both permitted and encouraged, but moving beyond that is meeting resistance due to concerns about product stability and fuel into engine oil transfer. Blends above 7% are available, subject to local market demands, with up to 20% available in some parts of the United States, and 30% being used in captive fleets, particularly in France. 100% biodiesel enjoyed some success in Germany, but largely disappeared when the focus turned to using biodiesel as a blend extender.

The conversion of other fossil fuels such as gas or coal into gasoline and diesel replacements or extenders has a long history, often driven by local shortages or restrictions on the supply of crude oil. The conversion of these molecules is generally undertaken using the Fischer–Tropsch process. The resulting fuels are highly paraffinic and generally of very high purity, lacking many of the residual materials left from distillation of crude oil. These fuels are sometimes referred to as GTL and CTL (gas and coal to liquid, respectively). Biomass can also undergo the same process to make BTL (biomass to liquid) or can be hydrotreated, either on its own or coprocessed with fossil fuels within a refinery to produce HVO (hydrogenated vegetable oil), which has a similar paraffinic nature and purity to those fuels produced by the Fischer–Tropsch process. Unlike biodiesel blends, there is no limit on the proportion of these components, the only restrictions being physical constraints such as density.

Simultaneously, engine and vehicle technology continues to evolve, with a range of changes to both hardware and mode of use/operation. A short list summarising potential additive influencing factors can seen in Table 9.1. All of these changes have been affecting the appetite of fuels for additives. Feedstocks and production processes other than those mentioned above are under development or even enjoy small scale production. However, it is unlikely that the properties of the fuels they produce will differ significantly from those already mentioned, so they are not discussed in this section.

7.2 Additive types and their impact on conventional and advanced fuels

7.2.1 Antioxidants and stabilisers

With the passing of time, fuel degradation is not uncommon. Degraded fuels can form gums and sediment that plug fuel filters, create deposits on Variable compression ratio Gasoline direct injection Homogeneous charge compression ignition (HCCI)

Diesel – HSDI Hybrids

Technology Potential cleanliness from additive use Cam phasing Variable valve lift Turbocharging Cylinder deactivation Variable charge motion Valve, injector, intake Valve, injector, intake Hotter engine, EGR, intake Valve and Injector Intake system, charge motion deflector

Intake Injectors

New fuel, injectors

Increased diesel use, injectors Valve, injector, intake

Table 7.1 Potential engine technologies for improved fuel economy and potential cleanliness needs to be addressed through use of fuel additives

sensitive fuel system components and interfere with the proper operation of motor vehicles. Due to these common features, fuel specifications often include performance tests, such as oxidative and thermal stability tests, to ensure the fuels are fit for use in the marketplace. Additives can be used to prevent the degradation, and are typically used at treat rates in the 25 to 100 ppm range. The type of degradation that occurs, however, is different between gasoline and diesel fuels.

Gasoline can undergo oxidative degradation and form gum deposits on engine components that lead to poor drivability and increased emissions, fuels with high olefin and diene concentrations being typically more susceptible to oxidation. To control gum formation in gasoline, refineries use phenolic and aromatic amine antioxidants. Antioxidants are most effective when they are added to reactive blend streams such as cat cracked (FCC), coker or polymeric gasoline streams as they are produced. Once the degradation process begins it becomes more difficult to control. One of the benefits of the desulphurisation of gasoline is the improvement in its oxidative stability. Many of the reactive species in gasoline are removed during desulphurisation and normally only low concentrations of antioxidants are needed to control oxidation in low sulphur gasoline. Gasoline blended with ethanol has similar stability properties as the hydrocarbon base gasoline.

Unlike gasoline, diesel fuel normally degrades by mechanisms other than oxidation. Frequently, heterocyclic and other sulphur and nitrogen containing molecules that result from cracking heavier streams to make diesel and gasoline blend components will react with each other or acidic components found in other diesel fuel blend components. The fuel in some diesel engines passes through the high pressure fuel system where it undergoes considerable heating before being returned to the fuel tank. Under such conditions, the above reactions may occur resulting in the formation of sediments, which can plug fuel filters and produce deposits in critical fueling locations, such as fuel injectors, resulting in power loss, smoke and higher overall emissions. Amine-based additives, sometimes combined with dispersants, are used to control the stability of diesel fuel. Deep desulphurisation or hydro-processing (normally used to make ultra-low sulphur diesel fuel) will eliminate most of the reactive species found in higher sulphur diesel fuel. Ultimately, hydroprocessing produces a very stable diesel fuel requiring the use of little, if any, stability additive. Biodiesel, while normally low in sulphur, degrades through an oxidative process that can be controlled, typically using hindered phenol antioxidants. It is important to treat biodiesel when it is produced, as any degradation will be largely irreversible.

Both gasoline and diesel fuel are occasionally contaminated with metals, such as copper, that can catalyse the formation of gums and sediment. Consequently, metal deactivators may be used at low levels to neutralise the catalytic effect of these metals. Most biodiesel antioxidant packages contain a metal deactivator to control catalytic degradation.

7.2.2 Cold flow improvers

Diesel fuel contains naturally occurring paraffinic molecules distilled from crude oil. The source, chemical composition and processing of the crude oil determines the concentration and distribution of these saturated, straightchained molecules in the fuel. Higher molecular weight paraffins have limited solubility in diesel fuel and precipitate from the bulk fuel in the form of wax crystals when the fuel cools. This separation occurs at the cloud point, the temperature where wax crystals precipitating from diesel fuel become visible to the naked eye. As more wax precipitates, large plate-like crystals begin to develop. These crystals can restrict the flow of diesel fuel from storage tanks and pipes and plug vehicle fuel lines and filters, resulting in an engine losing power or being rendered unable to start. In the winter, the refinery streams used to make road diesel are selected to reduce the higher MW paraffin concentration and lower the cloud point of the fuel. The resulting fuels have lower densities and viscosities than those found in summer. If such streams are not available or their use is uneconomical, cold flow improving (CFI) or wax crystal modifying additives can be used to improve the low temperature properties of diesel fuel.

CFIs can be divided into two different categories:

1. pour point depressants
2. operability additives.

Pour point depressants are used to keep diesel fuel fluid and transportable. They co-crystallise with paraffins coming out of solution as a fuel cools and disrupt the structure of the wax crystals. Pour point depressants can also reduce the size of wax crystals, but not necessarily to a size where they will pass through a vehicle fuel filter. Fuels are normally treated with pour point depressant at the refinery.

Operability additives are designed to improve the filterability of diesel fuel through diesel fuel filters at low temperatures. Effective operability additives function as wax crystal nucleators, crystal modifiers and crystal dispersants. When a fuel cools, nucleators precipitate from the fuel as wax crystals begin to precipitate and offer many sites for the initiation of wax crystal formation. With many nucleating sites, the crystals remain small and are able to pass through vehicle fuel filters without plugging them, or will have a different shape that that does not block fuel flow so readily. Operability additives normally treat at considerably higher treat rates compared to pour point depressants, usually in the 100 to 500 ppm range, but can be above for extreme requirements.

Biodiesel in general has a negative effect on the low temperature properties of diesel fuel and an additive's ability to improve the low temperature properties of biodiesel blends. This is also true with renewable fuels derived from refineries processing oils and fats in hydroprocessing units or with Fischer–Tropsch diesel fuel derived from biomass to liquids processes. Renewable diesel fuel and blend components have a high paraffin content. In all cases, greater concentrations of less soluble paraffin and biodiesel components precipitate when a fuel cools and are more difficult to control. Figure 7.1 illustrates the impact of treat rate of CFI additives on the cold filter plugging point (CFPP) of base diesel fuels and the base fuel containing soy methyl ester (SME) biodiesel.

7.1 Average CFPP response of 5 base fuels and biodiesel blends of the 5 base fuels and soy biodiesel treated with 3 levels of cold flow improver.

When fuel below its cloud point remains undisturbed for a period of time, wax crystals tend to settle to the bottom of the vessel containing the fuel, causing stratification of the wax in the fuel. This results in different cloud points between the top and bottom of the fuel in its storage or fuel tank. To control wax settling to the bottom of a diesel fuel tank, wax antisettling additives (WASA) are used to disperse the wax throughout the fuel tank. Typically, ethylene vinyl acetates (EVA), copolymers and terpolymers and their derivatives form the core of a today's cold flow packages. Other proprietary components enhance the performance of these polymers.

7.2.3 Lubricity improvers and friction modifiers

The diesel fuel fraction of crude oil contains sulphur and nitrogen compounds that provide natural lubrication to protect vehicle fuel pumps and injectors from wear. Fuel sulphur limits are becoming progressively more stringent and widespread, and the process of desulphurisation of diesel fuel tends to remove these naturally occurring lubricating components from the diesel fuel. Many diesel fuel injection systems rely on this lubricity and the absence of such compounds will result in fuel that gives increased wear and eventual failure of critical fuel-delivery system components.

Low sulphur and ultra low sulphur (ULSD) diesel fuels are frequently additised with lubricity additives to protect critical injection system components. Lubricity additives are generally classified as neutral or acidic. Neutral additives, esters and amides, may require higher treat rates compared to more cost-effective, mono-acid lubricity additives (see Fig. 7.2), and the mono-acid type has become the most commonly used, often being able to deliver the necessary lubricity improvements at the lowest treat cost and at treat rates of around 100 ppm.

Biodiesel fuel has chemistry similar to some lubricity additives and when blended with diesel fuel provides significant improvement to the fuel's lubricating properties, in many cases eliminating the need for additional lubricity additives. Figure 7.3 shows that if the biodiesel content exceeds 2–3% in the finished fuel, a lubricity additive would typically be considered unnecessary.

Renewable diesel, either from Fischer–Tropsch or hydrodrogenation of vegetable oils, contains negligible amounts of the compounds that would aid lubricity. Therefore, unlike biodiesel, it does not provide any lubricity benefits when incorporated into a mineral diesel fuel. The resulting fuel blend is likely to require either a lubricity additive or to be used in association with biodiesel. Similarly, where renewable diesel is used as a single component fuel, lubricity additives are necessary to protect the fuel injection system.

Friction modifiers are similar in chemistry and function to lubricity improvers, but are generally targeted at gasoline fuels. Friction modifiers

7.2 Mono-acid lubricity additive comparison to non-acidic (neutral) lubricity additives. HFRR is the high frequency reciprocating rig and MWSD is the mean wear scar diameter.

7.3 Effect of palm oil fatty acid biodiesel on lubricity of diesel fuel as measured by HFRR.

are transferred by fuel wetting into the lubricating oil layer on the cylinder walls. The friction between the piston and rings and the cylinder walls is one of the more significant sources of parasitic losses within an engine. A reduction in friction in this area will translate into a measurable improvement in engine efficiency, improving both fuel consumption and power whilst offering enhanced wear protection. The benefits can also spread beyond the cylinder area, with the friction improved oil layer being gradually moved into the bulk oil in the sump and eventually acting on other areas of friction such as the valvetrain. Diesel engines do not have the same level of cylinder wall wetting, and it has been established that the potential efficiency benefits are negligible.

7.2.4 Pipeline corrosion inhibitors

Most diesel fuel and gasoline at some point are transported through a mild carbon steel pipeline and stored in mild carbon steel storage tanks. Fuel commonly contains small quantities of dissolved water that can become free water when the fuel cools. Free water can cause internal corrosion in pipelines and storage tanks, thereby producing fine rust that coats the inside of pipeline and storage tank walls. This rust can increase drag in the pipeline resulting in decreased pumping efficiency and higher energy costs. Fine rust particles can also separate from the pipeline and storage tank walls and create unnecessary wear on critical pipeline metering and pumping equipment, resulting in high-maintenance costs and frequent cleaning. Rust inhibitors, such as dimer, trimer and alkyl succinic acids, are sometimes used to protect the distribution system from corrosion, and can deliver the necessary protection at relatively low treat rates, around 20 ppm. By forming a thin film on the pipeline wall, the rust inhibitors protect the distribution system from oxidative corrosion caused by water and oxygen. Other corrosion inhibitors have also been used, such as sodium nitrate and sodium hydroxide.

However, the trend is moving away from the use of corrosion inhibitors and instead controlling corrosion by good housekeeping, making sure that the fuels distribution network is as far as possible kept free of water and other contaminants. This trend has been accelerated by recent issues with diesel injectors suffering from internal deposits linked to the use of the sodium compounds mentioned above and also to some alkyl succinic acid corrosion inhibitors as discussed by Schwab et al. (2010). Where corrosion inhibitor use continues, the selection of product should be done in discussion with the relevant suppliers.

Biodiesel blends and gasoline containing ethanol can have corrosion issues if the renewable fuels are not free of contaminates which can come from the manufacture of the fuel or in the handling and distribution of the fuel. For instance, metal salts found in some biodiesels can increase the corrosivity of the fuel. This is best addressed by controlling the quality of the biodiesel being used, but where required, typical pipeline corrosion inhibitors can provide the necessary protection.

Unlike biodiesel, ethanol blends are not transported by pipelines, and instead the blend is made at the distribution terminal where delivery tankers are loaded. The ethanol is either blended with gasoline in a storage tank prior to loading, or by adding to the fuel during the tanker loading process. However, the use of corrosion inhibitors is recommended for protection of the ethanol distribution system, and should be added as early as possible after manufacturing the ethanol.

After blending ethanol with gasoline, there is a risk of phase separation. If exposed to moderate levels of water, including that absorbed from the atmosphere, the ethanol will separate from the gasoline producing a separate layer in the storage vessel, either in the filling station or in the vehicle. In these cases, ethanol corrosion inhibitors should be designed to not only protect from corrosion in the hydrocarbon phase but also the ethanol water phase if phase separation should occur. Figure 7.4 shows examples of the protection provided by a corrosion inhibitor when tested according to ASTM D665. In this test, polished steel pins are exposed to fuel mixed with water and a remarkable difference can be seen when compared to the fuel without inhibitor. The concerns about ethanol corrosion have led to their use being recommended by a variety of industry bodies including CONCAWE and CEN in Europe and the RFA in North America.

Some ethanol corrosion inhibitors provide buffering properties designed to control the pHe of the ethanol. Where specifications instead use conductivity limits as a surrogate for measurement and control of pHe and corrosivity, the presence of some inhibitors can be seen to affect conductivity. The ethanol conductivity should therefore be measured before the corrosion inhibitor is introduced. Care should also be taken in selecting ethanol corrosion inhibitors, with DuMont et al. (2007) noting that some can increase engine intake system deposits, especially in high ethanol content fuels such as E-85.

7.2.5 Other corrosion inhibitors

Although iron oxidation is a major cause of fuel system corrosion, other non-ferrous components are also susceptible to corrosion caused by reactive sulphur species found in some diesel fuels and gasoline. As a result, many fuel specifications have copper corrosion limits as determined by a copper strip test. Typically thiadiazole chemistry is effective at controlling copper

Base gasoline + 10% ethanol + corrosion inhibitor: 0% rust, A-rating

Base gasoline: 100% rust, E-rating

7.4 Corrosion protection recommended for fuel ethanol and blends (a) base gasoline + 10% ethanol + corrosion inhibitor: 0% rust, A-rating and (b) base gasoline: 100% rust, E-rating.

corrosion. More recently, there have been cases of silver contacts in fuel sending/sensing units suffering damaged from free sulphur and other sulphur species remaining in gasoline after the desulphurisation process. This problem has occurred sporadically in North America and other regions. In the US, the ASTM requirements now stipulate that gasoline pass a silver corrosion test, whilst in other markets, fuels distributors regularly check that products meet their own internal specifications. Figure 7.5 illustrates the effect of free sulphur on silver strip ratings. The protection provided by two different fuel additives is shown in Fig. 7.6.

7.5 Effect of elemental sulphur on gasoline silver corrosion ratings. Silver strip immersed in unleaded regular gasoline inside glass test tube sealed in stainless steel bomb at 50°C for 3 hours.

7.6 Treat rate of two different corrosion inhibitor additives needed to achieve zero silver strip rating for a given concentration of elemental sulphur in gasoline.

7.2.6 Conductivity improvers

In order to eliminate the potential for a spark to ignite fuel vapours, jet fuel, and in some cases, other distillate fuels, have for many years been treated with additives to dissipate static charge build-up. Static charge build-up is fairly common when diesel fuel is discharged into an improperly grounded vessel such as a fuel tanker. As a result, if the tanker switches back-and-forth between hauling diesel fuel and gasoline, the potential for gasoline vapours to remain in the container while loading diesel fuel creates the risk of an explosion and fire. Desulphurisation of diesel fuel significantly lowers diesel fuel's natural conductivity resulting in a greater risk of explosion or fire. To reduce the potential for static charge build-up during transfer of fuel from one vessel to another, conductivity additives are now frequently added to ultra-low sulphur diesel fuel. Fuel conductivity is temperature dependent with conductivity decreasing as a fuel cools (Fig. 7.7). Conductivity improvers are used at very low treat rates, often around 1 to 2 ppm, and can be added at either the refinery or at terminals before the fuel is loaded into a tanker. One benefit of biodiesel is its ability to increase the conductivity of diesel fuel. Fig. 7.8 shows that difference sources of biodiesel have different effects on conductivity, even when they are made from the same feed stock.

7.3 Impacts of additives on combustion characteristics

The combustion characteristics of fuel are generally controlled by the fuel's chemical composition. One of the key combustion properties is the ability to ignite the fuel and air charge at an optimum time in an engine. Fuel ignition is a radical-driven reaction, and properly designed fuel additives can have a significant impact on ignition, enhancing or retarding the propensity for ignition.

7.7 Fuel conductivity decreases with fuel temperature.

7.8 Conductivity of biodiesel blends as a function of biodiesel concentration for SME from two different sources.

7.3.1 Diesel ignition improving additives

Diesel engines function by compressing the air in the combustion chamber to a temperature at which the atomised fuel that has been injected into it will spontaneously ignite. The ease with which a fuel will ignite is measured as the cetane number and directly affects an engine's ability to start and efficiently burn fuel, especially whilst the engine is cold. The cetane number itself is determined by measuring the delay between fuel being sprayed into the hot combustion chamber and ignition starting, with a short delay denoting a high cetane. A long delay will lead to the eventual combustion being very rapid, causing an unacceptably high rate of pressure rise in the combustion chamber, producing significant noise, whilst incomplete combustion, loss of power and increased emissions will also occur. Conversely, reducing the ignition delay results in improved startability, less noise and lower overall emissions. Numerous studies have demonstrated that increasing the cetane number of a fuel will result in general reduction in NOx emission from a diesel engine. This is a result of the shorter ignition delay time for higher cetane fuels and subsequently lower peak temperatures within the diesel engine. The US EPA (2003) conducted a thorough analysis of cetane quality impact across a broad range of fuels and engine types, and demonstrated the typical levels of NOx reductions obtained using additives to increase cetane.

The chemical composition of diesel fuel determines its cetane quality. Fuels with higher paraffin content tend to have a higher cetane quality, while more aromatic and unsaturated components have a lower cetane quality, and this can be seen in the empirical calculation of cetane index, which looks at the fuel's density and distillation characteristics to predict cetane. It is possible to increase the cetane number of a fuel by the use of energetic additives that encourage the autoignition process. Figure 7.9 illustrates the delay in ignition observed for a low cetane base fuel versus base fuel to which a cetane improver additive was added to yield a ten cetane number increase. Commonly used additives with demonstrated capability to improve fuel cetane quality include alkyl nitrates and peroxides. Alkyl nitrates, particularly 2-ethyl hexyl nitrate, are the preferred commercial source of cetane improver due to low cost and favourable handling characteristics, and are typically used at up to 1500 ppm, both for base fuel improvements as mentioned below and as part of diesel performance additive packages, where they offer both performance benefits and a reduction in noise that will be readily perceptible to the consumer.

The use of cetane improver offers particular opportunites in territories using CEN standards and similar, as there is a significant difference between the minimum cetane index of 46 and the minimum cetane number of 51. It is possible to produce a fuel that meets the 46 index minimum and use cetane improver to meet the 51 number, offering savings for the fuel producer.

Depending on the source of FAME, the use of biodiesel can increase or decrease the cetane number of a diesel fuel. This is dependent on the cetane number of the base fuel to which the biodiesel is added. In areas with low cetane number specifications such as the US, biodiesel normally increases cetane number. In regions where cetane number is high such as Europe, some biodiesels will lower cetane number. Biomass to liquids and hydroprocessed

7.9 Reduced ignition delay for diesel fuel with 10 cetane number increase (+10 CN) compared to base fuel.

oils provide distillate blend stocks that have a very high cetane number, and coupled to their being mainly paraffinic, make good components for blending to produce high quality, low emissions diesel fuel.

7.3.2 Octane improving additives

Gasoline engines use a spark to initiate combustion, and unlike diesel engines, it is important for the fuel to resist auto ignition. The resistance of a gasoline fuel to auto ignition enables hotter conditions to be tolerated in the combustion chamber, allowing higher, more efficient, compression ratios and advanced ignition timings to be adopted. Resistance to auto-ignition is measured on the octane scale, with higher numbers denoting increased resistance. The chemical composition of a gasoline determines the base gasoline octane level. Molecules such as aromatics and olefins are less susceptible to spontaneous ignition under pressure and have a high octane value. A fuel's octane quality can be improved by the use of high-octane blend components such as oxygenates or aromatic components or by the use of octane-improving additives such as mmt® (methylcyclopetadienyl manganese tricarbonyl).

7.4 Diesel performance and deposit control additives

The need to maintain diesel injector cleanliness has been recognised for many years, and has generally focused on the effect of nozzle coking, i.e. deposits forming in the hole through which fuel is sprayed into the combustion chamber. However, over recent years, deposits forming inside the injectors have also become a significant issue. Being able to address both of these phenomena has become a necessity in modern diesel performance additives.

7.4.1 Injector nozzle coking

Allowing injectors to form nozzle coking deposits will result in flow loss and modify the spray pattern of the fuel. Figures 7.10 and 7.11 show the deposits seen at the tip of a modern diesel injector. If these interfere with the flow of the fuel through the nozzle hole, it may cause incomplete mixing of fuel and air, inefficient combustion and affect the ratio of fuel to the inducted air and EGR gases, leading to degraded driveability, reduced power, increased emissions and even to operational failure. Prior to the widespread use of deposit control additives within fuels, engine manufacturers responded to the coking issue by assuming that cleanliness would not be maintained, and deliberately optimised engine calibrations to account for some deposit formation. Older indirect injection engines with distributor pump injection

7.10 Deposits on the tip of a light duty diesel injector.

7.11 SEM close up of deposits around the injector nozzle hole.

systems and their pintle needle injectors are at their most efficient with some, although not excessive, deposits present. This was amply demonstrated in the paper by Reading et al. (1991), which identified that flow retention in the range 50% to 75% was the optimum. In addition to the measured emission improvements shown in that paper, it is well recognised that the formation of some nozzle deposits during the early life of the injector reduces engine noise.

To establish the deposit forming tendency of fuels in such applications

and to encourage the development of additive-based solutions, engine tests were introduced. For the heavy duty diesel engine market in North America, the Cummins L10 test (now defunct) described by Gallant et al. (1991) was available. In Europe, the focus was on light duty engines, where the Coordination European Council for the Development of Performance Tests for Fuels, Lubricants and Other Fluids (CEC) introduced a test based on the Peugeot XUD-9 engine (CEC F-23-01, 2001). This test uses a 1.9 litre indirect injection distributor pump type engine very much like the one used by Reading et al. (1991), and measures loss of flow through injectors after 10 hours of relatively low load and speed operation following a prescribed cycle. This test is still a fully approved and supported engine test and has, for many years, defined the performance of fuels and their additives in a range of markets.

The introduction of biodiesel has had some limited impact on the deposit forming tendency of base fuels and the response of fuels to additives within this test. Response testing in fuels containing up to 10% biodiesel has therefore become a common stage in the validation of additive packages. However, the nature of the response can be unpredictable with both improved and degraded test performance being reported with such blends.

Since the approval of the XUD-9 test in 1997, light duty/passenger car engines have changed significantly, with the base engine design moving from indirect to the direct injection and the fuel injection systems also changing to high pressure common rail (HPCR), enabling significant improvements in performance, emissions and fuel consumption. These systems use significantly higher injection pressures than their predecessors, with resulting increases in both fuel temperatures and the heat rejection within the systems. Recognising these changes, the CEC introduced a new test to help define the performance of fuels and additives in resisting injector coking for such engines. Designated CEC F-98-08 (2008) and more commonly known as the DW10, this test uses a 2.0 litre Peugeot engine equipped with deposit sensitive prototype injectors designed to meet Euro V emissions. The test itself measures flow loss indirectly by monitoring power reduction with time, and the test cycle is severe, with Hawthorne et al. (2008) observing that a significant amount of the engine running being done under full load conditions. Vehicle manufacturers have generally indicated that power loss of less than 2% is acceptable in this test.

Whilst field experience has shown that high mileage 'real world' vehicles are likely to have developed injector nozzle deposits, most mineral based fuels do not form coking deposits within the DW10 test. To encourage their formation and reduce the duration of the test, a zinc based compound is added to the fuel. However, as shown by Ikemoto et al. (2011), the resulting deposits are substantially derived from the zinc addition, a source of some contention within the industry as the relationship to 'real world' high mileage deposits has not been demonstrated. Conversely, some biodiesels have also shown they can also lead to coking deposits in the DW10. Generally attributed to questionable biodiesel quality, where such power loss does occur, the precise source of the problem within the biodiesel has not so far been successfully identified. Due to the biologically derived nature of biodiesel, reproduction of the problem fuels has not proved possible. Conversely, the presence of the biodiesel may even cause the zinc compound addition to have unpredictable effects regarding deposit formation. Figure 7.12 compares the power loss generated by a zinc treated mineral fuel with the same fuel plus addition of a deposit control additive, whilst Fig. 7.13 shows similar data, but in this

7.12 Power loss due to injector coking in high speed direct injection engine test (DW10) for mineral base fuel with zinc and for same fuel containing diesel additive.

17.13 Power loss due to injector coking in high speed direct injection engine test (DW10) for B10 (SME) base fuel and for same fuel containing diesel additive.

case with a B10 biodiesel blend as the base fuel. In both cases, the use of a suitable deposit control additive is able to prevent power loss.

Given that the operating envelope and conditions for the DW10 test are significantly different to the XUD9, and that the injector tip temperatures have increased greatly from circa 160 °C to nearer 270 °C, it is not surprising that the additive response has also changed. Additives that had been previously shown to give good performance in the XUD-9 were found to require significantly higher treat rates to provide good DW10 performance. The response of the additive industry has been to develop new deposit control additives that, if correctly designed, will give the correct performance in the XUD9, good cleanliness in the DW10 and also address coking and hence performance in 'real world' vehicles.

7.4.2 Diesel injector internal deposits

Starting around 2005, there were sporadic reports in Europe of problems with common rail fuel injector needles sticking. However, by 2009, the problems had become much more widespread and were also occurring in North America. The issue was identified as being mainly due to soaps and lacquers forming on the moving parts of high pressure common rail injectors. Where those components had a tight clearance relative to the injector body or other parts, the deposits could lead to those elements moving in a sluggish manner and affecting the injection timing, or becoming stuck in either a closed or open position. The effect of these deposits was to cause a range of problems including power loss, starting issues, rough running, fuel filter plugging, increased emissions and sluggish acceleration.

It was identified that there were two main types of the deposits, one being a white–tan crystalline deposit, as shown in Fig. 7.14, whilst the other was an orange 'lacquer'. A paper by Ullmann et al. (2008) postulated several mechanisms, whilst sodium carboxylate deposits and their likely source were specifically identified by Schwab et al. (2010), using a proprietory engine test for verification. This paper established that the root causes included the reaction between certain corrosion inhibitors being used by pipeline operators and the sodium contamination that is almost ubiquitous when handling fuel. A second mechanism was identified by Galante-Fox and Bennett (2012) as being based on organic amides with certain deposit control additives being implicated. In both cases, solubility of the material being deposited is seen as a key factor.

It is instructive to note that the chemical components held responsible for the deposits have a long history of use without such issues. However, it is believed that the operating environment has changed, with the move to lower sulphur diesel having affected the ability of the fuel to solubilise these materials, whilst the increasingly sophisticated common rail injection

7.14 Sodium carboxylate internal deposits on the needle of a diesel injector.

systems so critical to reducing emissions may be less robust against such deposits.

To address the issues of internal deposits, advanced additive systems are being developed that are capable of either preventing or removing the deposits. In Fig. 7.15, the individual exhaust port temperatures of an engine are being monitored immediately after cold start. The fuel in this case has been deliberately treated to produce sodium carboxylate deposits. In (a) at the start of test, all four cylinders warm up at the same rate. In (b) after only 3 hours of testing, two of the cylinders are malfunctioning due to insufficient fuel being injected, and the other two try to compensate by overfuelling. In (c) it can be seen that an attempt to clean using a conventional additive has no impact, whereas in (d) an advanced additive restores the failing injectors.

7.4.3 Diesel performance additive packages

Besides controlling deposit formation, diesel performance additives are frequently formulated with a number of other components to provide additional functionality to the package. Packages frequently contain cetane improvers, wax crystal modifiers or CFIs, combustion improvers, lubricity additives, demulsifiers and antifoaming additives. Since diesel fuel has a tendency to foam when fueling a vehicle, antifoams are frequently added to diesel fuel to control spillage and increase the rate of fueling diesel vehicles. The dispersant

7.15 Effect of internal deposits on injector cold start function – individual exhaust port temperatures (a) start of test, (b) two injectors stuck, (c) conventional additive and (d) advanced additive.

used in diesel fuel also tends to emulsify water, which can lead to plugged fuel filters. Therefore, incorporating adequate demulsifiers into the package to control water pickup by the fuel is highly important. Depending on the performance claims being targeted, treat rates for the additive packages can range from around 100 ppm, which is the minimum injection rate for many of the systems that dispense additive into the bulk fuel, up to and beyond 1000 ppm for the packages used in some premium fuels.

7.5 Gasoline performance and deposit control additives

7.5.1 Gasoline engine deposits

During the normal operation of a gasoline-powered vehicle, fuel related

When these deposits occur, they are likely to cause poor fuel delivery and volatilisation leading to reduced engine efficiency, increased emissions, drivability problems, and loss of power. Deposits also form in the combustion chamber leading to increased octane requirements, loss of performance, and in some cases, engine knock. In each of the above scenarios, deposit formation can also lead to increased fuel consumption.

Performance additives to control fuel system deposit problems have been around for over 50 years. Many early versions of gasoline additives were low molecular weight amine-based chemistries designed to keep carburettors clean and avoid carburettor icing. With the introduction of fuel-injected vehicles in the 1980s, the low concentration of additive needed to keep carburettors functioning was insufficient to keep injectors from fouling. While higher concentrations of these additives were effective in keeping fuel injectors clean, they also contributed to a growing problem caused by intake valve deposits (IVD). In Europe, several vehicle manufacturers were badly affected by those deposits, the primary symptom seen by the end user being degraded driveability. In this case, driveability is affected by the IVD making the fuel–air mixture become lean when the driver tries to accelerate and causing the engine to initially stumble and misfire. This behaviour is uncomfortable for the driver, leads to increased emissions and can even cause exhaust catalyst failure. To address this, the CEC introduced an engine test, CEC-F05-93 (1993), also known as the M102E, which allowed additive companies to design suitable packages to counter this issue. To control IVD, higher MW amine-based chemistry blended with mineral oil 'carriers' became the predominate method for controlling fuel injector and intake valve deposits. In some cases, additive treat rates to effectively control intake valve deposits approached 1000 ppm. These high concentrations soon led to significant increases in combustion chamber deposits and increases in engine octane requirements. As a result of the additives' inefficiencies, in the early 1990s, mineral oil carriers were phased out while synthetic carriers started becoming widely used in formulating gasoline performance additives (GPA) as we know them today.

7.5.2 Gasoline performance additive (GPA) packages

A typical GPA will consist of, at a minimum, an active deposit control component and a synthetic carrier. The carrier serves several functions in an additive package:

1. to provide a fluid medium through which the deposit control agent can disperse deposits precursors
2. to provide valve stick protection, and
3. to not contribute to combustion chamber deposits.

More usually, these packages also contain demulsifiers, corrosion inhibitors, antioxidants, and friction modifiers. Many of these components are surface active and may not readily coexist in their concentrated form, so to provide storage stability, solvents will to generally be incorporated in the package to address this. Similar to diesel performance additive packages, treat rates can range from around 200 ppm up to 1000 ppm and beyond.

7.5.3 Cleanliness and performance of port fuel injected gasoline engines

Port fuel injected gasoline engines have been in the market for many years and represent the engine technology utilised in the large majority of the gasoline vehicles. To significantly reduce emissions and improve fuel economy, manufactures have worked to improve design and control of the engine, specifically to control the air–fuel ratio. Deposits in the fuel intake system can have detrimental impact on preparation of the air–fuel charge leading to suboptimal engine performance. Deposit control additives added to gasoline can work to remove previously formed fuel system deposits and restore vehicle performance, lowering emissions and improving fuel economy.

The performance of the gasoline deposit control additive and its ability to clean-up fuel-intake system deposits in the standard M102E test is illustrated in Fig. 7.16 for a number of different fuels including a market fuel containing ethanol (E10). The severity of the three fuels differed, building different levels of deposits, but in all cases, subsequent operation on gasoline containing a deposit control additive provided significant clean-up of the valve deposits.

7.16 Intake valve deposit clean-up using base fuels containing gasoline additive in M102E IVD test. Clean-up is of deposit built from base fuel. Examples for three different base fuels.

Deposit control additives also deliver clean-up in vehicle operation, improving fuel efficiency and lower vehicle emissions. The effect of a gasoline deposit control additive on valve cleanliness for three vehicles designed to meet stringent Euro IV emission standards is illustrated in Fig. 7.17. An additive package at a fairly low commercial treat rate, reduced valve deposits by an average of almost 30%. The emission performance and fuel consumption of these vehicles was also measured before operation on the additised fuel and after the clean-up phase. After operation of the additised fuel and subsequent IVD clean-up, average hydrocarbon, carbon monoxide and nitrogen oxide emissions from the vehicles were all reduced. The vehicles also showed a reduction in fuel consumption.

Controlling fuel system deposits can also help maintain a vehicles power and acceleration closer to new vehicle design. Figure 7.18 shows the benefit of cleaning up intake system deposits on vehicle power and acceleration. Compared to a newly built engine, the power loss before and subsequent recovery of that loss after clean-up, is significant. In this case, the power recovery led to a 5% reduction in the average time required for the vehicles to accelerate from 100 km/hr to 145 km/hr. These examples illustrate the benefits of using gasoline fuel additives to keep clean or clean-up the engine intake systems.

7.5.4 Direct injection gasoline (DIG) engines and injector plugging

The direct injection gasoline (DIG) engine operates by injecting fuel directly into the combustion chamber and is capable of providing significantly improved fuel efficiency. The gains in fuel efficiency and ability to produce more power

7.17 Average emission and IVD reduction achieved after clean-up for three vehicles.

7.18 Improvement in power and acceleration on cleaning up fuel system deposits. Average of four different vehicles. Dirty-up for 8000 km on commercial gasoline and clean-up for 2400 km on premium level of additive in regular unleaded gasoline.

are achieved through the higher compression ratios associated with charge cooling, coupled to precise control over amount of fuel and injection timings which are varied according to the load conditions. The proper presentation of the fuel–air mixture into the cylinder is important to ensure robust operation of engine overall operating conditions. Engine designers utilise wall guided and more recently, spray guided systems to ensure proper formation of a combustible fuel–air mixture and robust engine performance. Both of these approaches rely on proper operation of the fuel injection system. Formation of fuel injector deposits can adversely impact fuel spray formation and reduce levels of fuel injected into the engine impacting engine operation.

Fuel additives play an important role in maintaining the operation of DIG engines at optimal levels and preventing loss in performance, and their selection is discussed at length in the paper by DuMont et al. (2009). The injectors in the DIG engine are exposed to higher temperature than those used in the PFI vehicles discussed above, and are potentially prone to the formation of deposits that restrict flow and adversely impact performance. Injector manufacturers design the injectors with the goal of reducing the propensity for deposit formation. However, injector deposits will often form, reaching an acceptable equilibrium but nonetheless at a degraded condition compared to the vehicle design intent. The ability to return the injector to a clean, more efficient condition is the key to recovering lost performance and the vehicle operating at its optimum. Figures 7.19 and 7.20 show results of Relative power (%)

7.19 Direct injection nozzle deposits with additive use.

7.20 Direct injection nozzle deposits – base fuel.

testing where injectors from an engine that has used a fuel with a tendency to form deposits is compared with injectors exposed to the same fuel, but dosed with a gasoline performance additive. A clear visible difference can be seen inside the injector nozzle holes.

Using a vehicle equipped with a four cylinder direct injection gasoline engine, the impact of deposit and the role of additives was demonstrated as shown in Fig. 7.21. To give good discrimination between additives, a bespoke fuel was used that was intended to give a known, high level of injector deposits. The vehicle was operated over a period of 48 hours at highway speeds and underwent periodic hot soaks. After this period of time the base fuel displayed about 9.4% loss in fuel flow through the injectors due to deposit build up. The test was then repeated, firstly with a historical deposit control additive known to give good PFI IVD control performance, and then with a more advanced additive that gives the same level of PFI IVD performance, but which had also been designed to provide DIG injector performance. The difference is obvious, demonstrating that providing good

7.21 Differences in intake valve deposit forming tendencies of E-85 fuel made from two fuel grade ethanol sources and commercial E-85. Tests conducted in a flexible fuel vehicle operating for 5000 miles.

7.22 Intake valve deposit formation can vary significantly with ethanol blends. Additives can be very effective even with severe base fuel.

DIG performance should be a fundamental consideration when developing additives for the modern vehicle fleet.

7.5.5 Effects of ethanol on deposit formation

Low levels of ethanol blended with gasoline can affect the amount of deposit formed, but typically have only limited impact on the additive performance, and should be accounted for by additive producers during product development. Figure 7.22 shows IVD performance of two commercially sourced E5s, with a significant difference being seen in base fuel deposition. However, even the most severe of these can be seen to respond to the use of a deposit control additive.

Higher level ethanol blends such as 85%, have been shown to increase injector fouling and can also add to intake valve deposits. Figure 7.23 compares the IVD performance of E85 fuels where different ethanol sources have been used. The variation in deposits formed is significant and was identified as being likely to have been due to the type of corrosion inhibiter used.

In addition to the need to ensure that gasoline deposit control additives give the correct performance in the presence of alcohol, high alcohol blends such as E85 require consideration of the solubility of the additive used. Where a product is not fully soluble, suspended matter will appear, obstructing light and producing a haze in the fuel. This can be measured as opacity and Table 7.2 compares this property for two different additive package chemistries. Opacity below 5 NTU (nephelometric turbidity units) would be considered

7.23 Cleanliness performance of additives in direct injection gasoline engine.

0.87 0.96 1.02

| Finished package
treat rate (ppm) | Gasoline detergent type | | | | |--------------------------------------|-------------------------|-------|--|--| | | PIBA* | PEA** | | | | 0 | 0.82 | 0.82 | | | | 120 | 4.31 | 0.85 | | |

6.66 13.28 38.00

Table 7.2 Gasoline detergent solubility in E85 based on detergent type

240 480 960

^* PIBA fully soluble at all treat rates up to E50

^** Low haze numbers indicate that PEA based package has complete solubility in E85

as clear. As can be seen, the polyether amine (PEA) type of additive has effectively complete solubility in E85 even at unusually high treat rates, unlike the more conventional PIBA. Conversely, at lower ethanol concentrations, PIBA is fully soluble and offers performance advantages that make it the preferred option.

7.6 Conclusions and future trends

The use of additives to enhance the performance of fuels and protect against the consequences of combustion has a long history. Fuels continue to evolve, with increasingly tight controls on the quality of the mineral base fuel being coupled to the incentivised introduction of new, renewable fuel streams, whilst in the search for lower emissions and greater efficiency, the appetite of engines and vehicles for fuels is changing. Biofuels in particular have introduced a range of opportunities for the additive producers to respond to with their different properties compared to mineral oil based fuels, ranging from solubilising performance to the types of contaminants present.

As engines increase their specific power and duty cycles change in response to regulatory drivers, the stresses on fuel are changing. Engines in hybrid vehicles operate for relatively short periods, but at higher load conditions than conventional powertrains. This will lead to more extreme thermal conditions that the fuel and additives will be exposed to, and to changing requirements of additives to support these engines. Concurrently, in the drive to further differentiate their products and give themselves a marketing advantage, fuel suppliers are becoming more demanding. Having products that address identified consumer needs is critical to the long term sustainability of the fuel additives business.

7.7 Sources of further information and advice

Chevron Diesel Fuels Technical Review (FTR-2). Chevron Products Co, USA, 1998.

Gairing, M., Marriott, J.M., Reders, K.H., Reglitzky, A.A. and Wolveridge, P.E. The Effect of Modern Additive Technology on Diesel Fuel Performance. SAE Technical Paper 950252, 1995.

Marie, E., Chevalier, Y., Eydoux, F, Germanaud, L. and Flores, P. Control of n-alkanes cystallization by ethylene-vinyl acetate copolymers. Journal of Colloid and Interface Science, 290, 406–418 2005.

Nikanjam, M., Diesel Fuel Lubricity: On the Path to Specifications. SAE Technical Paper 1999-01-1479, 1999.

National BioDiesel Board, Electrical Conductivity of Biodiesel. 2009.

http://www.biodiesel.org/pdf_files/fuelfactsheets/electrical_conductivity. pdf.

Stein, Y., Yetter, R.A., Dryer, F.L. and Aradi, A., Autoignition Behavior of Surrogate Diesel Fuel Mixtures and the Chemical Effects of 2-Ethylhexyl Nitrate (2-EHN) Cetane Improver. SAE Technical Paper 1999-01-1504, 1999.
Leedham, A., Caprotti, R., Graupner, O. and Klaua, T. Impact of Fuel Additives on Diesel Injector Deposits. SAE Technical Paper 2004-01- 2935,2004.
Udelhofen, J.H. and Zahalka, T.L. Gasoline Additive Requirements for Today's Small Engines. SAE Technical Paper 881644, 1988.
Lenane, D.L. and Stocky, T.P. Gasoline Additives Solve Injector Deposit Problems. SAE Technical Paper 861537, 1986.
Bitting, W.H., Gschwendtner, F., Kohlepp, W., Kothe, M., Testroet, C.J. and Ziwica, K.H. Intake Valve Deposits – Fuel Detergency Requirements Revisited. SAE Technical Paper 872117, 1987.
Noma, K., Noda, T., Ashida, T., Kamioka, R., Hosono, K., Nishida, T., Kameoka, A., Koseki, K., Watanabe, M., Takahashi, K., Koide, S., Suzuki, T., Fukui, H., Hirose, M., Ohta, S., Notsuki, Y. and Tsuboi, K. A Study of Injector Deposits, Combustion Chamber Deposits (CCD) and Intake Valve Deposits (IVD) in Direct Injection Spark Ignition (DISIS) Engines. SAE Technical Paper 2002-01-2659, 2002.
Aradi, A., Imoehl, B, Avery, L.N., Wells, P.P. and Grosser, R.W., The Effect of Fuel Composition and Engine Operating Parameters on Injector Deposits in a High-Pressure Direct Injection Gasoline (DIG) Research Engine. SAE Technical Paper 1999-01-3690, 1999.
Aradi, A.A., Colucci, W.J., Scull, H.M. and Openshaw, M.J. A study of Fuel Additives for Direct Injection Gasoline (DIG)) Injector Deposit Control. SAE Technical Paper 2000-01-2020, 2000.
Aatola, H., Larmi, M., Sarjovaara, T. and Mikkonen, S. Hydrotreated Vegetable Oil (HVO) as a Renewable Fuel: Trade-off between NOx, Particulate Emission, and Fuel Consumption of a Heavy Duty Engine. SAE Technical Paper 2008-01-2500

7.8 References

CEC F-05-93 (1993). Inlet Valve Cleanliness in the MB M102E Engine
CEC F-23-01 (2001). Procedure for Diesel Engine Injector Nozzle Coking Test (PSA XUD9A/L 1.9 Litre 4 Cylinder indirect injection diesel engine)
CEC F-98-08 (2008). Direct Injection, Common Rail Diesel Engine Nozzle Coking Test.
DuMont, R.J., Cunningham, L.J., Oliver, M.K., Studzinski, W.M. and Galante-Fox, J.M. (2007). Controlling Induction System Deposits in Flexible Fuel Vehicles Operating on E85. SAE Technical Paper 2007-01-4071.
DuMont, R.J., Evans, J.A., Feist, D.P., Studzinsky, W.M. Cushing, T.J. (2009). Test and Control of Fuel Injector Deposits in Direct Injected Spark Ignition Engines, SAE Technical Paper 2009-01-2641.
Gallant, T., Cusano, C.M., Gray, J.T. and Strete, N.M. (1991). Cummins L10 Injector Depositing Test to Evaluate Diesel Fuel Quality. SAE Technical Paper 912331
Galante-Fox, J. and Bennett, J. (2012). Diesel Injector Internal Deposits in High Pressure Common Rail Diesel Engines, Proceedings of the Institution of Mechanical Engineers, Fuel Systems for IC Engines, 14–15 March 2012, pp 157–166.
Hawthorne, M., Roos, J.W. and Openshaw, M.J. (2008). Use of Fuel Additives to Maintain Modern Diesel Engine Performance with Severe Test Conditions. SAE Technical Paper 2008-01-1806.
Ikemoto, M., Omae, K., Nakai, K., Ueda, R. et al. (2011). Injection Nozzle Coking Mechanism in Common-rail Diesel Engine, SAE Int. J. Fuels Lubr. 2011-01-1818.
Reading, K., Roberts, D.D. and Evans, T.M. (1991). The Effects of Fuel Detergents on Nozzle Fouling and Emissions in IDI Diesel Engines. SAE Technical Paper 912328.
Schwab, S.D., Bennett, J.J., Dell, S.J., Galante-Fox, J.M., Kulinowski, A.M. and Miller, K.T. (2010). Internal Injector Deposits in High-Pressure Common Rail Diesel Engines. SAE Technical Paper 2010-01-2242.
Ullmann, J., Geduldig, M., Stutzenberger, H. and Caprotti, R. (2008). Investigation into the Formation and Prevention of Internal Injector Deposits, SAE Technical Paper 2008-01-0926.
US EPA (2003). The Effect of Cetane Number Increase Due to Additive on NOx Emissions from Heavy-Duty Highway Engines. Final Technical Report, EPA420- R-03-002, 2003.

Part II

Improving engine and vehicle design

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Internal combustion engine cycles and concepts

J. D. Naber and J. E. Johnson, Michigan Technological University, USA

DOI: 10.1533/9780857097422.2.197

Abstract: This chapter discusses the various internal combustion engine operating cycles, including the standard two and four-stroke cycle, along with alternative cycles such as over-expanded and split-cycle engines. Additionally, real engine operating strategies are reviewed including spark-ignition and compression-ignition engines. The cycle performances are compared and limitations are addressed. The chapter concludes with a discussion on future trends for internal combustion engine operating cycles.

Key words: cycles, ideal, spark, compression, alternative.

8.1 Introduction

Internal combustion engines (ICE) are the primary transportation mover in today's society and will likely continue to be for decades to come (Pischinger et al., 2006). ICE's date back to the mid 1800s when Nicholas Otto, Rudolph Diesel and Jean Lenoir first developed and proved the concept of heat engines using internal combustion (Cummins, 2002). Although engines have changed considerably since this time, including efficiency and emission improvements, the basic operational principles of the ICE have not shifted significantly. Even as alternative fuels and alternative combustion engine technologies are developed and produced, the costs, along with integration and infrastructure requirements, result in long transition times. Even without alternative fuels, the sheer magnitude of transportation energy use and reliance on petroleum will require substantial changes to the ICE, with current focus being on engine efficiency improvements (Manley et al., 2008).

In the fundamental sense, an internal combustion engine converts the chemical energy available in the fuel (conventionally petroleum-based) through combustion to thermal energy, and, through expansion of the working fluid, converts the thermal energy to mechanical work with the crank-slider mechanism converting linear force/work to rotational torque/work to the output shaft. The process occurs with working fluids (air and fuel to combustion products) exchanged on a repetitive cycle with combustion occurring in a closed chamber. There are various engine technologies in use with differing applications. The two main engine technologies are compression ignition (CI) engines which typically use diesel fuel and spark ignition (SI) engines which typically use gasoline fuel. In the US, SI gasoline engines comprise 98% of the total light duty vehicles in 2008 (EIA, 2010) while in Europe in 2008 approximately 50% of the light duty vehicles employed SI gasoline engines (US DOE, 2010). The majority of medium and heavy duty vehicles employ CI diesel engines.

8.2 Ideal engine operation cycles

Conventional internal combustion engines use a crank-slider mechanism (Fig. 8.1(a)) to convert the linear work imposed on the piston by the gases in the combustion chamber to rotational work as torque (T) through the rotational

8.1 Crank slider mechanism (a) and volume versus engine position (b).

angle . The cylinder volume of crank-slider mechanism is maximum (*V*max) at bottom dead center (bDC) at 180° before top dead center (TDC) (Fig. 8.1(b)). The swept volume of the cylinder of the crank-slider mechanism is the displacement volume of the engine (*V*d = *V*max – *V*min) with the minimum volume equal to the clearance volume (*V*c = *V*min). Note that the piston dwells near TDC which increases the time for combustion but also for heat transfer. a fundamental relationship and design parameter of the engine is the compression ratio (CR) which is the ratio of the maximum volume to the clearance volume:

$$CR = \frac{V{\text{max}}}{V{\text{c}}} = \frac{V{\text{d}} + V{\text{c}}}{V_{\text{c}}}$$ [8.1]

Internal combustion engines operate on two conventional cycles, a two-stroke and a four-stroke operational cycle. The cycles differ based on how many crankshaft revolutions occur for each combustion event and expansion or power stroke. Therefore, the number of cycles directly impacts the engine's power output based on a given engine size and other parameters. each operating stroke of the engine involves the piston moving up or down in the engine cylinder equivalent to 180° of crank-angle rotation. In a two-stroke cycle, there is one rotation of the crankshaft for a power stroke (360° = 2p) and in the four-stroke cycle there are two rotations of the crankshaft (720° = 4p) for one power stroke. hence, the two-stroke cycle provides almost double the power density for a given engine size based on this operational strategy.

8.2.1 two-stroke cycle

The two-stroke operational cycle includes two strokes over one crankshaft revolution, the compression and power or expansion stroke. a common mechanism of gas exchange in two-stroke engines is the use of ports in the cylinder. as the piston moves, it exposes ports to control exhaust and inlet fl ows, as opposed to using separate intake and exhaust strokes, as will be discussed in the four-stroke cycle. In the compression stroke, the inlet and exhaust ports are closed, and the cylinder charge is compressed, drawing fresh air into the crankcase. The piston moves towards TDC, compressing the mixture, and then combustion is started. Next, the power stroke occurs, with an increased pressure resulting from the exothermic combustion of the fuel working to push down the piston. as the piston nears bDC, the exhaust port is uncovered, and an exhaust blow-down occurs where the exhaust gases exit the cylinder due to the pressure differential from combustion process to the environment. With the piston continuing to move down, the intake port is uncovered, letting the fresh charge which was compressed in the crankcase fl ow into the cylinder. As there is a time of overlap with both exhaust and inlet ports being open, the cylinder is designed to minimize outlet fl ow of the fresh charge gas through the exhaust port. The two-stroke engine operational cycle is applied to certain snowmobiles, outboard motors, chain saws, and other devices which require high power density, engines with low weight but high engine speed (RPM). Two-stroke engines are also common in large applications including locomotives, ships, and stationary applications. Limitations of the two-stroke engine are mixing of the incoming fresh charge with the exhaust residuals, and the passage of fresh intake charge directly into the exhaust system due to the scavenging process resulting in high emissions, particularly of unburned hydrocarbons. Tradeoffs, however, are engine mechanical simplicity, small size, and high output when compared with the four-stroke operating engine.

8.2.2 Four-stroke cycle

The four-stroke operational cycle of the ICE includes intake, compression, expansion/power, and exhaust. For the four-stroke cycle (Fig. 8.2) there are two crankshaft revolutions for one power stroke. The intake stroke involves movement of the piston from TDC (clearance, or minimum cylinder volume) to BDC, to induct the charge gas into the cylinder. If the engine is a CI engine, just air and, potentially, residuals are inducted; whereas for an intake port fuelled SI engine, a fuel–air mixture is inducted into the cylinder. The next stroke is the compression stroke with the piston moving from BDC to TDC, to compress the charge-gas (fuel and air mixture for SI engine, air and residuals for CI engine) to increase the temperature of the cylinder contents. Combustion occurs near TDC, with the next stroke being the expansion stroke or power stroke. In the power stroke, the piston moves from TDC to BDC, with the force of the high pressure products from combustion of the fuel–air mixture pushing the piston down. Of the four total strokes, this is the one stroke where work is extracted from the operational cycle of the engine. The final stroke of the cycle is the exhaust stroke, where the exhaust valve opens and the piston moves back up the cylinder (BDC to TDC) to push the combustion products out of the cylinder. Upon completion of the exhaust stroke, the intake stroke starts over, defining the start of the next engine operational cycle.

8.2.3 Ideal cycle analysis

Ideal cycles are simplified thermodynamic closed cycles to analyze the compression, combustion, and expansion process in an engine with a focus on extraction of work from combustion of the fuel–air mixture. Since the thermodynamic system is modeled as a closed system in these ideal cases, the combustion process is modeled as a heat addition to the working gases. There are three different ideal heat addition methods modeled:

8.2 Four-stroke engine cycle (intake ^Æ compression ^Æ expansion/power ^Æ exhaust).

1. constant volume (CV)
2. constant pressure (CP), and
3. limited pressure (LP).

The three heat addition methods are illustrated in terms of the pressure-versusengine position (Fig. 8.3(a)) and log of pressure versus log of volume (Fig. 8.3(b)). The analysis requires definition of thermodynamic properties for the working fluid (trapped gases in the cylinder = fuel + air) and application of thermodynamic conservation laws including conservation of energy and assumption of isentropic processes for compression and expansion. With constant specific heats (*C*p & *C*v) these isentropic processes lead to the following relationships between pressure, volume, and temperature:

$$p{\rm r} = V{\rm r}^{-\gamma} = T_{\rm r}^{\gamma/\gamma - 1} \tag{8.2}$$

8.3 Illustration of three ideal heat-addition methods compared to an actual SI engine (a) pressure versus engine position and (b) log of pressure versus log of volume.

where r is a ratio at two points of the stroke and is the ratio of specifi c heats (*C*p/*C*v). When plotting the isentropic compression and expansion in log pressure versus log volume coordinates with constant properties they are straight lines (see Fig. 8.3(b)) with a slope of –g. The combustion process is assumed ideal and the energy addition from combustion is equal to the mass of fuel (*m*f) times its lower heating value (*Q*LhV) and this is modeled as an input of heat energy (*Q*in):

$$Q{\rm in} = m{\rm f} \cdot Q_{\rm LHV} \tag{8.3}$$

The net work out is evaluated as the integration of the pressure (p) versus volume (V) over the two strokes:

$$W{\text{net}} = \int{\text{BDCIntake}}^{\text{BDCExpansion}} p \cdot dV$$ [8.4]

Details on the ideal cycle derivation and analysis are provided in heywood (1988) and stone (2002), with the key results and trends discussed here.

Constant volume ideal heat addition

The constant volume heat addition method assumes that combustion occurs infi nitely fast at constant volume at TDC (see the solid black trace in Fig. 8.3(b)). This will provide the highest ideal effi ciency at a given operating condition of the engine. additionally, since the volume is constant and there is no expansion, the constant volume ideal heat addition method results in the highest peak pressure. The net indicated fuel effi ciency (*h*f,i) for the constant volume cycle is only a function of compression ratio and g:

$$\eta{f,i} = \frac{W{\text{net}}}{Q_{\text{in}}} = 1 - \frac{1}{CR^{\gamma - 1}}$$ [8.5]

at a compression ratio of 10 with g = 1.35, the constant volume cycle fuel conversion effi ciency is 55%.

Constant pressure ideal heat addition

The constant pressure ideal heat addition cycle assumes that combustion occurs at constant pressure, which results in a volume increase during combustion (since pressure is constant) and, hence, an expansion process which results in a reduced effi ciency compared to the constant volume case. This effi ciency depends on the compression ratio, ratio of specifi c heats, and a value of b, which defi nes the ratio of the fi nal volume, *V*3b, to the initial volume during the constant pressure combustion phase, *V*3a:

$$\beta = \frac{V{3B}}{V{3A}} \tag{8.6}$$

When this ratio of b is one, this reduces to the constant volume ideal heat addition method effi ciency. The constant pressure ideal heat addition cycle effi ciency is defi ned as:

$$\eta{f,i} = \frac{W{\text{net}}}{Q_{\text{in}}} = 1 - \frac{1}{CR^{\gamma - 1}} \left[ \frac{\beta^{\gamma} - 1}{\gamma(\beta - 1)} \right]$$ [8.7]

Limited pressure ideal heat addition

The limited pressure ideal cycle is a combination of the constant volume and constant pressure heat addition methods, with a percentage of heat addition at constant volume and the remaining at constant pressure. This heat addition method, therefore, provides an improved representation of the actual engine combustion, with an effi ciency between that of the constant volume and that of the constant pressure. This effi ciency depends on the compression ratio, ratio of specifi c heats, a value of a which defi nes the ratio of peak pressure from the constant volume combustion, *p*3, to the starting pressure of the constant volume combustion, *p*2:

$$\alpha = \frac{p_3}{p_2} \tag{8.8}$$

additionally, there is the value of b, which defi nes the ratio of the fi nal volume to the initial volume during the constant pressure combustion phase. When this volume ratio is 1, this reduces to the constant volume ideal heat addition. When this pressure ratio is 1, this reduces to the constant pressure ideal heat addition. The effi ciency is calculated using the following equation:

$$\eta{f,i} = \frac{W{\text{net}}}{Q_{\text{in}}} = 1 - \frac{1}{CR^{\gamma - 1}} \left[ \frac{\alpha \beta^{\gamma} - 1}{\alpha \gamma (\beta - 1) + \alpha - 1} \right]$$ [8.9]

Ideal heat addition method comparison

The constant pressure, constant volume, and limited pressure ideal heat addition methods can be compared in terms of pressure-volume cycles and also effi ciency as a function of compression ratio. An analysis is undertaken assuming the engine is operating at a compression ratio of 10 (typical for spark ignition engines), with an inlet pressure of 100 kPa (1 bar) (naturally aspirated) and charge gas temperature at bDC of 323 K (50 °C). additional assumptions are that the engine is operating on gasoline, combusting in air, with a specific heat ratio of 1.356. The limited pressure cycle assumes that *p*max is 5 MPa (50 bar). The pressure-volume diagram comparing these three ideal heat addition methods is shown in Fig. 8.3(b). Additionally, a heat addition cycle with a realistic combustion rate for a SI gasoline engine is shown in the dash-dot line.

As observed in Fig. 8.3, the constant volume heat addition method provides the highest peak pressure, with the constant pressure heat addition method having the lowest peak pressure, and the limited pressure heat addition method, being a combination of the CV and CP cycle, having a peak pressure falling in between. If the percentage of fuel burnt at constant volume was increased from 50%, the peak pressure of the limited heat addition method would increase and approach that of the constant volume heat addition method. Also of interest is a comparison of the efficiencies for the different heat addition methods as a function of compression ratio (Fig. 8.4).

As compression ratio is increased, the ideal cycle efficiency increases. Two distinct regions are shown and labeled on Fig. 8.4 as SI (spark ignition) and CI (compression ignition) engines, based on their typical compression ratios. Spark ignition engines are limited by combustion knock and typically exhibit compression ratios of 8–12, with CI engines, which are material and durability limited, having compression ratios from 15–20. Clearly CI engines will exhibit increased efficiencies due to their elevated compression ratio. As discussed, the constant volume cycle has the highest efficiency since it has the largest work, or area, under the pressure-volume curve due to the highest peak pressure. The limited pressure heat addition method efficiency for a 50% constant volume and 50% constant pressure combustion process is also

8.4 Ideal cycle analysis – efficiency as a function of compression ratio for the different heat addition methods.

included on the figure, falling between that of the CV and CP cycle. This limited pressure cycle is most representative of the actual engine operation; however, it is still an overestimation of the efficiency achieved in actual engine operation. This is the case since factors such as heat transfer, finite combustion time, exhaust blowdown loss, crevice effects and leakage, and incomplete combustion are not considered in the cycle analysis (Heywood, 1988).

8.3 Alternative engine operating cycles

In addition to the standard two-stroke and four-stroke engine operating cycles, modifications can be applied to these as will be discussed.

8.3.1 Over-expanded cycle

The over-expanded engine cycle is a variant of the standard four-stroke engine cycle. In the standard cycle, compression and expansion strokes are symmetrical, in the over-expanded cycle, the expansion stroke is extended to provide increased expansion, to increase the expansion ratio, which functions to increase energy extraction from combustion and, therefore, improve engine efficiency. The over-expanded cycle is the general cycle providing this extended expansion. There are subsets of this cycle based on methods of cycle application, including the Atkinson cycle and the Miller cycle.

Atkinson cycle

In practice, the Atkinson cycle can be achieved by changing exhaust valve opening and intake valve closing timings, relative to BDC (Heywood, 1988). This change in timing allows the gases to fully expand back to ambient pressure. Although the Atkinson cycle will provide improvements in efficiency since there is additional work output (increased area under the pressure – volume curve), disadvantages include reductions in power density, peak torque, and indicated mean effective pressure as there is a reduction in volume being filled with fresh charge (Heywood, 1988; Stone, 2002). Despite these disadvantages, the Atkinson cycle finds use in applications with a secondary source of power, such as in hybrid vehicles (e.g., Toyota Prius (German, 2004)).

Miller cycle

The Miller cycle is a modification of an over-expanded cycle which provides a higher expansion ratio than compression ratio, with the advantage of providing improved thermal efficiency compared to conventional internal combustion engine operating conditions (Branyon and Simpson, 2012). In practice, this difference in expansion ratio can be achieved through a compression stroke which includes a late or early closing of the intake valve. This effectively reduces the compression stroke, but keeps the combustion and expansion process as normal enabling extraction of additional energy before the exhaust process, while increasing the expansion ratio to improve thermal efficiency with a tradeoff of reduction in brake mean effective pressure (BMEP) (Branyon and Simpson, 2012; Wu et al., 2003). The mean effective pressure is used as a quantifier of an engine's ability to do work, scaled by the engine's displacement, essentially defining a metric to quantify the effectives of engine operation over a wide range of engine sizes. The metric of brake mean effective pressure is used to define the work of the actual engine output, defined from its brake power. To combat this shortening of the compression stroke, turbochargers or superchargers are used to keep BMEP levels stable to uphold advantages of this cycle (Branyon and Simpson, 2012). Therefore, the Miller cycle uses boosting to recover the lost charge resulting from the smaller displacement on compression. This cycle also helps to minimize the combustion knock problem in SI engine operation in terms of pre-ignition, by providing cooling to the fuel–air mixture before combustion based on timing of the closing of the intake valve (Wu et al., 2003).

8.3.2 Split cycle engines

Split cycle engines rely on multiple cylinders to provide the operating cycle for the engine. A typical configuration is two paired cylinders, one for intake and compression and the second for power and exhaust. Two types of split cycles are the Scuderi and the Stirling, as will be discussed.

Scuderi split cycle

The Scuderi split cycle follows the conventional four-stoke cycle but differs in that it divides the four strokes over two paired cylinders; one which functions as an intake and compression cylinder (the compressor), the other as a power and exhaust cylinder (expander). The two cylinders are connected via a crossover port (Phillips et al., 2011; Meldolesi et al., 2011) with the TDC of the two cylinders phased by several crank angle degrees (Meldolesi et al., 2011). Benefits include separation of the compression and expansion strokes which provides flexibility to optimize both processes, while also allowing for Atkinson cycle operation (over-expansion), or supercharging operation (Phillips et al., 2011). Further advantages include enhanced combustion, along with potential pneumatic hybridization of the engine (Meldolesi et al., 2011). Limitations include pumping losses and heat losses (Phillips et al., 2011). Furthermore, development of the engine cycle results in several technical challenges, including the requirement of fast valve actuation (Meldolesi et al., 2011).

Stirling split cycle

Another split cycle engine is the internal combustion Stirling cycle. This engine includes two cylinders, an intake and compression cylinder and a power and exhaust cylinder, with mechanical compression, with a regenerator between the cylinders, acting as a counterflow heat exchanger (Patton and Bennett, 2011). During exhaust the regenerator cools hot exhaust gases and stores this heat for heating compressed gasses that will flow into the intake and compression cylinder, to provide elevated gas temperatures for diesel autoignition (Patton and Bennett, 2011). This Stirling cycle was developed to improve efficiency for light vehicles, with reduced friction and throttling losses, with drawbacks of low break mean effective pressure which can be partially combatted by high turbocharging (Patton and Bennett, 2011).

8.3.3 Wankel engine

The Wankel rotary type engine was first tested in 1957 (Heywood, 1988). In contrast to the standard reciprocating type IC engine with a crank-slider mechanism, the Wankel engine uses an oval-shaped housing with a triangularshaped rotor on an eccentric shaft. The rotary engine is advantageous as it is compact, has high engine speeds to provide higher power to weight ratios, and is well-balanced. However, the engine's downfalls include high heat transfer (higher surface-to-volume ratios) along with sealing and leakage issues, low efficiency due to smaller compression ratios, and poor emissions due to a non-optimal combustion chamber shape including large crevice volumes which can cause quenching (Heywood, 1988; Stone, 2002; Tartakovsky et al., 2012). In the rotary engine operation, there are three rotations of the eccentric shaft for one rotation of the rotor, with the rotor and housing forming three combustion chambers. This means that one working cycle in a Wankel engine occurs during 1080 degrees shaft rotation, whereas a standard four-stroke engine has 720 degrees of crankshaft rotation (Tartakovsky et al., 2012).

8.3.4 Free piston engine

The working principle of the free piston engine is a two-stroke engine cycle, using the linear motion of pistons sliding in a cylinder. The piston is driven by gas pressure and other forces on the pistons and, hence, this engine functions similar to a mass spring system (Fredriksson and Denbratt, 2004). The methods in which to generate this equilibrium of forces vary, but one example is the use of a linear alternator which varies the electric load to control the piston motion (Fredriksson and Denbratt, 2004). The free piston engine has no mechanical leakage from the piston and, therefore, enables simple variation of the compression ratio which facilitates multiple fuels and combustion strategies and provides opportunities for reduced engine friction and increased efficiency (Fredriksson and Denbratt, 2004). One application of a free piston engine is coupling it with a linear generator for hybrid vehicles (Goertz and Peng, 2000).

8.3.5 Actual engine cycles

The above ideal cycles are based upon idealized thermodynamic heat additions, compression, and expansion to provide insight to the factors impacting efficiencies and peak pressures. They, however, do not provide realistic efficiencies and to expound upon this further we will look at the two dominant engine technologies in use today. Internal combustion engines are commonly defined as compression ignition (CI) or diesel engines, and spark ignition (SI) or gasoline engines. These engines can have the same operational cycle strategies in terms of two-stroke or four-stroke (intake, compression, expansion, exhaust), but differ in how the fuel is ignited and combusted. Other differences exist in terms of operational properties, fuel type, emissions, etc.

Spark ignition (SI) engines

In the spark ignition engine, fuel and air are mixed before initiating combustion with fuel introduced either in the intake manifold or now commonly in cylinder with direct injection technologies. In either case the fuel vaporizes and mixes with the charge gases to an ignitable mixture. Prior to TDC of the compression stroke, a high energy electrical discharge (spark) initiates combustion, igniting the fuel–air mixture forming a flame kernel transitioning to a propagating turbulent flame. The compression ratio is limited by the autoignition of the fuel–air mixture ahead of the flame so as not to cause combustion knock. As a result of lower compression ratios, SI engines reach peak efficiencies of 35% (US DOE, 2010). For light duty applications it is typical to operate the engine at the stoichiometric air-to-fuel ratio (exact amount of fuel needed for combustion of all the air) to enable effective aftertreatment via a threeway catalyst in meeting stringent emission standards (Heck et al. 2009). This means that at part load the engine is throttled to control the amount of air charge and thus stoichiometric fuel charge to control the amount of energy addition. This results in a reduction in volumetric efficiency (which defines how effectively the displaced cylinder volume is used). With lower than atmospheric intake pressures from the throttle, the engine must pump

air from the intake to the exhaust which is a negative work (referred to as pumping work) lowering the efficiency. These pumping or throttling loses can be reduced or eliminated by phasing the valve timings and lifts to achieve over-expanded cycle operation using large internal residuals (Fontana and Galloni, 2009). This operation is physically achieved by recent advances in valve actuation including variable cam phasing (Cheever et al., 2012). SI engines are commonly fueled on gasoline, with the fuel characterized based on octane number. The octane number defines the fuel's ability to reduce autoignition or knock, with the higher the octane number, the higher the resistance to autoignition. The octane number is measured in an engine according to an ASTM standard with two methods for a research octane number (RON) and motored octane number (MON). The value given on the pump when purchased is the average of these two. In addition to gasoline, SI engines can run on alternative fuels including methanol, ethanol and their blends, and natural gas and propane.

Compression ignition (CI) engines

In a compression ignition engine only air or air plus residual combustion gases from the exhaust known as exhaust gas recirculation (EGR) is inducted into the chamber during the intake stroke, and compressed during the compression stroke. Near TDC of the compression stroke, fuel is injected directly into the cylinder using a high-pressure fuel injector. This fuel must then mix with the compressed, heated, air, and residuals already in the chamber to a combustible limit and a portion of the fuel–air mixture subsequently autoignites. The time between the injection of the fuel and ignition of the first fuel is referred to as the ignition delay. After ignition a portion of the fuel burns in a premixed mode combustion mode with the remainder of the fuel combusting in a diffusion combustion mode. CI engines can run at higher compression ratios since they are based on the autoignition principle and are not limited by the premixed fuel–air combustion knock in SI engines. CI engines achieve higher efficiencies than SI engines, peaking around 45% (US DOE, 2010). Load is controlled in a CI engine by the amount of fuel injected limited by the amount of charge gas oxygen. CI diesel engines combust fuel in a locally stoichiometric rich zone, but globally lean (excess air), which provides efficiency improvements.

Compression ignition engines are typically fueled with diesel fuel. Phased in from 2006 to 2010 in the US, diesel for on highway use has to have a low sulfur content (EPA, 2006) to reduce particulate emissions and enable use of advanced lean aftertreatment devices to reduce oxides of nitrogen to meet emissions standards. Compression ignition fuels are characterized by their cetane number with the higher the cetane number the lower the autoignition temperature and shorter ignition delay. Alternative CI engine fuels include biodiesel, which is composed of methyl esters and produced from vegetable oils and animal fats, green diesel produced in many different ways, and their blends, all with different advantages and disadvantages. Dimethyl ether (DME) is also an excellent diesel fuel with Volvo currently in an extended evaluation and development program for its trucks. Additional details of fuels are discussed in Section 8.4.2.

8.4 Comparison of engine cycle performance

The different engines and combustion cycles as discussed yield varying advantages and tradeoffs in terms of power density, efficiency, application, emissions, etc.

8.4.1 Limitations

The two-stroke cycle is one type of operational engine cycle which produces one power stroke per every crankshaft revolution, yielding almost double the power density compared to the four-stroke cycle. However, due to scavenging losses and inefficiency of filling the cylinder volume, power density is not quite doubled compared to that of the four-stroke cycle, which is a limitation of this engine cycle. There are various further cycle limitations that need to be discussed, including friction, heat transfer, throttling and boosting.

Friction

In engine operation, there is work transferred to the piston from the combustion gases, which defines the thermodynamic indicated work (work from a pressure volume diagram). This indicated work does not translate directly to the amount of work to propel the vehicle as there are various losses, including friction, reducing the work available at the drive shaft. Friction work can be from 10 to 100% of the indicated work at full load to idle (Heywood, 1988). Friction influences the coolant system size as friction is dissipated as heat. Increased friction losses directly result in a reduction in brake torque (work available at the output shaft) and increase in brake specific fuel consumption measured in units of fuel consumed per unit of work or energy (g/hp-hr, or g/kW-hr). Friction increases with increasing engine speed and nearly linearly with increasing cylinder peak pressure. Friction resistance comes from engine moving parts including piston rings, valve mechanisms, crankshaft, etc. The largest friction contribution for both SI and CI engines is the piston and crank assembly (Heywood, 1988).

Although not a force impeding motion due to sliding, the work to pump the working gases from the intake to the exhaust including the fresh-air charge and any recycled exhaust gases termed the pumping work is included in the friction losses. If the intake pressure is below the exhaust pressure, as it often is in a spark-ignition engine when throttled, this can be a significant negative work term. The pumping work is often grouped with other factors including losses through auxiliary engine accessories, which together give the total friction losses. Spark-ignition engines have larger pumping losses due to throttling, with compression-ignition engines having significant friction losses from the high pressure fuel injection pump.

Heat transfer

Heat transfer losses from the combustion chamber result in reductions in efficiency and pressures, and deviations from the ideal cycle. The proportion of heat losses increases at lower engine speeds as there is more time for heat transfer to occur. Heat transfer is most important from the burned combustion gases during combustion and the expansion stroke as combustion products can reach upwards of 2200 °C, depending on the operating strategy. In addition to phasing during the cycle, heat transfer largely depends on location within the combustion chamber, governed by the movement and location of the hot burnt combustion products. Engine heat transfer must be controlled to prevent knock or pre-ignition in spark ignition engines including combustion of the end gases, assist in emissions control, minimize thermal stresses on engine materials, ensure optimal exhaust gas power (minimum heat transfer) for operating the turbine of the turbocharge system, and for other reasons. Considering an energy balance on the basis of fuel heating value for a naturally aspirated compression ignition engine with an indicated efficiency of 45%, nearly 25% of the fuel energy is removed by cooling water, with about 2% due to friction, 8% heat loss due to combustion, 6% heat loss during expansion, and 9% heat loss during exhaust; values for spark-ignition engines are a larger fraction of the fuel heating value (Heywood, 1998). Heat transfer varies based on engine material properties, cooling uses, engine speed and load operating conditions, and other factors.

Throttling

During normal engine operation, in particular, in spark-ignition engine load is controlled by air into the combustion chamber, which is governed by the throttle. The throttle is an air flow restriction to reduce the intake pressure reducing the amount of air trapped in the cylinder and thus reducing the stoichiometric matching fuel. As a result of the reduction in fuel the work per cycle is reduced. An intake pressure below atmospheric reduces efficiencies due to increased pumping losses. In compression-ignition engines, the incoming air is typically not throttled so this volumetric efficiency reduction is not observed. An exception to this is the use of EGR in CI engines, with EGR loops containing flow valves and restrictions which act as a throttle and have negative impacts on efficiency, at the trade-off of improved emissions to meet standards.

Boosting

Boosting using a turbocharger or supercharger in engine operation increases the inlet air pressure and temperature, which in turn can increase the output of an engine. Turbocharging can be used on both SI and CI engines with most CI diesel engines for on-highway use boosted and a larger number of SI engines for vehicles becoming boosted. Boosting on compression engines function to improve the engines power density, which assists in overcoming lean operation limits while also improving fuel consumption. However, based on the knock limit of SI engines, this increase in fuel consumption is not always achieved (Stone, 2002). In light duty vehicles, using a properly sized boosting system while also downsizing the SI engine results in reduced fuel consumption, decreased throttling losses and smaller relative losses in friction. In incorporating turbochargers or boosting, it is vital to correctly size and implement the device to ensure optimum transient response and minimize the turbo lag of the turbocharger over a wide speed and load range, which is more difficult for SI engines (Stone, 2002).

Limitations of ideal cycle analysis

As discussed, engine combustion can be modeled using an ideal cycle analysis to provide an approximation to the cycle. However, there are several limitations to this ideal cycle analysis that limit this applicability. One limitation is heat transfer, as has been discussed earlier.

First, the ideal cycle analysis does not consider the finite time for combustion, which results in the piston not being at TDC at the maximum pressure and, therefore, results in lower work. Another limitation is intake and exhaust restrictions through the valves resulting in finite time for the gas exchange process. For exhaust, the gases must blow-down from the high pressure in the cylinder to the exhaust pressure. To account for this, the exhaust valve is opened before BDC which results in a decrease in expansion stroke work transfer. Finally, the ideal cycle analysis assumes complete and ideal combustion, with the actual engine operating cycle being incomplete, meaning the engine exhaust contains species that are combustible. This results in a reduced fuel chemical energy use compared to the available input energy and, hence, a reduced combustion efficiency, about 95%, and thus reduced pressures. Additionally, since complete and ideal combustion is assumed, minor species such as NO, NO2, or combined NOx are not considered in the ideal cycle analysis, with these species being an important combustion

process in terms of emissions. Refer to Heywood (1988) for further details on the ideal cycle limitations.

8.4.2 Impact of fuel type

The fuel used in the engine has a direct impact on the engine performance, emissions, and combustion process. Compression ignition and spark ignition engines operate using different fuels with varying properties as the method of initiation and combustion are different.

Spark ignition engines operate conventionally on gasoline and fuels with high octane numbers, which have high resistance to engine knock. Alternative SI engine fuels typically include alcohols such as methanol, ethanol, and butanol as liquid fuels, or hydrogen, biogas, natural gas or methane as gaseous fuels. These alternative fuels all have octane ratings which exceed those of conventional gasoline and, therefore, offer the potential to increase the knock limited gasoline compression ratio to improve engine efficiency. Limitations in the alternative liquid fuels include lower energy content when compared to gasoline. Additionally, gaseous fuels, although having higher energy contents when considered on a density basis, have reduced energy densities on a volumetric basis. In addition to octane rating, another important characteristic is its volatility; if the fuel is too volatile it could vaporize in fuel lines and cause vapor locks, if it is not volatile enough there are cold start issues. Cold-start issues are a particular problem with ethanol due to its higher fuel–air stoichiometry, lower vapor pressure and high latent heat of vaporization in comparison to gasoline. Ethanol, with its higher fuel–air stoichiometry, has a fuel to air mass ratio of 1 to 9, whereas gasoline has a 1 to 14.6 ratio, meaning 60% more ethanol must be vaporized to reach the stoichiometric ratio. The result is, without design changes, the fuel–air mixture will not be rich enough to combust (Davis et al., 2000). These effects can be minimized by fuel blending.

Compression-ignition engines use fuels with high cetane numbers, which define the propensity for autoignition, as combustion in a CI engine is initiated without a spark plug, with the most common fuel being diesel. Various alterative CI fuels have been proposed and used including biodiesel, Fischer–Tropsch (FT) diesel (green or synthetic diesel), and dimethyl ether (DME) (Silva-Petrobas, 2006; Goodger, 1975) with differences in terms of physical properties, energy density, and others. Biodiesel is a combination of methyl esters (C15–C17) and is an oxygenated fuel, being 11% by weight oxygen, exhibiting an 8% reduction in volumetric energy density in comparison with diesel fuel (NREL, 2009). The oxygen in the fuel reduces the energy density but results in reduced particulate matter emissions. Dimethyl ether (DME) is another oxygenated diesel fuel (34% oxygen by mass) which results in lower exhaust emissions, including particulate matter (Arcoumanis et al., 2008). It has a high cetane number, greater than 55 (ULSD is 40–55), but due to its physical properties of low lubricity and high volatility, use in conventional fuel systems is not feasible so specialty designs are needed (Arcoumanis et al., 2008). Because of these properties DME, which has a higher cetane number (lower ignition delay) and low boiling point, has enhanced fuel–air-mixing and reduced cold-start issues. This is in contrast to biodiesel which gels in cold environments and can form waxy deposits, and so may require use of a glow-plug and heated fuel lines to promote engine starting and initial operation at low temperatures.

Other alternative fuels that can be used include compressed gases such as compressed natural gas (CNG), liquefied petroleum gas (LPG), and hydrogen (Demirbas, 2002; Gligorijevic et al., 2009), with the ultimate goal of reducing emissions and improving combustion. Use of these fuels requires engine modifications, which are governed by the desired application. For example, these fuels can be used in compression ignition engines under a dual fuel mode, with a pilot diesel event used to initiate the combustion process. CNG fuel is currently used in vehicles such as heavy duty trucks and buses in public transportation being advantageous due to low carbon dioxide emissions (Sperling, 1995; Tzeng et al., 2005). LPG is another alternative that is found in public transportation, although much less common than CNG, this fuel is being adopted in European countries to minimize pollutants due to transportation (Sperling, 1995; Tzeng et al., 2005). Furthermore, these fuels can also be coupled with other propulsion modes such as hybrid electric bus with a CNG or LPG backup engine in case of electrical motor failure (Tzeng et al., 2005).

8.4.3 Convergence of SI and CI engines

In addition to the discussed engine technologies, new engine cycles and operating strategies are being developed to further reduce fuel consumption and meet stringent emission standards. Some of these strategies combine the advantages of the spark and compression ignition CI engines.

One of these types is a stratified charge spark ignition engine, which relies on having a mixture that will easily ignite at the spark plug and a leaner (potentially non-ignitable) mixture further from the plug. The benefits of this are reducing throttling losses by controlling the load through fuel (as done in a CI engine) as opposed to through the control of air (as typical in a SI engine) (Stone, 2002). An enabling technology for this engine is direct-injection technology termed DISI (direct-injection spark-ignited) or a GDI (gasoline direct injection) engine. In this technology gasoline fuel is sprayed directly into the combustion cylinder, similar to a CI engine. The fuel–air mixture is still ignited with a spark discharge to promote flame kernel growth and propagation. If the fuel is injected early in the cycle, often prior to the intake valve closing, the resulting cylinder charge is nearly homogeneous and operation is similar to port fuel injected engines. If the fuel is injected late in the cycle, the charge is stratified. Advantages of the GDI engine technology are improved fuel economy, improved transient response, reduced CO2 and un-burnt hydrocarbon emissions at cold-start, and more precise fuel–air control, among others (Stone, 2002; Zhao et al., 1999). Disadvantages exist in regards to high emissions of NOx and soot at high-load operation, reduced efficiency of three-way catalysts, complexity in injection system and injection fouling, and increased engine wear (Zhao et al., 1999). Various DISI engines exist in production today, including for example Volkswagen, General Motors, BMW, and others (Celik and Ozdalyan, 2010). One in particular is the Ford EcoBoost engine series. The Ford EcoBoost 3.5 L, using direct injection coupled with turbocharging to allow for engine downsizing, boasts a 12% improved drive cycle fuel economy and 15% lower emissions (Yi et al., 2009). Furthermore, Ford has developed a 1.0 L 3-cylinder EcoBoost engine, capable of producing approximately 123 hP (92 kW), using extensive downsizing (Friedfeldt et al. 2012). DISI engines rely on downsizing to minimize engine emissions and fuel consumption, which requires increased air induction, achieved using cooled EGR and higher boosting, while also allowing for higher torque at lower engine speeds (Kaiser et al. 2010).

Another alternative engine technology is homogeneous charge compression ignition (HCCI), which again provides advantages of both diesel and gasoline engines in an effort to optimize the benefits of each. The HCCI technology capitalizes on a well-mixed charge as representative of SI engines to minimize particulate emissions, while using lean compression ignition to minimize throttling losses while upholding efficiencies (Aceves et al., 2001). This combustion technology relies on compressing a charge of homogeneous fuel, air, and residual gases (as used in SI combustion processes), to its autoignition temperature (relying on the CI combustion process), resulting in nearly simultaneous ignition at various locations in the combustion chamber in comparison to a spark ignition (Aceves et al., 2001; Juttu et al., 2007). HCCI is dependent upon the chemical kinetics of the mixture and is therefore sensitive to fuel formulation, mixture and temperature. Under the appropriate conditions it yields low temperature combustion which significantly reduces PM and NOx emissions while still providing high efficiencies as expected from diesel engines. HCCI has limitations in current application due to control of combustion phasing, elevated unburned hydrocarbon emissions and carbon monoxide emissions (Aceves et al., 2001; Juttu et al., 2007), and a limited load range.

To overcome the limitations of HCCI, combustion stratified charge compression ignition concepts are under development that use DI gasoline technologies using multiple late cycle injections to achieve the desired charge stratification while still relying on autoignition (Nathan et al., 2009). This combustion strategy may incorporate spark assist in the autoignition process (Szybist et al., 2010). Another concept is reactively controlled compression ignition (RCCI) which uses fuel blending to control the autoignition through use of high and low cetane fuels; it relies on blending two fuels with differing autoignition characteristics, in-cylinder, to control fuel reactivity and, hence, combustion phasing (Kokjohn et al., 2011). Premixed charge compression ignition (PCCI) combustion is another low temperature combustion strategy which must have precise mixture preparation to ensure optimal combustion, as it relies on premixed combustion of a diluted or lean mixture, and is controlled by chemical kinetics (Kokjohn et al., 2011; Cao et al., 2009). Although these low temperature strategies of RCCI, PCCI, and HCCI offer promise to simultaneously reduce soot and NOx in CI engines, their use is limited currently because of the difficulty in controlling these combustion processes

A specific example of the advancement of engine technologies is that of Southwest Research Institute (SwRI) which has developed the combustion engine concept termed HEDGE (High Efficiency Durable Gasoline Engine), which relies on a high compression ratio and heavy EGR (Pirault et al., 2005). The goal of this engine is to extend the performance and fuel efficiency of the SI engine to match that of the CI engine while still enabling use of the traditional SI aftertreatment of three-way catalysts to meet emissions standards. Key combustion and engine strategies include high energy ignition, cooled EGR, variable valve actuation, and a pressure charger such as a variable geometry turbocharger (Pirault et al., 2005). The HEDGE concept has dilution limitations and issues with knock that limit its use with higher compression ratios. (Alger and Mangold, 2009; Cairns et al., 2006).

8.5 Advantages and limitations of internal combustion engines

Internal combustion engines dominate power plants in transportation today due to the high energy density of liquid petroleum fuels and continuous improvement in efficiency and reduction of emissions. Furthermore, their existence and improvements since the 20th century has helped to define their longevity, durability, and promote their acceptance in society. That being said internal combustion engines have developed to the current point with numerous limitations and trade-offs because of fuel and emissions.

Emissions regulations are a large driver of engine technologies including after treatment. Since the mid 1970s, emissions of carbon monoxide, unburned hydrocarbons, oxidizes of nitrogen, and particulate matter have been reduced significantly (Dieselnet, 2012). For example, particulate matter has been reduced by a factor of over 1000, oxides of nitrogen by a factor approaching 50, and carbon monoxide by a factor of nearly 21 for light-duty vehicles (US EPA, 2007). Therefore, combustion strategies and engine technologies must be continuously modified to meet these standards.

Spark ignition engines are commonly operated at stoichiometric conditions, meaning the chemically correct fuel and air mixture, with no excess fuel or no excess air. The reason for this is based on the emission aftertreatment device. Spark ignition engines use a three-way catalyst to reduce harmful exhaust emissions. This includes an oxidizing reaction to convert carbon monoxide and unburnt hydrocarbons into CO2 and water as well as a reduction reaction of NO and NO2. For the catalyst to have the optimum conversion efficiency to meet emission regulations, the engine must operate at near stoichiometric, which results in increased fuel consumption. As with lean burn, there is excess air which assists in reducing throttling losses that occur under normal steady state engine operation and reduces heat transfer both reducing fuel consumption. These lean burn or dilute engines can assist in improving fuel economy, reducing CO2 emissions (important as this is a greenhouse gas), and can still utilize oxidation catalysts to assist with HC and CO emissions, but difficulty lies in the reduction of NOx (Twigg, 2007). During lean burn engine operation, with the additional air, excess NOx is generated so a more complex catalytic converter system is needed. One method is to use a NOx absorber reduction catalyst in conjunction with the conventional three-way catalyst (Li et al., 2007). Another option is to use high rates of EGR (Li et al., 2007).

Compression ignition engines produce harmful pollutants of NOx, soot or particulate matter, CO (carbon monoxide), and unburnt hydrocarbons (UHC). An increased complication is the trade-off between NOx and soot that has continuously plagued diesel engines. Emissions can be reduced to regulated levels to meet emission standards through the use of internal engine methods and after treatment. The internal methods include EGR which dilutes the charge gas and helps reduce combustion temperatures. After treatment requires the use of different devices on the engine to reduce emissions to regulated levels, before they leave the tailpipe. Current devices include diesel oxidation catalysts which oxidize CO and HC into CO2 and H2O, lean NOx trap which absorbs NOx during lean operation and reduces it to N2 during rich operation; diesel particulate filters which filter exhaust PM and require regeneration; and selective catalytic reduction that converts NOx to N2 and H2O by dosing the exhaust gas with urea which through hydrolysis is converted to ammonia which is a reducing agent for the NOx in the exhaust. Additionally, for the case of compression ignition engines, the required engine aftertreatment has led to fuel regulations, including the development of ultra-low sulfur diesel fuel (ULSD), which enables application of these advanced engine after-treatment strategies (Bosch, 2006).

8.6 Conclusions and future trends

Internal combustion engines will continue to be the primary mode of power generation for vehicles for decades to come. As discussed, internal combustion engines operate on differing cycles depending on the application, technology, etc. Typical operating cycles are two and four stroke, with different applications and advantages in processes, including, for example, scavenging for chargegas intake and exhaust (two-stroke) and intake and exhaust strokes (fourstroke), along with compression and expansion strokes.

Internal combustion engines need to continue to adapt and improve in order to reduce emissions, reduce fuel consumption, and retain performance and power. Furthermore, as fuel sources change, engines will also need to transition to utilize these novel fuel sources. There are various trends for the future of internal combustion engines. First and foremost is improving engine efficiency and fuel economy (regulated through CAFE standards in the US (NHSTA, 2010)) and reducing greenhouse gas (GHG) emissions and reliance on foreign and imported petroleum. In addition to fuel economy/GHG regulations, toxic emission (NOx, CO, UHC, PM) reductions are required as regulated in the US by the Environmental Protection Agency (EPA, 2012a). These regulations are forcing the development of novel combustion and engine strategies, as will be discussed below.

Advancements to the internal combustion engine can be achieved through alternative combustion strategies which harness the advantages of both SI and CI engines. These technologies rely on low temperature combustion strategies which simultaneously reduce fuel consumption and emissions. These include HCCI, RCCI, and PCCI engines. Low temperature combustion defines a combustion strategy which reduces combustion temperatures to minimize NOx formation, with soot formation also being reduced due to longer ignition duration (lower temperature), enhanced fuel–air mixing, and reduction in fuel-rich combustion zones (Kokjohn et al., 2011). Additionally, with these emission reductions, there are reduced aftertreatment requirements, which can also assist in improving engine efficiency.

An additional strategy as discussed in earlier engine cycles is the overexpanded engine cycle, but the method of actual practical implementation is the challenge. A method to implement this over-expanded engine cycle is variable cam phasing or variable valve timing (VVT) or phasing. VVT is used extensively in light duty vehicle spark-ignition engines in a number of configurations. These include intake only, intake and exhaust linked, and independent intake and exhaust phasing. The valve phasing with respect to the piston position has a direct influence on engine efficiency and exhaust emissions as this controls the movement of gases in and out of the cylinder. VVT directly affects volumetric efficiency and, thus, pumping losses. Charge gas control with VVT is achieved through intake valve timing, with a focus on late or early intake valve closing in terms of reducing pumping losses (Hong et al., 2004). When emissions are considered, valve overlap and exhaust valve timing become increasingly important, and these strategies can promote internal EGR to minimize NOx emissions (Hong et al., 2004). Therefore, an optimized VVT strategy must be considered for minimizing pumping losses and emissions to enhance and promote engine efficiency, which requires the use of accurate and reliable valve timing mechanisms (Hong et al., 2004).

Another potential future trend to assist with engine performance, efficiency, and emissions is variable compression ratio (VCR) or a variable expansion ratio engine, which enables a change in engine cylinder volume. This strategy has potential since it enables the engine to optimize the compression ratio to provide thermodynamic benefits including optimum operating temperature and therefore high efficiency. At low power, the VCR engine runs at a high compression ratio to improve fuel efficiency; at high power, the engine operates at a low compression ratio to prevent engine knock (Shaik et al., 2007).

In addition to standard powertrains, future trends will include increased use of electrification of the power train including hybrid electric vehicles (HEV) which, although they include on-board electrical energy storage, still use fuel combustion in an ICE, with the ability to manage power in an effort to optimize engine efficiency. At higher levels of electrification, plug-in hybrid electric vehicles (PHEV) are HEVs with larger batteries and charging system to enable power from the grid. A PHEV extends the blended mode operation using a gas engine and electric motor and electric-only mode. When the state of charge of the battery reduces, these vehicles function as standard HEV's (Tuttle and Kockelman, 2012). In addition to hybrid vehicles, full electric vehicles will rely on electrical energy, displacing fuel energy from the ICE. One example is plug-in electric vehicles (PEVs), which became mass-market viable in 2010 (Tuttle and Kockelman, 2012). There are various electric vehicles, with one example being BEV or battery electric vehicles, which include an on-board battery charged while parked and connected to the power grid (Tuttle and Kockelman, 2012). Another example is extended range electric vehicles (eREVs), which are BEVs with an on-board ICE generator configurator, to mitigate the range issue for standard BEVs by matching diesel and gasoline vehicle ranges (Tuttle and Kockelman, 2012; Bassett et al., 2012). Examples of these engine technologies in production and commercially available are the Chevrolet Volt (eREV), Nissan Leaf (BEV), and Tesla Roadster (PEV).

Both eREV and PHEV extended range vehicles combat the range issue of typical HEVs by relying on the energy density of petroleum to provide extended range and enable reduced battery size (and reduced cost). However, these powertrains require the cost of a hybrid electric-gasoline powertrain (Tuttle and Kockelman, 2012). Standard HEVs have a market penetration of 2 to 2.5% in the US attributed primarily to the Toyota Prius (EPA, 2012b). PHEVs will be more difficult to sell to consumers due to complex grid charger and initial purchase costs, along with uncertainty in terms of maintenance cost; however, there are lower operation costs and as the price of gas increases, this adoption rate may increase (Tuttle and Kockelman, 2012).

The future of the internal combustion engine will continue to evolve and be refined. In addition to alternative fuels, engines will be optimized for differing combustion strategies. Furthermore, alternative powertrain technologies will be increasingly incorporated including hybrid systems. Despite this, the internal combustion engine is a well-accepted and adaptable technology, whose future will be largely driven by fuel efficiency and emission regulations.

8.7 Sources of further information and advice

Sources of further information on this topic include various automotive and internal combustion engine textbooks (examples include: Stone, 2002; Ferguson and Kirkpatrick, 2001; Heywood, 1988; Taylor, 1985a, 1985b; Obert, 1973). An excellent history of the internal combustion engine is given by Cummins (2002). Additionally extensive research papers and articles are available on the subject. Examples include the Society of Automotive Engineers (SAE), International Journal of Engine Research, and Proceedings of the Institution of Mechanical Engineers Part D: Journal of Automobile Engineering.

8.8 References

Aceves, S.M., Flowers, D.L., Martinez-Frias, J., Smith, J.R., Dribble, R., Au, M. and Giarard, J. (2001) 'HCCI Combustion: Analysis and Experiments', SAE 2001-01- 2077.
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Improving the environmental performance of heavy-duty vehicles and engines: key issues and system design approaches

Q. Xin and C. F. Pinzon, Navistar Inc., USA

DOI: 10.1533/9780857097422.2.225

Abstract: This chapter reviews key issues in improving the environmental performance of heavy-duty vehicles and engines. It discusses the impact of emissions and fuel economy regulations as well as the range of design options and technological innovations required to reduce emissions, maximize engine efficiency, and improve fuel economy. Subsystem interactions and trade-offs in product attributes are illustrated in an engine system design approach.

Key words: heavy-duty, light-duty, engine system design, subsystem interaction, performance, emissions, greenhouse gas (GHG), CO2, fuel economy, exhaust gas recirculation (EGR), selective catalytic reduction (SCR), advanced technologies.

9.1 Introduction: classifying engine and vehicle types

Internal combustion engines can generally be classified into three categories:

1. spark ignition engines
2. compression ignition engines
3. homogeneous charge compression ignition (HCCI) engines.

A spark ignition engine uses a high-energy spark plug to ignite the homogeneous air–fuel mixture (e.g., gasoline or natural gas) to achieve flame-propagation combustion, normally combined with using an intake throttle valve or a flexibly controlled engine intake valvetrain such as a variable valve actuation (VVA) device. Due to the concerns of auto-ignition, spark ignition engines are limited in the peak compression pressure and gas temperature in the cylinder by the octane number of the fuel, and therefore their engine compression ratio and turbocharging are restricted.

Diesel engines do not use a spark plug, and instead use a high compression

Note: unit conversions are provided in Section 9.11.

ratio to compress the air to a very high temperature to ignite the diesel fuel to produce pre-mixed and diffusion combustion. Therefore, diesel engines are also known as compression-ignition engines, and their high compression ratio and lack of intake throttle loss lead to high thermal efficiency. HCCI is a low-temperature combustion concept that relies on air–fuel mixing and kinetically controlled homogeneous combustion, rather than the flamepropagation or diffusion controlled combustion encountered in gasoline or diesel engines.

It should be noted that a given type of fuel can be used in different combustion modes. For example, natural gas can be used in either sparkignition engines or compression-ignition engines, and their combustion mechanisms and thermal efficiencies are different. Another example of fuel flexibility is the use of gasoline in reactivity controlled compression ignition (RCCI), where 20% of the total fuel can be directly injected diesel fuel and 80% of the total fuel can be intake-port-injected gasoline fuel, in a compression-ignition engine in order to achieve very low soot and high thermal efficiency.

Meeting the regulations controlling pollutant emissions (i.e., nitrogen oxides or NOx, particulate matter or PM, hydrocarbons or HC, and carbon monoxide or CO) has been a primary focus worldwide for approximately twenty years (Majewski and Khair, 2006; Liu, 2010; Mollenhauer and Tschoeke, 2010). Most internal combustion engines, whether light-duty or heavy-duty, on-road or off-road, have achieved great progress in reducing emissions to very low levels, especially in the US, Europe, and Japan. As the emissions control era approaches its end, a new age of technological innovation and product competition has started since 2011 with the focus of meeting stringent greenhouse gas (GHG) emissions regulations and achieving significant improvement in fuel economy.

Modern diesel engines are used in a wide variety of applications such as automotive on-road (on-highway) market (i.e., commercial trucks, buses, vocational vehicles, pickup trucks, vans, etc.), off-road mobile applications (e.g., agricultural, construction), stationary (e.g., electrical power generation), marine, and locomotive industries (Heywood, 1988; Challen and Baranescu, 1999; Xin, 2011a, b, c, d). The world transportation industry, which consists of road, air, water, and railway transportations, consumes a large amount of energy of 128 exajoules in 2007. It was estimated by extrapolation (Greene and Baker, 2011) that the energy consumption will increase to 309 exajoules in 2050 if there are no control measures taken to reduce fuel consumption. However, with active and aggressive legislations of CO2 reduction and fuel economy improvement, the energy consumption in 2050 can be reduced to 121 exajoules, which is basically the same as today's level. This indicates the urgent need for reducing fuel consumption of engines and transportation vehicles. Within the transportation sector, road transportation occupies the largest portion at approximately 75–85%, and therefore reducing fuel consumption of internal combustion engines used in road transportation has become a top priority.

Vehicles are usually classified into several classes by gross vehicle weight rating (GVWR), for example in the US according to the following:

∑ Class 1: 6,000 lbs and less (e.g., light minivan, light utility van, multipurpose SUV, and light full-size pickup truck)
∑ Class 2: 8,501–10,000 lbs (e.g., heavy minivan, heavy utility van, heavy full-size pickup truck, and step van)
∑ Class 3: 10,001–14,000 lbs (e.g., small walk-in truck, light conventional van, and light city delivery truck)
∑ Class 4: 14,001–16,000 lbs (e.g., medium walk-in truck, heavy conventional van, and medium city delivery truck)
∑ Class 5: 16,001–19,500 lbs (e.g., large walk-in truck, heavy city delivery truck, and bucket truck)
∑ Class 6: 19,501–26,000 lbs (e.g., beverage truck, single-axle van, rack truck, and school bus)
∑ Class 7: 26,001–33,000 lbs (e.g., refuse truck, furniture truck, medium conventional tractor, and city transit bus)
∑ Class 8: 33,000 lbs and above (e.g., dump truck, cement truck, cab-overengine sleeper tractor, and heavy conventional tractor).

Commercial vehicles usually refer to Classes 2–8. Heavy-duty commercial vehicles are normally powered by diesel engines, with very few exceptions of using gasoline engines and natural gas engines. Heavy-duty compressionignition engines can be classified into the following three categories or primary intended service classes:

1. light heavy-duty (LHD, 8,500–19,500 lbs GVWR, 70–170 hp engine power)
2. medium heavy-duty (MHD, 19,500–33,000 lbs GVWR, 170–250 hp engine power)
3. heavy heavy-duty (HHD, 33,000 lbs and greater GVWR, 250 hp and greater engine power).

In US GHG emissions and fuel economy regulations, engine manufacturers must identify a single primary intended service class for each compressionignition engine (DieselNet, 'Emission standards: United States').

Engine and vehicle product attributes can be classified into the following four categories (Xin, 2011a):

1. performance
2. durability
3. packaging
4. cost.

Emissions, fuel consumption, noise, vibration and harshness (NVH), transient response, and acceleration capability are important sub-attributes of performance. In the heavy-duty market, fuel cost is the largest operating cost for the end customer, and therefore fuel economy is extremely important. On the durability side, long life with minimum durability or reliability problems and minimized downtime is crucial. Hardware cost and cost of ownership are other important considerations in product design. Moreover, diesel engines consist of many interacting subsystems such as fuel, air, EGR, combustion, aftertreatment, and electronic controls. In design optimization of engines, vehicles, and fuels, properly balancing trade-offs or compromises between different product attributes or subsystems is the core of system integration. With different technical paths selected that favor different attributes (e.g., low cost design, no NOx aftertreatment), product differentiation reflects unique choice of trading one attribute or subsystem with another.

Looking from the perspective of GHG emissions (mainly CO2, directly correlated to fuel consumption (Fig. 9.1)), among all the mobile sources in the US in 2006 (including cars, light trucks, heavy-duty trucks, off-road, marine, aviation, railway, etc.), cars and light trucks emitted 54% of GHG emissions, and heavy-duty trucks followed as the second biggest pollution source and emitted 18% of GHG. This situation indicates the strong need of reducing GHG emissions and fuel consumption of heavy-duty commercial vehicles such as freight trucks, refuse trucks, transit buses, and school buses (Cooper et al., 2009; Dressler et al., 2010; Greszler, 2011; Hausberger et al., 2012).

9.2 The use of alternative fuels to improve environmental performance

Fuel technology is another cornerstone of an engine–vehicle–fuel combined system approach to address current and future needs of emissions control and fuel consumption reduction. The concept of 'fuel design' has been receiving increasing attention in order to reduce criteria emissions and CO2 at affordable cost. In high-efficiency clean combustion, fuel design refers to the effects of changing fuel properties such as ignitability, volatility, and chemistry, which are affected by cetane number, aromatic content, and T⁹⁰ distillation temperature, respectively (Kumar et al., 2008). More details of fuel data can be found at the US Department of Energy website of Alternative Fuels Data Center (see the references).

Looking back in the history of engine fuel technology development, there were several milestone events including the following:

∑ gasoline lead removal in the 1970s
∑ volatility controls in the 1980s
∑ low sulfur diesel in the 1990s
∑ low sulfur gasoline and ultra-low sulfur diesel in the 2000s
∑ emergence of alternative fuels and renewable fuels, again in the 2000s
∑ benzene reduction in the 2010s
∑ low-carbon fuel formulations for reducing GHG emissions beyond 2010.

For gasoline engines, criteria pollutants will essentially become zero after LEV III Tier 3. For diesel engines, criteria pollutants have been reduced to extremely low levels after the US 2010 and Euro VI regulations. Therefore, future focus of fuel development will need to shift from criteria

9.1 The relationships between CO2 emission, BSFC, engine thermal

9.1 Continued

emissions control to CO2 reduction and helping cost reduction of engine and aftertreatment.

The latest globe-wide emissions and fuel economy regulations have generated a renewed interest in various alternative fuels to fossil fuels, e.g., natural gas, propane, ethanol, biodiesel, and hydrogen (Murtonen and Aakko-Saksa, 2009; Xin, 2011d). The interest in alternative fuels rooted from the oil crisis of the early 1970s. Modern challenges in alternative fuel development include fuel designs for meeting the requirements of criteria pollution reduction, CO2 reduction, and enabling aftertreatment delete. Moreover, the selection of alternative fuels for a given vehicle application should be based on a life-cycle viewpoint and practicality assessment.

The effectiveness of CO2 reduction for each type of fuel should be evaluated

not only from engine emissions considerations, but also more importantly from a well-to-wheel basis (Stodolsky et al., 1999; Louis, 2001; Wu et al., 2006). Well-to-wheel analysis indicates that among various fossil fuels, the rank of energy efficiency from high to low generally is as follows:

∑ diesel
∑ compressed natural gas (CNG)
∑ dimethyl ether (DME) made from natural gas
∑ synthetic diesel made from natural gas.

The rank of GHG emissions from low to high generally is as follows:

∑ diesel or CNG (depending on applications)
∑ DME made from natural gas
∑ synthetic diesel.

From a life cycle perspective, DME made from natural gas and synthetic diesel are inferior to petroleum diesel in terms of total energy consumption and GHG emissions. CNG is the only fossil fuel that can compete with petroleum diesel for vehicle applications. Among various renewable fuels, the rank of energy efficiency from high to low generally is as follows:

∑ biogas (sewage)
∑ DME (made from wood)
∑ synthetic diesel (cellulose)
∑ fatty acid methyl ester or FAME (soy, rape)
∑ ethanol (cellulose)
∑ ethanol (grain).

The GHG emissions of all renewable fuels are much lower than those of fossil fuels (e.g., by more than 90%).

Regarding the practicality of various fuels for transportation use, from the perspective of 'energy density per volume' vs. 'energy density per mass' (Simnick, 2011), liquid fuels such as gasoline and diesel are superior because they possess the highest energy densities. Biodiesel has approximately 11% lower energy density compared to diesel. Methanol has approximately 59% lower density than diesel. Natural gas has 12% higher energy density per mass than diesel, but its energy density per volume is only 27% of diesel. This makes the driving range of natural gas vehicles much shorter than that of liquid fuel vehicles. The energy densities of the Li-ion and NiMH batteries are orders of magnitude lower than liquid fuels. The energy density per volume largely determines the practicality of an energy source for mobile vehicles.

In alternative fuels development, another important concept is low carbon fuels. It refers to either having less carbon content in the fuel molecule or being made from renewable or biological sources such as the biodiesel produced from plants or animal fats. California has taken actions to try to establish a low carbon fuel standard to reduce carbon intensity of fuels by 10% before 2020. The development of low carbon fuels is a systems engineering practice. Although clean combustion and energy saving are primary drivers for low carbon fuels, other factors should also be considered in a system approach, for example, matching the fuel with a specific application, engine and aftertreatment compatibility and durability, vehicle ownership cost, resale value of the vehicle, and availability of fueling stations.

There is abundant reserve of natural gas in the US, and natural gas engines are encouraged by the US government in order to reduce foreign oil dependence and achieve lower GHG emissions. Natural gas, which mainly consists of methane (CH4), is a type of sustainable fuel containing lower carbon content than diesel. In fact, it contains the lowest carbon content among all fossil fuels except for hydrogen (H2). However, it also contains much lower energy per volume than diesel, and therefore more frequent refueling for the vehicle is required.

Natural gas can be used in line haul tractors, refuse trucks, vocational trucks, specialty vehicles, transit and school buses, etc. (Litzke and Wegrzyn, 2001; Harrington et al., 2002; Canelada and Tischer, 2007). The natural gas vehicle (NGV) market is fast growing, especially in the heavy-duty segment, which consumes a large amount of fuel. CNG is very feasible for local transportations, but it has inadequate fuel energy density for line haul applications. Liquefied natural gas (LNG) is feasible for line haul if fueling infrastructure is available. In well-to-wheel analysis, if all the GHG emissions from extraction, processing, transportation, storage, and end user consumption are considered, CNG can actually provide similar GHG emissions with diesel fuel (e.g., sometimes approximately 10% more in light-duty passenger cars but probably less in heavy-duty buses than diesel). Natural gas engines are usually also compatible with bio-methane or renewable natural gas (RNG), which provides great reduction of GHGs compared to gasoline and diesel fuels.

Biodiesel is a type of renewable fuel produced from soy, canola, rape seed, etc. It contains fatty acid methyl ester (FAME), and its molecule generally is heavier than average diesel. Many of today's diesel engines are certified to run B20 (i.e., 20% biodiesel). However, significant changes in engine design are required in order to run B100 (i.e., 100% biodiesel). Use of biodiesel sometimes can lead to lubrication issues such as high and persistent fuel dilution, oil oxidation and bearing corrosion, and piston deposits, as pointed out by Lauterwasser (2009). Biodiesel also has certain impact on aftertreatment devices, especially diesel particulate filter (DPF) and SCR, through ash accumulation and alkali absorption (Toops et al., 2010). Quality of biodiesel fuels is another challenge to the automotive industry (e.g., glycerin impurity is a filter killer). Another type of fuel worth noting is bio-produced diesel such as algae or genetically engineered yeast from sugar.

Fossil alternative fuels mainly include synthetic liquids, methanol, and DME. Synthetic diesel fuels are produced by using the Fischer–Tropsch approach to convert natural gas to liquid (GTL) or convert coal to gas and then to liquid (CTL). Synthetic diesel can have excellent fuel properties like almost zero aromatics and sulfur to enable lower emissions than conventional diesel fuel (Xin, 2011d). Table 9.1 provides a summary on general characteristics of various alternative fuels and their major applications in heavy-duty vehicles.

Table 9.1 General characteristics and major vehicle applications of alternative fuels

Alternative fuel	Performance characteristics	Applications in heavy-duty vehicles
Natural gas	It is a clean burning alternative fuel that is stored onboard a vehicle in either compressed gaseous or liquefied state. Engines can operate solely on natural gas or on a combination of natural gas and diesel fuel (i.e., dual-fuel). Natural gas vehicles can generate significantly lower carbon monoxide, nitrogen oxide, non-methane hydrocarbons, particulate matter, and other toxic emissions, and the fuel is typically less expensive than diesel fuel.	• School bus (CNG) • Shuttle bus • Transit bus • Refuse truck • Delivery van
Propane	It is a clean burning, high-energy alternative fuel that is a byproduct of natural gas processing and crude oil refinement. It is characterized by low carbon monoxide and carbon dioxide emissions.	• School bus • Shuttle bus • Delivery van
Biodiesel	It is a clean burning alternative fuel usually derived from vegetable oils or animal byproducts. It can be used as a pure fuel or blended with petroleum in any percentage.	• Load-haul-dump vehicles used in mines • Same applications as in diesel engines with different biodiesel blend percentages.
Ethanol	It is a renewable fuel produced from corn and other plant materials. It is usually blended with diesel fuel for use in compression ignition engines (e.g., standard diesel plus up to 15% by volume of ethanol).	• Transit bus • Urban operations (delivery, refuse)
Hydrogen	It is potentially an emissions-free alternative fuel. Although it is not widely used currently, research is underway to develop hydrogen as a fuel for internal combustion engines and fuel cells. It can also be blended with natural gas to create 'hythane' for use in natural gas engines to offer significant reduction of NOx.	• Hydrogen fuel cell bus (research) • Fuel cells for auxiliary power generation

9.3 Electric, hydraulic, and flywheel hybrid powertrains for improved fuel economy

Hybrid powertrains mainly include electric and hydraulic types. Hybrid electric vehicles (HEVs) include a battery, an electric motor, and power electronics for controls. HEVs can be classified by degree of hybridization. The simplest hybrid electric design is a start–stop micro type that avoids energy losses by allowing the engine to shut down during idle and/or deceleration. In a more complicated mild HEVs, the electric motor provides assist to the engine during acceleration events, thus enabling engine downsizing. Mild hybrid works well in driving cycles that have frequent starts and stops, such as in medium-duty applications or garbage packer trucks. Full hybrid is required in applications that have less frequent starts/stops and need to store regenerative braking energy for longer time. Full hybrid can also help the engine operate at its most efficient speed–load mode by utilizing the stored energy to help propel the vehicle. In full HEVs, the engine can be momentarily turned off in certain vehicle speed ranges (e.g., low speeds). In plug-in HEVs, a significant all-electric range capability is added with the requirement of recharging the battery from an electrical outlet (Miller, 2010). In all of these types, the fuel consumption improvement by using hybrid powertrains results from regenerative braking (with the electric motor operating as a generator, especially in urban driving cycles), downhill battery charging, engine starts/stops, and optimization of engine operating conditions. Typically, increasing the degree of hybridization implies an increase in battery size, complexity, and cost.

There are three basic configurations of HEV powertrain architecture:

1. series
2. parallel
3. series–parallel.

In the series configuration, the engine does not directly power the vehicle, but instead is used to power a generator, and a motor is used to drive the vehicle wheels. Since there is no direct mechanical linkage between the engine and the wheels, series hybrid has the capability of operating the engine at a desired operating mode such as at the highest thermal efficiency or the lowest emissions (Pritchard and Johnson, 2005). In the parallel hybrid configuration, both the engine and an electric motor provide power to drive the vehicle. This configuration allows engine downsizing while usually keeping the same transmission as in a conventional vehicle. Launch performance can be improved by faster reaction of the electric motor compared to turbo-lag limitations of the diesel engine. In the series–parallel configuration (Grammatico et al., 2010), the engine thermal efficiency advantage of the series hybrid and the acceleration capability advantage (due to greater combined power) of the parallel hybrid can be combined, at the penalty of higher cost and complexity in hardware and controls (Muta et al., 2004).

Hydraulic hybrid vehicles (HHVs) also can be classified into series and parallel configurations (Kim and Filipi, 2007). Compared with a HEV, the main difference of a HHV is using accumulators as main energy storage and using hydraulic pump-motors to drive the vehicle. Hydraulic accumulators have the capacity to store a large amount of energy in very short time intervals, while a HEV is limited by energy absorbing rate. Unlike a HEV, which has high energy density but low power density, a HHV is characterized by higher power density, lower energy density, and higher energy conversion efficiency than a HEV. These characteristics make a HHV ideal for medium-duty and heavy-duty parcel delivery or refuse vehicles.

Although hybrid powertrains are more expensive than conventional engine powertrains due to challenges in production battery technologies, hybrid vehicles grow fast in the light-duty segment under the pressure of recent new corporate average fuel economy (CAFE) regulations for fuel economy improvement. Hybrid electric vehicles have been successfully introduced to the passenger car market since the late 1990s (e.g., Toyota Prius, Honda Insight), and these hybrid technologies are especially effective in energy recovery for city driving cycles. In the light-duty segment, using hybrid powertrains (stop/start, micro, mild, full, or plug-in) to achieve GHG reduction rather than criteria pollutant reduction has become a prevailing practice. Although hybrid technologies have not been regarded as a crucial element and a potential solution to the emissions and fuel economy challenges of the heavy-duty industry (i.e., not as strong as in the light-duty and gasoline segments), the interest in hybrid technologies has increased significantly during the past several years to a stage that almost every major truck/bus and engine manufacturer has introduced production commercial hybrid vehicles and is continuously developing both advanced electric and hydraulic hybrid vehicles (Chammas and Clodic, 2005; Van Amburg, 2007). For line haul vehicles (Karbowski et al., 2011), even a few percentage points of fuel consumption reduction may lead to large savings in vehicle operating cost. For stop-and-go type of pickup and delivery trucks and city buses, hybrid powertrains can be quite effective in reducing fuel consumption.

Electrification in powering auxiliary systems is another general trend for meeting either light-duty or heavy-duty GHG and fuel economy regulations. Hybrid configurations are well suited for this need, especially combined with the electrification in other waste heat recovery (WHR) technologies. For example, using turbocompounding, organic Rankine cycle, or thermoelectricity to recover exhaust heat energy has become more popular in heavy-duty vehicles. The electric motors and batteries associated with these WHR technologies provide great synergy with hybrid powertrains.

Although not as popular as HEV and HHV, flywheel mechanical hybrid

vehicles are slowly gaining acceptance as an additional alternative and efficient hybrid technology. This emerging technology uses the same energy recovery principle as other hybrid types, except that it is accomplished by using a rotational flywheel to store kinetic energy. In fact, the idea of using a flywheel as an energy storage device in the transportation industry is not new, and it was dated back to the 1950s when a vehicle known as Gyrobus in Switzerland, Belgium, and Zaire was introduced (Harlow and Johnson, 2012). Although that passenger bus carried a 3-ton rotating wheel that was attached to an electric motor, it serves as a good example for understanding the flywheel technology as an effective means of conserving energy in hybrid vehicles.

Nowadays, the flywheel technology has made significant advances, particularly due to improvements in materials (e.g., carbon fiber) and friction reduction (e.g., using magnetic bearings), which allow flywheels to become smaller, lighter, and more suitable for vehicular applications. Modern examples include some Formula-1 race cars and a transit bus – Flybus, which was a project developed by Torotrak, Ricardo, Optare, and Allison Transmission (Crosse, 2009; Harlow and Johnson, 2012). In the Flybus system, energy is recovered during deceleration and then transferred by a continuously variable transmission (CVT) from the vehicle wheels to the flywheel at speeds around 60,000 rpm. During acceleration, the CVT returns the energy stored in the flywheel to the vehicle wheels, thus reducing the engine power requirement during vehicle launch and fuel consumption.

A flywheel hybrid vehicle typically consists of a rotating flywheel, a CVT, and a driveline connection. The flywheel-based energy recovery system captures the kinetic energy of the vehicle as it decelerates, and stores it through a series of gears and the CVT. Moreover, as the energy is transferred from the driveline to the flywheel, the flywheel acts as a brake and helps the vehicle decelerate (Diego-Ayala et al., 2008). The role of CVT is critical due to the changes in the ratio between the flywheel speed and the vehicle speed. Gearing down a flywheel at a speed around 60,000 rpm to the vehicle wheel speed at approximately 3,000 rpm requires a very smooth and continuously variable ratio adjustment. Once the vehicle accelerates, the stored energy is transferred back to the vehicle, thus increasing the vehicle speed and decreasing the flywheel speed.

Like other hybrid systems, there are both advantages and disadvantages of using flywheel-based energy recovery systems. Two main advantages are high efficiency and low cost of production. The flywheel hybrid system keeps the vehicle's energy in the same form, i.e., the kinetic energy during deceleration is stored by the rotating flywheel, and then released to the vehicle during acceleration. This process avoids energy transformation or conversion losses from kinetic energy to for example electrical energy and finally to chemical energy as in the case of a HEV. Another advantage is the reduction of service brake wear since the flywheel essentially acts as a brake and can slow down the vehicle; however, this feature is actually also available in other hybrid systems. Furthermore, the ability of easily retrofitting existing vehicles with a flywheel system becomes an attractive solution for fleet owners. For example, it is more economically feasible to retrofit existing buses with a flywheel system compared with installing a hybrid electric system using expensive batteries and motors. Perhaps the main concern of flywheel hybrid systems is safety. Hayes et al. (1999), Forbes and Van de Ven (2008), and Harlow and Johnson (2012) described safety and vehicle handling challenges of the gyroscopic forces created when the vehicle is in motion. More importantly, the design must ensure appropriate protection to the vehicle and passengers from the potential failure of a flywheel rotating at high speeds (e.g., 50,000 rpm).

9.4 Vehicle emissions and fuel economy regulations

As mentioned earlier, commonly regulated diesel engine emissions include the following four criteria pollutants:

1. nitrogen oxides (NOx)
2. particulate matter (PM)
3. hydrocarbons (HC)
4. carbon monoxide (CO).

Emissions limits are not only required for new production diesel engines, but also required in emissions durability/warranty, usually given as a minimum mileage or time period over which the engine must meet the standard (DieselNet 'Emission standards: United States'; Dollmeyer et al., 2007; Nicol, 2007; Seger, 2010; Lei, 2012).

9.4.1 Emissions and greenhouse gas regulations in the USA

The global emissions control journey started in the 1990s. The US 2010 heavy-duty diesel engine emissions regulation basically marks the end of the 'emissions control era' in terms of technologies by near zero NOx (0.2 g/(hp. hr)), PM (0.01 g/(hp.hr)), and non-methane hydrocarbons (NMHC, 0.14 g/(hp. hr)), with the aid of ultra-low sulfur diesel fuel standards. The US 2014–2017 heavy-duty GHG and fuel economy regulations have unveiled the beginning of low carbon journey in the transportation industry towards energy issues such as oil consumption and dependence, national energy security, and global climate changes. Future directions of engine, vehicle, and fuel development will be under the pressures of both emissions and fuel economy regulations. Legislative requirements and customer expectations have been well aligned from now on (Subramanian et al., 2008; Seger et al., 2011).

The history of heavy-duty engine and vehicle emissions and fuel economy regulations has a similar footprint as their light-duty counterpart. In the US light-duty segment, the fuel economy of passenger cars will be required to increase from 27.5 mpg in 2010 to 37.8 mpg by 2016. The fuel economy of light trucks will be required to increase from 23.5 mpg in 2010 to 28.8 mpg by 2016. The combined average will need to increase from 27.3 mpg in 2010 to 35.5 mpg by 2016 in the US CAFE requirement. It is expected and agreed by major US automakers that the fuel economy average of cars and light trucks will be improved to 54.5 mpg by 2025 in CAFE (Jackson, 2012). There is a clear trend that the CO2 emissions from cars (in g/mile) will decrease by 4–5% every year from 2010 to 2040 as a directional GHG target of future legislations (spread to multiple phases over years). A similar largescale continuous reduction in future decades is expected for the heavy-duty segment as well. In fact, a very early diesel engine was run at 27% thermal efficiency in 1897. After more than 100 years of development, today's on-road heavy-duty diesel engines have approximately 42% thermal efficiency with almost zero emissions. It is clear that the automotive industry has entered into an accelerated era of emissions and fuel efficiency improvement. In the transportation industry, freight efficiency improvement requires not only engine advancements but also vehicle improvements, and optimized system integration will become increasingly important.

The US Environmental Protection Agency (EPA) and National Highway Traffic Safety Administration (NHTSA) developed in late 2011 a set of joint national regulatory rules regarding GHG and fuel economy for heavy-duty engines and commercial vehicles (US EPA 2011; US EPA Industry/EPA Workshop Compliance Division, 2011a, 2011b; US EPA Industry/EPA/ NHTSA Workshop, 2011a, 2011b; Bunker, 2011; Bunker et al., 2011; Lee et al., 2011). The 2014–2017 GHG regulation is the first ever medium-duty and heavy-duty fuel consumption standard in the US. It sets separate standards between engines and vehicles in order to ensure improvements in both. It intends to allow the industry to use existing off-the-shelf technologies in production engines and vehicles. The EPA's GHG emissions standard will become effective from model year 2014, followed by a more stringent standard in 2017. The NHTSA's fuel consumption standard will be voluntary in 2014, 2015, and 2016 model years, and will become mandatory in 2017.

The 2014–2017 GHG regulations set separate standards for CO2, N2O, CH4, hydrofluorocarbons (HFCs), and fuel consumption. The CO2 and fuel consumption standards are aligned from each other to ensure consistency. It should be noted that there are direct correlations (Fig. 9.1) among brake specific CO2 emission (g/(hp.hr)), engine brake specific fuel consumption or BSFC (g/(hp.hr)), engine brake thermal efficiency, and truck freight efficiency (gallon/(ton.mile) or ton.mile/gallon). The correlation between CO2 and fuel consumption is not affected by urea. However, using urea in selective catalytic reduction (SCR) aftertreatment may cause an increase in N2O emission, which is 296 times more severe than CO2 in greenhouse global warming potential. Note that the US GHG regulations do not regulate the consumption of aftertreatment working fluid such as the liquid urea used in SCR. In fact, the urea does not come from free in terms of energy consumption. Instead, urea production is directly related to consuming nonrenewable energy resources such as natural gas or coal.

Although reducing crude oil consumption is important for national energy security, using fluid economy (rather than only using diesel fuel economy) by including urea consumption is more reasonable and comprehensive in GHG ruling, and can better protect customer interest and encourage or promote advanced diesel engine technologies.

The key differences between the EPA's GHG rule and the NHTSA's fuel consumption regulation reside in the following aspects. The EPA rule regulates CO2 in the unit of g/mile, g/(ton.mile), and g/(hp.hr) as standard metrics for all heavy-duty vehicles (except for medium-duty passenger vehicles or MDPV) and engines with an effective year of 2014, and it also covers N2O, CH4, and air conditioning leakage HFCs. The NHTSA rule regulates fuel consumption in the unit of gallon/mile, gallon/(ton.mile), and gallon/ (hp.hr) with an effective year of 2017 as mandatory, and it does not cover recreational vehicles. Although the EPA rule considers the contribution of urea to engine tailpipe N2O emission, the NHTSA rule does not cover the consumption of N2O-related working fluid such as the urea used in SCR.

The US 2014–2017 heavy-duty GHG regulations apply to all on-road motor vehicles with GVWR of 8,500 lbs or greater and the engines that power them, except for MDPVs that are already covered by the GHG emissions standard and CAFE standard issued for model year 2012–2016 light-duty vehicles. The US 2014–2017 GHG regulations divide the medium-duty and heavyduty transportation industry into three distinct categories as follows:

1. combination or line haul tractors (Classes 7 and 8 vehicles) and their engines
2. heavy-duty pickup trucks and vans (Classes 2b and 3 vehicles) and their engines.
3. vocational vehicles (all the vehicles in Classes 2b to 8 that do not belong to the previous two categories) and their engines.

The third category includes delivery, refuse, utility, dump, and cement trucks; transit, shuttle, and school buses; emergency vehicles; tow trucks; and motor homes, etc.

The EPA's CO2 limits of different engines are stipulated according to vehicle application and engine power range in g/(hp.hr). The EPA's CO2

limits of vehicles are classified according to vehicle application, weight, and design features (e.g., day cab, sleeper cab) in g/(ton.mile). The GHG regulations require the manufacturer of combination tractors to certify the vehicle for aerodynamics, weight reduction, tires, idling, and speed limiters, etc. They require the chassis manufacturer of vocational vehicles to certify the vehicle for tires. Moreover, they require the engine manufacturer to certify the engine for NOx, PM, CO2, N2O, and CH4, etc. Trailer manufacturers are not regulated yet in the US 2014–2017 GHG regulations. The GHG and fuel economy certification methods are different for engines and vehicles. Moreover, the test cycle used in CO2 and fuel consumption measurements is also different for each category mentioned above. There are two defined types of test cycles: tractor duty cycle (i.e., the US SET 13-mode test) and vocational duty cycle (i.e., the FTP test cycle). Engine dynamometer steady-state Supplemental Emissions Test (SET) certification test needs to be used for the engines in the first category, while the EPA's Greenhouse Gas Emissions Model (GEM) (Bunker et al., 2011) needs to be used to certify vehicles. Chassis certification test needs to be used for the engines and vehicles in the second category. In the third category, the engines need to be certified by using the Federal Test Procedure (FTP) transient test cycle, while the vehicles need to be certified by using the GEM. The methane and N2O emissions of compression-ignition engines need to be certified by using the heavy-duty engine FTP test cycle.

The GHG regulations provide manufacturers flexibility of averaging, banking, and trading (AB&T), which is similar in concept to the AB&T rules used in previous criteria emissions standards. The regulations also provide incentive credits for advanced technologies (e.g., electric or fuel cell vehicles, hybrid powertrains, Rankine cycle waste heat recovery systems, alternative fuels). More details of the GHG regulations can be found on the website of the US EPA and www.DieselNet.com.

9.4.2 Emissions and greenhouse gas regulations in other countries

The ultimate goal of emissions regulations is to lead the engine industry towards zero pollution and zero carbon transportation by advancing technologies with affordable cost. Emissions regulations throughout the world vary from region to region, with the US, the European Union, and Japan having the most stringent regulations. Other parts of the world typically adopt one of these three heavy-duty engine emissions standards with a certain time delay. In general, the usage of ultra-low sulfur diesel (ULSD) fuel has become a prerequisite to achieve effective and strict NOx and soot emissions control. Some countries such as the US, Canada, Japan, and the European Union have adopted strict sulfur levels. Most industrialized countries have also established regulations addressing fuel economy and GHG emissions (including CO2, methane, and N2O) due to concerns of national energy security or global climate change. Moreover, fuel technologies including biodiesel and other low-carbon and clean-burning fuels have attracted much attention as a potential method for CO2 emission reduction.

The European Union's criteria emissions regulations for heavy-duty diesel engines are commonly known as Euro I to Euro VI. The first standard (Euro I) was introduced in 1992 (DieselNet, 'European Union'). The current newest legislation, Euro VI, is basically comparable in stringency to the US 2010 standard and became effective in 2013. It also introduces emissions limits in terms of particle number, new testing requirements including off-cycle and in-use testing, and more strict on-board diagnostic (OBD) requirements. Russia introduced regulations based on Euro IV beginning in 2010 and based on Euro V targeting for 2014. In 2007, the European Commission began to investigate and establish a program to address heavy-duty vehicle fuel consumption and CO2 emission. The program included studies on potential fuel economy improvements from both engine and vehicle technologies. In order to account for various vehicles and duty cycles, a common simulation tool for determining fuel efficiency and CO2 emission was established as a standard certification procedure. It is expected that the ruling of European GHG standards will be finalized by 2013 and implemented in 2015 (Bandivadekar, 2012).

Japanese emissions standards have been regulated since the late 1980s (DieselNet, 'Japan'). Effective from October 2009, the 'Post New Long Term Emission' (PNLT) standard for heavy-duty vehicles has been in place, corresponding to a level between the US 2010 and Euro V legislations. Onroad light-duty and heavy-duty vehicles are also subject to mandatory fuel economy standards. There are two sets of target in the history of Japanese fuel economy legislations. The first set was effective in November 2005, and was the world's first fuel economy standard for heavy-duty vehicles. It uses a 'top runner' approach, which essentially sets future targets based on the fuel economy of the most fuel-efficient vehicle on sale. The standard was set in kilometers per liter of fuel (km/L), and was based on vehicle weight category. Manufacturers are required to ensure that in each financial year the average fuel economy of their vehicles in each weight category meet the standard. The second renewed set of target was stipulated in 2007 for trucks and buses, and will become effective in 2015. The fleet average fuel economy of trucks is targeted at 7.09 km/L (396.6 g CO2/km), which corresponds to a 12.2% increase over the 2002 baseline. The fleet average for buses is targeted at 6.30 km/L (416.0 g CO2/km), 12.1% increase over the 2002 baseline. Similar to the European certification procedure, fuel consumption and vehicle performance is evaluated also through computer simulations. However, unlike in Europe, the primary focus in Japan is on engine technologies rather than

the vehicle as a whole. Therefore, the simulation tool allows for evaluating new engine and powertrain technologies, including hybrid powertrains via hardware-in-the-loop simulations.

The heavy-duty diesel engine emissions standards in China are based on the European legislations and typically adopted with a certain time delay (DieselNet, 'China'). China's national IV standard, based on the Euro IV regulation, is set to be implemented after 2013 with several years of delay due to fuel quality issues. China's rapidly growing heavy-duty market segment accounts for the majority of hazardous air pollutants including NO^x and particulate matter (PM). Based on fuel consumption and vehicle usage data, it is estimated that heavy-duty vehicles emit approximately three times more CO2 than passenger automobiles (China Greentech Initiative, 2011). In September 2012, the Chinese government proposed its first heavy-duty fuel consumption standard that will be effective in July 2014. This regulation applies to diesel-powered vehicles over 3,500 kg, and covers various applications such as trucks, construction vehicles, semi-trailers, coaches, and city buses. The fuel consumption limits in the standard are set per 100 km, in the range of 13–45.5 liters for trucks, 15–49 liters for construction vehicles, 33–48 liters for semi-trailers, 12.5–29.5 liters for coaches, and 14–49 liters for city buses.

India has begun adopting European emissions and fuel quality regulations for both light-duty and heavy-duty vehicles since 2000. The Bharat Stage III (based on the Euro III regulation) is currently applicable nationwide, with exception that some major cities have adopted the more stringent Bharat Stage IV (based on Euro IV) since 2010 (DieselNet, 'India'). India is also emerging as an important market for heavy-duty vehicles and has a great potential to benefit from cleaner and more fuel efficient engine technologies.

Most of the heavy-duty emissions standards and certification test cycles used in South America are largely based on the European counterparts. In the early 2000s, Brazil adopted its PROCONVE P5 and P6 standards (based on the Euro III and Euro IV regulations, respectively) with implementation dates during 2006–2009. However, the PROCONVE P6 standard was actually never implemented, and later a new PROCONVE P7 standard (based on Euro V) has been enforced since January 2012. Typically, Brazil leads the continent with the most stringent standards in the region, compared to other countries such as Argentina, Colombia, Chile, Peru, and Uruguay, where the Euro III or IV regulations are still currently in place (DieselNet, 'Brazil').

9.5 Improving vehicle design to meet environmental regulations

The US 2014–2017 GHG regulations require approximately 20% reduction in fuel consumption from combination tractors by 2017 (compared with 2010), 7–10% reduction from vocational vehicles, and 17% from heavy-duty pickup trucks and vans. It is expected that approximately 3–5% CO2 reduction is achieved by engines by 2014 compared to the 2010 industry average, and an additional 2–4% reduction achieved by 2017. The remaining part of the reduction will come from vehicle improvements.

Vehicle fuel economy improvement technologies usually include the following:

∑ aerodynamic profile tractor body shape (to reduce drag resistance)
∑ frontal area reduction
∑ roof spoilers
∑ aerodynamic bumper
∑ aerodynamic mirrors
∑ tractor mounted gap reducer
∑ trailer mounted gap reducer
∑ integrated cab roof fairing
∑ trailer side fairings/skirts
∑ fuel tank side fairings
∑ trailer rear fairing or boat tail
∑ low rolling resistance tires (e.g., single wide tires)
∑ idle reduction equipment
∑ aft body plates.

Using automatic mechanical transmissions, whose shift schedules can be adjusted to match the characteristics of the engine and the vehicle, will reduce the variability of fuel consumption among fleet drivers and hence reduce CO2 emission. Electrification of the accessories driven by the engine may also reduce fuel consumption, particularly by powering hotel loads or vehicle idle needs more efficiently.

Most of the vehicle technologies currently used or considered for fuel efficiency improvement and GHG reduction are similar in main global markets (e.g., the US, Europe, and Asia). Since many European engine and vehicle manufacturers export products to the US, technologies are expected to be similar (Law et al., 2011). In China, simple and low-cost general vehicle improvements (e.g., low-rolling-resistance tires, trailer skirts, and cabin fairings) and driver training are a near-term priority. On the other hand, technologies adopted may vary between different vehicle categories. For example, long haul trucks generally have a greater potential of fuel consumption reduction through aerodynamic streamlining and improving driver behavior. Some other applications such as refuse trucks and city buses may experience the largest GHG reduction benefit via engine improvements and powertrain hybridization. Coach vehicles may employ certain controls features such as brake assist and adaptive cruise control technologies.

Fuel cost typically represents the single largest operation cost for a fleet.

In the European Union and the US, fuel consumption can be monitored and reported in order to help fleet operators and drivers effectively manage fuel cost, although such a practice has not become a requirement yet. The impact of vehicle speed on fuel consumption has been given special attention in both Europe and the US. Simulations reported by Hill et al. (2011) show that fuel consumption reduces by 6% when 10 km/hr reduction is achieved from 90 to 80 km/hr for a heavy-duty vehicle during constant-speed cruising. In addition to vehicle speed, fuel consumption is also highly dependent on vehicle duty cycle.

Finally, it is important to point out the critical role of simulation tools used as a part of the fuel economy certification procedures in the US, Europe, and Asia, in addition to engine dynamometer testing. However, each governing agency may use different assumptions, powertrain configurations, duty cycles, and other input data in their models that would generate slightly different GHG limits and targets.

9.6 Improving engine design to meet environmental regulations

The US 2010 emissions regulation and 2014–2017 heavy-duty GHG and fuel economy regulations have been strongly influencing the engine and truck industry and will continue having a profound impact on engine technology selection in the next several decades. Moreover, engine design and calibration also need to meet OBD regulations. In engine and vehicle product development, properly classifying engines in the right service class or vehicle category and selecting the right technical path to meet both emissions and fuel economy requirements are critical.

The first 10–20 years of the new GHG era will focus on fuel consumption improvement of conventional diesel engines. Looking back in the history of on-road diesel engine technology evolution, it is noticed that the following six major technologies emerged at different times during the journey of NO^x and PM controls:

1. turbocharging in the 1970s–1980s
2. charge air cooling in the 1980s–1990s
3. electronic fuel systems in the 1990s
4. cooled EGR in 2002/2004
5. diesel particulate filter (DPF) in 2010
6. SCR in 2010–2012.

Now the question is 'what will be the next mainstream heavy-duty diesel engine technology in the CO2 age on top of those six technologies?' The answer probably is engine downsizing or waste heat recovery (WHR). Another possibility is to quickly adopt the mature production technologies of VVA from the light-duty gasoline engines and apply them to heavy-duty diesel engines to realize different mechanisms of improvement.

During the past ten years of diesel engine development, there have mainly been three technical paths as follows:

1. high EGR without SCR
2. SCR without EGR
3. SCR with medium EGR.

In engine technology selection, the engine-out emissions target has to be properly selected in combination with available aftertreatment technologies. Moreover, technical path selection needs to ensure good continuity or connectivity between criteria emissions compliance and fuel consumption compliance, and such connectivity comes from not only good tracking and estimate of emissions legislation progression, but also a deep understanding of subsystem interactions in engine performance and product attribute interactions. In fact, for meeting the US 2010 or Euro VI emissions regulations, DPF is usually required for soot control and cooled EGR is required for NOx control. These above three technical paths have great impact on engine design strategies in terms of engine-out NOx target, system configuration, performance, durability, cost, and method of meeting GHG regulations. These technical paths are also affected or constrained by the sulfur content in diesel fuel, SCR conversion efficiency, and desired urea consumption.

Cooled EGR is very effective in reducing NOx emissions. It also affects the combustion of diesel fuel and low-carbon alternative fuels. Moreover, it affects combustion noise, heat rejections, pumping loss, transient response, air system configuration, and fuel injection timing adopted, etc. Accordingly, EGR affects several critical system design performance parameters related to durability, e.g., maximum allowable peak cylinder pressure, exhaust manifold gas temperature and pressure, and compressor outlet air temperature. In the above-mentioned first technical path, only using high EGR without using SCR or other NOx aftertreatment (i.e., relying on advanced in-cylinder technologies to control NOx and PM emissions) has a very positive influence in leading technological advances of the diesel engine industry. However, this technical path is the most difficult or challenging one and hence requires most advanced technologies, for example, imposing the most stringent requirements on the engine air system, cooling system, and base engine designs (e.g., using a very small turbine area to drive a large amount of EGR at full load, using a very large radiator or even a low-temperature radiator to dissipate the high heat rejection coming from the large EGR cooler, and raising the peak cylinder pressure design limit to accommodate engine downsizing strategy for practical reduction of fuel consumption).

While deleting NOx aftertreatment and its working fluid offers an advantage in cost saving for this technical path, its engine fuel consumption is relatively high due to more retarded fuel injection timing that has to be adopted for in-cylinder NOx control, if no other design measures are taken to reduce fuel consumption. The fluid economy (i.e., counting both diesel fuel and urea liquid consumption) also may not be the best for in-cylinder 0.2 g/(hp.hr) NOx control. Two-stage turbocharging is usually required for this path due to the stringent requirement of high intake manifold boost pressure. Controlling the cylinder-to-cylinder variation of EGR distribution also becomes important. Electrically assisted boosting or supercharging technology or even stoichiometric combustion during transients is desirable or even has to be considered in order to resolve the issues of transient air shortage that is caused by using high EGR rate during transients to control both NOx and soot emissions.

On the contrary, the second technical path of only using SCR without EGR goes to the other extreme. Although the fuel injection timing of this path can be advanced, it often has to be restricted from being set very early. Otherwise, NOx emissions would become too high, unless extremely high SCR NOx conversion efficiency is available. The reason is this technical path only relies on high-efficiency SCR rather than EGR to reduce NOx. Therefore, its fuel consumption and NOx emissions cannot be reduced to a very low level. In fact, one major technical barrier of this path is to achieve very high NOx conversion efficiency in SCR (e.g., 97%). Actually, even when SCR efficiency becomes very high, there could still be a mismatching issue between the fuel consumption reduction potential of injection timing advance and SCR efficiency, i.e., there is a diminishing return or even penalty on BSFC when fuel injection timing is set too advanced (early) and engine-out NOx becomes too high (Fig. 9.2).

Moreover, in today's high brake mean effective pressure (BMEP) engines, the turbine area has to be small enough in many speed–load modes in order to deliver sufficient air-to-fuel ratio (air–fuel or A/F ratio), hence resulting in a positive engine delta P (i.e., exhaust manifold pressure greater than intake manifold pressure) and pumping loss, unlike the negative engine delta P or 'pumping work or gain' encountered in old diesel engines that have lower BMEP and no EGR. (Note that engine delta P is defined as exhaust manifold pressure minus intake manifold pressure.) In this case, using no EGR actually does not provide any advantage in pumping loss reduction, compared to an EGR engine. There actually is an optimum combination between EGR rate and required SCR NOx conversion efficiency for achieving the best engine system solution for NOx control, minimum pumping loss and BSFC, boost pressure requirement, heat rejections and cooling system capability. The system design method to handle such an optimization will be elaborated in Section 9.3.2. Other challenges of the second technical path include the following: successful conversion of urea to ammonia without excessive deposits; NO^x selectivity in ammonia slip oxidation catalyst; compact SCR packaging size

9.2 The effect and potential of fuel injection timing on BSFC reduction.

and low pressure drop; and fuel efficient thermal management for the warmup portion in SCR operation during the transient FTP emissions cycle (e.g., using effective fuel injection strategies as well as air and cooling system controls to assist).

The first and second technical paths both encountered difficulties, in more or less extent, for meeting the most stringent NOx emissions regulations such as the US 2010 or Euro VI standard. Currently, a more realistic, mature, and production-ready technical path is the third one (i.e., using SCR with a medium level of EGR rate), at the price of increased hardware cost and heat rejections. This technical path maximizes various advantages of EGR with reasonably controlled pumping loss, and utilizes very advanced fuel injection timing to reduce BSFC (as long as not exceeding the peak cylinder pressure limit, especially at full load). This technical path currently offers the best trade-offs between attributes and the most smooth transition from meeting stringent NOx/PM emissions regulations to meeting GHG regulations, based on the current situations that the easiest measure of reducing BSFC is simply advancing fuel injection timing and that other fuel consumption reduction technologies (e.g., turbocompounding, Rankine cycle waste heat recovery, significant downsizing or greatly increased BMEP) are not production-ready yet.

The third technical path of combining SCR and EGR may not be the best solution from the viewpoints of promoting diesel engine technologies, optimizing overall product attributes (e.g., cost), or enhancing product competitiveness. However, it is indeed the easiest path to meet both criteria emissions and GHG regulations by balancing the tradeoffs between different product attributes such as performance (including emissions), durability, packaging, and cost of various subsystems involved.

9.7 Developments in light-duty diesel engine technologies

Some previously published studies (e.g., Cackette, 2010) estimated that the production volume of gasoline-engine-powered light-duty vehicles using liquid fossil fuel and liquid bio-fuel will steadily and slightly grow from 2010 to 2020 and will sharply drop from 2020 to 2050 to approximately half in volume due to ever-stringent GHG regulations. In contrast, micro/mild hybrid, full hybrid, and plug-in hybrid electric vehicles will keep growing in volume from 2010 to 2030 and then maintain a steady large market share till 2050. Battery electric vehicles will continuously grow in volume from 2020 to a large share in 2050 once the breakthrough of energy storage technology occurs. Natural gas and bio-gas vehicles will occupy a small fraction of market share by 2050. Fuel cell vehicles may gain a small fraction by 2050 once the breakthrough of fuel cell stack and hydrogen storage happens and after hydrogen infrastructure is in place.

Direct injection diesel engines are far better than conventional portfuel-injection gasoline engines in thermal efficiency (Austin et al., 2008), although the cost of diesel engines is higher. The development direction of internal combustion engines is generally moving towards both higher thermal efficiency and higher cost. This trend is evident for several lightduty gasoline engine technologies (Steinberg and Goblau, 2004; Kuhn, 2007; Ryan, 2008; Eckerle, 2010; Johnson, 2010; Tatur, 2010; Jackson, 2012) such as downsized turbocharged direct injection spark ignition engines (Blaxill, 2011), high energy-ignition gasoline engines, high cold-EGR gasoline engines, and reactivity controlled compression ignition (RCCI) dual-fuel (gasoline plus diesel) engines (Wagner et al., 2011). On the other hand, the thermal efficiency and cost of diesel engines also move up under the pressure of GHG regulations. Although diesel engines have an advantage over gasoline counterparts in meeting GHG requirements, the high cost of diesel aftertreatment system for meeting the very stringent US light-duty emissions regulations (e.g., Tier 2 Bin 3 or Bin 5) is the main challenge when diesel engines try to penetrate into the light-duty market.

Heavy duty and light duty have different application scenarios. Understanding light-duty technologies and their production readiness for meeting emissions and GHG regulations and their differences compared with heavy duty is important for planning heavy-duty technologies. The technology trends of light-duty diesel engines and gasoline direction injection (GDI) engines mainly include the following for the 2013–2016–2025 time frame:

∑ downsizing and extremely downsizing engine displacement (at the same power or torque rating)
∑ down-speeding combined with a greater final drive ratio
∑ improved fuel injection
∑ advanced combustion mode including low-temperature diesel combustion
∑ improved air charging including electrical supercharging
∑ dual-loop EGR system
∑ VVA including de-throttling or Miller cycle
∑ exhaust WHR (e.g., using thermoelectric generator) and coolant WHR
∑ reduced engine parasitic loss and friction
∑ smart accessories for reduced power consumption (e.g., switchable piston cooling, variable speed water pump, smart alternator)
∑ engine and vehicle weight reduction
∑ stop–start micro hybrid management and reduced idle fuel consumption
∑ HEV and plug-in HEV
∑ high conversion efficiency aftertreatment system (lean NOx trap (LNT), SCR, etc.)
∑ closely-coupled diesel oxidation catalyst (DOC) and DPF
∑ advanced low-carbon fuel formulations.

As the counterpart to the heavy-duty SuperTruck research program, the US government (Department of Energy or DoE) is sponsoring a light-duty program called 'Advanced Technology Powertrains for Light-Duty Vehicles (ATP-LD)' (Singh et al., 2012) to explore advanced engine and vehicle technologies. Cummins, Ford Motor Company, General Motors Corporation, Chrysler Group, Delphi Automotive Systems, and Robert Bosch are active industry participating members. The ATP-LD program objectives include 25% fuel economy improvement for gasoline engines over the baseline of state-of-the-art port-fuel-injection gasoline engines, and 40% fuel economy improvement for diesel engines over the gasoline baseline.

Among the above-mentioned technologies, it is worth noting that engine downsizing and VVA are especially popular in light-duty production engines. Some companies (e.g., MAHLE) claimed that 50% engine displacement downsizing can provide up to 30% fuel economy benefit. It should be noted that downsizing in light-duty engines is actually easier than that in heavy duty because the durability requirements of light duty at full load are usually less stringent than heavy duty owning to much less time spent at full load in real-world driving cycles. The reason for the popularity of VVA is due to the fact that most gasoline engines used an intake throttle valve to regulate air flow rate in order to achieve stoichiometric combustion and load control, and hence there is a large potential to use intake VVA to replace the intake throttle in order to achieve reduction of pumping loss and BSFC.

As to the advanced low-NOx combustion modes used in light-duty diesel engines, the major part of emissions certification cycles is located at low engine speeds and low loads, and therefore low NOx targets can be met by using advanced low-temperature combustion. Only a small fraction of light-duty test cycles (i.e., high engine speeds and high loads) fall within the conventional diesel combustion mode. Thus, it is possible to reduce or even eliminate NOx aftertreatment requirements. In contrast, the majority of certification cycles of heavy-duty diesel engines is located at high speeds and high loads, and this feature makes using advanced combustion modes more difficult than in light duty.

Unlike the light-duty segment where gasoline engines, hybrid powertrains, and batteries can play a major role due to their short distance applications, in the heavy-duty segment (especially line haul and long distance applications) currently there are no better alternatives to diesel engines or compressionignition cycles and low-carbon liquid fuels. Although electrification is feasible in light-duty applications to largely reduce fuel consumption, heavy-duty vehicles have not seen such a trend and feasibility. Finally, it should be noted that technology selection for CO2 reduction (e.g., via hybrid or low carbon fuels) should not be restricted to only complying with the standards/limits stipulated by GHG regulations. Rather, a life cycle CO2 emission analysis including fuel/energy production, fuel consumption, electricity consumed, and disposal energy consumption is required in order to truly identify the differences and to reduce the entire life cycle CO2 emission.

9.8 A system design approach to address challenges in advanced engine and vehicle technologies

Heavy-duty diesel engines and their system design are critical for lowemissions and low-carbon transportation in the 21st century. Engine system design consists of three pillars of work:

1. generate and maintain system design specifications
2. develop advanced simulation models and analysis methodologies
3. develop advanced engine technologies with system integration and optimization.

Optimal design for engine, fuel, and vehicle development in Pillar 3 requires good understanding of product attributes and subsystem interactions as well as the mechanisms involved. As noted earlier, engine and vehicle product attributes include performance, durability, packaging, and cost. Emissions and fuel consumption are also important performance sub-attributes that are related to legislative compliance for engines.

There are always trade-offs between different attributes or subsystems, i.e., improving one aspect is usually accompanied by a penalty on another aspect. There are three levels of trade-offs:

1. attribute level
2. sub-attribute level
3. parameter level.

These trade-offs are governed by the physics of the technology or the product. The essence of system optimization is to reach the ultimate potential in parametric relationship and minimize the trade-offs or the penalty by seeking solutions from certain attributes which can be sacrificed, or by discovering affordable new measures that can break or alleviate the existing trade-off difficulty, rather than simply moving the risk from one place to another.

One example of this philosophical view of trade-offs is that a more expensive higher-fuel-injection-pressure system breaks the trade-off between NOx and soot emissions and reduces both emissions simultaneously. As fuel injection timing is advanced, engine-out NOx emissions increase, and BSFC and soot emission decrease until a diminishing return occurs when injection timing becomes too early. The increased engine-out NOx can be reduced in the tailpipe by adding a NOx aftertreatment device with a significant cost. In other words, an in-cylinder NOx emissions compliance technology gains a benefit of NOx aftertreatment delete in cost saving but may lose engine thermal efficiency due to disabling the mechanism of advancing fuel injection timing to reduce BSFC. On the other hand, as fuel injection timing is retarded, NOx decreases and soot increases. An increased injection pressure

and EGR rate can reduce soot emission while keeping the NOx and BSFC unchanged.

Technical path selection is a choice of trading different attributes or parameters, for better or worse, by moving the system design point within the factor space that is formed by the physics of the engine, fuel, and vehicle. Diesel engine emissions and BSFC are affected by engine speed, load, ambient temperature, and altitude conditions. Engine BSFC or thermal efficiency is affected by indicated power, pumping loss, mechanical friction, and waste heat recovery gain. Indicated power is mainly affected by engine compression ratio, cylinder heat transfer loss, fuel injection timing, and heat release rate duration. Pumping loss and its associated air system design have paramount importance in modern diesel engine design due to its large potential in thermal efficiency improvement (e.g., 0.9 bar engine delta P corresponds to 7% BSFC change at rated power) and the strong interactions among various subsystems (e.g., turbocharger, EGR, combustion, cooling, manifolds, valvetrain, exhaust).

Air system capability analysis is an important core task in diesel engine system design. The analysis is critical for pumping loss control (via hardware sizing and calibration) and the direction of hardware design improvement. Figures 9.3 and 9.4 illustrate air system capabilities and subsystem interactions of various engine technologies. Basically, turbine area (size) controls engine delta P, air–fuel ratio, and EGR driving. A fixed geometry turbine encounters greater difficulty in the trade-off between low speeds and high speeds for pumping loss control than variable geometry turbine (VGT). Turbine wastegate can regulate engine delta P and air–fuel ratio, but at a price of exhaust energy loss to the ambient. Higher turbocharger efficiency can greatly increase air–fuel ratio and slightly reduce EGR rate at almost constant engine delta P. Intake and exhaust throttle valves have similar functionality in air system capability. They both can greatly reduce air–fuel ratio and slightly increase EGR rate at almost constant engine delta P. Intake valve closing-variable valve actuation (IVC-VVA) is a competing technology to VGT in air–fuel ratio and pumping loss controls. Moreover, if EGR cooling is increased, in order to meet the same emissions, less EGR rate can be used and this may lead to selecting a larger turbine area for pumping loss reduction and BSFC improvement. Understanding these parametric relationships in the engine air system is critical for controlling pumping loss at desirable EGR rate and air–fuel ratio for meeting both emissions and GHG regulations.

Modern diesel engines need to be designed by using a highly integrated system design approach (Fig. 9.5). The systems engineering approach in diesel engine system design (Xin, 2010, 2011a) reconciles the requirements of combustion/emissions recipe, durability design constraints, engine electronic controls, and vehicle drivetrain matching for optimized precision design (Fig. 9.6).

9.3 Engine air system capabilities at peak torque.

As discussed earlier in engine technology path selection, design strategies for in-cylinder emissions control solutions and NOx aftertreatment solutions are very different in terms of EGR rate, air–fuel ratio, fuel injection timing, combustion noise, turbocharger matching, inter-stage cooling, heat rejections, and requirements on engine durability parameters (e.g., exhaust manifold gas temperature and peak cylinder pressure – with a trade-off nature between each other, as well as compressor outlet air temperature). In-cylinder low-NOx diesel engine system design is characterized by very high EGR rate (e.g., greater than 30% at full load and greater than 40% at part load), low

9.4 Engine air system capabilities at rated power (EGR rate = 33%).

frequent DPF regenerations, high intake manifold boost pressure, high pumping loss, high charge air cooler (CAC) and coolant heat rejections, high peak cylinder pressure (caused by high compression pressure), low in-cylinder temperature, high compressor outlet air temperature, and low exhaust manifold gas temperature. In contrast, SCR engines are opposite, e.g., with no EGR or a low to medium level of EGR rate (e.g., 20–25% at full load), but with two exceptions: low to medium air–fuel ratio, and high peak cylinder pressure (caused by high combustion pressure).

Successful system design of high-NOx SCR diesel engines relies on the

9.5 Simulation models and engine performance and system integration (EPSI) analysis in diesel engine system design.

right requirement on SCR NOx conversion efficiency (Figs 9.2, 9.7, and 9.8) and the right selection of EGR rate. In general, fuel consumption decreases as engine-out NOx level increases, but this is not always true and there is also a diminishing return, depending on the fuel consumption reduction mechanisms that are affected by the EGR rate (i.e., mostly air system related) and fuel injection timing (i.e., mostly combustion system related). Another design consideration in determining the optimum SCR NOx conversion efficiency is the balance between fuel and urea consumptions, and there is an optimum conversion efficiency corresponding to the minimum combined consumption of both (Stanton, 2009).

The subsystem interactions of SCR diesel engines between the air system and the combustion system can be well explained by the engine system design methodology and parametric simulation charts shown in Figs 9.7 and 9.8 on the sweepings of EGR rate and fuel injection timing at peak torque and rated power. NOx model plays a critical role in this universal system design approach of hardware sizing and virtual calibration for different engine-out

The '3-3-3-6' approach in diesel engine system design:

• 3 categories of analysis:
- 1. parametric analysis
- 2. design point analysis
3. failure/case analysis.
• 3 types of full-load engine system design analysis and data format:
Type 1: single hardware/controls configuration, single power rating, and multiple ambient temperature and altitude conditions
Type 2: multiple hardware/controls configurations, single power rating, and single ambient temperature and altitude condition
Type 3: single hardware/controls configuration, multiple power ratings, and single ambient temperature and altitude condition.
• 3 guidelines of handling combustion/emissions recipe, durability constraints, and hardware/controls assumptions:
Guideline 1: no change to realistic/existing hardware, de-rate to obtain maximum allowable power rating without violating durability
Guideline 2: no change to realistic hardware, do not de-rate, simulate target power rating to show which durability constraints are violated
Guideline 3: change hardware to unrealistic designs, do not de-rate, simulate target power rating to illustrate all durability design constraints and emissions recipe are not violated.
• 6 steps of design optimization for engine system design specifications:
Step 1: determine critical engine speed–load operating modes and ambient/ altitude conditions
Step 2: determine combustion/emissions recipe for engine system design targets
Step 3: determine durability design constraints
Step 4: determine engine controls logic, allowable changes in engine hardware and software calibration
Step 5: determine drivetrain matching requirements and constraints (if applicable)
Step 6: tune engine cycle simulation models, generate system design points based on the four quadrants shown below.

9.6 The four quadrants of engine system design and the need to balance all the trade-offs via system optimization.

NOx emissions levels. It expands the engine system design approach from the air system (Fig. 9.9) to the fuel injection and combustion system (Fig. 9.2), e.g., for evaluating BSFC improvement potential. Therefore, adding an emissions model into system design is a great leap in the way of handling

9.7 Diesel engine subsystem interactions at peak torque and system optimization (use VGT, wastegate fully closed, air–fuel ratio = 18.5).

9.7 Continued

9.8 Diesel engine subsystem interactions at rated power and system optimization (use VGT, wastegate fully closed, air–fuel ratio = 19.5)

9.8 Continued

emissions recipes. The NOx model used here is built with a neural network by using DoE test data. Figures 9.7 and 9.8 show that EGR is very effective in reducing NOx emissions, and using a medium EGR rate can significantly reduce the requirement on SCR NOx conversion efficiency, considering the fact that low NOx conversion efficiency during the cold portion of the FTP cycle is still a great challenge to the industry. The diminishing return in BSFC reduction by advancing fuel injection timing is clearly illustrated in Figs 9.7 and 9.8.

Moreover, there is a pumping loss raising threshold in EGR rate, beyond which engine delta P, pumping loss, and BSFC all sharply increase due to the excessively small turbine area used to try to maintain the required minimum air–fuel ratio. The three big circles in Figs 9.7 and 9.8 illustrate the comparison 'horizontally' (i.e., point A compared with point B at the same NOx level) and 'vertically' (i.e., point B compared with point C at the same EGR rate) when trying to select an engine system design point from

9.9 The effect of air–fuel ratio on engine delta P.

the parametric sweeping charts. They show that in order to reach a given engine-out brake specific NOx (BSNOx) target that is compatible with an achievable SCR conversion efficiency, the higher-EGR strategy (i.e., point B) is better than the lower-EGR strategy (i.e., point A) because point B can use more advanced fuel injection timing to reduce BSFC than point A.

In Fig. 9.7, the turbine 'Phi' corrected mass flow rate represents turbine area. The combination target of EGR rate and air–fuel ratio determines the required turbine area and EGR circuit flow restriction (e.g., with EGR valve fully open at peak torque). A lumped EGR valve opening flow coefficient equal to zero represents that the EGR valve is fully closed. A lumped EGR flow coefficient equal to 0.6 represents that the EGR valve is fully open in this case. A larger EGR circuit flow coefficient means less restrictive in terms of pressure drop across the entire EGR circuit and is usually more desirable. The required EGR circuit flow coefficient is determined based on the EGR flow rate and the engine delta P created by the turbine area that drives the target air–fuel ratio and EGR rate. Note that the greatest EGR circuit flow coefficient does not occur at very low or very high EGR rates. Instead, it occurs at medium EGR rates (e.g., 20–25% at peak torque). In Figs 9.7 and 9.8, engine delta P exhibits a 'flat' trend vs. EGR rate (e.g., at peak torque) or a low 'optimum valley' at 20–25% EGR rate (e.g., at rated power) as an air system characteristic. The reasons for the flat shape of engine delta P curves and BSFC curves at EGR rates less than the 30% threshold are due to the following:

∑ decreasing pressure drop of exhaust restriction due to colder exhaust temperature and lower exhaust volume flow rate at higher EGR rate
∑ increasing volumetric efficiency when EGR rate and intake manifold gas temperature increase
∑ increasing high-pressure-stage turbine and compressor efficiencies due to increasing boost pressure (a unique characteristic of two-stage turbocharging).

All of the above reasons depress the increase of exhaust manifold pressure as EGR rate and intake manifold pressure increase, and therefore result in basically constant or even decreasing engine delta P as EGR rate increases. Moreover, it should be noticed that lower EGR rate leads to a lower requirement of compressor pressure ratio and possible single-stage turbocharging. However, this low-EGR and low-boost strategy requires higher SCR conversion efficiency and higher durability design limit of exhaust manifold gas temperature, as illustrated in Fig. 9.8.

Lastly, it should be noted that the choice of engine system design point at full load (especially at rated power) is also limited by engine durability design constraints, particularly peak cylinder pressure, exhaust manifold gas temperature, compressor outlet air temperature, charge air cooler inlet air temperature, and heat rejections. The design point can be selected by checking the crossover between a constant-NOx target characteristic curve (e.g., the line linking points A and B) and a durability limit line (e.g., exhaust manifold gas temperature), or alternatively the crossover between two most stringent durability limits on the parametric design charts (Fig. 9.8). The engine system design methodology described in Figs 9.7 and 9.8 provides a powerful and universal approach to understanding complex subsystem interactions and optimization for not only in-cylinder low-NOx engines but also SCR high-NOx engines.

9.9 Summary of next-generation technologies for heavy-duty vehicles

Heavy-duty engine and commercial vehicle technologies are driven by legislative requirements and market demands. Bold goals of diesel engine development have been continuously proposed during the past 20 years till today, for example in the following milestone events (Fig. 9.10): US EPA 2004–2007–2010 NOx and PM emissions regulations, US 2014–2017 GHG regulations, and the research goal of 55% thermal efficiency of on-road diesel engines. Historically, diesel engine brake thermal efficiency had been steadily increasing from 34% in the 1960s to 44% in the early 2000s, and reduced to 42% in 2004 (due to the use of cooled EGR to meet the 2004 emissions) and then slightly recovered to 43% during 2004–2010 despite more stringent emissions regulations. The thermal efficiency recovery or large increase is more prominent for SCR engines due to their more advanced fuel injection timing. However, the BSFC reduction from 2007's 1.2 g/(hp.hr) NOx DPFonly engines to 2010's 0.2 g/(hp.hr) NOx DPF-plus-SCR engines was not large (only approximately 1–3% gain in thermal efficiency), mainly due to SCR's limited NOx conversion efficiency at that time.

Beyond 2012, engine thermal efficiency is expected to increase at an accelerated pace. People in the diesel engine industry start to talk about '30–300–3000' technologies or new bold performance goals as future trends, i.e., '30' representing 30 bar BMEP high power density; '300' representing 300 bar peak cylinder pressure to allow for very advanced fuel injection timing, high engine compression ratio, and aggressive downsizing in order to enable high thermal efficiency; and '3000' representing 3000 bar maximum fuel injection pressure for in-cylinder NOx and soot controls and better combustion efficiency. Other similar bold goals exist for durability/reliability and cost attributes such as a longer B10 life and 'NOx aftertreatment delete'.

Engine thermal efficiency improvement is critical for achieving great freight efficiency (Fig. 9.1). Potential freight efficiency improvement and CO2 reduction come from three parts:

1. vehicle
2. engine
3. operations (driver behavior).

They have different importance in distinct applications. By examining vehicle energy balance, it is found that at low vehicle speeds (e.g., less than 35 mph), engine losses dominate the total energy consumption. At medium speeds (e.g., 35–55 mph), tire friction dominates. At high speeds (e.g., greater than 55

9.10 North America on-highway diesel engine historical technology development.

mph), aerodynamic resistance dominates. In line haul applications, vehicle, engine, and operations have approximately equal shares (33%) in efficiency improvement potential. However, in vocational applications (e.g., refuse trucks), the vehicle and operations potential can reduce to approximately 10% for each, and the engine potential may increase to as high as 80%.

Current on-road diesel engines have approximately 42% brake thermal efficiency at full load, with 28% of fuel energy wasted in exhaust gas (including 4% going to pumping loss), 28% of fuel energy dissipating to cooling media as heat rejections to the ambient (including 4% going to mechanical friction and parasitic accessory power), and 2% going to miscellaneous heat losses. There are strong interactions between heat rejections and exhaust gas energy, mainly depending on EGR rate and pumping loss. When EGR rate increases, heat rejections become higher and exhaust energy becomes less. Pumping loss is related to EGR rate and air–fuel ratio (or essentially EGR circuit flow restriction, turbine area, and turbocharger efficiency). Such an energy distribution change affects the strategy of waste heat recovery. A roadmap for achieving 50–55% engine thermal efficiency can be planned by allocating improvement targets for each part of the energy distribution or each engine subsystem (Fig. 9.11).

With all the above-mentioned fast-paced dynamic changes over the diesel engine history and the bold goals in the industry as well as the complicated vehicle/engine physics for efficiency improvement, people ask the questions 'where will the next generation diesel engine and heavy-duty vehicle technologies head towards?', 'what will be the main stream technologies and how well are they ready for commercial production?', and 'what role can the government play to guide and help the industry?' These questions can be answered by understanding the current four-year 'SuperTruck' research program funded in 2010–2014 by the US government (Department of Energy) and industry participating partners (Stanton, 2009, 2010; Lei, 2009; Aneja et al., 2009, 2011; Amar et al., 2011; Zhang et al., 2009; Sisken et al., 2010; Eckerle, 2011; De Ojeda, 2011; Jadin and Taylor, 2011a, 2011b; Amar, 2012; De Ojeda and Rajkumar, 2012; Jadin and Nine, 2012a, 2012b; Koeberlein, 2012; Koeberlein and Newhouse, 2012; Rotz and Sisken, 2012). One lesson learned by the industry was a significant drop in engine thermal efficiency when the US EPA 2002/2004 emissions regulation was enforced and when EGR was introduced. With stronger support of government funded research after 2004 (Davis, 2011), large thermal efficiency drop was avoided in 2007–2010 emissions compliance. Therefore, strong support from government–industry alliance on advanced engine/vehicle research is necessary for guiding the industry to achieve super high engine thermal efficiency and freight efficiency.

The SuperTruck research program has established the following objectives:

9.11 Heavy-duty vehicle and diesel engine technology improvement options.

∑ Demonstrate 50% or greater brake thermal efficiency of a clean (the US EPA 2010 emissions) and efficient engine in a test cell at an operating condition indicative of a 65,000 lbs vehicle travelling on a level ground at 65 mph. The increased thermal efficiency is targeted for production in 2015. This vehicle operating condition corresponds to approximately 260 hp power for a heavy truck, i.e., at 1,000 rpm speed and 1,366 ft.lb torque (80% load or close to A75 US SET mode), or 1,500 rpm 910 ft.lb (55% load or close to B50 mode), or 2,000 rpm 683 ft.lb (55% load).
∑ Demonstrate 50% or greater improvement in freight efficiency (in freight-ton-miles per gallon) over a defined driving cycle using the above engine. The 50% efficiency improvement is split to 20% for the engine and 30% for the vehicle.
∑ Demonstrate 68% improvement in freight efficiency over a defined 24-hour duty cycle (i.e., the above driving cycle plus an extended idle period) representative of real world line haul applications.
∑ Demonstrate through technology scoping 55% brake thermal efficiency of the engine (targeting production in 2020).

Cummins, Daimler Truck North America, Navistar, and Volvo are active industry participating members in the SuperTruck program. The program has identified many cutting-edge technologies as follows. The data in the parenthesis below are a directional estimate of average percentage of improvement in SuperTruck driving cycles that cover the entire engine speed–load domain. *f*VFh is vehicle freight efficiency improvement percentage. *f*ETh is engine thermal efficiency improvement percentage. For example, *f*ETh = 5% means thermal efficiency increases from 42% to 44.1%. Different technologies have a distinct impact on fuel consumption at different engine speeds and loads. Because both US SET 13-mode (for line haul trucks) and FTP cycle (for vocational trucks) certifications in GHG regulations emphasize the fuel consumption at high-speed and high-load heavy-duty modes, technology assessment should focus on not only part load but also full load. Major advanced vehicle and engine technologies are planned in the following as a roadmap for fuel economy improvement (Fig. 9.11):

∑ vehicle tractor and trailer aerodynamic drag reduction (*f*VFh = 15%)
∑ tire rolling resistance reduction (*f*VFh = 3–4%)
∑ drivetrain, transmission, and axle improvement (*f*VFh = 3–4%)
∑ anti-idling technologies for idle reduction and electrified parking, energysaving auxiliary power unit (APU), and energy storage systems for idle management, e.g., power demand less than 3 kW (*fVF^h* = 8–10%)
∑ predictive cruise control using on-board GPS and 3D digital maps to anticipate road grade and speed limit changes to improve vehicle fuel economy (*fVF^h* = 2–3%)
∑ road speed limiting and driver operation management (i.e., predictive

torque management, smart gearing, acceleration aggressiveness control; *f*VFh = 2–3%)

∑ heavy-duty hybrid powertrains and electric drive batteries (*f*VFh = 4–6%)
∑ vehicle and engine weight reduction (*f*VFh = 3–5%)
∑ advanced fuel injection timing in engine calibration combined with SCR or other NOx aftertreatment for high engine-out NOx (*f*ETh = 10–12%)
∑ increased peak cylinder pressure enabled by cost-effective materials of engine cylinder head and block (to allow higher engine compression ratio, more advanced fuel injection timing, or higher EGR rate; *f*ETh = 1–2%)
∑ engine downsizing to achieve higher BMEP (*f*ETh = 4%)
∑ engine down-speeding with acceptable durability and vibration (*f*ETh = 1%)
∑ exhaust waste heat recovery with turbocompounding (*f*ETh = 2%)
∑ exhaust and/or EGR waste heat recovery with organic Rankine cycle (*f*ETh = 5–7%)
∑ advanced fuel system (e.g., higher injection pressure) and combustion system optimization (*f*ETh = 2%)
∑ advanced mixed-mode high-efficiency clean combustion (e.g., lowtemperature combustion, lifted flame combustion, stoichiometric diesel combustion; *f*ETh = 3%)
∑ dual-fuel (diesel plus gasoline) engine with reactivity controlled compression ignition (RCCI) combustion (*f*ETh = 7%)
∑ closed-loop feedback combustion controls (*f*ETh = 1%)
∑ engine and vehicle accessory power reduction (e.g., variable flow pumps, flexibly controlled electrically driven auxiliary components; *f*ETh = 1–2%)
∑ engine friction reduction (*f*ETh = 1%)
∑ lower oil viscosity and durability compatibility (e.g., engine thermatic oil cooler to control oil temperature, *f*ETh = 1% in cold weather)
∑ reduced pumping loss with optimized EGR rate, lower air–fuel ratio, low-restriction EGR system, and improved compressor and turbine efficiency (*f*ETh = 2–4%)
∑ mechanical or electrical supercharging boosting assist (*f*ETh = 1%)
∑ variable geometry turbocharger (*f*ETh = 2%)
∑ VVA and Miller cycle (*f*ETh = 2%)
∑ variable intake swirl (*f*ETh = 0.5%)
∑ lower in-cylinder heat transfer losses to coolant and better insulation of the cylinders and the exhaust system (*fET^h* = 1%)
∑ advanced electronic transient controls with online parameter estimation to optimize real-time (RT) emissions and fuel consumption (*fET^h* = 2%)
∑ reduced fuel consumption in aftertreatment thermal management (e.g., low-

temperature high conversion efficiency NOx aftertreatment, minimizing soot loading and active DPF regeneration; *f*ETh = 1%)

∑ pressure drop reduction in aftertreatment devices (*f*ETh = 0.5%)
∑ alternative NH3 delivery systems for NOx aftertreatment (e.g., solid ammonia NOx reduction)
∑ low carbon fuel formulations.

The total vehicle freight efficiency improvement *f*VTh (vehicle and operations only) is approximately 45%. The total engine thermal efficiency improvement *f*ETh (engine only) is approximately 55%. The combined total freight efficiency improvement from vehicle, operations, and engine is approximately 100%. In general, there are no giant leaps in each design or calibration measure for vehicle freight efficiency or engine thermal efficiency improvement, but the total potential is large. The only outstanding single large gain is heavy-duty truck aerodynamic resistance reduction, which has been reducing over past several decades (e.g., from the aerodynamic drag coefficient 0.85 in the 1950s to 0.55 in the 2000s), and the SuperTruck direction is moving towards the 'high-speed locomotive train' type of vehicle body design, which has a very streamlined and smooth body shape and ultra-low drag coefficient. A second large gain in fuel consumption reduction is due to fuel injection timing advance in combination with SCR, but it is accompanied by higher engine-out NOx and peak cylinder pressure.

Downsizing in engine displacement and down-speeding for revised engine torque curve are two other noticeable trends of future engine development. Downsizing provides higher BMEP and lower BSFC; enables lower pumping loss and friction; reduces engine weight and packaging space; and also gives optimized exhaust thermal signature and freight efficiency. Engine down-speeding provides higher BMEP, lower pumping loss and friction, and lower BSFC and engine noise. Moreover, limiting vehicle speeds can provide reduced cruise load and aerodynamic resistance.

In technology selection, cost reduction and durability have paramount importance for commercial success. Weight is also important, and adding weight (via adding a technology device such as Rankine cycle or hybrid powertrains) may improve engine thermal efficiency but reduce vehicle freight efficiency. On the other hand, certain technologies such as engine downsizing may help both thermal efficiency and weight reduction. Moreover, synergy effects must be taken into account in system design since different technologies are not necessarily addable, and some of them may actually conflict with each other or are redundant in function. For example, simultaneously using VGT and intake VVA may be redundant in the function of reducing engine pumping loss at high speeds.

9.10 References

Amar P (2012), 'SuperTruck development and demonstration of a fuel-efficient class 8 highway vehicle – vehicle systems', 2012 DOE Hydrogen and Fuel Cells Program and Vehicle Technologies Program Annual Merit Review and Peer Evaluation Meeting, Washington DC, 14–18 May. Available at: http://www1.eere.energy.gov/ vehiclesandfuels/resources/proceedings/2012_merit_review.html
Amar P, Gibble J and Brantley K (2011), 'Volvo SuperTruck overview and approach', Proceedings of the Directions in Engine-Efficiency and Emissions Research (DEER) Conference Presentations, Detroit, MI, 3–6 October.
Aneja R, Kalish Y and Kayes D (2009), 'Integrated powertrain and vehicle technologies for fuel efficiency improvement and CO2 reduction', Proceedings of the Directions in Engine-Efficiency and Emissions Research (DEER) Conference Presentations, Dearborn, MI, 3–6 August.
Aneja R, Singh S, Sisken K, Rotz D and Ziegler M (2011), 'SuperTruck 50% improvement in class 8 freight efficiency', Proceedings of the Directions in Engine-Efficiency and Emissions Research (DEER) Conference Presentations, Detroit, MI, 3–6 October.
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9.11 Appendix: units and unit conversion

Units

absolute absolute pressure

Btu/min British thermal unit per minute, a unit of power

deg degree or °, a unit of angle ft foot, a unit of length

ft.lb foot pound-force (lbf), a unit of torque

g gram, a unit of mass gauge gauge pressure

hp horsepower, a unit of power

hr hour, a unit of time in inch, a unit of length

inHg inch Hg or inches of mercury, in Hg, or "Hg, a unit of measurement

for pressure

K Kelvin, a unit of temperature

L liter, a unit of volume

lb pound-force, lbF or lbf, a unit of force

lb/(hp.hr) pound-mass (lbm) per horsepower per hour, a unit of specific

fuel consumption rate

lb/hr pound-mass (lbm) per hour, a unit of mass flow rate lb/min pound-mass (lbm) per minute, a unit of mass flow rate

lbs pounds, the plural form of lbm or pound-mass, which is a unit

of mass

m meter, a unit of length mile a unit of length or distance

mpg mile per gallon, a unit of fuel economy

mph mile per hour, a unit of speed

ppm parts per million, a unit of volume concentration

psi pound-force per square inch, or lbf/in² , a unit of pressure or

stress

psia psi, absolute pressure

278 Alternative fuels and advanced vehicle technologies

r/min rpm, revolution per minute, 1 r/min = (1/60) s–1

s second, a unit of time

°C degree Celsius, a unit of temperature °F degree Fahrenheit, a unit of temperature

Unit conversion

$$(^{\circ}F - 32) \times 5/9 = ^{\circ}C ; (^{\circ}F - 32) \times 5/9 + 273.15 = K$$

1 Btu = 1055.06 J

1 Btu/min = 0.0175843 kW

1 foot = 0.3048 m

1 ft.lb = 1.35582 N.m

1 g/(hp.hr) = 1.341 g/(kW.hr)

1 gallon = 0.00378541 m³ = 3.78541 L

1 hp = 0.7457 kW

1 inch = 0.0254 m

1 inHg = 0.0338638 bar

1 lb = 4.448 N

1 lb/(hp.hr) = 608.28 g/(kW.hr)

1 lb/hr = 1.26×10–4 kg/s

1 lb/min = 0.00756 kg/s

1 lbm = 0.4536 kg

1 mph = 1.609 km/hr = 0.447 m/s

1 mpg is equivalent to 2.352 L/km

1 mile = 1609.34 m

1 psi = 0.06895 bar

1 ton = 1000 kg

Improving the environmental performance of heavy-duty vehicles and engines: particular technologies

Q. Xin and C. F. Pinzon, Navistar, Inc., USA

DOI: 10.1533/9780857097422.2.279

Abstract: This chapter reviews various advanced technologies to improve the efficiency and environmental performance of heavy-duty engines, including design and operating characteristics of key subsystems such as fuel injection, conventional and advanced low-temperature combustion, turbocharging, exhaust gas recirculation (EGR), variable valve actuation (VVA), cooling, aftertreatment, waste heat recovery (WHR), and electronic controls. Engine downsizing, down-speeding, down-breathing, turbocompounding, and air system capability performance are summarized by engine system theoretical analysis and numerical simulation for fuel consumption improvement.

Key words: performance, combustion, air system, downsizing, downspeeding, down-breathing, turbocompounding, variable valve actuation (VVA), waste heat recovery (WHR), aftertreatment, N2O.

10.1 Introduction

This chapter builds on the previous chapter by reviewing various advanced technologies to improve the efficiency and environmental performance of engines. It begins by looking at fuel injection systems, conventional and advanced low-temperature combustion systems as well as air flow and turbocharging systems to improve performance. After discussing engine downsizing, down-speeding and down-breathing, this chapter discusses supercharging, turbocompounding, exhaust gas recirculation (EGR) systems, valvetrains and variable valve actuation (VVA), as well as cooling and thermal management systems to improve engine efficiency. It then goes on to review aftertreatment technologies for emissions control, waste heat recovery (WHR) systems, and engine mechanical friction reduction technologies. The chapter concludes by considering electronic controls and on-board diagnostic (OBD) systems to optimize engine performance.

Note: unit conversions are provided in Section 10.19.

10.2 Fuel injection systems and engine performance

Modern electronically controlled common-rail fuel injection systems allow high injection pressures that can be controlled independently of engine speed and fuel injection quantity. Advances in electronics have allowed complete control of injection timing, duration, and dosing processes, including multiple injections, tight tolerances, and high-accuracy fuel delivery (Dingle, 2010). Fuel injection system development has been focusing on improving the mechanical efficiency and capability of the injection system for achieving higher injection pressure and better rate shaping, optimizing swirl–fuel– combustion matching for low emissions and high thermal efficiency, and adding flexibility to handle alternative fuels. The number of spray holes and the nozzle hole size affect spray atomization, and they need to be optimized in combination with intake swirl, combustion chamber bowl shape, and fuel injection pressure. Wall impingement and oil dilution need to be avoided or minimized in fuel injection system development. Moreover, engines are traditionally designed and calibrated for one specific fuel. Changes in fuel type or fuel property may result in potential issues of durability and emissions compliance related to fuel viscosity, spray pattern, operating temperature, and material compatibility.

In conventional diesel combustion, lowering the initial peak in heat release rate curve or the initial combustion temperature can help reduce NOx. Shortening the duration of diffusion combustion and heat release rate can reduce fuel consumption, particulate matter (PM), and exhaust manifold gas temperature. In modern low-NOx engines, high EGR rate needs to be used. This causes low air–fuel ratio and high soot emission. In addition to air system technologies that control soot (e.g., VGT, higher swirl at low speeds, transient EGR control, transient supercharging assist), advanced fuel injection technology is critical in developing low-air–fuel-ratio-tolerant concepts. Soot reduction technologies mainly include increased fuel injection pressure, early main injection timing, early pre-injection, and injection timing strategies during transient conditions.

Pilot injection is very beneficial for combustion noise reduction and cold startability improvement. However, it may cause soot emission to increase. Therefore, optimization of pilot injection quantity and timing is required in order to minimize the penalty on soot. High main injection pressure is very effective in reducing soot emission without significant increase of NOx at high EGR levels, i.e., being able to move the NOx–soot trade-off curve (formed by injection timing sweep) to overall lower levels. Optimization of injection rate shaping can simultaneously improve brake specific fuel consumption (BSFC), NOx, and soot. Eckerle (2011) has shown that manipulation of the in-cylinder combustion event and heat release rate through injection rate shaping can significantly alter engine thermal efficiency and emissions and improve the trade-off curves of NOx–soot and NOx–BSFC. Lastly, a closecoupled post injection to the main injection can reduce soot when the post injection pressure is high enough to provide good atomization.

10.3 Conventional combustion technologies and engine performance

Conventional diesel combustion development for in-cylinder low NOx (close to 0.2 g/(hp.hr) level) has been focusing on the concepts of a low air–fuel ratio tolerant, equivalence ratio optimized, and low soot system via incremental optimization of low swirl ratio, two-step combustion chamber, and injection rate shaping. Low in-cylinder heat rejection, high geometric expansion ratio, optimal combustion phasing, high injection pressure, and short combustion duration are general directions for fuel economy improvement. On the other hand, revolutionary changes in combustion technologies have also made significant progresses, mainly in the following three areas: lifted-flame diffusion combustion, low-temperature combustion (LTC), and feedback control of combustion and fuel quality.

In conventional combustion optimization, one key parameter is engine geometric compression ratio. It is well known that higher compression ratio results in higher thermodynamic cycle efficiency. However, higher compression ratio also leads to higher NOx emissions due to increased in-cylinder gas temperature, higher peak cylinder pressure, and greater mechanical friction due to increased gas loading acting on tribological components. More advanced fuel injection timing can reduce BSFC and increase peak cylinder pressure. At a fixed structural design limit of peak cylinder pressure, higher compression ratio and more advanced injection timing have competing effects for BSFC reduction and therefore need to be balanced or optimized. On the other hand, at fixed fuel injection timing, peak cylinder pressure is affected by intake manifold boost pressure and compression ratio, and the boost pressure is affected by power rating, EGR rate, and air–fuel ratio. High air–fuel ratio can be achieved basically by two different measures, small turbine area or high turbocharger efficiency. These two methods have different consequences on pumping loss and engine thermal efficiency. Using a small turbine area to achieve high air–fuel ratio causes high engine delta P and pumping loss, while using high turbocharger efficiency does not have such a negative consequence. Therefore, in order to achieve high thermal efficiency, the purpose of raising the structural design limit of peak cylinder pressure should be set usually for accommodating more advanced fuel injection timing (which is very effective in reducing BSFC), rather than for accommodating higher compression ratio or higher boost pressure. These points can be clearly illustrated in Fig. 10.1 by the small and diminishing return in thermal efficiency gained by the increased peak cylinder pressure that

10.1 The effect of peak cylinder pressure on engine thermal efficiency at constant fuel injection timing: (a) at fixed EGR rate, parametric sweeping simulation of VGT vane opening and engine compression ratio with fixed turbocharger map at B100 mode and (b) at fixed EGR rate, parametric sweeping simulation of turbine efficiency and engine compression ratio with fixed VGT vane opening at B100 mode.

is caused by higher compression ratio or boost pressure. Figure 10.1 presents two sets of simulation analysis, one for achieving higher boost pressure by using practical turbocharger efficiencies (i.e., using fixed turbocharger maps), and the other for achieving higher boost pressure by using artificially high (sometimes unrealistic) turbocharger efficiencies. In fact, the former set of data show that there is a penalty in thermal efficiency when boost pressure and peak cylinder pressure become very high, owing to excessively high pumping loss. In summary, developing a higher peak cylinder pressure capability for engine thermal efficiency improvement needs to be carefully planned to differentiate the needs coming from accommodating higher power rating, more advanced injection timing (for higher engine-out NOx), higher EGR rate (for reduced NOx), or higher air–fuel ratio (for reduced soot). For some of these needs, if the turbocharger efficiency is not improved, the engine will encounter high pumping loss and BSFC.

While low-temperature combustion (e.g., premixed charge compression ignition (PCCI), homogeneous charge compression ignition (HCCI)) has achieved great progress in ultra-low in-cylinder emissions control at low brake mean effective pressure (BMEP) levels, at high engine speeds or loads advanced combustion research is moving from conventional diffusioncontrolled combustion towards lifted-flame diffusion-controlled combustion (Fig. 10.2) with very low soot emission (Flynn et al., 1999; Musculus et al., 2002; Pickett et al., 2005; Gehrke et al., 2008; Stanton, 2008; Vishwanathan and Reitz, 2008; Zhang et al., 2011). In conventional combustion, the liquid fuel penetration length is long, resulting in an air–fuel mixture core that possesses high equivalence ratios (e.g., 4–6, corresponding to low local air–fuel ratios) at the end of the penetration length and the beginning of the diffusion plume. This mixing core can form high soot emission. In contrast, in lifted-flame combustion, the length of liquid fuel penetration is

10.2 Illustration of lifted flame combustion (modified based on Stanton, 2008).

greatly reduced and air entrainment is enhanced prior to the 'lift-off length' to increase the 'lift-off length' and the volume of the mixing core so that the local equivalence ratios are greatly reduced (e.g., less than 2) to form much lower soot emission by an order of magnitude at the same NOx level. Lifted-flame combustion requires a small injector nozzle hole size, like the size required by PPCI or HCCI at lower speeds or loads. It also requires very high injection pressure (higher than conventional combustion, even 3,000–5,000 bar), matched with an optimal number of nozzle holes. Too many holes can cause unfavorable spray plume interactions that cause soot to increase. In contrast to premixed LTC that can be used usually only at low loads, lifted-flame mixing-controlled diffusion combustion is a research front in conventional clean diesel combustion for high BMEP. Similar to premixed low-temperature combustion (LTC), lifted-flame produces very low soot.

Closed-loop combustion feedback control (Schiefer et al., 2003; Beasley et al., 2006; Husted et al., 2007; Kaiadi et al., 2008; Nakayama et al., 2008; Hinkelbein et al., 2009; Tatur et al., 2009; Ciaravino et al., 2011; Guido et al., 2011; Kolbeck, 2011) and fuel quality sensing are two research fronts related to adaptive control strategies and sensor technologies for achieving greater emissions robustness and fuel economy at 0.2 g/(hp.hr) engine-out NOx. In future diesel engine controls strategies, NOx emissions, burn rate, combustion stability and noise will potentially be added into the list of controlled variables, along with existing signals such as boost pressure, air flow rate, and injection rail pressure, for the ECU to control fuel injection and combustion parameters more flexibly based on cylinder pressure feedback. Moreover, virtual and real fuel quality sensors can help achieve robust emissions performance while maintaining high thermal efficiency as fuel type or fuel property/quality variation increases (Stanton, 2008). In fact, adaptation to a wide variety of fuel types (including alternative fuels and bio-fuels) and maintaining robust performance and fuel efficiency gains are important for future engines designed and sold for a global market. For example, as cetane number varies, NOx, soot, and fuel consumption vary accordingly. As fuel quality varies (e.g., premium vs. non-premium), NOx and soot emissions also vary slightly, and the variation percentage becomes more prominent when NOx and soot levels become very low under stringent regulations. Moreover, based on exhaust oxygen content, virtual sensors can be developed to estimate the bio-content in the fuel for bio-fuel sensing (Stanton, 2008).

Another noticeable front of conventional diesel combustion research is stoichiometric combustion (Lee et al., 2006, 2007; Chase et al., 2007; Neely et al., 2007; Hoffman and Abraham, 2008; Kim et al., 2009; Sung et al., 2009; Solard et al., 2011; Wu et al., 2011; Maiboom and Tauzia, 2012). It operates compression-ignition engines at stoichiometry and uses a low-cost high-efficiency three-way catalyst (TWC) to control tailpipe NOx, HC, and CO. The high engine-out soot emission is handled by continuously regenerating the diesel particulate filter (DPF) to ensure tailpipe PM emissions compliance. Engine calibration of fuel injection timing and EGR rate can balance engine-out NOx and PM based on the efficiencies of the TWC and DPF. The issues with stoichiometric diesel combustion include fast oil degradation due to in-cylinder soot, high exhaust manifold gas temperature due to running at stoichiometric low air–fuel ratio, and transient controls to maintain stoichiometry during rapid load changes. Moreover, stoichiometric combustion without using a TWC has been explored for transient Federal Test Procedure (FTP) cycle NOx emissions control in ultra-low engine-out NOx strategies when high EGR rate and very low air–fuel ratio are used.

10.4 Advanced low-temperature combustion systems

Diesel engine combustion before the US EPA 2010 emissions regulation was dominated by conventional combustion for a wide range of NOx levels of 0.4 to more than 10 g/(hp.hr). For EPA 2010's 0.2 g/(hp.hr) NOx level and even lower, various LTC modes have been proposed to attempt to control both incylinder emissions and BSFC. LTC has been receiving great attention in the heavy-duty diesel engine industry. LTC includes a variety of innovative and different pre-mixed combustion mechanisms discovered by many researchers, such as PCCI (Kanda et al., 2005, 2006; Zhang et al., 2009; Murata et al., 2010), HCCI (Stanglmaier and Roberts, 1999; Epping et al., 2002; Zhao et al., 2003; Cracknell et al., 2008; Schleyer, 2006; Zhao, 2007), premixed compression ignition (PCI), modulated kinetics (MK), Unibus, and reactivity controlled compression ignition (RCCI) (Reitz, 2010), etc.

A common feature of LTC is to enhance the premixing of fuel and air and to keep combustion temperature low in order to simultaneously avoid NOx and soot formations (De Ojeda et al., 2007, 2008, 2009; Musculus et al., 2011). LTC can also potentially offer low fuel consumption due to short combustion duration. High thermal efficiency and low emissions of NOx, soot, HC, and CO require a precise control of the LTC process on auto-ignition and combustion timing in order to make the cylinder charge of reacting mixture combust in the region of concurrent low emissions on the famous *f*–T diagram commonly used in combustion analysis (Fig. 10.3). LTC usually uses a high EGR rate, high boost pressure, high compression ratio, lean mixture (but with high tolerance for high equivalence ratio or low air–fuel ratio), and fast burn rate to achieve extremely low engine-out NOx and PM emissions and is able to meet the US 2010 emissions standard with in-cylinder solutions. EGR and intake valve closing (IVC) timing are usually used in PCCI or HCCI to control optimum diesel combustion phasing.

10.3 Illustration of low-temperature combustion.

Early PCCI refers to injecting fuel far before top dead center (TDC), and the ignition and burning events occur generally before TDC. Late PCCI refers to injecting fuel after TDC, and the ignition and burning events occur far after TDC. Both early and late PCCI can rely on long ignition delays to achieve good mixing and produce very low NOx and soot at low BMEP. Early PCCI has good stability and low fuel consumption, but it requires a higher EGR rate, and generates higher peak cylinder pressure, higher combustion noise, and a more limited BMEP range than late PCCI. Late PCCI has a narrower combustion stability range and hence usually needs a combustion sensor to control it. Stanton (2008) shows that early PCCI is superior to late PCCI and smokeless rich combustion at low speeds and loads in terms of thermal efficiency at the same low NOx level of 0.2 g/(hp.hr).

LTC usually encounters problems of high HC and CO emissions due to complications in ignition control, and sometimes the problems are severe enough to lead to high BSFC. The high HC and CO emissions are due to relatively low volatility of diesel fuels, fuel condensation and flame quenching on the combustion chamber surface or in the crevice, and spray–wall impingement (Miles et al., 2010). Liquid fuel impingement on

Although HC and CO can be controlled by using a diesel oxidation catalyst (DOC), high BSFC and high CO2 emission are still challenges for LTC to meet greenhouse gas (GHG) regulations. The fuel efficiency benefit of diesel HCCI/PCCI is limited by the current inability of adequately controlling optimal combustion phasing and liquid fuel impingement, especially at high loads. In kinetically controlled LTC, there is only a small combustion window for simultaneous low emissions and high thermal efficiency, and this window is very difficult to control at various speeds and loads. The difference of BSFC between LTC and conventional diesel combustion resides from a complex combination of several aspects as follows. Controlled combustion timing, leaner and premixed mixture, less in-cylinder heat transfer losses, less intake oxygen of LTC may offer some combined advantages in thermal efficiency (e.g., total 7%). However, lower compression ratio, reduced combustion efficiency (related to excessive HC and CO emissions), and hotter intake charge temperature may offset the gain in thermal efficiency to a certain extent (e.g., 3%). Finally, there may be either a net gain or loss in BSFC for LTC, compared with conventional combustion (Musculus et al., 2011).

The low-end bound of load range in LTC operation is limited by ignition and combustion stability. Running LTC at high loads is also an unresolved challenge. The high-load operation of LTC is limited or prohibited by high equivalence ratio (low air–fuel ratio), high soot emission, and excessively high peak cylinder pressure and rise rate. The load span from the minimum to the maximum achievable in PCCI/HCCI is affected by the fuel cetane number. The challenge of implementing LTC not only comes from controlling stable combustion phasing (via EGR and VVA) and controlling the transitions between different combustion modes from low loads to high loads (and vice versa), but also comes from the fact that the combustion chamber and injector nozzle configuration must be compatible with conventional combustion. Although the speed–load range of LTC has been expanding through advanced combustion development, currently conventional diesel combustion still has to be used at high loads. It should be noted that high-load or full-load conditions often are critical modes used in diesel engine system design.

In kinetics-controlled PCCI, seeking an optimum fuel blend to control reactivity is an effective way of extending the BMEP range of HCCI/PCCI. It is worth noting a new emerging combustion mode, RCCI (Reitz et al., 2009; Reitz, 2010; Splitter et al., 2010, 2011a, 2011b, 2011c; Kokjohn et al., 2011; Wagner et al., 2011; Nieman et al., 2012). It is a combustion mode between diesel HCCI and gasoline controlled auto-ignition (CAI) in terms of combustion chemistry. The concept of RCCI is to achieve high thermal efficiency and low NOx and soot emissions across a wide range of engine loads by the mixing of fuels of varied reactivity in the cylinder. RCCI uses direct injection of diesel fuel plus port injection of gasoline or direct additized injection of gasoline (e.g., 75–90% gasoline plus 25–10% diesel) to control in-cylinder charge conditions and operate in a compression-ignition cycle. It is well known that the high volatility of fuel (e.g., diesel and gasoline mixture in-cylinder) can help mixing. As Reitz (2010) pointed out, diesel fuel ignites easily but is difficult to vaporize, while gasoline is difficult to ignite but can vaporize easily. Both fuels have benefits and drawbacks in terms of controlling HCCI/PCCI. Diesel is good for low-load premixed combustion, but can cause combustion to occur too early at high loads, and therefore diesel fuel encounters a load limit at high BMEP. On the contrary, gasoline gives poor combustion at low loads but can offer good combustion at high loads. Therefore, dual-fuel compression-ignition combustion may offer a viable path to resolve the load range limitation problem of HCCI/PCCI in order to properly control combustion timing and cylinder pressure rise rate, and extend the load limits of either pure diesel or gasoline.

It should be noted that adding a diesel-to-gasoline ratio into LTC control provides another powerful dimension of combustion control parameters. RCCI has much higher HC and CO emissions (just like gasoline engines) than conventional diesel combustion and therefore requires HC and CO oxidation catalysts. Although the combustion efficiency of RCCI is lower than conventional diesel combustion (e.g., 97% vs. 99% due to excessive HC emissions), the benefits of RCCI in terms of combustion timing, leaner mixture's equivalence ratio, much reduced EGR rate (e.g., zero EGR) and reduced pumping/heat losses, and less in-cylinder heat transfer can offer a net gain of several percentage points of thermal efficiency increase. It was reported (Reitz, 2010) that RCCI can offer approximately 20% improvement in thermal efficiency over conventional diesel combustion while meeting NOx and PM emissions without aftertreatment. Reitz (2010) also reported that RCCI can reach high thermal efficiencies greater than 50% for both heavy-duty and light-duty engines.

10.5 Engine air flow and turbocharging systems

The air system of diesel engines consists of an air filter, inlet ducts, an intake manifold, an exhaust manifold, intake/exhaust ports, a valvertrain, a turbocharger, an inter-stage cooler (ISC, optional), a charge air cooler (CAC), an EGR system (including an EGR cooler and an EGR valve), exhaust pipes, and intake/exhaust throttle valves. The ISC, CAC, and EGR cooler also belong to the cooling system regarding their coolant side. The amount of fuel that can be combusted to generate power depends on the amount of air available at an acceptable air–fuel ratio for meeting the requirements of emissions, durability constraints control, and engine performance. Turbochargers are typical charge air boosting devices used in internal combustion engines (Baines, 2005; Carter et al., 2010). In order to meet the requirements of the air–fuel ratio and EGR rate in the entire engine speed–load domain, certain turbine control is usually required, for example wastegating or variable geometry turbocharging (Arnold et al., 2005).

Diesel engine turbocharging is a system design topic that needs to be addressed together with other air system components such as the EGR system, intake/exhaust throttle valves, and intake valve timing. NOx and soot are mainly determined by air–fuel ratio and EGR rate for a fixed set of fuel injection parameters. In emissions-driven engine system design, with a variable geometry turbine, turbine area can be flexibly adjusted at any speed–load mode and any altitude condition to meet the EGR driving need and the air–fuel ratio requirement without the penalty of the energy loss due to wastegating flow. However, due to the reasons of cost reduction and durability concern, a fixed geometry turbine is often used. In this case, the turbine area is usually determined according to the most challenging combination of EGR rate, air–fuel ratio, and torque rating near the peak torque mode at sea level or a high altitude condition. Moreover, minimizing turbocharger lag for good vehicle acceleration also needs to be considered in turbine sizing.

At high speeds such as rated power, the wastegate is opened and the EGR valve is regulated to reach the target air–fuel ratio and EGR rate (Figs 10.4 and 10.5). Opening the wastegate can also prevent over-boosting (for controlling the structural limit of peak cylinder pressure) and turbocharger over-speeding. High intake or exhaust flow restriction or closing the throttle valve may slightly increase the EGR rate at a large penalty of air–fuel ratio reduction with almost constant engine delta P. Advancing or retarding IVC

10.4 The effects of EGR rate and air–fuel ratio on engine delta P at rated power at low or medium EGR rate.

10.5 The effect of air–fuel ratio and EGR rate on engine delta P at rated power at high EGR rate.

timing causes simultaneous reduction in both engine delta P and air–fuel ratio. Engine delta P is a good indicator of pumping loss. Engine delta P increases with a smaller turbine area, more closed turbine wastegate opening, and higher engine speed.

The goal of air system design is to balance all related components in terms of flow restriction, efficiency, and thermal characteristics to achieve a target EGR rate and air–fuel ratio at each engine speed–load mode with minimum engine delta P or pumping loss. The effects of different turbine control methods (e.g., VGT and wastegate) on air–fuel ratio, engine delta P, and BSFC are illustrated in Fig. 10.6, showing that there is a large benefit of reduced air–fuel ratio on pumping loss reduction. However, Fig. 10.6 also shows that the benefit reaches a diminishing return at a certain low air–fuel ratio threshold, below which BSFC actually increases due to increased cylinder heat transfer losses and reduced indicated power.

The role of turbocharger efficiency (either compressor or turbine) is that a higher efficiency gives a greater air–fuel ratio at almost the same EGR rate or engine delta P without increasing pumping loss. Turbocharger efficiency should not be under-designed or over-designed, and it needs to be designed together with turbine area and EGR circuit flow restriction in order to reach a target EGR rate and air–fuel ratio at each speed–load mode. Usually, high turbocharger efficiency is desirable because without any increased pumping loss it results in high tolerance of EGR rate, high air–fuel ratio, and low soot, as well as improved combustion, as long as NOx does not become excessively high or there is no over-boosting problem. The ranges of flow rate and pressure ratio of a compressor need to be Engine delta P (mbar)

10.6 The effect of air–fuel ratio on engine delta P.

sufficiently large so that there is no surge or choke for engine operating points.

Compressor efficiency, turbine efficiency, and turbine area (or wastegate opening if applicable) determine the air–fuel ratio. Moreover, high compressor efficiency means low compressor outlet air temperature, and high turbine efficiency provides low turbine outlet gas temperature. Total engine mixture flow rate (fresh air and EGR) and intake manifold boost pressure are related by the definition of engine volumetric efficiency. With a given intake valve Ó

ÔÓ

is determined, boost pressure can be calculated. Exhaust manifold pressure and engine delta P are largely determined by turbine area and EGR circuit fl ow restriction or EGR valve opening. These above described parametric relationships can be thoroughly explained by the following four core equations of the engine air system (Xin, 2011):

$$\eta{\text{vol}} = \frac{2(\dot{m}{\text{air}} + \dot{m}{\text{col}})T{2a}R_{\text{gas}}}{(p2 - \Delta p{\text{TT}} - \Delta p{\text{CAC}})N{\text{E}}V_{\text{E}}}$$

$$p{\text{EGRin}} - p{\text{EGRout}} = f(C{\text{d,EGR}}, \dot{m}{\text{EGR}}, T_{\text{EGRcoolerOut}})$$ [10.1a]

$$p{\text{EGRin}} - p{\text{EGRout}} = f(C{\text{d,EGR}}, \dot{m}{\text{EGR}}, T_{\text{EGRcoolerOut}})$$

$$\approx C_0 + \mathcal{L} \dot{m} \dot{m}$$ [10.1b]

$$\begin{cases} 1 - \left(\frac{p_2}{p_1}\right)^{\frac{\kappa_c - 1}{\kappa_c}} + \eta_C \etaT \eta{TC, mech} \left(\frac{\dot{m}_T}{\dot{m}C}\right) \left(\frac{c{p,T}}{c_{p,C}}\right) \left(\frac{T_3}{T_1}\right) \ \times \left[1 - \left(\frac{p_4}{p_3}\right)^{\frac{\kappa_t - 1}{\kappa_t}}\right] = 0 \end{cases} [10.1c]$$

$$\dot{m}{\rm T} = A{\rm T} \cdot \frac{p3}{\sqrt{R{\rm ex}T3}} \cdot \sqrt{\frac{2\kappa{\rm t}}{\kappa_{\rm t} - 1}} \cdot \sqrt{\left(\frac{p_4}{p3}\right)^{\frac{2}{\kappa{\rm t}}} - \left(\frac{p_4}{p3}\right)^{\frac{\kappa{\rm t} + 1}{\kappa_{\rm t}}}}$$ [10.1d]

where *h*vol is the intake manifold air–EGR mixture non-trapped volumetric effi ciency of a four-stroke internal combustion engine, m˙ air is the overall engine fresh air fl ow rate, m˙ EGR is the EGR mass fl ow rate, *T*2a is the intake manifold gas temperature, *R*gas is a gas constant, *p*2 is the compressor outlet pressure, D*p*iT is the pressure drop associated with the intake throttle valve, D*p*CAC is the pressure drop of the charge air cooler, *N*E is the engine speed, and *V*E is the effective engine displacement. In equation 10.1b, *p*EGRin is the EGR circuit inlet pressure, *p*EGRout is the EGR circuit outlet pressure, *C*d,EGR is the lumped EGR circuit fl ow restriction coeffi cient, *T*EGRcoolerOut is the EGR cooler outlet gas temperature, *C*0 and *C*1 are polynomial coeffi cients. Moreover, *p*1 is the compressor inlet pressure, *h*T is the turbine effi ciency, *h*C is the compressor effi ciency, *h*TC,mech is the turbocharger mechanical effi ciency, m˙ ^T is the turbine mass fl ow rate, *m˙*C is the compressor mass fl ow rate, *c*p,T and *c*p,C are specifi c heat constants of turbine and compressor fl ows, respectively. *T*3 is the exhaust manifold gas temperature, *T*1 is the compressor inlet air temperature, *p*4 is the turbine outlet pressure, *p*3 is the exhaust manifold pressure, k^t is the specifi c heat ratio, *A*T is the turbine area, and *R*ex is an exhaust gas constant.

Equation 10.1a is the defi nition of engine volumetric effi ciency. Equation 10.1b describes the EGR gas fl ow through a lumped EGR circuit fl ow restriction orifi ce. Equation 10.1c refl ects the turbocharger power balance between the compressor and the turbine. Equation 10.1d is the turbine flow equation (an example of axial flow turbines under subsonic flow conditions). Any four parameters, either design/calibration parameters or performance parameters, can be selected as unknowns in equation 10.1(a-d) to solve (e.g., *p*2, *p*3, *A*T, and *C*d,EGR), with all other parameters set as known inputs.

Equation 10.1 shows that in order to reach a given requirement of air and EGR flow rate, the following eight technologies or parameters need to match well within the engine system:

1. valvetrain design (*h*vol)
2. engine displacement (e.g., via cylinder deactivation or downsizing)
3. engine speed selection
4. turbocharging (via turbine area and turbocharger efficiency)
5. intake throttling (D*p*IT)
6. air/EGR charge cooling (*T*2a)
7. exhaust restriction (*p*4, related to aftertreatment or turbocompounding)
8. exhaust manifold gas temperature control (*T*3).

In particular, the joint effects of turbocharger efficiency and turbine area on engine delta P and pumping loss at a given target of air–fuel ratio and EGR rate can be analyzed by using equation 10.1. Increasing turbocharger efficiency can result in pumping loss reduction due to the fact that in order to reach the same air–fuel ratio, a higher-efficiency turbocharger can use a larger turbine area and hence generate lower engine delta P. Equation 10.1 will be used to provide a theoretical analysis of engine downsizing, downspeeding, and down-breathing in Section 10.6.

Properly matching engine operating points on the compressor map and the turbine map can help obtain high turbocharger efficiency. On the other hand, compressor design improvement has been focusing on flow range–efficiency trade-offs in diffuser, casing, and inlet guide vane designs. Optimal design of compressor impellers and advanced casing treatment can improve efficiency, e.g., via better manufacturing quality and controlling the clearance between the compressor wheel and the housing.

Turbine design improvement has been focusing on rotor blade design for efficiency increase while balancing stress, inertia, and durability life. A greater turbine pressure ratio corresponds to a lower tip speed ratio of the turbine. As more EGR is used in modern diesel engines and as the turbine pressure ratio becomes generally greater in the entire engine speed–load domain, the turbine operates more frequently at lower tip speed ratios. Moreover, in pulsed turbocharging, from a crank-angle instantaneous perspective, the pulsating turbine flow in engines that have low emissions and high exhaust manifold pressures causes the engines to operate more frequently at higher turbine pressure ratios and lower tip speed ratios than before. However, conventional turbine blade designs are usually optimized for achieving high turbine effi ciency at medium tip speed ratios, and turbine effi ciency is usually inherently low in the region of lower tip speed ratio.

Future turbine design should be improved to achieve high effi ciency at lower tip speed ratios. In turbine design, forward sweep of the blade angle can move the peak effi ciency to lower tip speed ratios, but this cannot be achieved in a radial turbine without increasing blade stress, and therefore requires mixed-fl ow geometry (Baines et al., 2009). Mixed fl ow turbines generally possess greater effi ciency than radial fl ow turbines. These turbocharger design improvements will lead to an increase in effi ciency and can help engine downsizing to achieve higher BMEP to meet GHG regulations.

10.6 Engine downsizing, down-speeding, and downbreathing

In the early 1990s, 11–13 L diesel engines dominated the heavy-duty truck market. In the 2010s, 13–15 L engines dominated with increased power ratings. in the Co2 age, engine size will reduce with the same or even higher power rating (i.e., higher BMEP or power density) in order to reduce brake specifi c CO2 consumption and engine weight.

It should be noted that pumping loss is basically proportional to the product of engine delta P and engine displacement. At a given target of EGR rate, in order to reduce engine delta P or pumping loss, a theoretical examination of equation 10.2 shows that down-breathing, downsizing, and down-speeding actually are three companion effective techniques. Equation 10.2 is obtained from equation 10.1a by re-arranging the terms in the defi nition of volumetric effi ciency:

$$\eta{\text{vol}} V{\text{E}} N{\text{E}} = \frac{2 (m{\text{air}} + \frac{1}{1000}) T{2a} R{\text{gas}}}{p2 - \Delta p{\text{IT}} - \Delta p_{\text{CAC}}}$$ [10.2]

The reason for these three technologies to be companion is that the term on the left side of equation 10.2 contains volumetric effi ciency, engine displacement, and engine speed in a mathematically interchangeable manner, and these three parameters all only show up once in equations 10.1a–d and therefore their effects on engine delta P are similar. This theory is illustrated in Fig. 10.7.

However, these three technologies also have different effects on pumping loss, air–fuel ratio capability, and BSFC through their own distinct mechanisms. For example, at the same EGR rate, air–fuel ratio, and power rating, downsizing gives a lower pumping loss due mainly to smaller engine displacement rather than lower engine delta P. The downsized engine (e.g., by 10%) requires a downsized turbine area (basically also 10%) to deliver the same EGR rate at peak torque, and therefore the engine delta P may

10.7 Unified theory of engine downsizing, down-speeding, and down-breathing in air system capability charts: (a) down sizing, (b) down-speeding and (c) down-breathing.

remain the same. Downsizing also provides BSFC benefits due to a higher load factor (i.e., higher BMEP, with less proportion of mechanical friction relative to indicated power), reduced mechanical friction (owing to smaller bore or shorter stroke), and less cylinder heat transfer loss. Figure 10.8 illustrates the status of power density and engine size of recent North American

10.8 Competitive benchmarking analysis of diesel engine power density and cylinder bore diameter: (a) 2008 North America heavyduty diesel engines and (b) 2008 North America on-highway heavyduty diesel engines.

heavy-duty diesel engines. Downsizing will move the data distribution in the figure towards smaller bore and higher power density.

Down-speeding (Mayer et al., 1982; Ostrowski et al., 2012; Keidel et al., 2012) can reduce BSFC via reduced engine delta P, increased load factor, and reduced mechanical friction. The turbine area may need to be downsized in some down-speeding strategy and therefore it may increase engine delta P and offset the gain in pumping loss reduction to a certain extent. There is a limit of BMEP for either downsizing or down-speeding. Beyond this limit, boost pressure and engine delta P will increase to an excessive level so that there is not any BSFC benefit, unless turbocharger efficiency improves. Moreover, above that BMEP limit, peak cylinder pressure, exhaust manifold pressure, and compressor outlet air temperature will also become unacceptably high. Figure 10.9 illustrates the status of power density and mean piston speed of recent North American heavy-duty diesel engines. Down-speeding will move the data distribution in the figure horizontally towards lower engine speeds.

Down-breathing refers to reducing volumetric efficiency by using an advanced thermodynamic cycle (e.g., Miller cycle), early IVC timing (i.e., reduced breathing duration), and ultra-high boost pressure. With a given turbine area or wastegate opening, advancing IVC timing provides simultaneous reduction in both engine delta P and air–fuel ratio. Downbreathing, especially coupled with turbine wastegating elimination, can reduce BSFC via decreased engine delta P, shorter intake valve event duration, lower pumping loss, good expansion/internal cooling effect at bottom dead center (BDC), and increased compressor efficiency due to better positioning of engine operating points on the compressor map caused by higher boost pressure. However, excessive down-breathing results in too low air–fuel ratio or too low engine delta P (negatively affecting EGR driving). A downsized turbine area increases engine delta P and air–fuel ratio and may offset the effect of down-breathing. However, down-breathing combined with wastegating elimination can maintain the same air–fuel ratio while achieving pumping loss reduction. Detailed analysis of down-breathing is provided in Fig. 10.24 in Section 10.10.

Downsizing is a very effective way of reducing BSFC in the entire engine speed–load domain, especially at low loads or high speeds (Mayer et al., 1982; Ecker et al., 2000; Ge et al., 2011; Saulnier and Guilain, 2004; Bargende and Berner, 2005; Banks et al., 2009; Fraser et al., 2009; Lumsden et al., 2009; Chadwell and Walls, 2010; Kirwan et al., 2010; Sisken et al., 2010; Thirouard and Pacaud, 2010; Gheorghiu, 2011; Pohorelsky et al., 2012). Downsizing (or even combined with down-speeding) at the same brake power means increased BMEP so that the engine speed–load modes re-defined by the US SET 13-mode or FTP cycle certification will all have elevated BMEP levels and hence lower BSFC and brake specific CO2 emission. In

10.9 Competitive benchmarking analysis of diesel engine power density and mean piston speed: (a) 2008 North America heavy-duty diesel engines and (b) 2008 North America on-highway heavy-duty diesel engines.

fact, there are two types of 'downsizing' in a broad sense, both requiring enhanced turbocharging and higher intake manifold pressure. The first type is called 'constant-power downsizing', i.e., keeping the same power rating and full-load torque curve while reducing engine displacement. The second type of downsizing should actually be called 'up-rating', i.e., raising the power rating or modifying the full-load torque curve shape while keeping the engine displacement unchanged. Both types have increased BMEP levels, but only the first type is true downsizing and can achieve a large reduction of BSFC. The differences between 'downsizing' and 'up-rating' are explained in detail as follows.

In constant-power downsizing (Table 10.1 and Fig. 10.10(a)), as explained earlier, if the turbine area is not changed, engine delta P will decrease so that the EGR driving capability at peak torque will become insufficient. In order to compensate this effect and maintain the same EGR rate and/or air–fuel ratio at peak torque, a smaller turbine area (basically proportional to engine displacement reduction) needs to be used to restore the same engine delta P (Fig. 10.10(b)). Because engine displacement becomes smaller, pumping loss becomes lower. BSFC becomes lower in the entire speed–load domain, especially at low loads or high speeds (Fig. 10.10(c)). Moreover, intake manifold boost pressure, compressor outlet air temperature, and peak cylinder pressure all become higher (Figs 10.10(d)–(f)).

In up-rating (Fig. 10.11), there are three possibilities or scenarios, depending on whether using a variable geometry turbine (VGT). If a VGT is used, the entire full-load torque curve including the peak torque mode can be raised to higher BMEP levels, and the engine sometimes may not encounter a pumping loss penalty at high speeds because the VGT can flexibly regulate the turbine area, as long as the BMEP level does not become too high and lead to excessively high engine delta P. If a fixed geometry turbine is used and if the peak torque is not increased when the rated power is increased (i.e., reducing the torque backup coefficient), there is usually no need to downsize the turbine area, and therefore the high-speed high-load modes (e.g., rated power) may not encounter much penalty of pumping loss. However, if a fixed geometry turbine is used and if the peak torque is increased when the rated power is increased (e.g., maintaining torque backup in torque curve shape), a smaller turbine area needs to be used in order to maintain the same air–fuel ratio at higher BMEP at peak torque, and this will result in a pumping loss penalty at rated power and will offset some BSFC gain at part-load modes in the overall SET composite BSFC.

As mentioned earlier, there is an optimum limit of BMEP, beyond which pumping loss and other limiting design factors will become excessive to offset all other gains in BSFC. Moreover, it should be noted that when compared at the same BMEP level, downsizing (i.e., reducing engine displacement while fixing rated power) and uprating (i.e., increasing rated power while fixing engine displacement) have a different impact on BSFC. Downsizing can achieve better BSFC (Fig. 10.10(c)), while up-rating may result in worse BSFC at high-speed full load (Fig. 10.11). The sharply increased BSFC at high BMEP levels at rated speed (1,700 or 1,600 rpm) in the heavy-duty diesel engine up-rating simulation in Fig. 10.11 is due to the small VGT area required and the associated high pumping loss, which is partly caused

Table 10.1 Design changes required by engine downsizing

En in ha dw g e r ar e io t as su m p ns d in us e im la io t s u n	Bo io t re ra la ive t to re ba l in se e	S ke tro -b to or e io t ra	P H b in tu r e ar ea l ip l ie t m u r	P L b in tu r e ar ea l ip l ie t m u r	P H co m p re ss or f lo l ip l ie t w m u r	P L co m p re ss or f lo l ip l ie t w m u r	P H b in tu r e io t ar ea ra la ive t to re ba l in se e	S C I f fe ive t e c ne ss d t r te a a ( % ) p ow er	En in g e io co m p re ss n io t ra
Ba l in se e	1	1. 3 1 7	1. 0 4	1. 0	1. 0	1. 0	1	0	1 7
0 % do iz in 1 w ns g	0. 9 6 5	3 1. 1 7	0. 8 8	0 1.	0 1.	0 1.	0. 8 6 4	0	1 7
2 0 % do iz in w ns g	0. 9 2 8	1. 3 1 7	0. 8 2 2	0. 8 3	0. 9	0. 8 8	0. 7 9 0	2 8	1 6. 3
3 0 % do iz in ns g w	0. 8 8 8	1. 3 1 7	0. 7 1 7	0. 6 7	0. 8	0. 7 7	0. 6 8 9	3 0	1 5. 7
% do iz in 4 0 w ns g	0. 8 4 3	1. 3 1 7	0. 6 2 3	0. 5 0	0. 7	0. 6 5	0. 5 9 9	4 0	1 5

Notes: 'HP' refers to high-pressure-stage, 'LP' low-pressure-stage, and 'ISC' inter-stage cooler. Engine valve size is proportional to cylinder bore size.

10.10 (a) The effect of diesel engine downsizing on BMEP, (b) the effect of diesel engine downsizing on engine delta P, (c) the effect of diesel engine downsizing on BSFC change relative to the baseline engine displacement, (d) the effect of diesel engine downsizing on intake manifold boost pressure, (e) the effect of diesel engine downsizing on peak cylinder pressure and (f) summary of BSFC reduction and design constraints of diesel engine downsizing.

10.10 (Continued)

Summary of peak cylinder pressure of heavy-duty diesel engine downsizing (at 1,700 rpm rated speed)

10.11 The effects of diesel engine power up-rating and down-speeding with VGT on turbocharging and BSFC.

10.11 (Continued)

by certain limitations of fixed hardware assumptions used in the modeling such as high exhaust restriction in a fixed-size aftertreatment system, high flow restriction (pressure drop) in a fixed charge air cooler, and the absence of an inter-stage cooler. In fact, these hardware devices can be re-sized larger and improved for increased power rating so that the high BSFC problem at higher BMEP levels can be partially alleviated.

A common challenge of downsizing or up-rating is increased peak cylinder pressure, which needs to be handled by strengthened structural design (Fig. 10.12). Another challenge of up-rating is increased heat rejection that the vehicle cooling system must handle due to increased fueling rate or power. Figure 10.13 illustrates the impact of engine torque curve shape change or down-speeding on vehicle drivetrain matching and selection of rear axle ratio. Proper matching adjustment can re-position the cross-over point between the engine torque curve and the vehicle resistance power curve in order to achieve acceptable engine fuel consumption and satisfactory hill-climbing or acceleration power reserve.

Diesel engines have an inherent advantage over gasoline engines to support downsizing due to diesel combustion's superior tolerance to high boost pressure of turbocharging. Moreover, two-stage turbocharging has a natural advantage over single-stage turbocharging in tolerating the higher boost pressure produced by downsizing. Downsizing is a popular trend in light-duty engines, even in knocking-sensitive and boosting-sensitive gasoline engines (Boggs and King, 2011). It starts to receive great attention in heavy-

10.12 The effect of BMEP on peak cylinder pressure.

10.13 The effect of engine down-speeding on drivetrain matching.

duty diesel engines for the purpose of meeting GHG regulations, e.g., in the SuperTruck research program.

The difference in degree of downsizing between light duty and heavy duty is that light-duty downsizing is very aggressive and receives assistance from hybrid powertrains. For example, when a 2.4 L naturally aspirated MAHLE gasoline engine was downsized to a 2.0 L Turbo GDI engine and 1.2 L Turbo GDI, the New European Driving Cycle (NEDC) CO2 emission was reduced by 9% and 26%, respectively (Blaxill, 2011). When a mild hybridelectric powertrain was added to the 1.2 L engine (with possible power rating change), CO2 was reduced by 35% compared to the 2.4 L engine. When the mild-hybrid engine was further downsized to 0.8 L, CO2 was reduced by 41% (all compared to the 2.4 L engine). The same study also reported that in the US06 driving cycle, when the 2.4 L was downsized to 2.0 L and 1.2 L Turbo GDI, CO2 was reduced by 11.8% and 12%, respectively. When a mild hybrid was added to the 1.2 L, CO2 was reduced by 15.4%. When the mild-hybrid engine was further downsized to 0.8 L, CO2 was reduced by 14.7%, indicating a limit of downsizing in terms of BMEP level for certain speed–load modes. The differences in CO2 reduction benefit between the above two driving cycles are probably due to gasoline engine's limitations in downsizing under certain high-load conditions. Heavy-duty diesel engines will not receive hybrid assist in power rating in the near future, and therefore the extent of downsizing will be more limited compared to light-duty engines.

Heavy-duty diesel engine downsizing benefit is illustrated in the simulation result of the entire engine speed–load domain in Table 10.1 and Fig. 10.10(a)–(f). The results show that engine downsizing is a very effective technology to reduce fuel consumption and CO2 emission. In contrast to some other BSFC reduction technologies (e.g., VGT or Miller cycle) that usually are only effective at high loads or high pumping loss regions, downsizing is very effective in reducing BSFC at low loads as a unique characteristic, and actually more prominent at low loads than at high loads. The BSFC reduction benefit is also more obvious at higher speeds than lower speeds. Compared to other emerging innovative engine technologies used for meeting GHG regulations (e.g., Rankine cycle), downsizing is a relatively more mature, production-ready, cost-effective, and evolutionary technology.

Developing high BMEP and small displacement engines is not an easy task and still faces great challenges in thermo-mechanical durability. When engine displacement is reduced, turbine area also needs to be proportionally reduced in order to drive the same EGR rate and air–fuel ratio (i.e., to maintain the same emissions to ensure analysis results comparable), e.g., maintain sufficient EGR driving capability at peak torque. When engine displacement is greatly reduced, the air–fuel ratio at low loads may become lower than that of the original engine displacement. The possible excessive soot emission problem caused by such air–fuel ratio reduction needs to be assessed carefully.

The simulation results shown in Figs 10.10(a)–(f) at different levels of engine downsizing identify and quantify the design challenges such as elevated peak cylinder pressure, intake manifold boost pressure, exhaust manifold pressure, engine delta P, high- and low-pressure-stage compressor outlet air temperature. There exists an ultimate limit of displacement downsizing and an optimum BMEP level for BSFC (Fig. 10.10(f)). The limit is essentially restricted by the design capability of sustaining peak cylinder pressure, the width (flow rate) and height (pressure ratio) of the compressor map that are related to surge, low-pressure-stage compressor outlet air temperature, and the magnitude of engine delta P that affects valvetrain operation. The trend of engine downsizing pushes engine system configurations towards two-stage turbocharging, inter-stage cooling, reduced engine compression ratio or strengthened capability of sustaining high peak cylinder pressure. Finally, it should be noted that cylinder deactivation is essentially variable engine displacement, an option of flexible downsizing. It can provide a large BSFC benefit in the region of very light loads. However, there are many challenges with cylinder deactivation as detailed in Xin (2011).

10.7 Mechanical and electrical supercharging systems for improved emissions control and performance

As diesel engine-out NOx level becomes lower and emissions control relies less on NOx aftertreatment, the problem of transient air shortage caused by using a high EGR rate in addition to traditional turbocharger lag needs to be resolved in order to raise transient air–fuel ratio to ensure satisfactory soot control and fast load response (e.g., during tip-in transients in highly transient bus cycles). Transient air boosting technologies generally include the following three types:

1. VGT turbocharging
2. air injection
3. supercharging.

The potential of VGT in transient air assist is limited due to turbocharger inertia and the limit of minimum turbine area. Air injection includes using an external air source (e.g., vehicle air tank) to inject high pressure air via short bursts into the intake manifold or injecting hot exhaust gas (e.g., via a burner that consumes fuel) into the exhaust manifold to speed up the turbine.

Mechanical or electrical supercharging (Cantore et al., 2001; Ueda et al., 2001; VanDyne and Wagner, 2008; Birckett et al., 2012) is usually the most effective measure for transient NOx and soot controls, fast transient load response assist, and reducing transient fuel consumption, but at the price of increased hardware cost. The slight transient fuel consumption reduction is believed due to higher air–fuel ratio and the associated improved combustion and reduced pumping loss during transients. Supercharging can also increase low-end torque (e.g., higher peak torque or down-speeding to 800–1,000 rpm by reducing the peak-torque speed) to enable a down-speeding strategy for CO2 compliance or enhanced drivability.

Supercharging has historically been regarded as an inferior technology compared to turbocharging due to its inherent disadvantages from the perspective of thermodynamic second law. This is true for steady-state fuel consumption (shown in Fig. 10.17). However, transient performance benefits and overall engine GHG strategy planning require a renewed and balanced look at the potential of supercharging for modern diesel engines. For example, the down-speeding effect may translate to BSFC reduction at overall SET and ramped mode cycle (RMC) 13 modes or in the FTP cycle. The reduced soot loading in the diesel particulate filter (DPF) may translate to a BSFC benefit of fuel dosing reduction due to increased regeneration intervals. On the other hand, the effectiveness of supercharging depends on how much EGR is used and hence how much exhaust energy is left to the turbocharger. Moreover, it also depends on the type of driving cycle. In certain driving cycles (e.g., line haul trucks), the extra power consumed to spin the supercharger may actually end up with a net penalty of fuel consumption. Furthermore, sometimes the supercharger might feature a clutch to allow engagement or disengagement, and the added complexity may present a durability concern.

10.8 Turbocompounding to improve engine performance

Today's typical on-road modern heavy-duty diesel engines have approximately 42–43% peak thermal efficiency. Most of the remaining fuel energy is dissipated to the ambient through cooling media and exhaust gas. If a lower EGR rate is used, less heat is rejected to the coolant and more heat is left into the exhaust gas. If a higher EGR rate is used, the situation is opposite. Recovering exhaust energy presents a large opportunity to reduce fuel consumption. Turbocompounding and bottoming cycles (e.g., Rankine cycle) are two primary WHR technologies. Through their different mechanisms, these two technologies have very different attributes and implications in performance, durability, packaging, and cost. They affect engine thermal efficiency through different subsystem interactions such as exhaust restriction, pumping loss, interference to turbocharger performance, air–fuel ratio capability, soot emission, bottoming cycle efficiency, and aftertreatment temperature.

Turbocompounding, recognized as an enabler for power density growth, has a long-standing history and was popular in several brands of on-road and off-road production diesel engines since the 1950s (e.g., Cummins NH engine in 1981, Caterpillar's 3406 in 1986, Scania's 11 L engine, Volvo's D12 500TC engine in 2002, Daimler's DDC 15 L engine in 2007, and some military diesel engines). There are generally four types of turbocompounding, in either production or research prototypes:

Type 1: mechanical turbocompounding using a power turbine (Fig. 10.14(a))

10.14 (a) Turbocompounding schematic of mechanical turbocompounding using a power turbine, (b) turbocompounding schematic of mechanical turbocompounding using a turbocharger turbine (also called a mechanical driven turbo), (c) turbocompounding schematic of electrical turbocompounding using a power turbine (also called a turbo-generator) and (d) turbocompounding schematic of electrical turbocompounding using a turbocharger turbine (also called an electrical driven turbo).

10.14 (Continued)

2. Type 2: mechanical turbocompounding using a turbocharger turbine (also called mechanical driven turbocharger, Fig. 10.14(b))
3. Type 3: electrical turbocompounding using a power turbine (also called turbo-generator, Fig. 10.14(c))
4. Type 4: electrical turbocompounding using a turbocharger turbine (also called electrical driven turbocharger or electric turbocharger, Fig. 10.14(d)).

In Type 1 turbocompounding, a power turbine is used at the downstream of the turbocharger turbine to produce mechanical power. The power turbine can be either axial flow or radial flow. The power turbine's shaft is connected to the engine crankshaft through a geartrain for speed reduction. The power turbine usually needs to be located close to the crankshaft due to limitations of mechanical linkage. An isolating viscous fluid coupling is usually required to prevent crankshaft torsional vibration from damaging the high-speed gears and the power turbine. A Type 1 turbocompounding device is usually large, heavy, complex, and expensive. From a thermodynamic perspective, the expansion ratio created by the power turbine, which depends on the speed and the area of the power turbine (modeled as a turbine mass multiplier in Figs 10.15 and 10.16), acts equivalently as an increased 'exhaust restriction' or back pressure for its upstream turbocharger turbine and therefore has a

10.15 (a) Engine performance simulation of mechanical turbocompounding using a power turbine (Type 1) with a wastegate two-stage turbocharger system – BSFC reduction from the baseline and (b) engine performance simulation of mechanical turbocompounding with a power turbine (Type 1) with a wastegated two-stage turbocharger system – engine delta P.

10.15 (Continued)

10.16 Sensitivity simulation of BSFC comparison between Type 1 (mechanical) and Type 3 (electrical) turbocompounding using a power turbine with a wastegated two-stage turbocharger system.

negative impact on turbocharger turbine's air boosting capability or pumping loss. Moreover, due to the fixed mechanical linkage between the power turbine and the crankshaft, at light loads where the exhaust energy is quite low, the power turbine can actually even become a mechanical power consumption device to increase fuel consumption, if the power turbine's output cannot offset its mechanical losses.

Turbocompounding performance also strongly depends on the EGR rate used, as shown in Figs 10.15–10.18. A higher EGR rate leads to less exhaust energy available to the power turbine or the turbocharger turbine. In all the turbocompounding simulation cases, 90% conversion efficiency is assumed for turbocompounding power transfer, and constant air–fuel ratio is assumed to ensure fair comparison (unless specially marked). Figure 10.16 shows that an optimum gear ratio (10 here) between the power turbine and the crankshaft in Type 1 turbocompounding needs to be selected based on air–fuel ratio and BSFC. The trend of air–fuel ratio vs. gear ratio is clear, while the trend of BSFC vs. gear ratio is complicated by the power turbine area at rated power.

In Type 2 turbocompounding, there is no power turbine. A geartrain is used to directly link the crankshaft to the shaft of the turbocharger turbine to control its rotational speed. At a given turbine shaft speed that can be adjusted through a set of gears, the power output or consumption of the turbocharger turbine is determined by the power balance between the turbine and the compressor. The compressor works as a load device to consume power to deliver certain air–fuel ratio, and the turbine is a power producing device whose power can be adjusted by either a turbine wastegate or turbine area (e.g., VGT vane). If the turbine power is greater than the compressor power, a net mechanical power output is transmitted to the crankshaft as in turbocompounding mode. If the turbine power is less than the compressor power, a net power input needs to be transmitted from the crankshaft to the turbine as in supercharging mode. The Type 2 turbocompounding presents an appealing idea of flexibly regulating between the two modes, and such a concept has a profound influence on today's Type 4 electrical driven turbocharger. In fact, the Type 2 device even appeared in the early history of turbocompounding in the Napier Nomad's 12-cylinder two-stroke diesel engine in 1954. Figure 10.17 shows the simulation result of Type 2 turbocompounding using a low-pressure-stage (LP) turbine, with either a high-pressure-stage (HP) wastegate or VGT, actually demonstrating little BSFC benefit. Figure 10.17 also shows that supercharging causes a BSFC penalty in steady-state operation, although engine delta P can be reduced. For example, the point of 20% EGR rate on the curve of running the lowpressure-stage turbine at 90,000 rpm receives 29 kW supercharging power from the crankshaft to boost the compressor to maintain the air–fuel ratio at 19.5, with a low engine delta P of 600 mbar but a BSFC penalty of 5.3%.

10.17 Engine performance simulation of mechanical turbocompounding using a turbocharger turbine (Type 2, mechanical driven turbo) with a two-stage turbocharger system.

10.18 Engine performance simulation of electrical turbocompounding using a turbocharger turbine (Type 4, electrical driven turbo) with a wastegated two-stage turbocharger system.

In Type 3 turbocompounding, the power turbine's output is transmitted to a generator to provide electrification power to drive electrical loads for the engine or the vehicle. Alternatively, the power turbine's output can be stored in batteries for flexible use (Hopmann and Algrain, 2003). Because it does not require complex and bulky mechanical linkages, the power turbine can be located far away from the crankshaft, even at the downstream of all aftertreatment devices in order to maximize the benefits of system performance and packaging. In fact, Type 3 (electrical turbocompounding) can regulate power turbine speed more flexibly at any engine speed and load without the restriction of a fixed mechanical gear ratio. In Fig. 10.16, at 17,000 rpm power turbine shaft speed, the difference of BSFC between Type 1 and Type 3 is caused only by the location of the power turbine relative to the aftertreatment devices.

In Type 4 turbocompounding, an electrical device (for power absorption and releasing) is attached to the turbocharger turbine's output shaft to flexibly realize a turbocompounding (generator) mode to generate power or a supercharging (electric motor) mode to help spin the turbocharger's shaft faster, and hence in theory is able to flexibly adjust engine delta P, pumping loss, EGR driving, and air–fuel ratio. If the generator or motor mounted on the turbocharger shaft is small, this type of turbocompounding can be a very compact package. The generator can be located either in front of the compressor or integrated into the bearing housing between the compressor wheel and the turbine wheel. In the former option, rotordynamics, stress, shaft bending, and packaging are usually challenges. In the latter option, thermal management of the stator and the magnet becomes a critical issue. Although the size is small, Type 4 turbocompounding usually still possesses high rotation inertia, and also has high current, high voltage, and stringent thermal constraints. Figure 10.18 provides the simulation result of Type 4 at constant air–fuel ratios, actually showing very little BSFC benefit, especially at part load.

Since electric auxiliaries can be flexibly modulated to meet system demands and energy storage can be used to run auxiliaries for hotel functions and driver needs to avoid idling, it is desirable to have vehicle electrification for fuel economy improvement (e.g., via large fuel savings due to idle reduction). Type 3 and Type 4 turbocompounding are especially suitable as a part of the electric hybridization and storage strategy. Turbocompounding power can be fed into vehicle electrical demands (e.g., cooling pump, oil pump, cooling fan, power steering, air compressor, air conditioning, cab hotel power supply) or stored in batteries. In fact, turbo-generator has been receiving attention in recent years' research (e.g., the SuperTruck program), and it has the synergy with electrification and hybridization needs of commercial vehicles where motors and generators are needed anyway in electric-hybrid powertrains. Moreover, it should be noted that Type 2 and Type 4 turbocompounding can realize all the benefits of supercharging such as low-end torque enhancement, enabling down-speeding, transient air assist, transient emissions and fuel consumption reduction, and fast load response.

In the evaluation of the BSFC benefit of turbocompounding, two most important issues are:

1. the available compressor power to maintain sufficient and constant air–fuel ratio
2. the increase of exhaust manifold pressure, which is caused by either elevated exhaust restriction due to power turbine's expansion ratio or the increase of turbine pressure ratio. The resulting pumping loss increase may finally produce very small net gain in engine thermal efficiency, even at full load.

It should be emphasized that BSFC comparison needs to be conducted at the same emissions level (or its equivalent in terms of the same air–fuel ratio and EGR rate) in order to ensure fair and clear comparison. In fact, many published literature on turbocompounding research has limitations of comparing different concepts or configurations on BSFC at different NOx and soot levels, which actually makes the comparisons and conclusions invalid or fuzzy.

In summary, turbocompounding has limited advantages in increasing power density and reducing fuel consumption for modern diesel engines, depending on turbocompounding type, turbine configuration, turbine area, transfer gear ratio, turbocompounding installation position, EGR rate, etc. (Fig. 10.19). The BSFC benefit is usually most prominent in high-load or high-speed applications or certain part load (B50) operation. In fact, it was reported in literature that the maximum engine thermal efficiency improvement by Type 1 turbocompounding is 3–5%, or 3% in line haul applications. The performance of power-turbine turbocompounding is determined by the power turbine's speed and area as well as mechanical/electrical energy conversion efficiency. The performance of driven turbochargers is determined by the wastegate opening or VGT vane opening of the turbine and energy conversion efficiency. A variable geometry power turbine can provide flexible control of turbocompounding output and may increase part-load or low-speed output. It may also help control engine air flow rate and transient response. At a given engine speed–load mode and EGR rate, there is an optimum turbine area or wastegate opening that provides the maximum benefit of BSFC reduction. This optimum is a delicate balance between back pressure and exhaust energy conversion. Higher turbocharger efficiency or power turbine efficiency can greatly alleviate the problem of high back pressure in turbocompounding, and can maintain sufficiently high air–fuel ratio. This efficiency requirement sometimes can even become an important prerequisite for realizing any BSFC benefit from a turbocompounding system without sacrificing emissions

10.19 Summary of turbocompounding steady-state performance on BSFC reduction benefit and aftertreatment inlet gas temperature: (a) benefit of BSFC reduction in steady-state operation and (b) aftertreatment inlet gas temperature in steady-state operation.

(Kruiswyk, 2008, 2010). Although mechanical or electrical supercharging can reduce engine delta P, it is accompanied by a large penalty of fuel consumption. Overall, turbocompounding using a power turbine seems more effective than without using a power turbine for engine BSFC reduction (Fig. 10.19). However, the extra weight of the power turbine has a negative impact on vehicle fuel economy.

One limitation of turbocompounding is the cool exhaust gas at the outlet of the power turbine or the turbocharger turbine and at the inlet of the aftertreatment devices (Fig. 10.19). The cool exhaust reduces the effectiveness of the aftertreatment system, e.g., resulting in more frequent active regenerations for DPF or more stringent thermal management requirements for selective catalytic reduction (SCR) and lean NOx trap (LNT). More information on diesel engine turbocompounding can be found in Assanis and Heywood (1986), Tennant and Walsham (1989), Kapich (2002), Hopmann and Algrain (2003), Millo et al. (2006), Hountalas et al. (2007), Greszler (2008), Kruiswyk (2008, 2010), McGuire et al. (2009), Patterson et al. (2009), Divekar et al. (2010), Edwards et al. (2010), Ismail et al. (2012), Eriksson et al. (2012), and Ishii (2009, 2012).

10.9 Exhaust gas recirculation (EGR) systems

Cooled EGR has been used as an effective approach to reduce NOx emissions in gasoline and diesel engines (both light duty and heavy duty) for many years. EGR can reduce NOx because it reduces combustion temperature and intake manifold oxygen concentration. The high soot emission accompanying high EGR rate is usually due to low air–fuel ratio and/or retarded fuel injection timing. EGR also affects the soot loading in lubricant oil and engine wear durability. EGR can reduce combustion noise and change ignition delay. EGR cooler heat rejection is a part of engine coolant heat rejection that needs to be dissipated via the vehicle cooling system. EGR can also reduce exhaust manifold gas temperature. At a constant air–fuel ratio, when EGR rate increases, intake manifold boost pressure, compressor outlet air temperature, and peak cylinder pressure all increase (Figs 10.4, 10.5, and 10.12).

Engine delta P and pumping loss are affected by EGR rate in a complicated way, depending on the changes in volumetric efficiency, exhaust volume flow rate, exhaust restriction, and turbocharger efficiency. As EGR rate increases, compressor pressure ratio needs to increase, and this requires turbine pressure ratio to increase as well. At low or moderate EGR rate (e.g., 10–25%), if increasing EGR rate results in a reduction in exhaust restriction and an increase in volumetric efficiency and turbocharger efficiency, the increase in exhaust manifold pressure can be depressed and hence engine delta P may not increase or may even decrease. However, at a high EGR rate (e.g., above 30–35%), an increase in EGR rate usually results in a large increase in engine delta P (detailed in Section 9.8 and Figs 10.20 and 10.21).

The required EGR rate and the associated EGR system design in terms of maximum mass flow rate capability, EGR cooler flow restriction and fouling, and heat rejection need to be optimized at the engine system level by considering the requirements of engine-out NOx target, turbine area, and intake manifold temperature, etc.

There are two main EGR system configurations commonly used:

1. high-pressure-loop (HPL) EGR
2. low-pressure-loop (LPL) EGR.

The cooled HPL EGR configuration extracts EGR gas from the exhaust manifold and then sends it back into the intake manifold by utilizing the engine delta P created by the turbocharger. The engine delta P for sufficient EGR driving at low engine speeds (e.g., A50 mode to peak torque) needs to be designed carefully in matching between the turbocharger and the EGR circuit flow restriction. Usually, using an intake throttle to drive EGR at low speeds should be minimized or avoided.

In the LPL EGR configuration (Chatterjee et al., 2003), EGR gas is picked up from the downstream of an aftertreatment device (e.g., DPF) and then introduced back into the compressor inlet. All of the exhaust gas flows to the turbine and the mixture of fresh air and EGR gas flows through the compressor. Due to these high flow rates, the compressor and turbine sizes in LPL EGR are larger than those used in HPL EGR. EGR cooling is still required for LPL EGR, and the compressor outlet air temperature can actually become very high. The LPL EGR system usually encounters problems of insufficient EGR driving due to its inferior nature of the available pressure differential across the entire EGR circuit.

Generally, the HPL EGR system is favored over the LPL EGR system due to better effectiveness of handling EGR requirements and simpler packaging arrangement. The Venturi EGR system, EGR pump, and hybrid or dual-loop HPL–LPL EGR system (Czarnowski et al., 2008; Shutty and Czarnowski, 2009) are variants of the HPL or LPL EGR systems. Xin (2011) provides detailed elaboration of EGR system performance and durability.

10.10 Improving conventional valvetrains and the use of variable valve actuation (VVA)

Conventional valvetrains in modern diesel engines mainly include the following two types:

1. pushrod
2. overhead cam (OHC).

10.20 Diesel engine subsystem interactions at peak torque and system optimization (use VGT, wastergate fully closed, air–fuel ratio = 18.5).

10.20 (Continued)

10.21 Diesel engine subsystem interactions at rated power and system optimization (use VGT, wastegate fully closed, air–fuel ratio = 19.5).

10.21 (Continued)

Pushrod valvetrains have been used in heavy-duty diesel engines for a long time for rated speeds around 1,500–3,500 rpm. OHC valvetrains have been usually used in higher-speed light-duty engines. However, recent design trends show that many heavy-duty engines with rated speeds around 2,000 rpm started to use OHC valvetrains in order to achieve better breathing performance.

Valvetrain system design is a complex balance between engine performance and valvetrain dynamics, particularly related to cam and valve spring designs. Engine volumetric efficiency is affected by both valve timing and flow restrictions across the valves and the ports. When valve timing changes, both volumetric efficiency and pumping loss change in a trade-off manner, with volumetric efficiency representing engine gas flow capability (for emissions) and pumping loss representing fuel consumption. Therefore, valvetrain design is actually also a balance between emissions and fuel consumption. Moreover, the operation of compression-release engine brakes used in heavy-duty diesel engines is usually controlled by a braking cam in the valvetrain. General considerations of valvetrain design are illustrated in Fig. 10.22. Engine performance characteristics of valvetrain optimization are illustrated in Fig. 10.23. In-depth discussions on the performance, durability, and design of different types of valvetrains and engine brakes are provided in Xin (2011).

Engine air and gas flows can be regulated at two different levels of resolution:

1. engine cycle average level
2. crank-angle-based instantaneous level.

10.22 Illustration of valvetrain design considerations.

10.23 Engine performance simulation of valve timing effects at constant fueling rate: (a) volumetric efficiency map at 2,100 rpm 80% load, (b) engine brake power map at 2,100 rpm 80% load, (c) volumetric efficiency map at 1,400 rpm full load and (d) engine brake power map at 1,400 rpm full load.

Historical focus of flexibly controlled air systems in heavy-duty diesel engines has been placed at the cycle average level, such as variable geometry turbocharger and the modulation of various gas flow control valves such as wastegate, EGR valve, intake throttle, and exhaust throttle. These cycleaverage flexible air control devices can alter the relationships of the pressures, temperatures, and flow rates between different components in the engine system's gas flow circuit, and hence affect the trade-off relationships between air flow and pumping loss (or fuel consumption).

What is missing from these cycle-average air control devices is the influence from the engine cylinder (i.e., another key element in the circuit) in the gas flow network in terms of the ability of changing the relationships between intake manifold pressure, exhaust manifold pressure, exhaust manifold gas temperature, and cylinder gas flow rate. This breathing influence from the cylinder can be best described by the equation of engine volumetric efficiency (see equations 10.1 and 10.2), which summarizes the 'macro behavior' of engine breathing at a cycle-average level, and is affected by the 'micro behavior' of engine valve flow rate at a crank-angle-based instantaneous level.

A VVA device can alter the 'instantaneous nature' of engine breathing and affect the macro global performance of the engine at different speeds and loads. Moreover, VVA can be used to enable low-temperature combustion (e.g., HCCI) through controlling in-cylinder mixture temperature and gas composition. The impact of VVA on engine performance can be summarized by the following four aspects:

1. exhaust valve opening (EVO) timing
2. exhaust valve closing (EVC) timing
3. intake valve opening (IVO) timing
4. intake valve closing (IVC) timing.

EVO timing affects the balance between expansion work and pumping work, and also affects the exhaust energy available to the turbine. EVC timing and IVO timing affect valve overlap size, residue gas fraction, and valve flow area at the exhaust valve closing side and the intake valve opening side (hence pumping loss). IVC timing significantly affects effective engine compression ratio and volumetric efficiency, as well as the trade-off relationship between air–fuel ratio and engine delta P.

In general, internal combustion engines use two types of VVA mechanisms:

1. cam-driven systems
2. camless systems.

As the currently most common VVA system, the cam-driven mechanism enables variable valve timing (VVT), lift, and duration (Pierik and Burkhard, 2000; Wang, 2007). These changes are typically accomplished by camshaft phasing, cam profile switching, or a combination of these mechanisms. In the camshaft phasing technology, valve events are altered by advancing or retarding the timing of the camshaft relative to the crankshaft in order to advance or retard the intake/exhaust cam events. This technology provides control of the amount of fresh air flowing into the cylinder and control of residual gas (i.e., internal EGR) as well as a change in pumping loss. On the other hand, the cam profile switching mechanism allows for independently changing valve event timing and valve lift by switching between two different camshaft profiles. Such a change in valve lift with engine speed can lead to torque increase over the entire operating range of the engine (Wang, 2007).

The camless VVA system (electromagnetic or electrohydraulic) can potentially provide the greatest control flexibility in the valvetrain, allowing for valve events to be optimized for every operating condition. VVA, especially camless VVA, can be used to greatly improve compression-release engine brake performance (e.g., via two-stroke braking). Despite the advantages of camless VVA, certain issues such as noise, wear, durability, reliability, power consumption, and cost have to be addressed in developing production-viable solutions.

VVA has become very popular in light-duty production gasoline engines but has not seen many applications in heavy-duty diesel engines. One major motivation of using VVA in gasoline engines is to eliminate the pumping loss caused by intake throttling. In fact, VVA can also offer great benefits to diesel engines, as elaborated in Xin (2011). In the SuperTruck program, variable IVC timing has been researched as one possible way of improving engine thermal efficiency to achieve flexible controls on engine compression ratio and low-temperature combustion timing (De Ojeda, 2010a, 2010b). In fact, the Miller cycle with early or late IVC timing has been always regarded as one of the very rare engine technologies that can simultaneously reduce NOx and BSFC (Ke and Pucher, 1996; Clarke and Smith, 1997; Edwards et al., 1998; Millo et al., 2004; Imperato et al., 2009; Yang et al., 2009, 2010; Murata et al., 2010).

For heavy-duty diesel engines using a fixed geometry wastegated turbine, using VVA with early IVC timing in combination with wastegating elimination (WE-VVA) can achieve the benefits of the Miller cycle (e.g., expansion/ internal cooling effect), reducing wastegating energy loss, and reducing engine delta P or pumping loss at high speeds and high loads. A detailed analysis of WE-VVA and its interactions with turbine area and wastegating at different engine speeds is provided in Fig. 10.24. In the simulation of Fig. 10.24, constant engine compression ratio, EGR rate, fuel injection timing, and compressor map are used for comparing different air system technologies. It shows that point A of WE-VVA using early IVC timing (i.e., down-breathing, *h*vol = 76%) on the IVC timing sweep curve of a medium high-pressurestage turbine with the wastegate fully closed and at a target air–fuel ratio (e.g., 19.6) gives 900 mbar engine delta P at rated power. This engine delta P is the same as that produced by a VGT (e.g., a larger turbine, point B) without using early IVC timing. In contrast, a wastegated turbine with fixed conventional intake cam timing (point C) gives 1,170 mbar engine delta P at the same air–fuel ratio, showing a pumping loss penalty. Moreover, point A gives significantly lower peak in-cylinder thermodynamic bulk temperature than both points B and C due to the Miller cycle's expansion cooling effect, and actually would allow for using more advanced fuel injection timing to further reduce BSFC to a lower level than the BSFC shown in Fig. 10.24. The simulation shows that by properly advancing fuel injection timing to

10.24 Engine performance simulation of Miller cycle with early IVC timing compared with VGT and turbine wastegating. WG = wastegate.

10.24 (Continued)

achieve the same NOx and soot emissions, WE-VVA can reduce BSFC by 3.5% at rated power, compared to a wastegated turbine. WE-VVA (point A) requires using higher intake manifold boost pressure to compensate for the shorter breathing duration of the intake valves in order to maintain the same air–fuel ratio. Moreover, it should be noted that using early IVC timing at low speeds may cause excessively low engine delta P and an EGR driving problem, as well as low air–fuel ratio, as shown in Fig. 10.24 at peak torque.

10.11 Heavy-duty diesel engine cooling and thermal management systems

The primary functions of engine cooling include:

∑ NOx emissions control
∑ achieving acceptable performance and fuel economy
∑ engine protection.

In addition to EGR rate, air–fuel ratio, fuel injection timing, and fuel injection pressure, intake manifold gas temperature (IMT) is another critical parameter that affects NOx emissions. If IMT is too high, a large amount of EGR or retarded fuel injection timing has to be used to reduce NOx, and these may adversely affect fuel consumption. On the other hand, if IMT is too low, there may be condensation problems at the cooler outlet and associated durability issues, especially under humid climate conditions. A proper cold IMT may provide durability benefits of controlling exhaust manifold gas temperature at full load and reducing compressor outlet air temperature via reduced requirements of EGR rate or boost pressure.

In order to achieve a given design target of IMT, there is an optimum tradeoff between the EGR cooler and the CAC in terms of cooler effectiveness (size), packaging space, and cost. In some modern low-NOx diesel engines, a low-temperature-radiator-cooled EGR cooler or even an air-cooled EGR cooler has been used in order to achieve very low IMT. Moreover, in two-stage turbochargers, there is often an inter-stage cooler between the low-pressure-stage and high-pressure-stage compressors, which can reduce the high-pressure-stage compressor outlet air temperature to an acceptable level under hot ambient or high altitude conditions and improve the overall thermodynamic efficiency of the turbocharging system.

Probably the most important parameter in cooling system design is heat rejection. Heat rejection management has become one of the most critical issues in modern heavy-duty diesel engine design. Vehicle front-end total heat rejection includes the heat rejections from base engine coolant, EGR cooler, inter-stage cooler, charge air cooler, fuel cooler, transmission cooler, and air conditioning heat load, etc. CAC outlet air temperature and top tank coolant temperature need to be carefully designed under full-load conditions at sea level and high altitude, as well as at normal ambient temperature and in hot ambient. Moreover, CAC inlet air temperature needs to be limited for the durability of the CAC and the compressor.

In EGR cooler and CAC designs, the gas-side pressure drop always needs to be minimized in order to minimize pumping loss. Cooler effectiveness (at clean or fouled conditions), size, and cost need to reach a balanced compromise to avoid diminishing return. Another important design consideration is cooler sink temperature (e.g., cooling air temperature for the radiator and the CAC, coolant temperature for the EGR cooler). Sink temperature control is a subject of vehicle underhood thermal management. Performance and operating strategy of other components such as transmissions, power electronics, and batteries (in the case of hybrid vehicles) are also greatly affected by the vehicle cooling and thermal management system. The vehicle layout configuration (engine position, floor height, air induction, fuel tank location, etc.) is another important factor that needs to be considered since it affects the flow around the engine. Integrating vehicle aerodynamics and the optimization of front-end openings, discharge areas, and ducting have become a major part of cooling system design since its performance can be limited by packaging space, cooling air flow, and vehicle styling constraints (Hughes et al., 2001).

10.12 Aftertreatment technologies for emissions control

This section reviews the following technologies and issues in emissions control:

∑ diesel oxygen catalysts (DOCs)
∑ control of PM
∑ SCR systems
∑ Lean NOx trap (LNT) technology
∑ control of N2O emissions
∑ integrated aftertreatment systems.

10.12.1 DOC performance

Catalytic converters have been used in vehicle emissions control for many years, e.g., three-way catalytic (TWC) converters in spark-ignition engines during the past several decades and DOCs in diesel engines since the 1990s. The substrate of the DOC is usually ceramic or metallic mesh, and its washcoat contains a catalyst (Pd/Pt combination). The DOC oxidizes HC and CO emissions by using the oxygen present in the exhaust gas flow, and converts them to CO2 and water vapor. The DOC needs to be lighted off to above a certain temperature threshold to activate the catalyst for good conversion efficiency. The catalyst efficiency reduces at high exhaust flow rate (i.e., high space velocity), and the catalyst ages over time. DOC performance is dependent on exhaust gas temperature, showing very little dependence at low temperatures (when not lighted off), while linearly increasing with exhaust temperature until leveling off at a relatively high oxidation rate. The HC and CO conversion efficiency in the DOC can be greater than 90% at high exhaust temperatures with the use of platinum-based catalysts. Additionally, some HC derivatives such as the soluble organic fraction (SOF) in diesel particulate matter and polynuclear aromatic hydrocarbons (PAH) can also be oxidized in the DOC, and these reactions result in a reduction of PM emissions.

However, in some cases, the oxidation reactions in the DOC may result in undesirable products such as sulfur trioxide and sulfuric acid when significant sulfur content exists in the fuel. Moreover, depending upon factors such as catalyst formulation and size, in certain applications like underground mining, oxidation of NO to NO2 may become undesirable due to the toxic nature of NO2. But, for heavy-duty diesel engines, NO2 is a very effective oxidant for soot oxidation in the DPF (Majewski and Khair, 2006), and high engine-out NOx engines (e.g., greater than 1 g/(hp.hr)) can enable such passive regeneration by using NO2. Therefore, the DPF will benefit from higher engine-out NOx-to-soot ratio (i.e., high NO2 and low soot loading), which is related to DOC design. In addition to the function of removing engine-out HC and CO emissions, the DOC can also oxidize the fuel dosed into the exhaust gas flow in order to achieve desirable gas temperatures for DPF regeneration. A properly designed DOC should have a low light-off temperature and low HC slip during DPF regeneration. Currently, reduction of precious metal loading in the DOC is a main design challenge.

10.12.2 Control of particulate matter (PM)

Diesel PM is composed of four main constituents (Khair, 2003; Majewski and Khair, 2006):

1. elemental carbon (solid soot particles)
2. ash (solid inorganic particles mainly from lube oil additives and worn metals)
3. SOF (liquid particles)
4. sulfate particulates (liquid particles).

SOF is controlled by the DOC. Sulfate particulates are controlled by using ultra-low sulfur fuel. Soot and ash are controlled by the DPF, with filtration efficiencies usually greater than 95%. The organic fraction and sulfate particulates in PM are ineffectively filtered by the DPF. In fact, a catalyzed DPF may even increase sulfate particulates through catalytic oxidation of sulfur dioxide.

DPFs are classified into different types such as flow-through, wall-flow, and irregular-flow (Zhan et al., 2006) based on flow directions, and classified into catalyzed or non-catalyzed based on the nature of chemical reactions. The current most common type is wall-flow monolith DPF. As soot and ash particles accumulate inside the filter, the pressure drop across the filter may reach a level that starts to adversely impact engine performance and fuel economy so that DPF soot regeneration is required (Van Nieuwstadt and Trudell, 2004). The soot loading in the DPF is estimated based on the pressure drop measurement across the DPF at different exhaust flow rates. A DPF soot loading model can be improved by adding an ash buildup model.

During DPF regeneration, soot particles experience oxidation reactions with NO2 (typically in a slow reaction at around 300 °C) or O2 (typically in a fast reaction at around 500–600 °C). In passive regeneration, the DPF oxidizes the soot continuously without any auxiliary means such as electric heating, full-flow burner, or selective heating. However, under normal engine operating conditions, the exhaust temperature is usually not high enough for the oxidation to naturally occur inside the DPF, thus preventing a passive regeneration. In this case, a periodic active regeneration must be achieved. During active regeneration, auxiliary means are used to intentionally add heat to the exhaust flow to increase the exhaust temperature.

Active regeneration occurs when the gas temperature inside the DPF is raised to a certain level (e.g., 500 °C) and carbon particles react with oxygen. Achieving such high temperatures requires fuel dosing in the exhaust stream, for example by using a DOC or burner. Fuel dosing has to be tightly controlled based on gas temperature and soot load in order to prevent uncontrolled soot combustion. Active regeneration can be triggered by either the soot load in the DPF or by travel distance, while other related criteria have to be met in order to start the regeneration (e.g., coolant/oil/fuel temperatures above a certain threshold). Moreover, active regeneration can exit based on soot load or limited time duration.

In active DPF regeneration controls, there are two stages:

1. warm-up
2. DPF temperature control.

In the warm-up stage, if the exhaust gas temperature is lower than the light-off temperature of the DOC (approximately 250 °C), the exhaust gas temperature is raised with aftertreatment thermal management by using calibration measures such as reducing air–fuel ratio by opening the turbine wastegate or closing the intake throttle valve, retarding main fuel injection

timing, increasing fueling rate, or increasing exhaust restriction by closing the exhaust back pressure valve. The calibration challenge is to maintain drivability and emissions while keeping the exhaust gas hot.

In the DPF temperature control stage, once the DOC reaches its light-off temperature, dosing fuel is injected into the exhaust stream at the inlet of the DOC. The injected fuel oxidizes inside the DOC, and the DOC outlet or DPF inlet gas temperature increases to the regeneration temperature in order to start to burn the soot inside the DPF. The precisely controlled dosing fuel can be injected via a separate injection unit that is mounted close to the exhaust pipe, or can be injected by using in-cylinder post injection during the exhaust stroke. There are several limitations with the post injection option, such as fuel–oil mixing on the cylinder wall (oil dilution), EGR shut-off to avoid fuel returning to the intake manifold, and high NOx penalty during regeneration due to EGR shut-off. The DPF can also be regenerated in stationary regeneration when the vehicle is parked and the engine speed is forced to increase during idle. However, the use of stationary regeneration should be minimized or eliminated as much as possible.

10.12.3 Selective catalytic reduction (SCR) systems

Urea-based SCR is a proven NOx reduction method that has been used in stationary diesel engine applications for decades. In order to meet the US 2010 emissions regulation, SCR needs to be used for on-road heavy-duty diesel engines (Song and Zhu, 2002; Nakayama et al., 2006; Adelmann et al., 2010). The NOx reduction process of SCR is realized by injecting a reducing agent (typically liquid urea, which decomposes into ammonia) into the engine exhaust stream just before the SCR catalyst. Once mixing occurs, the ammonia (NH3) reacts with NO and NO2 in the exhaust gas and produces N2 and H2O.

Using SCR also provides a smooth transition from the US 2010 emissions compliance to the US 2014–2017 GHG regulations. Euro-IV and V diesel engines may use SCR without EGR to achieve emissions compliance, but combined SCR and cooled EGR is required for the Euro-VI NOx level. SCR allows for early fuel injection timing and high engine-out NOx level with low BSFC. If the SCR NOx conversion efficiency is 80%, in order to achieve a tailpipe NOx standard at 0.2 g/(hp.hr), the engine-out NOx level can be relaxed to approximately 1 g/(hp.hr) by using SCR. If the SCR efficiency reaches 95%, engine-out NOx can be increased to 4 g/(hp.hr) when the tailpipe NOx needs to be 0.2 g/(hp.hr). The early fuel injection timing reduces soot loading in the DPF. As SCR NOx conversion efficiency further increases, more advanced fuel injection timing can be used to allow for higher engineout NOx, and consequently soot emission can be drastically reduced, and the DPF can operate more in the desirable passive regeneration mode.

Achieving very high NOx conversion efficiency in SCR (e.g., 97%) requires design and calibration efforts in the following areas:

∑ reduce or eliminate urea-derived deposits to ensure high conversion rate from urea to ammonia
∑ enhance the mixing between NH3 and exhaust gas
∑ use higher cell density in SCR while maintaining or reducing SCR pressure drop to keep exhaust restriction low
∑ conduct fuel efficient thermal management for acceptable transient FTP cycle emissions (e.g., using air system controls, throttling air, reducing EGR rate, using post fuel injection strategy, dosing hydrocarbons into the DOC to achieve exothermic reactions, using CAC or EGR cooler bypass, using compressor bypass).

Many thermal management methods used to improve SCR warm up actually increase fuel consumption because of heat energy demand. In these heating strategies, the fuel consumption penalty increases as SCR NOx conversion efficiency increases, exhibiting a correlation. One key issue in system design is to identify the strategy of the minimum fuel penalty.

Moreover, NOx conversion efficiency increases with the increasing NH3/ NOx ratio. However, the amount of ammonia slip also increases with the increasing NH3/NOx ratio (although it decreases with increasing temperature). Other factors such as low temperature and inaccurate dosing of urea may lead to production of secondary emissions such as ammonium nitrate (a hazardous byproduct). Promoting NOx selectivity of the ammonia slip oxidation catalyst to minimize slip is a major focus in SCR improvement.

SCR systems have certain serious disadvantages in high cost and weight, urea consumption, urea deposit formation, cold weather low-temperature performance, and secondary emissions, etc. They are bulky due to additional components required (i.e., a urea storage tank, an injection system, sensors, etc.), compromising vehicle packaging optimization efforts. Additionally, system complexity and cost increase with the need of precise injection and control system for the urea, and increase with the price of the reducing agent itself. Another challenge that requires attention is the need to establish a urea distribution infrastructure, which ultimately will affect urea price. Moreover, regulating the use of urea is of key importance because currently many diesel engines can run without urea, thus violating emissions regulations in place.

10.12.4 LNT technology

LNT is another type of NOx aftertreatment device used for lean-burn engines (Takahashi et al., 2004). Also known as NOx adsorber, LNT absorbs NOx from the exhaust gas during lean combustion periods and stores it, and releases and reduces the trapped NOx in a regeneration mode. This two-stage cyclic process occurs alternatively between lean and rich engine operations. During the adsorption stage, NO in the exhaust gas is oxidized over noble metal catalysts (e.g., platinum or Pt) to form NO2, which then reacts with the adsorbent material (e.g., alkali metals or alkaline earths like barium or Ba) to form an inorganic nitrate in a NOx storage stage. The storage capacity will eventually reach a saturation limit, and the stored NOx is then released and catalytically reduced to N2 with the assist of reductants such as CO, HC, and hydrogen (Kim et al., 2003; Wu et al., 2005). Such a process is known as regeneration, and it occurs in the environment exposed to the exhaust containing a rich air–fuel ratio. Optimal control of the transition between adsorbing and regeneration stages is critical in order to meet emissions standards and achieve good fuel economy. A precise control strategy is required to determine the amount of reducing agent for appropriate NO^x reduction and to switch between the adsorbing and reduction stages (Kim et al., 2003). LNT previously had a deNOx efficiency of 60–70%, but currently has reached as high as 80–90% in NOx conversion efficiency over a relatively large range of adsorber temperatures (200–500 °C) (Majewski and Khair, 2006). One of the biggest issues with LNT is its secondary emissions such as nitrous oxide (N2O) and hydrogen sulfide (H2S). LNT is usually used in low-NOx engines at a lower cost than SCR.

10.12.5 N2O emissions

Nitrous oxide (N2O) has become a regulated emission gas under the US EPA 2014–2017 GHG emissions regulation due to its strong global warming potential at 296 times of CO2 (Becker et al., 1999; Metz, 2001; Graham et al., 2008; Kamasamudram et al., 2011). N2O is produced mainly by ammonia oxidation and secondarily by ammonium nitrate decomposition in NOx aftertreatment devices, basically in all NOx reduction catalysts such as lean NOx catalysts, SCR, and LNT, as well as ammonia slip DOC.

Under certain conditions, NOx is reduced over oxidation catalysts to N² and N2O. N2O formation is affected by exhaust gas temperature and the concentration and composition of hydrocarbons in the exhaust. There is a temperature window where N2O concentration becomes high (e.g., peak at 200–250 °C). Outside the window (e.g., less than 150 °C or greater than 350 °C), N2O formation is very little.

In SCR, the global reaction of NOx reduction can be described as

$$NH_3 + NO_x \rightarrow N_2 + H_2O$$ [10.3]

In further detail, the standard reaction of NOx reduction is

$$4NO + 4NH_3 + O_2 = 4N_2 + 6H_2O$$ [10.4]

A 'fast' reaction refers to

$$NO + NO_2 + 2NH_3 = 2N_2 + 3H_2O$$ [10.5]

A 'slow' reaction refers to

$$6NO_2 + 8NH_3 = 7N_2 + 12H_2O$$ [10.6]

The reactions with NH3 can be described as follows:

$$4NH_3 + 3O_2 = 2N_2 + 6H_2O$$ [10.7]

Equation 10.7 is the desired reaction over the NH3 slip oxidation catalyst. Non-selective oxidation of NH3 is expressed by

$$2NH_3 + 2.5O_2 = 2NO + 3H_2O$$ [10.8]

N2O formation from NH3 is described as

$$2NH_3 + 2O_2 = N_2O + 3H_2O$$ [10.9]

Moreover, ammonium nitrate deposits are formed by

$$2NH_3 + H_2O + 2NO_2 + 0.5O_2 = 2NH_4NO_3$$ [10.10]

Ammonium nitrate decomposition at exhaust temperatures greater than 210 °C can lead to N2O formation through the following reaction:

$$NH_4NO_3 = N_2O + 2H_2O$$ [10.11]

NH3 selectivity to N2O (equation 10.9) is also strongly affected by the N2O/NOx ratio, especially above 0.5 (Kamasamudram et al., 2011). Since the DOC can convert NO to NO2, the DOC influences N2O formation in its downstream SCR catalyst. Different types of SCR catalysts possess distinct temperature characteristics (or selectivity) of N2O formation. Moreover, many precious metal based oxidation catalysts for ammonia slip control are prone to produce high NOx and N2O when NH3 is oxidized at the downstream of SCR. Special formulations need to be developed to depress the formation of NOx and N2O in the catalyst.

In the three-way catalysts used for gasoline engines, N2O is formed mostly at cold-start conditions during the first 200 seconds. However, in SCR systems, N2O is usually formed throughout the entire FTP cycle, not just only in the cold start portion. Minimizing N2O mainly relies on catalyst formulations of DOC and SCR, as well as engine and aftertreatment control strategies. The current SCR systems are usually able to meet the US EPA 2014–2017 N2O standards (0.05 g/mile for heavy-duty pickup trucks and vans, and 0.1 g/(hp.hr) for compression-ignition engines over FTP test cycles) without great difficulties (Kamasamudram et al., 2011).

10.12.6 Integrated aftertreatment systems

Several design trends of heavy-duty diesel engine aftertreatment exist as follows:

1. small DPF size enabled by lower engine-out soot and higher engine-out NOx and greater SCR conversion efficiency, along with reduced pressure drop or flow restriction of the DPF
2. enhanced capability of passive DPF regeneration due to reduced soot loading caused by higher engine-out NOx
3. elimination of ammonia slip oxidation catalysts
4. large reduction of precious metal loading of the DOC
5. integrated one package/can or element containing multiple aftertreatment functions (e.g., a two-way SCR/DPF system, a coupled DOC–DPF plus SCR system, or a DOC integrated into a LNT and a DPF) for reduced cost, size, and pressure drop (Gurupatham and He, 2008; He et al., 2009; Boorse et al., 2010; Rappe et al., 2011)
6. large cost reductions in SCR, DOC, and DPF.

Among these trends, integrated aftertreatment element has been receiving great attention in order to optimize the interactions and synergies between different individual aftertreatment devices in terms of operational strategies, geometries, reducing agents, catalysts, dosing quantities, regeneration times, etc. (Blakeman et al., 2003; He, 2007; Johnson, 2007, 2008, 2009, 2010a, 2010b, 2011a, 2011b, 2012). Moreover, engine performance simulation (Fig. 10.25) shows that every 1 inch of Hg flow restriction pressure drop of aftertreatment devices leads to 0.33% BSFC reduction at rated power due to pumping loss reduction.

10.25 The effects of air–fuel ratio and exhaust restriction on BSFC for line haul tractor diesel engine at rated power.

10.13 Waste heat recovery (WHR) systems

WHR presents a large opportunity to improve engine thermal efficiency. There are two basic types of WHR technologies (Mollenhauer and Tschoeke, 2010):

1. recovery of waste heat as mechanical or electrical energy (e.g., turbocompounding and thermodynamic bottoming cycles)
2. recovery of waste heat as thermal energy (e.g., cabin heating in vehicles and cogeneration).

In addition to turbocompounding, organic Rankine cycle (ORC) is another popular form of WHR to convert heat to engine power via electrical or mechanical connections (Fig. 10.26). This technology has been extensively investigated since the early 1970s and covered in literature (DiBella et al., 1983; Diehl et al., 2001; Crane et al., 2001; Chammas and Clodic, 2005; Stobart and Weerasinghe, 2006; Arias et al., 2006; Teng et al., 2006, 2007a, 2007b; Kruiswyk, 2008; Nelson, 2009; Ringler et al., 2009; Teng, 2010; Edwards et al., 2010, 2012; Hirschbichler, 2010; Briggs et al., 2010, 2012; Chiew et al., 2011; Arunachalam et al., 2012; Latz et al., 2012; Lopes et al., 2012). It can be applied to EGR heat rejection, charge air heat rejection, turbine outlet exhaust heat, and even engine coolant heat rejection. In fact, in WHR areas, the general trend in the heavy-duty industry from 2010 to 2025 is not only moving from mechanical turbocompounding towards

10.26 Illustration of organic Rankine cycle schematic.

electrical turbocompounding, but also adding Rankine cycle or Brayton cycle, thermoelectricity, along with electrical or mechanical hybridization.

WHR should be applied to the most promising and realistic potential of energy recovery sources, i.e., high-quality and large-quantity heat sources that are worth capturing with cost-effective approaches. The temperature difference between the heat source and the heat sink (coolant) needs to be large enough to ensure a reasonable heat exchanger size in packaging. Exhaust heat and EGR heat both have high quality due to their high temperatures. Exhaust gas, coolant flow, and sometimes EGR flow possess a large quantity of heat (Fig. 10.25). For example, if 20% of the fuel energy goes to the exhaust, 2% of the fuel energy in the exhaust flow can be recovered by a Rankine cycle as useful work. Similarly, if another 15% of the fuel energy goes to EGR cooler heat rejection, another 1.5% of the fuel energy can be recovered from the EGR gas flow by using a Rankine cycle. This total 3.5% of the fuel energy can be added to the engine crankshaft power to make engine thermal efficiency increase from 50% to 53.5%. At a medium EGR rate (e.g., 25%), EGR cooler heat rejection is usually less than turbine outlet exhaust energy. However, when the EGR rate increases to a very high level (e.g., 40%), the EGR cooler heat rejection may become greater than the turbine outlet exhaust energy. Applying WHR to the EGR gas may also reduce coolant heat rejection to ease vehicle cooling design, make the vehicle radiator smaller (but the Rankine condenser larger), and help achieve better aerodynamic design of the truck through reduced underhood cooling package size. Certainly, using WHR on the EGR gas increases the complexity of design and controls. Its gain in thermal efficiency strongly depends on EGR flow rate, which is determined by engine-out NOx target and the associated NOx aftertreatment conversion efficiency.

Major Rankine cycle components include an evaporator, an expander, a condenser, and a pump. The working fluid is usually ethanol or organic lowglobal-warning-potential refrigerant such as R245fa, whose thermodynamic properties allow a low boiling point and a robust high peak temperature of the fluid. The evaporator (boiler) is a high-efficiency heat exchanger to absorb heat from the exhaust gas or EGR gas and transfer the exhaust energy to the working fluid. The expander may be able to handle two-phase flows and produces mechanical or electrical power. The expander can be an axial turbine, a piston expander, a screw or scroll expander. The condenser also handles two-phase flows and rejects heat to the ambient.

For vehicle applications, the Rankine cycle should be kept as simple as possible in the following four basic processes:

1. working fluid compression in the pump
2. constant-pressure heating by an external heat source
3. isentropic expansion in the expander to generate power output
4. isobaric heat rejection in the condenser.

10.27 Illustration of heavy-duty diesel engine energy distribution by thermodynamic first law analysis in speed–load domain: (a) Percentage of EGR cooler heat rejection in fuel energy (%), (b) Percentage of base engine coolant heat rejection (excluding EGR cooler and ISC heat rejection) in fuel energy (%), (c) Percentage of sensible exhaust enthalpy at turbine outlet in fuel energy (%) and (d) Percentage of CAC plus ISC heat rejection in fuel energy (%).

However, in a real thermodynamic cycle, irreversibilities related to the expander and the heat transfer processes would alter the isentropic expansion and isobaric evaporation and condensation processes of the ideal cycle (Chammas and Clodic, 2005). The thermal efficiency improvement potential of WHR not only depends on the heat source utilized and vehicle condenser's capacity, but also depends on component efficiencies such as turbine expander (currently around 75%), pump (30%), power transmission efficiency (90%), valve and pipe flow losses, and flow losses in the evaporator and the condenser.

Compared with hybrid powertrains, WHR is more suitable for line haul type of applications that have very few start–stop events, while hybrid is more suitable for vocational applications that have frequent start–stop events. Line-haul heavy-duty vehicles have a good potential for WHR due to their operation at stable engine loads and speeds over long periods of time, which may translate to large fuel savings. Cummins reported (Nelson, 2009) that the potential of WHR from engine coolant heat is approximately 0.5% of relative improvement in engine thermal efficiency, and it is a low potential benefit compared to the extra system cost and complexity. It was also reported that WHR from charge air gives 1% relative improvement in thermal efficiency. WHR from exhaust gas provides 3–5% efficiency improvement at the EGR rate level of 2007 model year. WHR from EGR heat provides 2.0–2.5% efficiency benefit at 2007 EGR rate level.

The challenges of Rankine cycle include system complexity, weight, packaging size, and cost (Hountalas et al., 2007). Rankine cycle systems are much more expensive and heavier than turbocompounding systems. Moreover, exhaust waste heat recovery adds a significant burden to the vehicle cooling system because the majority of the tailpipe exhaust heat is rejected underhood via the condenser, and only a small portion of the exhaust heat is recovered as useful work. Re-sizing the radiator and re-designing the water pump and the cooling fan are required. If the vehicle cooling system cannot handle so much heat rejection, bypassing some waste heat from the condenser or limiting the condenser capacity needs to be considered, and these will reduce the thermal efficiency benefit offered by WHR.

10.14 Engine mechanical friction reduction technologies

In order to meet GHG regulations and improve engine thermal efficiency, accessory power reduction and mechanical friction reduction have become very important (Schwaderlapp et al., 2000). These benefits are especially prominent at low loads where typical driving conditions occur. The contribution from main engine components to mechanical friction losses can be broken down into (Taraza and Henein, 2000; Comfort, 2003):

∑ piston assemblies (45–50%)
∑ engine bearings (20–30%)
∑ engine accessories (20–25%)
∑ valvetrain (7–15%).

Most accessories undergo fluid friction losses and intermeshing friction losses due the rolling and sliding of the gear teeth (Comfort, 2003). The rubbing friction and lubrication regimes of engine components include hydrodynamic lubrication (piston skirts, bearings), mixed lubrication (piston rings), elastohydrodynamic and boundary lubrication (cams) (Sandoval and Heywood, 2003).

Using variable-power delivery options (smart accessories) or electrically driven accessories can reduce engine parasitic losses. Engine rubbing friction reduction has been mainly focused on three aspects:

1. tribological design of friction interfaces/surfaces
2. materials and coatings
3. lubricants and additives.

Tribological design options include reducing contact area (e.g., using fewer/ thinner piston rings or smaller bearings, shortening piston skirt length), and optimizing component surface profiles to promote hydrodynamic lubrication. Reducing boundary lubrication friction can significantly reduce asperity and mixed lubrication frictional losses and hence fuel consumption (estimated benefit of fuel consumption reduction being 0.5–1%). Wear-resistant materials, coatings, and anti-wear additives can also be considered.

Low viscosity lubricant is very effective in reducing hydrodynamic and mixed lubrication friction for engine bearings and piston assemblies (estimated benefit of fuel consumption reduction being 0.5–1%). However, reducing rubbing friction should not cause excessive degradation in wear durability when oil viscosity and oil film thickness are reduced (i.e., increased boundary lubrication and the associated wear). It should be noted that the overall industry trend from the 1960s to the 2010s is to move towards lower viscosity engine lubricants (e.g., from 10W-40 to 5W-20 and 0W-20). As the lubricant viscosity becomes lower, the contact severity of surface asperity increases on piston skirts, piston rings, and bearings, and this is especially prominent for severely loaded tribological engine components such as bigend bearings.

The challenge to lube oil manufacturers and engine manufacturers is whether a lubricant formulation can be successfully developed to simultaneously provide low viscosity (lower than OEM's normal recommendations), low boundary friction, and acceptable durability and reliability matched with an improved tribological design of frictional interfaces (Miller, 2010; Fenske et al., 2011). Another noteworthy aspect is aging and contamination of the engine lubricant oil by soot particles (Ajayi et al., 2008). The soot in the cylinder is mainly related to EGR rate, air–fuel ratio, and fuel injection timing. Oil aging may result in an increase in viscosity, which causes fuel consumption increase that can be many times greater than the fuel economy benefits achieved by lubricant formulation modifications (e.g., on viscosity index and friction modifier). Moreover, fast warm-up using recovered exhaust energy to increase coolant and oil temperatures is promising in reducing the high friction caused by cold oil and its high viscosity during driving cycles.

Considerations for future lubricant development will focus on fuel economy improvement, wear resistance, continuous emissions compatibility, alternative fuel compatibility, and alternative lightweight material compatibility. Advanced lubricant formulation also needs to reduce the degradation and poisoning of aftertreatment systems caused by sulfur, sulfated ash, and phosphorus.

10.15 Electronic controls and on-board diagnostic (OBD) systems to optimize engine performance

10.15.1 Model-based controls and predictive dynamic onboard controller

Modern heavy-duty diesel engines feature sophisticated control systems responsible for optimizing engine performance and durability protection while meeting emissions and fuel consumption regulations. This is achieved by advanced sensing technologies, actuator technologies, and control algorithms or predictive models to handle precise interactions between different subsystems at a system level. There are several trends in engine controls development. One direction is to transition from emissions compliance to CO2 and fuel consumption reduction and compliance. A second trend is that electronic controls have become predictive or model-based rather than 'reactive' or map-based. A third trend is to develop on-board real-time transient dynamic controls with an optimizer.

As emissions regulations have become more stringent and electronic controls had been introduced into heavy-duty diesel engines since the 1990s, the number of sensors and actuators have increased exponentially, from one actuator for fuel injection control in the late 1990s to six actuators (injector valves, EGR valve, VGT vane or turbine wastegate, intake throttle, HC doser, and urea doser) in 2010. Looking ahead for GHG regulations compliance, more sensors and actuators will be required, such as cooler bypass, VVA, and waste heat recovery.

A historical path and future outlook regarding heavy-duty diesel engine controls system complexity and control variables has been summarized by Atkinson et al. (2005, 2007, 2011) as follows:

∑ In the 1990s, only fuel injection timing control was implemented.

∑ In 2002–2004, as common rail and other advanced fuel systems emerged, controls of injection pressure, multiple injections, EGR, and VGT have been added.
∑ In 2007, DPF control was added to meet the US 2007 emissions regulation.
∑ In 2010, SCR control was added to meet the US 2010 emissions.
∑ It is predicted that in-cylinder combustion feedback control will emerge in production engines in the 2012–2014 time frame.
∑ Multiple injection strategies and their controls will become increasingly important for meeting future GHG regulations starting 2014.
∑ It is expected that multiple combustion regimes (e.g., LTC) and their controls will become feasible in production diesel engines starting 2014.
∑ Electronic controls development for waste heat recovery, hybridization, auxiliary electrification, and other energy recovery technologies will become mature around 2017.
∑ It is anticipated that fuel tolerance and sensing for fuel quality or alternative fuels (e.g., bio-fuels) will become production ready by 2020.
∑ By 2025, fully independent valve actuation controls, individual cylinder controls, active cylinder-to-cylinder variation controls on fuel and gas flows, and cycle-by-cycle controls will emerge in production.

The increased multiplication of control variables (actuators and sensors) makes engine controls more flexible, but also makes map-based controls and calibration increasingly difficult to be optimized. Not only does set point calibration optimization become very time-consuming, tuning control gains in control algorithms to balance between steady-state stability and transient response also becomes very difficult.

Model-based controls (MBC) have become highly desirable for modern diesel engines. It is also compatible with virtual sensing, OBD, and modelbased calibration (Nanjundaswamy et al., 2012b). It can alleviate the burden of developing control algorithms and intensive calibration work because algorithms can be replaced by fully predictive models; and calibration can be replaced by target setting of emissions and performance goals and realtime on-board optimization. The development of advanced engine controls has moved towards map-less and predictive engine controllers that include a real-time transient dynamic optimizer for on-board emissions and fuel consumption controls.

The controller contains an engine performance model, which can be built by thermodynamic principles, engine system dynamics, empirical surface-fit, or neural network models. The empirical models are built by using either steady-state or transient engine test data. The inputs of the model-based controller are performance targets such as emissions and fuel economy, and the outputs are set points or actuator signals such as air–fuel ratio, turbine wastegate opening, EGR valve position, fuel injection timing and pressure, aftertreatment fuel dosing, and urea dosing commands. A real-time dynamic online optimizer in the engine controller can compute optimum weighted targets of emissions and fuel efficiency as set points with forward predictive models and the corresponding actuator signals with inverse control models to minimize NOx and soot emissions and fuel consumption during both steadystate and transient operations. Detroit Diesel Corporation has reported much progress in this area, including their recent applications in the SuperTruck program (Allain et al., 2007, 2009; Aneja et al., 2009, 2011; Zhang et al., 2009; Allain and Atkinson, 2010).

10.15.2 OBD systems

In addition to the compliance of criteria emissions and GHG regulations, heavy-duty engines used in on-road vehicles over 14,000 lbs GVWR also need to comply with heavy-duty OBD regulations, starting in the 2010–2013 phase-in stage. Unlike the much earlier adopted light-duty OBD I (1988) for passenger vehicles and OBD II (1996) for all light-duty vehicles, heavy-duty OBD is relatively new to the diesel engine industry (Grimaldi and Mariani, 1999; Geraldo, 2006; Vitale et al., 2007; Fischer et al., 2009; Nanjundaswamy et al., 2011, 2012a). OBD development and in-use testing is an emphasis of post-2010 diesel engines.

OBD regulations consist of many requirements that can affect system design, hardware configuration, software algorithm, calibration, and overall warranty cost to the engine manufacturer. OBD can help ensure low emissions standards are met and emissions control components work properly under in-use conditions during the life of the vehicle. OBD emissions threshold monitors or functional monitors need to be designed to detect and execute a fault before emissions thresholds are exceeded for the air system, fuel system, exhaust gas system and emissions sensors, aftertreatment system, cooling system, crankcase ventilation system, etc. The subjects related to the air system may include over-boost, under-boost, or slow response of the turbocharger; low flow, high flow, or slow response of the EGR system; cooling performance issues of the EGR cooler and the charge air cooler; air charging performance issues of the VVA system, etc. The subjects related to the fuel system may involve injection pressure, timing, quantity, and misfire, etc. The subjects related to the aftertreatment system may involve conversion efficiency of SCR, working fluid dosing delivery, DPF filtration performance, and regeneration frequency. The subjects related to the cooling system may involve thermostat and engine coolant temperature sensors. The OBD monitoring system needs to be able to identify the systems, sensors, or actuators that are no longer functioning and affect emissions (i.e., in a failed state) and identify recovery from a failed state, and also need to contain features to minimize falsely passing a failed system or falsely failing a good system.

In model-based controls for OBD, an engine performance model is developed for every sensor to predict what should come out of the sensor. This virtual signal is compared with what actually comes out of the physical sensor to check for good or faulty signals. These on-board models of virtual sensors can predict transient response and provide sensor diagnostics by feeding the signals into the control logic, along with physical sensors' signals. Those gas and liquid performance models related to pressure, temperature, and flow rate used in OBD monitor design and OBD calibration are affected by engine system and component design changes and calibration settings. Therefore, designing and calibrating with OBD considerations upfront in product development is the most cost-effective approach.

10.16 Development of natural gas engines

Natural gas engine technologies have evolved to meet the strict US 2010 emissions and the 2014–2017 GHG regulations and the requirements of heavyduty vehicle applications, and may provide comparable or sometimes even lower GHG emissions than diesel engines. Major heavy-duty natural gas engine manufacturers in the US include Cummins-Westport, Westport Innovations, Navistar, Emission Solutions Inc., and Doosan. Automotive heavy-duty natural gas engines usually have 5–15 L in displacement and 100–500 hp in power rating. Industrial and power generation natural gas engines can be as large as 90 L in displacement and more than 3,000 hp in power. Natural gas engines operate with either natural gas or a combination of natural gas and diesel fuel (dual-fuel), with no major changes to the diesel engine.

There are mainly two types of natural gas engines used in heavy-duty vehicles:

1. stoichiometric spark-ignited engines (e.g., the Cummins-Westport's 8.9 L ISL G engine and the 11.9 L ISX)
2. lean burn compression-ignition engines.

Cooled EGR can be used for both types in order to reduce engine-out NOx and exhaust manifold gas temperature, increase power density, and improve thermal efficiency. Stoichiometric spark-ignited natural gas engines use electronically controlled gas injectors installed in the intake manifold. They may use an intake throttle valve or a turbocharger air bypass valve to quickly and accurately control air–fuel ratio. They need to use a three-way catalytic converter to reduce NOx, CO, and HC. In lean burn compressed natural gas (CNG) or liquefied natural gas (LNG) engines, SCR and DPF are used to achieve the US 2010 emissions level.

Lean burn natural gas engines usually have lower in-cylinder temperatures,

lower NOx emissions, higher power density, greater thermal efficiency, and better durability than their stoichiometric counterparts. In vehicle operation, natural gas vehicles perform similar to gasoline or diesel vehicles in terms of power capability and acceleration. In fact, in powertrain and drivetrain matching for natural gas engines, the torque curve needs to be compared with diesel engines to determine the need of changing gear ratios for the optimization of vehicle performance and fuel economy.

10.17 Future trends

Looking forward on powertrain development for heavy-duty vehicles in the next several decades, owing to their superior thermal efficiency and reliability, diesel cycle (compression ignition) engines will continue dominating as prime powerplants in freight transportation. Cleaner alternative fuels, dualfuel (e.g., diesel plus gasoline), natural gas, and biodiesel will emerge in many more applications. Engine thermal efficiency will be continuously and greatly improved by advanced low-temperature combustion and exhaust waste heat recovery (Rankine cycle), and these technologies are expected to enter production engines in the next 10–20 years. Simultaneously nearzero in-cylinder NOx and soot emissions and low fuel consumption in the entire engine speed–load domain will be achieved via breakthroughs in low-temperature combustion and advanced engine/combustion controls. SCR NOx aftertreatment will be gradually downsized and expected to be finally deleted in order to reduce hardware cost and urea consumption. Engine downsizing and higher BMEP will become necessary for meeting CO2 and fuel economy regulations. VVA and two-stage turbocharging will prevail in future high BMEP diesel engines. The technology trend of widespread applications of electrification will penetrate into heavy-duty vehicles and thus make electrical turbocompounding and electric-hybrid powertrains cost-effective and synergic choices.

Engine cost will significantly increase as more advanced technologies are adopted in production to ensure GHG regulation compliance, just like the significant cost penalty encountered for the US 2004–2007–2010 emissions compliance. However, looking from an operation cost perspective, the US EPA and NHTSA estimated that with GHG rules implemented, the percentage of fuel consumption reduction will be 10–23% for tractor trucks, 6–9% for vocational vehicles, and 12–17% for pickup trucks and vans in 2018, compared with the 2011 status. In EPA and NHTSA's estimate or expectation, the US 2014–2017 GHG rules will result in a cost of US \$6,220 for Class 8 combination tractors, \$380 for medium-duty vocational trucks, and \$1,050 for pickup trucks and vans (Bunker, 2011), while the lifetime fuel savings will be \$79,100, \$5,900, and \$7,200 for these vehicles, respectively. Therefore, the payback period will be approximately 1–2 years. However, under the pressure of reducing engine initial cost in market competition, low-cost GHG technologies (e.g., less than \$500) will actually dominate the technical path of heavy-duty diesel engines. These technologies include engine downsizing with increased peak cylinder pressure capability, low viscosity oil, advanced electronic controls, variable water and oil pumps, etc. Medium-cost technologies (e.g., less than \$1,000) such as VVA and mechanical turbocompounding will have a good chance of entering production if their use is fully justified for multiple advanced features or purposes. High-cost technologies (e.g., higher than \$2,000) such as Rankine cycle and electrical turbocompounding will receive continuous attention by the industry in research for their high GHG benefits, but their production application will be greatly challenged by other lower-cost production-ready conventional technologies of incremental or evolutionary improvement nature. The technology advances of fuel, engine, and vehicle in different subsystems and product attributes (performance, including emissions and fuel consumption; durability; packaging; cost) require a highly integrated system design approach to optimize the entire technical path and the product.

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Nakayama R, Watanabe R, Takada K, Odaka M, Kusaka J and Daisho Y (2006), 'Control strategy for urea-SCR system in single step load transition', SAE paper 2006-01-3308.
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Nelson C (2009), 'Exhaust energy recovery', Proceedings of the Directions in Engine-Efficiency and Emissions Research (DEER) Conference Presentations, Dearborn, MI, 3–6 August.
Nieman D E, Dempsey A B and Reitz R D (2012), 'Heavy-duty RCCI operation using natural gas and diesel', SAE paper 2012-01-0379.
Ostrowski G, Neely G D, Chadwell C J, Mehta D and Wetzel P (2012), 'Downspeeding and supercharging a diesel passenger car for increased fuel economy', SAE paper 2012-01-0704.
Patterson A T C, Tett R J and McGuire J (2009), 'Exhaust heat recovery using electroturbogenerators', SAE paper 2009-01-1604.
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10.19 Appendix: units and unit conversion

Units

absolute absolute pressure

Btu/min British thermal unit per minute, a unit of power

deg degree or °, a unit of angle

ft foot, a unit of length ft.lb foot pound-force (lbf), a unit of torque

g gram, a unit of mass

gauge gauge pressure

hp horsepower, a unit of power

hr hour, a unit of time in inch, a unit of length

inHg inch Hg or inches of mercury, in Hg, or "Hg, a unit of measurement

for pressure

K Kelvin, a unit of temperature

L liter, a unit of volume

lb pound-force, lbF or lbf, a unit of force

lb/(hp.hr) pound-mass (lbm) per horsepower per hour, a unit of specific

fuel consumption rate

lb/hr pound-mass (lbm) per hour, a unit of mass flow rate lb/min pound-mass (lbm) per minute, a unit of mass flow rate

lbs pounds, the plural form of lbm or pound-mass, which is a unit

of mass

m meter, a unit of length mile a unit of length or distance

mpg mile per gallon, a unit of fuel economy

mph mile per hour, a unit of speed

ppm parts per million, a unit of volume concentration

psi pound-force per square inch, or lbf/in² , a unit of pressure or

stress

psia psi, absolute pressure

r/min rpm, revolution per minute, 1 r/min = (1/60) s–1

s second, a unit of time

°C degree Celsius, a unit of temperature °F degree Fahrenheit, a unit of temperature

Unit conversion

(°F – 32) × 5/9 = °C ; (°F – 32) × 5/9 + 273.15 = K

1 Btu = 1055.06 J

1 Btu/min = 0.0175843 kW

1 foot = 0.3048 m

1 ft.lb = 1.35582 N.m

1 g/(hp.hr) = 1.341 g/(kW.hr)

1 gallon = 0.00378541 m³ = 3.78541 L

1 hp = 0.7457 kW 1 inch = 0.0254 m

1 inHg = 0.0338638 bar

1 lb = 4.448 N

1 lb/(hp.hr) = 608.28 g/(kW.hr)

1 lb/hr = 1.26 × 10–4 kg/s

1 lb/min = 0.00756 kg/s

1 lbm = 0.4536 kg

1 mph = 1.609 km/hr = 0.447 m/s
1 mpg is equivalent to 2.352 L/km
1 mile = 1609.34 m
1 psi = 0.06895 bar
1 ton = 1000 kg

Advanced and conventional internal combustion engine materials

L. L. Myagkov, Bauman Moscow State Technical University, Russia, K. Mahkamov, Northumbria University, UK, N. D. Chainov, Bauman Moscow State Technical University, Russia and I. Makhkamova, Northumbria University, UK

DOI: 10.1533/9780857097422.2.370

Abstract: The design of internal combustion engines is being continuously improved to ensure enhanced specific power output combined with the highest possible efficiency in both the steady and unsteady regimes of operation and to comply with ever more challenging emission regulations. This chapter describes advanced and conventional materials and manufacturing methods used for production of modern internal combustion engines. Advanced materials include compact graphite iron, graphite/carbon and carbon/carbon fibre-reinforced polymer composites, polyamides for manufacturing intake manifolds and alloys and ceramics for manufacturing valves and other components. Application of such materials allows engineers to create engines with reduced dimensions and increased durability.

Key words: internal combustion engines, advanced alloys, ceramics, polyamides, reinforced polymer composites.

11.1 Introduction

Modern internal combustion (IC) engines must have superior technical, economic and environmental characteristics to maintain technical and commercial competitiveness in the rapidly changing market for automotive vehicles. The design of internal combustion engines is being continuously improved to ensure enhanced specific power output combined with the highest possible efficiency in both the steady and unsteady regimes of operation and to comply with ever more challenging emission regulations.

These goals are mainly achieved by increasing the compression ratio in cylinders and the mean effective pressure through turbo-charging, high pressure fuel injection, better organisation of the air/fuel mixing process and improved combustion strategies. Advanced lightweight materials with a high specific strength are necessary to withstand the elevated dynamic and thermal loadings which result from these developments. These materials make a reduction of engine dimensions possible and increase their durability, thus reducing production and maintenance costs. New lighter materials also replace metals in components which are not exposed to high temperatures and dynamic loads. This chapter describes a number of advanced and conventional materials used in the manufacture of modern IC engines.

11.2 Advanced internal combustion (IC) engine materials: compact graphite iron (CGI)

As described above, there is a growing requirement for stronger materials for cylinder heads, blocks and cylinder sleeves. One of the new materials in widespread use is compact graphite iron (CGI). More than 500,000 CGI engines are produced annually. Current production is limited to diesel engines and is primarily based in Europe. It is estimated that more than 30 different engine designs are being produced with either a CGI cylinder block or a CGI cylinder head [1].

When compared to conventional grey cast iron, CGI provides the potential for:

∑ reduced wall thickness at current operating loads;
∑ increased operating loads (increased *P*max) at current design;
∑ reduced safety factors due to less variation in as-cast properties;
∑ reduced cylinder bore distortion;
∑ shorter thread engagement depth and therefore shorter bolts.

In comparison with aluminium, the mechanical properties of CGI have the potential for:

∑ smaller package size;
∑ reduced cylinder bore distortion and improved oil consumption;
∑ no cylinder liners or surface etchant/coating;
∑ lower production cost;
∑ improved recyclability.

The mechanical properties of CGI are pre-defined by its structure. The graphite phase in compacted graphite iron is presented as individual vermicular particles (see Fig. 11.1) [1]. The particles, as in grey iron, are elongated and randomly oriented but are shorter and thicker with rounded edges. The individual particles are connected to their immediate neighbours within the eutectic cell. The coral type graphite structure with rounded edges and irregular surfaces formed by compacted graphite particles (see Fig. 11.2) [1], results in strong bonds between the graphite and the iron matrix.

CGI also includes some nodular graphite particles. As the nodularity increases, the strength and stiffness also increase, but there is a reduction in castability, machinability and thermal conductivity. The microstructure specification therefore should be carefully selected, depending on the

11.1 CGI microstructure containing 10% nodularity (1).

11.2 Deep-etched SEM micrographs showing the 3-D coral-like graphite morphology (1).

production requirements and performance conditions of the product. The requirements for a CGI cylinder block or head are as follows:

∑ 0–20% nodularity, for optimal castability, machinability and heat transfer;

∑ no free flake graphite or flake type graphite (as in grey cast iron) which causes localised weakness;
∑ 90% pearlite, to provide high strength and consistent properties;
∑ < 0.02% titanium, for optimal machinability.

The typical mechanical properties for this CGI grade, in comparison to conventional grey cast iron and aluminium are summarised in Table 11.1.

Overall, the typical mechanical properties for cylinder blocks show an increase in tensile strength in CGI. The fatigue limit for grey iron is in the range of 62 to 79 MPa, depending on the carbon content, while for CGI, the fatigue limit is 175 MPa. The increase in fatigue strength allows a reduction in the weight of the cylinder block. CGI also shows a higher elastic modulus when compared with grey cast iron. The increase, from 100 GPa for grey cast iron to 150 GPa for CGI, results in diminished cylinder bore distortion as reported in [2], thus reducing oil consumption and emissions. When CGI is used instead of grey cast iron in the manufacture of cylinder blocks, the weight reduction is in the 10–30% range.

The Ford PSA Jaguar V6 is the first high volume engine with a CGI cylinder block, production having started in October 2003 (see Fig. 11.3) [3]. The number of CGI engines is expected to increase considerably in the future and Table 11.2 shows a list of production programmes from different engine manufacturers for various types of engines.

11.3 Graphite/carbon and carbon/carbon fibrereinforced polymer composites (CFRPs)

Carbon-fibre reinforced polymers offer good potential for the manufacture of conrods. Figure 11.4 shows a compound conrod made of such material. These materials are additionally reinforced by 5–20 micron dispersed

conventional grey cast iron (GJL) and aluminium (A) at 20°C [1]
Properties	Units	GJV 450		GJL 250 GJL 300	A390.0
Ultimate tensile strength	MPa	450	250	300	275
Elastic modulus	GPa	145	105	115	80

Elongation % 1–2 0 0 1 Rotating-bending fatigue (20 °C) MPa 210 110 125 100 Rotating-bending fatigue (225 °C) MPa 205 100 120 35 Thermal conductivity W/(mK) 36 46 39 130 Thermal expansion µm/(mK) 12 12 12 18

system)

Brinnell hardness 10–3,000 215–255 190–225 215–255 110–150

7.1 7.1 7.1 2.7

Density g/cc (CGS

Table 11.1 Mechanical and physical properties of CGI (GJV) in comparison to

11.3 Ford PSA Jaguar V6 high performance 2.7 L diesel engine [3].

Table 11.2 CGI series production programs [1]

Engine manufacturer	Engine details	CGI component
Audi	3.0 L V6 diesel	Cylinder block
Audi	4.2 L V8 diesel	Cylinder block
Audi	6.0 L V12 diesel	Cylinder block
Caterpillar	Heavy-duty engines	Cylinder liners
DAF	12.6 L I-6 diesel	Cylinder block
DAF	12.9 L I-6 diesel	Cylinder block and head
Ford-PSA	2.7 L V6 diesel	Cylinder block
Ford	3.6 L V8 diesel	Cylinder block
Ford-Otosan	9.0 L I-6 diesel	Cylinder block and head
Hyundai	3.0 L V6 diesel	Cylinder block
Hyundai	3.9 L I-4 diesel	Cylinder block
Hyundai	5.9 L I-6 diesel	Cylinder block
International	11.0 L I-6 diesel	Cylinder block
International	13.0 L I-6 diesel	Cylinder block
John Deere	9.0 L I-6 diesel	Cylinder liners
MAN	10.5 L I-6 diesel	Diesel cylinder block
MAN	12.4 L I-6 diesel	Cylinder block
Mercedes	12.0 L V6 diesel	Cylinder block
Mercedes	16.0 L V8 diesel	Cylinder block
Mercedes	10-15 L HDEP I-6 family	Cylinder head
Renault	3.0 L V6 diesel	Cylinder block
Volvo	Heavy-duty diesel	Cylinder block

11.4 A carbon-fibre-reinforced polymer conrod of R4 (82/71) engine.

ceramic nanoparticles, such as Si3N4, Al2O3 and B4C which may make up between 5 and 30% of the material volume. These materials have relatively high strength and stiffness at low density and the anisotropic distribution of mechanical properties is advantageous in the manufacture of certain engine components such as conrods. The mechanical properties of carbon-fibre reinforced polymer used in the compound conrod of the R4 (82/71) engine are compared with steel properties in Table 11.3. The material is made of two monolayer polymers which differ in the orientation of their fibres. The main part of the conrod and its cap are made of two monolayer polymers mixed in different proportions.

Significant efforts are currently being made to develop designs for pistons made of graphite/carbon fibre or carbon/carbon fibre type composite materials. These materials have a graphite or carbon matrix strengthened by carbon fibres. The strengthening of the material usually takes place in four directions, giving the material isotropic mechanical properties on the macro scale. The main advantage of these materials is their combination of comparatively low density and high strength. They also have stable mechanical properties over the typical temperature range for internal combustion engines. The high cost of these materials currently prevents their large scale application in engine production. Table 11.4 shows the mechanical properties of a graphite/ carbon fibre composite material used in the production of prospective piston designs.

11.4 Advanced polymers: polyamides for manufacturing intake manifolds

By the end of the 20th century, plastic had became a popular material replacing conventional aluminium for manufacturing engine intake manifolds.

Table 11.3 Mechanical properties of carbon fibreglass material

Properties	Material
	Monolayer (0°±90°)	Monolayer (0°±45°)	Steel	Main part of the conrod	Cap of the conrod
r, g/cm3	2.0	2.0	7.8	2.0	2.0
¥ 10–3 MPa Ex	125	121	200	92	80
¥ 10–3 MPa Ey	8.4	8.35	–	34.2	46
mxy	0.29	0.29	0.3	0.133	0.1085
myx			–	0.0494	0.024
¥ 10–3 MPa Gxy	4.3	4.5	77	6.6	6.98
s+ x/s– x MPa	1,050/950	900/850	1,000	226.7/666.6	195.6/592.2
s+ y/s– y MPa	20/120	30/118	–	82.1/258.5	111./348.3
txy MPa	60	60	800	111.2	–
Specific strength, s+ x/s– x, m ¥ 10–3	53.0/48.0	4.6/4.3	9.3	12/34	10.0/30.2
Specific strength, s+ y/s– y m ¥ 10–3	0.1/0.6	0.15/0.6	9.3	4.2/13.2	5.7/17.8
Specific stiffness, Ex/g*, m ¥ 10–3	6,400	6,200	2,600	4,700	4,100
Specific stiffness, Ey/g*, m ¥ 10–3	430	430	–	1,750	2.350

Table 11.4 Mechanical properties of a graphite-carbon fibre composite material

Bulk density, g/cm3	1.8–2.0
Young (elastic) modulus, GPa	13–16
Bending strength, MPa	80–120
Compressive strength, MPa	150–250
CTE, 10–6 K–1	5–7
Thermal conductivity, W/mK	40–60
Specific stiffness, km	6,800
Specific bending strength, km	5.0
Specific compressive strength, km	10.0

These are known as plastic air intake manifolds (PAIM) (see Fig. 11.5). The application of plastics provides around 40% weight savings which results in better fuel efficiency and lower emissions. Air intake manifolds made of Ultramid enhance the engine air supply and thus help to increase performance. The smooth internal walls of polyamide intake channels provide lower drag properties than aluminium. Plastic may also be more easily shaped to produce a favourable air flow design at reduced cost.

Ultramid is a typical example of plastic material and consists of glass-

11.5 A plastic air intake manifold (PAIM) with integrated adjacent components.

fibre reinforced polyamide which is resistant to hot air and heat ageing. It can withstand contact with hot air at 120 °C for sustained periods as well as peak temperatures of 150 °C. Plastic materials are currently optimised to cope with the increasing mechanical and thermal requirements of modern air intake systems and to improve their dynamic and acoustic characteristics. Active air intake manifolds with a variable runner length help to significantly improve engine performance and to allow the functional integration of adjacent components – such as air filter housings, cylinder head covers and engine covers. The consolidation of the number of parts also reduces weight and cost. Plastic air intake manifolds are currently installed on the majority of gasoline engine cars.

11.5 Advanced alloys and ceramics for manufacturing valves and other components

NIMONIC® 80A alloy (UNS N07080/W. Nr. 2.4952 & 2.4631) is a wrought, age-hardenable nickel-chromium alloy. It is strengthened by the addition of titanium, aluminium and carbon and has been developed for use at temperatures up to 815 °C. It is produced by high-frequency melting and casting in air for extruded products. Electroslag refined material is used for forged products. Vacuum refined versions are also available. NIMONIC® alloy 80A is currently used for gas turbine components (blades, rings and discs), bolts, nuclear boiler tube supports, die casting inserts and cores and also for automobile exhaust valves. Its chemical composition is presented in Table 11.5. Stellite alloys are generally used as coatings to increase the wear resistance of valves.

There is an ongoing search for new materials to make lighter valves. Ceramic materials and intermetallic compounds such as titanium aluminide TiAl (g) and alloys of the titan Ti3Al (a, a + b) have been the focus of a number of investigations. Among ceramic materials, Si3N4, SiC and В4С are considered to have potential for the production of valves. It is hypothetically possible to lower the weight of valves by a factor of 2.5–3 by using these materials. The main shortcoming of ceramic materials is the distribution of their properties over a very wide range. They are also prone to sudden breakage which can be characterised by Weibull modulus (7–8 for ceramics). The combination of complex technology and the high cost of manufacturing has restricted the large scale deployment of these materials in the manufacture of engine components.

The ceramic material Si3N4 has high temperature strength, wear resistance and resistance to corrosion. Its density is half that of other heat resistant alloys, making it the most prospectively useful material for such components as intake and exhaust valves [4–6]. A number of engine manufacturers, including Isuzu, Mazda, Nissan, Mercedes-Benz (concept engine) and GM (sports car) have developed ceramic valves. However, further investigation is required due to the material's insufficient level of destruction viscosity and its low crack resistance [7, 8]. The cost of machining silicon nitride ceramics is very high compared to other types of material.

One promising direction in the development of a new generation of construction materials is the creation of heat resistant inter-metallic compounds based on titanium aluminide, particularly TiAl (g). Alloys of this type exceed

Table 11.5 Chemical composition of
NIMONIC® alloy 80A (% by weight)

Carbon	0.10 max
Chromium	18.0–21.0
Silicon	1.0 max
Copper	0.2 max
Iron	3.0 max
Manganese	1.0 max
Titanium	1.8–2.7
Aluminum	1.0–1.8
Cobalt	2.0 max
Boron	0.008 max
Zirconium	0.15 max
Lead	0.0025 max
Sulfur	0.015 max
Nickel	Balance

the best existing metallic materials in specific strength properties under elevated temperature conditions. Their excellent casting properties [9, 10] also allow for the production of parts with a complex shape. Valves for internal combustion engines are produced from TiAl alloy by casting, profile stamping, extrusion and diffusive welding [9, 11–15]. According to [11], the replacement of valves made of heavyweight metals (high carbon steels and nickel alloys) with those made of lighter materials such as g titanium aluminides decreases the stress in intake and exhaust engine valves by a factor of two, thus increasing the durability of the valves and improving the engine use parameters. In developing the manufacturing technology, the main focus should be on achieving low cost valves and high overall efficiency. With this in mind, the cast shape method offers the greatest potential. Using this method combined with hot isostatic pressing and heat treatment of g TiAl, the following properties are achieved [16, 17]: tensile strength – 360–700 MPa; toughness, K1c – 12–30 (MPa) and elongation – 0.04–1.5%

A further increase in the plasticity (elongation) of g TiAl to values of 2.5–3.8% can be achieved by isothermal die forging with subsequent thermal treatment to create the conditions for the formation of fine-grained microstructures. The specific strength value remains the same when the material is heated up to 700–800 °C, but its plasticity level is considerably increased. The problem is therefore to maintain a high level of durability (TS> 500–600 MPa and plasticity EL>2%) when casting technology is used. A similar combination of properties g TiAl was obtained in prototypes.

The majority of petrol engine failures are due to valve breakages. These are caused by the high operating temperatures and stress levels in the heads of valves (particularly exhaust valves). Stresses are also caused by the uneven temperature distribution (which is 700–800 °C and 150 °C in the head and stems, respectively, of exhaust valves). Research has therefore been undertaken on manufacturing valves using a combination of two types of materials: valve stems of titanium alloy and heads made of g TiAl. The linear thermal expansion and thermal conductivity coefficients of these two materials are sufficiently close, making it possible to eliminate high thermal stresses in compound valves. The main shortcoming of g TiAl material is its low wear resistance and the application of coating made of materials with high wear resistance. Table 11.6 shows some of the physical and mechanical properties of materials used for manufacturing valves.

Inter-metallic titanium-based a + b – Ti3Al alloys provide superior tribological performance and the technologies for manufacturing components, including coatings, from these materials are well developed. Hundreds of thousands of valves are currently produced from these materials every year.

Table 11.7 presents the physical and mechanical properties of ceramics which have been considered for use in the manufacture of engine parts subject

Table 11.6 Physical and mechanical properties of materials used for manufacturing valves

	l (W/(mK))	aT (106К–1)	E, (GPa)	Poisson coefficient (m)
Alloy steel HNV3/40X9C2	20.0	14.3	200	0.3
Alloy steel EV12/55X20G9AH4	23.0	18.0	150	0.3
Titanium alloy	16.0	9.3	82	0.35
Ti-Al intermetallic	22.0	11.0	170	0.33
Si3N4 ceramic	21.0	2.7	310	0.27

Table 11.7 Physical and mechanical properties of ceramics

		HPSN (hot pressed silicon nitride ceramics) Si3N4, HP	Silicon nitride ceramics Si3N4, КВ	Silicon nitride ceramics SiC, Sint	Zirconium dioxide ZrO2	Al2O3TiO2
Bending, strength sB,	500 °C	650	300	450	300	40
MPa	1,200 °C	400	300	450	–	–
Compressive strength sCom, MPa	20 °C	2,500	1,500	3,500	2,000	550
Density r, g/cm3		3.2	2.6	3.1	5.7	3.1
Thermal conductivity l, W/(m ¥ K)		20.0	10.0	50.0	3.5	2.0
Specific heat c, kJ/(m3 ¥ K)		2,300	2,000	3,200	2,300	2,200
Coefficient of linear thermal expansion aT, 1/K ¥ 106		3.5	3.2	4.5	9.8	3.0
Elastic modulus E, MPa		2.9 ¥ 105	1.8 ¥ 105	4.1 ¥ 105	2.0 ¥ 105	2.0 ¥ 104
l ¥ c		46.0 ¥ 103	20.0 ¥ 103	160.0 ¥ 103	8.0 ¥ 103	4.4 ¥ 103
500 °C/E R1 = sB ¥ aT		640	520	240	150	890
500 °C/E R2 = lsB	¥ aT	12.8 ¥ 103	5.2 ¥ 103	12.0 ¥ 103	0.5 ¥ 103	1.8 ¥ 103
R3 = sCom/EDaTA1		465	440	490	820	1,930

to high thermal stresses. The bottom part of the table provides information on the values of some complex parameters, such as *R*1 = s^B ⁵⁰⁰ °C/E · aT which indicates permissible temperature gradients; *R*2 = ls^B ⁵⁰⁰ °C/E · aT, which indicates the resistance to thermal shocks and *R*3 = sCom/*E*DaT A1, which characterises the stresses arising from the difference in the coefficients of linear expansion DaT of ceramic inserts and the metallic piston parts (particularly those made of aluminium alloys).

11.6 Materials for particular components in IC engines

11.6.1 Crankshafts

Crankshafts are one of the most crucial components of internal combustion engines and are subjected to very high dynamic loads during engine use. The selection of materials and manufacturing methods depend on the type of engine and the geometry and design of the crankshaft. It is necessary to use high strength materials to ensure a long operational life. Steel crankshafts are usually manufactured by forging or die forging or by casting if the material is cast iron.

In low and medium speed engines, crankshafts are frequently made of low alloy carbon steels such as 1034, 1035, 1038 (UK: 060A35, 080A32, 080A35, 080A5, 080M36, 1449-40CS, 40HS, C35; Russia: 35); 1040 and 1042 (UK: 060A40, 080A40, 080M40, C40, C40E, CS40; Russia: 40); 1049, 1050, 1055 (UK: 060A52, 070M55, 080M50, C50E, C55; Russia: 50); 1037 (UK: Gr.A; Germany: GS20Mn5; Russia: 35Г); 1039, 1041, 1043 (UK: 150M36; Russia: 40Г); 1045, 1046 (UK: 080M46, C45, C45E; Russia: 45Г); 1053 (UK: 080M50, C50E; Russia: 50Г).

Crankshafts for automobile and tractor engines are made of 1044, 1045, 1045H (UK: 060A47, 080M, 080M46, 1449-50CS, 1449-50HS, 50HS, C45, C45E; Russia: 45); 1040, 1046 (UK: 080M46, C45, C45E; Russia: 45Г); 1050, 1053 (UK: 080M50, C50E; Russia: 50Г) steels and also of 5135, 5140, 5140H, 5140RH (UK: 37Cr4, 41Cr4, 530A36, 530A40, 530H36, 530H40, 530M40; Russia: 40Х); 3135, 3140H (UK: 640M40; Russia: 40ХН); 1144, 1345, 1345H (Germany: 46Mn27; Russia: 45Г2); 4340 (UK: 817A37, 818M40; Russia: 40Х2Н2МА) steel alloys. Thermal treatment of the steel after hardening and low temperature tempering provides the martensite structure of the material.

Steels such as chromium vanadium, chromium-molybdenum, nickelchromium and nickel-chromium molybdenum alloys 4130, 4130H (UK: 25CrMo4; Russia: 30XMA); 4340 (UK: 817A37, 818M40; Russia: 40Х2Н2МА); 25H2N4WA (Poland) or 25Х2Н4МА (Russia), 38ХН3МА (Russia) or SNC28 (Japan) are used for manufacturing crankshafts for highspeed, high power diesel engines for various applications.

Crankshafts in marine, stationary, locomotive and tractor engines are often made by casting modified cast iron, such as 60-40-18 (UK: EN-GJS-420-12; Russia: ВЧ40) with a globular graphite and pearlite-ferrite structure. Carbon steels and alloy steels may also be used for this purpose.

Cast iron crankshafts are cheaper and less time consuming to manufacture than those made from forged steel. Metal may be more economically used in the casting process when the design of the crankshaft becomes more complex. The wear resistance of the main and crankpin bearings increases due to the presence of graphite in cast iron. The reliability of the crankshaft is also improved due to the high cyclic strength of cast iron. However, it should be noted that the mechanical strength of cast iron crankshafts is inferior to those forged from steel and it is more difficult to determine internal casting defects.

When Babbitt bearings or hard alloys such as leaded bronze are used for manufacturing bearing shells, heat treatment is used to obtain the high levels of hardness required for increased wear resistance in the crankshaft journals.

11.6.2 Connecting rods

Engine connecting rods (see Fig. 11.6) are usually produced by press forging with subsequent heat treatment and machining. Carbon steels or alloy steels are the most commonly used materials. Alloy steels are used for manufacturing conrods for high performance engines as they provide the required cyclic strength without substantially increasing the dimensions of critical sections. They also offer uniform distribution of strength properties, such as metal hardness after thermal treatment.

11.6 An engine connecting rod.

Long experience of the manufacturing and operation of engines, has shown the following moderately alloyed steels to be suitable:

∑ for conrods of automobile engines with spark ignition 1035, 1039, 1040, 1041, 1043 (UK: 150M36; Russia: 40Г), 1144, 1345, 1345H (Germany: 46Mn27; Russia: 45Г2), 5135, 5140, 5140H, 5140RH (UK: 37Cr4, 41Cr4, 530A36, 530A40, 530H36, 530H40, 530M40; Russia: 40Х); 3135, 3140H (UK: 640M40; Russia: 40ХН)
∑ steels 832M13 and 835M15 (UK); 14NiCrMo134, GX19NiCrMo4 and X19NiCrMo4 (Germany); 18Х2Н4МА (Russia); 4340, 9840 (UK: 34CrNiMo6, 36NiCrMo4, 708M40, 816M40, 817A37, 817M37, 817M40, 818M40; Russia: 40ХН2МА) are used for the conrods of automobile and tractor diesel engines. Cold/hot forged pearlitic cast iron may also be used for the production of these components
∑ steels 832M13 and 835M15 (UK); 14NiCrMo134, GX19NiCrMo4 and X19NiCrMo4 (Germany); 18Х2Н4МА (Russia); 4340, 9840 (UK: 34CrNiMo6, 36NiCrMo4, 708M40, 816M40, 817A37, 817M37, 817M40, 818M40; Russia: 40ХН2МА), 4340 (UK: 817A37, 818M40; Russia: 40Х2Н2МА), 4340 (UK: 817M40; Russia: 36Х2Н2МФА), 4130, 4130H (UK: 25CrMo4; Russia: 30ХМА), 4140, 4142, 6135 (UK: 3111-5/1, 42CrMO4, 708M40; Russia: 40ХФА) are used for high load conrods with relatively large cross-sectional areas
∑ steels 832M13 and 835M15 (UK), 14NiCrMo134, GX19NiCrMo4 and X19NiCrMo4 (Germany), 18Х2Н4МА (Russia); 4340, 9840 (UK: 34CrNiMo6, 36NiCrMo4, 708M40, 816M40, 817A37, 817M37, 817M40, 818M40; Russia: 40ХН2МА), 4330, 4337 (UK: 34CrNiMo6, 816M40, 817M40; Russia: 38Х2Н2МА) are used for conrods of high-speed, high performance supercharged diesel engines
∑ steels 5135, 5140, 5140H, 5140RH (UK: 37Cr4, 41Cr4, 530A36, 530A40, 530H36, 530H40, 530M40; Russia: 40Х); 5145, 5145H (Germany: 41Cr4; Russia: 45Х), 3135, 3140H (UK: 640M40; Russia: 40ХН), 1144, 1345, 1345H (Germany: 46Mn27; Russia: 45Г2) are used for low-speed stationary and marine diesel engines.

Carbon steels are used for the production of relatively small conrods subjected to moderate loads and for conrods of very large dimensions.

In order to take full advantage of the high strength of alloy steels, it is necessary to take into consideration their high sensitivity to surface quality and condition on the macro and micro geometry scale. Up to 50–70% of the metal mechanical strength may be lost if the metal surface is not machined after the forging process. A further reason for such a significant reduction in the strength is the presence of weakened metallic layers in which a considerable part of the carbon and some alloying elements is burned out as a result of repeated heating during the forging and thermal treatment (hardening) processes.

Conrods made of alloy steels require machining or scraping to remove the decarbonised layers. Alternatively, they can be heated in a protective environment followed by closed-die forging. If the thickness of the remaining layers is 0.3–0.4 mm, the reduction of metal strength can be totally compensated by shot blasting.

In high-speed four-stroke engines, the conrod blades are subjected to tensile stresses up to 20 MPa and this results in a high possibility of fatigue cracks developing in the weakened and damaged surface layers of metal. This results in conrod breakdown. It is therefore important to remove the decarbonised surface layer fully and subject the cleaned surface to intensive shot blasting to eliminate possible residues from the defective surface layer.

Particular consideration should be given to the selection of manufacturing treatment processes for the blades of main rods in V-type four- or two-stroke engines as these are subjected to considerable bending stresses which are increased by additional dynamic loadings from knuckle conrods.

The thickness of the decarbonised surface layer of conrods following forging and heat treatment is defined by their dimensions and is proportional to the time they remain in the heating oven. For conrods with a mass of 100–150 kg, the thickness of the layer is around 1.0–1.5 mm.

Research and experience of engine use indicates that alloy steels with a low carbon content, such as 832M13 and 835M15 (UK); 14NiCrMo134, GX19NiCrMo, X19NiCrMo4 (Germany) and 18Х2Н4МА (Russia) cannot be recommended for the manufacture of conrod bolts or bolts used in other joints where considerable axial drawing-up forces are required in the assembly process if plastic deformation of the material is to be avoided over a long period of engine use.

Conrod blades in two-stroke engines are mainly subjected to compression stresses. The effect of the weakened surface layer is therefore negligible and additional treatment of the surface layers can be minimal, involving only surface scaling or scraping.

In some cases, the conrods of high-speed engines may be made of aluminium or magnesium alloys. The mass of these conrods is 20–25% less than that of steel. The application of titanium alloys for manufacturing conrods for high performance turbocharged engines is of considerable interest because of their good strength properties and low density (the density of titanium alloys is half of that of steels.) The fatigue properties of titanium alloys are also superior to those of steel materials.

Titanium alloys with an increased aluminium content (7.5–14% by mass) are currently available. In these alloys, an intermetallic compound, Ti3Al, is formed which reinforces the alloy and improves its rigidity. The increase of mass content of aluminium in the alloy from 7.5 to 14% results in a rise of the elasticity modulus from 120 to 150 GPa. The magnitude of compression stresses in conrods considerably exceeds that of the tensile stresses so conrods made of titanium alloys which can better withstand compression stresses are advantageous.

The disadvantages of titanium alloys are their lower wear resistance, a predisposition to creep at ambient temperature conditions and a rapid decrease in the fatigue limit (compared to steels) when the number of dynamic load cycles exceeds 10⁷ .

Bolts for assembling the compound heads of conrods in modern engines are largely made of structural alloy steels. In low speed (primarily two-stroke) engines, these bolts are made of carbon structural steels such as 1035 (UK: C35; Russia: 35), 1040 (UK: C45; Russia: 40); 1045 (UK: C45; Russia: 45) etc., and also of medium carbon nickel-chromium steels. The same materials are used for manufacturing cross-head rods in low speed engines (steels 1045 (UK: C45; Russia: 45), 1050 (UK: C50; Russia 50)). Medium carbon chromium-molybdenum and nickel-chromium steels are also used to manufacture cross-heads with subsequent heat treatment. The cross-heads of low speed engines are also manufactured by casting, using steels such as EN1A low carbon mild steel (UK: 430(A) – A732(4A); Russia: 20Л-50Л).

Conrod bolts for locomotive diesel engines and high-speed supercharged engines are made of the strongest and most ductile chromium-molybdenum and nickel chromium molybdenum vanadium steels such as 5130, 5130H, 5130RH (UK: 28Cr4, 530A30; Russia: 30Х); 4340, 9840 (UK: 34CrNiMo6, 36NiCrMo4, 708M40, 816M40, 817A37, 817M37, 817M40, 818M40; Russia: 40ХН2МА, 40Х2Н2МА, 36Х2Н2МФА).

Conrod bolts for automobile and tractor engines are made of alloy steels 5135, 5140, 5140H, 5140RH (UK: 37Cr4, 41Cr4, 530A36, 530A40, 530H36, 530H40, 530M40; Russia: 40Х), 5145, 5145H (Germany: 41Cr4; Russia: 45Х), 4130, 4130H (UK: 25CrMo4; Russia: 30ХМА); 4135, 4135H, 4137, 4137H (UK: 34CrMo4, 708A37; Russia: 35ХМ), 4340, 9840 (UK: 34CrNiMo6, 36NiCrMo4, 708M40, 816M40, 817A37, 817M37, 817M40, 818M40; Russia: 40ХН2МА, 40ХНМА).

Bearing bushes, which are the outer races of needle bearings, are made of steels 52100 (UK: 2S135, 534A99, 525A99; Russia: ШХ-15Ш) or 8Х4В9Ф2-Ш (Russia). A number of anti-friction materials are used for manufacturing bushes. These include tin phosphor bronze C52100 (UK: PB104; Russia: БрОФ7-0.2 with НВ 90-120) with maximum permissible load pressures up to 90 MPa; aluminium bronze C90500 (Russia: Br. АЖ9-4 with НВ 110); copper-zinc alloy brass C95200 (Russia: ОЦ 10-2 with НВ 110).

The following properties are taken into account in the selection process: specific strength sB/g, fatigue strength s–1/g and specific stiffness E/g (g = r · g, r and g are the material density, kg/m³ , and gravity constant (9.81 m/s² ), respectively).

The mechanical properties of materials produced in the USA and Europe are relatively easy to access through the relevant handbooks or designated websites. As an example, Table 11.8 presents the properties of materials used in Russia for manufacturing conrods for internal combustion engines. This table also presents information on the properties of Titanium alloy Ti-6Al-4V produced in the USA.

11.6.3 Pistons

Materials for the manufacture of pistons must meet the following requirements: light weight, sufficient mechanical strength over a range of working temperatures, low linear expansion coefficient, wear resistance, high thermal conductivity at elevated temperatures and good machining ability. The ductility of the material is also very important, as are the combinations of properties which determine the level of thermal stresses. The materials used will vary according to the design and purpose of the engine.

Table 11.8 Some materials used for production of engine connecting rods in Russia

	Alloy steel 40XH	Alloy steel 40XH2MA	Alloy steel 18X2H4MA	Titanium alloy Ti-6Al-4V (USA)	Aluminium alloy Д16T
Density r, g/cm3	7.85	7.85	7.95	4.43	2.8
Hardness, HBN	200–320	217–333	262–400	255–340	105
Tensile strength sB, MPa	910	1,000	940–1,470	≥930	460
Yield strength s0.2, MPa	710	850	710–1,170	≥862	300
Yield strength s–0.2, MPa (compression)	710	850	710–1,170	917	285
Elastic modulus ¥ 10–3 MPa	204	204	150	105–115	71
Fatigue limit s–1, MPa	392–490	447	475–774	560	135
Specific strength s0.2/g* , m ¥ 10–3	8.3	10.8	8.3–14.7	19.46	10.7
Specific strength (compression) s–0.2/g* , m ¥ 10–3	8.3	10.8	8.3–14.7	20.7	10.18
Specific stiffness E/g* , m ¥ 10–3	2,600	2,600	1,890	2,370– 2,600	2,530
Fatigue strength s–1/g* , m ¥ 10–3	5–6.24	5.7	6–9.7	12.64	4.8

^* g = r · g where r is density and g equals 9.81 ms–2

The pistons of high-speed engines, including automobiles and tractors, are made of light alloys. The methods used include gravity casting, die casting and hot die forging. Eutectic silumin type aluminium alloy A384 (Russia: АЛ25 with 10–13% Si) and hypereutectic aluminium alloy, which contains copper, nickel and manganese additives, are used for gravity die casting. Pistons cast using traditional/conventional technologies have insufficient thermal stability, fatigue durability, hardness and wear resistance and are therefore of limited application in modern high performance engines.

Die forging is used to increase the strength and wear resistance of pistons. Those made of aluminium alloy A04130, A14130 (UK: 4L33, LM20; Russia: АК12Д) with a silicon content of 13% using hot die forging, have high wear resistance of piston ring grooves and a coefficient of linear expansion which is lower by 10–12% than in the case of conventional aluminium wrought alloys 2618 (UK: 717, 724, 745; Russia: АК4) and AA2018, AA2218, AA2618 (Russia: АК4-1). Many engine manufacturers have proposed the production of aluminium alloy pistons by liquid metal forming technology in order to increase fatigue strength, resistance to corrosion and wear resistance. An increase in strength of 10–15% is obtained by a combination of liquid metal forming technology and reinforcement of the head of the piston with ceramic fibres.

Traditional aluminium alloys can be used for manufacturing pistons which work at a maximum temperature of around 350 °С and are used alongside light alloys, various cast irons and steels for piston manufacturing. Pistons for supercharged, medium speed engines are manufactured using grey cast iron and high-strength cast iron. Cast irons have a higher melting point than aluminium alloys and there is therefore a possibility of burnout in piston edges which face the combustion chamber.

The heads of compound pistons are made of high temperature steels such as 20Х3МВФ (Russia) for locomotive diesel engines; 310, 310S, 314 (UK: 310S16; 310S24, 314S25, 310S31, X8CrNi25-21; Russia: 20Х25Н20С2 and 20Х23Н18) for pistons in high-speed transport diesel engines; A387 Gr 12 CL 2; A335 P12 (K11562) (UK: 13CrMo4-5; Russia: 15ХМ), 4118, 4130 (UK: 25CrMo4, 708A25, 708H20; Russia: 20ХМ) for pistons in low speed two stroke diesel engines. There have been attempts to use ceramic (Si3N4; ZrО2) coatings on the heads of compound pistons in order to reduce the heat transfer from pistons to cylinder walls. However, low plasticity, high brittleness and difficulties in machining ceramics, combined with high costs, are drawbacks to the use of ceramics in full scale production.

High temperature steels are used for manufacturing pistons where the maximum temperature of the piston exceeds 450 °С. High nickel chromium alloy steels have been successfully used in the manufacture of piston heads for high-speed two stroke engines where the maximum temperature in the head of the piston is around 900 °С.

11.6.4 Piston pins

Piston pins are made of alloy carburised steel. For piston pins in low speed engines, carbon steels 1015 and 1020 (UK: C15E and C22E; Russia: 15 and 20) are used; in high performance supercharged or turbocharged automobile, tractor and locomotive engines the pins are made of alloy carbon steels such as A387 Gr 12 CL 2; A335 P12 (K11562) (UK: 13CrMo4-5; Russia: 15ХМ), 5140H (UK: 41Cr4, 530A36, 530M40; Russia: 38ХА), 3415 (UK: 655M13; Russia: 12ХН3А); 820A16 (UK) or 18Х2Н4ВА (Russia); 3120, 3316H (UK: 822M17; Russia: 20Х2Н4А), etc. The external surface of the pins is carburised to a depth of 0.5–1.5 mm, then hardened and tempered. Finally, the pin surface goes through a nitriding process to improve its hardness to a level of НRС ≥ 58. Induction hardening to the depth of 1.0–1.5 mm is applied to the pin surfaces where the pins are made of carbon steel 1045 (UK: C45; Russia: 45). Thermal treatment is more economical than cementation and provides a good surface quality. To achieve the necessary high surface quality, the pins should also undergo fine machining followed by polishing.

11.6.5 Piston rings

Piston rings are usually made of cast iron and steel. Wear resistance is an important requirement in the materials used for their manufacture. The material should also have a low friction coefficient to enable the rings to slide along the cylinder's surface and should be able to withstand high pressures at various temperatures under limited lubrication conditions. It should not be prone to seizing when rings are moving in contact with a cylinder and should have a high modulus of elasticity (to provide the required specific pressure upon a cylinder's surface), high yield strength and hardness. Grey modified cast iron meets the above requirements.

The properties of grey modified cast iron mostly depend on the structure formed during the casting process. In order to improve the mechanical characteristics of the rings, centrifugal casting with subsequent heat treatment is applied. To improve the material properties, a small amount of chrome, copper, molybdenum, vanadium and tin additives are used. It is therefore possible to achieve sTS = 480 МPа, Е = (1.4–1.7) · 10⁵ МPа and HBN of 230–285. High-strength cast irons are also used for manufacturing piston rings, and in this case sTS = 530 МPа, Е = (1.4–1.8) · 10⁵ МPа and НВ is 240–300 (perlite structure). The first upper piston ring in many types of engine is usually made of D2 tool steel (Germany: X165CrMoV12; Russia: Х12М), 1050, 1053 (UK: 080M50, C50E; Russia: 50Г); 1066 and 1566 (UK: 080A67; Russia: 65Г) strips. The rings of high performance supercharged heavy duty engines are made of 4Х5МФ1СШ steel (Russia) in order to increase their strength.

11.6.6 Engine cylinder valves

Engine cylinder valves are subjected to both high mechanical and temperature stresses. These conditions define the requirements of the materials used for manufacturing valves. The materials must provide valves of minimum weight; high thermal strength and thermal stability at temperatures up to 1000 °С (for exhaust valves). Martensitic stainless steels such as HNV3, HW3, S65007 (UK: 401S45, X45CrSi9-3; Russia: 40Х9С2); X40CrSiMo10-2 (UK and Germany); 40Х10С2М (Russia); S63005 (Russia: 30Х13Н7С2) are traditionally used for manufacturing valves; ferritic stainless steel such as 405, S40500 (UK: 403S17, X10CrAlSi13; Russia: 10Х13СЮ) are used for manufacturing inlet valves and austenitic steels such as 45Х22Н4М3 (Russia), EV12, EV8, S63008 (UK: 349S54, X53CrMnNiN21-9; Russia: 55Х20Г9АН4), S66009 (UK: 331S42; Russia: 45Х14Н14В2М) and NIMONIC 80A alloy are used for the production of exhaust valves. Table 11.9 shows recommendations on the application of different steels for specific temperature levels.

11.6.7 Camshafts and cams

Depending on the type of engine, these components are usually made of steels or cast iron. Camshafts manufactured by forging use medium carbon steels 1040, 1042 (UK: 060A40, 080A40, 080M40, C40, C40E, CS40; Russia: 40); 1044, 1045, 1045H (UK: 060A47, 080M, 080M46, 1449-50CS, 1449-50HS, 50HS, C45, C45E; Russia: 45), 5145, 5145H (Germany: 41Cr4; Russia: 45Х) or 5015, 5115 (UK: 17Cr3, 523M15; Russia: 15Х), 5117, 5120, 5120H (UK: 207; Russia: 20Х), 3415 (UK: 655M13; Russia: 12ХН3А) and 20ХН3А (Russia) alloy steels for high performance supercharged engines. The surfaces of camshafts (or cam lobes if they are manufactured separately) and their end faces, which are in contact with the thrust bearings, should have high wear resistance, and HRC 55–65 is obtained by thermochemical treatment and surface quenching. Camshaft journals are also polished. In automobile and tractor engines, the camshafts (or cam lobes) are made of chilled cast iron, which is comparable to the alloyed steels used in the manufacture of

Table 11.9 Recommendations on application of different steels for certain temperature levels

Material	Temperature level (°C)
HNV3/40X9C2	650
X40CrSiMo10-2/40X10C2M	650
45X22H4M3	850
S66009/45X14H14B2M	650
EV12/55X20Г9AH4	800
NIMONIC®80A	815

bearings. The wear resistance of chilled cast iron is considerably higher of that of ductile cast iron.

11.6.8 Tappets and pushrods

The tappets and pushrods of valve trains are produced from low carbon steels 1015 and 1030 (UK: C15 and C30; Russia 15 and 30); low alloyed steels 5015, 5115 (UK: 17Cr3, 523M15; Russia: 15Х), 5117, 5120, 5120H (UK: 207; Russia: 20Х), 5135, 5140, 5140H, 5140RH (UK: 37Cr4, 41Cr4, 530A36, 530A40, 530H36, 530H40, 530M40; Russia: 40Х) or grey pearlitic cast iron. The surfaces of tappets and pushrods made of low carbon steels are carbonised and if medium carbon steels are used, the surfaces are quenched to provide a surface hardness of HRC 50–60. Rocker arms are also made of alloyed steels 5117, 5120, 5120H (UK: 207; Russia: 20Х), and 5145, 5145H (Germany: 41Cr4; Russia: 45Х), or of cast iron with the contact surfaces hardened to a level of HRC 50–60. Tapper and/or rocker arm rolls are made of bearing (high alloyed) steels.

11.6.9 Valve springs

Valve springs are made of steel wire and their diameter is defined by the loading forces which act upon them during the engine operation. When two valve springs are installed co-axially, the external one takes up about 60–70% of the total load. The mean diameters of external and internal valve springs is usually 0.72–0.9 and 0.5–0.75 of the diameter of the valve's head.

Alloyed steels such as 1070, 1566 (UK: 060A67; Russia: 65ГА), 6145 and 6150 (UK: 51CrV4, 735A50, 735A51, 735A54, 735h51; Russia: 50ХФА) are used for manufacturing valve springs. The surfaces of the springs are treated by shot blasting to increase their durability. The surfaces of valve springs are oxidised and coated with enamel to improve their corrosion resistance. Zinc or cadmium coating is also used for the surface protection of valve springs. Valve keepers are made of carbon and low alloy steels.

11.6.10Cylinder blocks

The selection of materials for production of cylinder blocks depends on the type and function of the engine. Engine blocks are usually made from cast iron or aluminium alloys. Perlitic grey cast iron such as A48-30B (UK: Grade 200; Russia: СЧ20), A48-40B (UK: Grade 250; Russia: СЧ25) and A48-45 B (UK: Grade 300; Russia: СЧ30) and high strength ductile cast iron such as ВЧ35 (Russia), 60-40-18 (UK: EN-GJS-420-12; Russia: ВЧ40), 65-45-12 (UK: EN-GJS-450-10; Russia: ВЧ45), 70-50-05 (UK: EN-GJS-500-7; Russia: ВЧ50), 80-60-03 (UK: EN-GJS-600-7; Russia: ВЧ60) and 100-70-03 (UK: EN-GJS-700-2; Russia: ВЧ70) are used for this purpose. The high fluidity of cast iron makes it possible to produce engine blocks with very complex geometry. Cast blocks undergo machining and for that reason, their hardness should not be greater than НВN 230.

The weight of engine blocks is reduced by using aluminium alloys in the casting process. These alloys have lower fluidity than cast iron therefore the walls of aluminium alloy engine blocks are usually thicker. Steel insertions are used in parts of aluminium alloy engine blocks in which studs are installed for fastening the main engine components. Eutectic silumin type aluminium alloys A413.0 (Russia: АЛ2), АЛ4 (Russia), A356.0 (Russia: АЛ9) and hypereutectic aluminium alloys, which contain copper, nickel and manganese additives are used for casting with subsequent hardening and tempering for high performance machining. The casting process takes place at 12 MPa pressure and the moulding temperature is between 200 and 250 °C. The production of aluminium engine blocks is noticeably more expensive when compared to cast iron.

The casings of heavy duty medium speed diesels and locomotive diesel engines are made from cast iron or by welding cast components made of low carbon steels. Aluminium alloys are used infrequently due to relatively large cylinder diameters and high loads. In high performance, supercharged low and medium speed locomotive and marine engines, high strength cast iron with a relatively large damping capacity is used for casing components to reduce noise and vibration during engine use. Killed low carbon steels 1020, 1025 and 1030 (UK: C20, C25 and C30; Russia: 20, 25 and 30) are also used for manufacturing welded casing components.

11.7 References

1. Steve Dawson, 'Compacted graphite iron a material solution for modern diesel engine cylinder blocks and heads', China Foundry, 6(3) 241–246 (2009).
2. M. Tholl, A. Magata and S. Dawson. Practical experience with passenger car engine blocks produced in high quality compacted graphite iron. SAE Paper 960297, SAE Congress, Detroit, 1996.
3. S. Birch. 'PSA and Ford: the next diesels', Automotive Engineering International – SAE, October 2003, p. 32.
4. G. Woetting, L. Frassek, G. Leimer and L. Shoenfelder 'Application-oriented development of high-performance-Si3N4 materials and components', CFI, 70(6) 1–8 (1993).
5. J. Huber and J. Heinrich 'Ceramics in internal combustion engines, in 2nd European Symposium on Engineering Ceramics Proceedings, 1987, pp. 203–221.
6. R.R. Wills and R.E. Southam 'Ceramic engine valves', J. Am. Ceram. Soc., 72(7), 1261–64 (1989).
7. Y. Maniette, M. Iganaki and M. Sakai 'Fracture, toughness and crack bridging of a silicon nitride ceramic', J. Europ. Ceram. Soc., 7, 255–63 (1991).
1. T.-S. Shen 'Microstructure and mecanical properties of the in situ b-Si3N4/a-sialon composite', J. Am. Ceram. Soc., 77(9) 2345–53 (1994).
1. D.E. Larsen, D.A. Wheeler and B. London 'Near-net shape processing of gamma and XD™-gamma titanium aluminides by investment casting'. Titanium 92 Science and Technical Proceedings of the Seventh World Titanium Conference, June 1992, San Diego, California, pp. 1487–93.
1. C.M. Austin and T.J. Kelly 'Development and implementation status of cast gamma titanium aluminide', Proceedings of the First International Symposium on Structural Intermetallics, September 1993, Champion, Pensylvania, pp. 143–150.
1. P.E. Jones, W.J. Porter, D. Eylon and G. Golvin 'Development of a low cast permanent mould casting process for TiAl automotive valves', Abstract Booklet of the 124th TMS Annual Meeting and Exibition, February 1995, Las Vegas, Nevada, p.13.
1. D. Smits, D. Eylon and P.E. Jones 'High speed centerless grinding of TiAl using conventional grinding wheels', Abstract Booklet of the 124th TMS Annual Meeting and Exibition, February 1995, Las Vegas, Nevada, p.15.
1. S. Hurta, H. Clemens, G. Frommeyer, H.-P. Nikolai and H. Sabum 'Valves of intermetallic g-TiAl base alloys: processing and properties', Abstract Booklet of the Eighth World Conference on Titanium, October 1995, Birmingham, S 3a/3.
1. D.E. Larsen, D.A. Wheeler and R. Diehm 'Recent developments in investment cast processing, properties and applications for gamma titanium aluminide alloy', Abstract Booklet of the Eighth World Conference on Titanium, October 1995, Birmingham, S 6a/2.
1. P.E. Jones, D. Eylon and G. Golvin 'Development of permanent mould casting methods to produce Ti-48Al-2Nb-2Cr automotive valves through the vertically integrated "Emtec Project"', Abstract Booklet of the Eigth World Conference on Titanium, October, 1995, Birmingham, S 7a/3.
1. Y-W. Kim 'Microstructural evolution and mechanical properties in gamma titanium aluminides', Microstructure Property Relationships in Titanium Aluminides and Alloys, TMS, 1991, pp.91–103.
1. W.E. Dowling, B.D. Worth, J.E. Alisson and J.W. Jones 'The influence of microstructure on the mechanical behavior of g/a2 titanium aluminides', Microstructure Property Relationships in Titanium Aluminides and Alloys, TMS, 1991, pp.123–133.

Plate I (Chapter 1) Ford Focus active grille shutter, (*C*D= Drag coefficient).

Plate II (Chapter 12) Six-speed front-wheel-drive manual transmission of Mercedes-Benz SG6-310.

Plate III (Chapter 12) Seven-speed dual-clutch transmission of Mercedes-Benz 7G-DCT.

Plate IV (Chapter 16) Radial piston pump with digitally controlled solenoid valves (Rampen, 2006). Photo courtesy Bosch Rexroth Corporation. Used by permission.

Plate V (Chapter 16) Three-dimensional surfaces indicating the combined effect of pressure, relative displacement and shaft speed on hydraulic motor efficiency.

Plate VI (Chapter 16) Detailed schematic depiction of the of the parallel hybrid system for a six-by-six off-road truck.

Plate VII (Chapter 16) Schematic depiction of the series hybrid system for a truck with a 4 x 4 driveline.

Plate VIII (Chapter 16) Stochastic dynamic programming framework for generating implementable nearoptimal control strategy for a series hydraulic hybrid. Threedimensional engine torque and speed surfaces illustrate the result that can be directly programmed into the electronic control unit.

Plate IX (Chapter 16) Engine visitation points on the BSFC map, with a color scale indicating the relative amount of fuel consumed in a given zone during the city driving schedule for (a) SDP based S-HHV controller and (b) mild parallel hybrid electric vehicle with integrated starter alternator.

Plate X (Chapter 16) Packaging of the series hydraulic hybrid system in the passenger car: the three cylinder engine (red) is coupled to the pump (green) and mounted longitudinally, this frees up enough space for the traction motor integrated with the gear-box below and right.

Plate XI (Chapter 16) Schematic view of the Bosch Rexroth HRB parallel hybrid system for refuse trucks. Photo courtesy Bosch Rexroth Corporation. Used by permission.

Plate XII (Chapter 16) Configuration of the Peugeot's series HHV system for a passenger car called HybridAir

Plate XIII (Chapter 16) Schematic representation of the valve events required to achieve various modes of operation in a pneumatic hybrid; TC = top dead center, BC = bottom dead center (from Donitz et al., 2011 with permission of ASME).

Plate XIV (Chapter 17) Actual braking events shown under the optimum braking distribution curve.

Plate XV (Chapter 17) The operating range of the electric motor limits the amount of regenerative braking force available.

Plate XVI (Chapter 17) The fraction of regenerative braking used varies with speed and deceleration rate.

Plate XVII (Chapter 19) 2012 Nissan Leaf battery pack without active cooling. Image courtesy of A2Mac1, LLC and Ricardo Strategic Consulting, North America.

Plate XVIII (Chapter 19) Chevrolet Volt battery pack using liquid cooling. Image courtesy of A2Mac1, LLC and Ricardo Strategic Consulting, North America.

Plate XIX (Chapter 22) Operational principle of a C/LiCoO2 Li-ion battery [7, 8]. Source: S. Visco, M.S. Whittingham, R. Kelley, J.S. Vetrano, Solid-electrolyte Interfaces and Interphases in Chemical Energy Storage, in: J.B. Goodenough, H.D. Abruña, M.V. Buchanan (eds) Basic Research Needs for Electrical Energy Storage, Department of Energy, USA, 2007, copyright © Department of Energy, USA and B. Scrosati and J. R. Garche, Journal of Power Sources, 195(2010) 2419–2430, copyright © Elsevier.

Plate XX (Chapter 22) (a) Comparison of the different battery technologies in terms of volumetric (Wh l–1) and gravimetric (Wh kg–1) energy density (9). Here PLiON denotes polymer lithium-ion battery and (b) Ragone plot of specific power density versus specific energy density of various electrochemical energy storage and conversion devices.

Plate XXI (Chapter 22) Layout of a single cell PEMFC. Copyright James Coortney and Bruno G. Pollet.

Advanced transmission technologies to improve vehicle performance

S. N. Doğan, G. Henning, T. Gödecke, M. Sommer, K. Fronius, M. Krohn, J. Kiesel and J. Dorfschmid, Daimler AG, Germany

DOI: 10.1533/9780857097422.2.393

Abstract: This chapter describes advanced transmission technologies and state-of-the-art transmission concepts to improve vehicle performance. Different types of transmission in terms of concept and design have been reflected and evaluated to meet environmental requirements, emission legislation and customers' demands for driving pleasure in the next generation of vehicles. Since the introduction of the first four-speed automatic transmission at Mercedes-Benz, more than 50 years' experience in producing and developing automatic and manual transmissions have passed. These transmissions have undergone continuous development since their introduction with six decades of experience in the field of automatic and manual transmission development.

Key words: powertrain, transmission, transmission concept, transmission design, front-wheel-drive, rear-wheel-drive, dual-clutch transmission, hybrid transmission, manual transmission, automatic transmission, two-mode hybrid transmission, continuously-variable transmission, fuel consumption.

12.1 Introduction

Due to the global need to reduce fuel consumption and emissions, there have been continuous improvements in powertrain and transmission efficiency. Different types of transmission have been developed to meet environmental requirements, emission legislation and customers' demands for driving pleasure in the next generation of vehicles. Mercedes-Benz has over 50 years' experience in producing and developing automatic and manual transmissions with its first four-speed automatic transmission developed in-house back in 1961. With the six decades of experience that Mercedes-Benz has in the field of automatic and manual transmission development, rapid practical implementation of new transmission concepts in different vehicle platforms is achievable (Fig. 12.1).

Mercedes-Benz started to build automatic transmission back in 1961 in keeping with the company's commitment to maintain its innovative strength throughout the six generations of transmission development in the brand's history. Over decades the transmission has become a significant part of the

12.1 Mercedes-Benz six decades of experience in developing and producing transmissions.

powertrain influencing the character and the drivability of a vehicle. This chapter illustrates some advanced transmission types developed and produced by Mercedes-Benz.

12.2 Manual transmission: six-speed front-wheeldrive SG6-310

12.2.1 Technical concept and design

The six-speed front-wheel-drive manual transmission SG6-310 was developed in conjunction with the seven-speed dual-clutch transmission 7G-DCT (see Section 12.3). The manual transmission, with an installed length of 357 mm, is a three-shaft transmission with a very compact design. The dry weight is 46 kg. The axle shaft features a fixed-floating bearing, whereas both output shafts as well as the differential have tapered roller bearings. All tapered roller bearings are assembled in accordance with the 'set-right' principle. Separate adjustment is thus no longer necessary. The technical concept and design can be seen in Plate II (between pages 392 and 393).

Gears 1, 2, 5 and 6 are shifted on the lower output shaft, while gears 3, 4 and R are shifted on the upper shaft. A compact and comparably lowweight design was achieved due to the multiple usage of gear pairs. One shared fixed gear is also used for gears 4 and 5; a reverse shaft or reverse idler gear could thus be eliminated due to the direct meshing of idler gears 1 and R. The synchronisation of gears 1 and 2 is designed as a triple-cone synchromesh system in order to meet the high requirements with regard to shifting comfort. Double-cone synchromesh systems are used for all other forward gears, while reverse gear is designed as a single-cone system. Four weight-optimised cast aluminium shift forks contribute to reducing the overall weight in the SG6-310. They are guided by sliding bearings on just two shift rods. It must be possible to detect when the transmission is in neutral for the stop/start system to function. This is done by sensing a magnet attached to the gearshift shaft; this also triggers the activation of the reversing lamp. The torque capacity of the SG6-310 is currently 310 Nm and 330 Nm can even be achieved for a short time in over torque. Outputs of up to 115 kW can be transmitted. This transmission type is used in the compact class vehicles like A-Class in combustion engines with petrol and diesel. During the development stage the transmission was optimised to reduce fuel consumption and ease of shifting, as well as starting and acceleration response.

The gear lever is mounted in a plastic shift block with a mechanical lift lock for the reverse gear. It is connected to the shift module at the transmission side via two Bowden cables for the shifting and selection movements. Targeted, co-ordinated engagement of the internal gearshift allows, in connection with the carefully co-ordinated decoupling elements on the Bowden cables, a gearshift process that is perceived by the driver as being very fluid, yet precise. The high degree of smoothness in the gear changes is complemented by the specially designed clutch with low pedal forces, optimum sensitivity and excellent noise, vibration and harshness (NVH) characteristics.

12.2.2 Conclusion and advantages of SG6-310

The predecessor transmission of the SG6-310 had already achieved an extremely high degree of shifting ease and precision; the focus during the development of the new manual transmission was therefore on further optimisation. Improvements were achieved in the shift quality with an additional reduction in the shift force, while friction loss and weight were refined. On the whole, the SG6-310 contributes also significantly to meeting current and future CO² emission requirements. Furthermore, the variable gear set modular systems also ensure the future viability of the concept in additional drive variants within the new generation of compact cars (NGCCs).

12.3 Dual-clutch transmission: seven-speed frontwheel-drive 7G-DCT

12.3.1 Technical concept and design

The new seven-speed dual-clutch transmission 7G-DCT was developed to replaces the AUTOTRONIC continuously variable automatic transmission previously used in the compact A- and B-Class cars. This transmission is distinguished by its compact design, facilitated by the innovative gear set concept (Plate III between pages 392 and 393).

Similar to the transmissions already available on the market, the outer concentric clutch is connected to odd-numbered gears 1, 3, 5 and 7, while the inner clutch is connected to gears R, 2, 4 and 6. The decision in favour of an oil-cooled multi-disc clutch instead of the more established dry dual clutch is thermal robustness and low moment of inertia. Thermal robustness means, that the driver can depend on the high reliability of NGCCs in critical situations – as well as in the case of a permissible vehicle/trailer weight of over three tonnes – or when manoeuvring, stopping and starting off on inclines. SUV-like vehicles, which are currently in the planning stage as part of the NGCCs, place particular high demands on the thermal robustness of the starting device. Low moment of inertia is essential due to the demand for a torque capacity of 350 Nm has resulted, in the case of dry clutches, in significantly higher weights and moments of inertia than for the oil-cooled clutch – the result being reduced driving dynamics. Modification of the friction discs provides a simple method for increasing the torque of oil-cooled clutches.

The major distinguishing features of the new gear set are the high integration of reverse gear using the existing gear ratios as well as the use of two gear chains. The 7G-DCT thus has a very short layout in comparison with competitor transmissions with comparable torque capacity. Gears 5 and 7, as well as gears 4 and 6, each need only one drive gear. A smooth, progressive gear ratio step was achieved in conjunction with the final drive design in spite of the positive coupling in these gears.

A completely new design was chosen for reverse gear that does not require an additional reverse gear shaft found in many systems. In the 7G-DCT, the direction of rotation is reversed by transmitting the gear ratio from second gear to the other output shaft. However, the power transfer to the output shaft does not take place directly via the final drive. Torque is initially transmitted to the output shaft via the transmission ratio of third gear and transmitted from there via first gear to the output shaft. Despite the constraints due to gear chains 4/6 and 5/7, the reverse gear design creates an acceptable gear ratio. At the same time, it can be reversed by simply switching the clutches between gears 1 and R.

Adjustments to fulfil all requirements within the NGCCs are made possible through the combination of two gear set variants with two different final drive gear ratios. In the case of small petrol engines, the gear ratio spread lies at 7.16, while the optimal output of the larger diesel and petrol engines is obtained with a gear ratio spread of 8.

Innovative shift actuators, installed as modules integrating the hydraulic function, actuate the sliding sleeves for shifting the individual gears. Efficiency is improved by reducing the churning losses in the transmission by using an enclosure attached to the differential. Reduced oil foaming also leads to improved acoustics and high gearshift quality, even under the highest load scenarios.

The central element of the fully integrated 7G-DCT control unit is a 32 bit microcontroller with integrated flash memory and RAM. The basis is formed by a thick-film substrate, which acts as the circuit carrier securing the unenclosed components by means of a conductive adhesive. It was also possible to place the actuation and noise suppression directly on the circuit carrier in addition to the solenoid control system output stages and the sensor evaluation electronics. A very wide temperature range can be covered thanks to this assembly technology.

The main focus during the development of the 7G-DCT was reduction of the electrical interfaces. Using flex foil and aluminium wire bonding, five position sensors, three speed sensors and two pressure and temperature sensors were fully integrated into the control unit. The connections for the electrical oil pump, the nine solenoid valves and lift solenoid are similarly integrated in the control unit. There is thus no need for a complex wiring harness, and the on-board connection for the 7G-DCT is therefore reduced to a five-pin plug.

An electric auxiliary oil pump is also integrated in the 7G-DCT, similar to the Mercedes-Benz planetary gear systems currently in use. While it was an add-on solution in earlier applications, it is highly integrated in the 7G-DCT from both a design and a functional perspective. Not only is this auxiliary oil pump available for the stop/start system in order to permanently pressurise the transmission regardless of the engine-off time, but it can also be used to boost the mechanical pump during normal driving at low engine speeds. This allows the overall dimension of the mechanical vane cell pump to be further reduced. The total efficiency of the transmission can thus be increased considerably.

In addition to stop/start support and boosting, the auxiliary oil pump has two additional functions: when a significantly higher cooling oil requirement exists, it regulates the clutch coolant volume flow requirements. In a corresponding situation, the auxiliary oil pump takes over the function of an otherwise additionally required control valve. A further function is linked to the park-by-wire (P-b-W) system – an additional feature of the 7G-DCT. Each P-b-W function, in connection with stop/start-capable drives, requires an additional energy source that is independent of the engine speed so that the parking interlock can be engaged and released, even when the engine is off. This function is also taken over by the auxiliary oil pump without the use of an additional actuator. Thanks to the integration of the auxiliary oil pump, only eight control solenoid valves, one solenoid valve and one shift valve have to be installed in spite of the wide range of functions offered by the 7G-DCT (seven gears, wet dual clutch, P-b-W).

12.3.2 Conclusion and advantages of 7G-DCT

The newly developed transmission portfolio represents an important element of the technological leap forwards achieved with the NGCCs. It has been possible to integrate additional technological modules into the 7G-DCT. It combines the use of a highly efficient electro hydraulic control unit with an oil-cooled starting device, thus ensuring the highest possible degree of launch comfort and robustness. It was also possible to integrate the 'Park-by-Wire' and 'Stop/Start' functional modules for the first time. Due to the combination of electrically and mechanically driven oil pumps, the new 7G-DCT sets new standards for efficiency. This newly developed transmission contributes also significantly to meeting current and future CO2 emission requirements. Compared with its predecessor, the AUTOTRONIC, the overall concept of the 7G-DCT features fuel savings of 9% in the new European driving cycle (NEDC) and is thus at the level of the manual transmission for all NGCC applications. The variable gear set modular systems also ensure the future viability of the concept in additional drive variants within the NGCC. Future concepts of advanced dual clutch transmissions [1] in combination with hybrid drives are able to further optimise fuel efficiency.

12.4 Automatic transmission: seven-speed 7G-Tronic Plus

12.4.1 Technical concept and design

In the autumn of 2010, Mercedes-Benz Cars launched the latest version of its seven-speed automatic transmission 7G-Tronic [2] called 7G-Tronic Plus for passenger cars in the S-Class, R-Class, and E-Class with the V6 diesel engine and in the CL-Class with the new V8 petrol engine (Fig. 12.2).

Transmission components

The seven-speed inline transmission at Mercedes-Benz has two standard and one Ravigneaux planetary gears sets, seven multi-disc clutches, a torque converter, an internal gear pump and an electric auxiliary transmission oil pump, mounted externally to the torque converter housing for the transmission

12.2 Seven-speed automatic transmission, Mercedes-Benz 7G-Tronic Plus.

with start-stop system as well as a hydraulic control unit including control electronics. For shifting between P-R-N-D (park-reverse-neutral-drive), either an electric unit called direct select modul (DSM) is mounted at the transmission housing driving the manual shaft for shift-by wire applications, or a cable from the gear lever is connected to the manual shaft.

The overall length from the flange mounted to the rear face of block to the output flange connected to the drive shaft varies between 619 mm and 640 mm, depending on the internal combustion engine (ICE) in combination with the used torque converter. These components can transfer a maximum torque of 1,000 Nm (Table 12.1).

Mechanics

The clutch control mechanism offers a selection of seven gears for forward driving and two gears for reverse driving, considering the road conditions. The seven gears can be realised with a variety of combination of closed shifting elements. To provide a convenient, comfortable and fuel efficient ratio and shifting steps, the shift pattern shown in Fig. 12.3 is used. This transmission concept leads to four open elements in every gear. To find an optimum between packaging, requirements in respect to shifting/torque transfer capability and losses, every element has either double-side or single-side friction plates with a particular groove pattern and friction material.

The transmission oil is one of the most important components. After years of development the world's lowest-viscosity automatic transmission fluid (ATF) is used in the 7G-Tronic Plus. In respect to the former ATF134, the viscosity of the ATF134Fe is reduced down to 4.5cSt at 100 °C/212 °F (ATF134: 6.7cSt at 100 °C/212 °F).

The oil has to provide an optimum balance between reduced dynamic viscosity, temperature, friction level, and volumetric and mechanical efficiency. By harmonising the base oil, viscosity and friction characteristics, and

Feature	Value	Unit
Maximum sustainable transfer capacity of transmission input coupling downstream of converter	760/1,000	Nm
Maximum permissible output torque	2,700	Nm
Diameter of torque converter	270	mm
Total length (depending on joint flange and converter)	619–640	mm
Weight (including converter and fluid; depending on the engine, the optional shift-by-wire system, and optional start-stop system with auxiliary oil pump)	83–93	kg

Table 12.1 Technical data of the automatic transmission 7G-Tronic Plus

12.3 Layout and shift pattern 7G-Tronic Plus.

lubrication and pressure-transfer additives in the fluid with tribological and anti-leak adaptations made to various transmission components (for example, planetary gear set materials, seals, transmission oil pan) and temperaturerelated improvements to the control software, the oil contributes significantly to the efficiency of an automatic transmission [3].

To consider the contribution of the weight of a transmission to the overall fuel consumption of a car, since 2003 the material of transmission housings of nearly every variant of seven-speed automatic transmissions is magnesium. Intensive finite element and powertrain bending calculations are required to find the right geometry to ful fil the NVH and durability requirements and keep the weight to a minimum level.

Torque converter

The torque converter, assumes many functions in modern automatic transmissions. When the vehicle accelerates from stop, for example, the engine must be smoothly connected to the drive line and output efficiently transferred to ensure high performance (Fig. 12.4). Both tasks are carried out exceptionally well due to the hydraulic power delivery, which also bumps torque. An output unit, the torque converter must likewise convert power with as little frictional loss as possible. This is managed by a torque converter lock-up clutch, which splits the power as per the hydraulic characteristic curve. Efficient delivery via the lock-up clutch also means that imbalances created by the combustion engine are channelled directly into the drive line.

In order to provide a high level of comfort here as well, a performance torsional damper is required [3]. The development goal in defining the hydraulic characteristic curves is to find the ideal balance between a suitably high stall torque ratio and a high level of efficiency. Since the two variables cannot be optimised independently of the other, achieving this balance is the

12.4 Torque converter.

key to ensuring good start-off performance. Realising very high efficiency also facilitates a comfortable transition between the hydraulic characteristic curve and the torque converter lock-up clutch within the power split. The power curve is defined using turbocharged engines which result in flat to slightly upward sloping curves. The torque converter lock-up clutch design is based on the three-channel system, which means there is a separate hydraulic channel to apply the clutch, as well as to other channels for providing the cooling oil. The path that the cooling oil takes to the clutch plate set is designed to flow directly towards the coupling package. This provides for especially effective and efficient cooling. Another feature is to separate the clutch disks by waved springs so that the clearance of the coupling package can be optimally used and drag torque is significantly reduced. The result is much improved performance, especially under cold-starting conditions, when engaging a gear typically reduces the rotating speed of the engine.

Damper development is mainly influenced by three factors with respect to the torque converter, all of which target a reduction in CO2 emissions. First, the trend in engine design is toward increasing output and torque with ever-smaller engines (less displacement). This can only be achieved through heavy turbocharging or supercharging, which further increases rotational imbalance. This imbalance, in turn, is channelled directly into the drive line. Second, the good power delivery of engines at low speed, in conjunction with today's typically wide transmission ratio, makes it possible to introduce an exceptionally low-speed drive program, whereby the engine is principally driven at higher excitation levels. Last but not least, developers want to eliminate slippage of the torque converter lock-up clutch in order to promote efficient power delivery, which would be impossible using the damping characteristics of this setup. The answer to these three challenges

12.5 Centrifugal pendulum damper.

is given in the form of a centrifugal pendulum damper (Fig. 12.5), designed in conjunction with the company LUK. This damper initially comprises a conventional twin-turbine damper. Its distinctive characteristic, however, is the fact that a pendulum mass is connected to an intermediate damper mass and functions as an additional damper. A standard damper comprises a spring mass oscillator that responds to a specific excitation frequency, which is determined by spring stiffness and mass.

With a pendulum, however, the natural frequency depends on gravitational acceleration and the length of the pendulum. Since gravitational acceleration in the rotating reference system of the torsional damper can be replaced with centrifugal force, the natural frequency fluctuates with the speed of the engine. Designed appropriately, the natural frequency is always in line with the engine order, allowing the main excitation frequency to be almost fully compensated (dampened). This damper design makes it possible to operate the torque converter lock-up clutch without slip while ensuring maximum comfort with respect to the drive line noise caused by torsional excitation. Another advantage of the pendulum approach and the regulated zero slip associated with it is a more direct connection between the engine and the drive line, which increases driving pleasure while reducing CO2 emissions.

12.4.2 Transmission control

Transmission control is essential for function of automatic transmissions. The main tasks are oil distribution and activation of the clutches and torque converter clutch depending on driver request and operating conditions. These functions are ensured by the electro-hydraulic control unit, built up of the electric set with integrated transmission control unit and sensors, solenoids and hydraulic valve body (Fig. 12.6).

Further components of 7G-Tronic transmission control are an electrical oil pump for stop-start-operation and an electromechanical servo module for shift-by-wire as a substitute for the mechanical link between transmission and shift control lever. An important aspect of transmission control design is to assure fail safe mode when system parts or the complete control system fail and sufficient availability of transmission for the driver (limp home mode). Design and function of 7G-Tronic transmission control components are described in the subsequent sections.

Electric set and transmission control unit

Included in the electric set are the electrical connections between the integrated components. In the case of 7G-Tronic there is a flexible printed circuit board (FPCB) that provides compact design and all-round isolation against metallic particles and spoons. In addition this design reduces the amount of links between the components to a minimum and increases reliability.

The connector is the electrical interface from inside transmission to outside. The redundant o-ring sealing inhibits oil leakage to external over the whole operating temperature range and lifetime as well as ingress of water into the transmission. There are two contact pins for 12-volt supply and another two pins for (CAN) controller area network.

The completely integrated transmission controller unit (TCU) is connected to other controllers via the CAN and receives transmission internal signals from the sensors integrated in the electric set. Thermal losses of the TCU are dissipated by an aluminium carrier plate to continuous oil flow below the TCU.

12.6 Electro-hydraulic control unit, 7G-Tronic Plus.

Software of the microcontroller operates signals from the CAN and transmission internal sensors in combination with driving strategy and adaptive algorithms to balance mechanical and hydraulic tolerances as well as the long time change of these parameters. Closed loop current controllers drive the solenoids to operate the hydraulic circuits.

An auxiliary function of the electric set is ensured by two float gauges to avoid increase of oil level up to the rotating parts and subsequently oil foaming.

Sensors

There are three speed sensors integrated into the electric set to measure transmission input speed, transmission output speed and another transmission internal speed (Fig. 12.7). These signals are necessary to control clutches during gear shift and in steady-state mode as well as for closed loop control of the torque converter clutch. Furthermore these are used to monitor the proper function of transmission in steady-state mode.

One special feature of 7G-Tronic is that transmission input speed has to be captured through another rotating part. The resulting high distance between sensor and wheel causes the use of a permanent magnetic wheel in combination with the magneto-resistive sensor. This method is used instead of measuring magnet field intensity and provides robustness against air gap variation due to tolerances and operation. One more important signal for transmission operation is gear select signal, which defines the park, reverse, neutral or drive position in the electro-hydraulic control unit. This signal is captured by a permanent magnetic linear contactless displacement

12.7 Electro-hydraulic control unit with speed sensors and target wheels [4].

(PLCD) sensor (Fig. 12.8) and a magnetic pickup mounted on selector piston valve. This sensor design is made up of an inductor system of primary and secondary inductors, coiled on a ferromagnetic core. An alternating field induced to the primary coil induces a corresponding secondary field. The magnetic pickup leads to local saturation of the core depending on piston position and influences alternating voltage of secondary coils. Reaction of secondary voltage depending on local saturation of the core is evaluated by an electronic onboard device and transformed to proportional voltage that is used by the microcontroller software in the TCU. Finally gauging of oil temperature is done by a temperature sensor assembled onboard of the TCU.

Solenoids

To operate actuating variables of microcontroller software the 7G-Tronic transmission control uses eight solenoids. One of these is used for line pressure control and another for torque converter clutch control. The others actuate the six clutches of the 7G-Tronic. These solenoids convert the driven current according to a characteristic curve into hydraulic pressure. Due to limited operating pressure and flow rate these solenoids are known as pilot solenoids. Pressure and flow rate have to be amplified to requested levels by subsequent hydraulic circuits.

The solenoids are designed with a flat seat valve. The input port is fed by supply pressure. Flow profile of the internal flat seat is influenced by magnetic force which is induced by the electrical current. This variable bleed causes, in combination with an internal fixed bleed in supply line, the requested control pressure. Due to acting control pressure itself on the flat seat area, the solenoid includes an internal closed loop pressure control with current as input and control pressure as output. Combined with closed loop

12.8 PLCD-sensor for P-R-N-D-detection [4].

current control this system offers good signal chain performance with high independence against operating parameters like voltage, supply pressure and temperature. Solenoid design is shown in Fig. 12.9.

The solenoids of the 7G-Tronic contain an additional feature called closed-end-function. Integrated into the supply port there is a ball valve sealing supply line when operating at zero pressure, thereby avoiding the high leakage that has been characteristic for predecessor solenoid types and had a negative influence to power efficiency of the transmission. Due to reasons mentioned above with regard to safety against control system failures and limp home mode, there are solenoids with a normally low and normally high characteristic. While normally low characteristic leads to zero pressure at zero current the normally high characteristic has maximum control pressure at zero current. Thus, accordant clutches are actuated in case of electric system failures, transmission falls back into fail safe mode and enables limp home mode.

Hydraulic control

Hydraulic control of the 7G-Tronic is distributed in two die cast housings, separated by a steel plate with connecting passages and bleeds. An electric set with TCU and solenoids are mounted on the upper housing.

The hydraulic control unit provides several pressure levels with varying priority. The line pressure control valve is governed by a solenoid and controls line pressure. The level of desired line pressure is related to pressure of the clutch with highest demand, depending on active gear and transmitted torque.

12.9 Solenoid [4].

The line pressure control valve distributes delivered volume depending on operating conditions. As long as speed-related delivery volume is sufficient for hydraulic control, the appropriate volume is forwarded to the lubrication system and cooling. When the lubrication system is saturated, remaining volume is also redirected to injector line and pump suction.

Pilot pressure of solenoid valves for clutch control (Fig. 12.10) has to be amplified with respect to pressure and flow rate by subsequent piston valves. The gain is referred to pressure demand of the actuated clutch depending on the maximum torque to be transmitted. Piston geometry in combination with clutch line bleed and pressure ratio are responsible for quick and repeatable clutch filling. By providing a particular hydraulic circuit for each clutch there is maximum flexibility to control each clutch during gear shift. This control mechanism, also known as direct clutch control, is demonstrated using the example of one clutch in Fig. 12.10.

Not described here is an additional logical system implemented to provide fail safe and limp home mode of the 7G-Tronic in case of failure.

12.10 Clutch control schematic [4].

Software

The seven gears of the 7G-Tronic enable low consumption by the engine due to drive at low revolutions. In the case of driver requests dynamic operation mode, the transmission is able to ship gears by shifting more than one gear ratio in one step. Thereby the shift program provides shift patterns depending on velocity, engine speed and accelerator pedal position and further parameters like chassis and ascending slope. Possible gear changes of the 7G-Tronic are shown in Fig. 12.11.

Direct clutch control enables completely independent actuation of clutches and is used to operate the clutch quickly and smoothly just before engaging friction discs during the fill phase. The following overlap phase is used to increase clutch pressure in the slope shape and decrease pressure of the disabled clutch simultaneously. Depending on the gear changing type there is a pressure peak phase applied to the coming clutch to bring engine inertia to target speed. This can be supported by engine torque reduction requested from the engine control unit via CAN.

Gear changes (for example 7th gear to 4th gear) are proceeded as subsequent gear shifts with continuously speed change from old (7th gear) to new (4th gear) gear. For low fuel consumption of the powertrain it is necessary to control the torque converter clutch over a wide operation range and with

12.11 Possible direct gear shifts [4].

minimum of slip. Thereby, the torque converter clutch is engaged from 1st gear. For comfort reasons precise slip control and smooth engagement by software algorithms are required. Adaption algorithms implemented in the microcontroller software evaluate statistical spread due to tolerances and lifetime depending parameter variations of mechanical and hydraulic system like friction coefficient variation of friction discs, spring setting and drift of characteristic curves. The calculated adaption values are stored in the nonvolatile storage of the TCU and then used to calculate the current driver demand signal.

Electric oil pump

On demand the 7G-Tronic can be used in cars with stop-start-operation. In this case there is an electric oil pump mounted below the torque converter housing, which provides hydraulic control supply. The electric oil pump contains an integrated electronic device and is connected via CAN to other devices located in the car.

During the stop-phase when the engine is not operating, the electric oil pump has to be activated and the supplied volume of the pump takes care that the pressure level in the hydraulic control system is sufficient to keep the clutches engaged. Thereby the 7G-Tronic is ready for operation immediately after engine start and enables spontaneous and comfortable drive away.

Servo module for shift by wire

In cars that use the shift by wire system instead of a mechanical link between the transmission and gear selector it is possible to install an electromechanical device, called a servo module. This contains an integrated electronic device connected to the TCU and ignition lock device by CAN. Another main component of the servo module is an electric motor, driving a belt transmission to operate a spindle with spindle nut. The spindle nut is acting on a lever mechanism and thus chooses the park, reverse, neutral or drive position of the transmission.

To achieve safe parking of the car in case of loss of electrical power of the accumulator, there is a second electric path with a small auxiliary electric motor driven by a backup accumulator. This electric motor overrides the linkage between the spindle and spindle nut and a pre-loaded spring moves a lever mechanism to the park position.

12.4.3 Conclusion and advantages of 7G-Tronic Plus

The seven-speed automatic transmission 7G-Tronic Plus has been continually developed for passenger cars with rear-wheel-drive. The 7G-Tronic Plus also contributes significantly to meeting current and future CO2 emission requirements with approximately 7% better overall fuel economy than previous transmissions [2] as tested in the NEDC cycle. The contribution to reduce the CO2 emissions of a six-cylinder diesel engine in the S-Class, as an example, is 17 g of CO2 per km or 0.64 l per 100 km [5].

12.5 Continuously variable transmission: frontwheel-drive CVT AUTOTRONIC

12.5.1 Technical concept and design

AUTOTRONIC continuously variable transmission (CVT) was developed specifically for the powertrain of the A-Class. Particularly in terms of comfort and fuel consumption, this transmission sets new standards in the compact car class. Other development emphases include 'higher output, higher torque, more driving pleasure and very compact dimensions'. Compared to the preceding series, the outputs of the petrol and diesel models have increased by up to 38% and maximum torque by up to 46%. The innovative AUTOTRONIC is a belt-driven continuously variable transmission (CVT), which uses a pulley variator and steel thrust belt for variable ratio selection. The outstanding ratio spread, the greater all-round efficiency of the engine-transmission unit and the drive developed strategy have produced significant improvements with respect to ride and noise comfort, responsiveness and acceleration, fuel consumption and exhaust emissions. The mid-section of the AUTOTRONIC transmission with the power flow through the transmission and its major components is shown in Fig. 12.12.

As a starting device the transmission has a twin-channel torque converter with slip-controlled lockup clutch, a variator with a thrust link belt and a clutch with a downstream drive neutral reverse (DNR) set. Adaptation to the different engine variants is via an intermediate shaft with gear set upstream of the differential. The arrows in Fig. 12.12 show the power flow through the transmission and its sub-assemblies.

The new AUTOTRONIC has the following advantages in a direct comparison with the previous five-speed multi-step automatic transmission:

∑ the ride comfort is reached by smooth and variable ratio changes, reduced rpm when cruising with lower noise level, variable braking effect on downhill gradients and by optimal choice of ratio on uphill gradients
∑ the performance, acceleration and flexibility is achieved by ratio changes without torque interruption, downshifts with maximum response and shorter response times, and by running the engine at its optimal operating point
∑ the reduced fuel consumption and emissions are accomplished with a basic operating strategy geared to minimum fuel consumption using

12.12 Mid-section and with power flow through the sub-assemblies of the front-wheel-drive CVT, Mercedes-Benz AUTOTRONIC – WFC 280.

wider ratio spread, extended use of the torque converter lockup clutch and mechanics as well as hydraulics with an optimised degree of efficiency

∑ the driving pleasure is the result of the use of comfort and sport automatic modes with additional seven-speed manual program.

Reduced fuel consumption and emissions

In a direct comparison with the preceding transmission, AUTOTRONIC reduces fuel consumption by up to 9% in the legally prescribed driving cycles (NEDC). In practice this means that fuel consumption can be reduced to the level of a vehicle with manual transmission. This improved fuel economy is due to both the very high transmission efficiency and the low engine speed level obtained with the significantly wider ratio spread.

Adaptations to driving style

The transmission adapts to the individual style of driving with continuous ratio changes. Driver activity is monitored at the pedal value sensor and converted into a numerical index in the context of the vehicle's lateral and forward acceleration, then incorporated into the drive strategy. The transmission also responds to any change in the current driving conditions, i.e. a change in gradient, with a ratio adjustment. This depends on the chosen automatic shift program (C/S – comfort/sport), the position of the accelerator and the vehicle speed.

Manual program activated via one-touch function

If drivers wish to carry out gearshifts themselves, they can change to the manual (M) program. With the shift lever in position D, the manual mode can be activated by pushing the lever towards D+ or D–. This transmission has a total of seven gears or virtual gear ratios available. M-mode is cancelled as soon as the driver operates the C/S key, holds the shift lever in position D+ for at least 0.5 seconds or pushes it towards D+ repeatedly. When leaving M-mode, the previously selected C/S mode is reactivated. In M-mode the driver has a choice between being particularly fuel-efficient, relaxed driving or a more sporty style at higher engine speeds.

The different shift programs can be very well illustrated by the example of the engine speed curve during an overtaking manoeuvre at full throttle involving acceleration from 50 km/h to 130 km/h, followed by reduced throttle (Fig. 12.13).

In C-mode the engine speeds are lower and the ratio changes are most comfortable. There is discreet engine speed tracking as acceleration progresses.

12.13 Comparison of acceleration process in S, C and M modes and kick down.

In S-mode, downshifting to a higher engine speed is much more rapid when the accelerator is gradually depressed, and the engine speed increase as speed picks up is also more pronounced. There is an upshift at the end of the manoeuvre, though to a lower ratio than in C-mode. If the driver signals a requirement for maximum performance by operating the kick down switch, there is a very rapid downshift with engine speed tracking, followed by transfer of the maximum output to the drive wheels by continuous ratio control at the nominal operating point of the engine. In M-mode the driver is able to select the ratios as with a multi-step automatic transmission and during the above manoeuvre there are two automatic upshifts at approximately 85 km/h and 125 km/h.

12.5.2 Transmission components

Hydrodynamic torque converter

AUTOTRONIC uses a torque converter as a starting device. This concept is well-established at Mercedes-Benz and ensures optimal, reproducible starting comfort. The more stringent package requirements and considerably increased input torque levels required a new design. The well-proven threechannel design of the torque converter was abandoned in favour of a much more compact twin-channel design. Figure 12.14 shows a comparison of the key technical requirements between the AUTOTRONIC torque converter and the previous torque converter in the five-speed automatic transmission (W5 A180).

Implementation Damper area 180 Nm 280 Nm Damper characteristic 2-stage 3-stage

Requirements W5A180-converter WFC280-converter

Engine output Pmax 95 kW 142 kW Torque Mdmax 180 Nm 280 Nm Combustion space ø264 mm ¥ 84 mm ø264 mm ¥ 84 mm

Design 3-channel-converter 2-channel-converter Hydraulic circuit ø245 mm, circle ø 245 mm, circle Conversion ca. 2,0 ca. 2.0-2.2 Lockup clutch 2 linings. ø 176 mm 1 lining. ø 238 mm Clutch lockup mode Slip-controlled Slip-controlled

12.14 Requirement comparison of the torque converters.

Variator

In contrast to the CVTs of competitors, this CVT transmission features a downstream DNR set. The reasons and advantages can be described by the specific packaging situation, with reduced space on the primary side, the ability to reduce the constant pressure at the variator and thrust link belt. In addition protection against shockload (impact) from the transmission output side with this arrangement (Fig. 12.15) and the ability to adjust the variator when the vehicle is not moving are possible.

Thrust link belt

A thrust link belt with a width of 30 mm and 12 rings is used. Great attention was paid to the widest possible ratio spread when designing the variator and belt. The maximum engine torque is limited to 280 Nm. In technical terms this is a groundbreaking achievement in conjunction with the outstanding ratio spread of 6.41. The two sets of variator sheaves have a shaft distance of 180 mm.

Downstream DNR set

The downstream DNR set basically consists of a planetary gear set with two hydraulically operated multi-plate clutches. When moving forward, the forward clutch is locked while the reverse brake is open, causing the planetary gear set to rotate as a block. If the direction of rotation is to be reversed the forward clutch is opened and the reverse brake is locked, causing the planetary carrier to be braked. In this case the power flows to the ring gear via the centre gear and planetary gears, and therefore to the drive shaft. The

12.15 Locations of the variator and thrust link belt in AUTOTRONIC.

12.16 Dual stage vane pump.

direction of rotation is reversed by the stationary transmission ratio of the planetary gear set.

Dual-stage vane pump

Mercedes-Benz has used a dual-stage vane pump (Fig. 12.16) for the first time in the AUTOTRONIC transmission. This basically consists of a rotor with 12 vanes and housing with two separate chambers, with one chamber in operation full-time and the second only pressurised as required, i.e. part-time. The major advantage is that the power input of the pump can be practically halved, especially at high engine speeds. This has great benefits in terms of vehicle performance and fuel consumption. The dual-stage vane pump is bolted to the control unit and feeds the oil pressure and volume required for operation directly into the control unit.

12.5.3 Transmission control

Electro-hydraulic transmission control

The electro-hydraulic control unit (Fig. 12.17) was developed entirely by Mercedes-Benz and is produced in-house. The electro-hydraulic control unit of AUTOTRONIC consists of the hydraulic control assembly with 12 hydraulic sliding valves, four electronically actuated solenoid control valves and the electrical system, which includes sensors and a fully integrated control unit. As in all Mercedes-Benz automatic transmissions, the electro-hydraulic control unit together with the hydrodynamic torque converter allows purely hydraulic operation as an emergency running feature.

Electronic transmission control

As in all recent automatic transmissions, a hybrid concept was chosen for the electronic control unit in order to improve quality and reliability while lowering costs. This was mainly achieved by dispensing with wiring and invehicle connectors between the control unit and the transmission. Moreover, by already combining the control unit with the transmission at the production plant, the entire mechanical/hydraulic control system can be inspected and also adapted during special test runs, improving product quality.

12.17 The electro-hydraulic control unit, AUTOTRONIC.

The control unit (see Table 12.2) itself has a Motorola PowerPC MPC555 processor and is designed for a temperature range of –40 °C to +140 °C. It features two CAN controller modules and four current-controlled output stages for the solenoid control valves, as well as three engine speed sensors, one temperature sensor, the shift range sensor (consisting of four Hall sensors) and a pressure sensor. The control unit is connected to the onboard electrical system and the CAN by a six-pin transmission connector. The transmission control unit is networked with other onboard control units (e.g., engine control unit, ESP control unit, etc.) via a CAN bus.

12.5.4 Conclusion and advantages of AUTOTRONIC

The newly developed belt-driven continuously variable transmission AUTOTRONIC represents an important element of the Mercedes-Benz portfolio and was developed and tested in parallel with the new A-Class. This transmission contributes also significantly to reduce CO2 emissions and sets new standards of comfort and fuel consumption (reduction up to 9%) in the compact car class. Optimal integration into the overall vehicle concept was made possible by the most compact CVT in this torque category. The outstanding ratio spread of 6.41, the greater all-round efficiency of the enginetransmission unit and the drive developed strategy has produced significant improvements with respect to NVH and comfort.

12.6 P2 hybrid transmission

The development of hybrid transmissions has a long tradition at Mercedes-Benz. In 2009, engineers even brought two new hybrid systems onto the market.

	AUTOTRONIC	5-speed (W5A 180)-converter
In-house designation	ECC = electronic CVT control	VGS-FAG
Processor	Motorola PowerPC MPC 555, 32, bit, 32 MHz	Infineon SEC5 TC810, 8 bit, 4 MHz
RAM	26 kBytes	1,280 Bytes
ROM	448 kBytes	128 kBytes
E2 PR OM	4 kByte	512 Bytes
Output stage	4 solenoids (current controlled)	4 x PWM valves 1 x control valve
Sensors	3 x engine speed 1 x shift range sensor 1 x temperature 1 x pressure sensor	1 x engine speed 1 x starter lock-out contact 1 x temperature
Connector	6-pin, only 4 pins used	5-pin, all pins used

Table 12.2 Comparison of the control units

The NAG2-P1 seven-speed automatic transmission is sold successfully in the S-Class as 'S 400 Hybrid'. In this vehicle a mild hybrid is used which already reduces consumption effectively. Within the stop/start function the vehicle is also able to recuperate and support the acceleration electrically, which is also called 'boost'. At the same time a power split hybrid transmission was developed in the US Research and Development Centre, and is sold in the M-Class as 'ML 450 Hybrid' at present (see Section 12.7). This is a full-hybrid system and in addition to above mentioned features, ensures that several kilometres' electrical drive are possible.

The next step in the hybrid transmission area is the development of the NAG2 P2-20 transmission. This hybrid system will celebrate its debut in conjunction with a four-cylinder diesel engine in the E-Class. The main target during this evolution was to reduce fuel consumption even further without a loss in comfort. In fact the engineers have even been able to increase comfort in city traffic, because, in opposition to vehicles with a stop/start functionality alone, full climate cooling capacity can be provided even with the engine turned off. The electric motor in the transmission offers an additional output power of 20 kW with a maximum torque of 250 Nm. Combined with the efficient combustion engine, the power train has a system output of 170 kW and a maximum torque of 590 Nm. With the 0.8 kWh installed electrical energy the vehicle is able to drive up to 35 kph fully electrically. The average fuel consumption in the E-Class with this hybrid transmission is only 4.2 l/100 km with a CO2 emission of only 109 g/km.

12.6.1 Technical concept and design

Due to compact architecture the NAG2 P2-20 transmission is only marginally longer than the NAG2-FE+ (see Section 12.4) and fits into the conventional Mercedes-Benz E-Class undercarriage. The transmission's modular and compact architecture allows for assembly of the hybrid drive train in all vehicle platforms of Mercedes-Benz. The new hybrid transmission is based on the known seven-speed automatic transmission, which is used for inline applications (Fig. 12.18). In contrast to the 7G-Tronic, the hybrid transmission has no torque converter. In place of the converter a wet clutch integrated in the E-rotor engages the combustion engine, parallel to the E-motor, with the NAG2 seven-speed base transmission (Fig. 12.19). This is necessary to be able to disengage the combustion engine and minimise drag torque losses during electric powered driving and recuperation. In order not to lose the comfort benefits of a converter, two additional torsion dampers have been integrated between the combustion engine and the hybrid transmission as well as between the wet clutch and the base transmission. The dampers' elasticity eases torque peaks and effectively absorbs mechanical vibrations to avoid a transfer to the remaining power train. As a result, no unpleasant

12.18 Hybrid power train for inline applications at Mercedes-Benz.

12.19 NAG2-P2 power head with E-motor, wet clutch, torsion damper and positioning sensor.

noises are generated even in lower rpm-ranges. These two dampers and their spring and damper ratio also provide the room to perfectly adjust the vehicle power train. The technical data of the hybrid transmission is summarised in Table 12.3.

Unlike the regular NAG2 seven-speed automatic transmission two important components have been integrated to the hybrid transmission. Thus,

Table 12.3 Technical data of the hybrid transmission

Maximum torque (transmission input)	600 Nm (1st gear), 760 Nm ( 2nd gear to 7th gear)
Maximum torque (Start E-Motor)	250 Nm
Maximum power (E-Motor)	20 kW
Gear spread j1/7	6.0
Launch clutch	Wet clutch
TCU	TCU with integrated shift by wire and completely integrated in the transmissions control panel
Park-by-wire	P-b-W modul integrated in the electro hydraulic unit
Integrated additional oil pump	For E-drive and stop/start
Voltage level	120 V

12.20 Expanded view of the NAG2 hybrid transmission.

the parking lock is integrated as a park by wire module to the transmissions control panel. As a result, there is no mechanical connection to the outside which increases anti-theft security. The additional oil pump has also been completely integrated into the base transmission and contributes to a substantial improvement of the transmission's package situation in the vehicle tunnel due to a lack of interference contour. In Fig. 12.20 an expanded view of the transmission that highlights the most important renewals is shown

12.6.2 Transmission hybrid functions

With the new NAG2 P2-20 transmission a Mercedes-Benz is able to drive purely electric. Hence, the vehicle launches silently, without lag and with a turned-off combustion engine after stopping at a traffic light. With 0.8 kWh the lithium-ion battery provides enough energy to cover a distance of 1 km without consuming any fuel or producing local emissions. During E-drive the launching clutch is disengaged and the turned-off engine is separated from the drive train (Fig. 12.21).

To accelerate above 35 km/h, the combustion engine has to be turned on and engaged by the wet clutch to provide further propulsion. Given the fact that the engine comes with its own starter it does not have to be dragged up to start. As a result, no reserves need to be held in the electric motor and full performance is available at any time.

Up to a constant speed of 160 km/h the combustion engine can be turned off and the electric motor keeps constant velocity. With that 'sailing' feature the vehicle is able to drive on a highway emission free. Also in this case the clutch is disengaged and the engine turned off and separated from drive train. To use braking energy effectively the electric motor can also be used as a generator. At low decelerations the electric motor inherits the mechanical brakes' task while charging the battery with the vehicle's kinetic energy.

12.6.3 Conclusion and advantages of P2 hybrid transmission

The newly developed hybrid transmission NAG2 P2-20 has a modular and compact architecture with the ability to drive purely under electrical power and to launch silently without the internal combustion engine. Due to the modular concept the hybrid transmission can be easily integrated into all vehicle platforms of Mercedes-Benz. The main target during this evolution was to reduce fuel consumption even further without a loss in comfort. This hybrid system will be lunched in conjunction with the diesel engine in the E-Class with significant reduction of CO2 emissions and fuel consumption.

The contribution to reduce the CO2 emissions of a four-cylinder diesel engine in the E-Class, as an example, is only 109 g/km with an average fuel consumption of only 4.2 l/100 km, which makes it most efficient automotive in its class.

12.7 Two-mode hybrid transmission advanced hybrid system-cars (AHS-C)

12.7.1 Technical concept and design

The two-mode hybrid transmission AHS-C (Advanced Hybrid System-Cars) is the central component of the hybrid powertrain system and a part of the

12.21 Hybrid functions with power flow through the power head; E-motor stator (1), E-motor rotor (2), wet clutch (3) and torsion damper [4].

modular family of AHS transmissions which includes a range of transmission variants for front and rear-wheel drive vehicles and a variety of installation space designs with the common concept of power splitting in conjunction with four fixed transmission ratios or gears. This hybrid transmission is designed to be combined with the conventional mechanical four-wheel drive system and accommodated together with all electric cables within the available installation space in the M-Class body shell.

The two-mode hybrid transmission enables the vehicle to be driven under purely electrical power, by the internal combustion engine (ICE) alone or by a combination of both (power splitting). The hybrid transmission AHS-C has two electric motors (EM-A and EM-B), three planetary gear sets (PGS1, PGS2 and PGS3), an internal electric motor pump (EMP) to drive the oil pump, four clutches (C1, C2, C3 and C4) as well as a hydraulic control unit including control electronics (Fig. 12.22).

The transmission allows power to be split among an electrical and a mechanical path. Most conventional one-mode hybrid systems have no mechanical transmission ratios. Such systems consequently transmit the bulk of their output via the electrical path, requiring greater electrical power and resulting in lower efficiency. They operate most efficiently in the lower load and speed ranges. If power transmission were also to take place at greater loads and speeds, a correspondingly larger electric motor would be needed, but its energy supply and space requirements would be to the detriment of efficiency. In contrast, as the engine load increases and at higher speeds, the two-mode technology shifts power transmission variably from the electrical path to the mechanical drive path. This means that the system operates significantly more efficiently under all driving conditions, both in inner-city stop and go traffic and on fast long-distance journeys. One further advantage is that, relative to their output, electric motors of a significantly more compact design can be used.

The clutch control mechanism offers a selection between four fixed gears and two electric variable transmissions EVT 1 and EVT 2 (four virtual gears) depending on the traffic conditions (Fig. 12.23).

During launching and creeping at slow speeds an extremely smooth, comfortable and agreeable ride will be experienced by the driver by selecting EVT1. Meanwhile, in stationary driving conditions the efficient four fixed gears allow consumption-optimised operation of the AHS-C transmission. When driving at high speeds, an extremely comfortable acceleration and deceleration will be experienced by choosing the EVT2 electric variable mode.

The two-mode hybrid transmission can be operated with the E/S (Economy/ Sport) operating mode selector button which is included in the cockpit's lower control panel. This ensures that the driver is able to drive the transmission in economy mode, which feels like a continuously variable transmission or

12.22 AHS-C two-mode hybrid transmission.

C lu h io tc t op er a n		C 1	C 2	C 3	C 4
Lo E V T ra ng e w	E V T 1	x
1s t g ea r	F ixe d 1	x			x
d 2n g ea r	F ixe d 2	x	x
H ig h E V T ra ng e	E V T 2		x
3r d g ea r	F ixe d 3		x		x
4 h t g ea r	F ixe d 4		x	x

Cx Clutches C1, C2, C3, C4

A E-motor A E-motor B

Ring gear of planetary gear set

Planet gears/carrier of planetary gear set

Sun gear of planetary gear set

12.23 Clutch control mechanism of the two-mode hybrid transmission AHS-C.

in sport mode which offers handling characteristics comparable to those of a conventional automatic transmission with eight fixed transmission ratios (four fixed gears and four virtual gears) and very rapid gearshifts.

12.7.2 Transmission components

E-motors

The electric motors EMA and EMB have different arrangements within the two-mode hybrid transmission AHS-C due to their particular power requirements. Electric motor EMA benefits from the wider diameter of the housing at the front of the transmission and has been optimised for power and minimal losses. The high torque, high power density and great insensitivity to speed variations of electric motor EMB is due to the choice of a longer motor combined with a greater degree of integration within the transmission.

The electric motors EMA and EMB are compact three-phase synchronous motors with a permanently excited rotor field, and the integrated permanent magnets are designed and fixed in a V-position in the rotor core assembly, permitting systematic exploitation of the reluctance torque. This ensures great insensitivity to speed variations and, with the help of a special field-oriented control mechanism, optimises the available torque and power against losses at high engine speeds.

From the NVH perspective, the cogging torque was consistently avoided in the rotors to reduce noise emission of the electric motors. The stators are equipped with copper which are rectangular conductors arranged in a hairpin. To achieve the maximum torque and performance this was combined with the end winding and stator yoke cooling arrangement, which uses transmission oil as a reliable means of removing the waste heat due to copper and iron losses. Both electric motors produce a maximum power output of 62 kW at torque of 235 Nm and 60 kW at torque of 260 Nm and the maximum rotational speeds are between 10,500 rpm and 13,600 rpm. The electric motors are designed for maximum ease of integration and can assume additional functions such as supporting clutch and planetary gear set components. The systematic implementation and further development of the electrical motors is essential in achieving optimum values and meeting the specific requirements for the two-mode hybrid transmission AHS-C [6].

Power system components

In order to control the hybrid powertrain some additional components have to be considered and are essential to run and manage the hybrid system. These components are the power electronics box (PEB), the power distribution box (PDB) and the auxiliary power module (APM) located above the internal combustion engine (Fig. 12.24).

The PEB contains two inverters which supply the two electric traction motors EMA and EMB with alternating current while the vehicle is moving. During braking or the regeneration phases by the electric motors, the inverters convert alternating current into direct current. The PEB has fully integrated the controller board to communicate with the vehicle and to control the two inverters. A further inverter and an additional controller board have been integrated into the PEB for the transmission oil pump's electric motor EMP. In addition, a separate low temperature coolant circuit is installed to remove the generated waste heat. The integrated PDB distributes the current from the high voltage (HV) battery to the various HV components with different HV fuses and has safety functionality for shutting off the high voltage in the event of a malfunction. The APM is a bi-directional DC/DC converter to cover the needed average 12 V current and to charge the 12 V battery of the vehicle. The HV battery can also be charged using the 12 V vehicle power supply [6].

The integrated liquid-cooled HV battery is a nickel-metal hydride battery (NiMH) which is a type of secondary electrochemical cell similar to the nickel-cadmium cell. The NiMH battery can have two to three times the capacity of an equivalent size nickel-cadmium battery. Neither charging by an external power source nor regular replacement is necessary. Battery recharging is exclusively a matter for the control strategy while the vehicle is being driven. By optimising the design of the battery cells and the liquid cooling system the battery's efficiency and capacity has been significantly increased. The battery delivers 45 kW of power with a capacity of 2.4 kWh

12.24 Hybrid powertrain with the essential components (ML450 Hybrid).

and has a weight of 83 kg supplying the hybrid system with a voltage between 216 V and 390 V (rated voltage 288 V).

The battery control unit continuously calculates the current take-up and current consumption and the battery management system maintains it in a permanently stable charge stage. The HV battery fulfils all the requirements to allow driving under purely electric power as part of the hybrid system. The battery is installed in the spare wheel well in the vehicle rear section without changing the vehicle luggage compartment floor height and volume of the SUV's spacious trunk. To ensure long service life of the battery, the battery cell operating temperature is kept at a constant level which is a major factor influencing battery performance and ageing. Active cooling of the battery modules by the vehicle's air-conditioning system allows full power to be called upon even with extreme outside temperatures. The battery charge level is displayed, as required, either as part of the energy flow indicator in the ML 450 Hybrid COMAND system display or in the instrument cluster.

12.7.3 Operation strategy and fuel consumption

The operation strategy of the ML 450 Hybrid ensures the least possible fuel consumption by using a powerful processor which controls the hybrid system, performing up to 50 million computing operations every second. The electronic control centre calculates in real time the most efficient operating mode 160 times per second including the recording of consumption by the hybrid system components. The operation strategy of this vehicle is displayed on the COMAND system and shows the driver the current operating configuration like start-stop, cruising speed, acceleration, driving under electric power, regenerative braking and boost driving condition (Fig. 12.25).

The power steering pump and refrigerant compressor are integrated in the front of the vehicle and are electrically driven. This allows operation independently of the internal combustion engine. This electrifying makes it possible to reduce the power they consume when belt-driven from the combustion engine, with a resultant positive effect on fuel consumption. In all driving conditions, even under purely electric power, the power-assisted steering function remains fully operational. The climate control function for the vehicle interior and the essential continuous HV battery cooling requirements are also permanently assured. Reduced fuel consumption and reduced emission are achieved during stop, acceleration, braking, running the internal combustion engine at its most efficient operating points and reducing the system components by electrifying (i.e. no longer belt-driven).

During stops at a red traffic light, the internal combustion engine shuts down, thus saving fuel and reducing emissions. When braking, the electric traction motor that drives the rear wheels assists in the braking effort and generates electricity that is stored in the battery pack. This stored energy can

12.25 Operation strategy displayed on the COMAND system.

be released during accelerating. The internal combustion engine provides some of the power for acceleration, and in addition the stored energy from the braking process is used for this acceleration, thus reducing the amount of power the engine must provide. Since the internal combustion engine is used to generate electricity, it can be run at its most efficient operating points to minimise fuel consumption and emissions. Another essential part for fuel and emission reduction is the reduction in the number of belt-driven components. The refrigerant compressor, the power steering pump and the alternator are electrified and provide their respective part to the reduction.

12.7.4 Conclusion and advantages of AHS-C

The two-mode hybrid transmission AHS-C with four fixed and four virtual gears offers a comfort quality comparable to those of a conventional automatic transmission. With the two electrical traction motors EMA and EMB integrated in the transmission the vehicle is able to be driven under purely electrical power, by the ICE alone or by a combination of the two. The AHS-C implemented in the powertrain of the ML450 Hybrid vehicle offers great driver benefits, comfort and handling performance while retaining the same effective load space and making possible significant reductions in both fuel consumption and CO2 emissions. The ML 450 Hybrid produces virtually the same torque force as a V8 engine but with a four-cylinder engine's consumption. The fuel reduction [6] in the ML 450 Hybrid compared to a conventional vehicle powered by a V6 gasoline engine in the urban driving cycle is up to 30% and on highways up to 20%.

12.8 Automated commercial vehicle transmission: 16-speed G260-16

12.8.1 Technical concept and design

The automated commercial vehicle transmission G260-16 with 16 forward and two reverse gears is designed by Mercedes-Benz for the Actros heavyduty-truck as automated constant mesh transmission with a new ESP gearshift mechanism. The abbreviation ESP initially stood for 'electro-pneumatic gearshift system' and subsequently for 'electronic power shift'. 'Mercedes Power-Shift' is the collective name for the non-synchromesh, fully-automated 16-speed manual transmissions incorporating sophisticated control electronics to deal with the main functions such as changing gear and operating the clutch as well as numerous new control functions.

Instead of a clutch pedal operated by the driver's foot pressure, an Actros equipped with the Telligent automatic gearshift system has engine clutch control and electromotive clutch control. Both systems were integrated into the CAN data bus and communicated via the data bus with all the other onboard computers. All the driver had to do to start off was to move the joystick gearshift forwards out of neutral and simply depress the accelerator. The Telligent automatic gearshift system engaged whichever gear it considered most appropriate and let in the clutch as soon as the driver had increased the engine speed sufficiently. The onboard computer automatically calculated and prepared the gear changes during the journey; the truck's internal communication system coordinated and controlled the interaction between engine, clutch and electro-pneumatic gearshift system.

The transmission has three-ranges (2 x 4 x 2): the splitter unit, main gearbox unit and the range-change unit. While the splitter unit and the range-change unit are shifted via synchronisers, the gears of the main gearbox are engaged via dog clutches, which couple two rotating components not by friction but by interference. The 16-speed transmission is illustrated in Fig. 12.26.

12.8.2 Conclusion and advantages of G260-16

The automated commercial vehicle transmission with 16 gears has the main qualities of greater efficiency and faster, smoother gear changes which means added benefit for the driver. At the same time, the inherent advantages of the range-change/splitter gearshift has been kept light by dispensing with synchromesh in the main transmission. It is precisely in such applications that the benefits of a fully automated manual transmission can be fully

12.26 Automated commercial vehicle transmission: 16-speed G260-16 Mercedes-Benz.

brought to bear. Concrete advantages include further improved economy (added operator benefit) as well as faster and smoother gear changes (added driver benefit).

12.9 References

1. Naunheimer, H., Bertsche, B., Ryborz, J. and Novak, W. Automotive Transmissions – Fundamentals, Selection, Design and Application, 2nd edn, Springer Verlag (2009).
2. Greiner, J., Indlekofer, G., Nauerz, H., Dorfschmid, J., Gödeke, T. and Dörr, C. Siebengang-Automatikgetriebe von Mercedes-Benz, ATZ 105 (2003), Nr. 10, S. 920–930.
3. Dorfschmid, J., Döpper, W., Jäggle, G. and Heukelbach, K. Evolution to the sevenspeed automatic transmission 7G-Tronic Plus, ATZ 112 (2010).
4. Oberhauser, M. and Vetter, H. Mechatronische Getriebesysteme Band 595, Expert-Verlag (2005).
5. Henning, G., Dorfschmid, J. and Gödecke, T. 'Das Getriebe als Schlüsselelement im modernen Antriebsstrang'. Vortrag, AVL Konferenz Motor & Umwelt, Graz, Österreich, September (2003).
6. Weiss, M., Armstrong, N., Schenk, J., Lückert P., Ruhl T., Nietfeld, F. and Gödecke, T. 'Hybridsystem für die M-Klasse von Mercedes-Benz', MTZ 09I2009, Jahrgang 70, pp. 64–651 (2009).

Sustainable design and manufacture of lightweight vehicle structures

G. S. Daehn, The Ohio State University, USA

DOI: 10.1533/9780857097422.2.433

Abstract: Alternate fuels and powertrain systems will force an increased premium on vehicle mass reduction both to make vehicles more fuel-efficient and to enable greater range. An analysis will shows that life cycle analysis can be used to quantitatively assess how the use of alternate materials to reduce mass will contribute to energy savings and carbon footprint reduction. This justifies aggressive mass reduction driven by the use of alternative materials Also, a detailed look at auto construction shows that the vehicle of the future will likely be made from many materials. This poses many new challenges in design, manufacturing, joining and durability.

Key words: lightweight construction, materials selection, joining, steel, aluminum, carbon-fiber, magnesium, life cycle analysis, carbon footprint.

13.1 Introduction

Over the next two decades, personal transportation is going to undergo changes that are more dramatic than have been seen since Henry Ford's introduction of mass production in the decade following the introduction of the Model T in 1908. A perfect storm of sorts is brewing. The forces of sustainability, energy availability, carbon footprint reduction and new technological opportunities are impacting vehicle design and manufacture all at once. Each of these is a powerful force in its own right, but together they will drive upheavals in automotive design and production in the next decade.

Petroleum is becoming relatively scarce. This is driving the need for new powertrain and propulsion systems. Electric systems driven by batteries and fuel cells are of great interest. New combustion systems based on compressed or liquefied natural gas, or derivatives such as propane are also seeing significant use in commercial vehicles and these fuels are poised for much greater use in personal transportation. These new fuels (gaseous or electric) provide much less energy on a volumetric and/or mass basis. Therefore, significantly larger volumes must usually be carried to provide an acceptable driving range. If vehicle structural mass is reduced, more fuel or batteries can be carried while still maintaining an acceptable vehicle weight.

There is also worldwide concern for sustainability. Sustainability is a word that holds different meanings to different audiences, but generally sustainability is concerned with developing practices and technologies that conserve our resources, minimize our carbon footprint and support the dignity and values of both producers and consumers. Of course, vehicles that have greater mass will consume more energy to travel a given distance and for carbon-based fuels, this translates to increased carbon emissions. For electrically powered vehicles this means that larger batteries are needed, and larger motors are needed. These forces strongly favor lighter vehicles. Sustainability also demands that the materials that are used do not require excessive amounts of energy to produce, and can be efficiently recycled.

Technical opportunities also will drive change. Since the 1930s automobiles have relied on 'unibody' construction based primarily on stamped steel panels that are spot-welded together to produce high performance (stiff) and safe (crash energy absorbing) body structures. This fabrication technology is well suited to mass production and has produced structures with an excellent combination of performance and cost. Thanks to new higher strength steels, these structures are much lighter than before, but even lighter weight structures can be produced by utilizing low-density, high-strength materials such as aluminum, magnesium and polymer-matrix composites. Also, new shaping and joining technologies are starting to provide value propositions that challenge the dominance of the stamped and welded steel unibody construction. The technologies that will be used in the automobile of tomorrow will largely resemble those used in today's higher-value vehicles such as aircraft and racing bicycles. Also, the producers of these very light structures are eager to learn if there are ways of reducing cost in manufacture from the automotive industry, that has raised low-cost production to a high art.

This chapter presents a survey of the challenges and opportunities that will face the designers and manufacturers of automotive structures in the next decades as they struggle to reduce mass and enhance sustainability without sacrificing occupant safety. Of course, cost and economics will be the final arbiters of what solutions will be accepted and which will be considered failed experiments. This chapter first considers how we can quantitatively value mass reduction in ground vehicles. While there is an intrinsic value in minimizing carbon footprint and developing recyclable structures, cost will be the metric used, as this is the universal metric of business decisions. Next, we will describe in greater depth the constraints that automobile designers and manufacturers must work under. We will introduce the potential of new materials in light of these constraints. With this background, the potential of dramatically different vehicle design approaches will be considered with specific examples cited. Final remarks will again point to the great opportunities that this period of great change will produce for innovative engineers.

13.2 The value of mass reduction

Professor Michael Ashby of Cambridge University has developed a clear and systematic framework that can be used to analyze the effects of material usage and design a materials life cycle analysis that is in spirit very similar to those that have been done in the past (Gaines 1997; Peters 1997). The details and implications of this method are presented in a book (Ashby 2009) and software (Anon 2012a), but the approach will be introduced here and extended in some regards. The beauty of this approach is it allows quantitative assessment of the effects of a material or design change in any given component or structure. The life cycle of any structure or component can be broken into four discrete elements, each of which requires the consumption of resources (energy, feedstock, and transport) and produces waste (low grade heat, particulates, toxins and emissions such as CO2, NOx, SOx and so forth). With appropriate policy decisions these could also be expressed as costs through taxation. The five stages of the life cycle are:

1. Material production Any material requires some energy and cost to render it into a useful condition. For example, reducing the ore, bauxite, in an electrochemical cell, produces aluminum. This requires energy of about 250 MJ/kg, based on the mass of the finished aluminum, and this is considered to be an embodied energy. The production of any engineered material (e.g., steel, polycarbonate, carbon fiber, etc.) from natural materials requires cost and energy. Aluminum and carbon fiber have much larger embodied energies than steel, for example.
2. Manufacture Imagine that we have taken aluminum from bauxite and now want to produce an automotive wheel by casting. The material must be melted, cast and surface finished. All these stages represent cost and energy consumption.
3. Transport Often goods are shipped great distances between points of production and use. Different modes of travel have very different energy efficiencies. For example, rail or ocean shipping is much more efficient than airfreight. This can be accounted for quantitatively, and this will be touched upon later as well.
4. Use Most components or materials have aspects that consume energy and cost in use. For example, in a refrigerator good insulation can save significant energy over the life of the appliance. Similarly, by reducing the mass of an automotive component, fuel is saved because it takes less energy to accelerate the component on each stop-start cycle.
5. Recycling/reuse If a component can be recycled at the end of life, all or much of the embodied energy can be reclaimed. For example, if an aluminum wheel is recycled, all of the embodied energy on reduction can be taken as a credit. Many materials are not quite so forgiving, however.

As an example, thermosetting plastics cannot be easily recycled into similar high-value products.

All of these stages in a material's life can be carefully accounted for on the basis of energy, cost or both, and this system provides a framework whereby quantitative decisions can be made as to if a lighter but more expensive component or one with a higher embodied energy will pay back its value over its life cycle and a detailed formalism (Ashby 2009)with complementary software (Anon 2012a) exists.

For the materials that go into an automotive structure, the energy used in manufacture is often small relative to the embodied energy in the materials and the energy consumed in the use of the vehicle. The embodied energies, rough cost and recyclability of several potentially important automotive materials are provided in Table 13.1. Note that the values here are somewhat approximate, but this is not terribly important in that the values differ so widely between varied materials classes that each material provides a very different energy use scenario.

As an example, consider the materials selection choice between a steelintensive vehicle and an aluminum-intensive vehicle. Say 300 kg of steel is to be replaced by 200 kg of aluminum (which is in line with many estimates of the mass savings potential of aluminum). There is over 20 GJ of additional

| Table 13.1 Embodied Energy and Cost of Several Important Light Structural | | |---------------------------------------------------------------------------|--| | Materials | |

Material	Production energy (MJ/kg)	Approximate cost* (\$/kg)	Recycling notes
Steel	30	0.80	Easily and routinely recycled
Aluminum	150	2.00	Easily and routinely recycled
Titanium	700	50	High grades prone to contamination
Magnesium	350	3.00	Possible, but small waste stream
Carbon fiber	500	50	Possible, but small waste stream
Epoxy	150	5.00	High grade products impossible
ABS	90	3.00	Limited high grade recycle
PVC	80	2.00	Limited high grade recycle

^* Costs are particularly difficult to make general statements about and depend on precise chemistry, product form, surface condition, lot size, etc. However, without significant technology changes, titanium and carbon fiber will face large cost disadvantages versus steel, aluminum and other reinforced polymers.

embodied energy in the aluminum structure. The mass reduction must provide large energy savings in the use phase of the vehicle if there is to be a net environmental gain. As will be apparent soon, we can estimate the distance a vehicle must travel so that the energy savings due to lightweighting will offset those due to the extra energy that was required to produce the vehicle. As will be demonstrated in the next section, a typical passenger vehicle must be driven between about 50,000 and 100,000 km to offset this embodied energy. There are two important caveats in this simple assumption. First, this assumes that the energy used in material production is equivalent to that used in transportation. In fact, energy used in industrial production has a high probability of coming from sustainable sources (hydro, wind, nuclear, etc.) and land-based energy production can often employ more effective pollution abatement equipment, possibly including carbon sequestration. Second, the dynamics of this analysis change dramatically if we assume all or some of the vehicle structure is recycled at end of life. If the components are recycled fully, we can take full credit for the embodied energy of the materials on recycling and in this case the lightweight aluminum structure is a clear winner.

This issue of recycling and recyclability has aspects that touch on business practices, policy and technology. Some materials such as steel and aluminum are widely and effectively recycled. Because there are possibilities to develop high-grade products from recycled aluminum and steel and they are so commonly used, recycling for these materials is common and driven by market forces alone. Polymers are more problematic with respect to recycling. There are two basic types of polymers; thermoplastics and thermosets. While thermoplastics can in principle be re-melted and re-used, their properties do degrade and there are limits on how much recycled content can be used, and their recyclability decreases if thermoplastics are reinforced with fibers or fillers, as is common to increase strength or stiffness. Because of their permanent bonding chemistry, thermosetting polymers such as epoxy, urethane and rubber cannot presently be re-processed into high-value materials. The technology of vehicle structure recycling including issues of disassembly and separating scrap material into streams of like material is very important in the development of sustainable vehicles and a full treatment is well beyond the scope of this chapter.

It is important that we are able to make approximate but quantitative decisions about selecting optimal materials and structures for vehicles. In order to do so, we need a quantitative metric. Most commonly the appropriate metric involves fuel economy, and if we assume carbon-based fuels are used, this can be related to either cost or carbon footprint very simply. In the United States, the most common metric is miles per gallon. However, strict MPG may not present a fair comparison in that a bus may have lower miles per gallon than a sedan, but it carries 50 passengers. Therefore the bus is much 'greener' than one person driving a standard sedan. Thus, passenger miles per gallon may be a fairer metric.

To provide a fair comparison that does not consider how a vehicle is used, we will quantify the extra amount of energy to move an extra amount of mass some extra distance for a given vehicle type. For example, the airlines know well that if you carry an extra 50-pound suitcase it will cost them a little more fuel to travel a certain distance. If that weight can be saved either from the passengers and their cargo or from the aircraft structure itself, the airline will save fuel and money (and less carbon will be emitted into the upper atmosphere). Thus, we will approximate values of energy (kJ) that are required to transport a given mass (kg) a given distance (km). These assumed constants are shown in Table 13.2, and approximate CO2 emissions are also provided. As a matter of convenience in interpreting these numbers, a liter of gasoline contains about 35 MJ of energy (about 130 MJ/gallon).

With this assumed relationship between extra mass and energy or fuel consumption, we can ask the question of how much fuel would be saved by a material or design change. For example, if a magnesium seat frame in a personal automobile replaces a steel seat frame, this may save 10 kg. If this car has a lifetime of 200,000 km and has a marginal energy rate of mass of 3 kJ/km-kg, this will result in an energy saving of about 18,000 MJ of fuel energy over the life of the car, or about 170 liters of fuel saved over the vehicle lifetime. If gasoline costs about \$1 per liter, this results in a \$170 fuel savings. This can also be equated to about a 460 kg reduction in CO2 emissions. This demonstrates real and direct economic and environmental gains by mass reduction. The economics may justify many design and materials choices immediately, but there are also benefits in vehicle payload, handing and environmental footprint.

This approach can be generalized to other vehicles by considering their marginal mass-based costs of transportation, represented by C, and the lifetime of the vehicle expressed in distance of the reduced mass components travel (usually the life of the vehicle) which we represent as L. The fuel saved, DE, by a mass reduction of Dm is then simply:

Table 13.2 Marginal mass-based energy cost and CO2 production (assuming petroleum-based fuel for various modes of transportation)

Transport/vehicle mode	Energy (kJ/kg-km)	CO2 (g CO2/kg-km)
Diesel ocean shipping	0.15–0.3	0.009–0.018
Diesel rail shipping	0.2–04	0.012–0.024
Diesel heavy truck	0.4–1.0	0.024–0.06
Personal auto	1.5–5	0.090–0.3
Air freight	7–20	0.42–1.2
Helicopter	30–100	1.8–6

Source: Adapted from Ashby (2009).

$$\Delta E = \Delta m \cdot C \cdot L \tag{13.1}$$

as was shown by example in the last paragraph. This saving in transportation fuel can be translated to a petroleum saving, by a factor of about 35 MJ per liter of petroleum. This can be translated to cost savings by multiplying by the cost of fuel, and burning gasoline emits about 2.7 kg of CO2 for each liter of gasoline burned (Anon 2012b). Both the vehicle lifetime and the marginal mass-based costs of transportation are difficult to estimate with precision. Both depend strongly on the vehicle itself and how it is used and maintained. But it is instructive to make comparisons between different vehicle classes because there can be large differences between them. For example, a 737 aircraft has a marginal mass-based cost of transportation of about 15 kJ/km-kg. The same 10 kg saving over its lifetime (of about 100 million km) will produce a fuel saving of about 400,000 liters over the life of the aircraft. Thus, mass reduction in heavily used commercial aircraft pays tremendous environmental and economic benefits. It is instructive to make these kinds of comparisons for vehicles of different types. Since both the C and L terms are difficult to estimate and vary widely, they will be represented on logarithmic scales. There is an additional benefit of logarithmic scales; the product of the two axes will plot as a straight line. Vehicle classes can then be added to the plot. Such a plot is shown in Fig. 13.1.

Figure 13.1 shows how the life cycle economic benefits mass reduction can be quantified on varied vehicle platforms and that while strong benefits can be accrued for passenger vehicles, there can be even stronger benefits in commercial vehicles (largely because of their longer lifetimes) and extremely strong economic cases can be made using a life cycle analysis for weight minimization in commercial aircraft. While we may like to make choices in materials and design based on notions of sustainability, economic forces often drive human decisions. Even if an individual has an altruistic nature; bureaucracies often mandate lowest-cost options even if they are not the most sustainable. Keeping cost in mind, let us introduce another simple metric for vehicles: cost per mass, expressed most commonly in the United States as dollars per pound. This can simply be estimated by taking the purchase price for a new vehicle and dividing it by the mass. This is somewhat simplistic, but places interesting bounds on the kinds of materials that can be used extensively in various vehicles. The results of these rough computations are shown in the top half of Fig. 13.2 for a variety of vehicles.

Figure 13.2 also displays the cost, on a per mass basis, of the raw materials that may be used to build the vehicle structures shown. Material costs for predominant structure materials set a lower bound on the cost of the finished structures. The raw materials must be formed to shape into a finished vehicle, which takes investment in equipment, energy and manpower. In addition, the parts must be assembled, inspected and the whole process and structures needs to be designed, engineered and tested to meet standards. All of this

13.1 Plot of various vehicle types in terms of their approximate marginal costs of mass transport and service life. The ovals show approximate ranges for each value on logarithmic scales. The product of these two factors plots as lines on the graph, indicating the energy consumed by marginal mass during vehicle life. This also converted to the fuel and cost-savings provided by mass reduction over the lifetime of the vehicle. Iso-contours are labeled in liters of fuel saved per kilogram of mass reduction and dollars of operating cost reduced per pound of mass reduction assuming a fuel cost of \$1 per liter.

adds to total cost. This base materials cost, however, can tell much about how materials can be deployed in actual vehicles.

Production volume and performance both affect vehicle cost. The more costly vehicles tend to be more unique, that is to say there are fewer of them in the world and they each tend to be more built by hand than by mass production. These aspects add to cost. also, the more costly items tend to be very high performance in that they excel in some given mission. as examples of this a 737 aircraft will move people over millions of passenger miles. It will burn in the order of 500 million gallons of petroleum over its lifetime. Technologies that incrementally improve fuel economy clearly have great value. Similarly, the cost to launch a communications satellite is on the order

13.2 Logarithmic scale of cost per mass. Above the scale shows total approximate vehicle new purchase price divided by mass. Approximate dollar per pound costs for potential constituent materials are shown below the scale.

of \$100M and the craft itself has a similar cost. There is great economic value in eliminating mass. For example, very expensive technologies that reduce weight of the satellite or rocket may allow the insertion of one more transmitter module.

Figure 13.2 reinforces some of the trends seen in Fig. 13.1, in that the vehicles that provide large economic incentives for increased performance with mass reduction tend to have much higher costs on a per-pound basis. These vehicles can 'afford' the increased cost of advanced materials such as carbon-based structural composites, or diamond-based heat sinks. Furthermore, at about \$50 per pound, carbon-based composites will not economically find application in mass-produced consumer-based vehicles which have finished costs at about \$10 per pound simply because they are too expensive. Moreover, even materials modestly more expensive than steel (such as aluminum), have been relatively slow to gain acceptance in automotive applications. This may have more to do with the lack of accepted manufacturing practices and confidence in applying the materials than the cost of the material itself. This simplified introduction to life cycle economic analysis and its application to vehicles is largely in accord with those that come from other approaches (MacKay 2009), and more detailed analyses (Gaines 1997), (Peters 1997). Also alternative powertrains, such as those based on batteries, may impose additional incentives to light structures, because of the limited range and high mass of batteries (Joshi et al. 2011).

There are some important points that can be taken from this rough life cycle cost analysis:

∑ Economics are the final arbiter the materials, designs and technologies will be pervasively used over the long term. Life cycle costs should guide materials, design and manufacturing choices. They may also educate consumers to trade up to more initially expensive materials that will have lower life cycle cost.
∑ Sustainability will drive the design, materials selection and manufacture of products when economic incentives align with sustainability goals.
∑ All classes of vehicles share similar goals of maximizing performance, by minimizing mass and cost and these provide similar engineering trade-offs. Over time, as once expensive materials and technologies may become less expensive they may find application in lower-cost vehicles. Further, lower-cost manufacturing technologies may be applied to higher-cost vehicles. For these reasons, there are benefits in examining light vehicle structures as a class, including everything from bicycles to aircraft, with automobiles being a very important member of the class.

The next section of this chapter, guided by this life cycle analysis, will describe the engineering and challenges associated with reducing vehicle weight with a particular focus on adapting to the existing manufacturing paradigm.

13.3 General challenges and opportunities

The modern automobile is a highly evolved product. At any point of time, there are tens of companies worldwide, each employing hundreds of engineers focused on the design and manufacture of next generation vehicles. All are focused on continually developing new models, each of which is designed to be better than their last model and better than their competitors. This has lead to meticulous attention to many details in materials, stamping, welding, corrosion protection, paint and so on. Right now, the primary structures of over 90% of the world's passenger vehicles share a largely similar and highly refined design, and one that is economical to manufacture. These share a unified-body, or unibody construction where sheet steel is stamped into given shapes, and these are welded together to provide a structure that is economically produced and does a more-than-admirable job in in many ways.

The primary job of the automotive body structure is to comfortably carry people and goods from one place to another in a comfortable, safe and affordable way with good road handling characteristics. This can be translated into a number of more specific engineering criteria. These include strength, stiffness, crashworthiness, environmental resistance, overall conformation, affordability and sustainability. There are other important aspects, such as how such a vehicle might be repaired that is again a largely solved problem for steel-based unibody structures, that will need to be treated in the nextgeneration vehicles. Some of these categories overlap somewhat. First, let us describe what is required in each case and then this will be placed into the context of automotive design.

13.3.1 Stiffness

Stiffness is the resistance to reversible flexure of the vehicle or, more precisely, the force or torque required to present a given amount of elastic deformation. It is controlled both by the materials that the vehicle is constructed from and the shape or conformation of the vehicle structure. For example a polymer-based beam is much more compliant (less stiff) than a steel beam, but an I-beam section can be much stiffer in bending than a solid square or round section of the same weight per length. Stiffness is a very important parameter in giving a vehicle good road-handling characteristics. In one extreme, racecars must have very stiff chassis for good road manners. At the other extreme, while a cargo truck does not need great road handling, if it is not stiff enough, it may be impossible to close a door if one wheel is on a curb because the whole vehicle structure will flex and change shape. Exactly what stiffness is necessary is dependent on users' expectations, but vehicles that are comfortable and handle well generally have high stiffness.

Stiffness of particular panels also plays a dominant role in noise vibration and harshness. Large unconstrained panels can act something like the diaphragm of a drum, and will give large amounts of resonant noise.

13.3.2 Strength

Intuitively it would appear that the strength of auto body materials would be of paramount importance. While stregnth is important, the problem of designing an automobile structure has much more subtlety. While at first it may seem that if we could double the yield strength of the steel that makes up an auto-body, we may be able to cut the thickness of the structure in half while retaining the same structural strength. If this were true we could reduce mass by 50% as well. There are multiple problems with this logic. For example, all steels have elastic modulus values of about 210 GPa (or 30 ¥ 10⁶ pounds per square inch). Therefore with design unchanged, thinner sections of stronger materials will give vehicles structures of insufficient stiffness. Second, many properties such as dent resistance are complicated functions of material strength, shape and thickness. A sheet of twice the strength, but half the thickness, will generally have poorer dent resistance than the original sheet. Dent resistance is a primary design criterion for auto body exterior panels.

13.3.3 Crashworthiness

Crashworthiness is also highly dependent on how the materials, construction and design of the vehicle work together. From a collision point of view, a vehicle can be considered as two primary kinds of structure. First, there is the passenger cabin, within which the occupants should be belted to the seating. This compartment should represent a 'safety cage'. Ideally this cage will not distort or deform. Trapping feet with a collapsing firewall or having the roof structure deform onto heads is extremely poor from an injury point of view. Very strong passenger compartments are essential for these safety cages. The present material of choice for primary structural pillars is boron-containing hot stamped steels, which have very high strength and are easily manufacturable. The other important element is the crush zone that surrounds the cabin. This zone is tuned to absorb energy and provide deformation. Crash performance is improved by controlling the acceleration of an occupant's chest and minimizing a head injury criterion, which is also related to peak rates of acceleration or G-loading. Vehicle safety standards are based on these measures. To receive a 5-star crash rating, a passenger's chest should receive fewer than 48 Gs of acceleration when a vehicle impacts with a fixed rigid barrier. Therefore, from a design point of view, the crush zone should crush from its original shape to nearly zero thickness in front of the cabin compartment, ideally with a nearly constant force that is no greater than 48 times the vehicle weight while the safety cage remains undeformed. For the deforming members, high strength or stiffness is not desired, but what is important is the ability to absorb energy in a controlled manner and the ability to crush the full distance from the bumper to cabin, while the force remain near the peak value it maximizes the energy absorbed. Governmental regulation impose strict standards on occupant protection, and on various crash modes including side impact and roof crush. The mass of vehicles could be dramatically reduced if crashworthiness were not a consideration. This is part of the long-term potential promise of computerdriven autonomous vehicles. If the chance of vehicle collision is driven to nearly zero by fail-safe robot operators, then much lighter, non-crashworthy vehicles could be built.

13.3.4 Environmental resistance

Over the past three decades automobiles have been transformed from structures that would rust through on salty roads in a few years to vehicles that see very little environmental degradation over many years of service. This has largely been made possible by intelligent combinations of materials, design and manufacturing processes. In specific, modern vehicle designs are based on steel that is largely protected by surfaces galvanized with zinc. Also, painting and design methods have improved. New light structures threaten the environmental resistance that we have achieved. For example, the highly crashworthy design we discussed in the last section may ideally be produced from a hot-stamped steel occupant cabin. Even if relatively thin steel is used this construction gives remarkable strength and provides a cabin that is very difficult to deform, keeping the occupants from being crushed. Aluminum extruded beams are excellent with respect to absorbing energy with minimum mass. However, there is a significant problem in that if aluminum and steel are joined and electrically coupled, even if painted, water can diffuse through the paint layer and form galvanic couple that promotes corrosion, due to the dissimilar corrosion potentials of the two materials. This presents an Achilles heel for corrosion. There are practices that can isolate these metals in construction, but they are difficult to enact. In aircraft and boat construction, practical rules of thumb have evolved that allow some dissimilar metal connections. One key concept is that the corrosion potentials, found on the galvanic series, for dissimilar metals must be close to each other. Polymers also have issues with environmental resistance. Oxidizing chemicals and ultraviolet radiation can age or degrade these materials. This again can limit the safe lifetime of vehicle structures.

13.3.5 Affordability and manufacturability

At the time of this writing, General Motors' ambitious extended range electrical vehicle, Chevy Volt, which is by all measures a technical masterpiece and a compelling vision of sustainability and practicality is having its production temporarily reduced. Experts and pundits believe it is because the Volt costs about \$40,000 and a similarly featured and sized gasoline vehicle costs a little more than half that. This experiment with consumer tastes shows that while some consumers will pay for a sustainable vehicle, most will require final costs and sizes that are similar to the current generation of vehicles. Thus, both materials and manufacturing processes must be affordable. Hence, materials such as carbon fiber, and bonding processes that require autoclaves will probably not see mass production in the short term. Even with respect to less exotic materials, as strength levels in steel or aluminum increase, keeping shape tolerance (springback) becomes more problematic and joining methods through spot welding can be more difficult and expensive. There are many exciting and ambitious projects that are on-going that promise to reduce the costs of both radical and incremental mass reduction technologies.

13.4 Possible architectures of the next-generation vehicle

There is consensus that the automotive industry will undergo a time of dramatic and unprecedented change in the next one to two decades. This will be driven by the need to accommodate new propulsion systems that may feature electrical power plants, compressed or liquefied natural gas, fuel cells or other systems. This can produce fundamental changes in vehicle architecture. Weight reduction is even more important in these new systems as lower density fuels (CNG, batteries, hydrogen, etc.) challenge the range a vehicle can provide. Mass reduction not only benefits economy, but reduced structural mass can be replaced with increasing battery or tank mass without sacrificing design. As these changes occur, regulators and consumers will still expect the same or increased levels of safety, durability affordability and efficiency that they have become accustomed to. Customers may also appreciate sustainable features such as the use of plant-based materials for polymer replacement, but they will have to closely meet the cost and engineering properties of more traditional engineered materials. The overall conformation of the vehicle is very likely to evolve based on one of the established architectures, or some mixing of them.

Currently, virtually all mass-production sedans are based on a unibody design that uses steel stampings that are spot-welded together as described below. The process that has led to the common modern automotive unibody has many parallels to the process of evolution as Darwin described it, and the structures are correctly described as highly evolved, and most of the current structures share much of the same manufacturing and design 'DNA'. The core design and manufacturing methods, as well as the supply chain, are common to many different car models, or 'species'. Some changes in the coming years will be evolutionary and some may appear to be revolutionary, but will probably actually harken back to earlier stages of the evolutionary path, with new technology applied. With that in mind, it is worthwhile to see what the other options are for vehicle design and manufacture.

13.4.1 Unibody construction

In this design the structure is created from structural sheet steel that is stamped into shape and then spot-welded to form a single structural system. This is a system that scales extremely well to affordable mass-production and easily produces a great combination of strength, stiffness, crashworthiness and affordability combined with a reasonably high level of structural efficiency which then provides low mass. Figure 13.3 shows a typical construction of a body in white (BIW – the term used for the final welded body structure less closure panels, glass, interior and fenders).

The obvious path to mass reduction with a structure of this type is to use thinner sheets of stronger steel to produce structures of the same strength at reduced mass. This approach has been taken with great vigor by automakers and the steel industry. Consortia based around the steel

13.3 Schematic drawing of the construction of a stamped and spot welded, typical unibody vehicle construction. Individually shaded panels indicate discrete stamped parts. These are joined by spot welds.

industry including the Auto Steel Partnership have worked to demonstrate the potential of reduced mass using advanced steel-based vehicles. Much of this information and insight has been captured under the heading of the ultra light steel auto body (ULSAB) and the information is summarized at the website www.ulsab.org. As discussed earlier, there are a few essential problems with the theme of simply making steel stronger and lighter. Most importantly body stiffness and vibration modes are primarily related to the stiffness of the components and the elastic modulus of the component material. The elastic modulus of steel remains in a very narrow band of about 210 GPa (about 30 ¥ 10⁶ psi) for all grades. Similarly its density is nearly unchanged for all steel grades (about 8 g/cm³ ). Also, crashworthiness is complex and not necessarily improved by simply substituting higher strength metals.

13.4.2 Ladder frame

The earliest automobiles and trucks were built on frames that resemble ladders. There are typically two major frame rails that are often created from roll-formed carbon steel in C-shaped sections. Cross members connect the two major frame rails and then the suspension, engine and passenger compartment are mounted on top of this ladder frame. It is difficult to create acceptably stiff or sufficiently lightweight frames from ladder frames, but they are simple and inexpensive and present an agile platform for new or modified models. These still form the basis for most current truck frames as well as pickup and other light duty trucks. The advantage of this construction is that it is easy to implement and relatively inexpensive. As a result most low-volume commercial and military vehicles are based on this technology. Figure 13.4 illustrates a typical ladder frame.

With some enhancements, the basic ladder frame approach can be developed into high-performance auto structures. A very advanced extension of the ladder frame was applied to the 2006 Chevrolet Corvette Z06. This design featured two aluminum frame rails that run from the front to back bumper and are shaped to accommodate the wheels and provide a stiff and crashworthy final structure. This design incorporates a number of structure types including extrusions, castings and sheet structures. The primary structure uses a mixture of joining technologies to form a coherent whole from these pieces. These processes include metal inert gas (MIG) welding, laser welding and self-piercing rivets. A lightweight fiberglass body provides the exterior to this structural core. The Z06 Corvette's aluminum frame saves 136 pounds compared to a similar steel structure. Other innovations in this vehicle include use of a magnesium engine cradle and a balsa wood core, carbon-epoxy floor structure. This aggressive mass reduction technology comes at a cost. In 2006, the Z06 corvette cost approximately \$65,000.

13.4 Schematic drawing of a typical ladder frame. Two frame rails are joined by cross members. The cabin, powertrain and body are then mounted to this basic structure. This is a very common construction for vehicles with low production volumes.

13.4.3 Space frame and truss frame

Truss or space frame structures generally use triangular or tetrahedral arrangements of tubes. Ideally this configuration keeps the tubular elements in pure tension or compression. This can give very stiff, and potentially crashworthy structures a very low mass. These kinds of structures are commonly used for sport vehicles, fabricated from 4130 carbon steel tubes. These are very efficient structures, but their deficits are that welded tube frames are difficult to fabricate affordably in mass production and they are not as light as carbon-epoxy structures can be. A schematic diagram of this construction is shown in Fig. 13.5.

The essential problem with respect to both ultimate performance and affordability is related to the processes used to weld truss frames together. Fusion welding produces heat-affected zones that reduce the strength relative to the strength of the parent material. This somewhat limits the peak loads, crashworthiness and fatigue resistance of the structure. Probably more important; these structures are difficult to fabricate inexpensively. Relatively complex fixture systems are needed to hold individual elements in place and distortion tends to take place on welding and solidification. The spot welded unibody construction is much easier to scale up in mass production. There is something of a revival in elements of in frame structures, as many new

13.5 Typical configuration of a welded tubular space frame to create a small racing vehicle.

13.6 Schematic diagram of the backbone structure that is used in the Lotus Elan. In this case, the structure is fabricated from sheet aluminum.

designs can be described as hybrids with elements of unibody and frame construction. Often dissimilar materials and complex component shapes are used. The Audi A8 is an example of a structure that is often described as a space frame, but holds elements of other types of construction as well.

13.4.4 Backbone chassis

Figures 13.6 and 13.7 show 'backbone' approaches to creating the central vehicle chassis. Both these designs can be exceptionally stiff, strong and relatively simple. They use a central member that is stiff in bending and torsion and from this, attachments for wheels, powertrain and the passenger compartment are mounted. Energy absorbing crash features must also be mounted onto the basic structure. This approach has been used successfully in both elite performance vehicles such as the Lotus Elan (Fig. 13.6) and low-cost

13.7 Uni-chassis design. The central cylinder provides the heart of the structure and additional features are mounted to this to provide mounts for powertrain, cabin and energy absorbing features. Design (see: http://theodore-associates.com/unichassis.html).

vehicles such as the Volkswagen Beetle. Figure 13.7 shows a concept that has been promoted as a uni-chassis that uses the same basic concept. There is some great merit in such designs as they provide simple and agile platforms wherein a number of different vehicle configurations can be developed from a common base. They also present some interesting opportunities; for example the central cylinder in the uni-chassis design at once provides strength and stiffness, the cylinder design is almost ideally shaped to become a tank for storing compressed natural gas. Such multi-functional use of components is essential for minimizing weight at maximum utility.

13.4.5 Monocoque

The absolutely lightest present-day vehicles use a monocoque construction, which is basically the creation of a structural shell creating the structure itself. This is performed both in aircraft construction, usually using an aluminum shell and in the production of very high performance automobiles when a carbon-fiber reinforced epoxy is typically used. Figure 13.8 shows an example of a Lamborghini Aventador with the external non-structural panels stripped away. The monocoque is the central region around the cabin and to this, frame-like structures for suspension, powertrain and energy-absorbing features are mounted. Again, this demonstrates the blending of varied design ideas that always takes place as vehicle designs are refined and optimized. New manufacturing methods will be needed to make such carbon-polymer vehicles affordable.

13.8 Schematic example of monocoque construction. The prototype here is the Lamborghini Avenator which has a structural carbonepoxy shell around the cabin.

13.4.6 Multi-material construction

In order to balance the important, but divergent objectives of stiffness, crashworthiness, environmental resistance, and cost, most experts agree that next-generation vehicles will incorporate widely dissimilar materials. There is strong evidence for this trend in that both the US Department of Energy and the European Union are funding multi-material vehicle initiatives. As another practical example, Audi introduced the aluminum-intensive A8 in 1994 as the lightest all-wheel drive car in the full-size luxury segment. Until 2010 this vehicle used an aluminum-intensive space-frame concept, but in its most recent generation it also mixes materials and uses ultra high strength hot stamped pillars on steel to minimize cabin intrusion in a collision.

13.5 Specific lightweighting technologies

The vehicle structures shown in the previous section have to be constructed with specific materials, using particular assembly methods. The technical community is quite active in developing new technologies that can reduce the mass of vehicles. There is a significant and growing body of literature on technology for vehicle mass reduction. Significant among these are a recent NAE sponsored study on vehicle mass reduction (National Research Council, Committee on Benchmarking the Technology and Application of Lightweighting; 2012) and an extremely comprehensive analysis by Lotus Engineering (Lotus Engineering Incorported 2010) of mass reduction opportunities in a standard crossover vehicle. A similar, more comprehensive study funded by the US EPA conducted by EDAG is forthcoming in the very near future. In this brief section a few of the very recent and promising innovations are noted. This is broken into two sections. The first is related to emerging processing lightweight materials to appropriate shapes. And the latter is related to the critical question of how these will be joined. Presently spot welding dominates automobile construction, but this process will be inappropriate for most new material systems.

13.5.1 Shaping/fabricating technologies

A component has two primary attributes that give it fitness for structural service in automobile construction, regardless if the service is stiffness, energy absorption, fatigue resistance or strength. They are the intrinsic material properties (stiffness, density, and strength, primarily) as well as the shape of the component. It is also important that the shape can be fabricated affordably. The materials science of creating new materials is relatively slow and there are a number of materials and microstructures that are available for exploitation. The significant and shorter lead-time challenge is in creating optimal shapes from these materials for given service requirements. A number of the most exciting new or emerging technologies are noted below.

Warm forming

Many light alloys are difficult to form because they lack the formability of steels and this limits the shapes that can be created. Forming while the sheet is warm offers much improved formability because the strain rate sensitivity of the material is much greater and enhanced formability including superplastic deformation is possible. This has been envisioned for some time (Bolt et al. 2001) and has been developed into the quick plastic forming process that GM has successfully used on tens of thousands of auto parts ranging from mass-market automobiles to Cadillacs (Krajewski 2007).

Hot stamping

Hot stamping is a process that is thematically similar to warm forming in that it may use the improved formability of steel at high temperature to provide exceptional formability (Karbasian 2010; Bariani et al. 2008; Abbasi et al. 2012). However, prior to the final step, the steel is above its austenitization temperature and the dies are cold. This causes the formation of very hard and strong martensite on the combined quenching and forming process. Specially designed alloys are used in this process that develop exceptional strength levels. This allows the production of extremely strong and safe cabin structures.

Impulse forming

There has been recent realization that very high forming velocity (50–500 m/s) can provide a number of advantages relative to conventional metal forming. In particular, formability can be improved, precision shape control can be attained (counteracting the problems associated with springback for high strength materials), and high fidelity embossing and precision shearing with high quality is possible. These techniques have been reviewed recently (Psyk et al. 2011), and new work shows new methods of creating environments associated with explosive forming in small, controlled areas using a vaporizing foil method (Vivek et al. 2012).

High-integrity casting of light alloys

Die casting offers one of the most compelling value propositions possible with regard to being able to very inexpensively mass produce complex shaped components. The primary limit on this technology traditionally has been the relatively poor ductility and toughness and sometimes strength of the metals that are cast. There have been great commercial advances in evacuating the die-casting chamber prior to casting and using pressure to eliminate porosity (Niu et al. 2000; Luo et al. 2010; Luo and Sachdev 2010). This has greatly elevated ductility and strength for these materials and makes this a very exciting technology for a number of structural components.

Additive manufacture

In the past few years the technical community has experienced an epiphany that there are methods by which materials can be directly consolidated to desired shapes without dies, and potentially with great control over microstructure (Gu et al. 2012; Santos et al. 2006). For the near-term future, it is likely that this will remain the domain of very low-volume high-value components, but this could quickly provide value by allowing rapid innovation in vehicle structures.

Beam structures

As was shown in the previous section, frame structures are based on the use of high strength and stiffness columns that largely have a constant section which may be curved and these can be joined at their ends. There are a number of innovations that are in progress that could allow the use of much more efficient shapes, and materials based on what are essentially beams. There is a German program known as Transregio 10 (http://www.leichtbau. de/tr10/) that is creating directly shaped and reinforced light alloy beams. Many of the recent technical developments were captured in Weinert et al., (2008). Also, in the realm of polymer processing, pultrusion has offered the promise of creating long reinforced thermoplastic beams without autoclave processing. There are other technologies emerging that allow the fabrication of advanced composite structures with nearly continuous and shaped cross sections based on thermoplastic or thermosetting polymers. These offer fundamentally high strength and low mass, but are difficult to integrate into vehicle structures with current joining technology.

13.5.2 Joining technologies

As automobiles move from stamped steel components that are spot welded together to complex hybrids of many materials, new joining methods will be needed. This will likely be at least as limiting as the availability of materials or shapes. The key important new joining technologies are described below.

Friction stir welding

Friction stir welding is based on the solid state intense mixing of the metals of two components and can join materials in lines or spots (Mishra 2005; Nandan et al. 2008). It can join a number of dissimilar metals and is now used in the production of a small but growing number of automobiles. There will likely be further growth of the use of this technology.

Impact welding

If metals strike at a sufficiently high velocity (usually 100–400 m/s) and an appropriate angle (usually about 7° to 15°) they can form a solid-state weld upon impact. This impact can be generated by explosives or by magnetic pulse. This holds the same essential advantages as friction stir welding, but may be practiced more quickly. Very recently, PST Corporation demonstrated the application of magnetic pulse welding at the end of a robot (Schäfer and Pasquale 2012). Also, a new method of generating the workpiece velocity based on a vaporizing expendable metal has been demonstrated (Vivek et al. 2012).

Self-piercing rivets

For truly dissimilar materials, such as joining metals to polymer-based composites, welding techniques will not work, and other modes of joining must be relied upon. Fasteners are the most common other approach but only join at a point. They can be quite effective, are easier to install than conventional rivets (Abe et al. 2009; Sun 2007; Mori et al. 2006) and significant advantages have been noted if they are installed with high press speeds (Neugebauer et al. 2011). The latest Jaguar XJ has a rather conventional structure that is produced from several formed sheet metal components that are joined at points. However, in this case steel is replaced with aluminum and over 3,000 self-piercing rivets are used to assemble the structure.

Structural adhesives

Adhesives are often used to augment self-piercing rivets, spot friction stir welds and other joining methods and these allow the joining of truly dissimilar materials. Large quantities of structural adhesive are used on advanced aluminum-intensive vehicles such as the Jaguar XJ and the Lotus Elise. They are often not used as primary reliant joints as there are still concerns about long-term durability and the possible fracture modes (Curley et al. 2000; Harris and Fay 1992; Hadianfard et al. 2008). They are also still sometimes difficult to implement in manufacturing as fixtures may be required to prevent motion before the adhesive sets.

Conformal joining

Rather than relying on loading through points or along adhesive lines, structures can be designed such that features abut one another and eliminate motion. Snap fit joints are but one example of this. With accompanying deformation, structures can be formed that provide great strength by adjoining or interfering structures. Figure 13.9 shows one such example of this approach. This is a conformal interference fit that is created by deforming the outer aluminum tube onto a steel mandrel at high velocity. The impact provides a natural interference fit and the shape of the mandrel provides a system that gives great axial strength, exceeding the strength of the parent tube. The only way to separate the couple is to deform the aluminum. This mode of joining does not involve any chemical or metallurgical coupling of the two components of the pair and is created at near room temperature. Joints such as this do not suffer thermal distortion; heat affected zones can provide the full strength of the materials present. There is a growing body of literature on this mode of assembly (Daehn and Eguia 2004; Kleiner et al. 2006; Schulze et al. 2006; Psyk et al. 2006).

13.6 Future trends

The goal of this chapter was to provide some ideas that may spur further technical development. First, there are important large economic and environmental drivers that will require significant mass reduction in vehicles.

13.9 Example of a conformal interference joined 6061-T6 tube electromagnetically compressed onto a steel mandrel and extended in tension until failure. The strength of the joint exceeds that of the aluminum (unpublished research in progress, A. Sattler, A. Luscher, L. Rusli, The Ohio State University).

Second, this and the next decade will be times of dramatic changes in vehicle architecture both from a structural and powertrain point of view. Third, multi-material construction will be an important, if not dominant theme in vehicle structural construction. Lastly, the customer is always right, and accordingly affordability will be paramount. These changes will drive further technical development in post-collision repair and recycling technology, but these issues are outside the scope of this chapter. While lightweight multi-material structures may often have a small but beneficial impact on life cycle cost, consumers will not tolerate large increases in initial vehicle cost to save fuel.

The key question for the technical community is, how do we usher in this more sustainable lighter weight future and do this quickly so as to reduce our global carbon emissions before trends are irreversible, all while keeping costs low enough so that they will be adopted without resistance. In the author's opinion, there are several elements that must act in parallel and to conclude these are discussed in turn.

13.6.1 Customer appeal and affordability

Any changes in vehicle structural architecture or new powertrain systems will have to be embraced by and purchased by consumers. The Chevy Volt experience shows that only modest premiums will be paid and that strong variations from the status quo may initially be treated with skepticism.

13.6.2 Thought leadership

New ideas for reducing the mass of vehicles are plentiful, but sustained interest and financial support is required to bring them to commercial reality. Several organizations worldwide have shown an interest in this problem and actionable technical solutions are being developed and matured. One example of this comes from the European SuperLIGHT-CAR program that is a collaborative Research and Development Project funded by the European Commission. The United States has programs funded by the Department of Energy and the Environmental protection agency and other groups. Several significant reports have been issued (The Minerals, Metals and Materials Society 2012; Lotus Engineering Incorported 2010; National Research Council 2012) and others are forthcoming. There is significant present interest in this field. However, it is important to note that these studies represent only very early stages relative to commercial availability next-generation vehicles.

13.6.3 Conceptual and detailed design

The new ideas on how mass can be reduced need to be developed into both conceptual and detailed design, and as the general mass of a vehicle reduces, this often allows other lighter components to be used in other parts of the vehicle (i.e., a smaller engine can be used for a lighter weight vehicle and this reduces structural mass and so on…). This process, known as mass decompounding can be driven cyclically to successively remove mass from a vehicle design (Alonso et al. 2012). The conceptual design level is relatively simple, whereas more and more specificity is developed as design, manufacturing, crashworthiness, NVH and durability are considered. As production ready designs are approached, engineers must know more and more about the details of each material and process.

13.6.4 Materials and process maturation

In the introduction of new designs and vehicles the only processes that are truly well-behaved, consistent and solidly reliable will be used. The development of these production-ready processes is a slow and daunting process itself. The risks associated with new technology introduction slow technical innovation. This is all with good reason. For a line manager if 90% of a vehicle is being produced easily but one or two operations are slowing the production of that vehicle, the manager's career may be at risk, as will those of the champions of the unreliable technologies. A key enabler in speeding technical innovation may be the availability of near-scale pilot production facilities that can be used to debug a process prior to full implementation.

13.7 Acknowledgements

I am grateful for a gift from the Alcoa Foundation, supporting research on actionable solutions for sustainability that helped make this chapter possible. Collaborative research on conformal joining and many spirited discussions with Professor Tony Luscher have greatly enhanced my thinking in this area. Thanks are due to Emma Sanders and Kate Mansfield who prepared original graphics and Kathy Babusci who helped bring the manuscript into shape.

13.8 References

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Improving vehicle rolling resistance and aerodynamics

M. Juhala, Aalto University, Finland

DOI: 10.1533/9780857097422.2.462

Abstract: This chapter discusses the effects of aerodynamic properties and tire rolling resistance on the fuel economy of vehicles. The basic principles of vehicle aerodynamics and tire rolling resistance are described separately in the contexts of passenger vehicles, and heavy vehicles such as trucks and vehicle combinations. Recent developments and future trends in both the aerodynamic properties of vehicles and in tire development are reviewed.

Key words: aerodynamics, rolling resistance, fuel economy, passenger car, heavy vehicle.

14.1 Introduction

When a vehicle is travelling on the road there are many factors that contribute to its performance and the amount of energy used. Besides the mass of the vehicle, an important role is played by both its aerodynamics and tires. Other factors include the efficiency of energy conversion, i.e. the engine and transmission and driving habits, particularly in the context of frequency of breaking.

Today it is possible to estimate the relative contribution of the main performance factors using vehicle efficiency simulation tools. The role of aerodynamics becomes more important as speed increases, and conversely, rolling resistance is more important at lower speeds. The influence of mass increases with higher speed variations and while driving on hilly roads.

The role of transmission is a very important factor in relation to vehicle efficiency and its effects are twofold, the first relates to losses in the power line and the second to its influence on engine usage. Power line losses prior to the tires are not discussed in this chapter. However, it is important to note that the losses can be dependent on the power transmitted or they can be independent, as for example with oil pumping in the gearbox. In the latter case, the effect on total resistance during coast down is significant.

Figure 14.1 illustrates the simulated results for a typical passenger vehicle proceeding in two different driving cycles. As can be seen, the most important factor is the mass of the vehicle. A 10 per cent increase in vehicle mass corresponds to a 4 per cent increase in fuel consumption, or alternately in CO2

14.1 Effect of 10% change of mass, aerodynamic resistance, and rolling resistance on fuel consumption in NEDC and FTP-72 driving cycles.

14.2 Effect of speed on the influence of mass, aerodynamic resistance, and rolling resistance on fuel consumption.

emissions, in the New European Driving Cycle. In the US FTP-72 driving cycle, the effect of mass is even higher. In the European cycle the effects of aerodynamics and rolling resistance are approximately equal and little under half that relate to mass. It is also the case that the European cycle is characterized by milder accelerations and a higher maximum speed range.

In addition to the driving cycle, speed also has a significant impact. Figure 14.2 illustrates the effects on performance of 10 per cent variations in weight, aerodynamics, and rolling resistance as a function of driving speed. It is clear that speed is the most important factor affecting aerodynamic performance.

Where previously aerodynamic characteristics of heavy vehicles was focused on achieving higher speeds and the capability of maintaining cruising speed under a head wind, the current focus has shifted towards fuel economy issues. This is due in particular to the energy crisis of the 1970s which generated fuel efficiency concerns relating to all vehicles, and especially cars. Since then, development has been slow however, and passenger vehicles have generally grown in size, especially in the frontal area, and whilst aerodynamic performance is impacted equally by both size and form factor, a small advancement in form does not compensate for the growth in car size.

More recently, concerns regarding CO2 emissions and global warming have regenerated interest in aerodynamic performance and resistance lowering. There are, however, conflicting issues to consider such that the final vehicle design will be a compromise between environmental concerns and driver desires. Typically the compromise represents an attempt to find a good balance between vehicle safety, performance, environmental issues, maneuverability, economy and costs, etc. Figures 14.3–14.5 illustrate the impact of recent developments in aerodynamic coefficient, frontal area and the combination of both. The trend lines are based on publicly available aerodynamic data and are intended to be indicative.

14.2 Overview of vehicle aerodynamics

The drag experienced by a body in motion depends on its shape, its size and the medium through which it is flowing. The aerodynamic drag acting

14.3 Development of aerodynamic resistance coefficient, based on publicly available values.

14.4 Development of frontal area.

14.5 Development of aerodynamic resistance as combination of frontal area and aerodynamic coefficient.

on a vehicle is a result of the relative motion between the vehicles surface and the surrounding air. The front of the vehicle pushes the air aside resulting in a pressure increase. At the rear, the air molecules cannot flow together smoothly causing a drop in pressure. The sum of these pressure differences constitutes form drag. Airflow around a moving vehicle is asymmetric with respect to its longitudinal axis due to both wind and oncoming traffic. The driving speed and the speed of the wind combine to produce a relative flow speed. Aerodynamic drag is the component of the force acting in the direction of the relative velocity. It is the sum of friction drag, form drag and induced drag and is denoted by the coefficient *C*x in the following:

$$Fa = \frac{1}{2} \rho C{x} A v{r}^{2}$$ [14.1]

where r = air density, C^x = air resistance factor, A = frontal area and *n*r = relative air speed.

Friction drag results from tangential forces affecting on the surface of the vehicle. Induced drag is the portion of aerodynamic drag which is linked with the generation of lift. With road vehicles aerodynamic lift is an unwanted effect and is therefore usually minimized. Sometimes however, it is used to improve vehicle dynamics by its effect on tire forces. With heavy vehicles induced drag can also be linked to side winds which, at large areas cause forces in the transverse direction. Form drag is mainly due to the wake, which generates a relatively constant, low pressure and can be characterized by the wake pressure and area. When considering aerodynamic drag it is important to remember the influence that vehicle aerodynamics has in other areas such as anti fouling, prevention of splash and spray formation, temperature control and noise.

14.3 Rolling resistance in vehicles

The most usual way to represent energy losses relating to the tires is through the rolling resistance force which acts on the wheel axis. For a tire rolling on a flat surface without deformation there would be no resistance. Automotive tires are continuously deforming whilst rotating and the road surface also flexes during contact. To produce this deformation energy is needed and this energy cannot be completely recovered at the end of contact due to the damping of the materials. The rolling resistance is the product of the deformation processes which occur in the tire structure, contact patch and the road surface.

The rolling resistance of a vehicle on the road is larger than that measured on a smooth drum or belt. Vehicle vibration due to an uneven road surface leads to energy dissipation in the suspension. Dynamic vertical deflection causes frictional losses during contact due to micro slip. The effective radius of the tire changes, and whilst driving at constant speed, an angular acceleration occurs at the wheel. The distribution of the contact pressure with respect of centre of the contact zone is unsymmetrical under rolling conditions, and results in a torque at the rotation axis which constitutes the main component of rolling resistance. Further contributions derive from friction in the wheel hub, and tire and rim aerodynamics can also contribute.

Resistance depends on tire construction and on the operating conditions. In practice, it is difficult to analyse the significant parameters in detail as they are strongly correlated, however the amount of heat produced is indicative of total resistance. As heat dissipation is normally lower than heat production in a tire, temperature increases which will affect the hysteresis properties of the tire materials, and also therefore the rolling resistance.

The main factors influencing rolling resistance are as follows:

∑ Rolling speed has a small positive effect on rolling resistance. This is mainly due to inertial forces which at higher speeds cause vibrational energy to modify the hysteresis of the material and thus increase temperature. This tendency is more evident with passenger car tires than with the stiffer tires of trucks.
∑ Vertical load can have a small positive effect on rolling resistance due to higher deformations.

Experiments have shown that rolling resistance decreases with increasing inflation pressure which is due to a reduction in tire deformation. Inflation pressure itself increases with increasing temperature so that a comparison of results should always be made at a fixed temperature.

The temperature of the environment, including that of the road surface, affects heat transfer and thus also the rolling resistance. Also, road surface irregularities on a scale smaller than that of the contact area have an effect on the rolling resistance, although the mechanisms for this are not yet fully understood. It is likely due in part to the hysteresis of the material in the contact region adjusting to the irregularities. Whilst the effect of road deformation is likely to be small in built up areas, it can be significant in soft soil or deep snow. On wet or snowy roads rolling resistance increases due to the cooling effect of the water, a corresponding change in deformation behaviour, and also due to the increased energy required for the displacement of the water or snow.

Alignment of the axles in the form of toe-in and camber angles on the road can contribute significantly to rolling losses. If the longitudinal axis differs from the direction of driving, an effective side force develops. The vertical axis has an effect on the load distribution in the contact region and thus affects the hysteresis behaviour at the contact region and side walls.

Carcass and belt design affect the flexibility thus also the stresses between plies. The number of plies of the carcass also has a significant effect on losses. Cord wires display a dual influence; firstly, they absorb some deformation energy via hysteresis and secondly, they control the deformation amplitude. The effect of the cord wire dimensions is in fact negligible as instead they affect the behavior of the rubber lies between cords.

Tread design affects the hysteresis behavior of the tire. Micro level tread design components, called sipes, have an important impact on the behaviour of the tread, and it is claimed that the use of double compound layers can improve the resistance behavior without overly compromising other important factors. Tire geometry is another factor which significantly impacts the resistance value. An optimum value for rolling resistance can be obtained by optimizing the ratio between the width of the tire and that of the rim. This affects the behavior of the sidewall during deformation. It is also the case that a larger tire has a lower rolling resistance which is explained by the fact that the radius of the tire is correlated to the length of the contact region and thus to the nominal load of the tire. Thus a larger tire has a higher stiffness and lower deformation with the same vertical load. Another factor influencing stiffness, and thus rolling resistance, is the ratio between the height of the tire and its width. Further, a single wide base tire has a lower resistance than a pair of conventional tires with the same total load rating. This is due to the fact that there are only two walls in the former case and thus a lower amount of material participating in deformation, leading to reduced hysteresis.

The main materials employed for tire construction are cord materials, rubbers, and composites. When used for the carcass, cord material has some influence on rolling resistance although the effect is somewhat higher when used for the belt. Rubbers have a major effect on rolling losses due to their different hysteresis properties.

Tires on different axles make different contributions to the overall fuel economy. Changing trailer tires to low resistance types produces a larger effect than changing tractor tires.

How well tires are maintained also has a critical effect on their rolling resistance. Proper tire pressure is especially important in controlling rolling resistance. Tire misalignment and misbalancing are other factors that are claimed to increase vehicle energy consumption due to rolling resistance. Tire traction must also be considered as it too is controlled by the tread region and side walls. This results in a conflict between economy and safety such that a compromise must be sought.

14.4 Advanced vehicle design for drag reduction

14.4.1 Passenger car

In the case of passenger cars, aerodynamic development begins with the selection of the basic form. Schütz and Hünergarth (2011) define an aerodynamically favorable vehicle geometry as one which shapes the vehicle's contours in such a way as to both minimize resistance to airflow, and optimize aerodynamic lift in order to satisfy dynamic road-behavior requirements.

According to Hucho (1998) the front end of a car can be roughly approximated as a square block. A stagnation point is formed on the vertical front face and because of the close proximity of the undercarriage to the road, air tends to flow over and around a vehicle rather than under it. The flow is significantly deflected at the intersection of the front face and the hood and fenders. Thus essential parameters for aerodynamic performance are the slope of the hood, the slope of the front face, and the radii of the transition. The front end should be designed in such a way that the airflow remains in contact with the body sides as far as the wheel arch cut-outs and return back to contact at the beginning of door area.

Hugho (1998) states that the size of the separation at the base of the windshield is determined by the inclination angle of the latter. The larger the windshield angle becomes, the less air is forced outwards to the A-pillar, decreasing the vortices generated there. The drag can be reduced by arching the roof in the longitudinal direction although it should be noted that the form of the roof can change the frontal area and thus increase drag, even where the drag coefficient is decreasing.

The rear of the vehicle also has an important role in the formation of drag. Typically three basic forms are considered: square back, fastback and notchback. Today, space and safety requirements specify that the ideal forms are cut, and therefore the flow separates. Hugho (1998) indicates two types of separation: quasi-two-dimensional and three-dimensional. Depending on the form of the rear these two types may interact. For quasi-two-dimensional separation the governing parameter is boat tailing, which is determined by the angles of the roof, sides and undercarriage whereas the formation of three-dimensional separation is determined by the angle of the rear end. Boat tailing has two aims. The first is to minimize the static pressure at the end of vehicle body, and the second to keep the area as small as possible.

The formation of vortices at the fastback formed upper rear end of a vehicle is determined by the angle of inclination. Hucho (1998) indicates that the critical angle is about 50°. In addition to the angle, the length of the rear slope also has an effect on drag. The main parameters describing the geometry of a notchback are: the height of the notch, its length, and the angle of inclination of the rear window. For each of these parameters a saturation effect can be identified.

Recently attention has also been given to the design of the under carriage, the aim of which is to keep it as smooth as possible. In order to eliminate local vortices, small spoilers can be used in front of the components which are causing them – one example of which is the wheel bay. The contribution of the wheels to the drag coefficient is very high for a number of reasons. Tire rotation causes flow turbulence and the wheel area cannot easily be streamlined. The flow underneath the car spreads outwards influencing the angle of yaw for the flow contacting the tire. This flow contributes to the tire and wheel temperatures.

With a proper design of the front under body spoiler, according to Hucho (1998), three positive effects can be achieved: drag reduction, lift reduction on the front axle, and increased volumetric flow of the cooling air. The drag reduction effect of the front spoiler is based on the fact that it reduces the air speed under a vehicle, thus attenuating the contribution of the undercarriage airflow.

A rear spoiler also has three effects: drag reduction, rear axle lift reduction and reduction of dirt and snow accumulation at the rear surfaces. The effect of the rear spoiler is fundamentally different from that of a front spoiler, however. The former causes the pressure at the rear to increase whilst the latter maintains a constant pressure at the front, thus reducing the pressure difference.

Attachments such as mirrors or antennas also have an effect on drag formation. Typically their drag coefficient is high, but due to the small frontal areas involved, the total effect is relatively small. Recently attention has also been given to adjustable aerodynamic components such as closure of the airflow to the cooling radiator and spoiler design.

14.4.2 Heavy vehicles

The optimization of aerodynamic drag represents an important opportunity for a reduction in fuel consumption for heavy commercial vehicles. Regulation relating to vehicle dimensions and the logistic needs for maximum available space for cargo set limitations on the aerodynamic design of heavy trucks. It is interesting to note that the shape of a towing tractor in Europe and in America differ markedly as a result of the different regulations for dimensions and weights. Maintenance and maneuverability must also be considered in the design of heavy vehicles.

Vehicle length is especially important for trucks and for vehicle combinations in the presence of side winds, and the drag coefficient measured at zero airflow angles is not sufficient to define the aerodynamic characteristics of such vehicles as the drag coefficient increases significantly with increasing yaw angle.

Cabin, chassis, cargo and the presence of a trailer where relevant all contribute to the overall aerodynamic performance. These are often manufactured separately and require connection, for example, between the truck and the trailer. Cross flow of air between these two components of the vehicle increases drag through an increase in the yaw angle, although devices exist which attempt to control this. One solution, which considers the need for articulation, are cab extenders which effectively reduce the size of the gap but do not eliminate it entirely.

The size of the leading edge radii also has a substantial influence on drag. According to Hucho (1998), a radius of approximately 150 mm is sufficient to reduce drag. Loading requirements and the logistical standardization of container size and shape, however, make improvement of aerodynamic performance in this area difficult to implement.

In the interests of maneuverability, the undercarriage of a heavy vehicle often requires a large clearance between the chassis and the ground. In order to keep the aerodynamic drag as low as possible, this clearance should also be as smooth as possible. This can be achieved for example by protective covers around wheels, the undercarriage, and/or a front air splitter incorporated into the front bumper to pass air around the truck. Shields covering front and side areas may also be employed and dual ride height systems which lower the height at higher speeds further reduce drag.

Robinson et al. (2010) have analysed the aerodynamic frontal structure of trucks estimating the potential impact of nose cone frontal profiles on aerodynamic drag, fuel consumption and emissions. Whilst the nose cones had the desired effect in that they significantly reduced the aerodynamic drag of the tractor unit, the importance of considering the vehicle in combination with the trailer was highlighted by the fact that the overall drag of the entire vehicle was actually higher with a longer nose cone than on the standard vehicle.

The boat tail approach is an alternative solution to the issue of aerodynamic performance. Hirz et al. (2011) demonstrated that with an effective boat tile form, a reduction in fuel consumption of the order of 10% is possible. One means of achieving this is to use flaps attached along the edges of the trailer rear end and inclined inwards to decrease the size of the downstream wake.

Killström (2010) illustrated the use of trailing edge air blowing to achieve the performance associated with a long boat tail without significantly increasing the length of the vehicle, although the use of pressurized air for this purpose carries a financial overhead.

14.5 Advanced tire design and materials

14.5.1 Tire structure

The difference between radial ply and bias ply construction are the angle of body plies and the presence of belts. For a truck, a unisteel radial tire incorporates a single radial ply and a multiple belt system, whilst a bias ply tire has six to eight diagonally oriented plies and no belt system. The advantage of the radial tire is a relatively low internal friction which helps to minimize operating temperatures and rolling resistance.

One means of regulating temperature is the introduction of the two-layer side wall, which consists of an outer and inner side wall to minimize heat generation.

14.5.2 Contact area

As the tread becomes worn, tire rolling resistance decreases as the, main function of the tread is traction. Thicker and deeper treads generally have better traction on wet, snowy, or otherwise difficult road surfaces although a large amount of hysteretic material in the tread can also be a primary source of rolling resistance. According to the Transportation Research Board (2006), compared with an equivalent tire with a thicker tread, a tire with a thinner tread will yield fuel savings for a limited period. These savings will occur only until the thicker-treaded tire is worn down to the depth of the thinner-treaded tire. The thinner-treaded tire will wear out sooner, however, leading both to an increase in requirement for scrap tire disposal and earlier replacement for the motorist. Various tread-based technologies are under development with the goal of reducing rolling resistance without significantly affecting wear resistance. This can be achieved in practice through changes in tread depth, width, shoulder profile, and section width. The practical effects of these technologies on tread wear and other tire performance characteristics have not been established quantitatively; however, continuing advances in tire technology hold great promise.

14.5.3 Materials

Reducing hysteresis by selection of a suitable transition temperature with the best polymer filler interaction, is another key to reduce rolling resistance. Most passenger tread compounds are built with styrene butadiene rubber (SBR) and butadiene rubber (BR). According to Nair et al. (2008), many innovations in this area have been implemented due to rolling resistance and traction requirements, particularly through a change in the polymerization method from emulsion to solution, modifications to micro and macro structure of the polymer, star branching, and chain end modifications. These new technologies provide good control over styrene and vinyl content which impact both rolling resistance and traction.

Carbon black developments are also instrumental in meeting future requirements for natural rubber-based truck and bus treads. The aim is to try to improve the balance between abrasion resistance and hysteresis. A reduction in carbon black content would improve rolling resistance significantly, but at the same time would have a negative effect on the grip and wear rate of the tire due to compound damping. For good rolling resistance the damping should be low, but for good wet grip high damping is required. High performance carbon black materials can address this dual requirement.

Basak et al. (2011) has proposed that greater attention be paid to the use of precipitated silica as a reinforcing filler for tire tread formulations in order to produce high-performance tires which show improved rolling resistance and wet grip behavior. A key to the successful use of silica in automobile tire tread stock is the use of a silane coupling agent to improve bonding of the silica to the rubber, resulting in reduced rolling resistance. Solution SBR (S-SBR) is also widely used in tire treads to improve rolling resistance.

Truck tires are comprised primarily of natural rubber (NR) because of its exceptional resistance to heat build-up, and low cut and chip properties, it runs cooler and has lower rolling resistance, which translates to reduced fuel consumption. Furthermore, a decrease in running temperature yields better tire aging resistance. Truck tire treads also employ carbon black for maximum reinforcement and excellent resistance to abrasion. The replacement of carbon black by silica in truck applications has been complicated by the incomplete coupling of the silica to the polymer chain ends of the NR, increasing rolling resistance and decreasing abrasion resistance.

According to Nair et al. (2008), an important development has been the wide spread use of neodymium catalysed BR (Nd-BR), which gives excellent hysteresis properties which directly translate into lower rolling resistance. Also surface modified nanostructured blacks have shown good potential as fillers for reducing rolling resistance.

14.5.4 Pressure control

Tire pressure monitoring is another important consideration in keeping a vehicle as fuel efficient as possible by minimizing rolling resistance. The results of Jonsson (2007) have shown that a reduction in tire pressure at a level 15% lower than recommended can cause a 2.7% increase in fuel consumption.

Rodgers et al. (2006) states the modern radial tire was in many respects made possible through the introduction and use of halo butyl rubber inner liners. Use of this polymer in the inner liner compound enabled improvements in air retention performance, improvements in liner-to-tire casing adhesion and improvements in tire durability. Recently nano composites have shown potential for further development of the inner liner. They are typically based on isobutylene elastomers and clay, whose particles display large aspect ratios. A decrease in the natural rubber content of an inner liner reduces permeability and improves tire pressure retention.

In the monitoring of tire pressure two basic principles are used: direct pressure measurement and pressure evaluation via effective tire radius measurements. With the direct measurement, the effect of temperature also has to be considered. Another significant problem with this measurement is the weight of the sensor and the need for data transmission from a rotating tire to the chassis. When a sensor is rotating a fixed distance from the axle it is affected by the inertial force, which in the case of high speeds can be very high.

With the indirect measurement however, accuracy is still low due to disturbances from tire slip, inertial forces and also wear of the tire which effects the radius of tire rotation.

14.6 Conclusions and future trends

Aerodynamics and rolling resistance have an important role to play in reducing fuel consumption and improving the environmental impact of future vehicle designs. Increasing comfort and safety requirements are, however, driving a trend towards increasing mass. The latter is most apparent when considering the use of batteries or gaseous fuels, where fuel storage tends to weigh more.

Nonetheless, bearing in mind the enormous number of vehicles that are in use, even small improvements in performance can have a significant overall impact in respect of the environment.

14.7 Sources of further information and advice

McCallen R, Browand F and Ross J (eds), The Aerodynamics of Heavy Vehicles: Trucks, Buses, and Trains, Lecture Notes in Applied and Computational Mechanics Vol. 19, Springer-Verlag, Heidelberg, 2004.
Drugge L and, Juhlin M 'Aerodynamic loads on buses due to crosswind gusts – extended analysis', Vehicle System Dynamics 48(1), 2010, 287–297.
Juhlin M, 'Assessment of crosswind performance of buses', Doctoral thesis in Vehicle Dynamics, Royal Institute of Technology, TRITA-AVE, 2009.
Juhlin M, 'A study on crosswind sensitivity of buses; Licentiate thesis, Royal Institute of Technology, 2005.
Popov A A, Cole D J, Winkler C B and Crolla D 'Laboratory measurement of rolling resistance in truck tyres under dynamic vertical load', The European Tyre School.
Genta G and Morello L The Automotive Chassis, Volume 2: System Design, Springer Science+Business Media B.V., 2009.
Automotive Handbook; Robert Bosch GmbH, 2007.
Sainio P, Killström K and Juhala M 'Aerodynamic possibilities for heavy road vehicles – virtual boat tail', HVTT11 Congress, Australia, 2010.
Cooper K R and Leuschen J 'Model and full-scale wind tunnel tests of second-generation aerodynamic fuel saving devices for tractor-trailers', SAE 05CV-64, 2005.
Salari K and Ortega J (2010) 'Aerodynamic design criteria for Class 8 heavy vehicle trailer base devices to attain optimum performance', Lawrence Livermore National Laboratory LLNL-TR-464265.

14.8 References

Bhowmick A K, Bhattacharya M, Mitra S, Dinesh Kumar K, Maji P K, Choudhury A, George J J and Basak G C (2011), Morphology – property relationship in rubber-based nanocomposites', Advanced Polymer Science 239, 1–84.
Hirz M, Hirgchberg W and Stadler S (2011) 'Environmental impact of aerodynamic optimizations at heavy duty commercial vehicles', EAEC Congress Proceedings.
Hucho W H (1998) Aerodynamics of Road Vehicles, 4th edn, SAE.
Jonsson P (2007) 'Effect of tire pressure on fuel consumption methodology and results', Swedish National Road and Transport Research Institute, VTI Rapport 564 (in Swedish).
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Mechanical and electrical flywheel hybrid technology to store energy in vehicles

K. R. Pullen and A. Dhand, City University London, UK

DOI: 10.1533/9780857097422.2.476

Abstract: This chapter deals with flywheels and their applications as energy storage devices in automotive powertrains. A brief introduction about flywheels is given which is followed by the historical development of flywheel usage in automobiles. The important characteristics of the flywheel are discussed including the safety aspect. This is followed by an explanation of the types of transmission needed to connect the flywheel to the vehicle's driveline. Various types of utilisation strategies of the flywheel in the vehicle are discussed using specific examples. Finally the current challenges to flywheel technology, conclusions and future trends are presented.

Key words: flywheel, transmission, properties of flywheel, performance of flywheel hybrid vehicles.

15.1 Introduction

All vehicles act as energy storage systems by virtue of stored kinetic energy which increases with speed. It is therefore not surprising that flywheels have been considered as suitable energy storage devices for vehicles from an earlier time (Genta, 1985). Kinetic energy is stored in a similar way to the vehicle and, if energy is toggled between the two, the power needed for acceleration can be eliminated in theory, creating the effect of a zero inertia vehicle. Similarly, energy normally lost in braking can be captured and regenerated back into the vehicle. Given that the form of energy in the flywheel is mechanical kinetic, the same as that of the moving vehicle, the energy can be transmitted from one to the other and not transformed, maximising potential for high efficiency. By comparison, any electrical means of storing energy will fundamentally require transformation of the form of energy, this needs to be done twice in a round trip from vehicle to storage and back again for energy regeneration. If the electrical storage is an electrochemical battery, denoted a battery, then the two chemical transformations must then also be added. This is not a judgement against the use of electrical systems per se since it can sometimes make good sense to use electrical means to transmit power to and from a flywheel. However, good round trip efficiency of flywheels and transmission is often cited as a substantial benefit as compared to low efficiencies in the order of 30–40% when regeneration is done via electric motors, power electronics and batteries (Pullen and Ellis, 2008). The real challenge, however, is that as the vehicle speeds up, the flywheel must slow down such that the sum of squares of the vehicle and flywheel speeds is constant in the ideal system. There is no simple, non-friction drive gearbox able to create this relationship known to the authors so it must be done in different ways as described later in the chapter.

The notion of flywheels may conjure up images of large cast iron wheels on steam engines or the device fitted to internal combustion engines even in the minds of engineers. The other immediate issue that springs to mind is that of safety, the spinning flywheel being a like a bomb that is inherently unsafe. It is these kinds of notions that have perhaps dissuaded low carbon vehicle powertrain developers from giving this technology more serious attention, an attitude that has recently changed.

A simple calculation can show the amount of energy typically needed to be stored; this number being at the heart of the flywheel size and safety issue. The stored kinetic energy *E*Kin in any moving object is given by:

$$E_{\rm kin} = 1/2 \ MV^2 \tag{15.1}$$

where M is mass in kg and V velocity in m/s.

The kinetic energy stored in a one tonne vehicle at 70 mph (*V*veh = 31.3 m/s) is 489 kJ. If the flywheel is assumed to be a thin-walled hollow cylinder of steel with a feasible mean speed of the steel of 300 m/s, the mass of the flywheel needed to store the same energy as this vehicle is only 10.9 kg. This approximate calculation assumes a stress of 720 MPa and the dimensions of the cylinder for a 10 mm wall thickness of 0.2m diameter ¥ 0.2m length. This can further be illustrated by the analogy of moving a liquid volume, analogous with an amount of energy, between two tanks (Fig. 15.1). The long tank is the vehicle, the short one the flywheel, denoted as a surge power unit (SPU). The length of the tanks represents the mass in the energy equation and the radius represents the velocity. The energy is proportional to velocity squared hence the radial section tank. Kinetic energy flows to and fro between the vehicle and the SPU (grey tint in Fig. 15.1).

Taking into account casings and bearings, for the simple steel cylinder example, allowance for the shaft, bearings and casings would mean a flywheel package of less than 30 kg. The high speed flywheel even of this type can therefore store the energy of the vehicle in a unit of the order to 3% of the mass of the vehicle. Of course, better, lighter designs than steel cylinders are available.

There has been a notion that only one means of energy storage should be used on a vehicle and there have been attempts to see whether a flywheel or number of flywheels could be used as an alternative to a battery. In order to propel the vehicle for several miles, the amount of energy needed would be far higher than the amount calculated for regenerative energy recovery.

15.1 Flow of liquid between tanks energy analogy (Pullen and Ellis, 2008). The square law enables only a few tens of kilos of rotor to store most of the kinetic energy from the host vehicle.

The result would be a flywheel or flywheels of too substantial weight since the energy density of flywheels is several times lower than a battery. Of additional concern is the safety issue, which is discussed later but if the amount of kinetic energy stored is several times that of the vehicle, it is of far greater concern. Added to this is the issue of run down loss with flywheel charge for economic designs running down in the order of tens of minutes or hours not weeks and months. The notion of using flywheels with energy storage capacity much greater than the kinetic energy stored in a vehicle should thus be dismissed.

However, there is a very good case for using flywheels as a complement to batteries to overcome the shortcomings of the latter. These shortcomings include low regeneration efficiency, reduction in life and thermal issues when charging or discharging at high power levels. A flywheel can protect a battery from damaging surge demand particularly when the battery is either cold or hot. This is counterintuitive when the choice that is considered more obvious is the ultracapacitor, keeping everything electric rather than mixing mechanical with electric. However, the vehicle is a mechanical energy store so using a flywheel with a battery can make good sense. Ultracapacitors have lower energy density, and require dedicated power electronics to match a variable voltage to the approximate constant battery voltage. At present, they present an expensive solution.

The safety of flywheels is something often cited as a major concern and reason for dismissing this technology. The surface speeds may be cause for alarm approaching sonic and often above sonic for some designs. This has to be compared with speeds in the order of 300–500 m/s found in automotive turbochargers and much higher speeds found in aviation jet engines. In the turbocharger example, the rotor weight is less of course and for aviation jet engines, rotors are inspected and maintained carefully. However, in the latter, turbine discs cannot be contained in the event of a rotor failure due to the need to make the casings very light and thin. For the automotive flywheel, weight of the casing is an issue but having a containment system of around the same weight as the rotor is not a problem since the mass is still relatively small in comparison with the vehicle. Indeed, there are ways to ensure that the rotor fails in a more benign manner by minimising the size of rotor fragments in case of a full or partial rotor disintegration. Given this, the containment issue associated with these high speed rotors is not one simply to be dismissed but is also not an intractable problem. It has and can be solved by engineering solutions.

A further benefit of flywheel storage is the potential for low cost, what is often framed 'benefit for bucks'. Once production has exceeded the order of 100,000 units, the cost of most artefacts tends towards base materials cost plus the cost of any special processing for the materials, particularly if energy intensive. Much, if not, all the flywheel and transmission can be made of steel, plus it is fundamentally a simple technology. A material often used for the flywheel is carbon fibre composite, which is relatively expensive, but the amounts required are small and the shapes simple. This contrasts with variable speed and voltage electrical devices needed for electrical energy regeneration, which are expensive for high power levels. The power that can be stored or extracted from the flywheel is only limited by the transmission, if the mechanical type; hence the energy storage capacity and power can be chosen independently, something not true of most other storage means.

All of these fundamental benefits have led to recent resurgence of interest in flywheel energy storage for vehicles. However, getting it to work successfully is not as easy as might first be thought. It has taken a number of innovations both in flywheel design and in transmissions to get to a point where commercial exploitation of flywheel based vehicle powertrains is becoming reality. A brief history is now given as background to the more current developments described in the later sections of this chapter.

15.2 The development of flywheel technology

To date, the best historical account of flywheel technology inclusive of automotive applications can be found in Genta (1985). Probably one of the earliest applications of flywheels in vehicles worth mentioning is the Oerlikon Gyrobus which operated in the 1950s in several cities of Europe and Africa before it was phased out due to several technical challenges (Anon., 1955). In England in the 1960s Clerk presented designs of transmissions to couple energy storage flywheels to commercial and passenger vehicles (Clerk, 1964). These were relatively complex. In 1969 Rabenhorst presented the concept of the so called superflywheel, which used high uniaxial tensile strength material and could store energy of 30 Wh/lb (14 Wh/kg) which was deemed significant at that time (Rabenhorst, 1969).

In the 1970s the oil crises as well as other factors triggered a huge interest in flywheels with the US government funding numerous projects to evaluate flywheel-based vehicle designs. Unfortunately, interest in flywheel energy technology fell as oil prices stabilised towards the end of the 1970s and the research into flywheels reduced as programmes petered out by the mid 1980s. There was a revival in the 1990s with stricter emissions legislation coming into force worldwide. Some notable examples were electromechanical batteries developed by Lawrence Livermore Laboratory in the US (Post, 1996) and the University Eindhoven in the Netherlands (Thoolen, 1993). With the advent of production hybrid vehicles in the late 1990s, the interest in electric and hybrid vehicles became stronger. Since then, interest in vehicle flywheels has strengthened with the development of high speed motor generators made possible with advent of high frequency power electronics and high strength permanent magnets.

A major step for the flywheels was the declaration by the Federation Internationale L'Automobile (FIA) about the intention to employ recover and reuse the kinetic energy in Formula 1 (F1) race cars in 2006. The first season in which the F1 race cars implemented kinetic energy recovery system (KERS) was 2009. Some of the KERSs developed were flywheel based systems but, with relatively restricted power and energy limits coupled with packaging issues, the rules favoured battery based systems. Given large budgets, expensive batteries could be used and expended without concern, something not realistic for energy recovery systems for road vehicles. For reasons of lower cost and higher efficiency potential, a number of OEMs have started getting involved in developing flywheels for vehicular applications and the authors believe that flywheel-based powertrains might be seen in production vehicles in the near future.

15.3 Types and properties of flywheels

A flywheel is inherently a very simple device, essentially a disc or drum supported on bearings with an output shaft. An electrical motor-generator (M/G) may be integrated into the flywheel for electrical transmission but even here the complexity is little more than that of a standard electrical machine. However, there are some differences associated with the need for relatively high peripheral speeds that create particular engineering challenges. These take practical vehicle energy storage flywheels beyond routine design methods and are listed and treated one by one. These include:

∑ rotor design and materials for maximum ratio of stored energy to weight
∑ aerodynamic loss and vacuum system
∑ low loss bearings
∑ containment in the event of rotor failure
∑ gyroscopic moment potentially affecting vehicle dynamics.

15.3.1 Rotor design and materials for maximum ratio of stored energy to weight

The purpose of this section is to give a simple treatise on rotor design with rudimentary analysis and also discuss the issues with all metallic and composite designs. Clearly, the lower the mass of the flywheel rotor the better since any added weight to the vehicle increases the power needed for acceleration and rolling resistance loss. The higher the speed that the flywheel can operate for a given material density, the more energy will be stored and the lighter will be its weight for a given energy requirement. Maximum speed is governed by the maximum design stress in the rotor and this stress s is given by:

$$\sigma = k\rho V^2 \tag{15.2}$$

where r is the material density, V is the peripheral speed and k is a constant depending on the rotor shape. For a thin-walled cylinder, the value of k is unity and for other shapes k can be calculated. For a solid cylinder without a central hole, the value of k is 0.413 and can be determined by consideration of elasticity theory known as the Lamé formulae (Benham and Warnock, 1976). For a thick-walled cylinder with the radius of the central hole half of the outer radius, k is 0.869 and with the smallest central hole, k = 0.825 which is double that without the hole. The lowest values of k can be obtained for tapered discs as typically found in high speed axial turbomachines. Designs of this type are discussed later.

Given knowledge of material maximum allowable stresses and densities, calculations can be done to establish the relative merits of different materials. Table 15.1 presents a list of candidate materials that could be considered for flywheels for two shapes, a thin cylinder and a solid cylinder without central hole. The latter is not possible for composite materials since in filament form the have high strength in one direction only. The values of design strength include account of reductions that are necessary for practical designs in the experience of the authors. For metallic materials, there must be allowance for adequate fatigue life and methods for this are well understood. Composites are more resistant to fatigue but this must also be considered. There can be considerable variation in properties obtained and the strength of the composite is often controlled not by the fibre strength but the quality of the

Material and type	Density (kg/m3 )	Design stress (MPa)	Peripheral velocity (m/s)	Rotor mass (kg)
Aluminium cylinder T7075	2,800	350	354	7.8
Titanium cylinder	4,430	520	374	7.0
Steel cylinder	7,800	720	304	10.6
Aluminium solid T7075	2,800	350	550	6.5
Titanium solid	4,430	520	582	5.8
Steel solid	7,800	720	473	8.7
Glass epoxy cylinder	2,150	1,100	715	1.9
Carbon-epoxy cylinder	1,670	1,500	938	1.1

Table 15.1 Comparison of different materials and two basic flywheel shapes

matrix material. Also given in Table 15.1 is the weight of the rotor and its peripheral speed that would be required in order to store the figure of 489 kJ, the approximate energy requirement derived in Section 15.1.

What is immediately apparent is that most metals compared with each other have similar performance yet the composites, even with conservative strengths for reasons explained, are considerably lighter. However, a composite flywheel (a cylinder) must have a metallic shaft and discs which must be connected to the composite. Given that composites typically have a lower Young's modulus than metals when highly stressed, the radial displacement is high and it can be difficult to attach the composite part to the metal shaft via metal discs. Composite discs are more difficult to make due to the orthotropic properties of composites so this material is usually wound into a cylinder. For these reasons, most composite rotors typically include a considerable mass of metallic rotor parts and the ratio between weight of a practical composite-metal rotor and an all metallic rotor is reduced from around 1:8 in Table 15.1 to between ¼ to ½ of this depending on the design. For applications which are less cost sensitive, composites certainly hold the advantage but if cost is the major issue, steel can be highly attractive as long as the energy storage requirement is not too high.

It should also be noted that the energy stored depends on peripheral velocity and therefore a higher speed flywheel of lower diameter does not store any more energy than a larger diameter lower speed one as long as the peripheral speed, rotor shape and mass are the same. A higher speed flywheel will have a longer but lower diameter rotor like a drum with a low speed flywheel being more like a disc. The choice of speed will therefore affect the package shape not the flywheel rotor mass neglecting detail design effects. What is affected is the transmission, particularly if mechanical where a higher speed flywheel will require a greater speed reduction to get down to the order of a few thousand rpm range of gearbox input speeds or lower further down the drive train.

15.3.2 Aerodynamic loss and vacuum system

Given the high peripheral speeds of flywheels, the aerodynamic friction loss would be excessive unless the pressure inside the casing is reduced. Losses occur due to the effects of skin friction but also pumping by the discs as air near the rotor is pumped radially outwards. Not only would the loss of energy be excessive but the rotor might overheat if the rotor ran in under atmospheric conditions. Near complete removal of the air may be seen as a simple and attractive option by creating a very high vacuum but even in a hermetically sealed design with no shaft seal, preventing leakage can still present a major challenge and many of the materials particularly composite matrix materials, generate gas due to outgassing. Another issue is the lubricant for the bearings which will vaporise if the pressure is too low unless the bearings are kept outside the vacuum environment. If a shaft seal is required for mechanical shaft output then sealing a very high vacuum is a challenge. One solution to the leakage issue is to have vacuum pump on board the vehicle to run continuously or intermittently albeit with additional cost and weight.

A better starting point is to evaluate what level of windage loss can be tolerated without excessive loss of energy relative to the energy savings or other benefits the flywheel can provide. If the pressure is not too low then the losses can be calculated based on equations for continuum flow. There are numerous publications on this subject and the most comprehensive is Dailey and Nece (1960) which covers windage in enclosed chambers. If a very high vacuum is required then continuum flow gives way to molecular flow and a different set of equations are needed (Genta, 1985). The transition from continuum to molecular flow is gradual so this regime is denoted Knudsen flow. Windage losses are proportional to approximately the 5th power of diameter and the cube of the speed. However, these losses are proportional to pressure to the power of 3 /4. A higher peripheral speed composite flywheel will hence need to operate at a higher vacuum than a metallic one for the same energy storage.

15.3.3 Low-loss bearings

The choice of bearings for automotive flywheels is relatively limited to the rolling element type given the need for low loss, low cost and high capacity. Although the weight of the rotor and out of balance loads are relatively small, the bearings must support loads due to acceleration shocks and those imposed due to gyroscopic effects. A magnetic bearing might be highly suitable for a stationary application but would have to be substantially oversized to cope with these additional loads. One solution is to put the flywheel in a gimbal such that the flywheel and casing is free to precess without imparting gyroscopic moments on the rotor and bearings. As well as the added complexity and space with this solution there is an added problem of containing the flywheel and casing in the event of rotor failure when momentum is imparted into the casing.

Rolling element bearings offer low loss moments and when sized to give adequate fatigue life for normal running, can accept shock or short term loads many times the steady design load. Angular contact bearings are typically chosen and, depending on the cost considerations, can be all steel or ceramic balls of different levels of precision. Correlations for losses are available from most manufacturers and contain a fixed loss moment plus a loss moment element that is dependent upon speed. It can be noted that rolling element bearing technology has moved on considerably over the last few decades in terms of increased life and lower loss moments. The choice of lubricant depends on the vacuum pressure required with more limitations occurring when the vacuum is high. For such situations, silicon-based oils are needed albeit with inferior lubricating properties.

15.3.4 Safety in the event of rotor failure

Safety is of paramount importance and the approach to this issue and solutions are explained in Hansen and O'Kain (2011). The choice can briefly be stated as:

∑ design with such a high safety factor that failure will not occur
∑ ensure failure is incremental and contains fewer fragments than the entire rotor
∑ use instrumentation to detect and prevent failure
∑ incorporate containment tailored to the flywheel design.

It is the authors' opinion that for an automotive flywheel, the only option is one which guarantees containment under all foreseeable circumstances without reliance on instrumentation or other controls. Since the need for low cost means that materials cannot be controlled or inspected nor can the life of condition of the flywheel including instrumentation be controlled. Not only must the flywheel be contained but the rate of angular deceleration of the fragments not be too great lest a very high torque is transmitted which could cause the casing itself to break free or excessive torque be applied to the vehicle. Metallic flywheels tend to break into three large pieces if a crack grows to a critical size causing burst. Here, the entire energy of the flywheel is released in three large chunks that require a heavy casing to contain it. A solid flywheel without a central hole has the added problem of a triaxial stress at the centre, which promotes fracture over yielding. A solution to this is to use a laminated sheet type structure, laminated axially such that only a small fraction of the rotor can burst at any one instant; the probability of two or more failing at the same time being negligible. A flywheel of this type is shown in Fig. 15.2.

Composite flywheels have been promoted as safer on the basis that the material tends to turn into dust upon failure hence the containment vessel experiences a pressure of rotating mass more like a liquid rather than chunks. Other types of failure mode have been seen and what occurs is complex and requires extensive research in order to understand it. One of the challenges is how to create a realistic burst condition since composite flywheels tend to have such high safety factors and bursting with massive overspeed would be unrepresentative of a likely failure condition in service.

The methodology used for vehicle crash assessment can be adopted for flywheel burst containment evaluation based on dynamic finite element analysis simulation validated against results of highly instrumented experiments. There is a need for standards to be developed along these lines to create a clear framework for the design of automotive energy storage flywheels as the technology becomes established.

15.3.5 Potential effects of gyroscopic moment on vehicle dynamics

The issue of gyroscopic torque from flywheels on a vehicle is treated thoroughly in the paper by McDonald (1980). A gyroscopic torque will

15.2 A laminated steel flywheel designed at City University.

result if the axis of the flywheel is rotated and it acts perpendicular to the rotor axis. The magnitude of the torque is the product of the flywheel rotor moment of inertia, the flywheel angular velocity and the angular velocity of the flywheel axis. The vehicle will experience rotating in the yaw axis from cornering, roll axis also when cornering (at entry and exit) and pitch when riding through a road dip or crest of a hill. Yaw angular velocity is likely to be the greatest in the case of vehicle spin and effects on the flywheel can be eliminated by making the axis vertical. Sometimes this is less convenient particularly for mechanical transmission where an in line or transverse axis is preferred. If the axis is transverse, the gyroscopic torque can be used to counter roll moment and hence enhance road holding by virtue of transverse reducing weight transfer. However, the main conclusion from McDonald (1980) was that the gyroscopic torques generated were generally insignificant in comparison to other moments on the vehicle. It is an issue that should be considered but is not of great concern. Of more concern is the ability of the bearings to take the resulting loads to support the gyroscopic torque as mentioned previously.

15.4 Transmissions for flywheels

In the case of an automobile using a flywheel as an energy storage device, a suitable means to connect the flywheel to the driveline is needed that would allow the flywheel to change its speed continuously. As mentioned in the previous sections, the flywheel exchanges its energy with the automobile by increasing or decreasing its rotational speed when the vehicle's speed is changing in the opposite direction. In other words a continuously variable transmission (CVT) is essential for the flywheel. The requirements of the CVT used in a flywheel hybrid vehicle are slightly different to that used in a conventional vehicle. The main difference is that the CVT in a flywheel hybrid vehicle should be able to transfer energy in the forward as well as the backward direction at high efficiency. Another important difference is that the ratio range, which is defined as the ratio of the maximum to the minimum speed ratio, has to be relatively large especially when using a high speed flywheel with a low depth of discharge. The other requirements of low cost, light weight and ease of control are standard. The different types of transmissions considered for a flywheel energy storage system (FESS) are described in the next section

15.4.1 Hydrostatic transmissions

The hydrostatic transmission transmits energy using hydraulic fluid. The usual transmission has two hydraulic devices; one working as a variable displacement pump and the other working as a motor connected by hydraulic lines. The pump converts mechanical energy into pressure and the motor reconverts the pressure energy to mechanical energy. By varying the displacement of the pump a continuous ratio from zero to the maximum value can be obtained thereby forming an infinitely variable transmission (IVT). As a result the hydrostatic transmission does not require any starting clutch. The torque direction is reversed by the pump acting as the motor and the motor acting as the pump. The hydrostatic transmissions are simple to construct and offer flexibility; however, they are bulky and noisy, so they tend to be used more in heavy vehicles than in passenger cars. Figure 15.3 shows a typical hydrostatic transmission (Beachley and Frank, 1979).

15.4.2 Traction continuously variable transmission

Traction CVT transfers torque between two objects through adhesive friction. The transmission ratio is varied by changing the radius of the point of action of forces. The traction drives usually have a limited ratio range. There are predominantly two types of traction drives: the belt drives and the rolling traction drives.

Belt drives

The transmission takes place over belts which are placed between two pulleys. By varying the diameter of the pulleys the ratio change is achieved continuously.

15.3 Hydrostatic transmission (Beachley and Frank, 1979).

The belts are usually made of rubber or steel. Rubber v-belt CVTs have been used for many years. Van Doorne of Holland developed a v-belt made up of steel blocks linked by steel bands which was a significant improvement over the rubber belt drives (Walton, 1959). It is a compression belt, where the driver pushes the driven pulley. Figure 15.4 shows the schematic of this type of CVT. A similar type of belt was developed by Battelle Columbus Laboratories (Swain, 1980). Another popular CVT developed by Kumm Industries is the flat rubber belt design shown in Fig. 15.5. In this case the belt is positioned radially by drive elements located by guideways in the side of the two pulleys and the ratio change is achieved by changing the radial position of the belt along the guideways (Kumm, 1980).

Rolling traction drives

There are a number of designs of this type where the transmission takes place between two elements, which are loaded against each other, and are separated by a lubricant film. The speed ratio is varied by changing the point of action of forces or the effective pitch radius. A common design is the toroidal design in which the power is transmitted from the input discs to the output discs via rollers. The ratio change is achieved by varying the inclination of the rollers. This type of transmission has been developed in the past by Perbury of England (Stubbs, 1980) and is currently developed by Torotrak (Brockbank and Greenwood, 2009). Figure 15.6 shows the Torotrak design. Another rolling traction drive is the Milner CVT (Akehurst, 2001) which uses three or more spherical planetary rollers to transfer power

15.4 V-belt CVT (Beachley and Frank, 1979).

15.5 Kumm belt design (Kumm, 1980).

1. The input disc(s): powered by the engine
2. The variator roller(s): transfer power and match disc speeds
force 3. The output disc(s): transmit power to the drive shaft
4. Ratio change: rollers 'steer' like a castor to reflect the ratio change

15.6 Torotrak design (Brockbank and Greenwood, 2009).

from an inner race set to a carrier assembly. The ratio change is achieved by changing the axial displacement between the outer race halves. Figure 15.7 shows the Milner CVT.

15.4.3 Electrical transmission

The electrical transmission is formed by using two electrical machines in series, one functioning as a motor and the other as a generator. The mechanical energy is converted into electrical energy at one end and reconverted into mechanical energy at the other. The continuously variable transmission is achieved by controlling the torque of the machines by varying the voltage or the current of the machines. The electrical machines offer the advantage

15.7 Milner CVT (Akehurst, 2001).

of flexibility due to the absence of any rigidly connected mechanical links. However they tend to be on the expensive side. In the case of flywheels, it is common to attach a motor generator to exchange energy in and out of the flywheel and this is commonly called an electromechanical battery (EMB) (Post, 1996) or flywheel battery (FWB) (Hayes et al., 1999).

15.4.4 Mechanical transmission

The simplest way of achieving a pure mechanical continuously variable transmission is to use a slipping clutch in series with a stepped gear box. The gearbox has discrete ratios and at each step the clutch can be slipped to achieve the desired ratio. If a gearbox with a large number of ratios is used the power lost due to the slipping of the clutch can be reduced. Having a large number of gears would be expensive but Read developed a methodology of using a gearbox with a few steps plus a smaller gearbox to create the effect of a large number of gears (Read, 2010). Beachley suggested such a design as one of the ways to connect the flywheel to the driveline for their flywheel hybrid (Beachley et al., 1981). More recently a digital hydraulic system has been developed by Artemis Intelligent Power Ltd (Artemis, 2012) and has been applied to flywheel transmissions.

The other way of achieving a CVT through purely mechanical means would be by using the planetary gear set (PGS) as a two degrees of freedom device. In the conventional applications it is common to hold one of the elements of the planetary gear set stationary while the other two act as an input or output. However, in case of being used as a CVT all three branches of the PGS would be free to rotate. Since the PGS is a speed coupling device, by determining the speeds of any two branches the third one can be controlled. Diego-Ayala used a simple PGS with a brake to design a flywheel based powertrain (Diego-Ayala et al., 2008).

15.4.5 Power split principle

For the power split (PS) principle the previously mentioned CVTs will be called variators or the continuously variable unit (CVU) and the complete transmission will be referred to as CVT. Usually the variators suffer from poor efficiency and the power split principle is used to overcome this effect. It is used effectively in the Toyota Prius. A general PS-CVT is shown in Fig. 15.8. As the name suggests, the input power is split between the variator and the highly efficient direct mechanical path which is added up at the output. The PS-CVT can be input coupled which means that the torque is split at the input or it can be output coupled where the speed is divided at the input.

The PS-CVT usually has a higher efficiency than a variator only CVT; however, its ratio range is limited due to the fact that the power carried by

15.8 General PS-CVT (Beachley and Frank, 1979).

15.9 Different PS-CVT configurations (Martinez-Gonzalez, 2010). GV and GM are fixed gears and V is a variator.

the variator is less than the input power. However, in case the power carried by any of the branches of the PS-CVT is higher than the input power, it is called power recirculation. Power recirculation can be positive or negative depending on whether the power going through the variator is in the direction of input power or opposite to it. The case of power recirculation provides an interesting trade-off, whereby the ratio range can be increased at the expense of efficiency. White explains the power split principle in greater detail (White, 1967). Figure 15.9 shows the different schemes of the PS-CVT principle.

A drawback of power recirculation is that the bearings carrying the branches are overloaded and have to be designed to take the high power. However, another way of achieving a higher ratio range while avoiding power recirculation is by using multi regime CVT. Such kinds of CVTs have been discussed in literature (Mantriota, 2001). The basic idea is to use the variator multiple times by the use of clutches and gears.

The PS-CVT can be constructed using different types of variators explained in previous subsections. One of the common designs used in the 1970s and early 1980s was the hydromechanical design. A number of flywheel based powertrains conceived during that period use this type of design (Frank and Beachley, 1975). Another design that was developed to be used in a flywheel vehicle and patented in 1980 was the electromechanical CVT using a planetary gearset and two electric machines (Rowlett, 1980). As well as these, a number of designs have been developed using the traction rolling and belt variators in combination with planetary gearsets.

15.5 Performance evaluation of flywheel hybrid vehicles

The FESS can be applied in an automotive powertrain in different ways depending on the powertrain type and structure as well as the performance and the vehicle requirements. Not only is the design of the powertrain but also the energy management of the system crucial to the performance.

15.5.1 Pure flywheel-based powertrain

The FESS can be the sole energy source in the powertrain such as in the Oerlikon Gyrobus (Anon., 1955). In this case the flywheel is usually charged at stationary terminals during the journey. The flywheel is generally acting as a FWB which is charged and then is used to provide power to the traction motor. Since the whole of the propulsion needs to come from the flywheel, the flywheel has to be sized large enough to carry the vehicle a reasonable distance. This type of arrangement could be suitable for intra city bus or trolley coach application. However, beyond that, this type does not find much use. Figure 15.10 shows the Gyrobus.

15.5.2 Flywheel and internal combustion engine-based powertrain

This type is particularly useful since the conventional ICE vehicle does not have any means of capturing brake energy and adding a flywheel gives that option. The first application of this type was developed in England (Clerk, 1964) for bus and car application using a complex mechanical transmission consisting of integrating and differentiating planetary gear sets. The University of Wisconsin (Frank and Beachley, 1975) developed a similar powertrain and more were developed by University of Eindhoven (Kok, 1999) and Imperial College London (Diego-Ayala et al., 2008).

Usually non electrical transmissions would be used in such a powertrain. There are various configurations and operating modes possible. The flywheel

15.10 Gyrobus (Lawson, 1978).

can be connected in a parallel configuration or in a series configuration. At least one study explores the use of a flywheel with two CVTs in a series arrangement (Loscutoff, 1976). However, the common principle is the parallel configuration. One such system from Eindhoven University is illustrated in Fig. 15.11. In this type of configuration the flywheel is used in combination with the ICE up to a vehicle speed of 100 kph, and beyond that the ICE only is used. In the low speed hybrid mode (from 10 kph to 55 kph) the ICE is run at its most efficient operating point and is used to charge the flywheel up to its operational limit. As soon as the desired flywheel speed is reached, the ICE is shut down and the flywheel is used to drive the vehicle, thus the ICE operates in an on-off mode. From 55 kph to 100 kph the ICE is always running and the flywheel assists the vehicle. Beyond that the flywheel is disconnected and ICE powers the vehicle. According to the results shown by Kok (1999), the improvement in fuel economy is mainly the result of improved engine operation, and regenerative braking has limited impact. The parasitic losses of the flywheel system need to be optimised to improve fuel economy. The improved prototype of the flywheel hybrid driveline shows fuel savings up to 35 % in urban traffic.

Another configuration is the one developed at Imperial College London. Fig. 15.12 shows the schematic. Here the flywheel with a mechanical transmission is an add-on to the conventional ICE powertrain. The primary purpose of the flywheel is to capture the regenerative braking energy which otherwise would be lost. The flywheel is charged during braking and the energy is released

15.11 Eindhoven concept (Kok, 1999).

in the subsequent acceleration. The ICE is shutdown when the flywheel is operating. According to Diego-Ayala et al. (2008), the improvement in fuel economy using such a system is more to do with the engine operating less frequently rather than the engine operating at its most efficient range. It has been found that improvements in fuel economy and reductions in emissions up to 22% and 33% for a car and bus, respectively were obtained as compared to conventional vehicles. This approach differs to the earlier mentioned one. Here the flywheel and the CVT are not intertwined with the ICE operation and operate independently, whereas in the earlier one the flywheel and the ICE operations are coupled together.

Since 2009 F1 has been employing limited KERS in races. Most KERSs are electrical in nature but the Flybrid-Torotrak system employs a mechanical flywheel based KERS. This system is shown in Fig. 15.13. Projects are underway in various organisations to make this system implementable in road vehicles (Birch, 2011).

One such application is the project headed by Ricardo plc in the UK called 'Flybus'. The application involves a hermetically sealed flywheel coupled to a Torotrak CVT. The use of a hermetically sealed flywheel provides an opportunity to avoid the vacuum pump which is usually required to reduce windage losses, though it is difficult to predict if periodic service would be required to maintain the vacuum lost due to leakage. Another innovation in this work is that the flywheel is coupled to the transmission via a unique magnetic coupling thereby avoiding a moving seal. The CVT can transfer over 60 kW in a package that is less than 10 kg in weight. The vehicle also features an electrical power take off for recharging the batteries (Fuller et al., 2011).

15.12 Imperial College concept (a) brake-only hybrid version and (b) CVT-brake hybrid version (Diego-Ayala et al., 2008).

Flybrid Automotive Ltd in the UK has developed a small high speed flywheel for various applications including F1 and road cars. The flywheel is very light and spins at maximum speed of 60,000 rpm. It is made up of steel, aluminium and a small quantity of carbon fibre (Cross and Hilton, 2008). The flywheel has been used in various projects to develop road applications. Volvo is developing one such application to use the Flybrid flywheel with a Torotrak CVT. The flywheel is also being used by Jaguar in their development of hybrid vehicles (Birch, 2011).

Williams Hybrid Power (www.williamshybridpower.com) has developed a fully integrated electromechanical flywheel. The flywheel rotor is made up entirely of composite material and has over speed protection built into

15.13 Torotrak design (Brockbank and Greenwood, 2009).

it. It has been used in F1, in the Porsche 911 GT3 R hybrid race car where it drives the rear axle and in the Audi R18 e-tron Quattro race car. It is also being developed for commercial passenger bus application in the UK (www. willamshybridpower.com).

15.5.3 Flywheel in battery electric powertrains

It is a well-known fact that the crucial element that inhibits the electric vehicle (EV) is its batteries. The limited range and cost of the electric vehicle are its limitations. These limitations can be partially offset by the combination of a high power device such as the flywheel which would 'load level' the battery, and the battery can be designed as a high energy element. The battery would provide low average power to the vehicle and all the high power events such as acceleration and deceleration would be handled by the flywheel. This would not only lead to increasing the range of the vehicle but also improving the battery life as high transients are taken over by the flywheel. The cooling requirements of the battery are also reduced. The flywheel is usually added on the system as a FWB with very few known exceptions. The FWB combines a motor generator (MG) with a flywheel in three topologies (Hayes et al., 1999).

1. Fully integrated: in this version the flywheel and the MG operate as one unit and are fully contained in a single containment to give a highly compact system. There are no mechanical connections coming out of the FWB and it has only electrical connections as the output. However, since the machine is contained in a vacuum, heating is a problem.
2. Partially integrated: in this version the rotor of the MG is contained in the containment but the stator is outside. This partially offsets the heating problem from the stator but rotor cooling has the same limitations as the fully integrated one.

Non-integrated: in this design the complete MG is outside the containment and it requires rotating seals like in a mechanical connection. It is larger in size but the machine cooling is easier to engineer.

FWBs have been developed by a number of organisations including Eindhoven University (Thoolen, 1993), University of Texas at Austin (Hayes et al., 1999) and Lawrence Livermore Laboratories (Post, 1996). Magnetic bearings are commonly used in FWBs. Figure 15.14 shows a typical FWB.

Research has been carried out for a number of years on using FWBs in EVs as a load levelling device. In recent research Lundin (2011) used a FWB with double stator windings placed between the battery and the traction motor of an EV. The high voltage side is connected to the traction motor and the low voltage to the batteries. This allows the FWB to charge or discharge both the traction motor and the batteries at two different power and voltage levels. Results show a significant decrease in partial charge/discharge cycles, maximum current and battery resistive losses with the flywheel as compared to one without the flywheel.

The flywheel can be connected mechanically to the EV driveline, which gives the advantage of low cost, simplicity and avoids energy conversion. Some limited government funded research was carried out in the 1970s, notably in the US after the 1973 oil crises sparked an interest in EVs. However the EVs at that time were greatly inferior to the conventional vehicles and even with the flywheel were not deemed competitive to ICE conventional vehicles (Agarwal, 1982). However with the current technical situation in flywheels

15.14 Flywheel battery (Hayes et al., 1999).

and with the availability of production EVs today, there is research being conducted at City University London in this field. Figure 15.15 shows the EV with flywheel using electromechanical transmission built by Garrett.

15.5.4 Flywheel in a hybrid electric powertrain

Another common configuration is a hybrid electric series configuration which is shown in Fig. 15.16. Here the ICE is the prime mover connected to a generator and the FWB is there in place of the usual electrochemical batteries. Such an arrangement has been shown in the applications of the FWB developed by University Eindhoven, UT- Austin and University of Alberta (Wang et al., 2009). Similar to the battery based series HEV powertrain, this arrangement has been primarily implemented in public transportation such as transit buses. However, there are a few exceptions. The Chrysler Patriot sports car using a gas turbine engine alternator and a FWB developed in the early 1990s is such a system (Shortlidge, 1996).

An example of the parallel hybrid powertrain was developed at ETH Zurich and called the ETH-Hybrid III vehicle (Dietrich et al., 1999). Figure 15.17 shows the schematic of such a powertrain. It consists of an ICE, an electric asynchronous machine, batteries, a flywheel and a wide range CVT. Due

15.15 Garrett electromechanical transmission (Lawson, 1978).

15.16 Series configuration (Hayes et al., 1999).

Key: AHE: ambient heat exchanger B: batteries C: clutches	F: HC:	EWP: electric water pump flywheel heat and noise capsule	JS:	junction shaft LHS: latent heat storage
CAT: catalyst CVG: continuously variable transmission	HE: HV: ICE:	exhaust heat exchangers vehicle heater internal combustion	PE: SL: TF:	power electronics silencer/airfilter throttle liap
EM: electric motor/generator		engine	W:	wheel

15.17 ETH-III powertrain (Dietrich et al., 1999).

to the presence of various elements, a large number of operating modes are possible and in essence it combines the battery load levelling function and the strategy similar to the Eindhoven University concept described before. However, it is fairly complex in construction and control.

15.5.5 Energy management

Since there are various configurations for a possible flywheel hybrid vehicle no one control strategy fits all. One of the common strategies is to keep the total kinetic energy of the flywheel and the vehicle constant. This has been applied in various cases. It is a simple strategy though not optimal and suffers from the so called history dependence problem. As an example, after a light deceleration where the flywheel has not recuperated enough energy, the control strategy would demand additional energy from the prime mover to make up for the loss ignoring the other demands of the vehicle. Gilbert suggested that an anticipatory control strategy would avoid this problem (Gilbert et al., 1972). Martinez-Gonzalez (2010) showed with an example that a 'looking ahead' strategy using predictive journey estimation has the potential to improve fuel economy in a flywheel hybrid vehicle.

15.6 Technical challenges in flywheel development

The greatest technical challenge to the implementation of flywheels in road vehicles seems to be certainty of containment in the event of a failure. As stated earlier there is no fundamental issue but rather the need to mitigate against unexpected scenarios. Exactly the same type of issue applies to more or less all electric vehicles which can catch fire as voltages and battery energy storage systems are increased. It is a case of managing change at a time when vehicle health and safety is treated very differently to the period of the advent of the motor vehicle. Engineers do have at their disposal sophisticated tools for simulation and prediction and, in comparison to a battery, a flywheel is a relatively simple device even with composite materials.

For vehicular applications, the containment has to be low cost and more importantly low weight. One method is to design the flywheel in such a way that the ultimate stress point is never reached in practice or at least the risk is extremely low. Certain developments have shown promising designs in this aspect. One example would be the rotatable liner design by UT-Austin in which a free floating, cylindrical rotatable structure was able to mitigate the flywheel burst failure mode (Strubhar et al., 2003). Another example is the system from Ricardo where failure modes can be detected by the automatic monitoring of out of balance vibration by bearing sensors, which would result in shutdown of the system before any further damage is done (Hansen and O'Kain, 2011).

15.7 Conclusions and future trends

The view that mechanical solutions will be swept away by an entirely electric future is creaking in the face of the need to find pragmatic affordable solutions. There is an urgency for manufacturers to produce low carbon vehicles whose powertrain cost is not substantially greater than the conventional ICE with mechanical transmission. Despite considerable investment and advances in battery technology worldwide, cost of this key technology is currently too high and performance inadequate on its own. Added to this is the high cost of traction drives at torque levels demanded by the customer. Flywheels can offer a low cost means of bridging the gap and making electric vehicles with good performance more affordable.

Flywheels offer a low cost and efficient solution both in energy efficiency and use of recyclable benign materials. They may be a transitional technology but are more likely to remain complementary to other prime movers for a very long time. The reason for this stems from the fundamental physics that a vehicle is a kinetic energy storage system itself and it makes sense to use the flywheel to toggle energy to and from the vehicle in the mechanical form.

Interest and development of flywheels for automotive use has increased dramatically with a number of companies offering products in limited production. Investment has been substantial and has been kick started by the funding available for motor sport applications such as KERS in F1 and Le Mans. Now that the benefit of flywheels has been demonstrated, it is clear that this technology is becoming established and implementation in mass produced vehicles in only a matter of time. Key to this step is the development of low cost solutions in the order of a few \$100s manufacturing cost and this will likely mean flywheels with less composite content, all steel or steel with partial composite.

Safety is the area which requires further development; not that there is a fundamental concern but agreed methodologies and standards need to be established to ensure that rotor failures are contained to within an acceptable probability. No other prime mover or energy storage technology is completely risk free and there is no reason why flywheels cannot meet similar safety standards.

15.8 References

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Akehurst, S. (2001) Performance investigations of a novel rolling traction CVT, Transmission and Driveline Systems Symposium, Detroit, MI, USA.

Anon. (1955) The Oerlikon Electrogyro: its development and application for omnibus service. Automobile Engineer, December.

Artemis Intelligent Power Ltd, http://www.artemisip.com/ (accessed 26 January 2012). Beachley, N. and Frank, A. (1979) Continuously variable transmissions: theory and practice, Lawrence Livermore Laboratory Report, UCRL-15037.

Beachley, N.H. et. al. (1981) Continuously-variable transmission designs for flywheel-hybrid automobiles, Proceedings International Symposium Gearing Power Transmissions, Tokyo, pp. 393–398.

Benham, P. P. and Warnock, F. V. (1976) Mechanics of Solids and Structures, Pitman International, London, UK.

Birch, S. (2011) Volvo spins up flywheel technology research, Automotive Engineering International Online, June.
Brockbank, C. and Greenwood, C. (2009) Fuel economy benefits of a flywheel and CVT based mechanical hybrid for city bus and commercial vehicle applications, SAE paper 2009-01-2868.
Clerk, R. (1964) The utilization of flywheel energy, SAE paper 640047.
Cross, D. and Hilton, J. (2008) High speed flywheel based hybrid systems for low carbon vehicles, Hybrid & Eco Friendly Vehicles Conference, Coventry, UK.
Dailey, J. W. and Nece, R. E. (1960) Chamber dimension effects on induced flow and frictional resistance of enclosed rotating disks, Journal of Basic Engineering, March, 82(1), 217–230.
Diego-Ayala, U., Martinez-Gonzalez, P., McGlashan, N. and Pullen, K. R. (2008) The mechanical hybrid vehicle: an investigation of a flywheel-based vehicular regenerative energy capture system. Proceedings of the Institution of Mechanical Engineers Part D Journal of Automobile Engineering, 222(D11), 2087–2101.
Dietrich, P. et al. (1999) Results of the ETH Hybrid III vehicle project and outlook, SAE paper 1999-01-0920.
Frank, A. and Beachley, N. (1975) Improved fuel economy in automobiles by use of a flywheel energy management system, Flywheel Technology Symposium Berkeley, CA. USA.
Fuller, J. et. al. (2011) Hardware development of FLYBUS Flywheel Based Mechanical Hybrid Systems for bus and commercial vehicles, 2nd VDI Conference Transmission in Commercial Vehicles Friedrichshafen, Germany.
Genta, G. (1985) Kinetic Energy Storage: theory and practice of advanced flywheel systems, Butterworths.
Gilbert, R. et al. (1972) Flywheel drive systems study, Final report, contract no. 68-04- 0048, Lockheed Missiles and Space Company, Inc.
Hansen, R.G. and O'Kain D.U. (2011) An assessment of flywheel high power energy storage technology for hybrid vehicles, Oak Ridge National Laboratory Report ORNL/ TM-2010/280, December.
Hayes, R. et al. (1999) Design and testing of a flywheel battery for a transit bus, SAE Paper 1999-01-1159.
Kok, D. (1999) Design optimisation of a flywheel hybrid vehicle, PhD Thesis, Technical University Eindhoven.
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Lawson, L.J. (1978) Applications of kinetic energy storage to transportation systems, High Speed Ground Transportation Journal, 12(3), 1–27.
Loscutoff, W. (1976) Flywheel/heat engine power for an energy-economic personal vehicle, Prepared for the Energy Research and Development Administration under Contract E(45-1): 1830.
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Mantriota, G. (2001) Power split continuously variable transmission systems with high efficiency, Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, 215, January 357–358.
Martinez-Gonzalez, P. (2010) A study on the integration of a high-speed flywheel as an energy storage device in hybrid vehicles, PhD Thesis, Imperial College London.
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Hydraulic and pneumatic hybrid powertrains for improved fuel economy in vehicles

Z. Filipi, Clemson University, USA

DOI: 10.1533/9780857097422.2.505

Abstract: This chapter describes the principle of operation and main features of hydraulic hybrids, including the typical architectures. It includes details of component design, both pump/motors (P/M) and accumulators, and describes modeling of components and integration into a complete vehicle system simulation. Case studies illustrate the approach to design and control of the parallel and series configuration, and summarize lessons learned through application of advanced simulations and optimization algorithms. Reallife applications and results achieved so far with prototypes or production vehicles are also presented. Section 16.5 is devoted to air-hybrids, their principle of operation, options regarding the configuration of the system and necessary engine modifications, and assessment of the fuel economy potential.

Key words: hydraulic hybrid, hydraulic pumps, hydraulic motors, hydraulicpneumatic accumulator, vehicle simulation, supervisory control, optimal control, parallel hybrid, series hybrid, power-split hybrid, pneumatic hybrid.

16.1 Introduction

Concerns regarding the future of global energy security, the dependence of transportation on fossil fuels, and a need for rapid response to climate change concerns, provide a strong impetus for development of fuel-efficient vehicle propulsion. Hybrid powertrains provide exceptional opportunities for fuel economy improvements, much beyond gains that can be achieved through any single technology. The unique flexibility in design and control of the powertrain stems from the fact that a hybrid includes two power sources, one of which has to be reversible, and energy storage. The main mechanisms for improving vehicle fuel economy through hybridization are:

∑ regenerative braking
∑ engine downsizing/rightsizing
∑ optimization of engine operation, i.e. load leveling
∑ engine shut-off instead of idling when propulsion power demand is zero.

When fully utilized, these mechanisms allow defeating the otherwise tight correlation between vehicle mass and fuel economy, and significant leaps upward. The actual values depend on many factors, such as the choice of energy conversion and storage, powertrain architecture, size and design of individual components, and optimization of the system design and control. Of course, optimum solutions will differ depending on the vehicle category and dominant duty cycles. While hybrid electric vehicles have captured a lot of attention in the passenger car market, hydraulic hybrid vehicles (HHV) provide a compelling alternative in applications that require high power density, particularly those that experience aggressive duty cycles with frequent stops and accelerations. The early efforts focused on truck HHV applications, where the inherent high power-density of hydraulics provides an obvious advantage. However, recent efforts confirm outstanding opportunities for improving the fuel economy of passenger cars too. Cost is an advantage, as a hydraulic accumulator can be produced for significantly less than a modern Li-ion battery. The storage device is a high-pressure vessel with a sealed variable volume chamber filled with gas, typically nitrogen. When the hydraulic fluid is transferred into the accumulator due to high pressure provided by the hydraulic pump, the gas is compressed and acts as a spring, thus storing energy.

An even simpler hybrid concept is a pure pneumatic, or 'air-hybrid'. In this case, the engine itself is modified into a multi-function engine-compressormotor device, and the energy storage is a high-pressure air tank. As a quick illustration of the basic concept, during braking the flexible valvetrain in the air-hybrid turns the engine into a compressor and enables filling the storage tank. Details will be discussed in the remainder of this chapter. At this point, it is sufficient to say that the pressure levels in a hydraulic hybrid are several times higher than the achievable pressure in the air-hybrid system. Thus, the hydraulic systems offer much higher power density and can be considered 'stronger' hybrids, while the pneumatic hybrid offers opportunities for simpler, but very mild hybridization.

There are several reasons for considering hydraulic hybrids as a competitive alternative to other hybrids, and optimistic forecasts regarding the introduction of this technology into the global marketplace, namely:

∑ High efficiency and affordability of pump/motor technologies. Multiple manufacturers have developed refined variable displacement pump configurations specifically for mobile application, with efficiency numbers much higher than in generic components for machinery. Latest designs feature fast response hydraulic and solenoid valves, and digital control for unprecedented dynamic performance and controllability. Noise has also been reduced, thus addressing concerns regarding the driver perception and noise, vibration and hardness (NVH).
∑ High power density of both the pump/motors and the hydro-pneumatic accumulators (Backe, 1993). The charging of the accumulator amounts

to compressing the gas in the high-pressure vessel, and the device is capable of handling extremely high rates of charging, occurring with high frequency. This is in stark contrast to batteries, which feature lower power limits, and suffer from overheating in case of frequent and aggressive charging/discharging events. The energy storage capacity of the accumulator is comparatively less; therefore, the integration and control needs to be approached very differently than in the case of the electric hybrid.

∑ While the designs of modern pump/motors are very advanced, the manufacturing is well developed and enables extremely high quality with acceptable cost.
∑ Serviceability is an advantage, since many automotive facilities are equipped for work on hydraulics and there is a pool of qualified personnel educated in the construction machinery sector.

This chapter is organized as follows. The Section 16.2 describes the principle of operation and main features of hydraulic hybrids, including the typical architectures. This is followed by the details of component designs, both pump/ motors and accumulators, and a section detailing the approaches to modeling of components and integration into a complete vehicle system simulation. The description of models provides the background for subsequent analysis of system integration and optimization. Predictive simulation tools allow in-depth analysis of the interactions in the powertrain system, explanation of the design decisions, and development of supervisory control strategies. Next, case studies illustrate the approach to design and control of the parallel, series and power-split architecture, and summarize lessons learned through application of advanced simulation and optimization tools. Illustrations of selected real-life applications follow, including the results achieved so far with prototypes or production vehicles. The closing sections are devoted to air-hybrids, their principle of operation, options regarding the configuration of the system and required engine modifications, and experience gained through research and testing of prototypes.

16.2 Hydraulic hybrid principle of operation and system architectures

Fluid power is a mature technology, due to its extensive use in construction machinery, but its application as means of vehicle propulsion have garnered interest relatively recently (Backe, 1993). The hydraulic hybrid comprises an internal combustion engine (ICE) as the prime power source that converts fuel energy into mechanical work, one or more hydraulic pump/motors as reversible power components, and energy storage system. The configuration of the mechanical driveline depends on the selected architecture, e.g. parallel,

series or power-split. In contrast to some other options, the hydraulic hybrid energy storage system requires a minimum of two components, the highpressure pneumatic-hydraulic accumulator (main storage), and a low-pressure reservoir that enables transfer of fluid back and forth during charging and discharging events.

Illustrations of the three main hybrid powertrain architectures are shown in Figs 16.1–16.3. The parallel hydraulic hybrid vehicle (P-HHV) configuration features a reversible pump/motor coupled mechanically to the driveline. When the vehicle accelerates, the P/Mprop acts as a motor, provides power assist, consequently reducing the power requirement on the engine. During braking, the P/Mprop operation is switched to the pumping mode, therefore providing the braking torque on the wheels and eliminating or reducing the activation of friction brakes. Fluid is transferred to the accumulator, thus reducing the volume of nitrogen and increasing its pressure. This energy is available for subsequent acceleration. Pressure in the accumulator varies between predetermined limits. Typical pre-charge pressure, i.e. lower limit, is 100–120bar, and the upper limit determined by the strength of the vessel and the various fittings and valves is ~350 bar. The pressure in the reservoir

Mechanical power path
Hydraulic power path (high pressure)
Hydraulic power path (low pressure)

16.1 Parallel hydraulic hybrid, comprising an engine, mechanical transmission, pump/motor coupled mechanically to the driveline, and the energy storage.

Mechanical power path
Hydraulic power path (high pressure)
Hydraulic power path (low pressure)

16.2 Series hydraulic hybrid, comprising an engine, power generation pump/motor (P/M gen), reversible propulsion motor (P/M prop) and energy storage. There is no mechanical connection between the engine and the driveline.

is always at a low level; therefore, the variation of the fluid amount in the reservoir does not have significant impact on energy flows.

There is a lot of freedom in selecting the size of the hydraulic pump/motor for the P-HHV, since it acts as a power-assist device which is not expected to provide full power on demand. The implementation is relatively simple, as the vehicle retains the mechanical driveline designed for the conventional counterpart, and the hydraulic sub-system can even be approached as a retrofit option.

However, the incremental cost includes all hydraulic components, electronics and the transfer case that couples the pump/motor to the driveline, and there is no offset from components that would be removed. P-HHV enables regeneration, and some level of flexibility in controlling the engine, but not as much as a series HHV. Engine downsizing is also limited, due to relatively small energy storage capacity of the hydraulic accumulator. Therefore, the main objective of the control studies is to maximize the effectiveness of the energy regeneration process during braking and subsequent acceleration, and the fuel economy gains are most favorable in aggressive stop-and-go traffic. Several P-HHV systems are already in production, and details are provided in the Section 16.6.

Hydraulic power path (high pressure) Hydraulic power path (low pressure)

16.3 Power split hydraulic hybrid, with an engine, planetary power-split device, two pump/motors and energy storage system comprising the high pressure accumulator and the reservoir.

Series hydraulic hybrid vehicle (S-HHV) configuration removes a mechanical connection between the engine and the wheels, as shown in Fig. 16.2. Therefore, the powertrain includes two sub-systems: the power generation system, comprising hydraulic pump/motor coupled to the ICE, and the propulsion sub-system, comprising a reversible hydraulic traction motor coupled to a final drive. Architecture such as this one without the energy storage would be considered a hydrostatic drive, or an infinitely variable transmission. The addition of the hydraulic-pneumatic accumulator and the reservoir enables regeneration and transforms the powertrain into a full hybrid. Again, reservoir is kept at the low pressure and serves primarily to enable transfer of fluid to-and-from the accumulator, with minimal impact on overall energy conversion. In the case of the series HHV, the traction motor needs to provide the full power on demand, for any set of operating conditions. Given that the power of the hydraulic motor depends on the pressure, critical conditions such as low state-of-charge (SOC) in the accumulator have to be considered when determining the size of components. In turn, proper sizing needs to also ensure full capacity for regeneration during aggressive braking. Absence of the mechanical connection between the engine and the wheels implies full flexibility in optimizing engine operation, thus providing added benefits compared to a P-HHV. The engine can be downsized, since full power can be provided on demand. Frequent and extended engine shutdowns are expected in the S-HHV, as long as hydraulically or electrically driven ancillaries are integrated in the powertrain, such as power-brakes, power steering, etc.

Power-split hydraulic hybrid (PS-HHV) configuration, shown in Fig. 16.3, attempts to combine the best features of the parallel and a series system. The heart of the power-split device is a planetary gear, with an engine and two pump/motors interconnected in a way that allows significant flexibility in optimizing engine operation, and managing regeneration. The engine is coupled to the carrier, P/Mgen is connected to the sun, and P/Mprop is connected to the wheels via the ring and the final drive. In the power-split hybrid system, the classification of P/Mgen and P/Mprop becomes ambiguous because both P/M can be used for propulsion or generation. The main idea is to use the powersplit device to keep the engine at near-constant speed during acceleration or cruising, thus moving its operating points upwards, towards higher torque. High load means higher efficiency, as close as possible to the 'sweet spot' on the efficiency map. During braking the P/Mprop allows regeneration. Hydraulic-only operation is possible as long as there is sufficient SOC level in the accumulator. While the powersplit systems have been introduced in hybrid electric powertrains and machinery, the application to road-going hydraulic hybrid vehicles is somewhat hampered by kinematic features of the device. Kinematic relationships in the system lead to extremely high speeds of the sun and the P/Mgen during vehicle launch, when the wheel speed is zero. In contrast to electric motors, the maximum allowable speeds of hydraulic motors are typically in the range of 3000–4000 revolutions per minute (RPM), therefore additional gear sets and more complex designs are necessary for PS-HHVs. Nevertheless, hydro-mechanical transmission design offers multiple advantages, analogous to already demonstrated benefits of electro-mechanical power-split drivelines, and this is likely to stimulate significant development efforts in the future.

16.3 Hydraulic component design and modeling

This section presents basic features of hydraulic component designs, discusses options for hydraulic hybrid applications, and models suitable for generating a simulation of the complete powertrain. Discussion of modeling aids understanding of the key features, and their impact on system design and operation.

16.3.1 Hydraulic pump/motors

The hydraulic pump/motors transfer the fluid through the system, and in doing so either produce or absorb mechanical torque, depending on the pressure ratio across the machine. The torque level is directly related to the pressure difference and the internal displacement of the elements responsible for transferring the fluid. While there are wide varieties of hydraulic pumps for general purposes, the HHV pump/motors are predominantly piston type with variable displacement.

Two main architectures, axial and radial, are defined based on the arrangement of the pistons with respect to the drive shaft. In the axial piston category, two designs are most relevant for the HHV applications, the swash plate configuration shown in Fig. 16.4, and the bent-axis configuration shown in Fig. 16.5 (Ivantysyn and Ivantysynova, 2003). In both cases the angle of load-control actuator, i.e. the swash plate or the bent-axis mechanism, determines the effective stroke of the piston, and therefore the torque output of the machine. The swash plate design is somewhat simpler, and features input and output shafts along the same axis. Therefore, there is a possibility for in-line mounting in the mechanical driveline and construction of the retrofitting assembly. Such a system is illustrated in Fig. 16.6.

An innovative approach to radial pump design improves the high efficiency range, responsiveness and noise characteristics by using solenoid valves and digital control. The concept was first proposed by Artemis Group (Rampen, 2006), but after acquiring a license Bosch Rexroth continues to refine the product. The schematic of the cross-section of the pump is shown in Plate IV (between pages 392 and 393). Torque control is achieved by varying the number of active cylinders versus those that are idled. Active elements operate at full stroke and hence achieve high efficiency. The overall efficiency is

16.4 Swash-plate type, inline axial-piston hydraulic pump/motor (Hitchcock and Gannon, 2012).

16.5 Bent-axis type axial-piston hydraulic pump/motor (Hitchcock and Gannon, 2012).

16.6 Photograph of the axial-piston hydraulic pump installed in-line with the propeller shaft to form a so-called regenerative drive shaft (courtesy PermoDrive).

obviously affected by the idle elements too; nevertheless, carefully optimized design provides a favorably broad efficiency map compared to classic configurations. In addition, the dynamic response and noise characteristics are superb.

Mathematical model of the hydraulic pump/motor

Models based on physics and empirical constants will be presented in this section and used to characterize the operation of the typical hydraulic pump, and the effect of key variables on effi ciency. This will be the basis for approaching the analysis and optimization of integrated hydraulic hybrid systems.

Regardless of the design details, a piston-type hydraulic pump with variable displacement can be modeled using an updated version of Wilson's P/M theory (pourmovahed et al., 1992). The torque and fl uid fl ow is controlled by the displacement factor.

Ideal leak-free volumetric fl ow rate of P/M:

$$Q_{\rm i} = x\omega D \tag{16.1}$$

ideal frictionless torque of p/M:

$$T_{\rm i} = x \Delta p D \tag{16.2}$$

where x is displacement factor, i.e. the ratio of current p/M displacement to the maximum p/M displacement–its sign is negative for pump operation and positive for motor operation; w is angular velocity [rad/sec]; D is maximum p/M displacement per radian (m³ /rad); and DP is p/M pressure difference (pa) that typically varies in a wide range, e.g. 100 bar–350 bar. Therefore, the torque output of the pump/motor will vary signifi cantly depending on the accumulator SOC.

The difference between actual (Qa in (m³ /sec)) and ideal volumetric fl ow (Qi ) determines the volumetric effi ciency:

$$\eta{\text{v, pump}} = \frac{Q{\text{a}}}{Q{\text{i}}} = 1 - \frac{C{\text{s}}}{|x|S} - \frac{\Delta p}{\beta} - \frac{C_{\text{st}}}{|x|\sigma}$$ [16.3]

$$\eta{\text{v, motor}} = \frac{Q{\text{i}}}{Q{\text{a}}} = \frac{1}{1 + \frac{C{\text{s}}S}{|x|} + \frac{\Delta p}{\beta} + \frac{C_{\text{st}}}{|x|\sigma}}$$ [16.4]

where ^C x S s s p C ^t | | , and ^D b ^sx | ^| represent laminar leakage loss, compressibility and turbulent leakage loss, respectively. b is oil bulk modulus of elasticity (pa), and ^S ^P D p and 1/3 1/2 ∫ ∫ ^Ê mw ^s wr D D are dimensionless numbers in which r is oil

Ë ¯ density (kg/m³ ) and m is oil dynamic viscosity (Ns/m² ) Torque effi ciency relates actual (*T*a) and ideal torque (Tⁱ ):

^Á ^ˆ

$$\eta{\text{t, pump}} = \frac{T{\text{i}}}{T{\text{a}}} = \frac{1}{1 + \frac{C{\text{v}}S}{|x|} + \frac{C{\text{f}}}{|x|} + C{\text{h}}x^{2}\sigma^{2}}$$ [16.5]

$$\eta{\text{t, motor}} = \frac{T{\text{a}}}{T{\text{i}}} = 1 - \frac{C{\text{v}}S}{|x|} - \frac{C{\text{f}}}{|x|} - C{\text{h}}x^2\sigma^2$$ [16.6]

where C S x C x C x v f ^h 2 2 | | , | | and ^s represent the viscous loss, the mechanical friction and the hydrodynamic loss, respectively.

The coeffi cients are calibrated to match the particular P/M design, using available experimental data from the laboratory, or published articles.

The model captures variations of effi ciency with operating conditions, such as the displacement command, rotational speed or pressure. This is of great value in HHV studies. Figure 16.7 illustrates variations of the volumetric effi ciency and torque effi ciency with load, expressed as displacement command. As in many other mechanical power devices, the torque effi ciency increases with load and after a point fl attens out. Therefore, operating strategies that avoid lengthy operation at very low load are preferred. The torque effi ciency obviously dominates the behavior of the pump, since volumetric effi ciency

Pressure: 200 bar Speed: 2000 rpm Fluid temperature: 313K P/M size: 150cc/rev

16.7 Predicted variation of the axial-piston motor volumetric and torque effi ciency with displacement.

is much higher and not as sensitive to load. Figure 16.8 shows the variations of volumetric and torque efficiency across the shaft rotational speed range. Torque efficiency decreases with speed, due to the impact of the rotational velocity term on viscous and hydrodynamic losses. In contrast, the leakage losses decrease with speed, and volumetric efficiency increases, but the sensitivity is weak.

It is important to note that the efficiency of the pump also depends on the pressure difference across the device, as indicated by the terms in equations [16.3]–[16.6]. Therefore, in contrast to ICE or electric motors, the efficiency maps are three-dimensional rather than two-dimensional. An example is provided in Plate V (between pages 392 and 393). This has significant implications in development of the HHV control strategies, since pressure in the system varies in a wide range depending on instantaneous SOC. Accurate representation of pump efficiency trends is critical for achieving the correct behavior of the simulated hydraulic hybrid system.

16.3.2 Hydraulic-pneumatic accumulator

An accumulator is a device that provides energy storage in the hydraulic hybrid system through compression of gas sealed inside of the vessel. It

Pressure: 200 bar Displacement factor: 0.5 Fluid temperature: 313K P/M size: 150cc/rev

16.8 Predicted variation of the axial-piston motor volumetric and torque efficiency with speed.

is most often called the hydraulic-pneumatic accumulator, since it enables the transfer of fluid in and out of the vessel, but the mechanism for energy storage is compression of the gas. In most cases the gas is nitrogen. Two types dominate the practical applications, the piston-type accumulator shown in Fig. 16.9(a), and the bladder type accumulator in Fig. 16.9(b). In the piston type accumulator, the hydraulic fluid entering the vessel due to high pressure moves the piston and compresses the gas ahead of it. The design is straightforward, sealing is achieved via a piston ring set, but the accumulator is relatively heavy, and the direct contact between the gas and metal walls facilitates heat transfer.

In case of the bladder type accumulator, a bladder made of an elastomer contains the gas, and the fluid surrounds it. This design reduces the heat loss from the gas to the wall and the ambient, and facilitates application of advanced lightweight materials for the shell, such as carbon fibers. The accumulator can be designed for pressures above 400 bar, but the sealing in other parts of the system, including fittings, imposes a practical limit of 350 bar.

Reservoir design often mimics the design of the accumulator, but the pressure levels are much lower, on the order of 5 bar. Thus, the walls of the vessel can be significantly thinner. An open accumulator can be used as well, which keeps the fluid at an ambient pressure. This increases the packaging volume, but enables easy deaeration of the system.

16.9 Schematic drawings of (a) piston type accumulator and (b) bladder type hydraulic-pneumatic accumulator.

Thermodynamic model of the hydraulic-pneumatic accumulator

A hydraulic-pneumatic accumulator is a high-pressure vessel whose volume is divided into two parts: one part is designed to hold the hydraulic fl uid while the other part contains inert gas. When the hydraulic fl uid is pumped into the accumulator, the sealed gas is compressed and its internal energy increases thus allowing energy storage. The stored energy can subsequently be reused by allowing the hydraulic fl uid to fl ow out of the accumulator and drive the hydraulic motor.

The accumulator model captures the thermodynamic process resulting from the hydraulic fl uid fl ow in and out of the accumulator causing the compression and expansion of the gas. The approach is based on the application of the energy conservation to the gas in the accumulator, and it was fi rst introduced by Otis and pourmovahed (1985):

$$m{\rm g}\frac{du}{dt} = -p{\rm g}\frac{dv}{dt} - m{\rm f}c{\rm f}\frac{dT}{dt} - hA{\rm w}(T - T{\rm w})$$ [16.7]

The internal energy of real gas is given by

$$du = c{v}dT + \left[T\left(\frac{\partial p{p}}{\partial T}\right){v} - p{g}\right]dv$$ [16.8]

and the thermodynamic energy equation can be simplifi ed and rearranged to provide the differential equation characterizing the changes of gas temperature:

$$\left[1 + \frac{m{\rm f}c{\rm f}}{m{\rm g}c{\rm v}}\right]\frac{dT}{dt} = \frac{T{\rm w} - T}{\tau} - \left[\frac{\partial p{\rm g}}{\partial T}\right]_{\rm v}\frac{dv}{dt}$$ [16.9]

with time constant: ^t g v ^∫ m c hAw

where m^g is the mass of gas (kg), *m*f is the foam mass (kg), c^f is the specifi c heat of foam (kJ/kgK), T is the absolute gas or foam temperature (K), T^w is the accumulator wall or bladder temperature (K), c^v is the constant volume specifi c heat of gas (kJ/kgK), *p*g is the gas absolute pressure (kpa) and v is the gas specifi c volume (m³ /kg).

The Benedict–Webb–Rubin (BWR) equation relates the real gas pressure to the gas temperature and specifi c volume:

$$\begin{split} p_{\rm g} &= \frac{RT}{v} + \left(B_0 RT - A_0 - \frac{C_0}{T^2}\right) / \text{m} + (bRT - a)/v^3 + \frac{a\alpha}{v^6} \ &+ \left(c\left(1 + \frac{\gamma}{v^2}\right)e^{-\gamma/v^2}\right) / v^3 T^2 \end{split} \tag{16.10}$$

where R, *A*0, *B*0, *C*0, a, b, c, a and g are constants in BWR equation.

The formula for the thermal time-constant provides valuable insight. The larger the constant, the smaller are changes of gas temperature. This translates into better efficiency. The thermal time-constant increases if the heat capacity is increased, and heat loss reduced. This simple physical reasoning led to an idea to add elastomeric foam to the gas and increase the thermal capacity multiple times (Pourmovahed et al., 1988). Hence, the equations [16.7] and [16.9] contain mass and specific heat of foam (subscript f).

Figure 16.10 illustrates the round trip efficiency of the charging-discharging event calculated using equation [16.9] as a function of the thermal time constant. Three lines are shown, one calculated without the foam, and the other two assuming addition of the 2.1 or 7 kg of elastomeric foam, respectively. Efficiency of the standard accumulator depends strongly on the thermal time-constant, and increasing its value through hardware design is necessary. The addition of foam elevates the curve to much higher efficiency levels, in the 92%–95% range.

Figure 16.11 shows the estimated power density and specific energy values for common types of accumulators, in comparison with other energy storage devices, such as electric batteries or flywheels (Baseley et al., 2007). The power density is clearly advantageous, orders of magnitude better than the electric battery. However, the energy density is much lower. This has a significant impact on characteristics of the hydraulic hybrid powertrain, and requires a fresh approach to development of the control strategy.

16.10 Accumulator efficiency for a full charging/discharging event, as a function of the thermal time constant, and the mass of elastomeric foam added to the gas-side (*m*f).

16.11 Specific energy vs power density of different energy storage systems (Baseley et al., 2007).

16.4 Integrated hydraulic hybrid vehicle simulation

Design and control decisions require a tool capable of representing vehicle behavior under relevant driving conditions. Models of main components comprise necessary building blocks. Next, a software platform suitable for integration of individual models and control studies is selected. Several options are available, among which Matlab-SIMULINK has garnered wide acceptance. It enables integration in a common graphical software environment, provides a large library of standard modules, and facilitates design and implementation of controllers. The structure is hierarchical, therefore individual modules can have varying fidelity and extend several levels down if desired.

Figure 16.12 shows a top-level view of the integrated hydraulic hybrid vehicle system in SIMULINK. This is a series configuration, and the main components are the engine coupled to a P/Mgen, traction motors (in this case there is a front and a rear), drivetrain coupling the traction motors to the wheels (this may include a two-gear transmission), accumulator, reservoir, and the vehicle dynamics module. This is a forward looking model, therefore the flow of information is exactly as in a real vehicle: the initial command is provided by the cyber driver, and it is subsequently processed in the supervisory control module to generate command signals for the main power sources and actuators. Vehicle speed feedback is provided to the cyber driver and used to ensure that the vehicle follows the prescribed driving schedule.

16.12 Integrated series hydraulic hybrid vehicle simulation in SIMULINK.

Model fidelity is a choice that depends on the simulation goals. In case the simulation is used for preliminary assessments of the particular configuration and component choices, simple models based on torque and efficiency look up tables and a point-mass vehicle dynamics model will suffice. Deeper insight and final component design decisions require more fidelity, e.g. modules that allow physical scaling and predictions of dynamic behavior. Assessing the impact of component design details, and consideration of additional attributes such as exhaust emission or driveability, pushes the envelope further. In the case of the ICE, a one-dimensional engine system simulation with a predictive combustion model can be integrated in SIMULINK. In the case of hydraulics, full details of the pump internal design and fluid flow through the valves, fittings and lines can be captured in SIMULINK, or modeled using specialized software such as AMESim (Vacca et al., 2009) and a cosimulation technique. Finally, studies of vehicle stability during regenerative braking or hill-climb performance require implementation of the pitch-plane or a full three-dimensional multi-body vehicle dynamics model.

16.5 Design and control of hydraulic hybrid powertrains

Hybrids provide unparalleled flexibility in design and control of the powertrain. This leads to exceptional opportunities for improving fuel economy and/or other vehicle attributes. However, realizing the full potential for improvements depends strongly on design decisions and supervisory control. Particular vehicle categories and dominant duty cycles might dictate very different configuration, design parameters, and supervisory control strategy. The inherent advantages of hydraulics, such as the high power density of components and comparatively high efficiency, as well as the challenge imposed by the low energy storage capacity, require original solutions rather than translation from the world of electric hybrids. Variable pressure in the system affects performance, therefore even the basic sizing study becomes much more interesting in the case of hydraulic hybrids. As an example, an oversized accumulator will provide a larger capacity for regeneration or sustained acceleration, but the average pressure during normal operation will be lower, with implications regarding the on-demand performance and pump/motor efficiency. This section will define the HHV design problem, the supervisory control problem, and present optimization methodologies through cases studies. Analysis will illustrate solutions applicable to vehicles on the road.

Hydraulic hybrids were historically considered as a natural choice for heavier vehicles, trucks and buses. Hydraulics provide a compelling option due inherent power density, efficiency and cost advantages. Therefore, the initial case study of the parallel HHV focuses on the medium duty truck platform. However, recent investigations indicate equally attractive benefits in passenger car applications, and the case study of the series HHV will encompass a detailed look at both the truck and a small ultra-efficient passenger car.

16.5.1 Design and control of a parallel hydraulic hybrid

A schematic of the parallel hybrid powertrain configuration is shown in Plate VI (between pages 392 and 393. The motor is intentionally placed between the transmission output shaft and the final drive/differential. This is a preferred location for hydraulic hybrids, rather than a motor/generator coupled directly to the engine as is often the case in the parallel hybrid electric vehicles. The reasons stem from the consideration of basic pump/motor properties. High power density and limited speed range of hydraulics create a compelling case for integrating the pump/motor closer to the final drive, where it is possible to keep the speed within acceptable limits and maximize effectiveness of regeneration. Closer examination of Plate VI identifies the following design parameters that critically impact the overall performance of the HHV: engine size; pump/motor size; transfer case gear ratio, including a potential for two-speed transmission; and accumulator size, precharge and maximum pressure. The final gear ratio may be considered too.

System design study begins with considerations of constraints. As an example, the initial decision regarding the transfer-case gear ratio, or a possible need for a two-speed gear box will be driven by the desired torque and the maximum allowable pump speed. The initial guess for the accumulator size can be determined through the analysis of a representative braking/ acceleration event. However, decisions based solely on constraints do not account for efficiency, and may lead to missed opportunities. In addition, the interdependencies in the system make it impossible to arrive at a best solution through simple parametric studies. As an example, variation of the accumulator size will have impact on the pressure history during typical driving, and this will affect pump/motor performance and average efficiency. Therefore, achieving the best tradeoffs and maximizing the benefits requires application of the multi-variable optimization algorithm.

Before illustrating the approach to optimal design, it is important to recognize the interplay between the system design and supervisory control. The supervisory controller orchestrates the operation of multiple power sources and energy storage on-board of a HHV. Its actions clearly affect the duty cycles of components, and their dominant operating regimes. This will have impact on design. In turn, any major changes of design parameters will require re-assessment of the control strategy. Rigorous approach needs to consider coupling between the design decisions and supervisory control development (Filipi et al., 2004).

The sequence of critical steps is shown in Fig. 16.13. Hydraulic hybrid system simulation is developed based on the proposed system architecture and initial component design. Next, the vehicle mission is analyzed and a representative duty cycle is developed. Formulating the initial rule-based power management completes the preparatory steps, and enables evaluation of the performance and fuel economy of the initial hybrid configuration. Fully configured, predictive HHV system simulation is a critical element in the design the optimization process. For every function call, the optimization algorithm executes the HHV simulation over a driving cycle. The objective is to maximize fuel economy, subject to constraints defined to ensure the ability of the hybrid vehicle to fulfill the same mission as the conventional truck.

After determining the optimal (1st stage) values of the design variables, the supervisory control strategy is optimized in order to extract the most benefit out of the system operating dynamically over a driving schedule. Variables considered are a power split strategy in the parallel hybrid system and the transmission shift logic. An attractive possibility is to apply a dynamic programming (DP) algorithm, due to its proven ability to generate an optimal benchmark (Bertsekas, 1995). The second stage of the design optimization process captures the effects of the modified power management on optimum design, and guarantees that the final configuration

16.13 Methodology for sequential optimization of the design and power management (PM) of the hybrid system for best fuel economy (FE).

satisfies prescribed constraints. In the study of the off-road 15 ton vehicle, the systematic approach to optimal design (Filipi et al., 2004) allowed the incremental fuel economy improvement from the initial 21% to 32%.

Supervisory control methods and application to a P-HHV

The supervisory controller commands operation of the multiple power sources and actuators in a hybrid powertrain to maximize selected vehicle attribute(s). Minimizing the fuel consumption is the typical objective, but additional terms may be added. Supervisory strategy is critical for realizing the maximum potential of any given hybrid concept.

There are generally two directions that can be taken in the supervisory control development: (i) rule-based (i.e. heuristic), and (ii) algorithm-based. Rule based approach relies on the engineering intuition to develop a set of rules that define modes of HHV operation. In the case of the P-HHV, the main role of the supervisory controller is to split the power requirement for vehicle propulsion between the engine and the hydraulic pump/motor, and manage braking. The approach to development of intuitive rules is typically component-centric, e.g. the main goal is to operate the engine in the most favorable regime. This is illustrated in Fig. 16.14. The engine brake specific fuel consumption (BSFC) map is used to identify the most favorable regimes. Constant power lines are superimposed and used to define boundaries of the favorable BSFC zone. When the power request falls below the lowload limit, the engine shuts down and the vehicle is propelled using the motor only. If the power request exceeds the high-load limit, the traction motor assists the engine and keeps it running with high BSFC. Since the accumulator characteristics vary in a very fundamental way from those of the battery, the SOC sustaining logic needs to be changed. Low energy density of the accumulator favors relaxing the SOC limits to expand the capacity for regeneration, e.g. SOC = 0.3-1. Another fundamental difference is often overlooked. In the case of the HHV, the secondary power source often has the power output comparable to the main auxiliary power unit (APU). Therefore, neglecting its own operating regimes and focusing solely on the engine efficiency clearly leaves part of the fuel economy potential untapped.

Advanced strategies apply methodologies based on a mathematical algorithm, supported by the predictive HHV simulation. Options include the equivalent consumption minimization strategy (Paganelli et al., 2002), dynamic programming (Sciarretta et al., 2003, Lin et al., 2001, Wu et al., 2004), neuro-fuzzy control (Matheson and Stecki, 2003) etc. Dynamic programming will be highlighted here, since it has been proven to provide an optimal benchmark for any non-linear system. The algorithm searches for optimal decisions at discrete points in a time sequence. An illustrative

16.14 Definition of hybrid powertrain control rules on the engine brake specific fuel consumption (BSFC) map.

example is the application of the DP to a medium duty truck with a parallel hydraulic hybrid driveline (Wu et al., 2004). The baseline truck weighs 7,340 kg when fully loaded, and it is powered by a V8, 7.3 L turbocharged engine with a rated power of 157 kW, and four-speed automatic transmission. The hydraulic pump has a nominal displacement 150 cm3 /rev, and the accumulator capacity is 50 liters of fluid. The estimated weight penalty due to additional components is 274 kg.

The benchmark obtained through application of DP indicates a potential to increase city fuel economy up to 76% compared to a conventional baseline with a four-speed automatic transmission. Unfortunately, the optimal benchmark is produced for the known future, i.e. prescribed driving schedule, and it is not implementable in a practical controller. Rather, results need to be studied in order to extract knowledge and develop improved rules. Figure 16.15(a) and (b) illustrate the process. Figure 16.15(a) shows histories of key variables in the system for a 400–560 second segment of the US federal urban driving schedule obtained using standard engine-centric rules. Power histories given in the middle plot indicate that the hydraulic motor serves primarily to assist the engine during launch or acceleration. Therefore, it often operates in a part load, inefficient regime. Power traces during braking confirm the ability

16.15 Parallel HHV system behavior with different power management strategies during the 400~560 second interval of the US city driving schedule with: (a) rule-based control and (b) DP optimized control law.

of the hydraulic pump to harvest large amounts of energy. However, the top graph in Fig. 16.15(a) shows that the accumulator hits the upper SOC limit frequently, thus preventing full regeneration.

Contrast between this set of results, and the behavior of the system controlled by the DP algorithm (see Fig. 16.15(b)) is stark. DP attempts to use the motor aggressively at the beginning of each vehicle launch. Engine power supplements a portion of power if the demand exceeds the maximum available motor power. Generally, DP tends to use the motor and engine exclusively. Whenever the power demand exceeds maximum motor power, propulsion is switched to the engine, and in most cases the engine becomes the only power source. By avoiding simultaneous use of both the engine and the motor, the DP is able to keep propulsion components operating at high-load and high-efficiency, e.g. motor displacement factor time history in Fig. 16.15(b) shows frequent values above 0.8. Finally, analysis of the SOC profile in Fig. 16.15(b) indicates that DP ensures:

∑ enough accumulator storage capacity for the future regeneration event, and
∑ appropriate hydraulic energy reserve for the next vehicle launch.

Analyzing results such as those shown in Fig. 16.15 allows extraction of sub-optimal rules implementable in a practical controller. In the case of the P-HHV discussed here, the controller will call upon the motor to satisfy the total power demand whenever there is energy available in the accumulator. If the power requirement is more than what the motor can provide, the engine will supplement the motor power. When the accumulator is empty the engine becomes the sole power source. The additional module in the controller modifies the shift logic used in the conventional vehicle (Johri et al., 2011).

In summary, fuel economy improvement of the P-HHV truck with a classic rule based strategy is 35.5%, while the optimal benchmark indicates up the 76.8% fuel economy potential. Implementable rules extracted from the DP results guarantee achieving 47.4% increase of fuel economy (Wu et al., 2004). This amounts to incremental improvement of 12% over the standard rule-based case, without any hardware modifications.

16.5.2 Design and control of a series hydraulic hybrid

A schematic of the series hybrid powertrain configuration is shown in Plate VII (between pages 392 and 393). The power generation sub-system comprises the engine coupled to an adequately sized pump/motor. The propulsion system includes one or more reversible hydraulic traction motors. In this particular case two motors are coupled to differentials in the front and the rear, thus enabling all-wheel drive capability. There is no mechanical connection between the power-generation and the propulsion side. The accumulator and the reservoir are integrated into the hydraulic circuit. The following design parameters critically impact performance of the S-HHV: engine and pump size; traction motor(s) size, transfer case gear ratio, including a potential for two-speed transmission; accumulator size, precharge and maximum pressure. The approach to initial sizing is quite different compared to the P-HHV. The propulsion sub-system has to be capable of satisfying all performance constraints. Engine size can be reduced due to the fact that it can be operated at rated power regardless of the vehicle speed. It is important to note that the pressure in the S-HHV system varies, therefore the overall design, including the sizing of the accumulator needs to take into account the dependency of the traction power on instantaneous SOC.

The main portion of this section is devoted to a case study of the medium-duty truck. Key concepts in the S-HHV design and control will be presented using this platform as an example. However, the refined techniques for supervisory control will be highlighted through application to a light, ultra-efficient passenger car for urban environments. It will be shown that the hydraulic hybrid technology can be just as effective in the case of light vehicles.

Design optimization of the S-HHV propulsion system starts with the selections of initial values. For example, the pump (P/Mgen) should be big enough to absorb the maximum engine power at minimum fluid pressure, and the engine has to satisfy requirements for maintaining continuous speed on the grade. The P/Mprop size is dictated by the required vehicle performance, but the rotational speed constraint may dictate addition of the two-speed gear box, thus altering the boundary conditions. The accumulator size requires careful assessment of the tradeoff between size and the average pressure in the system. Reaching the best S-HHV fuel economy requires setting up a design optimization framework similarly to the procedure described in the section devoted to P-HHV. An illustrative example is found in Kim and Filipi (2007), where the optimized solution provided a 10% incremental increase of fuel economy over the best initial design.

Rule-based control of a series hydraulic hybrid

In a series hybrid configuration, driver's command is used for controlling the P/Mprop power and ensuring that the vehicle follows the desired speed profile. The SOC is the primary variable used for determining engine power. The particular challenge in developing the supervisory control for a hydraulic hybrid stems from the low energy density of the accumulator. While the battery serves as a huge buffer in the HEV system, the accumulator charge undergoes rapid transients and can easily be depleted. In that case the control needs to ensure that the power-generation sub-system directly fulfills the needs of the propulsion system.

The traditional rule-based approach yields a thermostatic SOC control algorithm illustrated in Fig. 16.16. As long as the SOC is above the threshold value, e.g. 0.4, the engine power command is zero. When the SOC drops to the threshold value, the engine starts charging the accumulator with the predetermined threshold power. The dead band is implemented to prevent frequent engine on-off switching. If power required for propulsion exceeds

16.16 Schematic illustration of the thermostatic SOC power management concept.

16.17 BSFC map of a diesel engine, with constant power lines (dashed lines), and the best BSFC trajectory (thick black line). The gray dot indicates a 'sweet spot'.

the threshold level, the SOC will instantaneously drop below the lower limit and the engine power will be progressively increased. Further increase in demand essentially keeps the system operating in the hydrostatic drive mode, and ensures that the SOC remains above the absolute minimum. In the limiting case, during full power acceleration or hill climbing the engine is operated at rated power. Of the four key parameters, the threshold power and threshold SOC have the strongest impact on fuel economy.

Engine-level strategy determines the combination of torque and speed for the power demand signal generated by the top-level controller. A common approach is to generate a trajectory of the best efficiency points on the BSFC map (see Fig. 16.17), and use the flexibility offered by the S-HHV system to keep the engine operating points on that line.

Conventional wisdom states that the objective of the supervisory control should be to keep the engine running at the sweet spot of best BSFC. However, this completely neglects the system effects. As a minimum, the combined efficiency of the engine and the P/Mgen should be considered. Other complex interactions should be investigated using the predictive simulation tool. Results of the parametric study in Fig. 16.18 (Kim and Filipi, 2007) illustrate the importance of approaching the problem from a system point of view. When the threshold power is varied, a much higher fuel economy is achieved with reduced Pthreshold values of 60 kW, compared to the 'sweet spot' power of ~100 kW. Explanation lies in very short and frequent charging events when Pthreshold is high. Very rapid load transients, e.g. a sudden 0–100 kW increase, come with a penalty, as there is an energy loss associated with rapid engine acceleration. Multiple lines in the graph correspond to different threshold SOC values, and the lowest value of 0.3 produces best system efficiency, because in ensures maximum regeneration capacity under most operating conditions. The best fuel economy value represents a 68% improvement over a conventional baseline, under city driving conditions, and with engine shut-downs whenever the engine power signal is zero. Highway driving represents a challenge for any series hybrid due to multiple energy conversions in the system and accompanying losses, but refined rule-based control enabled a 12% improvement for the vehicle studied in this section.

Advanced algorithms for control of the series hydraulic hybrid

Insights from the studies of the thermostatic SOC control indicate a critical role of approaching the S-HHV supervisory control from the system level, rather than simplifying it through component-centric approach. Application of dynamic programming can provide the optimum benchmark, but rather than pursuing the rule extraction, algorithm can be modified to generate a

16.18 Vehicle fuel improvements as a function of S-HHV threshold charging power and threshold SOC, with engine shut-downs whenever the engine power demand is zero.

sub-optimal, but directly implementable strategy. The latter approach uses a probability of driver action rather than a pre-determined driving cycle to determine the optimal control policy, and is therefore called the stochastic dynamic programming (SDP). This section highlights the benefits of using such an approach, but does that using a small passenger car platform rather than a truck. Consequently, the results indicate the potential of the S-HHV concept when applied to a light vehicle for urban transportation. A short summary of the vehicle specification is: vehicle mass 723 kg, frontal area 1.9 m2 , air drag coefficient 0.25, engine rated power 49 kW, axial piston pump 25 cm³ /rev, and two hydraulic traction motors of 20 cm³ /rev each.

The objective of the SDP control algorithm is to find the optimal control policy that maps observed states, such as the accumulator SOC and vehicle speed, and disturbance to the control decision. The control decision can be engine power, and in that case the engine-level logic keeps the engine operating on the best BSFC trajectory. A better option is to allow the algorithm to find the best combination of engine torque and speed (Johri and Filipi 2009).

An illustration of the methodology and the typical results are shown in Plate VIII between pages 392 and 393. A transition probability matrix is generated from a group of driving cycles and used to generate disturbance. An approximate policy iteration algorithm is used in an iterative fashion until the optimal cost function converges. Ultimately, state-feedback look up tables are produced and can be directly implemented to control engine torque and speed based on the instantaneous propulsion power demand and SOC (see Plate VIII). Look up tables effectively synthesize the system-level and the engine-level control, as they determine both the strategy to maintain the SOC and the best way to operate the engine. Plate IX(a) shows that the algorithm deviates from the best BSFC line, and tends to operate the engine at somewhat higher loads. In contrast, engine operating points in the baseline vehicle with a five-speed transmission and an integrated electric starter alternator are concentrated in the less efficient low-load region (see Plate IX(b). The S-HHV fuel economy improvement demonstrated in this simulation study is 74% for city driving, and ~19% on the highway, relative to the mild electric hybrid as a baseline. Proposed packaging of the S-HHV powertrain for such a vehicle is illustrated in Plate X.

16.6 Examples of practical applications

This section presents brief highlights of some of the products already introduced in the marketplace or announced for near future. P-HHV systems are in regular production for several truck applications. S-HHV systems have been demonstrated in the light-duty and medium-duty truck market, and a European OEM has announced a production-intent S-HHV system for a passenger car.

Bosch Rexroth has launched production of the parallel system for the refuse truck called the hydrostatic regenerative braking system. Configuration of the hydraulic power-assist system, integrated with an accumulator and an open reservoir is shown in Fig. 16.19. Plate XI illustrates packaging in the vehicle and the flow of energy. The system is compact and can also be retrofitted. Estimated fuel consumption improvements are up to 25% in refuse trucks or urban buses. The system is claimed to be almost maintenance-free and wear-free compared with electric hybrid drives. Additional benefits include extended life of friction brake pads, and improved engine reliability.

Eaton has developed a P-HHV system called hydraulic launch assist (HLA) – see Fig. 16.20. It is intended for vehicles that operate in stop-andgo duty cycles, such as refuse trucks and buses. The design is focused on maximizing the regeneration of braking energy. The propulsion system can operate in the economy mode or the performance mode. When in the former, the HLA system uses the hydraulic power exclusively during launch. During the latter, the system combines the power of the hydraulic motor and the engine. Eaton estimates 20 to 30% improvement in fuel economy, longer brake life, and increased productivity.

Collaboration between Eaton and the US Environmental Protection Agency (EPA) led to development of the series hydraulic hybrid diesel delivery vehicle. A schematic of the system is shown in Fig. 16.21, and the first fleet of prototype vehicles was used by the US Parcel (UPS) service. Projected

16.19 Hydraulic regeneration system designed by Bosch Rexroth for integration in a parallel hybrid powertrain.

16.20 Hydraulic sub-system designed by Eaton for integration in a parallel hybrid powertrain.

16.21 Series hydraulic hybrid architecture developed by EPA and Eaton for a delivery truck application.

improvements of the fuel economy are 60 to 70% in the stop-and-go delivery mission.

French manufacturer Peugeot has announced the HybridAir vehicle for 2014. This is a series hybrid system, utilizing a 1.2-liter three-cylinder gasoline engine and a hydraulic motor to drive the front wheels (see Plate XII). Operating modes are as described in the Section 16.5, i.e. regenerative braking, when the motor operates as a pump, transfers the fluid into the accumulator located longitudinally, and stores energy as a compressed gas. Company literature talks about 'compressed air' for clarity, but the gas is in fact nitrogen. During acceleration or cruising, the system uses the stored energy to power a hydraulic motor and engages the engine to sustain SOC. Estimated fuel economy improvements in city driving are 45%.

16.7 Pneumatic hybrids

Pneumatic hybrid, or 'air-hybrid', is a concept that transforms the ICE into a multi-function device, instead of adding the secondary power source. Energy storage is a high-pressure air tank connected to the engine via a special valve, as shown in Fig. 16.22. The valve actuation strategy changes depending on the desired mode of operation, and switches engine operation from 'compressor' to 'air motor' or standard ICE. The idea was explored by Ford (Tai et al., 2003), ETH in Switzerland (Donitz et al., 2009), University D'Orléans in France (Vasile et al., 2006) and Lund University in Sweden (Trajkovic et al., 2008).

Two general approaches are possible: development of a flexible actuation system for one of the existing engine valves, or design of a cylinder head with an additional port and valve, dedicated solely to air-hybridization. Irrespective of the engine design, the pneumatic hybrid is a very low cost option compared to other hybrids, since there are no additional power devices, and the air tank is much cheaper than the battery or the hydraulic accumulator. However, pressure level in the storage tank is much lower than the pressure in the hydraulic accumulator; therefore, energy capacity and power are limited, and the pneumatic hybrid falls into a category of very mild hybrids.

16.22 Schematic illustration of the pneumatic hybrid concept (Donitz et al., 2009)

16.7.1 Principle of pneumatic hybrid operation

The pneumatic hybrid relies on the flexible valve actuation system to convert the engine into the compressor during vehicle braking, and store the compressed air in a tank. During acceleration, the air can be used either to propel the vehicle by operating the engine in the air-motor mode, or for short term supercharging.

Figure 16.22 illustrates the basic idea, while Fig. 16.23 shows the proposed redesign of the cylinder head to include the additional port and a charge valve (CV) for communication with the storage tank (Ivanco et al., 2009). Realization of the pneumatic hybrid strategy is determined by the details of design. Options can be assessed by examining the schematic in Plate XIII, showing valve events associated with various pneumatic hybrid engine modes of operation. Achieving both the 4-stroke regular IC operation and 2-stroke compressor/motor operation requires a fully flexible valvetrain, since both the charge valve and the inlet/exhaust valves need to be actuated every crankshaft revolution. This is favorable from the standpoint of hybrid functionality, but increases the cost of hardware.

In case the regular camshafts are retained and only the charge valve has flexible actuation, the compressor/motor and the air-motor are limited to 4-stroke operation. The bottom of Plate XIII illustrates the possibility of using the compressed air in the tank for supercharging, rather than utilizing

16.23 Cross section through a cylinder head illustrating a pneumatic hybrid engine concept with an additional valve per cylinder: (1) charge valve (2) engine cylinder volume and (3) pressure air tank (Vasile et al., 2006).

air-motor operation. This improves engine transient response during hard acceleration, and therefore facilitates engine downsizing.

16.7.2 Pneumatic hybrid system examples and key findings

This section presents two examples of the pneumatic hybrid development in the research laboratory based on comprehensive simulation studies that guided hardware design and test cell demonstration. Only a limited amount of material can be presented here; hence, these two examples were selected as representative of the two very different approaches that include experimental verification. Other efforts are documented in the references listed at the end of the chapter.

Conversion of the heavy-duty diesel engine into a pneumatic hybrid using a fully flexible valve actuation (FFVA) system was published by Lund University (Trajkovic et al., 2008). Interestingly, engine valves are actuated pneumatically, rather than using electromagnetic or hydro-mechanical devices. This configuration allows engine operation as a 2-stroke compressor during vehicle braking, and as an air-motor driven by the compressed air previously stored in a tank during acceleration. It was shown that the valve events have to be carefully optimized in order to achieve efficient compressor and air-motor operation. One of the inlet valves of the engine was modified to allow communication with the tank, and basic modes of operation were demonstrated in the laboratory. A powertrain model was used as a basis for generating a complete vehicle simulation in SIMULINK, and the fuel economy improvement potential was explored for a city bus example. The fuel consumption reduction varied between 8% and 58% depending on the driving cycle, the highest values corresponding to the most aggressive stopand-go cycle. Simulation results indicated that even in the best case, only ~12% of fuel consumption improvement stems from regeneration, and the rest comes from the engine start-stop system.

After analyzing the pros and cons of various pneumatic hybrid concepts, Donitz et al. (2009, 2011) selected the fixed camshaft (FCS) configuration with one fully flexible valve for air-hybridization, but added a turbocharger. Pneumatic hybrid engine configuration is given in Fig. 16.24. A simulation study of the pneumatic hybrid vehicle indicated very limited benefits from the hybrid modes, such as regeneration and power assist, due to low energy density of the pneumatic storage. However, it was shown that availability of compressed air for supercharging enables severe downsizing of the turbocharged SI engine. Whenever the driver commands harsh acceleration, 'free energy' stored in the tank is used to supercharge the engine and eliminate turbo lag. A combination of very mild hybridization and very aggressive engine downsizing demonstrated a potential for 32% improvement of engine average

16.24 Schematic of the two-cylinder pneumatic hybrid engine with turbocharging and intercooling. One valve per cylinder is modified to serve as a charge valve with camless actuation (from Donitz et al., 2011 with permission of ASME).

fuel efficiency compared to a naturally aspirated conventional counterpart. A two-cylinder 750 cm³ engine was modified for experimental investigations, and the basic pneumatic modes of operation were demonstrated in the laboratory, including the switching between modes and supercharging.

16.8 References

Backe, W. (1993), 'The present and future of fluid power', Proceedings of the Institution of Mechanical Engineers. Part I, Journal of Systems and Control Engineering, 207(4), 193–212.

Baseley, S., Ehret, C., Greif, E. and Kliffken, M. (2007), 'Hydraulic hybrid systems for commercial vehicles', SAE Technical Paper 2007-01-4150.

Bertsekas, D. P. (1995), Dynamic Programming and Optimal Control, Athena Scientific, Nashoa, NH.

Dönitz, C., Vasile, I., Onder, C. and Guzzella, L. (2009), 'Realizing concept for high efficiency and excellent drivability: the downsized and supercharged hybrid pneumatic engine', SAE Technical Paper 2009-01-1326.

Donitz, C., Voser, C., Vasile, I., Onder, C. and Guzzella, L. (2011), 'Validation of the fuel saving potential of downsized and supercharged hybrid pneumatic engines using vehicle emulation experiments', Journal of Engineering for Gas Turbines and Power, 133(9), 13.

Filipi, Z., Louca, L., Daran, B., Lin, C.-C., Yildir, U., Wu, B., Kokkolaras, M., Assanis, D., Peng, H., Papalambros, P., Stein, J., Szkubiel, D. and Chapp, R. (2004), 'Combined optimization of design and power management of the hydraulic hybrid propulsion system for the 6 ¥ 6 medium truck', International Journal of Heavy Vehicle Systems, 11(3/4), 371–401.
Hitchcock, A. L. Gannon, M. C. (2012), Fluid Power Handbook and Directory Hydraulics & Pneumatics, A1–A118.
Ivanco, A., Colin, G., Chamaillard, Y., Charlet, A. and Higelin, P. (2009), 'Energy management strategies for hybrid-pneumatic engine based on sliding window pattern recognition', Oil and Gas Science and Technology – Rev, IFP, France.
Ivantysyn, J. and Ivantysynova, M. (2003) Hydrostatic Pumps and Motors, Principles, Designs, Performance, Modelling, Analysis, Control and Testing. New Delhi, Tech Books International.
Johri, R. and Filipi, Z. (2009), 'Low-cost pathway to ultra efficient city car: series hydraulic hybrid system with optimized supervisory control', SAE Paper 2009-24- 0065, ICE2009 Conference, Capri, Italy.
Johri, R., Baseley, S. and Filipi, Z. (2011), 'Simultaneous optimization of supervisory control and gear shift logic for a parallel hydraulic hybrid refuse truck using stochastic dynamic programming', Proceedings of the 4th Annual ASME Dynamic Systems and Control Conference, ASME, Arlington, VA.
Kim, Y. J. and Filipi, Z. (2007), 'Simulation study of a series hydraulic hybrid propulsion system for a light truck', SAE paper 2007-01-4151, SAE Transactions and Journal of Commercial Vehicles, 116, 147–161.
Lin, C., Filipi, Z., Wang, Y., Louca, L., Peng, H., Assanis, D. and Stein, J. (2001), 'Integrated, feed-forward hybrid electric vehicle simulation in simulink and its use for power management studies', SAE Technical Paper 2001-01-1334.
Matheson, P. and Stecki, J. (2003), 'Modeling and simulation of a fuzzy logic controller for a hydraulic-hybrid powertrain for use in heavy commercial vehicles', SAE Technical Paper 2003-01-3275.
Otis, D. R. and Pourmovahed, A. (1985), 'An algorithm for computing nonflow gas processes in gas springs and hydropneumatic accumulators', Journal of Dynamic Systems, Measurement, and Control, 107(3), 93–96.
Paganelli, G., Delprat, S., Guerra, T. M., Rimaux, J. and Santin, J. J. (2002), 'Equivalent consumption minimization strategy for parallel hybrid powertrains', Vehicular Technology Conference, VTC Spring 2002 IEEE 55th, Vol. 4, 2076–2081.
Pourmovahed, A., Baum, S. A., Fronczak, F. J. and Beachley, N. H. (1988), 'Experimental evaluation of hydraulic accumulator efficiency with and without elastomeric foam', Journal of Propulsion and Power, 4(2), 185–192.
Pourmovahed, A., Beachley, N. H. and Fronczak, F. J. (1992), 'Modeling of a hydraulic energy regeneration system – Part I: analytical treatment', Journal of Dynamic Systems, Measurement, and Control, 114(3), 155–159.
Rampen, W. (2006), 'Gearless transmissions for large wind turbines the history and future of hydraulic drives, Proceedings, 8th German Wild Energy Conference DEWEK 2006, Bremen, http://www.artemisip.com/news-media/papers
Sciarretta, A., Guzzella, L. and Onder, C. H. (2003), 'On the power split control of parallel hybrid vehicles: from global optimization towards real-time control', at Automatisierungstechnik, 5, 195–203.
Tai, C., Tsao, T-C., Levin, M., Barta, G. and Schechter, M. (2003), 'Using camless valvetrain for air hybrid optimization', SAE Technical Paper 2003-01-0038.
Trajkovic, S., Tunestål, P. and Johansson, B. (2008), 'Investigation of different valve geometries and valve timing strategies and their effect on regenerative efficiency for a pneumatic hybrid with variable valve actuation', SAE Technical Paper 2008-01- 1715.
Vacca, A., Franzoni, G. and Bonati, F. (2009), 'An inclusive, system-oriented approach for the study and the design of hydrostatic transmissions: the case of an articulated boom lift', SAE Int. J. Commer. Veh. 1(1): 437–445.
Vasile, I., Higelin, P., Charlet, A. and Chamaillard, Y. (2006), 'Downsized engine torque lag compensation by pneumatic hybridization', 13th International Conference on Fluid Flow Technologies, Budapest, Hungary.
Wu, B., Lin, C.-C., Filipi, Z., Peng H., Assanis, D. (2004), 'Optimal power management for a hydraulic hybrid delivery truck', Journal of Vehicle System Dynamics, 42(1–2), 23–40.

Integration and performance of regenerative braking and energy recovery technologies in vehicles

P. Tawadros and N. Zhang, University of Technology, Sydney, Australia and A. Boretti, RMIT University, Australia

DOI: 10.1533/9780857097422.2.541

Abstract: In the quest to meet ever-tightening regulation on vehicle emissions, one option available to designers in the technology 'toolbag' is regenerative braking. With the increasing hybridisation of vehicles, regenerative braking is often a free addition. This chapter explores the issues involved with the design of regenerative braking, how multiple brake systems interact together on a single vehicle, the regulation applying to production vehicles, and the various ways that regenerative braking may be implemented. A case study is undertaken using data from a common hybrid production vehicle to highlight these concepts.

Key words: regenerative braking, hybrid and electric vehicles (HEV and EV), braking regulation; kinetic, energy recovery system (KERS), brake control.

17.1 Introduction

It goes without saying that in order to make a vehicle go, one must also figure out how to make it stop. The most basic safety system in a vehicle, the brake, has evolved from a simplistic wooden block on an iron rim, into a computer-controlled electrohydraulic system with multiple levels of redundancy, and the ability to actively control vehicle attitude by making adjustments to brake torque distribution thousands of times a second. This evolution has been driven by ever-improving road interface technology, which has moved from iron belts of the 19th century through to complex rubber compounds on a radial steel-belted construction. As tyre technology advanced, so too did braking technology, in order to supply the necessary braking force required to make full use of the available tractive force. The wood block gave way to metal-on-metal drum brakes, then hydraulically assisted actuation, disc brakes and composite linings.

The state of evolution of brake technology today has reached the point where the limitation on braking force is now no longer the dynamic properties of the road interface, nor the properties of the brake mechanism itself, but actually the ability of the human element of the system to withstand the deceleration possible under heavy braking, whilst retaining safe control of the vehicle. Formula 1 vehicles typically reach maximum deceleration of 5.97g (Brembo S.p.A., 2012).

In the quest to meet ever-tightening regulation on vehicle emissions, a convenient intersection between braking technology and emissions legislation exists in the form of regenerative braking. This concept, whilst almost as old as the automobile itself, has seen renewed interest since the fuel crisis of the 1970s and the resultant research avenues into alternative powertrains. It has in turn, facilitated a change in the direction of braking technology, swinging the focus away from the dissipation of energy and towards the recuperation of it. Whilst traditional friction braking systems are still very much a required part of automotive hardware, contemporary technological advancement in the field considers not just the ability to stop, but how best to store the energy of the retarding vehicle.

In this chapter, we will explore the types of regenerative braking and some common properties. We will also investigate integration and optimisation of energy regenerating systems with particular reference to incorporation in hybrid powertrains, the advantages and limitations of the technique, and future trends in the technology.

17.2 Types and properties of regenerative braking and energy recovery

With the increasing hybridisation of vehicles, the alternative power source typically already includes a second propulsion component as well as an additional energy storage device. These components can be configured to store or expend energy, making regenerative braking a 'free' addition. Regenerative braking systems are designed to recover energy that would be otherwise dissipated during a braking event. In their most fundamental form, they are a bi-directional power transmission system, with a power source and sink at one end, and an energy storage device on the other (Cross and Brockbank, 2008).

Regenerative braking systems are typically mechanical, electrical or

17.1 Regenerative braking systems are composed of only two basic, interconnected elements.

hydraulic in nature; however, this is not strictly necessary, providing there is a way to store and later re-use energy. In illustration of this point, many stations along the London Underground network are (famously) built atop a slight incline. This means that as a train approaches a station its kinetic energy is converted to gravitational potential energy, slowing it down. As it leaves, the reverse occurs. This has benefits of increased safety, and decreased maintenance and power consumption.

17.2.1 Electrical regeneration properties

Electrical energy recovery systems are at their core, based on a motor/generator (electric machine) driven from the kinetic energy of the moving vehicle. The electricity generated is used to charge a battery, and then fed back to the machine in the acceleration phase. The energy is transformed from kinetic energy to electrical energy, and then to chemical energy in the regenerative braking phase. These transformations occur in reverse during acceleration. Due to the large number of energy conversions, electrical regeneration has a relatively poor round-trip efficiency, even in the most efficient systems (Cross and Brockbank, 2008). The specific power (kW/kg) of such systems is very highly dependent on the size and chemistry of the battery, but is typically much poorer than both hydraulic and mechanical systems. However, the specific energy storage is much better than hydraulic systems, and the main benefit of a fully-electric system lies in the packaging freedom that the designer is allowed (Cross and Brockbank, 2008, Boretti, 2011).

Electrical regenerative braking may be implemented in any electric, or part-electric vehicle. In fully electric vehicles, the power flow is reversed

17.2 A four-wheel drive hybrid electric vehicle in regenerative mode.

through the entire powertrain, whereas in part-electric vehicles (hybridelectric vehicles), power flow is reversed through only the electric portion of the powertrain.

17.2.2 Mechanical regeneration properties

Mechanical regeneration (known as M-KERS, or Mechanical Kinetic Energy Recovery System) is based on a flywheel and clutch system, in which a flywheel is clutched into the driveline at the beginning of a braking event, and the momentum of the vehicle is used to accelerate the flywheel, thereby slowing the vehicle down. The reverse is true in the acceleration phase, where the momentum of the flywheel is used to aid acceleration. This mechanical energy storage has advantages over other types of regeneration. Its key advantage is that by mechanically coupling a flywheel to the already-spinning driveline, there are no energy conversion losses. Further, as an energy storage medium and power delivery medium, it also boasts far better energy density (kJ/kg), and power density (kW/kg) than the other types of regenerative systems discussed herein (Boretti, 2011).

The key weaknesses of mechanical regeneration are two-fold. Firstly, packaging constraints have historically been a problem with the integration of flywheel systems in small vehicles. The Belgian 'Gyrobus' of the mid-20th century exemplifies an extreme case of this, utilising a 1,500 kg flywheel spinning up to 3,000 rpm, mounted in a low-pressure hydrogen chamber between the axles (Moglestue, 2008). The second problem, is the mechanical connection between the compact high-speed flywheel required for light automotive use, and the low-speed drivetrain.

Recently, materials and construction of flywheels has enabled speeds of 40,000 to 100,000 rpm, and mass reductions (down to the order of ten kilograms). The modern flywheel is typically of composite construction, compact in dimensions, and spins on contactless bearings in a hermetically sealed chamber (Liu and Jiang, 2007, Cross and Brockbank, 2008). The latest generation of mechanical regeneration systems uses a high-speed flywheel connected to a continuously variable transmission and clutched into and out of the driveline, as in the torotrak/flybrid devices. Whilst this

17.3 Mechanical energy storage involves a physical connection between a flywheel and the driven wheels often through a CVT.

eliminates inefficiencies due to energy conversion, the CVT is the least efficient transmission architecture currently available (Kluger and Long, 1999).

Another alternative is embodied in the Williams Hybrid Power Flywheel, which is an electromechanical device for use in hybrid electric vehicles. The flywheel itself forms the rotor of an electric motor, and is constructed of carbon fibre 'doped' with magnetic particles. The stator of the motor is the flywheel housing, and when the vehicle is braking, the traction motor generates electricity. This electricity is used to accelerate the flywheel. In reverse, the flywheel operates as a generator, sending electricity back through to the traction motor as required. The Williams system has a round trip efficiency comparable with purely mechanical systems, due to the elimination of the low-efficiency chemical energy conversion found in chemical battery systems.

17.2.3 Fluid-based regeneration properties

Fluid-based regeneration is either pneumatic or hydraulic, although due to the compressibility of air, pneumatic braking is limited to the use of retarders, which are non-regenerating devices. A hydraulic regenerative braking system utilises a low-pressure reservoir, a hydraulic pump/motor, and high pressure accumulators to store energy in compressed fluid. Under braking, the pump is powered from the driven shaft and pumps fluid from the low-pressure reservoir to the high-pressure accumulator, which is charged with an inert gas to exert pressure on the working fluid. The pressurised fluid is then used in the motoring phase by reversing the flow, allowing the fluid to flow through a hydraulic motor (in some cases the pump doubles as the motor), which is connected to the powertrain at a convenient location.

Hydraulic regeneration systems are quite cost-effective (owing to the mature technologies used), but are not particularly energy-dense (Achten et al., 2008). As such, they are typically used in heavy vehicles (trucks and buses) due to the low initial cost and the high relative mass of the vehicle compared to the system, meaning that the large mass of a high energy storage

17.4 Hydraulic accumulators are used for the storage of energy in fluid form.

capacity does not severely affect the performance of the vehicle. However, hydraulic systems, in comparison to mechanical and electrical systems, are much higher-maintenance propositions during their service life. Hydraulic fluids and accumulators must be replaced at regular intervals. The systems also have large numbers of moving parts – typically a multi-speed gearbox and a clutch connects the pump/motor to the powertrain, and if a combined pump/ motor is not used, then multiple fluid and mechanical paths are required.

17.2.4 Governance and regulation

In automotive use, the most common regenerative braking systems are electric, and as such there are two regulations existing worldwide for electric regenerative braking systems on light vehicles. To date, no regulations exist for regenerative braking using either mechanical or fluid storage, although the regulations for electric systems may be suitably applied with little modification for the other types.

As implemented on hybrid electric, and electric vehicles, regenerative brakes in most parts of the world are regulated by a document produced by the United Nations Economic Commission for Europe (UNECE, 2011) known as 'Regulation 13H – Agreement concerning the adoption of uniform technical prescriptions for wheeled vehicles, equipment and parts which can be fitted and/or be used on wheeled vehicles and the conditions for reciprocal recognition of approvals granted on the basis of these prescriptions'. The corresponding document for US-manufactured vehicles is known as 'Federal Motor Vehicle Safety Standard 135 – Light Vehicle Brake Systems' (US Department of Transportation, 2005). However, Regulation 13H is the more stringent of the two, with particular reference to stability, split-m surfaces, and response to driver input. At present most regulatory bodies are harmonising towards ECE regulations, therefore the US regulation is only mentioned here for reader interest.

Regulation 13H defines two categories of regenerative braking systems. 'Category A' systems are systems which do not form part of the service braking system. These systems must only be controlled by the accelerator and/or the gear neutral position, which limits their use to 'throttle-off' systems, which typically power engine accessories such as the alternator, air conditioning compressor, or power-steering pump during engine overrun, by clutched means.

'Category B' systems are systems which do form part of the service braking system. Category B systems may also satisfy the requirements of a Category A system – that is, they may be actuated on 'throttle-off' events, and would thus be 'Category A and B' systems. Category B systems may be actuated concurrently with the foundation brake (simultaneous, or parallel phasing), or before or after the foundation brake (sequential, or series phasing). If anti-lock control is fitted, Category B systems must be controlled by the anti-lock device, and any variation in torque output from the regenerative braking must be automatically compensated for by the foundation brakes, providing there is sufficient traction available to do so (Curry, 2012).

17.3 Hybridisation with energy recovery: design and performance issues

There are several original design and performance issues that must be considered when creating a hybrid powertrain with regenerative braking. These issues include basic design criteria, control algorithm optimisation, and even seemingly inconsequential issues arising from the changing usage patterns of particular components of hybrid vehicles as compared to their internal combustion counterparts.

17.3.1 Powertrain design

Traditionally, a powertrain is completely disconnected from the braking performance of a vehicle, as its power delivery is one-way. The sizing of the powertrain in this case is a comparatively simple exercise. It is required to meet certain benchmarks of acceleration, emissions, fuel consumption, and other functional considerations, such as size or layout (see Table 17.1).

It is therefore a relatively simple exercise to work backwards from the weight of the vehicle to calculate power requirements, aiming for an appropriate torque curve and complementary gearing in order to fit as many of the criteria as possible, as closely as possible.

An energy recovery powertrain differs from a traditional powertrain significantly; however, due to its bidirectional power flow. The effect of this is that when considering an energy recovery powertrain in a vehicle, the traditional powertrain design paradigm no longer strictly applies. This is due to a number of reasons:

∑ The energy recovery powertrain often includes more than one power source (and always includes more than one in the case of hybrid vehicles), and always includes more than one operating mode. The power needs in each

Table 17.1 Traditional powertrain design criteria

0–100 km/h time 80–120 km/h (passing) time 0–140 km/h Maximum acceleration Maximum speed

Maximum cruise grade at 90 km/h

operating mode must be quantified, and components sized accordingly. This may involve multiple sets of powertrain design criteria, one set considering each powertrain mode.

∑ The energy flow in the energy recovery powertrain is at least bidirectional, and often multi-path. Although the flow paths in power delivery modes and power storage modes may be simply reversals of one another, the power handling capability in each direction is almost always different. For instance, both electric and hydraulic regen systems typically have higher specific power delivery than they do power absorption (Zhang et al., 2011; Rydberg, 2009). This is not usually the case with mechanical flywheel systems, which can deliver essentially the same power that they can absorb; however, these systems have other limitations due to traction-limited braking and power-limited acceleration.
∑ The energy recovery powertrain and the braking system are no longer separated from one another, and although a foundation (dynamic) brake is fitted, the powertrain must be designed as much for the delivery of power as for the storage of it, in order to take the fullest advantage of the property of reversibility. Components may not be strictly linear. For instance, a battery could be oversized for its function of maximum acceleration in order to recover more braking energy by being able to accept greater charge current, or in order to reduce charging losses.

It is therefore necessary to consider how much energy (and power) may be recaptured, and examine the net energy consumption in a given drive cycle, focusing on peak power delivery and peak power absorption, rather than just sizing the powertrain for the power delivery.

This shift in focus necessitates an iterative approach to powertrain design, in which the reversible components are initially selected on their ability to provide power delivery required for acceleration, and then sized up or down, to take advantage of any higher gain in recovered energy as compared to the power required to accelerate the additional mass. This process is executed in vehicle design software such as ADVISOR, VEHLIB, or Autonomie.

17.3.2 Braking control

Given the higher order of complexity of regenerative braking, the braking control algorithm becomes much more complex than that of a traditional hydraulic friction braking system. Consideration must be given to three main areas of performance. These are energy recovery, vehicle stability, and driver feel. Each of these areas is directly related to the co-ordination between foundation and regenerative braking.

Category B systems that are simultaneously phased offer lower energy recovery benefits than sequential phasing, however in two-wheel drive vehicles there are clear benefits in vehicle stability and driver feel metrics (Ehsani et al., 2010). This is because simultaneous phasing means that the front–rear braking torque split remains at the optimal line, and therefore no redistribution of braking torque is required when a wheel or axle locks. The anti-lock controller may simply be used to control the regenerative braking torque as easily as it does the foundation braking torque.

Category B braking systems that are sequentially phased offer the greatest scope for energy recovery, however in two-wheel drive vehicles this is achieved by bringing an axle (or wheel) closer to its traction saturation point (i.e., lock-up) in order to recover more energy from it. This approach is problematic if the friction interface co-efficient (m) drops unexpectedly and causes wheel lock. In order to maintain braking performance it is necessary to automatically compensate for lock-up by reducing the brake power at the locked wheel, and applying a higher braking force at another wheel. This must be accomplished whilst keeping the vehicle deceleration rate related to the driver's braking demand. Further, whilst regulation 13H specifies that the compensatory force must rise to 75% of its final value within one second (UNECE, 2011), in practice many drivers will expect unnoticeable variation in the deceleration rate of the vehicle, therefore the system response must be many times faster than this.

Therefore in two-wheel drive vehicles with regenerative braking, energy recovery may be maximised at the expense of stability, and careful attention must be paid to driver feel in order to deliver an acceptable level of performance, with respect to both the regulatory requirements as well as the end-user experience.

The third important area of system performance is in driver feel. Regulation 13H specifies that 'the braking rate must remain related to the driver's braking demand'. In practical terms, this means that the measureable driver input parameters on the brake pedal (pedal velocity, pedal displacement) must correlate to a repeatable rate of deceleration, having regard to available traction (UNECE, 2011). This has implications in terms of not only control systems, but also componentry. When sequentially phased brake systems are used, it is no longer possible to maintain a direct hydraulic link between the pedal and the physical braking hardware under normal operating conditions. The role of a pedal-stroke simulator becomes critical as it must provide feedback to the driver that is proportional to the expectation.

The control of brake blending is also critical to driver feel. Unlike a traditional hydraulically-actuated foundation brake, the torque of regenerative brakes is speed dependent. It is therefore necessary to modulate hydraulic braking such that for a given constant pedal input, the sum of regenerative and friction torque remains essentially constant. This is a particularly difficult control problem when combined with stability and anti-lock control. The current standard practice is to simply disable regenerative braking and use only friction braking during a lock-up event (Toyota Motor Corporation, 2012). However, this is inefficient and control solutions that do not disable the regenerative braking under these circumstances may offer both better efficiency as well as greater safety, due to the elimination of the response time between disabling regenerative braking and using foundation brakes to compensate, as well as the elimination of the inevitable torque fluctuation that will occur during braking transition (Curry, 2012).

17.4 Design integration and operational optimisation

There are three primary considerations to make in the optimisation of a regenerative braking system. The first is the consideration of energy recovery – that is, the system must recover sufficient energy so as to be both financially and technically viable. The second is the consideration of vehicle stability – that is that the operation of the system must not interfere with the stability of the vehicle in any way. Finally, the system must give the driver a predictable and repeatable outcome when a braking demand is required.

With regard to the first consideration, there are no standards mandating particular levels of performance and so the consideration may be made from engineering principles alone. However, the consideration of vehicle stability is not purely an engineering problem to solve, as it also carries regulatory burden in the commercial marketplace. At the time of writing this regulation has been accepted as UNECE Regulation 13H (for cars and light vans) and 13.11 (for all other types of vehicle). The regulation specifies certain technical requirements that must be adhered to, which shapes both the way the regenerative braking system operates, as well as (indirectly) how much energy it can absorb. These requirements refer to both the stability of the vehicle, as well as the response to driver-issued commands.

To explore these issues, we will first briefly treat the fundamentals of braking from first principles, and then explore how the principles and the regulations interact with the design and operational requirements for a regenerative braking system using a case study.

17.4.1 Braking fundamentals

To explore the fundamental physical principles of braking systems we will carry out all analysis in this section based on a mid-size vehicle. The characteristics of the vehicle are listed in Table 17.2.

Regenerative braking

An emergency stop is the most demanding requirement of the brakes on a vehicle. The vehicle with specifications as above needs to be able to stop at

Mass	1,600 kg
Split front/rear	53/47
Centre of gravity height	500 mm
Wheelbase	2680 mm
Frontal area	2.09 m2
Cd	0.29
Tyre size	215/45/17 (rolling radius = 0.303 m)
Vmax	210k m/h (58.333 m/s)

0.95g from its maximum speed in an emergency. The energy that needs to be dissipated into the brake system in order to do this is:

$$E = \frac{mv^2}{2} = \frac{1600(58.333)^2}{2} = 2.722 \text{ MJ}$$ [17.1]

The brake on time required to stop the vehicle is given in seconds as:

$$t = \frac{v}{ag} = \frac{58.333}{0.95(9.81)} = 6.259 \, s \tag{17.2}$$

Therefore the average power required to be dissipated into the brake system is:

$$P = \frac{E}{t} = \frac{2.722 \times 10^6}{6.259} = 434.929 \text{ kW}$$ [17.3]

and the peak power may easily double this value. it is plain to see that no regenerative braking system has this power capacity at present. even the Formula 1 KERS fl ywheels only supply 60 kW peak for 6.67 seconds (Fry, 2009). This is equivalent to a stored energy of 400 kJ, and is less than 15% of the power and energy requirements for an emergency stop in a passenger vehicle. Therefore, until research and development allows for these levels of power absorption, regenerative braking systems must operate in tandem with foundation brakes.

in the ecosystem of passenger cars, however, it is fortunate that most braking occurs at under 0.3g. This means that other than heavy braking or emergency braking, it is entirely possible to replace the operation of foundation brakes with regenerative braking during the normal operation of the vehicle (ehsani et al., 2010).

Braking stability

The stability of a vehicle under braking depends upon the applied braking force at each wheel not exceeding the available tractive force at the same wheel. Provided that this condition is always met, the wheel will not reach a locked state, and the vehicle will therefore not begin to yaw undesirably.

The available tractive force is simply calculated by the formula F = mN, where N is the normal force acting on the road, and m is the friction interface co-effi cient. However, as the centre of gravity of a vehicle is always some distance higher than the road surface, this means that the higher the vehicle deceleration, the greater the load on the front axles, with the rear axle load decreasing by the same amount. For a given deceleration, the front/ rear load split will always be the same, and can be calculated by taking the sum of moments about a contact patch. a generalised formula is given below:

$$M{\rm d,f} = \frac{a \times M{\rm s,t} \times h{\rm cg}}{L{\rm wb}} + M_{\rm s,f}$$ [17.4]

where *M*d,f is the dynamic front load (kg), a is the acceleration in (units g), *M*s,t is the static total mass (kg), *h*cg is the height of the centre of gravity (m), *L*wb is the length of the wheelbase (m) and *M*s,f is the static front mass (kg). The result may be graphed to give a curve of optimal brake force split for a given vehicle with a given m.

in the curve in Fig. 17.6, the sum of the x value and y value at each point on the curve is equal to the deceleration rate at that point in the curve. at points above the curve, braking is said to be rear-biased, and is unstable. The rear axle will lock up before the front axle, and this will cause an unstable yaw moment around the centre of gravity. at points below the curve, the front axle will lock before the rear (front-biased braking), and the vehicle will track straight ahead regardless of steering input after the lock event. at any point on the curve, all wheels will lock up at the same time once the tractive force has been exceeded. For this reason, production vehicles are designed to be slightly front-biased in braking. Traditionally, this is achieved by using a proportioning valve on the hydraulic circuit, which splits pressure

17.5 Free-body diagram of a vehicle under braking.

17.6 Optimum braking distribution is governed solely by the geometry of the car.

along a piecewise linear function that closely approximates the curve whilst staying underneath it.

Regenerative systems

The physics of regenerative braking systems in terms of stability is no different to hydraulic friction braking. However, for reasons of cost and complexity, regenerative braking is often only deployed on one axle (the driven axle). This has implications in terms of creating a necessary trade-off between optimum stability and optimum energy recovery.

In order to recover most energy, the braking effort must bias towards the regenerating axle. In front wheel drive vehicles, this means that the braking effort (up to the maximum available regenerative braking) must be front-biased. This bias is acceptable providing that there is sufficient traction available to prevent slip, and if the traction is not available, the bias must be immediately restored to a point where slip is controlled or eliminated. A similar statement can be made about rear drive vehicles, however the imperative to control slip is even greater, owing to the positive-feedback nature of yaw induced by rear-wheel lock up.

The blending of brakes in a Category B sequentially phased braking system will always prioritise energy recovery until a slip event is detected. Therefore the algorithm will follow the sequence in Fig. 17.7.

In simultaneously phased systems, the sum of regenerative braking on the driven axles plus foundation braking on non-driven axles is applied to brake the vehicle according to the dynamic braking split required. A fundamental

17.7 A simplified representation of a braking algorithm satisfying the requirements of a Category B sequentially phased braking system.

difference of regenerative braking systems to friction braking systems is that whilst a friction braking system can apply constant braking torque at all speeds, the available torque in a regenerative braking system is related to the state of charge of the energy reservoir, and the speed of the vehicle.

Stability and skid control

Regulation 13H has been written for vehicles whose regenerative braking system is electric only. However, the general concepts can be translated easily to other types of regenerative system, and therefore it should be noted that in the following section, where reference is made to UNECE regulation 13H, the reader should infer similar guidelines for non-electric regeneration. Regulation 13H specifies 'For vehicles equipped with an anti-lock device, the anti-lock device must control the electric braking system' (UNECE, 2011). This requirement has implications for regenerative braking systems, particularly Category B systems with sequential phasing. In order to satisfy the requirements of this section of the regulation, the industry standard practice is to control regenerative braking by switching it off during a slip event. A compensatory force is then applied using the friction brakes. This compensatory force must rise to within 75% of the value of commanded braking within one second (UNECE, 2011).

The superior method towards which research is progressing, is to modulate regenerative braking and foundation braking together, however at present this results in unacceptable variation in torque output for a given braking demand (Curry, 2012). This is because during a slip event, it is necessary to change the mode of operation of the braking system, and also to modulate both hydraulic and regenerative braking systems in response to slip information from wheel speed, steering angle, and yaw sensors, and increasingly, radar, proximity or charge-coupled device (CCD) image sensors. These operations must all be controlled to provide the best possible braking performance, as well as providing meaningful feedback to the driver in order to prevent confusion or unintended inputs, which is a tall order for current technology.

Response to driver input

Another area regulated by Regulation 13H is the system response to driver input. This is regulated by paragraph 5.2.7.1, which states 'Intrinsic variations in the torque output of the electrical regenerative braking system (e.g. as a result of changes in the electric state of charge in the traction batteries) are automatically compensated by appropriate variation in the phasing relationship… and, wherever necessary, to ensure that braking rate remains related to the driver's braking demand, having regard to the available tyre/ road adhesion, braking shall automatically be caused to act on all wheels of the vehicle'.

The practical interpretation of this paragraph means that regardless of the speed of the vehicle or the state of charge of the energy reservoir, the system response to identical braking demand inputs must remain the same. Additionally, under split-m conditions, where some wheels have greater adhesion than others, if one wheel begins to slip, the braking force on that wheel must be reduced an automatic compensation must be applied (up to the limit of adhesion on all wheels) in order to maintain the same braking rate as demanded by the driver input (Curry, 2012; UNECE, 2011).

17.4.2 Case study

A front-wheel drive hybrid electric vehicle with a power-split transmission is studied. The vehicle uses a chemical battery as its electric energy reservoir. The study involves performing instrumented braking tests on a sealed, dry road with m = 0.75 (approximate). The braking region is shown in Plate XIV (between pages 392 and 393). The region is bordered in black, with the ideal braking distribution curve superimposed above it in grey. The traces within the bordered region are all acquired braking data, and show that the vehicle is equipped with a Category B sequentially-phased regenerative braking system. A deceleration of 0.4 g corresponds to a braking torque of 800 N.m distributed across all wheels. Up to this limit, braking may be entirely regenerative, entirely foundation braking (this would only occur in the case of brake failure), or may be a mix of foundation and regenerative braking. The braking may also be entirely via the front axle. At above 800 N.m, or 0.4 g deceleration, braking is always blended and cannot be entirely regenerative. As deceleration exceeds 0.4 g, the amount of frontbias in the braking falls away steeply so that at high rates of deceleration, the braking split is almost in line with the optimal brake force distribution curve.

The traction motor's operating range reveals that between 17.5 and 33 rad/s (equivalent to 18.7 to 35.3 km/h) the full 800 N.m is available at the driveshaft. Therefore 0.4 g of regenerative braking is only possible between these speeds. Between 7.5 rad/s (8 km/h) and zero, no meaningful regenerative braking is possible, and above 33rad/s, regenerative braking torque is inversely variable with speed (Plate XV). Below 0.4 g, regenerative braking is always active to some degree, and often supplies all of the mechanical braking force required to stop the vehicle. Above that limit, unless a slip event is detected, the maximum available regenerative braking is blended with foundation braking. This is shown in the linear reduction in the upper boundary of the regenerative braking envelope in Plate XVI. The minimum achievable deceleration of 0.05g is achieved through aerodynamic and rolling resistance alone.

The characteristics of a stop from 100 km/h at 0.4 g are shown in Fig. 17.8. Notice that the modulation of foundation and regenerative braking must happen automatically and maintain the same sum of braking torque at all times, which in this case is 800 N.m. At different times during the braking event, the regenerative braking provides the full braking torque, the foundation braking provides the full braking torque, and a dynamically shifting ratio of foundation-to-regenerative braking is occurring, whilst the driver braking demand remains fixed. The change in these phases of braking must occur seamlessly and with no fluctuation in torque sum at the wheels, in order to satisfy regulatory requirements, as well provide stable and predictable feedback to the driver.

17.8 Characteristics of a typical 100km/h-zero stop at constant 0.4g – regen and foundation blend.

17.5 Advantages and limitations of regenerative braking

The state of current production car technology in electric regenerative braking allows the regeneration of a maximum of approximately 45% of braking energy (Tawadros et al., 2012). An optimum use of available technology could see this figure rise as high as 66%. Ideally, save for irreversibilities in the system, all non-emergency braking energy should be targeted for regeneration. This target is not unachievable in the foreseeable future.

The advantages of regenerative braking are clear. In addition to the emissions and fuel savings brought through use of the technology, mechanical and electrical regenerative braking systems are essentially maintenance-free. This saves on vehicle maintenance costs in terms of replacing perishable and wear items on foundation brake systems, including friction pads, discs and drums, hydraulic fluid, and rubber fluid seals.

Although, for the time being, foundation brakes must still be present on a vehicle with regenerative braking, research is underway on developing electrically-actuated foundation brakes (Faul et al., 2012). This would further reduce maintenance of the system to just replacement of the friction interface, as the hydraulic system could be dispensed with altogether. These advantages are available at minimal cost to original equipment manufacturers. As fuel- and emissions-saving measures become increasingly mandatory, the equipment required to implement regenerative braking in either a 'Category A' or 'Category B' system is generally already present in the vehicle. The major cost to the manufacturer lies only in development time to create suitable controls.

As technology progresses, more and more of the limitations of regenerative braking are being overcome, and in the future it may be possible to almost entirely eliminate inefficient friction braking hardware from hybrid or electric vehicles, save for small systems required at low velocity. In order to reach this point, there are several limitations in the current technology that must be overcome or developed:

∑ power absorption limits
∑ mixed-mode regenerative systems
∑ all-wheel regenerative systems
∑ active ABS control of regenerative braking
∑ control of regenerative braking under split-m conditions
∑ physical limitations

It is important to note that energy storage solutions are already adequately sized to accommodate most regenerative braking systems, and that the main limitation imposed by these to date is related to travel-range, rather than energy recovery.

In the future, with the increase in power density available in regenerative systems, the ceiling of maximum braking power will rise, although short of superconducting elements, practical technology for the power absorption rates required for emergency braking (with peaks for light vehicles approaching and occasionally eclipsing one megawatt) does not yet exist. Power absorption rates may be increased relatively inexpensively through use of mixed-mode regenerative systems, using flywheels or supercapacitors combined with chemical batteries.

As emissions and fuel consumption legislation tightens, it will become economically and technically feasible to develop hybrid systems which may regenerate energy on all four wheels (Hori, 2002). This will be supported by incremental increases in power absorption ability in the hybrid powertrains of the day. This development will naturally segue into development of active regenerative braking under anti-lock, stability, and split-m control regimes (Kim and Kim, 2006). This is in contrast to the current control strategy of switching regenerative braking off in these situations.

Finally, the physical limitations of the systems may be examined. These limitations are often peripheral to the function, and include such factors as tyre deformation and energy absorption, control at low vehicle speeds, component efficiencies, and the physical size, weight, and packaging of componentry. For all the benefits of regenerative braking, the fundamental principles of thermodynamics must still be given due regard – a joule of energy not used is worth more than a joule of energy returned.

17.6 Conclusions and future trends

Research continues into each of the architectures discussed in this chapter, and it cannot be said that any one architecture stands out above the others, as each architecture has its own strengths and weaknesses and is best suited to particular requirements. However, the key benefit of electrical systems in terms of energy density, can be combined with any other architecture to produce favourable results.

For instance, flywheel systems in automotive applications are presently best for short periods of high power – as best evidenced by their use as 'push-to-pass' boost systems in Formula 1. Whilst the technology is also energy dense, in order to get true long-range functionality, very large or very heavy flywheels must be used. However, this is not without its issues. For example, a chemical battery of 200 kg in a light vehicle poses no particular design challenge other than packaging, however a 200 kg mass spinning at some high RPM is associated with difficult dynamic problems. As such, flywheels may be used standalone in large vehicles, or combined with battery technology to give both high power and high energy density in a single vehicle.

Similarly, electrical systems relying on batteries are quite heavy and, whilst giving useable range, the batteries are not well-suited to high power demands. By combining these systems with supercapacitor or flywheel technology (Dixon and Ortúzar, 2002) it is possible to load-level the battery, increasing longevity and capturing more of the available braking energy.

Further, hydraulic systems are typically limited to heavy vehicles owing to their high power density but poor energy density. Hydraulic pump/motors using 'floating cup' principles have been recently designed to achieve greater than 90% efficiency (Vael et. al., 2009); however, whilst this brings the conversion efficiency up quite significantly (compared to swashplate designs), the fundamental problem of poor energy density is simply a matter of higher system pressures or higher fluid volumes. Both of these solutions add weight to the system.

The latest developments in electrical regenerative braking are being driven with strong research into battery technology, and motor/generator architecture. New battery constructions are being developed using sodium and lithium chemistries, and polymer electrolyte technology (Armand and Tarascon, 2008) to increase specific energy, specific power, packaging flexibility, which are currently the three major limitations to electric hybrid technology.

Lithium-air batteries promise specific energy levels similar to that of petrol used in traditional internal combustion (12 kWh/kg for Li-air as opposed to 13 kWh/kg for petrol). Currently, charge/discharge efficiency is still poor compared to Li-ion and the older Ni-Cd chemistries however, as well as

there being safety issues associated with the highly reactive nature of some of the materials being used.

Supercapacitors are also being investigated for use in hybrid and electric vehicles for their high specific power. Supercapacitors have high charge/ discharge rates but are not particularly energy-dense. This precludes their use as a main source of traction power as they would require a large volume in a vehicle in order to provide all the power requirements. However, their quick charge/discharge makes them ideal for use in combination with a battery pack. The supercapacitor can absorb a larger proportion of regenerated power during a braking event, storing it temporarily to charge the main traction battery over a longer time. This power could be discharged over a similarly short period of time if required for high power demands such as hill-climbing or high-acceleration manoeuvres.

Mechanical regenerative braking has still much progress to be made in flywheel materials, high-speed bearings, and transmission devices. At present the most efficient and commercially viable bearings for high speed systems are magnetic bearings. Magnetic bearings are contactless and able to run in a vacuum; however, they require external power to operate, and are relatively bulky due to this. Additionally, the magnetic bearing often requires a traditional bearing to support the shaft at low speeds. A recent development is the electrodynamic bearing (Filatov et al., 2006) which uses permanent magnets and eddy-currents to support a high speed load. The eddy-current provides a restorative force in the bearing to stabilise the spinning load where it becomes slightly eccentric, but when it is in its operating position no net force is applied (Sandtner and Blueler, 2004).

These developments in bearing technology, combined with new high-strength materials for flywheel construction will allow higher angular velocities and in turn shrink typical flywheel mass. This will allow the compact packaging and higher energy densities required for light vehicles. Additionally, new hermetic seals allow the flywheel to spin in a high-vacuum environment and remove the need for vacuum pumps (Cross and Brockbank, 2008). Keeping the vacuum sufficiently high can be viewed as a maintenance issue (the slow leak-by of atmospheric air may necessitate vacuum pumping the chamber periodically).

Already, flywheel systems can achieve up to 85% round-trip efficiencies (excluding transmission losses) using magnetic bearings and vacuums (Beacon Power, 2012). In terms of increasing efficiency the returns are very much diminishing for the effort, particularly when compared to efficiency improvements in other forms of regenerative braking. However, greater speeds, better bearings, and higher vacuums should yield some efficiency gains.

The main locus for efficiency improvement lies in the interconnecting transmission system. At present the best option for transmissions with flywheel systems is the CVT. However, the CVT offers poor transmission efficiency of around 80%. Typical multi-ratio clutched transmissions offer higher efficiency than a CVT in this application, at the expense of decreased resolution of input and output torque control as compared with CVT, due to the discrete step ratios that must be employed. An ideal solution in the laboratory is to match a turbine with a flywheel in a hybrid transmission. This would result in much narrower transmission ratios required, which could then utilise a standard gear transmission, and yield much higher efficiency.

Hydraulic systems stand to benefit from research into high-efficiency pump/motors and quick-response switching valves. Hydrostatic pump/motors typically have poor efficiency due to high internal friction and pumping losses, and hydrodynamic pump designs are not suited to the high system pressures required for these drive systems. Pumps, like any piston device, exhibit peak efficiency at a certain system pressure and operating speed. The best peak efficiencies are around 92% for floating cup variable displacement pumps; however, efficiency can be as low as 75% if not operating at ideal load conditions (Vael et al., 2009). However, unlike flywheels which must spin at very high RPM in order to deliver useful power, a pump/motor spins at similar angular velocity to the rest of an automotive powertrain. This means that keeping a pump/motor in the area of highest efficiency can be accomplished using relatively simple architecture. For instance, simply placing it on the output shaft of a multi-speed transmission already installed in a vehicle may be enough for most cases. This strategy is non-optimal, however, where low-speed operation is required, as both hydraulic pumps and internal combustion engines exhibit poor start-up characteristics. At low speed pumps are inefficient and also exhibit undesirable pulsation and noise, which is not necessarily a concern in heavy vehicles due to increased separation from the driver, but poses problems in the passenger vehicle market, where noise, vibration and harshness (NVH) is a key consideration. Finally, due to the large time constants associated with hydraulic systems, it is tricky to integrate them into any sort of torque-vectoring, active brake control, or stability control programme. They therefore require fully-functioning foundation brakes to be fitted in parallel in case of emergency.

Finally, related technologies have synergies that may be exploited to boost the energy savings made by regenerative braking. By linking GPS (already commonplace in production vehicles) and climate information with powertrain control systems, it is possible to optimise the energy flow for the particular operating environment dynamically. This could mean, for instance, that an energy buffer could be built prior to entering mountainous terrain or during hot weather. Conversely, during city driving, stored energy levels could be reduced to optimise the efficiency of powertrain components.

Regenerative techniques can be used in applications other than braking. A novel technique patented by researchers at Tufts University (Goldner and Zerigian, 2001) uses linear electric motor/generators in place of shock absorbers to generate electricity whilst damping the suspension system of heavy vehicles. Another team of researchers at Massachusetts Institute of Technology have patented another regenerative damping technique, using oil-filled dampers and passing the oil through a turbo-generator (Chandler, 2009).

17.7 Sources of further information and advice

Ehsani, M., Gao, Y. and Emadi, A., 2010, Modern Electric, Hybrid Electric, and Fuel Cell Vehicles: Fundamentals, Theory and Design, 2nd edn, CRC Press, Boca Raton, Florida USA, ISBN: 9781420053982.
Husain, I., 2005, Electric and Hybrid Vehicles: Design Fundamentals, CRC Press, Boca Raton, Florida USA, ISBN: 0849314666.
Mi, C., Abdul Masrur, M. and Gao, D.W., 2011, Hybrid Electric Vehicles: Principles and Applications with Practical Perspectives, John Wiley & Sons Ltd., Chichester UK, ISBN: 9780470747735.
Limpert, R., 2011, Brake Design and Safety, 3rd edn, SAE International, Warrendale, Pennsylvania USA, ISBN: 9780768034387.
Jurgen, R.K., 2010, Braking Systems and NVH Considerations, SAE International, Warrendale, Pennsylvania USA, ISBN: 9780768057201.

17.8 References

Achten, P., Vael, G., Sokar, M.I. and Kohmäscher, T., 2008, 'Design and Fuel Economy of a Series Hydraulic Hybrid Vehicle', Proceedings of the 7th JFPS International Symposium on Fluid Power, Toyama, Japan.
Armand, M. and Tarascon, J.M., 2008, 'Building Better Batteries', Nature International Weekly Journal of Science, 451, 652–657.
Beacon Power LLC, 2012, 'Fact Sheet Frequency Regulation and Flywheels', http:// www.beaconpower.com/files/Flywheel_FR-Fact-Sheet.pdf, retrieved 17/09/2012.
Boretti, A., 2011, 'Improvements of Vehicle Fuel Economy using Mechanical Regenerative Braking', International Journal of Vehicle Design, 55(1), 35–48.
Brembo S.p.A., 2012, Brembo Brake Circuit Identity Cards FORMULA 1 Home, http://formula1.brembo.com/index.php?lang=en, retrieved 15/09/2012
Chandler, D., 2009, *'*More Power From Bumps in the Road', MIT TechTalk, 53(15), p 4.
Cross, D. and Brockbank, C., 2008, 'Mechanical Hybrid System Comprising a Flywheel and CVT for Motorsport and Mainstream Automotive Applications', Proceedings of the 2009 SAE World Congress, Detroit, Michigan USA.
Curry, E., 2012, 'Design and Effectiveness of Regenerative Brake Systems for Electric and Hybrid Vehicles', Proceedings of the 2012 FISITA Eurobrake Conference, Dresden, Germany.
Dixon, J.W. and Ortúzar, M.E., 2002, 'Ultracapacitors + DC-DC Converters in Regenerative Braking Systems', IEEE Aerospace and Electronic Systems Magazine, 17(8), 16–21.
Ehsani, M., Gao, Y. and Emadi, A., 2010, Modern Electric, Hybrid Electric, and Fuel Cell Vehicles: Fundamentals, Theory and Design, 2nd edn, CRC Press, Boca Raton, Florida, USA.
Faul, B., Wunsch, C. and Morgan, J.E., 2012, 'The VE Elektro Mechanical Brake with Fully Integrated Actuators in a SUV Class Vehicle', Proceedings of the 2012 FISITA Eurobrake Conference, Dresden, Germany.
Filatov, A., McMullen, P., Davey, K. and Thompson, R., 2006, 'Flywheel Energy Storage System with Homopolar Electrodynamic Magnetic Bearing', Proceedings of the 10th International Symposium on Magnetic Bearings, Martigny, Switzerland.
Fry, C., 2009, *'*On the Grid for Green', Engineering & Technology, (10), 48–51.
Goldner, R.B. and Zerigian, P., 2001, 'Electromagnetic Linear Generator and Shock Absorber', United States Patent No. 6952060, filed May 7, 2001.
Hori, Y., 2002, 'Future Vehicle Driven by Electricity and Control Research on Four-Wheel Motored "UOT Electric March II",' Proceedings of the 7th International Workshop on Advanced Motion Control, Maribor, Slovenia.
Kim, D. and Kim, H., 2006, 'Vehicle Stability Control with Regenerative Braking and Electronic Brake Force Distribution for a Four-Wheel Drive Hybrid Electric Vehicle', Journal of Automobile Engineering, 220(6), 683–693.
Kluger, M. and Long, D., 1999, 'An Overview of Current Automatic, Manual, and Continuously Variable Transmission Efficiencies and their Projected Future Improvements', Proceedings of the 1999 SAE World Congress, Detroit, Michigan USA.
Liu, H. and Jiang, J., 2007, 'Flywheel Energy Storage An Upswing Technology for Energy Sustainability', International Journal of Energy and Buildings, 39, 599–604.
Moglestue, A., 2008, Gyrobus: A Great Idea Takes a Spin, http://photo.proaktiva.eu/ digest/2008_gyrobus.html, retrieved 15/9/2012.
Rydberg, K., 2009, 'Energy Efficient Hydraulic Hybrid Drives', Proceedings of the 11th Scandinavian International Conference on Fluid Power, Linköping, Sweden.
Sandtner, J. and Blueler, H., 2004, 'Electrodynamic Passive Magnetic Bearing with Planar Halbach Arrays', Proceedings of the 9th International Symposium on Magnetic Bearings, Kentucky, USA.
Tawadros, P., Zhang, N. and Boretti, A., 2012, 'Quantitative Experimental Study of HEV Regenerative Braking Systems', Proceedings of the 2012 FISITA Eurobrake Conference, Dresden, Germany.
Toyota Motor Corporation, 2012, Toyota Hybrid System Diagnosis Course 072, TMC Training Document.
UNECE (United Nations Economic Commission for Europe), 2011, 'Agreement Concerning the Adoption of Uniform Technical Prescriptions for Wheeled Vehicles, Equipment and Parts which can be Fitted and/or be Used on Wheeled Vehicles and the Conditions for Reciprocal Recognition of Approvals Granted on the Basis of These Prescriptions', UN ECE Regulation 13H, Geneva.
US Department of Transportation, 2005, FMVSS 135 Light Vehicle Brake Systems, Washington DC, USA.
Vael, G.E.M., Achten, P.A.J. and Van Den Brink, T., 2009, 'Efficiency of a Variable Displacement Open Circuit Floating Cup Pump', Proceedings of the 11th Scandinavian International Conference on Fluid Power, Linköping, Sweden.
Zhang H., Yu, X. and Braun P., 2011 : 'Three-Dimensional Bicontinuous Ultrafast-Charge and Discharge Battery Electrodes', Nature Nanotechnology, 6(5), 277–281.

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Part III

Electric/hybrid vehicle technologies

T. Hofman, Eindhoven University of Technology, The Netherlands

DOI: 10.1533/9780857097422.3.567

Abstract: This chapter provides a classification of electric hybrid systems for cars and describes the conflicting design challenges involved in designing advanced vehicle propulsion systems. In addition, the chapter provides an analysis of the solution methods currently provided in literature on the coupled control and system design level.

Key words: hybrid vehicles, optimization, multi-objective design, control, efficient vehicles, electric machine, battery.

18.1 Introduction

Ongoing emission legislation and increasing fuel prices have forced many leading vehicle manufacturers and their suppliers to develop and manufacture more efficient and cost-effective drive train technologies. Hybrid drive train technologies are considered to be short- and mid-term solutions in reducing automobile fuel consumption and emissions without compromising vehicle performance. These new types of vehicle propulsion systems combine two power converters, usually an internal combustion engine and an electric motor (Hofman, 2007).

Due to the variety of types of vehicles and their usage, there have been many types of hybrid propulsion systems. Many researchers have also developed sophisticated control systems to optimize the power distribution between the two power sources, and these 'energy management' systems are reviewed in Sciarretta and Guzzella (2007). Over the last two decades, different modelling and simulation tools for optimization of hybrid vehicle propulsion systems have been trialled; however, these tools require complex and lengthy computation times due to the complexity of the problems addressed.

This chapter provides an overview of different hybrid drive train solutions and examines how they can be classified as a reflection of their hybrid functionalities (Hofman, 2007). This chapter also addresses the hybrid (electric) powertrain design problem and provides a review of the (mathematical) methods currently found in literature (Hofman, 2007; Hofman and Steinbuch, 2014).

Over the last two decades, EU nations have placed focus on tightening the pollutant limits (see Fig. 18.1(a)), while the regulation of CO2 emissions

18.1 EU emission limits and GHG emission targets (a) evolution of the NOx and PM emission limits (g/km) for light duty diesel engines in Europe and (b) comparison of CO2 emission targets, expressed as (g/km) for the NEDC cycle; the series includes historical performance, enacted values for 2015 and proposed targets (Payri et al., 2012).

only began in 2009. Figure 18.1(b) shows the historical records and the emission targets for cars in several world regions (Payri et al., 2012). From this chart we can see that the 2020 target for CO2 emissions in the EU is 95 g/km, which is similar to a reduction of 25%.

In general, the average CO2 emissions of passenger cars over the last decade has decreased; however, the CO2 reductions in new conventional vehicles is tempered by the increasing trend in the average vehicle mass and the maximum engine power. The vehicles become heavier as a result of the increasing demand for vehicle safety and comfort, but at the same time vehicle performance must not be compromised. For example, the average vehicle mass and engine output power of new passenger cars in The Netherlands increased from 1998 to 2005 by 11% and 20%, respectively. The 'cost' of the increased average vehicle mass reduces the 'benefits' of the development of more fuel economic (downsized) engines and more aerodynamic vehicles; however, it should be noted that a full-electric drive train increases the weight of the vehicle even further (20–30% for the same vehicle in its class), yet the energy loss (due to the higher vehicle loads) is compensated by the lower conversion losses from battery to wheels compared to classical combustion engine propelled vehicles (Rijksdienst voor her Wegverkeer (RDW), European Committee (2006)).

Advanced hybrid vehicle propulsion systems (including electric vehicles with range extenders) are very promising technologies for significant fuel consumption reduction and for reducing emission, without compromising vehicle performance (safety, comfort, drivability, range, etc.). This is discussed in the following section.

18.2 Hybrid vehicle configurations and classification

Hybrid vehicles such as the Citroën C3, Honda Civic IMA, Toyota Prius, and Lexus RX400h are readily available, and some of these vehicles have almost the same 'hybrid functions'. These functions include engine off during vehicle standstill (start-stop function), brake energy recuperation (BER), motor-assisting, charging during driving, and pure electric driving. However, the drive train topology, engine size, electric machine size, transmission technology, and control system of these hybrid vehicles are all completely different. Hybrid vehicles are categorized in the following three main types: series (e.g. Karma Fisker), parallel (e.g. Honda Civic IMA), or series-parallel hybrid configuration (e.g. Toyota Prius, or Chevrolet Volt). Figure 18.2 visualizes schematically these different configurations and describes the main topological characteristics of how the power sources are connected to the drive train.

Since the start of the twentieth century, (Koehler and Oehlers, 1998) 'series hybrid configurations' (Fig. 18.2, top) have been successfully used in other engine-powered machines (1899), such as diesel-electric powered ships (1903), and trains (1913). An advantage of a series hybrid is that the engine operates at its maximum efficiency point(s) thanks to the buffering of excess power; however, one disadvantage is the relatively low transmission efficiency at relatively high vehicle loads compared to other hybrid configurations. Unless the vehicle is used mainly for city driving (frequent use and fairly low power demands), relatively large electric machines (kW) are required to transmit power from the battery and engine to the vehicle wheels at high vehicle speeds. Often an internal combustion engine is used as generator, and in the case of the Mercedes B concept a fuel cell is used as a primary power source.

The 'parallel hybrid configuration' (Fig. 18.2, bottom) has a higher transmission efficiency due to the mechanical connection between the engine, electric machine and vehicle wheels, and the engine and electric machine

18.2 Different hybrid drive train configurations (Hofman, 2007).

supply the propulsion power very efficiently. The 'series-parallel configuration' (Fig. 18.2, middle) is a combination of the parallel and series configurations, which benefits from most of the advantages of both configurations. The engine propulsion power is divided into two power-flows, which are transmitted to the wheels over a mechanical and an electrical branch. The parallel and series-parallel configurations can be classified as a micro-hybrid up to a fullhybrid vehicle, and the transition from 'micro'- to 'full'-hybrid indicates an increase in the number of hybrid functions. Typically, the series configuration performs all the hybrid functions and, as a result, falls into the full-hybrid class.

Figure 18.3 lists some typical hybrid system specifications for different hybrid vehicle classes. As an example, the figure shows a number of different hybrid vehicles which fall inside different hybrid vehicle classes, including the Citroën C3, Honda Civic IMA, Toyota Prius, Fisker Karma, and Chevrolet Volt. The top part of Fig. 18.3 shows the estimated range of the vehicle as a function of the (typical) storage capacity for different vehicle classes. A hybrid vehicle or electric vehicle with a range extender is capable of reaching ranges of 500–800 km, while a full-electric vehicle (depending on the battery capacity) reduces this by approximately 30–50%. An increase of hybridization (micro-mild-full), where the maximum engine and electric machine power decreases and increases, respectively causes reduced fuel consumption and CO2-emissions (by 3–10%, 15–25%, 20–30%, respectively). Moreover, the full-electric vehicles have, in principle, a zero-emission range (100% fuel

18.3 Hybrid and electric vehicle classification.

savings). In the bottom part of Fig. 18.3 it is shown how the component sizes are a function of the system voltage. In principle, all vehicles shown have a system voltage lower than 500 V; therefore, the larger battery capacity of the electric vehicles is caused by the larger number of strings of battery cells/modules connected in parallel.

Generally, it can be concluded that there is a large variety of hybrid vehicles. However, fuel saving potential and the additional cost of hybrid vehicles strongly depends on the drive train configuration and the degreeof-hybridization (sizes of the power sources). Selection of the right solution depends on both the functional requirements and the projected production volumes related to the manufacturing investment. Hybrid systems that can be added on a conventional transmission may provide the most benefit for the least cost, depending on the amount of change required (van Berkel et al., 2012).

Plug-in hybrid electric vehicles such as the Toyota Prius and the Chevrolet Volt are equipped with a larger battery than a usual hybrid vehicle, allowing range extension without any local polluting emissions. As a result, these vehicles are seen as promising future solutions for transport in cities and suburbs (Toledo et al., 2006; Winkel and van Mieghen, 2006). After depleting

the secondary power source, the control strategy switches to charge sustaining mode using the primary power source (an internal combustion engine or a fuel cell). Figure 18.4 shows this control strategy as a function of the battery state of charge, vehicle speed and range. In this example, at low speeds (below 50 km/h) and with sufficient battery charge capacity (above 20%) the vehicle is driven purely on electrical power.

The required drive energy could be charged overnight when the demands are low, resulting in a load-levelling effect on the electric grid, and the idle generating capacity during off-peak hours would be brought into productive use. Allowing plants to operate with less variability and closer to optimum output could enhance the overall efficiency of the electrical system. The introduction of these relatively new types of hybrid vehicles to the market depends on the progress in the development of new batteries, which tolerate frequent and fast load changes. Moreover, the additional production costs of the electrical components strongly dictate when these types of hybrids can be introduced into the market.

The following section gives an overview of the design problem areas behind the hybridization of a vehicle propulsion system.

18.3 The challenges of hybrid vehicle design

Let us denote the internal combustion engine as the primary power source (P) and the battery system and the electric machine as the secondary power source (S). Hybridization implies adding S to P in order to improve the vehicle's overall performance. The vehicle (including the drive train), which is denoted as V, is defined as an energy source that stores kinetic energy during vehicle acceleration, and retrieves it during braking. The kinetic energy retrieved during braking is stored in the battery of S for later use. The brake energy storage is done at moments where the road load losses (air drag, roll resistance losses) are not sufficient enough to decelerate the

18.4 Energy management strategy of a plug-in hybrid vehicle (example: Mercedes S400 concept).

vehicle as desired. Usually, for vehicle certification and comparison reasons, the acceleration, braking, and steady-state driving periods of the vehicle are prescribed by a modal standardized drive cycle such as the New European Drive Cycle (NEDC). A drive cycle is a series of data points representing the speed of a vehicle versus time. A model at a high level of abstraction for the hybrid vehicle propulsion system is schematically visualized in Fig. 18.5. In Fig. 18.5, S is parallel connected with P at the primary- or engine-side of the transmission and the main mechanical transmission part is a push-belt continuously variable transmission (CVT). The transmission technology between P, S, and V, which may also include a wet-plate clutch, a torque converter, or a set of gear wheels, is denoted as T.

One of the reasons for the reduction in fuel consumption and emissions for a hybrid vehicle tested on a drive cycle is due to the fact that P is shut down during vehicle standstill (start-stop) and certain driving periods, which eliminates the idle and part load losses. Accordingly, S is used for restarting the engine quickly or to propel the vehicle. The latter driving mode (where the vehicle is only propelled by the secondary power source S and the engine P is shut off) is defined as the (electro-)motor-only (M) mode. Both hybrid functionalities (i.e. start-stop and propulsion-only by S) depend on the power specifications of S, and the energy needed for this is mainly recovered during braking; this driving mode is defined as the brake energy recovery (BER) mode.

In this section the balance of energy as depicted in Fig. 18.6 is often used. In Fig. 18.6, the 'energy blocks' on the energy balance represent the gross energy content at the input of S, where the discharged energy exactly

18.5 A model at a high level of abstraction for the hybrid vehicle propulsion system. In this topology example the secondary power source S is coupled at the engine side of the transmission T (Hofman, 2007).

18.6 Energy balance for the secondary power source S (Hofman, 2007).

counterbalances the charged energy. Due to storage, conversion, transmission losses, or by component power specifications, the total (dis-)charged energy at the output of S may not be balanced.

The energy-flow for each hybrid mode over a whole drive cycle in a parallel hybrid vehicle propulsion system is schematically visualized in Fig. 18.7. It is important to note that the (dis-)charge hybrid modes are not necessarily limited to the combinations shown in Fig. 18.7, but nevertheless, the figure provides a schematic overview of a realistic energy management strategy. The propulsion-only mode by P (Fig. 18.7(a), top) is also defined as the engine-only (E) mode, which is similar to a conventional vehicle, however, in this mode the secondary power source (S) is not used.

The fuel savings and costs increase with each hybrid mode from Fig. 18.7(b)-(d) and the BER and the M mode are depicted in Fig. 18.7(b). It is important to note that the energy needed for restarting P in the figure has been omitted, since the engine-restart energy is relatively small. The M mode results in 'fuel saving', as mentioned before, due to engine shut-off during certain driving periods. Using charged energy (which has been retrieved from P during driving) for propulsion-only by S may save further fuel (even in the absence of stored kinetic energy, thus without utilizing the BER mode). This mode, as shown in Fig. 18.7(c), is defined as the charging 1 (CH1) mode.

The charged energy represents a certain 'fuel cost', as the charged energy is delivered by the engine. Furthermore, the use of S during vehicle operation in combination with P further optimizes the energy exchange between P, S, and V, and the energy management strategy behind this is to charge at moments where a large amount of accumulator energy costs only a relatively small amount of fuel (fuel cost). This energy can then be used at moments where it is beneficial to assist P with S, i.e. where a small amount of energy can save a relatively large amount of fuel (fuel savings). These (dis-)charging conversion ratios are also referred to as the 'incremental fuel costs' (or savings). The hybrid driving modes where the charged energy is used over a drive cycle for motor-assisting are defined as the CH2 and the MA2 modes, respectively (Fig. 18.7(d), bottom).

18.7 Energy-flows over a whole drive cycle in a hybrid vehicle propulsion system (a) propulsion-only by P (b) propulsion-only by S (with BER = charged energy retrieved from V during braking) (c) propulsion-only by S (with CH1 = charged energy retrieved from P during driving) and (d) hybrid propulsion by S and P (Hofman, 2007).

In summary, the 'fuel saving potential', which is defined as the difference between the 'fuel savings' and the 'fuel costs', is determined by the vehicle, the drive cycle, the component sizes (e.g. the ability for propulsion-only by S), the component efficiencies, and the path efficiencies between the power sources. The 'fuel savings' are defined as the difference between the fuel consumption without and with the use of S for propulsion (pure electric driving) or during driving (motor assisting). Thereby, one of the design problems (also referred to as the 'energy management problem') is:

∑ how to determine the optimal power distribution between the power sources P and S among the different driving modes during vehicle operation.

The word 'optimal' refers to an as yet undefined criterion. In Koot et al. (2005) the minimization of fuel consumption and/or emissions is used, and the boundary condition requires a balanced charged and discharged energy over a drive cycle. This condition is used for vehicle comparison and certification reasons (similar to the use of standardized drive cycles). The component efficiencies and the path efficiencies between the power sources are affected, respectively by the component sizes, technologies, and the drive train topology. As a result, in addition to the above-mentioned control design problem the following design problems can be formulated (see also Fig. 18.8):

∑ how to determine the optimal hybrid drive train topology and moreover, given the topology
∑ how to find the optimal sizes of the hybrid components
∑ how to select the optimal hybrid component technologies.

The drive train topology determines where and how the power sources are connected to the drive train. As a result, the topology also determines where the energy losses occur during energy exchange between the energy sources.

18.8 Interlinked design problem areas (Hofman, 2007).

This also applies to the selection of a front- (FWD), a rear- (RWD), or an allwheel driven (AWD) vehicle, which implies a different topology choice.

Figure 18.9 provides an overview of some example topologies. The power sources are usually able to operate simultaneously and therefore adding a secondary power source S reduces the power specifications of the primary power source P for propulsion. Moreover, in the case of an engine it reduces the volumetric displacement and therefore the pumping, compression, and friction losses. The optimal size of the power sources is a trade-off between maximizing the average efficiency of the primary power source P and the hybrid propulsion system. This is performed by down- and up-sizing of the primary P and secondary power source S, respectively; however, the solutions are usually limited by vehicle performance constraints such as maximum vehicle acceleration, speed, or hill climbing specifications, and, therefore, the total combined maximum (peak or continuous) output power of the power sources.

18.4 Solutions to the design problem

The design of a hybrid vehicle propulsion system is a complex task (Hofman, 2007). The problem is often multi objective (for example, minimizing emissions

18.9 Hybrid vehicle drive train topologies (Hofman, 2007).

and maximizing acceleration performance while minimizing system and component costs) and requires solving control and design problems that have continuous and discrete dynamics and static or variational variables (Hofman and Steinbuch, 2014). In the literature the most common case studies feature a fixed technology or topology choice and the size of components is also chosen as the main design variable. This simplifies the problem at hand, yet may still impose difficulties and challenges related to the control design.

Analytical methods based on optimal control theory (Pontryagin's Maximum Principle (Kirk, 1970)) are very efficient for solving the control problem for a class of hybrid vehicle systems (Delprat et al., 2004; Serrao et al. 2011; Sciarretta et al., 2004; Sciarretta and Guzzella, 2007; Koot et al., 2005). However, difficulties arise when discrete state variables such as engine on/off, or gear position with a stepped gear box are included in the design problem (Ngo et al., 2012; Hofman et al., 2012). Practically, these problems are circumvented by calculating, for example, separate Hamiltonian functions (or values) related to each discrete controlled state; there these are individually and accordingly solved for the admissible control input, which minimizes this set of Hamiltonian functions (Ambühl et al., 2010). Inherently, singular solutions for the optimal control may occur with this method (Wei et al., 2007), and heuristics is required in choosing a particular mode, which results in a suboptimal solution.

The control problem is nonconvex, and often the engine on/off and gear position state are optimized a priori in this way or by using simplified rules. The remaining control problem and constraints are sometimes convexified to speed up the design process (Murgorsti et al., 2011).

Some authors tend to address the combined control and propulsion system design (Murgovsti et al., 2011; Patil, 2012; Sinoquet et al., 2011), and often the design space related to technology choice or topology variation is limited to one or two main possibilities of reducing the design space. This process of coupled system and control optimization can be done utilizing a sequential, iterative, bi-level or simultaneous strategy (Fathy et al., 2001).

In Filipi et al. (2004) the vehicle system design and control are optimized in an iterative manner: first, the independent system design parameters are selected using a Latin-Hypercube Sampling (LHS) method and given a (heuristic) controller, so the fuel economy (subject to performance constraints) is optimized. Accordingly, at the optimal set of design parameters for the propulsion system the control rules for the system are optimized using dynamic programming (DP) (Bertsekas, 2000). In a second iteration, the optimal set of design parameters is again found using the updated controller. The process converges until a stopping criteria relating to the relative improvement is satisfied.

The control and system design problems are coupled and sequential or iterative strategies fail to guarantee system-level optimality (Fathy et al., 2001). Additionally, so far in literature bi-level or simultaneous strategies that have been applied to the hybrid vehicle system design are limited. In Ebbesen et al. (2013) the multi-objective design problem is nested; for every feasible system design (topology and technology fixed) the control design is optimized separately using DP. In Assanis et al. (1999) a multi-objective co-design (control and propulsion system) for a parallel hybrid problem was also addressed instead using a rule-based control strategy (ADVISOR) to limit the achievable control performance. The nonlinear Pareto frontier is computed and the sensitivity of the main design parameters to the targets is systematically investigated. Particle swarm optimization (Ebbesen et al., 2012), generic algorithms, and deterministic search methods, such as the Nelder–Mead simplex algorithm were also suggested as efficient search methods of solving optimal sizing problems; yet the authors in Ebbesen et al. (2013) claim computing the separate objective values for different sizes is more efficient, since the sizing problem can be solved repeatedly with minimal computational effort for any combination of weights.

18.5 Conclusion

Towards the development of the ultimate eco-friendly car many different solutions are being developed in parallel, ranging from hybrids, plug-in hybrids towards pure electric vehicles. This chapter describes an overview of classification of these type of systems and describes the conflicting design challenges involved in designing advanced vehicle propulsion systems. In addition, an overview is given on solution methods found in the literature on the coupled control and system design level. The design problem is expanding and so far no single superior solution method can be found in the literature which puts challenges in developing suitable design methods and tools reducing development time and costs towards the future.

18.6 References

Ambühl, D., Sundström, O., Sciarretta, A. and Guzzella, L. (2010) 'Explicit optimal control policy and its practical application for hybrid electric powertrains,' Control Engineering Practice, 18(12), 1429–1439.
Assanis, D., Delagrammatikas, G., Fellini, R., Filipi, Z., Liedtke, J., Michelena, N., Papalambros, P., Reyes, D., Rosenbaum, D., Sales, A. and Sasena, M. (1999) 'An optimization approach to hybrid electric propulsion system design,' Journal of Mechanics of Structures and Machines, Automotive Research Center Special Edition, 393–421.
Bertsekas, D. (2000) Dynamic Programming and Optimal Control, Athena Scientific, Belmont.
Delprat, S., Lauber, J., Guerra, T. and Rimaux, J. (2004) 'Control of a parallel hybrid powertrain: optimal control,' IEEE Transactions on Vehicular Technology, 53(3), 872–881.
Ebbesen, S., Donitz, C. and Guzzella, L. (2012) 'Particle swarm optimisation for hybrid electric drive-train sizing,' International Journal of Vehicle Design, 58(2/3/4) 181–199.
Ebbesen, S., Elbert, P. and Guzzella, L. (2013) 'Engine downsizing and electric hybridization under consideration of cost and drivability,' Oil & Gas Science and Technology Review, 68(1), pp. 109–116.
Fathy, H., Reye, J., Papalambros, P. and Ulsog, A. (2001) 'On the coupling between the plant and controller optimization problems,' in Proceedings of American Control Conference, ACC, Arlington, VA, USA, pp. 1864–1869.
Filipi, Z., Louca, L., Daran, B., Lin, C.-C., Yildir, U., Wu, B., Kokkolaras, M., Assanis, D., Peng, H., Papalambros, P., Stein, J., Szkubiel, D. and Chapp, R. (2004) 'Combined optimization of design and power management of the hydraulic hybrid propulsion system for the 6x6 medium truck,' International Journal of Heavy Vehicle Systems, 11(3/4), 372–402.
Hofman, T. (2007) 'Framework for combined control and design optimization of hybrid vehicle propulsion systems,' Ph.D. thesis, Eindhoven University of Technology.
Hofman, T. and Steinbuch, M. (2014) 'Topology optimization of hybrid powertrains,' Optimization and Optimal Control in Automotive Systems, Springer, (to appear).
Hofman, T., Ebbesen, S. and Guzzela, L. (2012) 'Topology optimization for hybrid electric vehicles with automated transmissions,' IEEE Transactions on Vehicular Technology, 61(6), 2442–2451.
Kirk, D. (1970) Optimal control theory: an introduction, Prentice-Hall, Englewood Cliffs.
Koehler, H.W. and Oehlers, W. (1998) 95 Years of diesel-electric propulsion form a maekeshift solution to a modern propulsion system, 2nd International Diesel-Electric Propulsion Conference, Helsinki, Finland, pp. 1–24.
Koot, M.W.T. et al. (2005) Energy management strategies for vehicular power systems IEEE Transactions on Vehicular Technology, 54, 771–782.
Murgovski, N., Johannesson, L., Hellgren, J., Egardt, B. and Sjöberg, J. (2011) 'Convex optimization of charging infrastructure design and component sizing of a plug-in series HEV powertrain,' in Proceedings of the 18th World Congress The International Federation of Automatic Control, IFAC, Milano, Italy pp. 13 052–13 057.
Ngo, V., Hofman, T., Steinbuch, M. and Serrarens, A. (2012) 'Optimal control of the gearshift command for hybrid electric vehicles,' IEEE Transactions on Vehicular Technology, 61(8), 3531–3543.
Payri, F. Lujàn, J. M. Guardiola, C. and Pla, B. (2012) 'A challenging future for the IC engine: new technologies and the control role', IFAC Workshop on Engine and Powertrain Control, Simulation and Modeling (ECOSM12), Rueil-Malmaison, France, 2012.
Patil, R. M. (2012) 'Combined design and control optimization: Application to optimal PHEV design and control for multiple objectives,' Ph.D. dissertation, University of Michigan.
Rijksdienst voor het Wegverkeer (RDW), European Committee (2006) CO2-emissie per voertuigkilometer van personenauto's, 1998–2005 http://www.mnp.nl/mnc/i-nl-0134. html.
Sciarretta, A. and Guzzella, L. (2007) Control of hybrid electric vehicles, IEEE Control Systems Magazine, 27(2), 60–70.
Sciarretta, A., Back, M. and Guzzella, L. (2004) 'Optimal control of parallel hybrid electric vehicles,' IEEE Transactions on Control Systems Technology, 12(3), 352–363.
Serrao, L., Sciarretta, A., Grondin, O., Chasse, A., Creff, Y., di Domenico, D., Pognant-Gros, P., Querel, C. and Thibault, L. (2011) 'Open issues in supervisory control of hybrid electric vehicles: a unified approach using optimal control methods,' in Les rencontres scientifiques dIFP energies nouvelles internat scient conf hybrid and electric vehicles, Rueil-Malmaison, France.
Sinoquet, D., Rousseau, G. and Milhau, Y. (2011) 'Design optimization and optimal control for hybrid vehicles,' Optimization Engineering, 12, 199–213.
van Berkel, K. et al. (2012) Optimal control of a mechanical-hybrid powertrain, IEEE Transactions on Vehicular Technology 61 (12), 485–497.
Toledo, F. (et al.) (2006) Charging systems for tomorrow's plug-in hybrids and electric vehicles, Proceedings of the 22nd International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium, Yokohama, Japan, pp. 2236–2245.
Wei, X., Guzzella, L., Utkin, V. and Rizzoni, G. (2007) 'Model-based fuel optimal control of hybrid electric vehicle using variable structure control systems,' Journal of Dynamic Systems, Measurement, and Control, 129(13).
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Battery technology for CO2 reduction

N. M. Johnson, Ricardo, Inc., USA

DOI: 10.1533/9780857097422.3.582

Abstract: This chapter focuses on battery design and the opportunities of CO2 reduction in battery usage for transportation applications. Battery functionality and the various chemistries available, including lithium ion, are discussed and this is followed by a description of high voltage battery pack design and battery management systems. An outline of future trends concludes the chapter.

Key words: battery, battery design, battery functionality, battery chemistry, battery management systems, CO2 reduction, fuel economy, electric, EV, high voltage battery pack design, hybrid, lead acid, lithium, plug-in.

19.1 Introduction

In this chapter, battery design and function for CO2 reduction is discussed. In general, this chapter focuses on electrified passenger cars, but the ideas can be readily applied to other disciplines. This chapter begins with a review of CO2 reduction opportunities through the use of batteries and then proceeds through a review of battery functionality followed by an overview of several chemistries used in transportation, including a more detailed look at lithium ion chemistries. This leads to a discussion of the high voltage battery pack and battery management system requirements. Finally, the chapter will conclude with a look at future trends.

19.1.1 Other potential battery benefits

It is worth noting that there are additional reasons to utilize batteries in vehicles that are beyond the scope of this chapter on CO2 reduction. One significant benefit is reduction of vehicle emissions that are short-lived (compared to CO2). These include emissions such as soot, carbon monoxide, nitrous oxides and hydrocarbons. These emissions are contributors to human health problems, are often regulated and are increasingly significant in urban areas.

Another potential benefit is noise reduction through vehicle electrification. Commercial tasks in urban areas such as road construction and refuse collection are difficult to perform during working hours due to the quantity of vehicles on the road. These tasks can be moved into the pre-dawn hours if vehicles are sufficiently quiet. Lastly, heavily electrified vehicles may have lower maintenance costs than traditional vehicles in certain applications.

582

19.1.2 Definitions

This chapter warrants some particular attention to a few definitions. For the purposes of this chapter, the following definitions will be used:

∑ Cell the simplest device that can act as a battery, an electrochemical device with one voltage step. Cells are typically housed in an environmentally protective container that contains only one cell. 12 V lead acid car batteries and 9 V alkaline consumer batteries are exceptions where multiple cells are packaged in a single container.
∑ Module a collection of cells with electrical connectivity and additional structural support. A module may also include thermal control components and control electronics.
∑ Battery pack a group of cells or modules combined within an enclosure that provides the required protection from both the environment and people. The battery pack usually includes additional thermal control system components, additional structural components and a battery management system to control safety and performance of the battery pack.

Battery – a generic reference to any electro-chemical energy storage device, including cells, modules, and battery packs.

19.2 CO2 reduction opportunities of using batteries

An understanding of battery technology for electrified vehicles requires both an understanding of the desired performance as well as their capabilities and limitations. It is instructive to use passenger vehicle drive cycle characteristics and the resulting battery demands in various operating modes. This section will include a review of passenger car driving cycles, including power levels, durations and duty cycles. This will be followed by an overview of three broad electrified powertrain strategies used for CO2 reduction and how each powertrain strategy drives battery requirements. The powertrain strategies are:

∑ idle reduction (start/stop)
∑ separation of engine response from driver demand
∑ fuel displacement by plugging in.

19.2.1 Introduction

In the simplest view point, CO2 reduction for a given trip is only possible in one of three ways:

reduce vehicle energy required to accomplish the trip

2. increase fuel conversion efficiency during the trip
3. displace carbon fuel energy with another energy source.

Improvements in all three areas can be accomplished with a battery when used in combination with one or more electric machines. It should be said that a battery is not the only means of CO2 reduction. Smaller, lighter vehicles and/or smart infrastructure such as timed lights can be used to reduce energy consumption for a given trip. Improved engine designs and high gear count transmissions can improve fuel conversion efficiency. Lastly, the use of plantderived fuels can lower net CO2 generation for carbon based fuels. However, electrification can further reduce CO2 emissions in many driving cycles.

It is also worth noting that displacing vehicle fuel through the use of electricity from the power grid does not guarantee CO2 reduction due to the variety of sources of electricity. It does, however, provide a pathway for significant CO2 reduction as power plants continue to modernize and are mixed with generating sources that do not use fossil fuels.

19.2.2 Review of passenger car driving cycles

A review of passenger car driving cycles provides a framework for discussing vehicle operating modes and the CO2 reduction opportunities in each mode. Driving cycles from the United States are used for the majority of this discussion because they closely resemble real world driving. Key characteristics of US driving cycles and select international driving cycles are provided in Table 19.1.

No driving cycle encompasses the full performance expectations of a modern vehicle, but they are instructive for understanding powertrain demands, and thus the demands on a battery used to reduce CO2 production from the powertrain. Figures 19.1–19.3 show speed versus time for the US FTP72, US Highway and US06 cycles representing city, rural highway and high speed motorway driving, respectively. Time and speed axis scales are held constant across all three figures to allow easier comparison.

Initially, the functional requirements of a battery pack seem very simple. At the highest level, a battery pack is simply an energy storage device that provides (or accepts) power when requested. The energy content and power capabilities of the battery pack are the two primary design boundaries and are the primary characteristics that determine CO2 reduction capabilities. The required power and energy are a result of the desired functionality of the electrified vehicle design.

The United States Council for Automotive Research (USCAR 2012) is a cooperative research and development organization whose membership includes Ford, GM and Chrysler. The organization's United States Advanced Battery Consortium (USABC) subgroup has developed numerous battery test

Table 19.1 Summary of key statistics of common global driving cycles

Dr ive le c y c na m e	Du io t ra n ( ds ) se co n	Le h ( km ) t ng	M im ax um d sp ee ( km / hr )	Av er ag e d sp ee ( km / hr )	M im ax um le io t ac ce ra n 2) ( / m se c	M im ax um de le io t ce ra n 2) ( / m se c	I d le im t e @ 0 d sp ee % ( )	Lo i d le t ng es ( ds ) se co n
U S F T P 7 2	1, 8 7 4	1 7. 8	9 1. 2	3 4. 1	1. 5	–1 5	1 9	3 9
U S U D D S	1, 3 6 9	1 2. 0	9 1. 2	3 1. 5	1. 5	–1 5	1 9	3 9
U S H ig hw ay	7 6 5	1 6. 4	9 6. 0	7 7. 2	1. 6	–1 5	1	5
U S 0 6	6 0 0	1 2. 9	1 2 9. 2	7 7. 2	3. 8	–3 1	7	8
U S N Y C C	5 9 8	1. 9	4 4. 6	1 1. 4	2. 7	–2 6	3 5	4 2
U S H D U D D S	1, 0 6 0	8. 9	9 3. 3	3 0. 3	2. 0	–2 1	3 3	6 6
U S S C 0 3	6 0 0	5. 8	8 8. 2	3 4. 5	2. 3	–2 7	1 9	2 4
U S I M 2 4 0	2 4 0	3. 2	9 1. 2	4 7. 1	1. 5	–1 6	5	5
C A R B L A 9 2	1, 4 3 5	1 5. 8	1 0 8. 1	3 9. 6	3. 1	–3 9	1 6	3 0
C A R B L A 9 2– S ho t r	9 6 9	1 1. 3	1 0 8. 1	4 1. 8	3. 1	–3 9	1 8	3 0
U N E C E E le Ur ba ta m en ry n	1 9 5	1. 0	5 0. 0	1 8. 3	1. 1	–0 8	3 3	2 2
U N E C E Ex Ur ba tra n	4 0 0	7. 0	1 2 0. 0	6 2. 4	0. 8	–1 4	1 0	2 1
U N E C E Ex Ur ba L P V tra n	4 0 0	6. 6	9 0. 0	5 9. 3	0. 8	–1 4	1 0	2 1
Ja 1 0– M de p an o	1 3 5	0. 7	4 0. 0	1 7. 6	0. 8	–0 8	2 9	2 1
Ja 1 5– M de p an o	2 3 1	2. 2	7 0. 0	3 3. 7	0. 8	–0 8	3 3	6 6
Ja 1 0– 1 5 M de p an o	8 9 2	6. 3	7 0. 0	2 5. 6	0. 8	–0 8	3 3	6 7

CARB = California Air Resources BoardECE = Economic Commission for Europe FTP = Federal Test ProcedureIM = Inspection and Maintenance LA = Los Angeles LPV = Low Powered Vehicle

NYCC = New York City Cycle

SC = Supplemental Cooling

UDDS = Urban Dynomometer Driving Cycle

UN = United Nations

US = United States.

19.1 US federal test procedure (FTP) 72 driving cycle.

19.2 US highway driving cycle.

procedures and has continued to set goals for battery energy storage systems on vehicles. Table 19.2 is a summary of those goals and provides a broad overview of the demands on battery systems across a wide array of vehicle designs.

For large batteries, such as electric vehicle (EV) batteries, the primary performance goal is energy per weight and per volume. Small batteries,

19.3 US 06 driving cycle.

such as hybrid electric vehicle (HEV) batteries, place a greater emphasis on power per weight and per volume. In all cases, the batteries must operate across a wide temperature range from roughly –40 °C to +60 °C and meet cost and reliability targets. Most importantly, all requirements must be accomplished while meeting the rigorous safety and life/durability demands of the transportation industry. The following sections will look at the three aforementioned powertrain strategies and evaluate the demands placed on the battery.

Idle reduction (start-stop, micro-hybrid)

The first operating strategy described above is the implementation of an idle reduction or start/stop strategy. This strategy reduces CO2 by increasing average fuel conversion efficiency. When a traditional vehicle is stopped, the engine continues to run at very low power levels, only providing useful work to engine accessories such as power steering, power brakes, the alternator, and the air conditioning/heating system. Gasoline engines are very inefficient at these power levels; diesel engines are better but are still significantly less efficient at idle than higher power levels. The very low efficiency in combination with significant idle time in urban areas provides motivation for reducing engine idle operation, even though the fuel consumption rate is low.

Table 19.3 presents a number of statistics relevant to start/stop systems for US and international driving cycles, including engine starts per 100 km and engine starts per hour based on each driving cycle. The actual number

Table 19.2 USABC battery goals

		E V	ls g oa	P H E V	ls g oa		H E V g	ls oa
	Un i ts	Lo te ng rm l g oa	M in im um l g oa	M i le 4 0	M i le 1 0	H ig h en er g y	M d iu e m en er g y	H ig h low p ow er ; en er g y	S / ta t r to s p ( V ) 4 2
Pu lse d isc ha rg e p ow er ( 2 d lse ) se co n p u	k W	N / S	N / S	N / S	N / S	N / S	N / S	5 5	6
Pu lse ive t re g en er a p ow er ( 2 d lse ) se co n p u	k W	N / S	N / S	N / S	N / S	N / S	N / S	4 0	N / S
Pu lse d isc ha rg e p ow er ( 1 0 d lse ) se co n p u	k W	8 0	8 0	3 8	4 5	4 0	2 5	2 0	3
Pu lse ive t re g en er a p ow er ( 1 0 d lse ) se co n p u	k W	8 0	8 0	2 5	3 0	3 5	2 0	3 0	2. 4
To l a i la b le ( ha ta va en er g c rg e y de le in le l i fe ) t p g cy c	k W -h rs	4 0	4 0	1 1. 6	3. 4	N / A	N / A	N / A	N / A
To l a i la b le ( ha ta va en er g y c rg e fe in in le l i ) ta su s g cy c	k W -h rs	N / A	N / A	0. 3	0. 5	0. 5	0. 3	0. 1 6 5	0. 2 5
Co @ °C l d k in 3 0 cr a g p ow er – 2 d k, 1 0 d se co n cr an se co n 3 in t; re s a ro w	k W	N / A	N / A	7	7	7	5	5	8 % f f ( 5 0 ) e
f f ic M in im d- ip ie tr um ro un e nc y S C Cy ( U A B H E V le ) c	%	N / A	N / A	9 0	9 0	9 0	9 0	9 5	9 0
C fe ha de le in le l i t rg e p g cy c	Cy les c	1, 0 0 0	1, 0 0 0	5, 0 0 0	5, 0 0 0	N / A	N / A	N / A	N / A
C ha in in le l i fe ta rg e su s g cy c	Cy les c	N / A	N / A	3 0 0, 0 0 0	3 0 0, 0 0 0	3 0 0, 0 0 0	3 0 0, 0 0 0	3 0 0, 0 0 0	4 5 0, 0 0 0
Ca le da l i fe n r	Ye ar s	1 0	1 0	1 5	1 5	1 5	1 5	1 5	1 5

M im l low b le ax um a a l f- d isc ha te se rg e ra	W -h / da rs y	N / S	N / S	0 5	0 5	0 5	0 5	N / S	N / S
M in im iva b le te tu um s ur v m p er a re	°C	0 –4	0 –4	6 –4	6 –4	6 –4	6 –4	6 –4	6 –4
M in im in t te tu um o p er a g m p er a re	°C	0 –4	0 –4	–3 0	–3 0	–3 0	–3 0	–3 0	–3 0
M im in t te tu ax um o p er a g m p er a re	°C	8 5	5 0	5 2	5 2	5 2	5 2	5 2	5 2
M im iva b le te tu ax um s ur v m p er a re	°C	8 5	5 0	6 6	6 6	6 6	6 6	6 6	6 6
Sp i f ic ec e ne rg y	W -h / kg r	2 0 0	1 5 0	1 0 0	6 0	8. 2 5	7. 5	8. 2 5	2 5
Sp i f ic ( d isc ha ) ec p ow er rg e	W / kg	4 0 0	3 0 0	3 0 0	7 5 0	6 5 0	6 2 5	2, 7 5 0	6 0 0
Sp i f ic ( ) ec p ow er re g en	W / kg	2 0 0	1 5 0	2 0 0	5 0 0	6 0 0	5 0 0	2, 0 0 0	2 4 0
En de i ty er g y ns	W -h / L r	3 0 0	2 3 0	1 4 5	8 5	1 1	9	1 0	2 8
Po de i ty w er ns	W / L	6 0 0	4 6 0	4 7 5	1, 1 2 5	9 0 0	8 0 0	1, 5 5 0	3 3 3
M im ig h t ax um w e	kg	2 0 0	2 6 5	1 2 0	6 0	6 0	4 0	2 0	1 0
M im lu ax um v o m e	L i te rs	1 3 5	1 7 5	8 0	4 0	4 5	3 2	1 6	9
Pr ice les lu i f ie d t s a a o m e sp ec v in fo l low in g ro w	\$	4, 0 0 0	6, 0 0 0	3, 4 0 0	1, 7 0 0	8 0 0	5 0 0	4 0 0	1 5 0
Sa les lu o m e v	Un i / ts ea y r	2 5, 0 0 0	2 5, 0 0 0	1 0 0, 0 0 0	1 0 0, 0 0 0	1 0 0, 0 0 0	1 0 0, 0 0 0	1 0 0, 0 0 0	1 0 0, 0 0 0
Da f p b l ica io te t o n on u	/ d / m y	8 / 1 4 / 2 0 0 6	8 / 1 4 / 2 0 0 6	4 / 4 / 2 0 0 7	4 / 4 / 2 0 0 7	1 0 / 1 2 / 2 0 0 6	1 0 / 1 2 / 2 0 0 6	2 / 2 9 / 2 0 1 2	8 / 1 5 / 2 0 0 6
C U S A B W bs i te e

N/A = Not applicable N/S = Not specified

Source: Adapted from USCAR, 2012.

Table 19.3 Start stop metrics for US and select international driving cycles

Dr ive le c y c na m e	Du io t ra n ( ds ) se co n	Le h t ng ( km )	I d le im t e @ 0 d sp ee ( % )	Lo t ng es i d le ( ds ) se co n	Po ia l te t n in ta ts en g e s r du in le r g cy c	En in ta ts g e s r 1 0 0 km p er ( ) av er ag e	En in ta ts g e s r ho p er ur ( ) av er ag e	En d er g y us e du in lo t r g ng es i d le io d p er ( hr ) * t t- w a s
S U F T P 7 2	1 8 7 4	1 7. 8	1 9	3 9	2 2	1 2 4	4 2	3 3
S S U U D D	1 3 6 9	1 2. 0	1 9	3 9	1 7	1 4 2	4 5	3 3
S U H ig hw ay	7 6 5	1 6. 4	1	5	1	6	5	4
S U 0 6	6 0 0	1 2. 9	7	8	5	3 9	3 0	7
S C C U N Y	5 9 8	1. 9	3 5	4 2	1 8	9 4 8	1 0 8	3 5
S S U H D U D D	1 0 6 0	8. 9	3 3	6 6	1 4	1 5 7	4 8	5 5
S S C U 0 3	6 0 0	5. 8	1 9	2 4	6	1 0 4	3 6	2 0
S U I M 2 4 0	2 4 0	3. 2	5	5	2	6 3	3 0	4
C A R B L A 9 2	1 4 3 5	1 5. 8	1 6	3 0	1 6	1 0 1	4 0	2 5
C A R B L A S ho 9 2- t r	9 6 9	1 1. 3	1 8	3 0	1 0	8 9	3 7	2 5
U N E C E E le Ur ba ta m en ry n	1 9 5	1. 0	3 3	2 2	3	3 0 2	5 5	1 8
U N E C E Ex Ur ba tra n	4 0 0	7. 0	1 0	2 1	1	1 4	9	1 8
U N E C E Ex Ur ba L P V tra n	4 0 0	6. 6	1 0	2 1	1	1 5	9	1 8
Ja M de 1 0- p an o	1 3 5	0. 7	2 9	2 1	2	3 0 1	5 3	1 8
Ja M de 1 5- p an o	2 3 1	2. 2	3 3	6 6	1	4 6	1 6	5 5
Ja M de 1 0- 1 5 p an o	8 9 2	6. 3	3 3	6 7	8	1 2 6	3 2	5 6

CARB = California Air Resources BoardFTP = Federal Test ProcedureLA = Los Angeles LPV = Low Powered Vehicle NYCC = New York City Cycle SC = Supplemental Cooling UDDS = Urban Dynomometer Driving Cycle UN = United Nations. US = United States

ECE = Economic Commission for Europe IM = Inspection and Maintenance

^* Assumes 3kW average accessory load

of engine starts/stops can, of course, vary depending on the start/stop implementation strategy. Table 19.3 also indicates idle periods reach 20% of driving time in stop and go urban driving; experience indicates higher values in some cities. Figure 19.4 highlights the idle periods during the US FTP 72 driving cycle and provides an indication of their relative significance across the driving cycle.

Figure 19.4 is illustrative for developing initial energy and cycle life requirements for a start/stop battery. The battery must supply all vehicle power once the engine is turned off. The power delivery must be sufficient for modern convenience features such as interior lighting, exterior lighting, the navigation and entertainment system, electric seat adjustment motors, power window motors, windshield wipers, power mirrors, and the electric window defroster. This electrical load can reach 1 kW in subcompact cars and extends past 3 kW in a modern luxury vehicle (Robert Bosch GmbH 2011). The battery must also have enough reserve power to rapidly restart the engine; this can require another 2.5 kW for a warm engine or significantly more for cold engines. Lastly, engine driven accessories (power steering, power brakes and air conditioning) are often electrified and can add further demands to a start/stop battery. In a traditional vehicle, many electrical loads are turned off while the engine is being started. However, during the start/ stop operation, it is not acceptable to regularly turn items like the radio or

19.4 Illustration of idle time during US FTP 72 driving cycle.

headlights off for short periods of time to restart the engine. Thus, the entire electrical load and engine starting loads must be serviced for brief periods of time. The USABC set a power output target of 6 kW for start/stop systems which can be easily justified based on the previous discussion.

Energy requirements are developed by estimating an average power requirement and developing a maximum engine off duration. Using the 3 kW draw of a luxury car and a maximum engine off duration of 5 minutes produces an energy requirement of 0.25 kW-hours of usable energy. This agrees with the goals set by the USABC.

Lastly, the required number of start/stop cycles can be estimated. In the FTP 72 cycle, there are up to 22 opportunities for engine stop/start functionality (depending on the exact operating strategy). The cycle is approximately 18 km long, resulting in more than one stop/start cycle per kilometer. Thus, it is not unreasonable to expect more than 250,000 stop/start cycles across 10 years of use at 25,000 km per year. The USABC set a goal of 450,000 cycles across a 15-year life.

Separation of engine response from driver demand

A second method for CO2 reduction uses an electric machine that can exchange power with the driveline. This allows for CO2 reduction through two of the previously mentioned methods: reducing the amount of energy required on a given trip and delivering a higher percentage of the fuel energy to the vehicle. The reduction in required energy is accomplished through recovery of braking energy rather than discarding that energy as heat through the brakes (often called regenerative braking). Improved fuel conversion efficiency is accomplished by allowing the engine to operate more frequently at high efficiency points and allowing short term power transients to be accommodated through one or more electric machines. The benefits of stop/start are gained as well with this type of vehicle.

The size of the electric machine(s) is/are chosen based on the vehicle's design goals. Industry has developed a number of vehicle architectures with electric machines placed in many different locations. From the battery perspective, the different architectures define the power, energy and duty cycle required from the battery.

Figures 19.5 and 19.6 show the percentage of the US FTP72 and US 06 driving cycles that can be driven with various power levels for four different classes of vehicle. This provides an indication of the required battery charge/ discharge power levels for these vehicles. When designing a vehicle, the cost of a higher power battery and electric machine must be weighed against diminishing CO2 reduction benefits. Figures 19.7 and 19.8 provide peak acceleration and deceleration power levels for a number of different United States and international driving cycles. From these figures, it should be

19.5 Per cent of US FTP 72 driving cycle that can be completed versus power delivered to the road for several vehicle classes. Graph does not go through origin due to idling time.

19.6 Per cent of US 06 driving cycle that can be completed versus power delivered to the road for several vehicle classes. Graph does not go through origin due to idling time.

19.7 Peak acceleration power requirements by drive cycle and vehicle size.

19.8 Peak braking power requirements by drive cycle and vehicle size.

apparent that power in the 30 kW to 60 kW range can accomplish much of the CO2 reduction functionality in passenger vehicle applications by meeting both the regenerative braking and acceleration power demands. This drives similar battery power requirements.

The energy available for recovery in a single braking event is shown in Fig. 19.9; the maximum theoretical braking energy available for several vehicle classes and speeds (assuming flat ground) is shown in Fig. 19.10. These figures show that the usable energy storage necessary to take full advantage of regenerative braking is on the order of 125 to 250 W-hrs. Additional functionality, such as fully electric driving may require additional energy storage.

The last design parameter is cycle life. A cycle is defined as a single discharge and charge event. The number of transitions from negative to positive tire torque during the US FTP is on the order of 80 transitions (the value varies some times depending on vehicle drag). This equates to roughly 4.5 cycles per kilometer. A vehicle that is designed for a 150,000 km life could require 500,000 to 750,000 cycles from the battery if driven solely in stop and go traffic. The energy swing in one cycle varies from just a few watt hours to more than 100 watt hours.

19.9 Maximum energy content of single braking event by drive cycle and vehicle size.

19.10 Theoretical braking energy available by vehicle speed and vehicle size.

Fuel displacement by 'plugging in'

The most significant change in vehicle CO2 emission can be realized using a plug-in vehicle. Plug-in vehicles allow a reduction or even elimination of engine power output while the battery contains energy that can supply an electric machine. This is called charge depleting operation. At some point the battery becomes depleted and hybrid vehicles revert to a charge sustaining mode that utilizes the engine. Fully electric vehicles would need to be recharged.

Plug-in vehicle architectures cover a broad range of design. At one end are the pure electric vehicles such as the Nissan Leaf, Mitsubishi i-MiEV, and the Tesla Model S. For these vehicles, there is no engine; thus the battery must provide all propulsion and accessory power across all operating conditions. Spanning a range of electrification are plug-in hybrids like the Chevrolet Volt, Ford Energi vehicles, and plug-in Toyota Prius. These all use a battery and electric machine to provide propulsion power for some of the driving cycle. However, an engine can be utilized any time the driver demand exceeds the power capabilities of the electric drive system. The engine may turn on regularly or infrequently, depending on the size of the battery and performance capabilities of the electric machines.

For plug-in hybrid vehicles, their additional reduction in vehicle CO2 emissions is determined by the percentage of the drive cycle energy that is displaced by electric energy. This percentage grows with more powerful electric machines, larger battery pack capacities, and increasing plug-in frequency. The high cost of electric machines and batteries have encouraged a number of different architecture design decisions that attempt to meet market expectations.

Battery power requirements can be evaluated from Figs 19.5–19.10 above. Average propulsion energy requirements per kilometer at the tireroad interface are shown in Fig. 19.11. Accessory power requirements and energy conversion losses between the battery and tire add to the battery energy required per mile.

Cycle life requirements for pure EVs can be estimated as 1–2 charge events per day given the multi-hour charge times required with readily available charging equipment. This is between 500 and 1,000 full cycles per year (or less for many drivers). Thus, a 10-year life would require between 5,000 and 10,000 cycles. Plug-in hybrid vehicles will require a similar number of so-called 'charge depleting' cycles, plus a 'charge-sustaining' cycle life similar to the traditional hybrid vehicle cycle life discussed previously.

19.2.3 Summary

As mentioned earlier in this section, utilizing plug-in vehicles for CO2 reduction does not currently guarantee a decrease in CO2 production due to the variety of sources for the plug-in vehicle's electricity. However, it

19.11 Average propulsion energy required per km by drive cycle and vehicle size.

does provide a clear pathway leading to a much lower CO2 footprint from transportation. Figure 19.12 from Lawrence Livermore National Laboratories within the United States Department of Energy presents a very clear image of the benefit of transitioning to electrified vehicles*. Today, the United States' energy usage within the transportation sector is almost entirely derived from petroleum. A switch to electrified vehicles effectively decouples transportation from the debate about the 'best' source of energy because almost all energy sources can be effectively used to make electricity.

19.3 Battery functionality and chemistries for vehicle applications

19.3.1 Introduction

A battery is a device built to extract energy from a chemical reaction by allowing the participating ions to move and react while forcing the electrons through an external circuit to deliver energy (or absorb energy if the battery

19.12 US annual energy flow in 2010 in quadrillions of BTUs Source: LLNL 2013 – data based on DOE/EIA-0035 (2013).

^*Distributed electricity represents only retail electricity sales and does not include selfgeneration. EIA reports consumption of renewable resources (i.e., hydro, wind, geothermal and solar) for electricity in BTU-equivalent values by assuming a typical fossil fuel plant 'heat rate.' The efficiency of electricity production is calculated as the total retail electricity delivered divided by the primary energy input into electricity generation. End use efficiency is estimated as 65% for the residential and commercial sectors 80% for the industrial sector, and 21% for the transportation sector. Totals may not equal sum of components due to independent rounding.

is being charged). A rigorous treatment of the chemical processes involved in even the simplest battery is beyond the scope of this chapter. One particularly good reference that addresses a wide variety of chemistries is Linden's Handbook of Batteries (Reddy 2010).

This section will provide a high level understanding of battery functionality. Following the battery functional discussion, this section will review several battery technologies with application to the transportation market. These technologies will be compared and contrasted with regard to performance.

19.3.2 Cell functionality

A cell is an electro-chemical energy storage device. As such, it is capable of exchanging energy and power directly with an electrical circuit. The electrical energy contained in a cell is typically expressed in watt-hours. The energy that can be extracted from a cell is a result of the number of ions that can be stored at an elevated energy state and their motivation to migrate back to their discharged state. When the ions move through the interior of the cell, electrons will accompany them through an external circuit outside the cell.

The quantity of electrons that move between cell terminals is typically expressed in amp-hours. It is important to realize that discharging a battery involves ions physically migrating from one electrode to the other. In other words, mass is physically rearranging within the battery as it is charged and discharged.

In addition to the amp-hours or quantity of electrons, the other important parameter is the 'motivation' of the ions to leave their elevated energy state in one material and return to their discharged state in another material. This motivation is typically expressed in volts.

In summary, the energy content of a cell is the product of the number of electrons that migrate during charge or discharge (expressed in amp-hours) and the voltage motivating the ions to migrate. When multiplied together, these values describe the cell's energy content in watt-hours. The voltage varies as the cell is discharged or charged, and the energy content value is an integration of the product of voltage and current across the full discharge process. Figures 19.13 and 19.14 illustrate the flow of electrons, ions and current in a cell during discharge and charge.

The positive ions are more strongly attracted to the positive electrode (cathode¹ ) than the negative electrode (anode). The charging process stores

Technically, the anode and cathode are defined based on current flow; the cathode receives electrons and the anode looses electrons. This is clearly confusing as the materials switch between anode and cathode depending on whether the battery is charging or discharging. Thus, the function during discharge is commonly used in materials circles.

Ions migrate to most 'attractive' material Electrons move to maintain net zero charge on material

19.13 Battery processes during discharge.

Energy is used to push electrons into negative material Ions move to maintain net zero charge on material

19.14 Battery processes during charge.

energy in the cell by using electrical power to force electrons from the positive electrode into the negative electrode. As the electrons are moved, the charge unbalance will attract positive ions away from the positive electrode and into the negative electrode.

When an electric pathway is provided between the positive and negative electrodes, electrons 'want' to return to the positive electrode as the external electric circuit allows them to. The voltage is a measure of the level of motivation for the electrons to return to the positive electrode and is material dependent. Positive ions will also return to the positive electrode at the same rate as the electrons to maintain charge balance within the electrode materials.

There are several functional details that should be noted in the figures above:

∑ a cell consists of two different materials that are intentionally reactive, and are in close proximity to each other
∑ physical mass movement occurs within the cell (ions move back and forth between the anode and cathode as the cell is discharged and charged)
∑ the electrolyte is a material (usually a fluid) that enables the mobility of ions between anode and cathode
∑ the separator is a barrier preventing direct electron movement between positive and negative active materials
∑ the current collectors are conductive metals used to provide a low resistance path for electrons as they enter or exit the active materials.

19.3.3 Performance metrics

In transportation applications, there are four primary performance metrics that are used to compare various cell chemistries (and complete battery systems):

1. specific energy = stored energy per mass and expressed in W-hr/kg
2. energy density = stored energy per volume and expressed in W-hr/L
3. specific power = short term, peak power per mass and expressed in kW/ kg
4. power density = short term, peak power per volume and expressed in kW/L.

The maximum theoretical values for the above energy metrics are determined by the choice of battery materials (the chemistry). However, most practical cells do not come close to reaching these theoretical limits (and are often only 25% to 50% of theoretical limits). Practical cells require many components that add weight and take up space, but do not provide any energy storage. These include the electrolyte, current collectors, the case, the separator and various additives to the active materials that improve other characteristics such as safety, life and manufacturability. Values listed throughout this chapter represent actual cells rather than theoretical limits unless otherwise noted.

The cell designer has the ability to tailor the design of each of these components to meet the needs of the cell's end use application. Two of the more significant operating parameters include the desired operating temperature range and desired power output.

Extreme low or high temperature operation often dictates specific material additives to either improve the desired ion mobility at low temperatures or suppress undesired chemical reactions at high temperatures.

High power output often requires thicker current collectors to enable high current flow as well as thinner electrode coatings to reduce ion diffusion distance. This results in an increased ratio of current collector volume to active material volume and reduces energy metrics. The section on lithium ion cells contains further discussion of design tradeoffs for performance, safety and life within that family of chemistries.

19.3.4 Chemistries

To date, no commercialized electrochemical system has approached liquid hydrocarbon fuels in either energy density or specific energy. However, industry has developed a number of electrochemical systems suitable for transportation applications – each with its own benefits and weaknesses. This section will provide a brief overview of several of those systems and discuss their benefits and weaknesses.

Lead acid (PbA)

Lead acid batteries have been installed in almost every type of vehicle made for the past century because they have proven to be a very cost effective method for storing sufficient power and energy to start a vehicle and provide short term power while the engine is off. Lead acid battery strengths and weaknesses are well understood and their internal design has been highly optimized over the years. In addition, a very successful recycling stream has developed.

The lead acid battery consists of lead (Pb) plates attached to the negative terminal and lead dioxide (PbO2) plates attached to the positive terminal. These plates are immersed in a water and sulfuric acid (H2SO4) electrolyte. The reactions within a lead acid battery are somewhat complex. During discharge, the following compound reactions occur:

∑ On the negative lead plate, lead is dissolved by the sulfuric acid to form lead ions (Pb2+) in solution, leaving free electrons on the negative plate that can travel through an external circuit. The Pb2+ ions combine with the sulfuric acid's sulfate ion (SO⁴ 2–) to form lead sulfate (PbSO4).

Pb Æ Pb2+ + 2e– (lead is dissolved by the sulfuric acid to form lead ions)

Pb2+ + SO4 2– Æ PbSO4 (lead ions combine with sulfate ions to form lead sulfate)

∑ Meanwhile, on the positive lead dioxide plate, the lead oxide is attacked by the positive hydrogen ions in the sulfuric acid, stripping the oxygen from the lead oxide to initially form OH ions in solution. The OH– ions then combine with another H+ ion to form water. This process leaves excess lead ions in solution. Then, like the lead ions from the lead plate, these lead ions combine with sulfate ions to form lead sulfate.

$$PbO_2 + 4H^+ + 2e^- \rightarrow Pb^{2+} + 2H_2O$$

$Pb^{2+} + SO_4^{2-} \rightarrow PbSO_4$

The above reactions combine to produce a nominal cell voltage of 2V. Six cells are connected in series for a 12V battery.

One of the earliest modern era electrified vehicles in the United States was the General Motors EV1. The EV1 initially used valve regulated lead acid cells from Panasonic. The Panasonic specifications for that battery (the EC-EV1260) show the battery had a specific energy of 35 W-hr/kg and would lose > 30% of its capacity after 1,000 cycles at room temperature and 80% depth of discharge. Later EV1 vehicles utilized NiMH cells. Comparing the battery packs for the lead acid EV1 and the NiMH EV1 indicates the weight advantage of NiMH:

∑ lead acid: 18.7 kW-hrs capacity 595 kg weight 31.4 W-hr/kg ∑ NiMH: 29.2 kW-hrs capacity 481 kg weight 60.6 W-hr/kg
Lead acid may still be the solution to the most common type of hybrid in the near term. Start/stop technology is an inexpensive method to lower fuel consumption. Some analysts predict more than two in three vehicles sold in Europe will have start/stop solutions by the next product cycle. The most common battery solution in the near term will be some form of lead acid battery technology. The cycle life of traditional flooded lead acid batteries is not sufficient to consistently last through the vehicle warranty period. Additional development of lead acid technology is underway to optimize designs for start/stop duty cycles. However, there is no plausible pathway for lead acid to reach the specific energy values already attained by lithium ion cells and thus this chemistry will likely be limited to applications with small batteries (stop/start), that are minimally weight sensitive (some heavy

duty hybrid applications) or applications that are highly sensitive to initial cost such as e-bikes or similar vehicles in developing countries.

Nickel metal hydride (NiMH)

The nickel metal hydride chemistry was introduced to hybrid electric vehicles in the US in 2001 through the introduction of the Toyota Prius and Honda Insight (see Figs 19.15 and 19.16). The nickel metal hydride chemistry has proven to be a very robust chemistry well suited to hybrid vehicle systems.

The positive electrode consists of nickel oxyhydroxide, NiOOH, and the negative electrode is a material that stores hydrogen called a metal hydride.

19.15 2008 Toyota Prius NiMH battery pack.

19.16 2010 Honda Insight NiMH battery pack.

The metal hydride is a metallic structure (M) that can absorb hydrogen atoms forming M+xH (MH). The electrolyte is an aqueous solution of metal hydroxides such as KOH, LiOH and NaOH.

During charge, extra electrons are pushed into the metal hydride alloy (M). The charge imbalance attracts hydrogen atoms from the electrolyte into the metal hydride, forming MH. The electrolyte then regains its hydrogen from the Ni(OH)2, converting the Ni(OH)2 to NiOOH. The overall charge equation is:

$$Ni(OH)_2 + M \rightarrow NiOOH + MH$$

where M represents one of many different engineered alloys for use in storing hydrogen in NiMH cells and MH represents the same alloy after absorbing hydrogen atoms. NiMH cells have an open circuit voltage of approximately 1.2 V near 50% state of charge.

NiMH cell performance metrics are significantly better than lead acid. Panasonic offered a high power module with six prismatic cells in the Toyota Prius with the following claimed performance:

∑ specific power 1300 W/kg
∑ specific energy 46 W-hr/kg
∑ power density 2200 W/L
∑ energy density 80 W-hr/L.

A module optimized for energy from Saft (NHE 10-100) has the following claimed performance metrics:

∑ specific power 150 W/kg
∑ specific energy 66 W-hr/kg
∑ power density 310 W/L
∑ energy density 137 W-hr/L.

The two modules above provide a typical example of the tradeoff between power and energy when designing a cell. The Panasonic module has much higher power per kg but significantly less energy per kg when compared to the Saft cells. Neither of these products represents the endpoints of the power vs energy tradeoff. Linden provides a theoretical maximum specific energy of 240 W-hr/kg for NiMH cells and specific power is based on engineering tradeoffs for which there is no theoretical maximum.

Cycle life for NiMH cells is also significantly better than the lead acid chemistry. For hybrid applications, with <5% state of charge swing, NiMH batteries can continue for 300,000+ cycles while maintaining sufficient performance for the hybrid application. Cycle life in excess of 2,000 cycles is possible, even at 80% state of charge swing typical of electric vehicles.

NiMH cells have some safety benefits when compared to lithium ion cells. In particular, NiMH cells use an aqueous electrolyte rather than the highly flammable solvents used in lithium ion cells. NiMH cells are also more tolerant of overcharge than lithium ion cells because the chemical products of overcharge are able to recombine in the cell resulting in no net chemical change or degradation for modest charging rates. However, all excess energy is converted directly to heat. Thus, uncontrolled overcharging can still generate enough heat to damage cells or cause more significant safety concerns. NiMH cells can also tolerate operation across a broad temperature range. Performance suffers significantly at low temperatures, but cells can remain operational down below –30 °C and up past 100 °C.

Lastly, NiMH cells have demonstrated reliable, long life operation in both hybrid and fully electric vehicles since the late 1990s. Their most significant shortcomings are their weight metrics (compared to lithium ion) and their low cell voltage that necessitates roughly three times the number of cells in series to reach the same voltage as a lithium ion pack. Some forecasts also predict lithium ion cells will begin to undercut NiMH cells on price.

Sodium nickel chloride

The sodium nickel chloride chemistry is a high temperature chemistry that has been implemented in a few transportation applications. Typical cell operating temperature varies between 275 °C and 300 °C.

Sodium nickel chloride (NaNiCl2) cells utilize nickel chloride salt in sodium tetrachloroaluminate (NaAlCl4) as the positive electrode and liquid sodium metal as the negative electrode. These are separated by a ceramic aluminum oxide separator. The overall chemical reaction during charge is:

$$Ni + 2NaCl \rightarrow NiCl_2 + 2Na$$ .

NaNiCl2 batteries have high energy per kg metrics – on par with today's lithium ion packs. MES-DEA produced a 21 kW-hr battery pack (Model # Z5- 278-ML3P-76) for transportation applications with the following metrics:

∑ specific power 170 W/kg
∑ specific energy 120 W-hr/kg
∑ power density 250 W/L
∑ energy density 175 W-hr/L.

The low power to energy (P/E) ratio of 1.5 is typical of NaNiCl2 batteries. Efficiency degrades significantly as power increases above a P/E ratio of 1.0. Cycle life appears sufficient for transportation applications.

The safety of NaNiCl2 cells is dependent on the structural integrity of the cell. The high cell temperature poses a potential safety issue if the cell is breached. The high operating temperature range of NaNiCl2 batteries is also one of the positive attributes for this chemistry. The high operating temperatures make the chemistry relatively insensitive to the broad range of ambient conditions experienced in transportation applications. As a result, these batteries are quite capable of operating across the full –40 °C to +50 °C ambient temperature range seen in ground based transportation.

There are two significant hurdles that prevent adoption of this chemistry in most transportation applications. First, a significant amount of power is consumed maintaining the high cell temperature. This is particularly problematic for vehicles that may not stay plugged in when parked because the battery pack will continue to use energy to remain warm. This results in a completely discharged battery pack in a number of days. The second hurdle is the limited power to energy ratio. The high temperature liquid sodium electrode requires a unique separator solution. The separator must have sufficient structural strength to withstand pressure loading from the liquid sodium in a vehicle's high vibration environment. The combined high operating temperature and required structural strength lead to the design of a thick separator made of aluminum oxide. The separator thickness appears to be the primary obstacle to higher power to energy ratios for this chemistry.

Lithium ion cell chemistries

The lithium ion family represents a broad class of anode and cathode chemistries wherein lithium shuttles between the electrodes. Lithium ion chemistries were first developed relatively recently; the patent for the lithium cobalt oxide chemistry was filed by Dr Goodenough in Great Britain in 1979 (GB 11953/79). Since that time, three different classes of lithium ion chemistries have been commercialized when categorized based on the differing molecular structure of the positive electrode. They are the layered, spinel and olivine structured materials. It is relevant to categorize the chemistries by material structure because the material structure has significant influence on many aspects of performance.

Lithium ion cells all share one aspect of their chemical process – the transfer of lithium ions between positive and negative electrodes. While the positive electrode can have one of several structural arrangements, they are primarily metal oxides. The negative electrode is typically a carbon structure (graphite or hard carbon), but a lithium titinate electrode has also been commercialized. A generic chemical reaction can be written as follows for the discharge process:

$$\text{Li}{x}\text{C}{6} + \text{Li}{i-x}\text{MO} \rightarrow \text{C}{6} + \text{Li}_{i}\text{MO}$$

where MO symbolizes one of many metal oxides including CoO2, Ni(1/3) Mn(1/3)Co(1/3)O2, Mn2O4, LiFePO4, LiMnPO4.

Characteristics of lithium ion cells are as follows:

∑ specific energy for production cells varies from <60 W-hr/kg to >200 W-hr/kg

∑ specific power for production cells regularly exceeds 1,000 W/kg and can be pushed beyond 10,000 W/kg when necessary, such as in motorsports or military applications.
∑ energy density varies from <115 W-hr/L to >500 W-hr/L
∑ power density regularly exceeds 1,000 W/L and can also be pushed beyond 10,000 W/L when needed for the application.

The broad range of lithium ion chemistries combined with a breadth of design goals results in a wide range of performance metrics. It is important to note that the highest power and highest energy metrics are mutually exclusive because they are driven by cell construction decisions rather than the choice of chemistry. The highest power cells tend to have low energy metrics and vice versa. A discussion of life, safety, and temperature range will be developed in Section 19.4.

19.3.5 Cost

A discussion of battery technologies is never complete without a discussion of costs. Lead acid batteries benefit from a century of technology development, design optimization, supply chain growth and a comprehensive recycling process. In 2012, lead acid battery pack cost was in the \$75–\$125 per kW-hr range depending on pack size and whether high voltage safety components are required. Valve regulated lead acid batteries are more expensive due to additional design complexity and lower production volumes, although their cost penalty will likely diminish as start/stop production volumes drive further research. Costs of both are sensitive to the lead price because lead accounts for up to 70% of battery weight and up to 50% of battery cost. Lastly, the upfront costs for lead acid do not represent the total cost of ownership for many vehicles because lead acid batteries have limited life and require replacement in many applications.

Nickel metal hydride packs have substantial performance benefits compared to lead acid. However, those benefits come with a significant cost increase. Nickel metal hydride battery pack costs were in excess \$1,000 kW-hr in the 2009–2010 timeframe for production volumes of tens of thousands of vehicles. A very small number of vehicles, including the Toyota Prius, have achieved production volumes in excess of 100,000 vehicles per year but the prices paid are not publicly available. These prices are dependent on the cost of nickel which has fluctuated significantly over the last 10 years – between \$7,000 and \$50,000 per metric tonne. A cell can exceed 40% nickel content by weight thus nickel content alone exceeds \$300 per kW-hr of capacity at the high price range. It is difficult to determine what nickel cost is associated with the pack costs above; the historical volatility of nickel prices may also mean that cell and pack pricing is inflated to protect against nickel price uncertainty. Lastly, a single supplier (Panasonic/Sanyo) produces the vast majority of nickel metal hydride cells for automotive applications. This further obscures the true cost of this technology.

The high pack costs can also be explained in part by their design. The nickel metal hydride chemistry has been applied to high power, low energy HEV packs. These packs are more expensive per kW-hr than the larger plug-in hybrid electric vehicle (PHEV) and EV packs because the fixed cost burden of high voltage safety and cooling system components is spread over a small capacity pack. Additionally, both the cells and the pack have proportionally higher copper content than PHEV and EV equivalents due to the high power throughput.

Lithium ion cell and pack costs are also difficult to quantify because the market is still immature. Industry has invested heavily in this technology with billions of dollars spent to develop transportation specific cells, manufacturing facilities and supply chains. A similar amount of money has been spent developing the pack technologies, including the battery management system hardware and software plus the additional pack components for the electrical and cooling system. These development costs will all be recovered through the sale of product and will thus elevate near term costs. The rate of investment recovery varies across the industry. Additionally, numerous joint ventures were established between vehicle suppliers and cell suppliers; these restricted free market forces, further obscuring transaction costs.

Lithium ion cell costs to vehicle manufacturers have decreased to the \$300 to \$500 per kW-hr range for production volumes on the order of tens of thousands of vehicles per year. The additional cost for the pack components and pack assembly (including pack manufacturing equipment amortization) can vary significantly depending on the size and cooling method. The high power to energy ratio packs typical of hybrid vehicles are still above \$1000 kW-hr while the large electric vehicle packs are approaching \$500 kW-hr, again for production of tens of thousands of vehicles per year.

At the moment, it appears that lithium ion will displace nickel metal hydride in all light duty applications although displacement may be slow for the small, high power hybrid vehicle packs. Lithium ion's weight savings will ensure its use in packs above several kW-hrs of capacity in on-road transportation applications.

19.3.6 Summary

For the majority of the history of the transportation industry, lead acid batteries were the only choice for electrical energy storage. Nickel metal hydride solutions were introduced in the 1990s and gave very reliable service in hybrid vehicle applications while improving both power and energy metrics beyond those of lead acid. High temperature sodium nickel chloride cells were also introduced but they do not appear to have a future in light duty vehicle applications. There are some sodium nickel chloride development programs underway in very large vehicles such as locomotive applications. It is unclear if they will develop a niche in those applications. Lithium ion cells are now displacing nickel metal hydride in most new automotive applications, and are even displacing 12 V lead acid batteries in some light duty vehicle applications.

The transition from lead acid batteries to nickel metal hydride and then to lithium ion chemistries has been driven by the substantial improvements in performance metrics. Table 19.4 summarizes these metrics, showing the improved power metrics when moving from lead-acid to either nickel metal hydride or lithium ion technology. The improved energy per kg metric of lithium ion is the primary enabler of electric vehicles and plug-in hybrid vehicles.

19.4 Lithium ion cells

19.4.1 Introduction

Lithium ion chemistries have begun to show significant acceptance in the transportation industry and thus warrant a more in depth discussion than other chemistries. This section will discuss the construction, safety and life of lithium ion cells and provide the necessary background for the sections on battery pack design and battery management systems.

19.4.2 Lithium ion cell construction

A lithium ion cell is a highly engineered product; almost all aspects of cell design continue to receive significant research. Inside the cell, there are several components that must work together.

∑ positive active material and thin foil current collector
∑ negative active material and thin foil current collector
∑ separator
∑ electrolyte and solvent
∑ container.

Figure 19.17 illustrates the main components of a lithium ion cell, their relative locations and example thicknesses.

Lithium ion cells are manufactured by either winding both the electrode material and separator around a central core (which can be round or oblong), or by cutting the electrode material and separator into sheets and stacking the sheets like a stack of paper. The winding process is a mature process; it was used for many chemistries prior to lithium ion introduction. However,

Table 19.4 Comparison of performance metrics across cell chemistries

Lithium ion

C he is try m	Sp i f ic ec en er g y ( W -h / kg ) r	En er g y de i ty ns ( W -h / L ) r	S ho t- te r rm i f ic sp ec ( W / kg ) p ow er	S ho t- te r rm p ow er de i ( W / L ) ty ns	Cy le l i fe c (~ 8 0 % D O D )	Sa fe ty c on ce rn s
Le d i d a ac	3 0– 4 0	5 0– 7 5	2 5 0– 5 0 0	5 0 0– 1, 0 0 0	2 5 0– 1, 0 0 0	Su l fu ic i d lec ly tro te r ac e Hy dr lu io du in ha t og en e vo n r g ov er c rg e He in du in ha t a g r g ov er c rg e
N ic ke l m l hy dr i de ta e	0– 4 7 5	2 2 7 5– 5	0– 0 0 1 5 1, 5	3 0 0– 2, 0 0 5	6 0 0– 2, 0 0 5	Hy dr lu io du in ha t og en e vo n r g ov er c rg e He in du in ha t a g r g ov er c rg e
So d iu ic ke l c h lo i de m n r	8 0– 2 1 5	2 2 0 0 1 5–	0– 2 0 1 5 5	2 2 2 5– 7 5	3, 0 0 0+	H ig h l l o in t te tu ce p er a g m p er a re

60–200 115–500 500–10,000 1,000–5,000 300–3,000+ Flammable electrolyte

Cell damage tolerance Minimal overcharge tolerance cylindrical cells do not efficiently fill a volume of space. Cylindrical cells also have a low surface to volume ratio for heat extraction. Stacked cells improve multi-cell volumetric efficiency and improve surface area per volume for well controlled cooling.

19.4.3 Active materials

A detailed discussion of each active material's strengths and weaknesses is beyond the scope of this chapter. For further information, please refer to the journals and references listed at the end of this chapter. This section will attempt to provide enough understanding of the variety of chemistries, as well as physical attributes to enable a discussion regarding safety and life.

The active materials are the materials that actively participate in the storage of lithium ions. There is a positive and a negative active material. The positive material is the material that is most attracted to lithium ions, and is thus the material that holds the lithium ions in the discharged state. To date, commercial cathode materials have consisted of lithiated metal oxides (or mixtures of multiple metal oxides). Successful materials have included: LiCoO2 – 'LCO', LiMn2O4 – 'LMO', LiFePO4 – 'LFP', LiFexMg1–xPO4, several ratios of Li(NixMnyCo1–x–y)O2 – 'NMC' and several ratios of Li(NixCoyAl1–x–y)O2 – 'NCA'. The negative active material is usually a carbon structure, although lithium titinate, Li4Ti5O12 – 'LTO', has also been commercialized.

The active materials are placed on the current collector as a thin film. Typical dimensions are in the order of a 50 micro meter thick coating on each side of a 10 micro meter thick foil. The active material is made of small particles whose size and shape are varied to develop different properties. The electrolyte fills the voids, wetting the particles and separator as shown in Fig. 19.17. The particle size, shape and their variations are important for two reasons. The diffusion rate of lithium within the electrode particles is generally slower than the diffusion rate through liquid electrolyte. Thus, the power capability of the cell is influenced by the surface to volume ratio of the particles, assuming all else remains the same. In addition, the particles exhibit some strain when lithium is inserted and extracted. This continuous cycle by cycle strain can crack the particles and slowly degrade the cell's capacity. The net strain on bonds along the outer surface is again a function of surface to volume ratio. Thus, the particle size and shape influences the cycle life capabilities of the cell. Lastly, the safety of the cell at high temperature is also affected by surface area. As the cell gets warmer and warmer, the positive electrode begins to react with the electrolyte. This reaction is a very strong function of temperature, but also a function of surface area. Thus, particle size has safety implications.

19.17 Lithium ion cell construction.

19.18 Lithium ion separator. Courtesy Celgard LLC.

19.4.4 Separator and electrolyte

The separator (Figs 19.18 and 19.19) and electrolyte are the other major components of a cell. The separator has a seemingly simple primary function to let ions through but not allow electrical conductivity between the two electrodes. This leads to the design of a porous non-conductive material. However, there are several additional requirements that limit the variety

19.19 Tri-layer lithium ion separator cross section. Courtesy Celgard LLC.

and flexibility to tolerate the manufacturing process. The separator must remain intact during the life of the cell; a breach in the separator results in a direct internal short circuit. This can lead to significant localized heating, and ultimately trigger further exothermic chemical reactions that lead to an uncontrolled temperature rise or thermal runaway. Some separator designs function as a safety device by partially melting at a particular temperature. This closes the ion transfer pathways and can mitigate certain thermal runaway mechanisms. Separators in lithium ion batteries are typically polyethylene, polypropylene or a tri-layer design incorporating both polymers. Some manufacturers also incorporate a ceramic coating on the separator to further prevent separator failure. A thorough treatment of separators can be found in Arora (2004).

The electrolyte in a lithium ion cell is primarily an ion transfer media. Lithium ion electrolytes are typically a mixture of salts for ion transport and solvents that dissolve the salts and provide mobility. The solvents are often a mixture and must remain in the liquid phase across a wide range of temperatures. Common electrolyte solvents include numerous organic carbonates as well as dimethylether. The most common salt in use today is lithium hexafluorophosphate – LiPF6. Xu (2004) provides a thorough treatment of electrolyte properties and requirements.

19.4.5 Lithium ion cell safety

Lithium ion cells present new safety hurdles when compared to previous chemistries. The chemical and fire hazards must be thoroughly understood at both a practical and theoretical level by anyone developing a battery pack. This section is just a cursory overview intended to motivate the reader towards a more thorough understanding of lithium ion safety.

As a quick review, three components are required for a fire: fuel, oxidizer, and a heat source. If any of those three items are unavailable, a fire will not start.

A lithium ion cell has a significant amount of fuel inside. The solvent for the electrolyte is made of organic carbonates such as ethylene carbonate - C3H4O3, propylene carbonate – C4H6O3, diethyl and dimethyl carbonates – C5H10O3, C3H6O3, respectively, and dimethyl ether – C2H6O. The plastic separator is made of hydrocarbons such as polypropylene and polyethylene – (C3H6)n and (C2H2)n, respectively. There is also binder in both the positive and negative active materials – typically PVDF: (C2H2F2)n and/or PTFE (C2F4)n. Lastly, the negative active material is often made from carbon. From this list, it should be clear that a lithium ion cell contains plenty of fuel to support a fire.

Oxygen for combustion is usually readily available from the surroundings if a cell container is breached. However, even a sealed cell can release oxygen from the positive electrode material or electrolyte compounds at elevated temperatures. This oxygen can be sufficient to sustain exothermic reactions that can accelerate and lead to catastrophic cell failure.

The key component necessary for a lithium ion fire that must be prevented is the heat source. Chemical reaction rates are exponentially dependent on temperature. Decomposition reactions can become fast enough in the 100 °C range to generate further heating at a rate greater than the heat is removed through the external cell surfaces. The temperature and heating rate continue to grow simultaneously and can result in the catastrophic failure of the cell. The failure temperature varies with the cell chemistry, electrode particle sizes, electrolyte mixture and additives.

Fortunately, heat does not just show up spontaneously. An overheated lithium ion cell must get overheated in some manner. Evaluation and mitigation of the many possible heating mechanisms (high current flow, overcharging, external heat, etc.) is a critical aspect to battery pack design and a major function of the battery management system.

It is worth noting that sales of consumer lithium ion batteries have surpassed 3 billion units per year (BAJ 2012). The majority are used in low cost applications. While there have been a number of high profile product recalls due to product flaws, the recalls represent a very small percentage of the total market – indicating lithium ion cell safety is possible even in very price sensitive applications.

19.4.6 Lithium ion cell life

A thorough understanding of a lithium ion cell's life in a given application is critical to commercial success due to the combination of high pack costs and long warranty expectations in vehicles. Lithium ion cell degradation can be split into two categories – loss of capacity and increased resistance. Loss of capacity occurs when the amount of mobile lithium decreases – due either to lack of mobile lithium ions or lack of storage (intercalation) locations within the electrodes. Increased resistance can occur due to decreases in electrolyte conductivity, or degradation of the electrode particles.

One degradation pathway is through unintended chemical reactions (termed side reactions). Chemical reaction rates are strong functions of temperature. Thus, the life of a lithium ion cell is very sensitive to operating temperature because degradation reactions whose rates are inconsequential at room temperature can limit cell life to less than a year at temperatures in the range of 60 °C. Some side reactions can be further accelerated at the edges of the cell voltage window (very low or high states of charge).

The other primary degradation mechanism is mechanical degradation of the particles within the active material. These particles strain when lithium is inserted/removed. This strain causes cracking and slowly decreases the amount of bonding sites for lithium ions. This strain induced degradation scales with number of insertion/removal cycles. In addition, the strain is exacerbated at very high and low states of charge where the lithium begins to fully populate or fully vacate the surrounding structure. This excess strain at high and low states of charge, when combined with the acceleration of side reactions at voltage extremes explain why many system developers have chosen to limit the state of charge swing.

The complex interaction of cell operating temperature, state of charge, and cycle count leads to a statistical analysis of the vehicle fleet in various geographic locations and across expected drive cycles. This allows statistical estimation of battery pack degradation rates across a fleet of vehicles.

19.4.7 Summary

Lithium ion cell chemistry knowledge is continuing to grow at a rapid pace. Numerous journal articles are authored every month with ideas for improved safety, lower cost and higher performance. This provides optimism that future lithium ion cells will realize significant additional improvement.

19.5 High voltage battery pack design

19.5.1 Introduction

To date, on-road vehicles have had battery packs built with lead acid, nickel metal-hydride, sodium-nickel chloride and lithium ion cells, and likely others. A battery pack must perform a number of tasks. The foundation of battery pack design is formed by the need for high levels of safety consistent with transportation applications. The pack design then balances performance, reliability, cost, weight, and volume within the design envelope that ensures safety. High voltage battery packs with lithium ion cells are the most demanding battery pack application to date, and will be the subject of this section. However, much of this section is broadly applicable across other chemistries.

There are four primary systems within a battery pack – the high voltage system, the thermal control system, the environmental enclosure and the battery management control system. The battery management system is discussed in Section 19.6; the remaining topics will be discussed here.

19.5.2 High voltage system

The battery pack high voltage system is designed to control power flow to and from the cells and to maintain the power level within the design envelope. This is accomplished through the use of the following components whose functionality will be discussed below:

∑ contactors
∑ pre-charge circuit
∑ fuse
∑ manual service disconnect
∑ high/hazardous voltage integrity/interlock loop (HVIL) circuit.

Contactors

Contactors are electrically actuated switches that are primarily used to disconnect the battery pack from the vehicle's high voltage system when the vehicle is not in use. However, contactors are also a safety item that can be opened to disconnect the battery pack if some part of the vehicle malfunctions and is forcing the battery pack outside its design envelope. For example, the contactors can be opened by the battery management system if a charger failure allows the charger to begin overcharging the battery pack. The secondary safety function qualifies the contactors as a critical safety device in a modern high voltage battery pack. Their functionality and operating limits such as interrupt current rating and voltage rating must all be thoroughly understood.

Pre-charge circuit

As a battery pack closes the contactors and transitions from a disconnected state to a connected state, there is a possibility for momentary but very large currents. Thus, the transition must be a controlled process. That control is provided by the precharge circuit. The precharge circuit consists of an additional contactor and a resistor that controls the flow of current into the high voltage vehicle bus as capacitance on the vehicle bus is charged up to the battery pack voltage. Without the precharge resistor, today's low resistance batteries can supply many thousands of amps if suddenly connected to a low resistance load. This can weld the contacts within the contactor, or damage external circuitry. Lastly, the precharge process allows detection of external faults such as external short circuit faults before allowing full battery pack current to enter the high voltage bus. Thus, the pre-charge resistor is also a required safety component in battery packs.

Fuse

The fuse is another important safety device. The fuse is used to protect components from excess current, including current above the contactor interrupt rating. Fuse selection is not trivial and must be coordinated with the other high voltage system components such that all necessary failure modes are covered by either the fuse or the contactors.

Manual service disconnect (MSD)

A fourth safety device used within a high voltage battery pack is called a manual service disconnect (MSD). This device is a method for breaking the electrical continuity within the pack for the purpose of vehicle service. The MSD is often a touch safe, removable enclosure containing the fuse. The MSD is not designed for removal while there is current flowing through the battery pack; the contactors must be opened prior to MSD removal. Removal of the MSD is useful for servicing the high voltage system of the vehicle because the positive and negative connections to the pack are no longer electrically connected.

HVIL

One final safety mechanism used in some high voltage packs is the HVIL. This is a continuous electrical circuit that crosses high voltage access points in a manner that signals the battery management system if any access points are open. This is further discussed in Section 19.6 on the battery management system.

19.5.3 Thermal management system

A carefully controlled thermal environment is critical to the safety, performance and longevity of a lithium ion battery pack. Electrical performance drops off markedly with temperature; power output can vary by an order of magnitude between –20 °C and +40 °C. Although cell electrical performance increases with temperature, life-limiting chemical reaction rates also increase with temperature. High temperature cell life and low temperature performance are the reasons for investing in a thermal control system. Thermal control systems vary significantly in complexity depending on the system design, duty cycle, and cell attributes. The Nissan Leaf electric vehicle does not have any active cooling, instead relying on thermal mass and conduction/ convection from the external battery pack surfaces (see Plate XVII between pages 392 and 393). The GM Volt, on the other hand, utilizes liquid coolant meticulously distributed to every cell in the battery pack through carefully designed cooling plates adjacent to each cell as shown in plate XVIII and Fig. 19.20.

Liquid cooling is often employed in spatially dense, high power packs that generate significant heat. Cooling fluid is provided to each cell – typically flowing through a metal cooling plate or jacket adjacent to the cell. Liquid coolants are, to date, electrically conductive. A coolant leak inside the battery pack is a serious failure from both a reliability and safety aspect.

A second solution utilizes fans to force air through the battery pack. The air must be clean to ensure conductive contaminants are not introduced into the pack where they may eventually coalesce into a short circuit pathway. Passenger compartment air can be used as a source of clean air but an engineering solution must be devised to prevent unintended fluids from entering the pack, and prevent any vented gases from a failed cell from flowing back into the passenger compartment because the gases can be toxic and highly flammable. The Mitsubishi i-MiEV uses air cooling as shown in Fig. 19.21.

19.20 Chevrolet Volt cell assembly showing liquid cooling plate. Image courtesy of A2Mac1, LLC and Ricardo Strategic Consulting, North America.

19.21 2012 Mitsubishi i-MiEV battery pack showing air cooling ducts. Image courtesy of A2Mac1, LLC and Ricardo Strategic Consulting, North America.

Both the liquid and air cooled systems can be augmented by the incorporation of a refrigeration cycle to remove heat from the cooling fluid.

A third option is available. In some cases, no active cooling is used at all. In these designs, cooling depends on thermal mass in combination with conduction from the cells through the pack structure to the pack surface where air convection removes heat. This allows a completely sealed pack with no chance of a coolant leak inside the pack or contaminants entering the pack.

Some prototype packs have utilized Peltier cooling. This allows active heat removal while maintaining a fully sealed pack. However, Peltier systems are less efficient and more expensive than other methods; the performance requirements rarely justify this solution.

In addition to cooling, there can also be a need to heat batteries (some hybrids can revert to the engine if the battery pack is too cold). When heating is deemed necessary, it can have a dramatic impact on range. The battery pack may require in excess of 5% of its energy to heat itself 10 °C. Thus, heating is best accomplished while a vehicle is plugged in.

19.5.4 Environmental enclosure

The environmental enclosure is the third major system associated with high voltage battery packs. This enclosure must perform two tasks. The enclosure must keep objects, liquids, and debris out of the pack and it must protect the pack sufficiently in crash events. Foreign objects, liquids and debris can all cause internal short circuits in the battery pack. These can lead to pack failure or fires. Common concerns include liquid ingress when driving through deep water, salt ingress in the winter, small animal or insect ingress, and dust ingress. Fully sealed packs allow better control over the interior environment.

The second enclosure requirement involves crash response. Vehicle crash is one of the few vehicle failure modes that occur regularly across a broad range of severities. It is not possible to design around all possible crash scenarios. Instead, the battery pack enclosure and structure must be coordinated with the vehicle structure to ensure both regulatory compliance and sensible cell protection in non-regulated crash scenarios. In depth knowledge of the cell response to crush processes is required.

19.5.5 Summary

This section provided an overview of several systems within a high voltage battery pack and some of their functions. It should be apparent that there are many safety systems within a high voltage battery pack that must all work together. This adds significant design and development costs as well as component costs. However, they are necessary systems to meet the high levels of safety in modern transportation systems.

19.6 Battery management systems

19.6.1 Introduction

The battery management system (BMS) is a sophisticated hardware and software system which is generally a required part of any high voltage battery pack. The common functions of the BMS include:

∑ cell voltage measurement and control
∑ contactor control
∑ isolation monitoring
∑ temperature measurement and control
∑ state of charge/health calculation
∑ communications.

These functions are necessary to ensure vehicle safety and balance vehicle performance with battery life. Each of the above functions will be reviewed in this section in the context of lithium ion battery packs.

19.6.2 Cell voltage measurement and control

Voltage monitoring is possibly the most important function of the BMS. The BMS typically monitors the voltage across each series group of cells. Voltage excursions due to overcharge, overdischarge, or high power pulses can lead to significantly reduced life and safety issues. Cell voltage is also a critical input to the cell balancing algorithm and the state of charge and state of health algorithms.

The most important reason for monitoring cell voltage is overcharge avoidance. Lithium ion cells must not be overcharged. Excess overcharge will result in a combination of chemical reactions and temperature rise that leads to cell venting. This, obviously, ruins the cell and creates a serious safety concern. The vented gases are, to date, highly flammable. In addition, severe overcharge can heat a cell sufficiently to cause it to catch on fire. Overcharge can be prevented if the cell voltage is monitored and the charging power source is disabled before the voltage rises to an overcharge value. The design of a battery pack must include a robust method for monitoring cell voltage to ensure overcharge and overdischarge are very rare, detectable occurrences.

If any cell reaches the edge of the allowable voltage range, the BMS typically has multiple responses. First, the BMS will request a change in power flow in/out of the battery pack to bring the voltage back within limits. If this does not work due to some component failure, the BMS typically has the authority to open the contactors on the battery pack and stop all power flow in/out of the battery pack. Disabling a vehicle is also an unsafe condition for the vehicle occupants and should be a last resort. Thus, BMS design should include a variety of strategies to ensure consistent, safe operation even with at least one failure.

19.6.3 Contactor control

As described in the cell voltage control section, the BMS often controls the pack's contactors. This includes both the pre-charge contactor and main contactor(s). Some battery packs utilize a separate high voltage circuit for plug-in charging to limit the number of energized high voltage wires on the vehicle during charging. This architecture will require an additional set of contactors and a more complex contactor control algorithm.

The contactor control algorithm must also receive some feedback of the contactor state (open/closed) and must be able to confirm that the pre-charge process worked as expected. This algorithm should be able to identify an inoperative contactor, and should be able to identify a fault (short circuit or other) outside of the battery pack that prevents the precharge process from bringing the high voltage system up to operating voltage.

Another related circuit that is often employed is called an HVIL circuit. This is a circuit that is employed to ensure people do not gain access to the high voltage system while it is energized. This is accomplished through the use of switches at access panels and through additional electrical connector pins that are used to electrically determine if a connector has been removed. In the simplest implementation, the power for the coil that closes the high voltage contactors travels along a circuitous path through each user access point in the high voltage system. If any access point has been breached or opened, the conductor is broken and the contactor coil power is removed resulting in immediate opening of the contactors and ensuring the high voltage system is de-energized. This simple implementation is not typical of today's vehicles because it generates undesirable failure modes. Any faulty connection or switch will de-energize the high voltage system without warning. This can put occupants at risk while on the road if the vehicle propulsion system stops unexpectedly.

19.6.4 Isolation monitoring

Isolation monitoring is another critical safety function of any high voltage vehicle system. In today's vehicles, the high voltage system is fully isolated from the vehicle chassis. This ensures that a single fault in the high voltage system does not expose a person to electrical shock.

This process works because an electrical shock can only occur when current is supplied to your body from an electrical circuit, and there is a return path from your body to a lower voltage portion of the electrical circuit. A fully isolated circuit requires two faults before current can flow through the body – one fault allows electrons to leave the circuit and flow into the body and a second fault must exist to allow the electrons to leave the body and flow back into the circuit.

Isolation monitoring circuits regularly check the resistance between the high voltage system and the chassis. Typical guidance indicates this value should be greater than 100 ohm/volt for direct current (DC) systems (ISO 2011). Thus, a 350 V system should maintain a resistance above 35 kohms between the high voltage system and the chassis. This ensures that current will be limited to approximately 10 milli-amps which is uncomfortable but not generally life threatening (IEC 2007).

There are a number of methods for measuring isolation on the vehicle. The circuit used to measure isolation should not have any failure modes that allow the isolation monitoring safety circuit to become the source of isolation failure.

19.6.5 Temperature control

The BMS is usually responsible for battery pack and cell temperature control. Temperature is yet another characteristic that has significant safety and life implications as described earlier. The strategy for monitoring and controlling cell temperatures is unique to each application. But, the BMS must have sufficient information about temperature variation across the pack to allow the control process to adjust heating, cooling or pack power levels as required.

19.6.6 State of charge/health calculation

Possibly the most complex algorithm within a battery management system is the state of charge/state of health algorithm (SOC/SOH). In the simplest terms, SOC is the percentage of electrons available to do work compared to a fully charged battery whereas SOH is a broader measure of the overall performance capability of the battery compared to its initial performance when new.

An accurate SOC calculation is required for numerous dependencies, including indicated vehicle range, engine operating mode/calibration, and electrical power limits including charge rate. These dependencies are important vehicle characteristics and thus drive the need for an accurate SOC value.

SOC cannot yet be measured directly. Thus, SOC algorithms estimate state of charge utilizing quasi-static cell voltage measurements, current integration, cell voltage transient response and cell temperature. The core of a state of charge algorithm is the cell's voltage versus state of charge curve. For many chemistries, this curve is used to recalibrate the state of charge after the vehicle has been parked for a number of hours and cell voltages have stabilized. Cell voltages typically do not equilibrate fast enough to use this curve alone while the vehicle is operating. Lithium iron-phosphate cells have a particularly flat voltage versus state of charge curve throughout the majority of the operating range, varying less than 35 mV over as much as 35% of the operating range. This presents further hardware and software difficulties when using this chemistry.

To augment the cell voltage measurements, SOC algorithms integrate the current entering or leaving the battery pack as a second input. Many additional techniques have been developed to provide further state of charge accuracy, including characterization of cell transient response as a function of temperature, state of charge and power level.

The BMS must also perform cell balancing. The process of cell balancing requires accurate cell voltage measurement. Cell balancing is the process of equalizing the individual cell's state of charge. This is necessary due to variation in both the cells and their on-vehicle environment. Manufacturing variation can result in differing cell capacity and resistance values. Likewise, on-vehicle operating environment variation (temperature for example) can also result in differing cell resistances. These effects cause the cells to slowly diverge in their state of charge.

Two types of cell balancing can be implemented – passive and active. Passive balancing uses a simple resistance across a cell to remove energy from higher SOC cells until they more closely match lower SOC cells. Active balancing uses a more complex circuit to transfer energy from higher SOC cells to lower SOC cells.

SOH is a more complex algorithm but does not require the same minuteto-minute accuracy. The SOH algorithm estimates overall performance loss as the battery degrades with time. SOH algorithms typically include calculations for loss of capacity, increase in resistance and the resulting loss of power and energy content.

There are many state of charge and state of health patents; manufacturers must ensure they do not unknowingly infringe on existing patents while developing software that meets their needs. Battery SOC and SOH algorithms will also continue to develop as chemistries evolve and their performance is further characterized.

19.6.7 Communication

One final BMS function is the need to communicate with additional vehicle controllers. The BMS often requests changes in vehicle operation in response to monitored battery pack conditions and can provide some forecasting of battery pack capability in the near future. Communication is also necessary to provide information to the driver, such as vehicle range, operating mode, and any faults that arise. Lastly, off-vehicle communication is required when using an off-board DC charger. Off-board DC chargers will enable higher re-charge rates without burdening the vehicle with the cost, weight and complexity of a high power on-board charger.

19.6.8 Summary

The battery management system for a lithium ion battery pack is both a complex system and a significant contributor to safety, reliability and performance. As a result, the battery management system requires careful hardware and software design; the development cost and timeline are often underestimated.

19.7 Future trends

19.7.1 Introduction

One of the more exciting aspects of the battery industry is the amount of unexplored area within the design space. Materials have been proposed that far exceed today's leading edge technology. Research is spread across all aspects of battery performance including energy and power improvement, cycle life improvement, cost reduction, packaging flexibility, and safety improvement, to name a few areas of active research.

19.7.2 Energy metric improvements

Current battery technology is far from replacing liquid fossil fuels in either volume or weight based energy metrics. Our transportation infrastructure and societal expectations evolved around the capabilities of liquid fuels; thus, there is strong motivation to improve energy metrics for battery technologies to allow further adoption and the resulting CO2 reduction opportunities. The energy stored in a cell is a function of the cell's operating voltage and the capacities of the anode and cathode materials (expressed in milli amp-hours per gram or milli amp-hours per cubic centimeter).

Negative electrodes today typically utilize carbon structures. Zhang (2011b) reviewed the performance of a number of these materials where he provided theoretical capacity and density metrics for carbon as 372 mA-hr/ gm and 837 mA-hr/cm³ . Recently, there has been significant activity in the development of silicon anodes whose metrics are roughly ten times better than carbon but with approximately 10% decrease in cell voltage compared to carbon. However, silicon undergoes more than a 300% volume change as it progresses from silicon to the fully lithiated state of Si5Li22 (equivalent to 4.4 lithium atoms per silicon atom). This volume change tends to destroy the silicon particles due to strain after a small number of cycles. Mitigating this effect through control of particle shape and size is a primary area of research (Whittingham 2012, for example).

Positive electrode development is also promising. Whittingham (2004) provided a thorough review with an historical perspective. Fergus (2009) provided a thorough review of performance reported in the literature. Maximum capacities presented were on the order of 190 mA-hr/g for Ni1/3Co1/3Mn1/3O2 materials. Near term development has included the so-called 'layered-layered' cathode material where Li2MnO3 is layered within another material. This combination has been shown to provide capacities beyond 250 mAhr/g (Amine 2012, Lopez 2012). Ceder (2009, 2011) provided an overview of theoretical capacities of many materials based on first principle calculations; those results included numerous materials with capacities above 300 mAhr/g and voltages in excess of 4.0 V, indicating there is plenty of room for further exploration.

19.7.3 Electrolyte improvements

Electrolytes and solvents for lithium ion cells also have many areas of potential improvement. To date, hexafluorophosphate (PF⁶ ) is often used with a mix of solvents such as ethylene carbonate, propylene carbonate, ethyl methyl carbonate and dimethyl carbonate (Reddy 2010, Table 26.9). There are a number of desirable improvements under investigation.

A safer electrolyte is highly desirable. Many of the solvents in today's electrolytes are flammable and thus present a hazard if the cells are breached and release electrolyte. This is of particular interest for crash hazard reduction where it is difficult to ensure cell containment will remain intact in all cases.

In addition, research continues for a chemical mechanism that will limit overcharge to a particular voltage and cycle the excess energy through a circular chemical reaction mechanism called a redox² shuttle (Zhang 2011a). This limits the effects of overcharge to a problem of heat removal equivalent to the charge power – rather than permanent cell damage or potentially a catastrophic chemical decomposition.

Another area of interest is the development of electrolytes that withstand higher cell voltages without decomposition. Today's electrolytes begin to oxidize (or lose electrons) in the range of 4.5 V (Zhang 2012). This limits the development of higher voltage electrochemical systems that allow higher power and energy metrics.

Today's electrolyte performance is also limited at the low and high temperature limits of transportation applications. Ionic conductivity is noticeably degraded at the lowest temperatures which results in decreased cell power capability. Chemical decomposition rates are higher than desired at the upper temperature range, limiting calendar life and forcing some designs to incorporate expensive cooling systems.

Lastly, there has been ongoing activity in the development of two fundamentally different cell designs from today's two-dimensional layered designs utilizing coated aluminum and copper foil. The first is the development of three dimensional batteries, often using nano-scale features grown on a substrate. An overview of this area of work was provided by Long (2004)

The second area of interest is solid state batteries. These are batteries comprising only solid material (no liquid electrolyte). To date, power metrics have been limited because the ionic conductivity of solids is generally much lower than liquids (by several orders of magnitude). However, this can be overcome to a degree by building the cell with very thin electrode and separator layers. This may be possible through the use of semiconductor manufacturing techniques. Infinite Power Solutions has commercialized a solid state cell but with capacities in the micro A-hr range; thus they are not yet useful for transportation. Nancy Dudney's group at Oak Ridge National Labs has published numerous results in this area (Dudney 1999, 2005). Lastly, the Department of Energy's ARPA-E has recently funded four solid state battery programs as part of its RANGE program (ARPA-E 2013).

19.7.4 Further developments

Additional developments are occurring in the areas of battery packaging. Some vehicle manufacturers are investigating the ability to distribute their

Redo is short for reduction-oxidation – the two processes in a circular chemical reaction.

battery pack around the vehicle to allow for easier packaging of physically large batteries. Structural batteries are also under investigation. This could allow batteries to both store energy as well as provide necessary vehicle structure. If the battery pack displaces otherwise necessary structural material, the effective performance metrics could improve substantially when using the incremental weight and volume of the battery pack beyond the necessary structural material.

19.8 Conclusions

This chapter attempts to provide an overview of battery requirements for CO2 reduction in transportation applications. Battery systems are quite complex and become more complex when developed into high voltage designs that are further subjected to the broad environmental extremes seen in transportation. Maintaining reliability at a tolerable cost without compromising safety is a significant undertaking that requires a talented team encompassing electrical, mechanical, software and manufacturing expertise. However, vehicle electrification appears to provide a viable pathway towards significant reduction of both vehicle CO2 production and ultimately global CO2 production.

19.9 Sources of further information and advice

Industry standards organizations

Automotive Research Association of India (www.araiindia.com) Institute for Electrical and Electronics Engineers (www.ieee.org) International Organization for Standardization (www.iso.org) International Electrotechnical Commission (www.iec.ch) Japan Automobile Research Institute (www.jari.or.jp) National Fire Protection Association's Electric Vehicle Safety Training (www.evsafetytraining.org) SAE International (www.sae.org) Verband der Automobilindustrie (VDA) (www.vda.de)

Journals

Chemistry of Materials Electrochemical and Solid State Letters Electrochemistry Communications Electrochimica Acta Journal of Applied Electrochemistry Journal of Power Sources

Journal of Solid State Electrochemistry Journal of the Electrochemical Society Materials Chemistry and Physics Solid State Ionics

Lithium ion cell manufacturers

A123 Systems (http://www.a123systems.com/)

AltairNano (http://www.altairnano.com/)

American Lithium Energy (http://americanlithiumenergy.com/)

Automotive Energy Supply Corporation – Nissan/NEC joint venture (http:// www.eco-aesc-lb.com/)

Bosch (http://www.bosch-kraftfahrzeugtechnik.de)

Boston Power (http://www.boston-power.com/)

BYD (http://www.byd.com.cn/)

Cobasys (http://www.cobasys.com) – owned by Bosch

Dow-Kokam (http://www.dowkokam.com/)

EIG (http://www.eigbattery.com/)

Envia Systems (http://enviasystems.com/)

Electrovaya (http://electrovaya.com/)

Ener1 (http://ener1.com/)

GAIA/Lithium Technology Corporation (http://www.gaia-akku.com/)

Gold Peak (http://gpbatteries.com/)

GS Yuasa – including Yuasa and Lithium Energy Japan (http://www.gsyuasa.com/)

Hitachi Automotive Systems (http://www.hitachi-automotive.co.jp/)

Johnson Controls (http://www.johnsoncontrols.com/)

K2 Energy (http://k2battery.com/)

LG Chemical (http://www.lgchem.com/)

Lishen (http://www.lishen.com.cn/)

Lithium Energy Japan (http://www.lithiumenergy.jp/)

Molicel (http://www.molicel.com/)

NEC (see Automotive Energy Supply Corporation joint venture with Nissan)

Panasonic Electric Vehicle Energy (http://www.peve.jp/)

SAFT (http://www.saftbatteries.com/)

Samsung (samsung.com)

Sanyo (majority owned by Panasonic as of December 2009)

SK Innovation (http://www.skinnovation.com/)

Sony (no known transportation efforts to date) (h ttp://www.sony.net/)

Toshiba (http://www.toshiba.co.jp/)

Valence Technology (http://www.valence.com/)

19.10 References and further reading

Amine, K., 2012. Engineering of High Energy Cathode Material. In: United States Department of Energy Annual Merit Review and Peer Review Meeting, Washington DC, 14–18 May.
ARPA-E, 2013. 'ARPA-E Project Selections Robust Affordable Next Generation EV-Storage (RANGE).' 21 August 2013 Available at: www.arpa-e.energy.gov/sites/ default/files/documents/files/RANGE_ProjectDescriptions_082013.pdf> (accessed 25 August 2013).
Arora, P., 2004. Battery Separators, Chemical Review, 104, 4419–4462.
BAJ, 2012. Total Battery Production Statistics. Available at: http://www.baj.or.jp/e/ statistics/01.html. (accessed 2 June 2012).
Ceder, G., 2009. First Principles Calculations (and NMR spectroscopy of electrode materials). In: United States Department of Energy Annual Merit Review and Peer Review Meeting, Washington DC, 18–22 May.
Ceder, G., 2011. First Principles Calculations and NMR Spectroscopy of Electrode Materials. In: United States Department of Energy Annual Merit Review and Peer Review Meeting, Washington DC, 9–13 May.
Dudney, N., 1999. Nanocrystalline LixMn2-yO4 cathodes for solid-state thin-film rechargeable lithium batteries. Journal of the Electrochemical Society, 146, 2455– 2464.
Dudney, N., 2005. Solid-state thin-film rechargeable batteries. Materials Science and Engineering B-Solid State Materials for Advanced Technology, 116, 245–249.
Fergus, J., 2009. Recent developments in cathode materials for lithium ion batteries, Journal of Power Sources, 195, 939–954.
IEC, 2007. IEC 60479-5 Effects of current on human beings and livestock Part 5: Touch voltage threshold values for physiological effects. Geneva; International Electrotechnical Commission.
ISO, 2011. ISO 6469-3 Electrically propelled road vehicles Safety specifications Part 3: Protection of persons against electric shock. Geneva. International Standards Office.
LLNL, 2013. Estimated US Energy use in 2012: ~95.1 quads. Report by Lawrence Livermore National Laboratory.
Long, J., 2004. Three-dimensional battery architectures, Chemical Reviews, 104, 4463–4492.
Lopez, H., 2012. Development of High Energy Lithium Batteries for Electric Vehicles. In: United States Department of Energy Annual Merit Review and Peer Review Meeting, Washington DC, 14–18 May.
Primearth EV Energy Co. Ltd. 2012. Plastic Case Prismatic Module, Available at: http:// www.peve.jp/e/hevjyusi.html (accessed 3 June 2012).
Quiet Car Notice of Intent, National Highway Traffic Safety Administration (NHTSA). 2012. Quiet Car Notice of Intent, National Highway Traffic Safety Administration (NHTSA). [ONLINE] Available at: http://www.nhtsa.gov/Laws+&+Regulations/ Quiet+Car+Notice+of+Intent (accessed 27 May 2012).
Reddy, Thomas 2010. Linden's Handbook of Batteries, 4th edn, McGraw-Hill Professional.
Robert Bosch GmbH, 2011. Bosch Automotive Handbook, 8th edn, Bentley Publishers.
US EPA. 2012. Dynamometer Driver's Aid, Testing & Measuring Emissions, Washington DC. Available at: http://www.epa.gov/nvfel/testing/dynamometer.htm (accessed 1 February 2012).
USCAR, 2012. US Advanced Battery Consortium. Available at: http://www.uscar.org/ guest/teams/12/U-S-Advanced-Battery-Consortium (accessed 27 May 2012).
Whittingham, M., 2004. Lithium batteries and cathode materials, Chemical Reviews, 104, 4271–4301.
Whittingham, M., 2012. Metal-Based High Capacity Li-Ion Anodes. In: United States Department of Energy Annual Merit Review and Peer Review Meeting, Washington DC, 14–18 May.
Xu, K., 2004. Nonaqueous liquid electrolytes for lithium-based rechargeable batteries, Chemical Review, 104, 4303–4417.
Zhang L., 2011a. Novel redox shuttle additive for high-voltage cathode materials, Energy and Environmental Science, 4, 2858–62.
Zhang W., 2011b. A review of the electrochemical performance of alloy anodes for lithium ion batteries, Journal of Power Sources, 196, 13–24.
Zhang, Z., 2012. High Voltage Electrolyte for Lithium Batteries. In: United States Department of Energy Annual Merit Review and Peer Review Meeting, Washington DC, 14–18 May.

M. Ehsani, Texas A&M University, USA

DOI: 10.1533/9780857097422.3.632

Abstract: This chapter presents an introductory overview of the basic hybrid power trains for vehicles. The three fundamental topologies of hybrid power plants, series, parallel, and series-parallel, are introduced. Some variations of these basic topologies, such as, plug-in hybrids are also discussed. Finally, basic behavior and performance of example hybrid vehicles is presented for further insight. The objective is to familiarize the reader with the basic concepts of hybrid electric vehicles and to serve as a gateway for more detailed study of the subject.

Key words: hybrid electric vehicles (HEVs), plug-in hev, fuel cell vehicles, series HEV, parallel HEV, series-parallel HEV, design of HEV, control of HEV, performance of HEV, HEV power train, HEV batteries, HEV engines.

20.1 Introduction

Fossil fuels have been used as primary energy sources in vehicles for more than a hundred years. Their high energy density, availability, and low hazard level make them the perfect choices for on board energy storage. Due to the characteristics of internal combustion engines, conventional vehicles suffer from the disadvantages of poor fuel economy and pollutant emission. The reasons for these disadvantages are the mismatch between engine fuel efficiency distribution and real time operational points, and frequent stop-and-go drive patterns in city driving causing energy dissipation through braking. To address these problems, more complex transmissions have been developed to match the engine characteristics with drive cycle requirements, but thus greatly increasing purchase and maintenance costs making transmission repairs one of the most formidable aspects of repair work. The idea of coupling a secondary drive train that can compensate for the low efficiency working region of engines was proposed. A large variety of choices have been explored including fuel cell-motor, battery-motor, ultra-capacitor-motor and a combination of one or more of these. Vehicles employing two or more power sources are referred to as hybrid vehicles. In most cases, secondary power sources are coupled to the conventional power train through electric motors, thus these vehicles are usually called hybrid electric vehicles or HEVs.

The concept of HEVs is almost as old as the automobile itself. In 1906 there were more battery electric hybrids operating on the streets of Detroit than conventional internal combustion engine cars, with the Baker Electric car company of Ohio claiming to be the largest car factory in the world. But the primary purpose of introducing electric drive was to assist internal combustion engines to provide an acceptable level of performance for the early hybrids. The reason for this was the relatively weak engines back then combined with inferior transmissions and the lack of a starter motor, but most of the early hybrid models suffered from a lack of control over the electric motor, formidable weight and low reliability of the additional drive train. Only the 'modern' HEV will be discussed in this chapter including basic concepts, architecture and operating principles.

20.2 Basic components of a hybrid electric vehicle system

First, the basic concepts of a vehicle system need to be specified. The most important part in a vehicle system is the drive train or power train. A drive train usually consists of an energy source or energy storage, an energy converter and the necessary connector parts from the energy converter to the drive shaft or transmission. In the conventional vehicle system, the energy storage is the fuel tank, the energy converter is the internal combustion engine and the connector is the transmission. In later parts of this chapter, this is referred to as the internal combustion (IC) engine drive train. In a hybrid vehicle system, there usually exists a second drive train. In most cases, the second drive train is an electrical power train using an electrical motor as the final energy convertor, but energy conversion can go through multiple stages such as in a fuel cell hybrid, from chemical energy to electrical to mechanical. Different means of energy storage can be used, light fuel, such as hydrogen and natural gas, battery pack and ultra-capacitors are a few examples of popular choices. In some systems different types of storage can be combined in one drive train, one example is the combination of battery and ultra-capacitor.

In Fig. 20.1, energy conversion in a vehicle is shown. The solid line indicates the energy conversion in a parallel hybrid, the dashed line shows energy conversion in a series hybrid, and the dotted line shows energy conversion in regenerative braking.

20.3 Architectures of hybrid electric drive trains

There are three major classifications of HEV architectures (as shown in Fig. 20.2):

20.1 Energy conversion.

Mechanical connection Electrical connection

20.2 Classification of HEVs (a) series hybrid drive train (electrical coupling), (b) parallel hybrid drive train (mechanical coupling) and (c) series-parallel hybrid drive train (electrical and mechanical coupling).

1. series hybrid
2. parallel hybrid
3. series-parallel hybrid.

Usually, for a conventional HEV, energy flowing in the drive train can be classified into two types: mechanical energy and electrical energy. Power merging and splitting only happens within same power type; there are two major types of power couplers: mechanical power coupler and electric power coupler. Due to the existence of the power coupler, two or more power flows can be merged together or one power flow can be split into two or more power flows. Therefore HEV architecture should be classified by power coupling features such as an electrical coupling drive train, a mechanical coupling drive train, and an electrical-mechanical coupling drive train[1].

Figure 20.2 shows the configurations of the major hybrid drive train architectures which are series, parallel and series-parallel. The configuration of a conventional series hybrid drive train is shown in Fig. 20.2(a). This is an electrical coupling configuration. A power converter can function as an electrical coupler; the key feature of this configuration is that power flow delivered to the electric motor can be formed by controlling the battery power flow and the power flow from the IC engine/generator, or in the opposite direction, from the electric motor back to the battery. Figure 20.2(b) shows the configuration of a conventional parallel hybrid drive train. This is a mechanical coupling configuration. Similar to the series hybrid drive train, there is a mechanical coupler in this configuration. The mechanical power of the IC engine and the mechanical power of the batteries/motor can be controlled at the mechanical coupler.

Figure 20.2(c) shows the configuration of a conventional series-parallel hybrid drive train. It can be seen that the distinguishing feature of this configuration is the ability to handle both mechanical coupling and electrical coupling. This configuration is the combination of series and parallel structures, not only possessing the major advantages of both but also having more operation modes than those of the series or parallel structure alone, due to the combination. However, the drawbacks of this configuration are obvious – increased complication and higher cost.

20.3.1 Use of gasoline or diesel engines

Most IC engines used in conventional vehicles are gasoline or diesel engines. For the same power level, the gasoline engine has a larger operating speed range and a smaller operating torque range than diesel. For conventional vehicles, these differing torque-speed characteristics offer different choices. For example, most passenger vehicles use gasoline engines because this kind vehicle needs a better speed range than torque range. In contrast, trucks and heavy duty vehicles benefit from the diesel engine due to its better torque performance. Therefore, different engines in different hybrid architectures will have different design purposes. Diesel engines require more costly exhaustafter-treatment systems to meet emissions legislation and this greater cost makes them significantly less affordable for consumers in hybrid applications. The higher costs of diesel hybrids makes it difficult for consumers to recoup the greater purchase price even if they are very high mileage users.

20.4 Series hybrid electric drive trains (electrical coupling)

A typical series hybrid train consists of an IC engine-generator drive train and a battery-motor drive train. The two drive trains are connected in series through the battery with a by-path from the generator to the electric motor. Power from either or both drive trains can be controlled to fulfill traction requirement. The classical configuration of a series hybrid drive train is shown in Fig. 20.3. In this configuration a unidirectional power source, the IC engine drive train is coupled to the bidirectional electric motor power source utilizing the charging-discharging ability of battery. The main electric voltage is set by the interface between the battery and the DC bus. In order to connect different voltage sources, a controllable rectifier is employed between the output of the electric generator and the DC bus. The fuel tank, the IC engine, the generator and the rectifier constitute a unidirectional energy flow path. A bidirectional energy source battery pack and a bidirectional power electronic DC/DC converter constitute a bidirectional energy flow path. Both

20.3 Configuration of a series hybrid electric drive train.

of these two electrical power flows are connected through the DC bus. The electric motor can serve either as load, traction motor or source, regenerative braking generator depending on operational conditions. The interface between the electric motor and the DC bus can either be a DC/DC converter or DC/ AC inverter depending on the different types of motor used. One easy way of conceptual understanding of the series hybrid is a pure electric car with an IC engine as a range extender.

In order to control the operation and power flows to meet the vehicle power demand and collect information from on board components, a vehicle controller is necessary which can control the engine throttle, on board power electronics and traction motor to produce propelling torque or recapture regenerative braking torque. Compared with the conventional IC engine vehicles, the vehicle is propelled by a traction motor, which is powered by a battery pack and/or an IC engine-generator unit. There are many available operation modes to meet the power demand of the vehicle.

The series hybrid drive train has following operational modes:

∑ Pure electric traction mode: the battery supplies the whole vehicle traction power demand with the IC engine turned off.
∑ Pure engine traction mode: the vehicle traction power is only drawn from the IC engine-generator, while the battery neither supplies nor receives any power from the drive train.
∑ Hybrid traction mode: both the IC engine-generator and the battery, merged through an electrical coupler supply the vehicle traction power demand.
∑ Engine traction with battery charging mode: the IC engine-generator power, split into two power flows at the electric coupler supplies power to charge the battery and to propel the vehicle simultaneously.
∑ Regenerative braking mode: the traction motor functions as a generator and recaptures the vehicle kinetic energy while the IC engine is turned off. The energy recovered is stored in batteries for reuse.
∑ Battery charging mode: the IC engine-generator operates only to charge the battery while supplying no power to traction motor.
∑ Hybrid battery charging mode: the power for charging the battery is drawn from both the IC engine-generator and the traction motor functioning as generator for regenerative braking.

From this configuration, it can be seen that the engine-generator system is only supplying power to the driving wheels through the electrical system with no mechanical coupling. The operating speed and torque of the engine are independent of vehicle speed and traction torque demand, and can be set to work at any operating point on its speed- torque plane. Therefore, the engine should be operated in its optimal operating region where fuel consumption and emissions of the engine are minimized. Due to the mechanical decoupling of

the engine from the driving wheel, this optimal engine operation is realizable. However, it heavily depends on the operating modes and control strategy of the drive train.

The series hybrid drive train has the following advantages:

∑ Because of the complete mechanical decoupling between the IC engine drive train and the driving wheel, an IC engine can operate at any point on its speed-torque map as the controller requests. Therefore, one major advantage of the series hybrid drive train is being able to always operate the IC engine at its optimal working point, given a power demand and operation mode; thus, fuel consumption and emission of the IC engine can be minimized. Furthermore, various engines can be used in a series hybrid drive train due to mechanical decoupling; the benefit of employing novel engines such as high speed turbine or star rotor engine in automobiles can be investigated.
∑ Due to the torque-speed characteristics of electric motors which are very close to ideal for traction, multi-gear transmission may not be needed in a series hybrid drive train. Therefore, the drive train structure can be simplified which also means lower cost. Furthermore, four in-the-wheel motors can be used to significantly enhance the vehicle's drivability, each one driving a wheel. In such a configuration, the speed and torque of each wheel can be independently controlled. This is very important for off-road vehicles which usually need to be driven on difficult terrain, such as ice, snow, and soft ground.
∑ Compared with other types of drive train configuration, the control strategy of the drive train is simple because there is no direct mechanical connection between the IC engine and driving wheels.

However, series hybrid electric drive trains have some disadvantages, such as the following:

∑ Energy from the IC engine output has to be transformed from mechanical to electrical and electrical back to mechanical before reaching the driving wheel. More attention needs to be paid to motor/generator control, because inefficiencies of the generator and traction motor may cause extra losses.
∑ The generator adds additional weight and cost.
∑ Because the vehicle is only propelled by the traction motor, it must be sized properly to produce enough power for designed vehicle performance, such as acceleration, gradeability and so on.

20.4.1 Design principles

Unlike other architectures, in the series hybrid drive train, the desired vehicle performances such as acceleration, gradeability, and maximum speed are highly dependent on the traction motor performance. Besides the traction motor, as the power source the iC engine and battery pack are also important to the series hybrid drive train. The design of these major components should be estimated at the system level for high operation effi ciency.

in order to design the power rating for the traction motor in a series hybrid drive train, acceleration, maximum speed, the vehicle grade ability and motor characteristics need to be determined. according to the performance requirements, the following equations can be used to estimate power rating of the traction motor:

$$P{\text{acc}} = \frac{\delta M}{2t} \left( V{\text{f}}^2 + V{\text{o}}^{\hat{\mathbf{a}}} \right) + \frac{2}{3} Mg f{\text{r}} V{\text{f}} + \frac{1}{5} \rho{\alpha} C{\text{D}} A{\text{f}} V_{\text{f}}^3$$ [20.1]

$$\mathbf{P}{\text{spd}} = \left( Mgf{\text{r}} + \frac{1}{2} \rho{\alpha} C{\text{D}} A{\text{f}} V{\text{m}}^2 \right) V_{\text{m}}$$ [20.2]

$$\mathbf{P}{\text{grade}} = \left( Mg f{\text{r}} \cos \alpha + \frac{1}{2} \frac{\alpha C{\text{p}} A{\text{p}} V{\text{g}}^2}{4 \pi^2 g^2} + Mg \sin \alpha \right) V{\text{g}}$$ [20.3]

where *P*acc is the demanded average electrical power during acceleration in w; *P*spd is the demanded electrical power for maximum speed in W; *P*grade is the demanded electrical power for grading at grade speed in W; Vm is the maximum speed of designed vehicle in m/s; *V*g is the grade speed of designed vehicle in m/s; d is the rotation inertia factor; M is the total vehicle mass in kg; t is the required acceleration time in s; *V*f is the fi nal speed of the vehicle for required acceleration in m/s; *V*b is the vehicle corresponding speed of motor based speed in m/s; g is the gravity acceleration of 9.80 m/ s ; *f*r is the tyre rolling resistance coeffi cient; *r*a is the air density of 1.202 kg/m³ ; *A*f is the front area of the vehicle in m² and *C*D is the aerodynamic drag coeffi cient.

For different performance requirements, equations [20.1]–[20.3] could estimate the traction motor power rating. in the engineering design, tradeoff may be made between the vehicle performances and cost.

an iC engine-generator in a series hybrid drive train can be designed to supply electrical power to the traction motor and charge the battery. in the design stage, there are two driving conditions which must be considered: long term driving at high speed such as on a highway and a frequent stop and go pattern driving such as in an urban setting. For the highway driving pattern, the vehicle should use the engine-generator to provide enough power to maintain performance at high speeds. For a frequent stop-go driving pattern, the engine-generator should produce average power to prevent the battery from being completely depleted and to remain within a healthy state of charge (sOC) level in order to deliver enough power for vehicle acceleration or grade climbing. Based on the design driving cycle, the power rating of the iC engine-generator has to be properly determined. it is interesting to note that the energy form has to experience transformation twice between the mechanical and electrical, thus the generator and traction motor effi ciencies need to be considered:

$$P_{\text{high}} = \frac{\left(Mgfr + \frac{1}{2}\rho{\alpha}C{\text{D}}A{\text{f}}V{\text{ig}}^2\right)V{\text{ig}}}{\eta{\text{ge}}\eta{\text{m}}}$$ [20.4]

$$P_{\text{urban}} = \frac{1}{T} \int0^T \left( \frac{Mg f{\text{r}} + \frac{1}{2}}{+ \delta M \frac{dV{\text{urban}}(t)}{dt}} \right) \frac{V{\text{urban}}(t)}{\eta{\text{ge}} \eta{\text{tm}}} dt$$ [20.5]

where *P*high is the demanded electrical power for highway driving in W; *V*high is the constant speed on the highway in m/s; *P*urban is the demanded average electrical power for urban driving in W; *V*urban is the constant speed in urban in m/s; *h*ge is the effi ciency of the generator; *h*tm is the effi ciency of the traction motor and T is the driving period in s.

There are two important factors for designing the battery pack: one is the power rating, the other is energy capacity. The battery pack has to support suffi cient power when it is needed while storing enough energy to avoid being completely depleted. in order to design the power rating of the battery pack, the traction motor maximum power rating is needed, the total power comes from the iC engine-generator and battery pack and should be at least equal to the maximum power of the traction motor:

$$P{\text{battery}} \ge P{\text{tm,max}} - P_{\text{IC}}$$ [20.6]

where, *P*battery is the power rating of battery pack in W; *P*tm,max is the maximum power rating of the traction motor in W and *P*iC is the power rating of iC engine in W.

in most frequent stop and go driving patterns, typical drive cycle information is necessary because the design is based on energy variation which is closely associated with the drive cycle and the control strategy:

$$\Delta E{\text{battery}} = \int{0}^{T} (t)dt$$ [20.7]

$$E{\text{battery}} = \frac{\Delta E{\text{battery}}}{\Delta_{\text{SOC}}}$$ [20.8]

where T is the time period that the battery energy is being consumed; *P*battery is the battery pack power, Δ*E*battery is the energy variation of the battery pack, Ebattery is the energy capacity of the battery pack. Here, ΔSOC is the operating range of the battery pack which is the difference between the maximum and minimum SOC.

Energy variation represents the total energy drawn from the battery during the pure electric drive mode. The whole battery pack energy capacity can be estimated based on the ΔSOC and the ΔEbattery. The choice of battery size is discussed in Section 20.2.1 that addresses utility factor (UF).

One commercially available series hybrid vehicle is the Chevrolet Volt which is formally known as a range extended electrical vehicle. It employs typical series hybrid architecture with a 16 kwh battery pack and offers about 40 miles of pure electrical range with a 111 kw electrical motor and a 63 kw IC engine. It also has a plug-in feature that allows battery recharging from low cost off peak mains electricity. The size of battery pack is carefully selected according to the UF which will be introduced in Sections 20.7.1. One notable feature is the mismatch between power ratings of different components in the power train. This gives an opportunity for future version optimizations in terms of cost and performance.

20.5 Parallel hybrid electric drive trains (mechanical coupling)

A typical parallel hybrid drive train is a drive train which consists of an IC engine mechanical power source and a battery/motor mechanical power source to propel the vehicle as shown in Fig. 20.4. A unidirectional energy

20.4 Configuration of a parallel hybrid electric drive train.

source fuel tank and a unidirectional energy converter IC engine constitute a unidirectional energy flow. A bidirectional energy source battery pack and a bidirectional power electronic DC/DC converter coupled to an electric motor with the output of the electric motor connected to a mechanical coupler forms a bidirectional energy flow. Both of the two flows connect at the mechanical coupler. The mechanical coupler directly drives the wheels through drive shaft. We can consider the parallel hybrid electric drive train as a conventional drive train with an additional batteries/motor which is used to improve the fuel economy of the IC engine. The distinguishing feature of this architecture is that two mechanical powers from the engine and electric motor are added together by a mechanical coupler.

Generally, there are three types of mechanical coupling: torque coupling, speed coupling, and torque-speed coupling. In torque coupling, the IC engine torque combines with electric motor torque at the mechanical coupler, and then the total is delivered to the driving wheel. However, there is a fixed ratio relationship between the speeds of the engine, motor, and driven wheels. Due to this speed link the speeds cannot be independently controlled because of the power constraints. Similarly, in speed coupling, the IC engine speed and the electric motor speed can be combined together while the IC engine torque and electric motor torque are linked. By combining torque and speed coupling, there is a hybrid drive train in which torque and speed coupling states can be alternatively chosen usually through a few sets of planetary gears.

The separated axle torque-speed coupling architecture, which can be used to convert a conventional vehicle to a hybrid, is another parallel hybrid drive train. In this architecture, one axle is driven by the IC engine mechanical connection while the other is driven by a traction motor powered by a battery pack. Therefore, the total traction power comes from two separated axles to the power train on the vehicle chassis. By adding electrical traction to another axle of a conventional vehicle whose IC engine and transmission are left intact, the two-wheel drive vehicle is converted to a four-wheel drive which has better traction performance. The major disadvantages of this architecture are:

∑ the mechanical system is more complicated due to the two axles' torque coupling.
∑ regenerative braking is the only way to charge the battery without another mechanical connection between the motor and IC engine.

The operating modes are the same as in the series hybrid drive train. There are two advantages of the parallel hybrid drive train compared with the series one:

∑ for the IC engine there is only one energy conversion, thus at one operating point the energy efficiency may be higher.

∑ there is no need for an additional generator.

The major disadvantages are:

∑ the mechanical connection from the engine to the driven wheels does not allow the engine to operate in the most effi cient speed and torque region
∑ more attention should be paid to the complex structure and control.

20.5.1 Design principles

Because the parallel hybrid drive train is using a mechanical coupling, the design principles of the parallel hybrid drive train which include the iC engine, the battery pack and the electric motor are totally different from the series one. a proper design ensuring that all the components work harmoniously is necessary for the vehicle to produce the desired performance.

20.5.2 Engine power design

The iC engine is the primary energy source of the parallel hybrid drive train to propel the vehicle at constant speed on the highway without the assistance of the battery pack/electrical motor. For power requirements on the highway at a constant speed, the equation is the same as before:

$$P{\text{high}} = \left( Mgf{\text{r}} + \frac{1}{2} \rho{\text{a}} C{\text{D}} Af V{\text{ig}}^2 \right) V{\text{ig}} / \eta{\text{trans}}$$ [20.9]

where, *P*high is the demanded electrical power for highway driving in W; Vhigh is the constant highway speed in m/s and *h*trans is the effi ciency of the transmission.

as in the frequent stop and go driving pattern in the series hybrid drive train, the engine/generator should produce average power to avoid battery discharge and to maintain a healthy sOC level in order to produce enough power for vehicle acceleration or grade climbing. Based on the design drive cycle, the power rating of the iC engine/generator has to be properly determined. The power needed is expressed as:

$$P{\text{urban}} = \frac{1}{T} \int{0}^{T} \left( \frac{Mgf{\text{r}} + \frac{1}{2} - \left(t\right)^{2}}{t + \delta M \frac{dV{\text{urban}}(t)}{at}} \right) \frac{V{\text{urban}}(t)}{\eta{\text{crans}}} dt$$ [20.10]

where T is the urban driving period in s; *P*urban is the demanded electrical power for urban driving in W and *V*urban is the constant speed in urban in m/s.

in a parallel drive train, the mechanical coupling between the iC engine and the driven wheels is achieved through the transmission. hence, the engine rotating speed varies with vehicle speed. On the other hand, engine power is also associated with the speed which means the engine power variation also corresponds with the vehicle speed. Thus, unlike the series hybrid drive train in which the engine operating point can be fi xed, the operation of engine power in the parallel hybrid drive train to support the average power is not as straightforward as in a series hybrid.

20.5.3 transmission design

at low and medium speeds, the multi-gear transmission could help the engine to improve signifi cantly the tractive effort of the driven wheels which enhance vehicle acceleration performance and gradeability. With the assistance of the electric motor, the acceleration and gradeability can be further improved. another advantage of the improved tractive effort is enhancing the charging ability of the battery pack which could reduce the charging time by providing higher charging current. however, the multi-gear transmission system will increase the complexity of the drive train and control strategy.

The purpose of the electric motor in the parallel hybrid drive train is to provide peak power to the drive train. Thus, peak power in the drive cycle and the desired acceleration performance are major factors for designing the electric motor for the parallel hybrid drive train. The typical process is as follows. First, assume that the engine supplies the steady state power demand such as the rolling resistance and the aerodynamic drag, and the motor handles the dynamic power demand such as the acceleration. Based on this assumption, the power rating of the motor can be expressed by the equation:

$$P{\rm m} = \frac{\delta M}{2\eta{\rm trans}t} \left( V{\rm f}^2 + V{\rm b}^2 \right)$$ [20.11]

where *P*m is the necessary electrical power rating of the electric motor.

it is interesting to note that the obtained power rating of the electric motor based on equation [20.11] is overestimated because at maximum speed the engine power is always more than the steady state power during acceleration which means the engine could support more power to help the motor during acceleration. The electric motor power rating could be estimated more accurately based on different transmission types and different engine requirements.

The battery power rating and energy capacity are major concerns for battery design. Because the battery pack is the only energy source coupled to the electric motor to generate the mechanical power, the power rating of battery pack is straightforward and should be larger than or, at least, equal to the electric motor power rating:

$$P{\text{battery}} \ge \frac{P{\text{m}}}{\eta_{\text{m}}}$$ [20.12]

where *P*m is the electric motor power rating; *h*m is the motor effi ciency and *P*battery is the power rating of the battery pack.

The energy capacity of battery pack is designed based on the electrical energy consumption in different driving patterns which normally are the acceleration cycle and the urban driving cycle. The consumed electrical energy during the acceleration period could be calculated by the following equation:

$$E_{\text{battery}} = \int0^{t{\text{ac}}} \frac{P{\text{m}}}{\eta{\text{m}}} dt$$ [20.13]

where *E*battery is the energy capacity of the battery pack for acceleration in W; *t*ac is the acceleration time in s and *h*m is the electric motor effi ciency.

The energy capacity of battery pack also needs to provide enough energy during the urban driving pattern. actually, the energy stored in the battery pack cannot be fully used because a battery with low sOC has limited electric power performance and even damages the battery lifecycle:

$$E{\text{battery}} = \frac{\Delta E{\text{battery}}}{\Delta_{\text{SOC}}}$$ [20.14]

where Δ*E*battery is the energy variation of the battery pack during the driving period which consumes the battery energy and *E*battery is the energy capacity of the battery pack. Here, ΔSOC is operating range of battery pack which is the difference between the maximum sOC and minimum sOC.

The defi nition of hybridization degree was introduced to evaluate the amount of electromechanical power introduced by adding the electrical power train. It is defi ned as the ratio between the power of the electrical power train and the total power of both powertrains combined. Based on this defi nition, concepts such as mild and full hybrid are defi ned. There are no clear boundaries between mild and full hybrids. Usually, hybrids with less than 20 kw power rating for the electrical drive are referred to as mild hybrids, e.g. honda insight and CRZ. automotive manufacturers prefer mild hybrids due to the lower add on cost and less modifi cation to the original vehicle design. in most mild hybrids, battery banks of 32 V–42 V are used which is a much lower voltage level than full hybrids, mostly because of the 42 V standard introduced in the late 1990s. The fuel economy improvement of mild hybrids is not as good as full hybrids, but the initial cost payback time is considerably shorter with mild hybrids, which make them an easier choice for customers.

In most full hybrids the power rating of electrical power train is only slightly smaller than the IC engine power train with a hybridization degree close to 50%; a few examples are the Ford Fusion with a 116 kw IC engine and 79 kw motor, Toyota Camry with a 116 kw IC engine and 105 kw motor (these two can be classified as a full engine full hybrid), and Toyota Prius with a 73 kw IC engine and 27 kw motor (which can be classified as a reduced engine full hybrid). Neither of these are optimal solutions. The full engine full hybrids suffer from high add on weight, which may defeat the purpose of fuel saving, while the reduced engine full hybrid suffers from poor highway performance. The concept of the full engine optimal hybrid was developed at Texas A&M University.

20.6 Series-parallel hybrid electric drive trains (electric and mechanical coupling) and plug-in hybrids

The series-parallel hybrid drive train is a drive train which has both mechanical and electrical coupling working either individually or simultaneously. In this drive train an IC engine can not only operate as a mechanical power source, assisted by batteries/motor, to supply power to the driven wheels, but also can be coupled with a generator to function as an electric power source combined with batteries to supply electric power to a traction motor to propel the vehicle. The advantage of this drive train is the IC engine operation point can be controlled like series hybrid drive train without the torque and speed constraints of driven wheels while part of the IC engine mechanical power can still be delivered to the driven wheels like a parallel hybrid drive train but without energy conversion.

A good example of this drive train is the Toyota Prius by Toyota Motor Company[2]. The Toyota Prius also uses the series-parallel architecture to reduce the size of the required IC engine compared with a conventional power train to provide the same performance. It operates a Miller (Atkinson) cycle IC engine to reduce the cost of the exhaust-after-treatment system to meet emissions legislation and also uses an electric motor to drive the IC engine during decelerations to avoid lean NOx excursions that would otherwise result from lean operation during overrun.

The design principles of the series-parallel hybrid drive train components such as the IC engine, electric motor, and the battery pack power ratings and energy capacity have more freedom because of the dual coupling system, but are very similar to those in the series and parallel drive train mentioned above. Thus, they are not discussed further here.

20.6.1 Plug-in hybrids

Plugging into the utility power grid is another way of charging the on-board batteries in an HEV as well as using the on-board IC engine to charge. Energy from the utility grid is much cheaper than gasoline or diesel. As the HEV battery size grows, the HEV can benefit more from the pure electric traction mode, which means zero pollution at point of use and lower fuel cost. In the plug-in HEV, the battery pack plays a critical role. Based on the driving pattern, the PHEV 10, PHEV 20, PHEV 40 and the UF concepts are introduced in Section 20.7.1.

20.7 Control and performance

Hybrid drive trains are more complex than conventional drive trains. For example, different operational modes exist in the hybrid system. The hybrid power train usually consists of one or more energy storage devices, one energy conversion device and one drive shaft acting as the path of the power output. Using this definition, most of the hybrid vehicle operation can be divided among two power trains, one an electrical power train consisting of a battery, a converter and a motor/generator; the other a mechanical power train consisting of a fuel tank, a transmission, and an IC engine. These two power trains drive the vehicle load on the wheels. Different coupling ratios of the two power trains define the interactions between the power trains for various operational modes. Nine operational modes are defined below. For simplicity, the mechanical power train is referred as Power train 1 and the electrical power train as Power train 2[1].

1. Power train 1 delivers its power to the load alone.
2. Power train 2 delivers its power to the load alone.
3. Power train 1 and 2 deliver their power to the load simultaneously.
4. Power train 2 obtains power from the load (regenerative braking).
5. Power train 2 obtains power from Power train 1.
6. Power train 2 obtains power from Power train 1 and the load simultaneously.
7. Power train 1 delivers power to the load and to Power train 2 simultaneously.
8. Power train 1 delivers its power to Power train 2, and Power train 2 delivers its power to the load.
9. Power train 1 delivers its power to the load, and the load delivers the power to Power train 2.

Different HEV architectures and different control strategies will employ a different set of modes from the above list:

∑ Mode 1 is the engine alone mode which is usually employed in highway driving in parallel hybrids.

∑ Mode 2 is the pure electrical operation which can appear in both parallel and series hybrids, depending on driving conditions and the state of the vehicle.
∑ Mode 3 is the parallel propulsion mode which usually sums the torque or speed coupling in the parallel HEV, and sums the power in series/ series-parallel HEV with the shaft driven by the electric motor only.
∑ Mode 4 is regenerative braking which is a major reason for HEV fuel savings.
∑ Mode 5 is when the mechanical power from the IC engine drives the generator to charge the battery pack when excess and efficient engine power is available or when idling.
∑ Mode 6 is much like regenerative braking and 'engine brake' combined.
∑ Mode 7 and Mode 5 differ only in the amount of excess engine power and the vehicle state.
∑ In Mode 8, the IC engine works as a range extender in the series hybrid by charging the battery and the battery supplies power to the traction motor to drive the wheels. This mode suffers from the energy losses of too many stages of energy conversion as shown in Fig. 20.1.
∑ Mode 9 can be considered as a special case of regenerative braking where mild and short deceleration is needed.

The above operational modes offer flexibility for the control of hybrid vehicles which may have the potential of improving performance and fuel economy.

20.7.1 Control

The basic principle of control in the hybrid vehicle is to operate both drive trains at their highest efficiency operating points. Differences between the characteristics of the two power trains make this a complex optimization problem. From Fig. 20.1 it can be easily seen that the IC engine suffers from low efficiency at lower speeds and large deviation from the optimum operating locus. For the electric motor, although its efficiency varies with operating points or regions, the lower efficiency boundary is considerably higher than the IC engine. This makes the use of the electric motor favorable. Further, the electric motor's high torque at low speed also makes it a better choice for traction. Therefore, the control problem becomes an energy management strategy issue.

Usually, energy management is associated with the operational mode selection to prevent over-discharge and over-charge of the battery which is generally the more fragile component in the system. Management of the battery charging regime is essential to optimize battery life and reliability. Therefore, the control strategy can be designed so as to make the entire drive train operate efficiently while maintaining a safe state of charge in the battery. Research has established that good battery management and a large variety of strategies have been proposed. Here, the most popular ones will be introduced: charge depletion and charge maintaining. These two control strategies apply to different architectures and have their specific advantages and disadvantages. Charge depletion strategy refers to using the battery energy as much as possible by utilizing the electrical only propulsion mode. This is the default energy management strategy in pure electric vehicles. Pure electric battery systems can also be hybridized by employing ultra-capacitors, fuel cells and other sources of electrical energy. However, these are beyond the scope of this chapter.

Charge maintaining means that the state of the charge of the battery should be kept within a narrow band leaving enough room for regenerative braking and enough energy for assisted acceleration or grade climbing. This strategy is sometimes referred to as engine on-off control strategy, meaning an on-board IC engine is turned on to function as a charger when the battery reaches its lower SOC boundary and is turned off upon approaching the SOC upper boundary. Various battery technologies determine the safe band of the SOC. Further, limiting the SOC within a small band relaxes the requirement of interfacing the battery with the electric voltage bus.

Charge maintaining is usually employed in parallel and series-parallel architectures, mostly to compensate for the low efficiency regions of the IC engine. On the other hand, charge depletion is not preferred for conventional series hybrids without the plug-in feature. Series hybrids gain their fuel economy from operating the IC engine in its most efficient operating points at almost constant power. With charge depletion strategy, to protect the battery from over-discharging, the IC engine has to track the power demands from the wheels. This means the IC engine has to supply the flow of power through all the energy conversion stages with its associated energy losses. However, the SOC band for the charge sustaining strategy can be relaxed for series hybrids.

To illustrate the above concepts, simulation results for a parallel hybrid drive train, using each strategy with an identical drive cycle is shown in Fig 20.5. Although the fuel efficiency cannot be evaluated here it is true that better fuel efficiency can be gained by employing charge depletion strategy provided the drive cycle is short enough. This essentially makes the vehicle a pure electric or a plug-in hybrid.

The capacity of the battery should be chosen prior to choosing proper control strategy in the design stage. In most parallel and series hybrids, capacity gives way to power rating which has been optimized to gain maximum fuel efficiency in various design drive cycles. In most plug-in hybrids, charge depletion strategy is employed to make the most of the lower cost electrical energy from the utility grid. Thus, the energy storage capacity of the onboard battery has to be evaluated accordingly in addition to its power rating. However, more battery capacity means more weight and higher cost. On the other hand, too little capacity means the gasoline IC engine has to be used as a range extender even for short drives. The concept of UF has been introduced to address this dilemma in Section 20.4.1.

For plug-in hybrid electric vehicles (PHEVs), the assumption is that operation starts in the battery charge-depleting mode and eventually changes to the battery charge-sustaining mode. Total distance between charge events determines how much of the driving is performed in each of these two modes. An algorithm describing the portion of driving in each mode can be defined. Driving statistics from the US National Highway Transportation Survey are used as inputs to this algorithm to provide an aggregate UF applied to the

20.5 Simulation for different control strategies (a) charge sustaining and (b) charge depletion.

20.5 (Continued)

charge-depleting mode results[3]. Thus, terms such as PH10, PH20 and PH40 have been defined, depending on the regional UFs. The numbers refer to the number of miles that can be driven in pure battery electric mode from the plug-in electricity.

20.7.2 Performance

Due to the favorable speed torque characteristics of the electric motor in using the low speed, high torque region of the motor to compensate or the low torque, low speed region of the IC engine, the vehicle acceleration and gradeability can be improved. The architecture of using electrical and IC engine drive shafts on different wheel axles can convert a one axle drive vehicle to an all-wheel drive vehicle which will further improve the performance.

20.7.3 Emission reduction

Hybrid vehicles with an IC engine deliver emission improvement from always working the IC engine in its most efficient regions and also employing regenerative electrical braking. In urban drive conditions, a large portion of energy is dissipated by frequent braking. Thus, significant fuel savings (up to 10% of total fuel consumed) from regenerative braking will help to reduce greenhouse gas and other pollutant emissions. However, to evaluate the true environmental benefits of hybrid vehicles, well-to-wheel and life cycle analyses have to be performed.

Well-to-wheel analysis means energy consumption and emission have to be considered from the crude oil production stage. The word well refers to oil production – the stages of energy transportation, refinement, and conversion have to be considered – all the way to energy delivery at the wheel. By this way of thinking, grid electricity in pure electric and plug-in hybrids is merely an energy vector, transferring energy from the grid to onboard storage, with its associated emissions from the power generation plant to the vehicle.

Although, utility generators usually have higher efficiencies and lower emissions than IC engines, when the efficiency of the transmission lines are considered, benefits gained by pure electric and plug-in hybrids cannot be easily judged. The idea has been proposed to deploy renewable energy resources, such as wind turbines and photovoltaic panels, as the source of electrical energy for maximum environmental benefit. This involves construction and controls of distributed generation systems, which is beyond the scope of this chapter[4].

Since the initial energy investment of building a hybrid vehicle is higher than that of a conventional vehicle, it is important to compare its total energy savings within its life time with the conventional vehicle. Thus, the idea of life cycle analysis has been proposed. The problem with life cycle analysis is that most of the results are specifically data-based and not assertable in a general way. Many assumptions have to be made which makes the results contestable.

20.7.4 Auxiliary load

Auxiliary loads include all electrical loads outside of the power train. Heating, ventilation and air conditioning, power assisted steering, and lights are a few examples. Traditionally, all these loads have been supplied by a 14 V low voltage system based on the lead acid battery. A major problem with this low voltage system is that high current has to be drawn from the battery to supply the load power requirements. This high current requirement makes the total wiring weight and copper losses undesirable. In recent years some of the loads have been converted to more efficient technologies, such as lights with LEDs. But the problem remains[5].

In hybrid vehicles, the battery pack can be seen as a high voltage source in the system. With this high voltage it is reasonable to assume the current in the electrical bus can be greatly reduced, along with its associated wiring gauge. To what level should this voltage should be increased to gain the maximum efficiency is still under investigation in many places (e.g. the Power Electronic and Motor Drives Laboratory at Texas A&M University). 400 V is the maximum voltage that is generally acceptable in any vehicle application with appropriate safety design considerations.

Another trend in the industry is swapping to 'motorless' hybrids; one example is the new Mazda 6 hybrid. In the Mazda 6, a regenerative braking system called i-ELOOP is employed to recover energy while braking with an electric machine acting as a generator; the energy recovered is then stored in an ultracapacitor to drive other electrical loads. The idea of motorless means no motor is used in traction. This greatly reduces the complexity of the drivetrain system compared with conventional hybrids while gaining greater fuel economy compared to IC engine vehicles.

20.8 Future trends

The design philosophy of hybrid vehicles has been under continuous review and improvement in many places including the Power Electronic and Motor Drives Laboratory at Texas A&M University. One idea under investigation has been to improve the fuel efficiency and performance of conventional vehicles with minimum incremental hybridization costs. Favorable results have been obtained by computer simulations which will find their way into exciting new vehicles in the near future. By addressing the question on how much hybrid power needs to be introduced to reach the goal of a cost effective superior vehicle a universal electrical drive train rating of about 25 kw was found for passenger vehicles of all sizes. This is the amount of power required to propel an average car at a steady speed on the flat at around 60 miles per hour. This result when used in mass production can be of great value[6].

The plug-in hybrid has a power electronic interface between the on-board storage and the utility grid. This makes intelligent interaction between the two, usually referred to as V-to-G, possible. On board storage battery packs can be seen as part of the smart grid energy storage. This storage can be used to alter the energy demand from the utility grid by peak shaving and voltage stability and power quality improvements. The benefits of this strategy need to be carefully evaluated because frequent charge-discharge cycles may speed up the degradation of the battery pack which may lead to premature battery failure. Research has been conducted at the Lincoln Laboratory in Massuchetts by integrating a number of plug-in hybrids with the laboratory power supply. This research has rendered some interesting results[7].

Hybridizing the vehicle battery storage with ultra-capacitors is also an interesting area of research. Due to the ultra-capacitor's low energy density and high power density compared to batteries they are not favorable as the main on-board energy storage. However, the ultra-capacitor's high efficiency and fast charging-discharging cycle time make it a good 'energy buffer'. Using ultra-capacitors to recover energy during braking and reusing this energy for acceleration may help to improve the efficiency of the overall hybrid drive train[8]. Ultra-capacitors also have the advantage of not having a finite lifetime as that exhibited by batteries with a limited number of charge/ discharge cycle capability.

Great effort has been focused on the design and control of battery packs. Large numbers of battery cells put in series and parallel will inevitably cause charge imbalance. To prevent damaging the battery pack, the total usable charge is usually determined by the weakest cell. Charge imbalance may also cause abnormal heat accumulation in battery packs which may lead to thermal combustion (fires) and other hazardous situations[9].

20.9 References

1. M. Ehsani, Y. Gao and A. Emadi, Modern Electric, Hybrid Electric, and Fuel Cell Vehicle, Second Edition, Taylor & Francis Group, Boca Raton, FL, 2010.
2. Available at http://www.toyota.com, Toyota Motor Company, visited in September 2003.
3. SAE Standards, Utility Factor Definitions for Plug-In Hybrid Electric Vehicles Using Travel Survey Data.
4. Zhonghai Sui and Zhenpo Wang, 'Technical and economic analysis of pure-electric vehicles based on the life-cycle cost theory', Business Management and Electronic Information (BMEI), 1, 125–129, May 2011.
5. Ali Emadi, M. Ehsani and John M Miller, Vehicular Electric Power Systems, Marcel Dekker, Inc., New York, NY, 2004
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K. T. Chau, The University of Hong Kong, People's Republic of China

DOI: 10.1533/9780857097422.3.655

Abstract: This chapter discusses key technologies of pure electric vehicles. It first describes their system configurations when adopting various energy storage systems, electric propulsion systems and in-wheel transmission systems. Then, it discusses the existing and advanced electric drives for electric propulsion, and elaborates the energy storage devices and their energy management. Subsequently, it delineates viable charging infrastructure facilities and latest battery charger technologies. Then, it introduces the emerging vehicle-to-grid technology. It also comments on their energy and environmental benefits as well as their technological and societal limitations. Finally, it summarizes their current status and future development trends.

Key words: system configuration, electric propulsion, energy storage, charging infrastructure, vehicle-to-grid (V2G).

21.1 Introduction

The main theme of this chapter is to discuss key technologies of pure electric vehicles (EVs) which refers to those vehicles in which the energy is only sourced from the power grid and the propulsion is solely driven by an electric motor. In Section 21.2, various system configurations due to variations in energy storage, electric propulsion and in-wheel transmission will be described. In Section 21.3, different electric drives for electric propulsion, including existing and advanced technologies, will be elaborated. Also, they are compared and evaluated for EV application. In Section 21.4, main energy storage devices, namely batteries, ultracapacitors and ultrahigh-speed flywheels, will be briefed. Meanwhile, the corresponding state-of-charge indicators and energy recovery management will be discussed. In Section 21.5, home and public charging facilities including opportunity charging stations, fast charging stations and battery swapping stations as well as various battery chargers including conductive, inductive and wireless types will be discussed. In Section 21.6, the vehicle-to-grid concept will be delineated and the principle of vehicle-to-grid operation will be explained, hence exploring the new role of EVs. In Section 21.7, the energy and environmental benefits of EVs will be discussed while their technological and societal limitations will be identified. In Section 21.8, the current status of EV development will be summarized and the future trends will be commented on. In Section 21.9, relevant help and support will be acknowledged. In Section 21.10, a brief commentary on key references and websites will be given. In Section 21.11, useful references will be listed.

21.2 System configurations

Figure 21.1 shows the general configuration of the EV, consisting of three major subsystems: energy storage, electric propulsion and auxiliary. The electric propulsion comprises the electronic controller, power converter, electric motor, mechanical transmission and driving wheels. The energy storage involves the energy storage device, energy management unit and charging unit. The auxiliary consists of the power steering unit, temperature control unit and auxiliary power supply. The electrical link is represented by thick arrows and the control link by thin arrows. Based on the control inputs from the brake and accelerator pedals, the electronic controller provides proper control signals to switch on or off the power devices of the power converter which functions to regulate power flow between the electric motor and energy storage device. The energy management unit cooperates with the electronic controller to control regenerative braking and its energy recovery. It also works with the charging unit to control charging

21.1 General EV system configuration.

and to monitor usability of the energy storage device. The auxiliary power supply provides the necessary power with different voltage levels for all EV auxiliaries, especially the temperature control and power steering units. The steering wheel is another key control input. Based on its angular position, the power steering unit can determine how sharply the vehicle should turn (Chan and Chau, 2001).

21.2.1 Variations in energy storage

Figure 21.2 shows various EV system configurations due to different arrangements of energy storage, where B is the battery, C the ultracapacitor, F the ultrahigh-speed flywheel, and P the power converter. Among them, the single-source battery powered configuration shown in Fig. 21.2(a) is widely adopted by existing EVs. The battery may be distributed around the vehicle, packed together at the vehicle back or located beneath the vehicle chassis. This battery should be able to offer reasonable specific energy and specific power as well as being able to accept regenerative energy during braking.

Instead of using a compromised battery design, two different batteries (one is optimized for high specific energy while another for high specific power) can be used simultaneously in an EV. As shown in Fig. 21.2(b), this arrangement not only decouples the requirements on energy and power but also affords an opportunity to use those mechanically rechargeable batteries which cannot accept regenerative energy during braking or downhill.

21.2 EV configurations due to variations in energy storage (a) battery only, (b) two different batteries, (c) battery and ultracapacitor and (d) battery and ultra-high-speed flywheel.

Because of the feature of very low specific energy, the ultracapacitor has to work together with the battery for EVs. As shown in Fig. 21.2(c), the battery is aimed to have high specific energy while the ultracapacitor inherently offers exceptionally high specific power. Since the ultracapacitor is of relatively low voltage level, an additional DC-DC power converter is needed to interface between the battery and ultracapacitor terminals.

The ultrahigh-speed flywheel is an emerging energy storage device which can offer high specific power and high energy receptivity. The corresponding configuration is shown in Fig. 21.2(d) in which the battery is selected to offer high specific energy. Since this flywheel is generally coupled with an AC machine for electromechanical energy conversion, an additional AC-DC converter is needed to interface between the battery and flywheel terminals.

21.2.2 Variations in electric propulsion

Figure 21.3 shows two EV system configurations due to different arrangements of electric propulsion, where D is the differential gear, G the fixed gear, and M the electric motor. The longitudinal single-motor configuration shown in Fig. 21.3(a) is actually borrowed from the existing vehicle, namely replacing the variable gearbox with fixed gearing and hence removing the clutch. Differing from the existing vehicle, the EV does not need to employ the variable gearing, hence significantly reducing the overall complexity, size, weight and cost of the gearbox. Borrowing the concept from the transverse front-engine front-wheel drive of the existing vehicle, the transverse singlemotor configuration shown in Fig. 21.3(b) takes the advantages of more compact and higher efficiency than its longitudinal counterpart. In fact, this arrangement is most commonly adopted by modern EVs.

A differential is a standard component for conventional vehicles and this technology can be carried forward to EVs. When a vehicle is rounding a curved road, the outer wheel needs to travel on a larger radius than the inner wheel. Thus, the differential adjusts the relative speeds of the wheels; otherwise, the

21.3 EV configurations due to variations in arrangement of electric propulsion (a) longitudinal arrangement and (b) transverse arrangement.

wheels will slip which causes tire wear, steering difficulties and poor road holding. Figure 21.4(a) shows the principle of mechanical differential driven by an electric motor M, in which the differential gear D can allow axle side gears to turn at different speeds. Notice that this mechanical differential is not only bulky and heavy, but also complicated and lossy. For EVs, it is possible to dispense with a mechanical differential. By separately coupling two electric motors to the driving wheels, the speed of each wheel can be independently controlled in such a way that the differential action can be electronically achieved when cornering. Figure 21.4(b) shows the principle of electronic differential in which two electric motors M are employed. This arrangement can totally eliminate the mechanical differential, hence reducing the overall size and weight as well as improving the cornering control and transmission efficiency. The use of a single-motor drive with a mechanical differential or a two-motor drive with electronic differential is still controversial. Positively, the removal of a mechanical differential can reduce the overall size and weight while the electronic differential can accurately control the wheel speeds so as to achieve better performance during cornering. Negatively, the use of an additional motor and power converter causes an increase in the initial cost while the reliability of the electronic controller for the dual motors arouses a safety concern.

21.2.3 Variations in in-wheel transmission

In order to further shorten the mechanical transmission path from the electric motor to the driving wheel, the electric motor can be placed inside a wheel. This arrangement is the so-called in-wheel motor. Figure 21.5(a) shows the two-motor geared in-wheel configuration in which fixed gearing is employed to reduce the motor speed to the desired wheel speed. By fully abandoning the mechanical gearing, the in-wheel drive can be realized by installing a low-speed outer-rotor electric motor inside a wheel. Figure 21.5(b) shows the two-motor gearless in-wheel configuration in which the outer rotor is directly mounted on the wheel rim. Thus, speed control of the electric motor is equivalent to the control of the wheel speed and hence the vehicle speed.

21.4 EV configurations due to variations in differential of electric propulsion (a) mechanical differential and (b) electronic differential.

21.5 EV configurations due to variations in in-wheel transmission (a) two-motor geared transmission and (b) two-motor gearless transmission.

21.6 In-wheel motor topologies (a) high-speed inner-rotor topology and (b) low-speed outer-rotor topology.

For the geared in-wheel configuration, the high-speed inner-rotor motor is directly coupled with the high speed-reduction planetary gear which is mounted between the motor shaft and the wheel rim as depicted in Fig. 21.6(a). Typically, this motor is purposely designed to operate over 10 000 rpm so as to give a high power density. Thus, the corresponding planetary gear ratio is of about 10:1 to provide the wheel speed ranging from zero to about 1000 rpm. Inevitably, the use of mechanical gearing for speed reduction involves transmission loss, wear-and-tear problem and regular lubrication. Figure 21.6(b) depicts the low-speed outer-rotor motor for the gearless inwheel configuration. This motor has the definite advantages of simplicity and being gearless, hence offering high transmission efficiency and being free from maintenance. However, because of the inherent low-speed requirement of wheel rotation, typically less than 1000 rpm, the electric motor has to adopt the low-speed design which suffers from the drawbacks of increased size, weight and cost.

21.3 Electric propulsion

Electric drives that have been developed for industrial application are fundamentally different from those for electric propulsion (Zhu and Howe, 2007; Chau, 2009). In essence, EV drives need to offer high torque and power densities, high efficiency over wide torque and speed ranges, good controllability, high reliability and be free from maintenance.

21.3.1 Existing electric vehicle (EV) drives

Among many types of electric machines, four of them have been adopted for EV drives: the DC, induction, switched reluctance (SR), and permanent magnet (PM) brushless motors. They possess different topologies as illustrated in Fig. 21.7.

DC drives

DC drives used to be widely accepted for EVs. As determined by the mutual interconnection between the field winding and the armature winding or the

21.7 Existing EV drive topologies.

use of PM excitation, the whole family consists of the separately excited DC, shunt DC, series DC and PM DC types.

All DC drives suffer from a problem due to the use of commutators and brushes. Commutators cause torque ripples and limit the motor speed, while brushes are responsible for friction and radio-frequency interference. Moreover, due to the wear and tear, periodic maintenance of commutators and brushes is always required. These drawbacks make them less reliable and unsuitable for maintenance-free operation. The major advantages of DC drives are their maturity and simplicity. However, because of their relatively low efficiency and need of maintenance, DC drives are no longer attractive for modern EVs.

Induction drives

At present, induction drives are the most mature technology among various maintenance-free drives. There are two types of induction motors, namely the wound-rotor and cage-rotor. Because of high cost, need of maintenance and lack of sturdiness, the wound-rotor induction drive is less attractive than the cage-rotor counterpart. Actually, the cage-rotor induction drive is loosely named as the induction drive possesses the definite advantages of low cost and ruggedness.

Speed control of induction drives is considerably more complicated than that of DC drives because of the nonlinearity of the dynamic model with coupling between direct and quadrature axes. Two representative control operations, namely the variable-voltage variable-frequency (VVVF) control and the field-oriented control (FOC) which is also called vector control or decoupling control (Novotny and Lipo, 1996), have been developed for electric propulsion. The VVVF control strategy is based on constant volts/ hertz control for frequencies below the rated frequency, whereas variablefrequency control with constant rated voltage is for frequencies beyond the rated frequency. Because of the disadvantages of air-gap flux drifting and sluggish response, the VVVF control is not suitable for high-performance EV operation. The FOC enables the induction drive being controlled alike the separately excited DC drive. By using coordinate transformation, the torque can be effectively controlled to offer the desired fast transient response. With the advent of powerful low-cost microcontrollers, the FOC-based induction drive has been widely adopted for modern EVs.

Switched reluctance (SR) drives

SR drives have been recognized to have considerable potential for EVs. They offer the definite advantages of simple construction, low manufacturing cost and outstanding torque-speed characteristics (Krishnan, 2001). The operation principle of SR drives is based on the 'minimum reluctance' rule. The corresponding direction of torque is independent of the polarity of the phase current. However, the torque can be produced only in half a rotor pole-pitch, thus creating large torque ripples. Also, because of the heavy saturation of pole tips and the fringing effect of poles and slots, they usually exhibit acoustic noise problems.

The SR drives have basically two operation modes. When the speed is below the base speed, the current can be limited by chopping (so-called current chopping control (CCC)). In the CCC mode, the torque and thus the constant-torque characteristic can be controlled by changing the current limits. During high speed operation, however, the peak current is limited by the back electromotive force (EMF) of the phase winding. The corresponding characteristic is essentially controlled by phasing of switching instants relative to the rotor position (so-called angular position control (APC)). In the APC mode, the constant-power characteristic can be achieved.

Permanent magnet (PM) brushless drives

PM brushless drives are becoming attractive for EVs. Since the magnetic field is excited by high-energy PMs, the overall weight and volume can be significantly reduced, leading to high torque and power densities. Because of the absence of rotor copper loss, their efficiency is inherently high. Since the heat loss mainly arises in the stator, it can be more easily dissipated to the surroundings. Nevertheless, the PM brushless drives suffer from the drawbacks of relatively high PM material cost and uncontrollable PM flux.

Based on the waveforms feeding into the motor terminals, the PM brushless drives can be divided into two main types – the PM brushless AC (BLAC) and the PM brushless DC (BLDC) (Krishnan, 2009). The PM BLAC drive is fed by sinusoidal AC current, whereas the PM BLDC drive is fed by rectangular AC current. Actually, the PM BLAC drive is usually called the PM synchronous (PM Syn) drive. Since the interaction between trapezoidal field and rectangular current produce higher torque product than that produced by sinusoidal field and sinusoidal current, the PM BLDC drive possesses higher torque and power densities than the PM Syn drive. Meanwhile, the PM BLDC has a significant torque pulsation, whereas the PM Syn produces an essentially constant instantaneous torque or so-called smooth torque.

For the PM Syn drive that operates with sinusoidal current and sinusoidal air-gap flux, the corresponding control operation is similar to that of the induction drive. For the PM BLDC drive, the operation waveforms are no longer sinusoidal so that d-q axis transformation is ill-suited (Krishnan, 2009). In the absence of d-q axis transformation, the operation is based on conduction angle control.

Comparison of existing EV drives

In order to evaluate the aforementioned drives for application to EVs, they are compared in terms of their power density, efficiency, controllability, reliability, maturity, cost level, noise level and maintenance requirement. A point grading system (1 to 5 points) is used in which 1 is the worst and 5 is the best. As listed in Table 21.1, the key problem of the DC drive is the need for regular maintenance while the key drawback of the SR drive is the high acoustic noise, whereas the key merit of the induction drive is the low cost while the key advantages of PM brushless drives are the high power density and the high efficiency. This evaluation indicates that the DC drive is unattractive, whereas the induction drive and the PM brushless drives are most attractive. Between the two types of PM brushless drives, the PM BLDC drive can potentially offer better performance than the PM Syn drive.

Table 21.2 summarizes the application of existing drives to flagship EVs. For the DC drive, the application is either obsolete or limited to those versions aspiring simplicity. For the SR machine, the application is rare. Currently, the applications of the induction drive and the PM Syn drive almost equally share the market of EVs. With ever increasing concern on environmental protection and hence the demand of high energy efficiency, it is anticipated

| Power density | 2 | 3 | 3.5 | | |---------------|---|---|-----|--| | Efficiency | 2 | 3 | 3.5 | |

Table 21.1 Evaluation of existing EV drives

	DC	Induction	SR	PM Syn	PM BLDC
Power density	2	3	3.5	4.5	5
Efficiency	2	3	3.5	4.5	5
Controllability	5	4	3	4	4
Reliability	3	5	5	4	4
Maturity	5	5	4	5	4
Cost level	4	5	4	3	3
Noise level	3	5	2	5	5
Maintenance	1	5	5	5	5
Total	25	35	30	35	35

Note: A point grading system (1 to 5 points) is used in which 1 is the worst and 5 is the best.

Table 21.2 Application of existing drives to flagship EVs

Drive types	Car models
DC	Fiat Panda Elettra, Citroën Berlingo Electrique, Reva G-Wiz DC
SR	Chloride Lucas
Induction	GM EV1, BMW Mini E, Tesla Roadster, Reva G-Wiz i
PM Syn	Nissan Leaf, Mitsubishi i-MiEV, Ford Focus Electric, Citroën C-Zero,
	Peugeot iOn, BYD e6
PM BLDC	Smart Fortwo ED

that the induction drive will be phasing out while the PM BLDC drive will be phasing in. Currently, the corresponding peak output power ranges from 13 kW to 225 kW, depending on the vehicle weight and performance, while the norm is 47–80 kW.

21.3.2 Advanced EV drives

Since the PMs of the existing PM brushless drive are located in the rotor, they suffer from the problems of mechanical integrity and thermal stability. In recent years, the development of stator-PM drives has attracted a wide attention (Chau et al., 2008). Their topologies are with PMs located in the stator, and generally with salient poles in both the stator and the rotor. Since the rotor has neither PMs nor windings, this class of machines is mechanically simple and robust, hence very suitable for vehicular operation. According to the location of the PMs, it can be categorized as the doubly-salient PM (DSPM), flux-reversal PM (FRPM) and flux-switching PM (FSPM) types. With the inclusion of independent field windings in the stator for flux control, the class can further derive the flux-controllable PM (FCPM) type. Their typical drive topologies are shown in Fig. 21.8.

The DSPM drive is relatively the most mature type of stator-PM drives.

21.8 Stator-PM drive topologies.

Although it has salient poles in the stator and rotor, the PM torque significantly dominates the reluctance torque, hence exhibiting low cogging torque. Since the variation of flux linkage with each coil as the rotor rotates is unipolar, it is more suitable for the BLDC operation. The torque density of this DSPM drive is inherently higher than that of the SR drive.

The FRPM drive exhibits the feature of bipolar flux linkage variation because the flux linkage with each coil reverses polarity as the rotor rotates. Since the bipolar flux linkage variation can have better utilization of iron core than the unipolar counterpart, the FRPM drive inherently offers higher torque density than the DSPM drive. However, since the PMs are attached on the surface of stator teeth, they are more prone to partial demagnetization. Also, significant eddy current loss in the PMs may result.

In the FSPM drive, each stator tooth consists of two adjacent laminated segments and a PM, and each of these segments is sandwiched by two circumferentially magnetized PMs, thus enabling flux focusing. Also, it has less armature reaction, hence offering higher electric loading. Since its back EMF waveform is essentially sinusoidal, this drive is more suitable for the BLAC operation.

In general, stator-PM drives suffer from the difficulty in air-gap flux control. One basic type of FCPM drive is the hybrid-field DSPM topology. The stator incorporates two types of windings, namely the 3-phase armature winding and the DC field winding, and the PM poles. By varying the polarity and magnitude of the DC field winding current, the air-gap flux density becomes easily controllable. When realizing flux strengthening, the drive can offer exceptionally high torque for EV launching, or providing temporary power for EV overtaking and hill climbing. When realizing flux weakening, the drive can offer the wide-speed constant-power feature for EV cruising. However, its stator is relatively bulky and the continual excitation of DC field winding for flux control increases the copper loss, hence degrading the inherent merits of high power density and high efficiency.

All these advanced PM drives have their own merits and demerits. The importance of their merits and demerits also varies with various EV applications. For instance, the DSPM drive is readily applied to the latest EVs because it has been well developed and is relatively the most mature one. The FSPM drive is attractive for near-term application to EVs because of its all-round performances. Meanwhile, the FCPM drive is attractive for those high-performance EVs which desire a wide constant-power operating range.

21.4 Energy storage and management

In the present status of energy storage technology, there are three viable devices for EV application, namely batteries, ultracapacitors and ultrahighspeed flywheels (Burke, 2007; Chau et al., 1999). Notice that fuel cells are considered separately as the energy generation device for fuel-cell vehicles. Since the energy storage capacity is so precious for EVs, the corresponding energy management is of utmost important.

21.4.1 Energy storage devices

Batteries

There are many types of batteries that have been developed for EVs. Those viable EV batteries consist of the lead-acid (Pb-acid), nickel-metal hydride (Ni-MH), zinc/air (Zn/air) and lithium-ion (Li-ion) types.

The Pb-acid battery has a number of advantages contributing to its application for EVs: proven technology, high cell voltage (2 V), high specific power (200–300 W/kg), and low cost (about 150 US\$/kWh). On the other hand, it suffers from some disadvantages that limit its application to modern EVs: low specific energy (30–45 Wh/kg) and short cycle life (400–600 cycles).

The Ni-MH battery is a kind of nickel-based battery. The key component is the hydrogen storage metal alloy, either the rare-earth alloy based around lanthanum nickel or an alloy consisting of titanium and zirconium. It offers the advantages of relatively high specific energy (60–120 Wh/kg), high specific power (150–400 W/kg), long cycle life (600–1200 cycles), rapid recharge capability and environmental friendliness. However, it suffers from the problem of relatively low cell voltage (1.2 V), high initial cost (200–350 US\$/kWh), and the homogeneity of the hydrogen storage metal alloy.

The Zn/air battery is featured on low cost (90–120 US\$/kWh) and environmental friendliness. With a mechanically rechargeable design, it can avoid the need for a bidirectional air electrode and the shape change problem of a zinc electrode, hence offering high specific energy (about 230 Wh/kg). The mechanical recharging system for EV application is to remove and replace the depleted zinc negative electrode cassettes robotically at a public service station. The discharged fuel is then recharged at central facilities. However, this battery suffers from low specific power (about 105 W/kg), and possible carbonation of alkali electrolyte due to carbon dioxide in air.

The Li-ion battery is based on the principle that lithium ions are swinging through the electrolyte between the positive and negative electrodes during discharge and charge. The relatively more mature types include cobalt based, nickel based and manganese based ones. Their general advantages are the highest cell voltage (3.7 V), high specific energy (90–160 Wh/kg), high specific power (250–450 W/kg), and relatively long cycle life (1200–2000 cycles). Recent development includes the iron phosphate based, nickel manganese cobalt based and titanate based types, aiming to achieve safer design, higher cell voltage and longer cycle life. However, the Li-ion battery still suffers from a drawback of relatively high cost (600–1000 US\$/kWh).

Table 21.3 summarizes the application of various batteries to flagship EVs. For the Pb-acid battery, the application is either obsolete or limited to those low-cost versions. For the Ni-MH battery, the application was successful in some flagship EVs but becomes unattractive due to the patent encumbrance. For the Zn/air battery, the application is limited to experimental demonstrations only. Currently, almost all flagship EVs prefer the Li-ion battery. The corresponding installed capacity ranges from 10 kWh to 56 kWh, depending on the vehicle weight and performance, while the norm is 16–24 kWh. Meanwhile, the battery voltage level ranges from 50 V to 375 V, typically 330–360 V.

Ultracapacitors

The ultracapacitor or sometimes called supercapacitor is generally based on the double-layer capacitor technology to remarkably increase the capacitance to the order of farads (currently, up to 3000 F). It has advantages of high specific power (over 6 kW/kg), high energy efficiency (over 95%), practically unlimited cycle life (over 1 million cycles), simple to charge, and simple measurement of energy storage capacity. However, it suffers from low specific energy (about 6 Wh/kg), low maximum voltage (2–3 V), and very high cost (30–100 US\$/Wh).

Because of inherent low maximum voltage, a number of ultracapacitor cells are connected in series to obtain a reasonable voltage level for operation. Thus, voltage balancing between them is required. Unlike batteries, the ultracapacitor suffers from significant voltage drop as it discharges so that a DC-DC power converter is usually necessary to regulate the discharge process. Additionally, since the ultracapacitor is preferred to batteries to accept the high-rate charging current during regenerative braking of EVs, the DC-DC converter should allow for bidirectional power flow.

Battery types	Car models
Pb-acid Ni-MH Zn/air Li-ion	Fiat Panda Elettra, Reva G-Wiz i GM EV1, Honda EV Plus, Ford Ranger EV, Toyota RAV4 EV Arotech Electric Bus, Yangtze Bus Nissan Leaf, Mitsubishi i-MiEV, Ford Focus Electric, Tesla Roadster, Smart Fortwo ED, BYD e6, Citroën C-Zero, Peugeot iOn, BMW Mini E, Toyota RAV4 EV (2nd Gen), Reva L-ion

Ultrahigh-speed flywheels

The ultrahigh-speed flywheel has the rotor made of material with high tensile strength, suspended by magnetic bearings, and spinning at speeds up to 50 000 rpm in a vacuum enclosure. It takes the advantages of practically unlimited cycle life, high theoretical specific energy (100–130 Wh/kg), high energy efficiency (over 90%), rapid charge/discharge capability, environmentalfriendliness, and simple measurement of energy storage capacity. However, it suffers from the possibility of 'flywheel explosion' when the tensile strength is exceeded or under an EV accident, and the problem of gyroscopic forces whenever an EV departs from its straight-line course. Consequently, a strong containment vessel is required to protect the flywheel while two flywheels spinning in opposite directions are adopted to compensate the gyroscopic forces, leading to significantly increase the total mass and hence deteriorate the overall specific energy.

The key component of this flywheel is the electric machine for efficient bidirectional conversion between electrical and mechanical energies. Among many machine types, the PM brushless machine is most preferable because it offers high power density, high efficiency, and essentially zero rotor loss which is essential when working in a vacuum environment. Another important component is the power inverter for efficient conversion between DC and AC voltages. Particularly, soft-switching techniques may be employed to suppress the switching loss under high-frequency operation.

Instead of facing the two challenges as an on-board energy storage system, the ultrahigh-speed flywheel is preferably used as an off-board energy storage system to provide rapid recharging for EVs. The reason is so obvious that the flywheel offers the ability to release high power for rapid EV recharging while minimizing the corresponding peak power demand on our power system network. Meanwhile, the flywheel can be embedded in the ground to totally eliminate the problems of safety hazards and gyroscopic forces.

Energy hybridization

Based on the fact that the available energy storage devices, including batteries, ultracapacitors and ultrahigh-speed flywheels, cannot fulfil the demand of high specific energy and high specific power simultaneously, EVs can adopt the concept of using two energy storage devices, the so-called energy hybridization (Chau and Wong, 2001). The definite advantage is the decoupling of energy and power requirements, hence fully utilizing the individual high specific energy and high specific power. Of course, this is accompanied by the disadvantage of increased complexity and cost.

The principle of operation of this hybrid energy system, consisting of both high specific energy and high specific power sources is illustrated in Fig. 21.9. During normal driving, the high specific energy source supplies the necessary energy to the electric motor via the power converter. To enable the system to be ready for sudden power demand, this source can pre-charge the high specific power source in the light-load period. During acceleration or hill-climbing, both the sources need to simultaneously supply the desired energy to the electric motor. During braking or downhill, the electric motor operates as a generator so that the regenerative energy flows back to recharge the high specific power source via the power converter. If this source cannot fully accept the regenerative energy, the surplus can be diverted to recharge the high specific energy source provided that it is energy receptive.

There are two viable energy hybridization schemes for EVs, namely batterybattery hybrids and batteryultracapacitor hybrids. For the batterybattery hybrids, the Zn/airNi-MH hybrid is a natural choice since it combines the merits of the 230 Wh/kg of Zn/air for long driving range and the 400 W/kg of Ni-MH for acceleration or hill climbing. It also overcomes the incapability of the mechanically rechargeable Zn/air which cannot accept regenerative

21.9 Operation of a hybrid energy system.

energy. For the battery-ultracapacitor hybrids, the Li-ion-ultracapacitor hybrid is attractive since it incorporates the individual merits of 160 Wh/kg of Li-ion and 6 kW/kg of ultracapacitor.

21.4.2 State-of-charge (SOC) indicators

Similar to the fuel gauge of gasoline vehicles, the (SOC) indicator functions to provide the EV driver accurate information on how much energy content remains. Hence, the driver can plan the future driving range before recharging. Among the three energy storage devices for EVs, the SOC indicators for ultracapacitors and ultrahigh-speed flywheels are simple. Namely, the measurement of capacitor voltage V can directly deduce the ultracapacitor SOC using E = 1 /2CV² where C is its capacitance; while the measurement of flywheel speed can directly deduce the ultrahigh-speed flywheel SOC using E = ¹ /2Jw² where J is its moment of inertia. However, the determination of the battery SOC is much more complicated and uncertain.

Theoretically, the SOC of batteries is written as (*Cr/C*r) ¥ 100% where C^r and C^t are the residual capacity and total capacity, respectively. In general, *C*t is not only significantly affected by the battery discharging rate, but also influenced by temperature and ageing. Sometimes, C^t is roughly estimated by the Peukert equation *C*tI = KI(1–n) where I is the discharge current, and K and n are empirical constants. Although there are many methods that have been developed to measure *C*r, the only practical one for EV application is to directly count the watt-hour that has been taken out or put into the battery. This method does provide reasonable accuracy for short-term estimation of the battery SOC; however, it suffers from the accumulation of errors over a long period of estimation. On the other hand, the measurement of opencircuit voltage can directly reflect the SOC for some kinds of batteries such as the Pb-acid and Li-ion. The key drawback of this method is due to the fact that the open-circuit voltage generally needs a lengthy stabilization period after charging or discharging. Since the watt-hour capacity method takes the merit of sufficient accuracy under short-term operation while the open-circuit voltage takes the merit of high accuracy after stabilization, a combined approach is preferable.

Recently, some estimation methods have been developed to directly determine the available battery capacity, rather than the SOC, for EVs (Chau et al., 2004). This kind of method can estimate the available battery capacity of different kinds of batteries, including Pb-acid, Ni-MH and Li-ion, by using artificial neural networks and fuzzy logics in which the variable discharge current regimes as well as the temperature and ageing influences are taken into account. The accuracy of these estimation methods heavily relies on the training process, namely the longer the service of the estimator for the same battery pack the higher the accuracy of the estimation.

21.4.3 Energy recovery management

Regenerative braking is unique to EVs and enables the vehicle's kinetic energy to be converted back to electrical energy during braking (deceleration or downhill running). The converted electrical energy is stored in energy storage devices such as batteries, ultracapacitors and ultrahigh-speed flywheels to extend the driving range by up to 10%.

In general, EVs are equipped with the regenerative-hydraulic hybrid braking system. Whenever the regenerative braking torque is insufficient to offer the same deceleration rate as available in conventional vehicles, the hydraulic braking torque is applied. The control of their distribution is depicted in Fig. 21.10, which aims to provide the driver with the same braking feel as that experienced in conventional vehicles while maintaining maximum regenerative braking. During low brake pedal force, only the regenerative braking torque is applied on the driving wheels, and is proportional to the pedal pressing force. The braking torque on the non-driving wheels is always due to the hydraulic braking which is also proportional to the pedal pressing force. When the pedal force is beyond a certain limit, the maximum regenerative braking torque is applied on the driving wheels, and the hydraulic braking torque is simultaneously applied on the driving wheels to top up the desired braking torque. Meanwhile, the maximum regenerative braking torque is kept constant to fully recover the kinetic energy.

21.10 Distribution of regenerative and hydraulic braking torques.

21.5 Charging infrastructure

The charging infrastructure of EVs is composed of two main areas: charging facilities and charging devices. The charging facilities can be categorized into home charging and public charging. The public charging facilities can be further identified as opportunity charging stations, fast charging stations and battery swapping stations. Meanwhile, the charging devices can be categorized as conductive chargers, inductive chargers and wireless chargers.

21.5.1 Home charging

Charging at home is the most preferable way for people to charge their EVs since it can be done by simply plugging the EV on-board charger to the outlet that is installed or nearby the parked car. People can get their EVs fully charged up after a night's parking, hence allowing for over 100 km driving range. As the batteries are slowly recharged, the power requirement is just a few kilowatts and the charging time is from five to eight hours. In general, home charging favours an effective utilization of electricity since EVs are usually charged at night or off-peak periods. Thus, power utilities are willing to impose an incentive tariff and rebate to attract EV users to charge their EVs during off-peak periods.

The basic requirement of home charging is the availability of a garage or a parking lot that is fed with electricity. For those houses with a private garage, an indoor socket outlet can be installed for recharging. For those apartments and multi-storey buildings with car parks attached, an outdoor socket outlet can be installed. The outdoor socket outlets should have individual protection circuits, and can be independently operated.

21.5.2 Public charging

Basically, the public charging facilities refer to a distribution of public charging stations which can be readily accessed by EV drivers to recharge their EVs when necessary.

Opportunity charging stations

Conceptually, EVs are expected to be fully charged at night or after five to eight hours' parking at home. Practically, EVs may be used both day and night or desired to travel a long distance per day. Opportunity charging is a concept that EVs can be recharged whenever there are such opportunities. These charging stations are normally located in places where EVs are likely to park for half an hour to several hours such as airports, schools, shopping malls, supermarkets and tourist attractions.

EV drivers should not always expect to fully charge up their EVs within the parking period. Instead, they should consider that this is a chance to increase the battery usable capacity for the upcoming travel or simply as an additional reserve. Typically, the driving range of EVs may be extended by about 40 km for an hour of opportunity charging.

Fast charging stations

Fast charging is also known as rapid charging or quick charging and aims to recharge EV batteries within a short period similar to that for gasoline refuelling of conventional vehicles. The time necessary for fast charging is about 20 minutes for charging up 80% capacity. Thus, the total travelling distance of EVs can be greatly extended, provided that there are sufficient fast charging stations on the way. The key to fast charging stations is the off-board fast charging module, which can output 35 kW or even higher. The corresponding voltage and current ratings are 45–450 V and 20–200 A, respectively. As both power and current ratings are so high, such recharging facilities have to be installed in supervised stations or service centres.

Although fast charging enables EVs to have a driving range similar to that of conventional vehicles, it creates adverse impacts on our power system, namely harmonic contamination and high current demand superimposing on the peak-hour consumption, violating the principle of demand-side management.

Battery swapping stations

Instead of charging the batteries immediately, there is another way to refuel the energy source of EVs: mechanically swapping the discharged batteries with fully charged batteries. Of course, all these batteries should be owned by the service station or battery company while the EV driver is only a battery borrower. The discharged batteries will either be charged at the service station or centrally collected and charged. Since the battery swapping process involves mechanical replacement and battery recharging, it is also named as mechanical refuelling or mechanical recharging. These battery swapping stations combine the merits of both slow charging and fast charging, namely slowly recharging the EV batteries at off-peak periods while quickly refuelling the EVs within a very short time. With the use of robotic machinery, the whole battery swapping process can be carriedout within a few minutes, directly comparable to the existing refuelling mechanism for conventional vehicles.

There are many obstacles to practically implementing battery swapping. Firstly, the initial cost to set up this battery swapping system is very high, involving expensive robotic machinery to swap the battery and a large number of costly batteries for necessary operation. Secondly, due to the need to store both discharged and fully charged batteries, the necessary space to build a battery swapping station is much larger than that for a charging station. Thirdly, the EV batteries need to be standardized in physical dimensions and electrical parameters before the possible implementation of automatic battery swapping.

21.5.3 Battery chargers

Depending on whether the battery charger is installed inside or outside the EV, it is generally classified as on-board and off-board. The on-board charger is designed with a low charging rate, and is dedicated to charge the battery for a long period of time (typically 5–8 h for full charge). Due to the limitation of allowable payload and space of the EV, the on-board charger needs to be lightweight (typically less than 5 kg) and compact. On the other hand, the off-board charger is designed with a high charging rate, and has virtually no limitation on its weight and size. Based on the modes of power transfer from the power supply to the EV, the battery chargers can be described as conductive, inductive or wireless chargers. In principle, both on-board and off-board chargers can be based on these three modes of power transfer.

Conductive chargers

The conductive charger for EVs has the advantages of maturity, simplicity and low cost because it simply makes use of plugs and sockets to conduct electrical energy via physical metallic contacts.

According to the SAE J1772 standard, two AC levels (AC level 1 and 2) are defined while two DC levels (DC level 1 and 2) are proposed for EV conductive chargers. Currently, AC level 3 and DC level 3 are under active discussion. As listed in Table 21.4, AC level 1 and 2 are designed for single-phase on-board chargers, whereas DC level 1 and 2 are dedicated

Table 21.4 Standard power levels of conductive chargers

AC level 1	DC level 1
Single-phase, 120 V, 16 A, 1.9 kW (max)	200–450 V, 80 A, 36 kW (max)
AC level 2	DC level 2
Single-phase, 240 V, 80 A, 19.2 kW (max)	200–450 V, 200 A, 90 kW (max)
AC level 3	DC level 3
Single-phase or 3-phase, > 20 kW	200–600 V, 400 A, 240 kW (max)

for off-board chargers. These charging levels essentially satisfy all charging needs of various EVs (SAE Hybrid Committee, 2011).

Safety is the key concern for conductive chargers. The SAE J1772 standard includes measures to avoid electrocution. Namely, when the connector is mated, the power pins are not accessible; when not mated, the power pins are not energized. Also, there is a pilot pin to control the charging process and to detect the presence of the EV. There are no voltages at the power pins when the pilot pin is decoupled and the control signal does not activate power transfer.

Inductive chargers

Inductive charging allows electrical energy transfer from chargers to EVs by magnetic induction. The principle of inductive charging is based on the magnetic coupling between two windings of a high-frequency transformer. SAE published the J1773 standard, also termed the Magne Charge, for EV inductive chargers. Specifically, the primary winding is installed in the charger coupler while the secondary winding is embedded in the inlet of the EV. The main AC supply with a frequency of 50–60 Hz is rectified and converted to a high-frequency AC power of 80–300 kHz within the charger, then the high-frequency AC power is transferred from the charger coupler to the EV inlet by magnetic induction, and finally this high-frequency AC power is converted to DC power for battery charging. It can deliver high power at an efficiency of 86% (6.6 kW for normal charging and 50 kW for fast changing). The whole process is free from any metallic contacts between the charger and the EV, hence offering a distinct merit over the conductive one: inherently safe under all-weather operation (including rainy, snowy and dirty conditions). The main drawbacks are the high investment cost, inevitable induction loss and high switching loss.

To promote the concept of park-and-charge without plug, the inductive charger is further extended to the plugless charger. Specifically, the primary winding is installed on the floor of a parking lot while the secondary winding is installed on the vehicle. When the two pieces are brought together in close proximity, power transfer is activated to charge the batteries. It offers the advantages of better convenience for users and larger tolerance for industry standardization. However, it further decreases the overall efficiency of battery charging.

Wireless chargers

The most ideal situation for charging EV batteries is to charge the vehicle while it is cruising on the road, the so-called move-and-charge. Thus, there is no need for an EV driver to find a charging station, park the vehicle and then spend time recharging the batteries. The power transmitter is placed underneath the surface of a section of roadway, called the charging zone, and the EV wirelessly picks up the power for battery charging.

The wireless power transfer technique for move-and-charge is based on magnetic resonant coupling which is the near field wireless transmission of electrical energy between two coils that are highly resonant at the same frequency. Differing from the magnetic induction which is adopted by inductive chargers, the resonance at the primary can enable high current operation in the primary circuit without suffering from high losses while the resonance at the secondary can boost the efficiency of the power transfer (Kurs et al., 2007). So, the resonant objects, namely the power transmitters beneath the roadway and the receivers mounted on EVs, having the same resonant frequency can wirelessly transfer power efficiently with high power density, while dissipating relatively little energy in non-resonant objects such as vehicle bodies or drivers.

Definitely, the wireless move-and-charge needs much research in both technological and biological aspects, such as the varying displacement between the transmitter and the receiver, the misalignment between the transmitter and the receiver, and the biological response of humans to strong magnetic field. Nevertheless, this charging concept has a promising future as it can potentially solve the fundamental problem of EVs: short driving range per charge.

21.6 Vehicle-to-grid (V2G) technology

The vehicle-to-grid (V2G) technology is one of the most emerging systemcrossover technologies for EVs. It is a crossover of EVs, power system and information technology. The EV is no longer a simple transportation means, but serves as a mobile power plant generating electrical energy to the power grid when necessary. Thus, this V2G technology relies on a smart power grid, termed the smart grid, which involves smart metering, dynamic pricing, automated control and real-time information exchange.

21.6.1 V2G concept

The V2G concept describes a system in which EVs communicate with the power grid to sell services by delivering electricity into the grid or by controlling the charging rate for EVs. Since most EVs are parked with an average of 95% of the time, their batteries can be used to let electricity flow between the vehicles and the grid. When there is a reasonable penetration rate of EVs (such as 20–40% of vehicles are EVs) and each EV can store or generate electrical energy of 10–56 kWh, the V2G concept will have a significant impact on power system operation. Economically, the V2G concept will be a new business; namely the energy arbitrage between the power utilities and the EV drivers.

21.6.2 V2G operation

Since an EV can store only 10–56 kWh, an individual V2G operation of each EV with the power grid is ineffective and inefficient. So, an aggregator is introduced which is responsible for gathering a number of EVs and communicating with the power grid. Based on the willingness of EV drivers and the battery capacity of EVs, the aggregator controls proper EVs to achieve smart charging and discharging (Guille and Gross, 2009).

When an aggregation of EVs is ready for energy arbitrage, it may not be necessary to perform V2G operation. Actually, the corresponding energy arbitrage can be performed internally by the aggregator. This dual-grid framework is depicted in Fig. 21.11 in which the ESP is the energy service provider that markets and sells power directly to homes and businesses, the ISO is the independent system operator that oversees the operations of a particular section of the power grid, the RTO is the regional transmission organization that integrates the ISOs into larger operations, and the aggregator

21.11 Aggregated dual-grid V2G framework.

functions to aggregate the EVs to deal with the ESP and the ISO/RTO. Firstly, the aggregator coordinates the intragrid power flow, minimizes the total power demand and total power loss, optimizes the voltage deviation and total harmonic distortion, and calculates prices to maximize the profit of intragrid operation. Secondly, the aggregator coordinates the intergrid power flow, deals with the ISO/RTO to sell power and energy, deals with the ESP to buy power and energy, and calculates prices to maximize the profit of intergrid operation.

21.6.3 V2G application

The V2G operation has been identified to have two important applications. Firstly, since renewable power generations, such as wind power and solar power are intermittent in nature, the use of standby generators to backup the intermittent power outage is expensive, inefficient and sluggish. Although the battery energy storage system can perform the desired efficient and fast backup, it is too expensive and bulky. The V2G operation can fully utilize the EV batteries to backup the intermittent power outage. Secondly, since the power generation capacity has to match with the load demand, a large fluctuation of load demand will significantly increase the capital cost and operating cost of the power system. As shown in Fig. 21.12, the V2G operation can utilize the EV batteries to absorb or buy electrical energy from the grid during the off-peak period (called load levelling), but generate or sell electrical energy to the grid during peak period (called load shaving). Also, the corresponding charging and discharging processes are much faster than the shutoff and startup processes of standby generators.

However, at the present status of battery technology, the EV batteries still suffer from a limited cycle life. The V2G operation will inevitably degrade

21.12 Load levelling and load shaving.

their life for normal vehicular operation. Also, since the charging efficiency from the grid to the battery is 70–80% while the discharging efficiency from the battery to the grid is 80–90%, the overall charging-discharging efficiency for V2G operation is about 60–70% which needs to be considered against the potential cost savings.

21.7 Benefits and limitations of EVs

The use of EVs enjoys two major benefits. Namely, the energy benefit resulting from better energy diversification and higher energy efficiency, as well as the environmental benefit resulting from better air quality. Nevertheless, there are some limitations, especially the high initial cost and the short driving range per charge that hinder their popularization.

21.7.1 Energy benefits

Deriving from crude oil, gasoline and diesel are the major liquid fuels for existing vehicles. Although the development of biofuels has taken on an accelerated pace in recent years, our road vehicles still heavily depend on crude oil. EVs are an excellent solution to rectify this unhealthy dependence because electricity can be generated by almost all kinds of energy resources, including coal, natural gas, solar, nuclear, hydro, wind, geothermal, oceanic and biomass.

Besides the definite merit of energy diversification resulting from the use of EVs, the most important advantage is the high energy efficiency offered by them. In order to compare the overall energy efficiency of the EV with the conventional internal combustion engine vehicle (ICEV), their energy conversion processes from crude oil to road load are depicted in Fig. 21.13, indicating that the EV is more energy-efficient than the ICEV (Chan and Chau, 2001). Moreover, the EV possesses one distinct advantage over the ICEV in energy recovery, the so-called regenerative braking. Namely, the EV can recover the kinetic energy during braking and utilize it for battery recharging, whereas the ICEV wastefully dissipates this kinetic energy as heat in the brake discs. With this technology, the energy efficiency of the EV can be further boosted by up to 10%.

21.7.2 Environmental benefits

In many metropolises, ICEVs are responsible for over 50% of harmful air pollutants and smog-forming compounds. Although the engine of ICEVs is continually improved to reduce the emitted pollutants, the increase in the number of ICEVs is much faster than the reduction of emissions per vehicle. Thus, the total emitted pollutants due to ICEVs, including carbon monoxide

21.13 Comparison of energy efficiencies between EV and ICEV.

21.14 Comparison of overall harmful emissions between EV and ICEV.

(CO), hydrocarbons (HC), nitrogen oxides (NOx), sulphur oxides (SOx) and particulate matter (dust), continue to grow in a worrying trend.

In order to reduce or at least slow down the growth of air pollution due to road transportation, the use of EVs is the most viable choice. Definitely, the EV offers zero local emissions. Even taking into account the emissions generated by refineries to produce gasoline for the ICEV as well as the emissions by power plants to generate electricity for the EV, the overall harmful emissions of the EV are still much lower than those of the ICEV as indicated in Fig. 21.14 (Chan and Chau, 2001).

Whether the use of EVs can reduce overall carbon dioxide (CO2) emission is controversial. With the improvement of engine technology, European ICEVs have reduced CO2 emission from 172 g/km to 153 g/km over the period from 2000 to 2008, and need to reach 120 g/km by 2015 (Kojima and Ryan, 2010). Meanwhile, the CO2 emission of EVs heavily depends on the fuel mix for electricity generation. For instance, the average well-to-wheel CO2 emission of EVs in the UK is about 80 g/km based on the fuel mix of coal (28.9%), natural gas (44.2%), nuclear (17.3%), renewables (7.9%) and other (1.7%). According to the relationships between CO2 emission and fuel mix in different countries (Van der Hoeven, 2011), there is a large variation between countries in the CO2 emission of EVs. For example, the CO2 emission of EVs in Norway and China are estimated to be 3 g/km and 132 g/km, respectively, because Norway adopts over 98% hydroelectricity whereas China relies on over 66% coal-fired electricity generation. Therefore, the CO2 emission can be improved by using EVs and energy-efficient power plants. This improvement may be further increased with the use of higher percentages of clean or renewable power generation, but may even be negative when adopting inefficient coal-fired power plants.

21.7.3 Limitations

There are two major limitations hindering the popularization of EVs: high initial cost and short driving range. These limitations are mainly due to the high cost and low capacity of batteries, which cannot be easily solved by the foreseeable energy storage technology. Socially, the present EV market is under a vicious circle: high initial price Æ low demand Æ lack of investment Æ no mass production Æ no large-scale sales high initial price …

21.8 Conclusions and future trends

The commercialization of EVs is still slow, much slower than the hybrid vehicles (Chau, 2010). The major reasons are due to the high initial cost and short driving range per charge which cannot be easily solved in the near future. It also explains why most automakers focus on promoting hybrid vehicles, especially the emerging plug-in hybrids (Mi et al., 2011; Chau and Chan, 2007). For instance, the two representative plug-in hybrids, namely the GM Chevrolet Volt with a 1.4 L engine and a 16 kWh Li-ion battery and the Toyota Prius Plug-in with a 1.8 L engine and a 4.4 kWh Li-ion battery, can run 610 km and 870 km, respectively, which are much further than the driving range of 117 km offered by the Nissan Leaf EV with 24 kWh Liion battery. Moreover, these plug-in hybrids can reduce the initial cost of EVs due to the reduction of battery capacity, and also overcome the range anxiety problem of EVs due to the availability of gasoline.

Apart from the spin-off to plug-in hybrids, the future trends of EV development will be on the concept of a commuter EV system. Since the commuter EV is designed to be a compact size and is required to travel a short distance, it just needs a relatively small number of battery packs, thus significantly cutting the initial cost. In order to fundamentally solve the problems of high initial cost and short driving range, the commuter EV system aims to expand the role of EVs and change the ownership of EVs. Namely, when a large fleet of commuter EVs is owned by the municipal government, a power utility or even a private company, the driver just needs to hire the EV in any hire station and drive to another hire station that is near to their destination. Thus, each EV will be automatically charged up at the hire station and the required energy storage is just for the distance between two hire stations. Also, each hire station can serve as an aggregator for V2G operation which definitely expands the role of EVs.

21.9 Acknowledgements

The author would like to express heartfelt thank to all group members of the International Research Centre for Electric Vehicles for their contributions to this chapter. He must express his indebtedness to Joan and Aten for their hearty support.

21.10 Sources of further information and advice

The key sources of further information can be found in the international electric vehicle symposium (EVS) which is the most prominent forum in the field of EVs organized by the World Electric Vehicle Association (http:// www.wevaonline.net/) annually or biennially. More academic information on EV technologies can be found in the learned journals such as the International Journal of Electric and Hybrid Vehicles, the Journal of Asian Electric Vehicles, and the World Electric Vehicle Journal. More information on EV development can be found from the websites of the Electric Drive Transportation Association (EDTA), the Electric Vehicle Association of Asia Pacific (EVAAP) and the European Association for Battery, Hybrid and Fuel Cell Electric Vehicles (AVERE).

21.11 References

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B. G. Pollet, University of the Western Cape, South Africa, I. Staffell, Imperial College London, UK, J. L. Shang, University of Birmingham, UK and V. Molkov, University of Ulster, UK

DOI: 10.1533/9780857097422.3.685

Abstract: Decarbonising transport is proving to be one of today's major challenges for the global automotive industry due to many factors such as the increase in greenhouse gas and particulate emissions not only affecting the climate but also humans, the increase in pollution, rapid oil depletion, issues with energy security and dependency from foreign sources and population growth. Major breakthroughs in low and ultra-low carbon technologies and vehicles are urgently required. This chapter highlights the current status of hybrid, battery and fuel cell electric vehicles from an electrochemical and market point of view.

Key words: automotive energy storage devices (ESD), hybrid electric vehicle (HEV), battery electric vehicle (BEV), fuel cell electric vehicle (FCEV), hydrogen

22.1 Introduction

Decarbonising transport is proving to be one of the largest R&D projects of the early 21st century. There are around 1 billion automobiles in use worldwide, satisfying many needs for mobility in daily life [1]. The automotive industry is therefore one of the largest economic forces globally, employing nearly 10 million people and generating a value chain in excess of \$3 trillion per year [2]. As a consequence of this colossal industry, the large number of automobiles in use has caused and continues to cause a series of major issues in our society:

∑ Greenhouse gas (GHG) emissions the transportation sector contributes 13.1% of GHG emissions worldwide (5 billion tonnes of CO2 per year). More than two thirds of transport-related GHG emissions originate from road transport [3]. Reducing the GHG emissions of automobiles has thus become a national and international priority.
∑ Air pollution tailpipe emissions are responsible for several debilitating respiratory conditions, in particular the particulate emissions from diesel vehicles. The increasing number of diesel vehicles on Europe's roads would further worsen air quality.
∑ Oil depletion oil reserves are projected to only last 40–50 years with current technology and usage. Transport is already responsible for almost 70% of the EU's oil use and this share continues to increase [4].
∑ Energy security Europe's dependence on foreign sources for more than 80% of its oil and reserves of conventional oil is increasingly concentrated in politically unstable regions [4]; dependency on fossil fuels for transportation therefore needs to be reduced.
∑ Population growth It has recently been declared (31 October 2011) that there are 7 billion inhabitants in the world with an estimated figure of 9 billion in 40 years' time. This will obviously have an important impact on climate change, food security and energy security.

The ever increasing demand for personal mobility and near total dependence on liquid hydrocarbons means that emission reductions from this sector will be particularly difficult.

The development of alternative fuels to petrol and diesel has been ongoing since the 1970s, initially in response to the oil shocks and concerns over urban air pollution. Efforts have gained momentum more recently as the volatility of oil prices and stability of supplies, not to mention the consequences of global climate change, have risen up political agendas the world over. Low-carbon technologies are therefore rapidly advancing, with petrol and diesel hybrids, battery electric, hydrogen fuel cell, and hybrids of the two being developed by nearly every major manufacturer. Concerns about up-scaling production and the 'true' environmental and social costs of biofuels means that hydrogen and electricity are widely regarded as the sustainable transport fuels of the future.

This chapter aims to highlight the current status of hybrid, battery and fuel cell electric vehicles from an electrochemical and market point of view. The chapter also discusses the advantages and disadvantages of using battery, hydrogen and fuel cell technologies in the automotive industry and the impact of these technologies on consumers.

22.2 Energy storage devices (ESDs) for the transport sector

Energy storage devices are systems which store energy in various forms such as electrochemical, kinetic, pressure, potential, electromagnetic, chemical, and thermal; using, for example, fuel cells, batteries, capacitors, flywheels, compressed air, pumped hydro, super magnets, hydrogen, etc. The principal criteria of an ESD required for a specific application, in this case automotive, are:

∑ the amount of energy in terms of specific energy (in Wh.kg–1) and energy density (in Wh.kg–1 or Wh.l–1)

∑ the electrical power (in W.kg–1 or W.l–1), i.e. the electrical load required
∑ the volume and mass
∑ reliability
∑ durability
∑ safety
∑ cost
∑ recyclability
∑ environmental impact.

When choosing an ESD, the following characteristics should be considered:

∑ specific power
∑ storage capacity
∑ specific energy
∑ response time
∑ efficiency
∑ self-discharge rate/charging cycles
∑ sensitivity to heat
∑ charge-discharge rate life time
∑ environmental effects
∑ capital/operating cost
∑ maintenance.

For battery electric vehicles (BEVs), batteries with stored energies of 5–30 kWh for small urban BEVs (up to 85 kWh, e.g. the recently launched Tesla Model E) and up to 100 kWh for electric buses are required; whereas hybrid electric vehicles (HEVs) hold 1–5 kWh of stored energy, and focus more exclusively on high power discharge. Table 22.1 shows several types of electrochemical ESDs and their characteristics.

22.3 Batteries

A battery is an electrochemical cell (also known as a galvanic cell) that transforms chemical energy into electrical energy; it consists of an anode and a cathode, separated by an electrolyte (an ionic conductor which is also an electronically insulating medium). Electrons are generated at the anode and flow towards the cathode through the external circuit while, at the same time, electroneutrality is ensured by ion transport across the electrolyte. Table 22.2 shows characteristics of various types of battery.

The two main types of battery used in BEVs are nickel metal hydride (NiMH) and lithium-ion (Li-ion) batteries. NiMH batteries are in most cases used as secondary energy sources in HEVs (e.g. Toyota Prius) where they are used in conjunction with an internal combustion engine (ICE), whereas

Table 22.1 Electrochemical ESD characteristics

Characteristics	Supercapacitors (electrochemical capacitors)	Batteries	Fuel cells
Charge/discharge time Operating temperature/°C Operating cell potential (DV)/V	ms–s –40 to + 85 2.3–2.75	1–12 hrs –20 to + 65 1.25–4.2	1–300 hrs + 25 to + 1,000 0.6–1.0
Capacitance/F Life time Weight/kg Power density/kW.kg–1 Energy density/Wh.kg–1	0.1–2 30,000 + hrs 0.001–2 10–100 1–5	– 150–1,500 cycles 0.001–10 0.005–0.4 5–600	– 1,500–10,000 hrs 0.02–10 0.001–0.1 300–3,000

Table 22.2 Characteristics of various types of battery [5]

Battery chemistry	Type (primary (P)/secondary (S))	Cell potential (DV)/V	Theoretical (practical) specific energy/ Wh.kg–1	Useful energy density/Wh.l–1
Alkaline zinc manganese dioxide (Zn/MnO2)	P	1.5	358 (145)	400
Lithium iodine (Li/I2)	P	2.8	560 (245)	900
Alkaline nickel cadmium (NiCd)	S	1.3	244 (35)	100
Nickel metal hydride (NiMH)	S	1.3	240 (75)	240
Lead acid (Pd/A)	S	2.1	252 (35)	70
Sodium sulphur (Na/S)	S	2.1	792 (170)	345
Sodium nickel chloride (NiMH)	S	2.6	787 (115)	190
Lithium-ion (Li-ion)	S	4.1	410 (180)	400

Li-ion batteries are used as primary energy sources in BEVs such as the Nissan Leaf and Mitsubishi iMiev (Fig. 22.1). The \$35,000 5–door Nissan Leaf BEV is powered by 12 ¥ 4 cells (48 modules) providing a capacity of 24 kWh and taking up to 8 hours to fully charge from a standard domestic outlet from zero state-of-charge (SoC), or 30 minutes from a three-phase AC socket.

22.3.1 Nickel metal hydride (NiMH) batteries

NiMH batteries are used in over 95% of all HEVs, and major manufacturers have so far invested substantially in the last 10 years. The major advantage

22.1 Mitsubishi iMiev.

from a manufacturing point of view is the safety of NiMH batteries compared to Li-ion batteries, and, so far, no incidents have been reported in the press. Furthermore, NiMH batteries are preferred in industrial and consumer applications due to their design flexibility (e.g. ranging from 30 mAh to 250 Ah), environmental acceptability, low maintenance, high power and energy densities, cost and most importantly safety (in charge and discharge modes, especially at high voltages). NiMH batteries are currently priced at \$250 to \$1,500 per kWh, hence the total price of the battery pack for a hybrid (e.g. Toyota Prius, although the newer models use 5.2 kWh Li-ion battery packs) varies anywhere between \$600 and \$3,000 per vehicle.

The NiMH battery was patented in 1986 by Stanford Ovshinsky, founder of Ovonics, when researching hydrogen storage materials. Ovshinsky also described the NiMH battery as the 'hydrogen ion' or 'protonic' battery by analogy with lithium-ion batteries as the NiMH electrochemical reaction involves the transfer and 'insertion' of H+ . The components of NiMH batteries include an anode of hydrogen absorbing alloys (MH), a cathode of nickel hydroxide (Ni(OH)2) and a potassium hydroxide (KOH) electrolyte (Fig. 22.2).

The general electrochemical reactions are as follows:

22.2 Schematic diagram of the electrochemical reaction processes of a NiMH battery. For the charge process, the hydrogen atom dissociates from Ni(OH)2 and is absorbed by the MH alloy. For the discharge process, the hydrogen atom dissociates from the MH alloy and joins with NiOH to form Ni(OH)2 [6]. Source: F. Feng, M. Geng and D.O. Northwood, International Journal of Hydrogen Energy, 26 (2001) 725–734, copyright © Elsevier.

Anode (-) $$M + e^- + H_2O \rightarrow MH + OH^-$$ [22.1]

Cathode (+) $$\text{Ni(OH)}_2 + \text{OH}^- \rightarrow \text{NiO(OH)} + \text{H}_2\text{O} + \text{e}^-$$ [22.2]
Overall reaction $M + \text{Ni(OH)}_2 \rightarrow MH + \text{NiO(OH)}$ [22.3]

where 'M' is an intermetallic alloy capable of forming a metal hydride phase.

22.3.2 Lithium-ion (Li-ion) batteries

Lithium-ion batteries are light, compact and operate with a cell voltage of 4V with a specific energy in the range of 100–180 Wh.kg−1. In these type of battery both the anode (graphite, e.g. mesocarbon microbeads – MCMB) and cathode (lithium metal oxide – LMO2, e.g. LiCoO2) are materials into which, and from which, lithium (as Li +) migrates through the electrolyte (typically a lithium salt, e.g. lithium hexafluorophosphate (LiF6) in an organic solvent, e.g. ethylene carbonate–dimethyl carbonate (EC–DMC) in a separator felt), then is inserted (intercalation process) or extracted (deintercalation process) into the electrodes (Plate XIX between pages 392 and 393).

Thus when a lithium-ion battery is discharging, Li is extracted from the anode (–) and inserted into the cathode (+) and when it is charging, the reverse process occurs according to the following reactions:

$$yC + LiMO_2 \rightarrow Li_xCy + Li{(1-x)}MO2$$ where $x = 0.5, y = 6$ , and $V{cell} = 3.7 \text{ V}$ [22.4]

For the case of LiCoO2:

$$LiCoO2 \rightarrow Li1-xCoO2 + xLi+ + xe-$$ [22.5]

$$x \text{Li}^+ + x \text{e}^- + 6\text{C} \to \text{Li}_x \text{C}_6$$ [22.6]

The overall reaction is:

$$x\text{Li}^+ + x\text{e}^- + \text{LiCoO}_2 \rightarrow \text{Li}_2\text{O} + \text{CoO}$$ [22.7]

During recharging, Li + ions are removed and the oxidation of Co3+ to Co4+ occurs. The Co3+/Co4+ couple supplies a cell voltage of about 4.0 V vs. metallic Li.

Li-ion batteries store more energy than NiMH; however, they suffer from major issues such as cost (\$1,000/kWh), wide operational temperature ranges, materials availability (e.g. Li), environmental impact and safety. For example, LiCoO2 batteries are unsafe as they are thermodynamically unstable although they are kinetically stable in practice. It is often observed that these batteries suffer from electrolyte decomposition leading to the formation of oxide films on the anode, thus blocking extraction sites of lithium and severe oxidative processes at the cathode due to overcharging, in turn causing dissolution of protective films on the cathode and excess and continuous oxidation of the electrolyte (gas evolution). Table 22.3 gives a summary of the main secondary batteries and their characteristics and Plate XX (between pages 392 and 393) shows a diagram comparing the various battery technologies in terms of volumetric and gravimetric energy density [9].

22.3.3 Current issues

Lithium-ion 'chemistry' for batteries has not progressed much since their introduction to the market in the early 1990s by Sony and Asahi Kasei following the pioneering work from Whittingham, Tarascon, Armand and Scrosati [9]. Therefore, breakthroughs in lithium-ion battery technology are urgently required, with innovative, performing and durable material chemistries for both the electrodes and the electrolyte sub-components. The

Table 22.3 Various secondary batteries and their reactions [10]

Lithium-ion

Li + C

Ba t te te ry s y s m	An de (– ) o	E lec ly tro te	C O An d ic ( A ) / ho d ic ( ) / l l ( ) io t t o ca ov er a re ac ns	Ca ho de ( ) t +
Le d / i d a ac	P b	H2 S O4	2– A: P b S O4 P b S O4 2e i – + +	P b O2
			2– C: P b O2 H + + S O4 2e P b S O4 2 H2 O 4 – i + + +
			O: P b O2 2 H2 S O4 P b 2 P b S O4 2 H2 O i + + +
N i C d	C d	K O H	A: C d 2 O H C d ( O H )2 2e – i – + +	N i O O H
			C: 2 N i O O H 2 H2 O 2e 2 N i ( O H )2 2 O H – i – + + +
			O: 2 N i O O H C d 2 H2 O N i ( O H )2 C d ( O H )2 i + + +
N i M H	Hy dr og en	K O H	A: M H2 O M H O H – i – + e + +	N i O O H
	ds be d a or		C: N i ( O H )2 O H N i O ( O H ) H2 O – i – + + + e
	l loy a		O: M N i ( O H )2 M H N i O ( O H ) i + +

electrolyte + C: Li+ + e– + CoO2

O: Li(C) + CoO2

Organic A: Li(C) i Li+ + e– CoO2

ⁱ LiCoO2

Li salt

principal objective is to identify materials exhibiting higher performance and durability than those currently offered. Currently worldwide R&D efforts focus upon the replacement of (i) graphite and LiCoO2 with alternative high capacity and low cost materials and (ii) ethylene carbonate-dimethyl carbonate with other electrolytes which do not suffer from decomposition under oxidative regimes.

According to a report from the Strategy Consultancy Roland Berger [11], the supply of Li-ion batteries will exceed demand by more than 100% by 2015, in other words the market will grow from circa \$1.5 billion in 2011 to over \$9 billion in 2015. It is speculated that the market for Li-ion batteries in the automotive sector will reach over \$50 billion by 2020 [11]. At present Li-ion batteries are expensive but it is anticipated that the price will decline rapidly and that they will be the cheapest rechargeable batteries in 10 year's time. Furthermore, the USA are supporting BEV technology with approximately \$10 billion, while China are spending \$15 billion on R&D for BEVs [12].

Scarcity of lithium was once thought of as a looming concern for the electrification of vehicle fleets. However, it should be noted that only around 1% of a lithium-ion battery is Li by weight, implying around 0.08 kg of Li per kWh of storage capacity (approximately 1–2 kg per BEV) [13]. At present, this lithium is not recycled due to excessive cost and energy requirements; however, if future supply shortages lead to increasing material prices, the recycling of these batteries will become standard.

22.4 Hydrogen and fuel cells

22.4.1 Hydrogen energy

Hydrogen facts, production, storage and usage

Worldwide, 50 million tonnes of hydrogen is produced, mainly through reformation of fossil fuels, with 100,000 tonnes of H2 produced in the UK. Recent worldwide hydrogen production totals show that 48% of hydrogen is produced from natural gas, 30% from oil, 18% from coal and only 4% from renewable sources [14]. Nowadays, hydrogen is used in chemical processing, the petroleum industry, fats and oils, metals, electronics, space flights, utilities, glass manufacturing, etc.

Hydrogen storage on board vehicles is the key factor for achieving market success for fuel cell electric vehicles (FCEVs). To be competitive with ICE vehicles, hydrogen fuel cell vehicles (HFCVs) should have a similar driving range. As the volumetric energy density of hydrogen is very low, storing enough hydrogen on board remains a challenge in terms of weight, volume, kinetics, safety and cost. However, the efficiency of a hydrogen ICE is circa 25% and that of a HFCV is 60%; this is three times better than today's petrol-fuelled engines (18–20% for a petrol ICE reaching 40% at peak efficiency). The low volumetric energy density can only be increased by storing the hydrogen either under increased pressure, at extremely low temperatures as a liquid or in metal hydride systems.

Indeed, there are various methods for storing hydrogen in vehicles:

∑ Liquid hydrogen the energy density of liquid hydrogen is high, but to store hydrogen in a liquid state, it is necessary to maintain it at –253°C and at ambient pressure. Therefore a highly insulated liquid hydrogen tank is required. Furthermore, a quarter of the chemical energy of hydrogen itself is consumed in the liquefaction process.
∑ Compressed hydrogen the most popular method chosen by leading FCEV manufacturers. Honda and Nissan use 350 bar, whereas Toyota prefers 700 bar. The energy density is relatively low and the energy is highly consumed in compression.
∑ Metal hydrides These are the safest method, but weight is a major issue for transportation applications (although not for maritime applications), in addition a lot of time is required to store the hydrogen (i.e. long refuelling time), and it has an insufficient release rate.
∑ Hydrogen absorbed onto carbon nanotubes (CNT) and metal organic frameworks (MOF) – still at developmental stages.

As stated previously, hydrogen has a the highest energy content of any fuel by weight (33,320 Wh.kg–1) – i.e. about three and seven times more than gasoline (12,700 Wh.kg–1 or 8,760 Wh.l–1), natural gas (13,900 Wh.kg–1 or 5,800 Wh.l–1) and coal (7,200 Wh.kg–1), but it has a very low energy content by volume (about 3,000 times less than gasoline at STP) [15]. This makes storage and distribution to the point of use costly. 5 kg of hydrogen is equivalent to 5 gallons (or 22 litres) of petrol, but to store it under ambient conditions would require a 5 metre diameter vessel which is impractical! Table 22.4 compares the density of hydrogen to conventional fuels under different conditions.

Table 22.4 Comparison of density and energy densities of various fuels
	Density				Energy Density (LHV)
Fuel	kg.L–1	kg.m–3	MJ.kg–1	MJ.L–1	kWh.kg–1	kWh.m–3
H2 at 20 °C, 1 atm	0.0000899	0.0899 120		0.01006	33.3	2.79

H2 at 20 °C, 350 bar 0.025 25 120 2.8 33.3 775.86 H2 at 20 °C, 700 bar 0.039 39 120 4.4 33.3 1,210.34

1 atm 0.0708 70.8 120 7.92 33.3 2,197.24 Petrol 0.702 702 42.7 31.2 11.86 8,666.67 Diesel 0.855 855 41.9 36.5 11.64 10,138.88

H2 liquid at boiling point,

So far, compressed hydrogen is the simplest and most cost effective method at present, and it is seen as the most viable solution for automobile use; it only requires a compressor and a pressure tank. Because of the low storage density under standard temperature and pressure (STP), it has to be compressed up to 10,000 psi. Nevertheless, few leading fuel cell vehicle manufacturers have been successfully demonstrated in several prototype models, for instance, Honda and Nissan opted for a 350 bar (5,000 psi) pressurised tank, whereas Toyota uses 700 bar (10,000 psi). All pressurised tanks are made of aluminium and carbon-fibre-reinforced composite materials (e.g. Dynetek – www.dynetek.com); however, the extremely high pressure causes public concern about their safety, although the 10,000 psi composite tanks have been demonstrated to be very safe (a 2.35 safety factor, i.e. a 23,500 psi burst pressure) as required by the European Integrated Hydrogen Project (EIHP) specifications. In addition, the cost to manufacture such containers is still high and the tank dimensions require more space than conventional petrol tanks. Currently, both 350 bar and 700 bar tanks are commercially available at low production volumes but at high costs. In addition, compressing the hydrogen to 350 bar and 700 bar requires an additional 7%–15% energy, although this can be lower if the hydrogen is produced at high pressure (350 bar = 10% wt)

Hydrogen can be liquefied for on-board vehicle storage. The density of liquid hydrogen is 70.8 kg.m–3, hence the energy density of liquid hydrogen is quite high, but to store hydrogen in a liquid state, it is necessary to maintain it at –253°C at ambient pressure. Therefore a highly insulated liquid hydrogen tank is needed. Furthermore, a quarter of the chemical energy of hydrogen itself is used through a number of steps of the liquefaction process; hence the cost is much higher compared to compressed hydrogen, although the transportation costs are much lower. To avoid hydrogen 'boil-off', a wellinsulated container has to be used, which adds more cost. A few decades ago (1980s), BMW developed and tested liquid hydrogen ICE vehicles whereby 8 kg of liquid hydrogen was stored on-board enabling the vehicle to reach a distance of more than 200 kms (125 miles), with a fill up time of the hydrogen tank of less than 8 mins. However, after more than 2 million accumulative miles testing, BMW recently suspended the project.

Many metals and alloys are capable of repeatedly absorbing and releasing large amounts of hydrogen and store it at solid state under low pressure. The process chemically 'bonds' the hydrogen in the interatomic lattice of the metal, absorb hydrogen into the lattice through cooling and releases hydrogen through heating. It is probably the safest method, but it is very heavy and has a low energy density. Metal hydride storage systems are unlikely to successfully achieve the US DoE target of 9%wt by 2015. In addition, the refuelling time is quite long, and the release kinetics are too slow for automobile applications.

Thus, for transportation applications the challenge is how to store the necessary amount of hydrogen required for considerable driving range (>300 miles), within the constraints of weight, volume, durability (>1,500 cycles), efficiency and cost. A hydrogen fuelled vehicle may need to carry 5–13kg hydrogen on-board in order to match the current conventional vehicles' performance.

Another possibility is to produce hydrogen on-board the vehicle. For example, hydrogen can be produced from hydrogen-rich resources such as methanol, ethanol, natural gas, and petrol/diesel fuels [15]. While hydrogen distribution infrastructure and on-board storage still pose some difficulties for automotive purposes, generating hydrogen on-board via a reformer is considered to be a potential 'solution' for automotive applications [15]; obviously a successfully integrated on-board reformer would have many benefits, such as reducing the complications and initial capital investments. But the main disadvantages for on-board reforming systems are cost, size, weight, response time, efficiency and durability. Generally, on-board vehicle reformers would be required to produce sufficient hydrogen for its use in the vehicle. Although tremendous progress has been made towards achieving technical targets for on-board fuel processing, it is unlikely that it will improve sufficiently to support the transition to a hydrogen economy in the short term. For example in 2004, the US DoE terminated the on-board hydrogen reforming programme; the focus then shifted towards the development of lightweight hydrogen storage materials and tanks as well as a hydrogen infrastructure.

Current hydrogen distribution methods

Hydrogen is currently transported/stored in gaseous form using tube trailers or cylinders and in liquid form in cryogenic liquid hydrogen tankers and to a very limited extent via pipeline. Liquid hydrogen, cooled to –253°C, is transported by road in super-insulated cryogenic tankers with capacities of up to 60,000 litres. Pipelines present the most cost effective means of transporting large quantities of gaseous hydrogen over long distances. Currently the hydrogen infrastructure in the world is limited. The US DoE target for hydrogen distribution is <\$1/Gge by 2015, whereas currently it is \$4–9/Gge depending on the amount and quantity. If 40 million hydrogen fuel cell vehicles were on the road by 2030 in Europe, this would equate to 19,000 hydrogen refuelling stations, costing €6–24bn (\$8–33bn) which is comparable to the investments made in mobile phone and broadband infrastructure. Small-scale distributed facilities would produce from 100 to 1,500 kg of hydrogen per day at fuelling stations. This capacity could satisfy most fuelling stations' need; providing hydrogen for more than 300 hydrogen fuel cell vehicles per day; medium-scale plants would produce from 1,500 to 50,000 kg per day on the outskirts of cities cost effectively; and the large-scale plants would produce more than 50,000 kg of hydrogen per day and could offer the most cost effective solution, but would lead to higher distribution costs. Nevertheless, current hydrogen production is mostly based at the large scale plants, producing circa 30 million tons of hydrogen per year in the US.

Needless to say that the hydrogen cost is the key for a successful hydrogen economy as it must be produced economically. Unfortunately, there is no natural source of hydrogen, so the cost not only includes capital costs for the process, the utility costs, and indirect costs for labour, infrastructure, but also includes the raw materials (such as natural gas, coal and electricity), hence the cost of hydrogen is expected to be more expensive than conventional fossil fuels (at today's price).

Distributed on-site production of hydrogen is the most viable solution for introducing hydrogen as a fuel. Generally speaking, it requires less initial investment for smaller capacity productions, without hydrogen transport and delivery infrastructure requirements. Two distributed on-site hydrogen production technologies that have the best potential for development and commercialisation during a transition to a hydrogen economy are:

1. Reforming of natural gas or liquid fuels, including renewable liquids such as ethanol and bio-oil; these technologies are mature and widely available, and close to the US DoE hydrogen production cost targets. When reforming with renewable liquids, it offers near-zero net greenhouse gas emissions. The hydrogen has to be up to 99.999% pure for proton exchange membrane fuel cell (PEMFC) vehicle uses.
2. Small-scale alkaline or proton exchange membrane (PEM) or electrolyser systems present the opportunity for non-carbon-emitting hydrogen production when a renewable electricity source is used. When off-peak electricity is used, greater economic opportunities may be presented by energy stations that produce both fuel and electricity.[10]

The distributed stations may need hydrogen storage to meet the variable demand for hydrogen during a 24-hour period.

Centralised production of hydrogen represents lower cost opportunities and allows management of GHG emission through technologies such as carbon capture and storage (CCS). Central production of hydrogen could potentially include a more diversified feedstock base, such as fossil fuels, nuclear and renewable, for example:

∑ Coal and natural gas they are possibly the least expensive feedstock, and CCS could be used to reduce or even eliminate GHG emissions.
∑ Centralised electrolysis systems are viable solutions as they are fairly inexpensive and renewable electricity may be available; arguably, turning

the primary energy into electricity and then electricity into hydrogen is not an energy friendly method – the cost of hydrogen will heavily depend upon the cost of electricity.

∑ Hydrogen can be produced by concentrated solar heat via high-temperature thermo-chemical process; it may become viable with the development of appropriate water-splitting chemical process cycles and materials.
∑ Photoelectrochemical and biological hydrogen production are long-term technologies that have the potential to produce hydrogen with sunlight, but currently only produce small amounts of hydrogen at high costs.

For all centralised hydrogen production methods, it would require the development of a distribution infrastructure for hydrogen to be commercially viable. It would also require storage for 1 to 5 days' supply of production to accommodate demand fluctuations and short-term outages.

While many hydrogen fuelled vehicles are expected to be in use within the 2015–2025 timeframe, it is more than likely that local distributed generations using small-scale natural gas reformers or electrolysers will be the most economical way before large centralised plants are geared up to distribute hydrogen.

22.4.2 Fuel cells

When oil, one of the most important energy sources in the history of mankind, was first discovered in Pennsylvania almost 150 years ago, the fuel cell had already been known for 20 years – invented by Sir William Grove, 'father of the fuel cell', in 1839. Back then it was an idea that was far ahead of its time. Today, however, it is the most important development in the history of decentralised energy supply.

Today, fuel cells are widely considered to be efficient and non-polluting power sources offering much higher energy densities and energy efficiencies than any other current energy storage devices. Fuel cells are therefore considered to be promising energy devices for the transport, mobile and stationary sectors [16–19]. A fuel cell is an 'electrochemical' device operating at various temperatures (up to 1,000 °C) that transforms the chemical energy of a fuel (hydrogen, methanol, natural gas, etc.) and an oxidant (air or pure oxygen) in the presence of a catalyst into water, heat and electricity. Furthermore, the power generated by a fuel cell depends largely upon the catalytic electrodes and materials used [16–19].

There are currently six main groupings of fuel cell available [14, 16– 20]:

1. PEMFC including direct methanol fuel cell (DMFC)
2. alkaline fuel cell (AFC)
1. phosphoric acid fuel cell (PAFC)
1. molten carbonate fuel cell (MCFC)
1. solid oxide fuel cell (SOFC) and
1. microbial fuel cell (MFC).

PEMFC, AFC, PAFC, and MFC operate at low temperatures in the range of 50–200°C and MCFC and SOFC at high temperatures in the range of 650–1,000°C (Fig. 22.3).

The heart of a fuel cell consists of a non-conductive electrolyte material sandwiched between two electrodes – the anode and cathode [14]. The fuel and the oxidant are fed continuously to the anode and the cathode sides, respectively. At the anode side the fuel is decomposed into ions and electrons. The insulator electrolyte material allows only ions to flow from both the anode and cathode sides. The free electrons generated at the anode flow to the cathode side through an external electrical circuit. The recombination of the ions with the oxidant occurs at the cathode to form 'pure' water. In contrast to water electrolysis, the polarity of a fuel cell on the anode is negative and on the cathode is positive [14].

PEMFCs

PEMFC is a technology which was initially developed for military and spacecraft applications at GE (General Electric, USA) in the 1960s but was

22.3 Schematic representing all available and existing fuel cells. Copyright: Bruno G. Pollet.

abandoned in the 1970s, due to high cost and poor durability issues. From the 1980s, a revival in PEMFC R&D occurred, particularly in industry and mainly in portable and vehicular applications with the Canadian company Ballard Power Systems. PEMFC technology has now been extended to wider applications, with the potential to power a portfolio of devices e.g. mobile phones, PDAs, laptops, cars, buses, boats, houses, telecommunication stations as well as space shuttles. In recent years, the PEMFC has been extensively demonstrated worldwide in many application fields and is now on the verge of commercialisation.

A single cell PEMFC consists of a membrane electrode assembly (MEA) sandwiched between two flow field (FF) plates (or bipolar plates if in a PEMFC stack) as shown in Plate XXI (between pages 392 and 393). Each cell produces ~1.1 V, so to obtain the required cell voltage, the single cells are combined to produce a PEMFC stack. The MEA is typically made of (i) a PEM material (a semi-permeable ionomer with a perfluorinated backbone like Teflon, e.g. Nafion®, designed to conduct protons (H+)); (ii) anode and cathode electrodes or catalyst layers (CLs) made of an electrocatalyst supported on carbon and PEM material (mainly ionomer) and (iii) gas diffusion layers (GDL – allowing reactants to diffuse to the active sites on the electrocatalyst and facilitating water management by allowing water vapour to diffuse out, and allow the liquid water produced on the cathode side to flow out of the fuel cell). The EC in the MEA has an important function; the electrochemical reactions involved are the electro-oxidation of hydrogen or hydrogen oxygen reaction (HOR) at the anode:

$$H_2 \to 2H^+ + 2e^- E^\circ = 0.000 \text{ V vs SHE HOR}$$ [22.8]

and the electro-reduction of oxygen or ORR at the cathode:

$$O_2 + 4H^+ + 4e^- \rightarrow 2H^+ + 2e^- E^\circ = +1.229 \text{ V vs SHE ORR} [22.9]$$

where *E*° are the standard reduction potentials (SRPs) or formal potentials vs the standard hydrogen electrode (SHE reference). Thus the overall reaction is:

$$H_2 + \frac{1}{2} O_2 \rightarrow H_2 O E^{\circ} = +1.229 \text{ V vs SHE}$$ [22.10]

The oxygen reduction reaction (ORR) at the cathode is a kinetically slow process, which has a more dominant effect on the performance of the PEMFC than the HOR. Consequently, developing active catalysts for the ORR is the focus of PEMFC electrocatalysis R&D. Currently a few approaches are underway to address the issues of the sluggish ORR activity, high cost and poor stability.

Cost, performance and scarcity

As all fuel cells are normally distinguished by the materials used, e.g. the electrolyte and catalyst materials, although the manufacturing of the fuel cell electrodes is also different in each case, the main objective in fuel cell technologies is to develop low-cost, high-performance and durable materials. As well as reducing the cost, the main target in automotive PEMFC is to operate the system above 100 °C with low humidification of reactant hydrogen and air. Large-scale deployment of PEMFCs for the transportation sector demands the development of low-cost high-performing MEAs.

Currently, fuel cell systems are too expensive and not durable [19]. Platinum (Pt) is the key catalytically-active component in three of the six main fuel cell types (PEMFC, DMFC and PAFC) currently being used commercially in portable, stationary and transport applications, and pt offers benefits in terms of activity and selectivity over novel alternative materials. However, several avenues have been explored with the aim to reduce the cost and increase the performance of fuel cells. These can broadly be listed as [14, 16–20]:

∑ reducing the electrocatalyst loading in fuel cell electrodes
∑ developing novel nanostructured thin-film Pt (e.g. 3M's NSTF electrode)
∑ decreasing the electrocatalyst nanoparticle size
∑ reducing Pt dependence by developing metallic alloy (either as binary and ternary) and Pt-free electrocatalysts
∑ improving electrocatalyst dispersion by using novel fabrication methods
∑ developing MEA fabrication methods to enable better catalyst dispersion and utilisation
∑ using new techniques to increase mass-transport at the fuel cell electrode surface, and
∑ improving the performance of carbonaceous electrocatalyst support and exploring novel non-carbonaceous electrocatalyst support materials.

Although the degradation of Pt is now well understood, there are a few solutions under investigation to prevent this important effect. For example, many researchers have successfully modified Pt with gold or other base metals, optimised its crystallographic structure and modified as well as strengthened the support material-catalyst interaction.

The automotive industry is a major user of Pt, particularly for diesel ICEs, and it has always aimed at reducing or removing its use in catalytic converters. Although Pt loadings in PEMFCs have reduced by more than 80% in the last 10 years or so, the other major issue is the high-cost of the preciousmetal catalyst (~\$60/g on 14 October 2012). Pt contributes approximately one third of the stack cost [21], down from over half in 2008 [22]. Major automotive manufacturers such as Honda (FCX Clarity), Nissan (FCV X-trail), Hyundai, Daimler-Chrysler, etc. have spent large R&D investment in 'pure' fuel cell electric vehicles (up to 100 kW at circa \$300,000 (DoE website)) for demonstration and with a price-tag of up to \$2 million. Many sceptics believe that 'pure' FCEV is not the way forward for two main reasons:

1. high cost of manufacturing and components and
2. availability of components.

For example, if 600 million 'pure' FCEVs running at 100 kW were produced worldwide, circa 12,000 tonnes of Pt would be required. In other words, the Pt industry has the potential to meet the requirements of a scenario where FCEVs achieve 50% market penetration by 2050, while 80% market penetration could exceed the expansion capabilities of the industry, assuming a DoE R&D target of 0.2 g/kW by 2015. To achieve FCEV sale volumes of, for example, 500,000 units per year using Pt catalyst, an additional 8 tonnes will have to be produced for the new units, assuming the 2015 US DoE target is met and assuming that the 0.2 grams will work well in a 100 kW system (Fig. 22.4). Furthermore, unless the Pt can be economically recycled to the purity needed, additional Pt will have to be mined to replace the Pt in the refurbished fuel cells in the existing fleet.

However, from an 'electrochemical point of view', Pt is still the best electrocatalyst for PEMFC as it is stable, 'durable' and very active towards the HOR and the ORR. There have been many attempts at developing nonprecious electrocatalysts to replace Pt [23–25]. Although promising in the laboratories, the new nanomaterials have shown poor durability and stability in acidic and aggressive environments and cell voltage cycling, i.e. under 'real' operating conditions (although it has been shown that iron-based nanostructures on nitrogen functionalised mesoporous carbons have exhibited durability and stability comparable to Pt alone).

22.4 Evolution of total platinum loadings and power densities in a PEMFC stack.

In addition to the high material cost, the precious-metal Pt catalyst is extremely sensitive to poisoning by CO, H2S, NH3, organic sulphur-carbon and carbon-hydrogen compounds in the H2 stream, and NOx and SOx in air. Pt is also prone to dissolution and/or agglomeration resulting in performance degradation. More significantly, Pt is rare and is mined in a limited number of countries worldwide, with >80% mined in South Africa. Pt deposits are likely to become scarce in the coming years (~40 years reserves at the present rate). However, other minerals have historically seen a 'falsification' of resource scarcity, as the quantity of known reserves tends to increase in line with, or in excess of, increases in the rate of consumption due to economic factors [26].

In the last five years, pt utilisation in automotive stacks has increased five-fold to over 5 kW.g–1 of platinum group metal (PGM). General Motors, for example, have reduced Pt usage in their newest generation of fuel cell stack from 80 g down to 30 g, and believe they can achieve Pt loadings of 10 g by 2020 [12]. This compares more favourably with Pt usage in conventional ICE vehicles of around 4–5 g (1 g for petrol, 8 g for diesel) for the catalytic converter [27]. Around 70 tonnes of pt are used by the EU auto industry each year; however, loadings are decreasing due to soaring metal costs, and the risk of opportunistic theft of catalytic converters. Uptake of fuel cell electric vehicles will certainly increase demand for Pt, but not by an unfeasible amount.

22.4.3 Hydrogen and fuel cell challenges

Hydrogen and fuel cell technologies have been identified as priorities for direct investment by many governments such as those of the UK and other EU countries. These technologies will contribute to tackling the UK/EU climate change targets (80% reduction in CO2/GHGs by 2050) and energy security, whilst providing significant market opportunities to a strong UK/ EU capability base. Many governments have set targets for fuel cell and hydrogen technologies, e.g. increasing durability and performance levels up to 8,000 hours for transport applications and 40,000 hours for stationary applications with target costs of longer term sustainable hydrogen below €5/kg (\$6.80/kg). The expected impact will be through a breakthrough in materials bringing costs towards commercial cost and performance targets of €45/kW (\$62/kW) (mobile applications) and €1,500/kW (\$2,000/kW) (small scale stationary applications) with power density above 1W.cm–2 (2015 US DoE target (see Fig. 22.4).

There are a few challenges related to hydrogen generation, storage and utilisation and governments, together with academic and industry researchers, are currently looking to tackle them. These include:

∑ the design and development of low-cost and efficient hydrogen production systems using novel technologies, with a particular emphasis on the production of 'green' hydrogen from renewable sources (biomass, biowaste, electrolysers powered by photovoltaic and wind energy systems, etc.)
∑ novel technologies associated with low carbon emission hydrogen production and utilisation from fossil fuels (including hydrogen separation technologies) and distributed hydrogen technologies (in terms of transport, this would include innovative on-site vehicle refuelling systems) and
∑ the development of novel systems, materials and solutions for hydrogen storage and transportation, with low costs and high energy efficiency (with a focus on systems offering storage solutions suitable for integration with HFC vehicles).

22.5 Electrochemical capacitors (ECs)

Electrochemical capacitors (EC), also called supercapacitors or ultracapacitors are high-energy-density devices. There are two energy storage mechanisms for ECs:

1. electrochemical double layer capacitors (EDLC), i.e. double-layer capacitance arising from the charge separation at the electrode/electrolyte interfaces – they consist of activated carbon with high specific area as electrodes and an organic electrolyte able to reach a specific capacitance in excess of 7,000 F.kg–1 (or 9,000 F.l–1) [28] and
2. pseudo-capacitors, i.e. pseudo-capacitance arising from fast, reversible faradaic reactions occurring at or near solid electrode surfaces – they contain transition metal oxides, nitrides and polymers possessing relatively high surface areas (1,500–2,400 m² .g–1), such as hydrous RuO2, Fe3O4, porous NiOx, CoOx, MnO2 and SnO2 as electrode materials [29].

The use of RuO2 for pseudo-capacitors has been extensively investigated as it is highly conductive, possessing three oxidation states <1.2 V with fast electron transfer. The process involves fast kinetics and H+ adsorption on RuO2 where the Ru varies from Ru2+ to Ru4+ according to equation 22.11 [29]:

$$RuO2 + xH^+ + xe^- RuO{2-x}(OH)_x$$ where $x = [0-2]$ [22.11]

The change of x during the entry or exit of H⁺ occurs at a potential of circa 1.2 V which leads to a capacitive behaviour with ion absorption of the Frumkin isotherm type with specific capacities >600 F.g–1. Due to the high cost of Ru-based ECs and their limitations, i.e. a maximum cell voltage of 1.0 V (ideal for small electronic devices), ECs based on MnO2 have been studied as they offer higher specific capacities (~150 F.g–1) and are cheaper [29].

In all cases, ECs rely upon the separation of chemically charged species at an electrified interface between a solid electrode and an electrolyte. The electrolyte between the anode and cathode is ionic (usually a salt in an appropriate solvent). The operating cell voltage is controlled by the breakdown voltages of the solvents (i.e. the decomposition cell voltages) with aqueous (1.1 V) and organic electrolytes (2.5 to 3 V).

ECs are currently being proposed by many automotive manufacturers (VW, Honda, Toyota, Nissan, Hyundai, etc.) for HEVs, BEVs and FCEVs to (i) load level energy demand, (ii) quickly inject or absorb power to help minimise voltage fluctuations in the electronic systems, and (iii) provide pulse power well over 1,000 W.kg–1 with a cycle life reaching more than 500,000 cycles. When ECs are fully charged or discharged in seconds their energy density may reach up to ~5 Wh.kg–1, and higher power delivery or uptake, up to 10 kW.kg–1, can be achieved in a few seconds. In contrast to batteries and fuel cells, the lifetime of ECs is virtually 'indefinite' and in some cases their energy efficiency rarely falls below 90% provided they are kept within their design limits. Only ECs can provide a combination of high power density and relatively high energy density. However, unlike batteries and fuel cells, almost all of this energy is available in a quasi-reversible process. Figure 22.5 shows the specific power (W kg−1) against specific energy (Wh kg−1), also called a Ragone plot, for various electrical energy storage devices.

Finally, it is speculated that the next generations of ECs are expected to come close to Li-ion battery technologies in energy density while maintaining their high power density. This will only be possible if (i) room temperature ionic liquids (RTIL) with a voltage window of more than 4 V are used and (ii) novel nanomaterials combining both double-layer and pseudo-capacitance are discovered [29].

22.6 Current status of low-carbon vehicle technologies

Many automotive manufacturers have now shifted their research effort to focus on high energy efficiency and renewable energy vehicles due to the problems caused by conventional vehicles. There are numerous potential solutions to eliminating these problems, such as hybrid vehicles, bio fuel vehicles, battery electric vehicles and fuel cell electric vehicles. Currently, there is no clear answer as to which one could dominate the future low carbon vehicle market. Many automobile companies have an equal interest in all four power-trains and scholars broadly agree that all currently viable technologies are likely to play a part in a future sustainable transport system [30].

22.5 Specific power (W kg–1) against specific energy (Wh kg–1), also called a Ragone plot, for various electrical energy storage devices. If a supercapacitor is used in an electric vehicle, the specific power shows how fast it can go, and the specific energy shows how far it can go on a single charge. Times shown are the time constants of the devices obtained by dividing the energy density by the power [29]. Source: P. Simon and Y. Gogotsi, Nature Materials, 7 (2008) 845–854, copyright © Nature Publishing Group.

22.6.1 Conventional internal combustion engine (ICE) vehicles

The ICE powered vehicle is one of the greatest achievements in human history; the design has been perfected over 150 years and become the most popular power plant for vehicles. This is because the ICE vehicle had one dominant feature; it used petrol/diesel as a fuel, and so had a range that was far superior to other vehicles, for instance, battery vehicles. This can be explained by the energy density of petrol/diesel fuel compared to batteries – see comparison chart Ragone plot (Plate XX). The ICE dominated and will continue to dominate automobile technology for many years, even in today's most advanced hybrid vehicles the internal combustion engine is still the first choice as the main power supply. However the hybrid vehicle's ICE is different from the conventional vehicle's ICE: the engine in the hybrid vehicle is normally smaller and runs at its high efficiency point for longer periods of time, hence achieving better fuel economy [31].

Continuing to improve the efficiency of ICE is still the primary task for automotive engineers; some popular methods of improvement such as engine downsizing have doubled engine efficiency over the last 20 years. However, most engines are still not equipped with the most efficient technologies such as turbo chargers or variable valve timing, mainly because of their increased cost.

22.6.2 Advantages of HEVs

Broadly speaking, a hybrid vehicle power-train can combine any two power sources. Possible combinations include, but are not limited to, diesel/petrol ICE with a battery, capacitor or flywheel, or fuel cell with a battery or capacitor. Typically one component is for storage and the other is for the conversion of a fuel into useable energy. Conventional vehicles with ICE provide good performance and long operating range by combusting liquid fuels, with the advantage of high energy density. However, conventional ICE vehicles have the disadvantages of poor fuel economy and environmental pollution. The main reasons for their poor fuel economy are:

∑ mismatch of engine fuel efficiency characteristics with real-world driving conditions
∑ waste of vehicle kinetic energy while braking, especially when driving in urban conditions
∑ energy wastage during engine idling and standby
∑ low efficiency of hydraulic transmission (automatic) in current vehicles under stop-and-go driving conditions.

BEVs on the other hand possess some advantages over conventional ICE vehicles, such as high energy efficiency and zero tailpipe emissions. However, the limited range and long time charging makes them far less competitive than ICE vehicles, due to the much lower energy density of the batteries compared to liquid fuels. HEVs, which combine the best features of the two power sources, gain the advantages of both ICE vehicles and BEVs, and overcome their individual disadvantages. HEVs are more expensive than conventional vehicles because of the extra components and complexity required, but less expensive than BEVs due to the high cost of batteries.

22.7 Battery electric vehicles (BEVs)

22.7.1 History

The first BEV was built by Thomas Davenport in 1834; even a few decades earlier than the first ICE vehicle. The first vehicle to surpass the 100 km/h barrier was also a battery vehicle, namely the 'Jamais Contente' which was driven by Camille Jenatzy in 1899 [32]. In comparison with ICE vehicles, BEVs were comfortable, quiet and clean. However, due to the limited energy storage capacity of the battery, the range was very limited, and at the same time, the ICE was improving dramatically. As a consequence, the BEV almost vanished by the 1930s [33]. But, due to the energy crisis and oil shortage in the 1970s, automakers and policy makers started to re-think the BEV, as it offered high energy efficiency and allowed the diversification of energy resources, as well as having zero local emissions and helping to improve urban air quality.

22.7.2 Current status

Battery vehicles use an electric motor for traction instead of an ICE, and use batteries for their energy source instead of liquid fuels. BEVs have many advantages over conventional ICE vehicles, such as no tailpipe emissions, high efficiency and potential for independence from fossil fuels, and quiet and smooth operation. The characteristics of the BEV and HEV battery packs are very different. The BEV battery pack has high specific energy while the HEV battery pack has high specific power. Since the motor in a powerassist (grid-independent) HEV is used intermittently and must be capable of producing high power for short periods of time (e.g. during maximum acceleration), its battery pack should be optimised for high power.

The battery vehicle drive train consists of three major sub-systems:

1. Electric motor propulsion system vehicle controller, power electronic converter, the electric motor, and transmission.
2. Battery system batteries, battery management system (BMS), and charging unit
3. Auxiliary system heating/cooling, electronic pumps, and other electronic auxiliaries.

The principle of the BEV is very straight forward, based on the control inputs from the accelerator and brake pedals, the vehicle controller provides proper control signals to the electronic power convertor, which functions to regulate the power flow between the electric motor and battery. The motor can also play the role of a generator, converting the braking energy to electrons and charging the battery. The energy management unit cooperates with the vehicle controller to control the regenerative braking and its energy recovery. The electric motors produce a great amount of torque from rest to give amazing performance. In terms of acceleration and power, BEVs are superior to ICE vehicles.

22.7.3 Future developments

While significant progress has been made in developing automotive batteries, major challenges remain, as follows:

∑ Reducing cost currently a Li-ion battery with 35 kWh storage capacity costs around \$30,000 to manufacture, while a few organisations (ANL, IEA, EPRI, and CARB) project future prices around one third of this. Reducing the cost of battery packs is therefore the key challenge for BEV development [34].
∑ Improving safety current nickel and cobalt-based oxide Li-ion cathode materials have potential issues with overcharging, clearly; and voltage control at cell, module and battery level is critical to prevent overcharging of automotive Li-ion batteries, but these are all factors that will inevitably increase Li-ion battery cost further. Lithium iron phosphate cathodes offer a promising future but with lower specific energy and power density.
∑ Prolonging the life-span as an automotive battery, it should last at least 10 years or 150,000 miles under a variety of conditions, whereas for example the current average life of vehicles registered in the UK is 14 years.
∑ Shortening the charging time to a matter of minutes, and providing better charging facilities.
∑ Reducing the size and weight of the battery pack.

Compared with the problems posed by batteries, the challenges for the motor/ controller for electric vehicles may seem relatively small, with remaining developments aimed at improving the efficiency and reliability.

22.8 Fuel cell electric vehicles (FCEVs)

22.8.1 Introduction

FCEVs share many of the same components as BEVs, such as electric motors and power controllers or inverters; however, the major difference is the main energy source. While BEVs use energy stored in the battery, FCEVs use fuel cells as they are superior to batteries in many ways. The major advantages are that fuel cells are lighter and smaller and can produce electricity as long as the fuel is supplied. Due to the clear similarities between batteries and fuel cells, both these technologies will coexist in the future, while the BEV is suitable for short range and small vehicles, the FCEV is suitable for medium-large and long range vehicles.

The PEMFC is the best choice for automobile use. The principle of how FCEVs work is simple; they use low temperature fuel cells to generate electricity from hydrogen; the electricity is then either used to drive the vehicle or stored in an energy storage device, such as batteries or ultra capacitors. Since fuel cells generate electricity from chemical reactions, they do not combust fuel and therefore do not produce pollutants and produce much less heat than an ICE. The by-product of a hydrogen fuel cell is water. Fuel cells have no moving parts or irregular shapes, so they have the potential for high reliability and low manufacturing cost. Although FCEVs possess many advantages, they also have certain limitations; these relate to the fuel cell stack itself and its fuel, hydrogen production, transportation and storage.

22.8.2 Proton exchange membrane (PEM) fuel cell stack

The PEM fuel cell stack is like the 'engine' in a conventional vehicle, which is the most important part. PEM fuel cell technologies have been developing at a good pace recently, however, two key limitations still remain: cost and durability. The cost of an ICE power plant is about \$25–35 per kW, but current fuel cell systems are estimated to be about 5 times more expensive, even taking into account cost savings for high-volume manufacturing [35]. This is because materials and manufacturing costs for catalysts, bipolar plates, membranes, and gas diffusion layers are currently too high. On the other hand, the fuel cell stack as an 'automotive engine' is expected to be as durable and reliable as current automotive engines, i.e. 5,000 hours lifespan or 150,000 miles equivalent under a range of operating conditions, including different temperatures and climates.

22.8.3 Hydrogen as fuel for fuel cell hybrids

Fuel cells prefer to operate in a consistent condition and achieve their maximum efficiency at partial load. However, a vehicle requires a variety of power outputs according to the road and traffic conditions. Hybridisation of a fuel cell with peak power sources (PPS) could solve these problems. For instance, by using batteries or ultra capacitors when power demand is high, such as with higher loads or acceleration, PPS could help the fuel cell to provide the boost while power demand is low, such as during deceleration or when travelling downhill. The extra energy can be replenished to PPS; this allows the fuel cell system to be operated more efficiently. On the other hand, a smaller fuel cell can be used, for instance, to maintain a vehicle at 70 mph continuously requires 15 kW at the wheels, so a 20 kW fuel cell stack should able to cope with this.

So a hybrid fuel cell with a PPS could offer a viable solution for electric vehicles. Such a configuration will offer the following advantages compared to full fuel cell vehicles:

∑ smaller fuel cell means lower cost
∑ fuel cell could operate at optimum efficient point most of time under appropriate control strategy
∑ fuel cell life will be extended
∑ the fuel cell designer will be able to optimise the cells for power instead of cycle life
∑ deep discharging from the battery will be eliminated; therefore improving the batteries' life
∑ allows fast start-up of the fuel cell
∑ allows capture of regeneration energy.

The disadvantages of hybridisation are the complexity of the vehicle system, weight increase, complexity of the control system, and extra battery cost.

22.8.4 Example of hybrid battery FCEV

In 2006 the Birmingham Centre for Hydrogen and Fuel Cell Research (UK) received support from Advantage West Midlands to buy a prototype plus four improved vehicles which could then be trialled using the university campus as a test track (Fig. 22.6(a)). The Air Products refueller was opened at the School of Chemical Engineering in April 2008 and the five cars were then used to test design calculations over a two-year test programme undertaken by a consortium of West Midlands' suppliers part-funded by the UK government department Business Enterprise and Regulatory Reform (BERR). The results [36–38] showed that the initial campus design gave the desired acceleration, with a top speed of 20 mph and range of 60 miles on a campus drive cycle, but could not achieve the ECE15 urban cycle. This was not surprising because the fuel cell was only the 1.2 kW Ballard Nexa PEMFC stack (less than 1 g of Pt is used compared with 60g for a 100 kW system), with a hydrogen tank of 0.6 kg at 350 bar and a 2 kWh lead acid battery. The main problem was the battery capacity which was insufficient to give regenerative braking and plug-in range. Other issues such as the need for a lightweight aluminium chassis, the space requirements for fleet vehicles, the performance of lightweight body panels and better heat management for window and cab temperature control were addressed in 2010 when the next batch of eight cars was assembled.

So far, Microcab Industries, in collaboration with the University of Birmingham have made very good progress on the new hydrogen hybrid fuel cell vehicle (Fig. 22.6(b)) [39], for example:

∑ the range of the vehicle has been extended to 200 miles (a current stateof-the-art lithium-ion battery vehicle can only do 60 miles)

22.6 (a) Microcab hydrogen hybrid fuel cell vehicle 2nd generation and (b) new Microcab generation.

22.6 (Continued)

∑ a short refuelling time of three minutes (battery vehicles have a domestic charging time of 5–8 hours).
∑ the lifetime of the fuel cell stack has been increased to 5,000 hours with a high temperature PEMFC.

Economically, if 500,000 hydrogen hybrid vehicles were produced per annum, less than 0.5 tonnes of Pt would be required assuming a Pt loading of 0.6 g/kW. Of course, the key issues are cost, lifetime and reliability in both start-up and duty cycles, which still require some attention. Based on a PEMFC stack production cost of \$225/kW (mainly attributed to Pt and membrane cost) with a DoE target of \$30/kW by 2015, fuel cell-electric hybrids become a very attractive option for commercial deployment.

Of course, another option is to convert a conventional petrol combustion hybrid car to run on hydrogen in order to test the filling station infrastructure and the performance/costs of the new systems. In 2006, Statoil did this in Norway by opening the first of its HyNor fuelling stations in Stavanger with 15 modified Toyota Prius hybrids running on hydrogen gas. By 2009 there were seven hydrogen stations connecting the 600 km from Oslo to Stavanger and the number of hydrogen vehicles had increased to 50 with manufacturers Mazda, Think and Toyota [40]. The advantages of demonstrating this technology in Norway were substantial:

∑ car taxes are high and can be reduced to incentivise consumers
∑ hydrogen cars can drive in bus lanes and are exempt from road tolls
∑ hydrogen is readily available from the Statoil plants
∑ hydrogen can be produced from renewable sources such as hydropower to give green energy.

22.8.5 Future developments

The following issues need to be addressed:

∑ Reducing the cost of hydrogen the cost of hydrogen, which includes the cost of production and delivery, has to be competitive with conventional fuels
∑ improving hydrogen storage technology the low volumetric energy density of hydrogen makes storage a challenge; the storage needs to meet vehicle packaging, cost, and performance requirements
∑ reducing fuel cell cost and improving durability the cost of fuel cell power systems has to be reduced and durability must be improved for fuel cells to compete with conventional technologies.

22.8.6 Comparisons with BEVs

Table 22.5 compares the different types of vehicles in terms of cost, performance, and CO2 emissions; here the models chosen are representative vehicles within their class, and represent the most advanced technologies to date.

22.9 Technical prospects and barriers

Many of the technical advantages and limitations of batteries, fuel cells and capacitors are rooted in the fundamental electrochemical mechanisms they employ. It is unlikely that the performance of these three technologies will ever converge, and so their suitability for particular applications will always remain different [41].

22.9.1 Cost

Upfront cost is a key factor for public acceptance and uptake, and is proving to be one of the steepest challenges for electrochemical energy systems. These alternative technologies must overthrow a century of refinement and extreme economies of scale that ICEs benefit from. The advantage these nascent technologies possess is that, while still in their early years, technological progress has been impressive and enabled costs to fall rapidly. Toyota reports

Table 22.5 Comparison of vehicle specifications from a consumer's perspective

	ICE (VW GOLF 1.4TSI)	Hybrid (Toyota Prius III)	BEV (Nissan Leaf)	FCEV (Honda FCX Clarity)
Power supply	IC engine	ICE, electric motor	Battery and electric motor	PEM fuel cells and electric motor
Fuel	Petrol, diesel and alternative fuel	Petrol/diesel as main fuel	'Electricity'	Hydrogen
Top speed, (mph)	124	112	94	100
Acceleration, (s)	9.5	10.4	7	10
Range (miles)	552	716	73–109	240
Purchase price	\$29,400	\$33,400	\$41,250 (including \$8,000 government incentive)	\$80,000 (estimated)
Running fuel price (per mile)	\$0.22	\$0.14	from \$0.02	From \$0.07
Fuel economy (mpg or mpg equivalent)	45.6	72.4	99	81
Tailpipe CO2, emission (g/km)	144	89	0	0

to have cut the cost of making its fuel cell vehicles by 90% since 2005, from \$1 million per vehicle down to around \$100,000. They expect to halve their current costs before retail sales begin in 2015 [42].

Major organisations share a common view on the pace of fuel cell cost reductions – centred around \$75,000 at the start of commercialisation in 2015, under \$50,000 after 5 years, and a floor level of around \$30,000 by 2025 [12]. Battery prices are expected to fall less rapidly, by 50% within 10 years compared to 75% for fuel cells. By 2020, a 25 kWh battery pack is expected to cost \$6–10 thousand, giving a similar premium over conventional vehicles as FCEVs [12]. These costs are still a long way off \$1,000 for the ICE they replace; however, they represent only a modest increase in the cost of a complete vehicle, and are widely thought to be low enough to encourage the development of a substantial vehicle market.

22.9.2 Durability and degradation

The prospects for capacitors are excellent in this respect, with calendar lifetimes measured in decades and charge/discharge cycles in the millions. The calendar life of lithium-ion batteries is still a problem, as the rate of capacity loss has not improved in seven years, at approximately 5% per year [43, 44].

Lifetimes for lithium-ion chemistries are in the order of 2,000 cycles to 80% depth of discharge (DoD) before 20% of power is lost. The number of cycles is approximately reciprocal with the DOD, meaning that ~4,000 cycles to 40% DOD can be expected. With the 75 mile range of current BEVs, this suggests that the battery should last for at least 100,000 miles, comparable to an internal combustion engine. This level of durability has not yet been verified for lithium-ion chemistries, but has been obtained in real-world usage of NiMH battery packs produced some 14 years ago [45].

Fuel cells lifetimes are assessed by the number of hours until 10% power is lost. The latest generation of FCEVs from Ford, Daimler and GM are projected to last 800–1,100 hours, falling short of the DoE's 2009 target of 2,000 hours [36]. Based on these vehicles' driving patterns around California, this corresponds to 21,000–28,000 miles, and so a five-fold improvement in durability is required to compete with battery technologies. The reliability of current FCEVs must also be improved, as some vehicles are seen to lose 10–25% performance within the first 300 hours of driving (~8,000 miles) [36].

22.9.3 Energy and power density

Energy density is often cited as the largest problem for electrochemical storage devices. The specific energy and energy density of batteries and capacitors are unlikely to ever compete with liquid hydrocarbons holding around 12 kWh.kg–1 [15]. This is, however, an unfair and misleading comparison. While the fuel in a conventional vehicle weighs relatively little by itself, plenty of additional equipment is required to convert petrol into motion: an engine, radiator, transmission system, fuel delivery lines, an exhaust and catalytic converter. In contrast, the batteries of an electric vehicle comprise the majority of the power-train mass, as the motor, wiring and transmission are relatively small, light systems. FCEVs lie in between the two extremes, as the fuel cell stack and its ancillaries are a similar weight and volume to the high pressure storage tank.

A second issue is that of conversion efficiency. The 'value' of hydrocarbons, hydrogen and electrons lies in how much of their energy can be converted into a useful form of energy, namely motion at the wheels, as this determines the range of the vehicle and cost per mile travelled. Modern combustion engines (without hybridisation) are approximately half as efficient as a practical fuel cell engine, and a quarter as efficient as battery charge-discharge cycles. Table 22.6 demonstrates the impact that system weight and conversion efficiency have on the specific energy of petrol, hydrogen and lithium-ion batteries, using data from four modern vehicles. In all cases, the effective specific energy is substantially reduced from the values that are typically reported.

In order to compete with conventional vehicles in terms of 'usable energy density' and thus offer a similar driving range, batteries require a five-fold increase in specific energy, while hydrogen only requires a 30% improvement. A better battery that could hold 666 Wh.kg–1 at the cell level, or a hydrogen tank that held 6 wt.% would therefore give comparable performance from the driver's perspective. Hydrogen has strong potential to meet this target, as 'Type 4' all-composite compressed hydrogen tanks are currently approaching 5.2 wt.% at 700 bar [37]. Cryo-compressed hydrogen can presently attain 6–8 wt.%, however it suffers from a significant energy cost of liquefaction. Several

Table 22.6 Comparison of petrol, hydrogen and electrical storage systems in four leading vehicles

	Conventional	Hybrid	Hydrogen	Battery
Reference vehicle	Volkswagen Golf VI	Toyota Prius III	Honda FCX Clarity	Nissan Leaf
Fuel weight (kg)	40.8	33.3	4.1	171a
Storage capacity (kWh)	500	409	137	24
Specific energy (Wh primary/kg fuel)	12,264	12,264	33,320	140a
Storage system weight (kg)	48	40	93	300b
Specific energy (Wh primary/kg of storage)	10,408	10,261	1,469	80
Net power (kW)	90	100	100	80
Power plant and auxiliary weight (kg)	233	253	222	100
Specific energy (Wh primary/kg total equipment)	1,782	1,398	315	60
Average conversion efficiency	21%	35%	60%	92%
Effective storage capacity (kWh useable)	105.0	143.1	82.0	22.1
Specific energy (Wh useable/kg total equipment)	374	489	260	55

Notes: a: bare laminated lithium-ion cells; b: Including battery management and cooling systems.

forms of metal hydride storage are also on the horizon, with ammonia-borane (AB) complexes and metal-organic frameworks (MOF-5) also expected to exceed 6 wt.% [37]. Toyota are targeting 8 wt.% storage by 2015 using a hybridisation of compressed gas and solid hydride storage tanks [38].

It is unlikely that Li-ion batteries will attain a specific energy of 666 Wh.kg–1, as the optimisation of current chemistries has reached practical limits [8]. Several alternative chemistries are under development which could reach this goal in time; with lithium metal, lithium-sulphur, lithium-air, and non-lithium intercalation chemistries all offering the potential of systemslevel specific energies of at least 500 Wh.kg–1 [39].

There are no near- or long-term developments in capacitor technology that are expected to deliver such an improvement in specific energy. Novel graphene electrodes have demonstrated specific energies up to 86 Wh kg–1 of bare electrode, implying around 20 Wh.kg–1 for a complete system [40]. These developments are likely to spill over into battery electrodes, with graphene anodes offering the potential to double energy density in lithiumion batteries [46].

22.9.4 Efficiency

A high charge/discharge or conversion efficiency is also important as it impacts on the overall energy efficiency, running costs and life-cycle CO² emissions of the vehicle. Maximising efficiency must be done at the expense of other goals, notably power density and charge/discharge rates, which in turn impact on system size, weight and cost. Capacitors currently hold the advantage, with cycle efficiencies in the range of 90–98%, leaving little room for improvement. Battery efficiency has also improved substantially with the transition from lead acid (75–85%) and NiMH (65–85%) to lithium chemistries. Under ideal conditions, cycle efficiencies of lithium-ion batteries now rival those of capacitors at 90–94% in automotive use [47–49].

Battery system efficiency is closely related to operating conditions, decreasing with both current and temperature. The desire of consumers to move to rapid charging systems is therefore cause for concern. When charged with a 3 kW household charger the charging efficiency of the Mitsubishi i-MiEV is around 90%; however, the 50 kW quick-charge option can be as little as 60% efficient, due to the additional energy required to cool the battery [47]. Inductive charging is another promising option, due to its speed and inherent safety; however, it reduces efficiency by a further 10% [50].

The US Department of Energy (DoE) and Japan's NEDO hydrogen energy roadmap both set a target of 60% efficiency for FCEVs circa 2015. It appears from the latest field trials in California that this target is close to being attained, with fuel cell vehicles from five major manufacturers averaging 52%, and the top manufacturer achieving 57% [36].

A vehicle's well-to-wheel (WTW) pathway traces from the original energy source through conversion, distribution and storage, to the wheels of the car. Table 22.7 summarises the typical well-to-tank (WTT) and WTW efficiencies for conventional and electrochemical vehicles. The efficiency of these electrochemical systems only plays a part in determining the amount of energy required by the vehicle, and thus its fuel economy and maximum driving range. The energy required to move a vehicle scales linearly with its mass [51, 52], and so doubling the vehicle's efficiency will have a negligible impact if its weight also doubles. This is clearly evident with the range of FCEVs produced to date. Despite 40 years of technical development, the Honda FCX Clarity has virtually the same fuel economy as the Austin A40 converted to run on an alkaline fuel cell developed by Professor Karl Kordesch in the 1970s [53–56]. Figure 22.7 compares these vehicles with 15 other models, demonstrating the relationship between mass and fuel economy.

22.10 Improving the safety of hydrogen-powered vehicles

22.10.1 Introduction

High priority research directions for the hydrogen economy include safety as a technological, a psychological and sociological issue [57]. Here we consider some technological safety issues relevant to hydrogen-fuelled vehicles. Hydrogen is not more dangerous nor safer compared to other fuels [58]. Safety of hydrogen and fuel cell systems and infrastructure fully depends on how professionally it is handled at the design stage and afterwards. Hydrogenpowered vehicles are one of the main applications of hydrogen and fuel cell technologies. Hazards and associated risks for hydrogen-fuelled cars should be understood and interpreted in a professional way with full comprehension of accident consequences by all stakeholders starting from system designers

| | Table 22.7 Typical well-to-tank, tank-to-wheel and well-to-wheel efficiencies for | | |-------------------------|-----------------------------------------------------------------------------------|--| | each vehicle technology | | |

Vehicle type	Well-to tank			Tank-to-wheel			Well-to wheel
BEV	32%– 100%	Charger 90%	Battery 92%	Inverter 96%	Motor 91%	Mechanical 92%	21.3%– 66.5%
H2 FCEV	75%– 100%	Fuel cell 51.8%		Inverter 96%	Motor 91%	Mechanical 92%	31.2%– 41.6%
Hybrid	82.2%	30.2%					24.8%
Diesel	88.6%	17.8%					15.8%
Petrol	82.2%	15.1%					12.4%

22.7 Energy consumption (MJ km–1) measured from 'real-world' testing of 23 hydrogen fuel cell vehicles plotted against vehicle mass (kg) [51]. Source: I. Staffell and K. Kendall, International Journal of Low Carbon Vehicles, 7(2012) 10–15, copyright © Oxford University Press.

through regulators to users. Today the first series of hydrogen-fuelled buses and cars are already on the road and refuelling stations are operating in different countries around the world. But the question still remains…How safe are hydrogen-powered vehicles?

22.10.2 Current safety issues

Compressed gaseous hydrogen is currently the main stream for on-board storage. Maximum storage pressure varies from 35 to 70 MPa. Tanks of Type 3 (aluminium liner) and Type 4 (polymer liner) made of carbon fibrereinforced resin are in use due to their weight. Unfortunately, fire resistance of these tanks is low. For example, reported time to catastrophic failure of Type 4 tanks is from 3.5 to 6.5 minutes [59]. The current 'solution' to prevent catastrophic failure of these tanks in fire conditions is to release hydrogen quickly through a thermal pressure relief device (TPRD) of comparatively large diameter of about 5 mm (before the catastrophic failure). However, this 'solution', which is probably acceptable in the open atmosphere, generates serious problems for life safety and property protection when a vehicle is in confined space like garage, tunnel, etc.

22.10.3 Some large-scale experimental results

A HFCV was equipped with a TPRD with a vent pipe internal diameter of 4.2 mm. A small storage vessel of 36 litres volume at pressure 70 MPa was used. The fire spread from a gasoline vehicle to a HFCV was investigated to address scenarios when different types of vehicles are catching fire in a car collision or natural disaster like an earthquake. The experiment revealed that when the TPRD of the HFCV is activated by a gasoline fire a fireball of more than 10 m diameter is formed.

In another test by Tamura et al. [60], two vehicles were parked approximately 0.85 m apart and the spread of fire from the HFCV to the gasoline vehicle was investigated. It can be concluded that self-evacuation from the car or safeguarding by first responders with such design of hydrogen release system is impossible. Tamura et al. [60] concluded that in car carrier ships and other similar situations with closely parked HFCVs, the test results point to the possibilities of a fire in an HFCV to activate its TPRD and thereby to generate hydrogen flames which in turn may cause the under floor TPRD activation in an adjoining HFCV. To minimize damage by HFCV fire, therefore, the authors suggested that it is important to detect early and extinguish the fire before TPRD activation. It is known that hydrogen fire is difficult if not impossible to extinguish in many practical situations. Hopefully, car manufacturers will develop appropriate safety engineering solutions, including reduction of flame length from hydrogen-powered vehicle in a mishap; thus excluding the 'domino' effect in accident development and assisting first responders to control such fires and successfully perform rescue operations. Experiments by Tamura et al. [60] have clearly demonstrated that hydrogenpowered vehicle fire consequences can be very 'challenging' both from life safety and property loss points of view.

The modern definition of risk is provided by ISO/IEC Guide 73:2002 [61] that states that it is the 'combination of the probability of an event and its consequence' while safety is defined as the 'freedom from unacceptable risk'. This means that safety is a societal category and cannot be numerically defined while risk is a technical measure that can be calculated [62]. Society, in consequence, establishes acceptable levels of risk or risk acceptance criteria.

The general requirement is that the risk associated with hydrogen-fuelled vehicles should be the same or below the risk for today's vehicles using fossil fuels. Currently this requirement is not yet achieved throughout the whole range of hydrogen and fuel cell systems. Indeed, the consequences of a hydrogen-powered car fire to life safety and property loss, e.g. in confined spaces such as garages and tunnels, are more 'costly' compared to the consequences of a fossil fuel vehicle fire given the current level of fire resistance of hydrogen on-board storage (from 1 to 6.5 minutes only) and the design of pressure relief devices. In fact, the probability of external to vehicle fire, e.g. at home garages and general vehicle parking garages will be the same independent of the vehicle type.

The garage fire statistics from National Fire Protection Association (NFPA) are as follows. During the four-year period of 2003–2006 an estimated average of 8,120 fires per year started in the vehicle storage areas, garages, or carports of one or two-family homes [63]. Vehicles were involved in the ignition of only about 13% of structure fires. This statistic makes it clear that safety strategies and solutions, including those developed by car manufacturers, have yet to be improved to rely on a firm engineering design rather than a general risk assessment of which uncertainties are impossible to define for emerging technologies.

22.10.4 Car in a garage scenario

The Commission Regulation (EU) No. 406/2010 [64] requires on-board hydrogen storage to be equipped with pressure relief devices (PRDs). The PRD (TPRD) is fitted to the fuel tank and starts to release hydrogen when high temperature of about 110 °C is reached, e.g. in fire conditions. The PRD can provide rapid release of the hydrogen, if a large orifice diameter is used, thus minimising the possibility of tank catastrophic failure during long exposure to fire. However, the hazards resulting from a rapid release indoors were studied only recently.

Let us consider a release from a typical on-board hydrogen storage tank at pressure of 35 MPa, through a 5.08 mm diameter 'typical' PRD [65]. The release is assumed to occur in a garage of size LxWxH = 4.5 ¥ 2.6 ¥ 2.6 m (SAE J2579, [66]) and volume of 30.4 m³ with a single vent equivalent in area to a typical brick LxH = 25 ¥ 5 cm located flash with the ceiling. A conservative approach is taken, i.e. a constant mass flow rate of 0.39 kg.s–1 is applied (ignoring a pressure drop in the storage tank) after the PRD opening. The transient pressure load in the vented enclosure according to the selected scenario is given in Fig. 22.8.

Figure 22.8 illustrates how the overpressure within the enclosure resulting from the injection of hydrogen reaches a level above 10 kPa capable of rupturing the garage [67] within only 1 s. Evacuation of people in such circumstances is impossible and this life safety issue has yet to be addressed by car manufacturers by reconsidering requirements to fire resistance of onboard storage and PRD use. There seems only one engineering solution to this problem, i.e. to reduce the mass flow rate from on-board storage during fire. This is turn will require a higher fire resistance level of on-board storage tanks compared to current 3.5–6.5 minutes for Type 4 vessels [59].

If the garage does not rupture first, then the pressure within the garage for this typical scenario reaches a maximum level in excess of 50 kPa. This

22.8 Pressure peaking phenomenon for release of hydrogen with mass flow rate 0.39 kg/s into the enclosure of 30.4 m³ with a vent of typical brick size 25 ¥ 5 cm compared with pressure dynamics for releases of methane and propane at the same conditions.

maximum pressure then drops off and tends towards a steady state value; considerably lower, and equal to that predicted by the simple steady state orifice equations for pure hydrogen release from the garage. The maximum pressure is reached in less than 10 s. Within this time the entire garage would be destroyed with missiles flying around and creating more damage. These are consequences of pressure build up without even considering the combustion of the released hydrogen. Pressure peaking phenomenon of hydrogen release in a vented enclosure is a new aspect of life safety provision that hydrogen safety engineers and manufacturers of hydrogen and fuel cell systems have to address [65].

Figure 22.8 illustrates the predicted overpressure in the garage versus the time for three fuels with the same mass flow rate of 0.39 kg.s–1: hydrogen, methane, and propane with the molecular mass of 0.002, 0.016 and 0.044 kg.kmol–1, respectively. It is clearly seen that the maximum overpressure drops with the increase of molecular mass. There is a very small pressure peak for methane release as its molecular mass is below that of air. The pressure peaking phenomenon is absent for propane altogether as its molecular mass is higher than air. This should be taken into account when designing PRDs for use with different gases for indoor systems. The same PRD used for compressed natural gas (CNG) or liquefied petroleum gas (LPG) should not be assumed to behave in the same way for hydrogen.

The pressure peaking phenomenon, i.e. existence of the maximum in pressure transient that is above the steady state overpressure value, during a release of lighter than air gas into vented enclosure, should be accounted for when performing safety engineering for indoor use of hydrogen or helium systems. Overall the conclusion is drawn that PRDs currently available for hydrogen-powered vehicles should be redesigned, along with essential increase of fire resistance rating of on-board hydrogen storage tanks, and regulations, codes and standards (RCS) updated if the vehicle is intended for garage parking.

22.10.5 Hydrogen jet flame length

Current engineering solutions for hydrogen-powered vehicles imply very long flames up to 15 metres. Hydrogen safety engineers have to be equipped with tools for calculation of jet flame length as a function of storage pressure and PRD diameter. Figure 22.9 shows the nomogram for graphical calculation of hydrogen jet flame length [68]. The nomogram is derived from the best fit line of the dimensional flame length correlation [68]. The use of the nomogram for calculation of flame length is demonstrated in Fig. 22.9 by thick lines with arrows. Firstly, an actual nozzle diameter of a leak, e.g. 3 mm in this example, and a storage pressure, e.g. 35 MPa, are chosen. Secondly, a horizontal line is drawn from the diameter axis at point 3 mm to the right until it intersects with the pressure line denoted as 35 MPa. Thirdly, a vertical line is drawn from the intersection point upward until the second intersection with the only line available in coordinates (*L*F, (m. D) 1/2). Finally, to obtain a flame length, a horizontal line is draw from the second intersection to the left until the final intersection with the axis denoted 'Hydrogen flame length *L*F, m'. In the example considered, the leak of hydrogen at pressure 35 MPa through the orifice of 3 mm diameter will result in the flame length of 5 m. The conservative estimate of the flame length is 50% longer, i.e. 7.5 m.

The nomogram incorporates a special feature based on the results by Mogi et al. [69] and others, i.e. no stable flame was observed for nozzle diameters 0.1–0.2 mm as the flame blew off although the spouting pressures were as high as 40 MPa (denoted as 'No flame area' in Fig. 22.9). For example, a stable jet flame cannot exist at pressures equal or below 35 MPa if the leak orifice diameter is below 0.3 mm. It should be noted that the nomogram does not account for conditions when flow losses in a leakage pathway cannot be ignored. In such cases a straight forward use of the nomogram gives a conservative result.

22.10.6 Separation distances

Hydrogen releases from high pressure on-board storage or equipment will create highly under-expanded jets. This could lead to formation of a large

22.9 The nomogram for hydrogen jet flame length [68].

flammable hydrogen-air envelope (unignited release, e.g. due to failure of the PRD) or long jet flame. The size of the flammable envelope is the deterministic separation distance from the release source. Indeed, if the flammable envelope (hydrogen concentration in air equal to the lower flammability limit of 4% by volume) reaches a location of air intake into high-rise buildings, then consequences for occupants and building structure can be catastrophic. Here we do not consider separation due to pressure separation distance is the minimum separation between a hazard source and an object (human, equipment or environment) which will mitigate the effect of a likely foreseeable incident and prevent a minor incident escalating into a larger incident.

Separation from unignited release

The nomogram for graphical calculation of hydrogen concentration decay by the similarity law and the under-expanded jet theory without losses is shown in Fig. 22.10. The nomogram includes four core graphs ('Volumetric to mass fraction', 'The similarity law', 'Choose leak diameter' and 'Choose density in the nozzle exit') and one additional graph 'Calculate density in the nozzle exit by storage tank pressure and temperature'. Let us demonstrate how to use the nomogram to calculate the distance from the nozzle of 1 mm diameter to concentrations of hydrogen in air 4% (dashed line, corresponds to the lower flammability limit of hydrogen in air) and 11% (solid line, corresponds to

22.10 The nomogram for concentration decay calculation in unignited jets.

the average location of jet flame tip; scattering of flame tip location is from 8% to 16% by volume) by volume along the axis of a release from a storage tank at pressure 70 MPa and temperature 300 K.

Firstly, the line from the axis 'Hydrogen volumetric fraction' is drawn downward from the chosen concentration of interest (4% and 11% by volume in our example) until the intersection with the line of the graph 'Volumetric to mass fraction'. Secondly, the horizontal line from this intersection point is drawn to the intersection with the similarity law line at the right top graph 'The similarity law'. Thirdly, the vertical line is drawn downward until intersection with line '1 mm' at the graph 'Choose leak diameter'. There are eight lines on the 'Choose leak diameter' graph which correspond to the following leak diameters (from top to bottom): 15 mm, 10 mm, 5 mm, 3 mm, 2 mm, 1 mm, 0.5 mm, 0.1 mm). These labels are shown on the right hand side of the graph.

The fourth stage is to calculate the density using the additional graph 'Calculate density in the nozzle exit by storage tank pressure and temperature' at the bottom of the nomogram using given pressure (70 MPa) on the ordinate axis and a line corresponding the chosen temperature (300 K). This is shown by two thick grey lines on the 'Calculate density in the nozzle exit by storage tank pressure and temperature' graph. Calculated graphically, density at the nozzle exit for 70 MPa and 300 K is about 23 kg.m–3.

The calculated value of density of 23 kg.m–3 is applied at the fifth stage of calculations to draw the horizontal line from the intersection with the '1 mm' line at the 'Choose leak diameter' graph to the left until the intersection with an imaginary line corresponding to 23 kg.m–3 at the 'Choose density in the nozzle exit' graph (between the two shown on the graph lines 20 kg.m–3 and 50 kg.m–3). There are five lines at the graph corresponding to densities 1 kg.m–3, 3 kg.m–3, 10 kg.m–3, 20 kg.m–3, and 50 kg.m–3 from top to bottom respectively. These labels are shown at the left-hand side of the graph.

The final stage is to draw a vertical line downward from the intersection with the line 23 kg.m–3 to the intersection with abscissa axis of the 'Choose density in the nozzle exit' graph. Thus, the calculated graphically distance from the nozzle exit to hydrogen concentration of 4% by volume is about 7.7 m, and to concentration of 11% by volume is about 2.7 m. The error of graphical calculations is estimated to be at the acceptable level of below 10%.

This nomogram is a performance-based tool for calculation of separation distance for unignited releases as the alternative to prescriptive codes. For instance, the International Fire Code (IFC) [70] presents a table of prescribed separation distances between a hydrogen system and various potential targets, including air intake to a building. However, this code does not provide or require any information either on the hydrogen storage parameters (pressure and temperature) and leak size from the system. This IFC code is an example of an archaic prescriptive approach that contradicts modern requirements for codes and standards to be performance-based.

Three separation distances from a jet fire

Figure 22.11 shows measured axial temperature of a hydrogen flame [71,72] as a function of distance from the nozzle, x, normalised by the flame length, *L*F. Three accepted criteria for three different separation distances are presented by horizontal lines: 70 °C – 'no harm' separation distance;

22.11 Measured axial temperature as a function of distance expressed in flame calibres [71, 72, 74] and three criteria for jet fire effects (lines).

115 °C – pain limit for 5 min exposure; 309 °C – third degree burns for 20 s ('death' limit). Comparison between the axial temperature profile and named criteria provides three separation distances: x = 3.5*L*F for 'no harm' separation (70 °C), x = 3*L*F for pain limit (115 °C, 5 min), x = 2*L*F for third degree burns (309 °C, 20 s).

Recently an 'unexpected' conclusion has been drawn [66, 73, 74] from the performed analysis that in the conservative case all three separation distances for jet fire are longer or equal to the separation distance from unignited release from the same source based on the lower flammability limit (LFL). In particular, the separation distance from a hydrogen leak source to a location with axial concentration equal to the LFL, e.g. to prevent ingress of flammable mixture into a ventilation system of building, is practically equal to the 'death' separation distance for jet fire. Two other separation distances for jet fires ('no harm' and 'pain' limits) are longer than the separation distance to LFL in the case of unignited release.

22.10.7 Concluding remarks

Numerous knowledge gaps have been closed by the international hydrogen safety community (www.hysafe.org). However, more issues appear as we learn more. The key issue at the moment that is directly related to hydrogenpowered vehicles is low fire resistance of on-board storage. The increase of fire resistance from the current 3.5–6.5 minutes for Type 4 tanks to above 30 minutes could drastically change public perception of hydrogen technologies. Indeed, flame length from TPRDs could be reduced from current 10–15 m to about 1 m. Destruction of civil structures due to the pressure peaking phenomenon during accidental release of hydrogen would be excluded. Moreover, small length hydrogen jet flames could be considered not to be a hazard but a source of extinguishing agent, i.e. water, to control fire in an enclosure like a garage. High fire resistance would allow essential mass flow rate reduction from TPRDs and exclude formation of large flammable clouds in tunnels and bring deflagration pressure below harmful levels. A higher fire resistance rating on hydrogen storage tanks and shorter flame length would allow self-evacuation of civilians from the accident scene, efficient intervention of first responders to control the accident, etc.

Thus, the way forward with hydrogen safety is to increase the fire resistance of on-board storage and reduce the mass flow rate from the TPRD. The issue of fire resistance of on-board storage has yet to be addressed by car manufacturers to allow longer time for blow-down of tanks. Unfortunately, self-evacuation of passengers from currently designed cars or safeguarding by first responders with such design of hydrogen storage and PRD systems is impossible and car manufacturers in close cooperation with experts in hydrogen safety have yet to address this customer safety issue.

22.11 Conclusions

There has been a lot of progress in electrochemical storage devices during the past decade, and the near future holds prospects for many further advances. Cost, durability and energy density are the main areas where improvements are required to compete with conventional fossil fuels.

Costs have been falling exceptionally rapidly, and are expected to continue doing so for the next 5–10 years. For example, the cost of manufacturing FCEVs has decreased by a staggering 90% since 2005, and the publicly acceptable goal of \$50,000 for a luxury sedan is now within reach. BEVs have already reached such a price-point – with current vehicles retailing at around \$40,000 – with \$20,000–30,000 attainable within 10 years.

The durability of battery and capacitor technologies is already expected to be sufficient for automotive use, giving 10 year's calendar life and 150,000 miles. Fuel cell stacks appear to still be falling short of the US DoE's 2009 target of 2,000 hours operation, corresponding to approximately 25,000 miles before a 10% drop in power output. New MEA designs with improved catalysts and reduced crossover currents are expected to improve this situation in the future.

Energy density is still the Achilles heel of batteries and capacitor technologies. It is a sad fact that even the latest nanotechnology-based ultracapacitors are not expected to offer close to the energy density of battery technologies, and so they are unlikely to ever be used for more than peak power provision. However, the high efficiency and relatively light ancillary systems required by electric power-trains mean that batteries need only offer 666 Wh.kg–1 at the cell level to offer the same driving range and consumer acceptability as current petrol vehicles. The next generation of lithium-based chemistries are expected to approach this value, and so the perennial problem of 'range anxiety' may soon be overcome. Hydrogen storage will, however, be the first technology to exceed petrol in terms of the range offered, as only 6 wt.% hydrogen is required, and several forms of metal-hydride storage are on the horizon which are expected to beat this.

Japanese automakers announced at the 2010 Fuel Cell Expo in Tokyo, a programme designed to deploy 2 million FCEVs in Japan by 2025 – a point at which the industry estimates FCEVs would be fully competitive. It has also become apparent that several (but not all) OEMs had redeployed their technical staff from fuel cell R&D to focus on Li-ion batteries. In addition, some of these automotive companies also stated that technology investment in the fuel cell supply chain is in serious decline. Thus, these two factors combined reduce the likelihood of the very fast progress in a hydrogen economy that was expected a few years ago. It is unlikely that global market forces alone will be able to achieve the Intergovernmental Panel on Climate Change (IPCC) 80% carbon reduction goal by 2050, the policy makers are therefore required to give an aggressive push towards this objective.

22.12 Acknowledgements

The authors would like to thank Advantage West Midlands (AWM), EPSRC (Contract No: EP/E034888/1) and the RCUK CDT in hydrogen, fuel cells and their applications for their kind financial support. JLS would also like to thank the Birmingham Centre for Hydrogen and Fuel Cell Research, College of EPS and The University of Birmingham for its generous support.

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22.14 Appendix: abbreviations

AB ammonia-borane AFC alkaline fuel cell

BERR Business Enterprise and Regulatory Reform

BEV battery electric vehicle BMS battery management system

CL catalyst layer

CNG compressed natural gas CNT carbon nanotube DoD depth of discharge DoE US Department of Energy

DMFC direct methanol fuel cell EC electrochemical capacitor

EC–DMC ethylene carbonate–dimethyl carbonate EDLC electrochemical double layer capacitor

ELHP european Integrated Hydrogen Project

ESD energy storage device FCEV fuel cell electric vehicle

FF flow field

GDL gas diffusion layer GHG greenhouse gas HEV hybrid electric vehicle HOR hydrogen oxidation reaction HFCV hydrogen fuel cell vehicle IFC international fire code

ICE internal combustion engine

IPCC Intergovernmental Panel on Climate Change

LFL lower flammability limit Li-ion lithium-ion battery LPG liquified petroleum gas MCFC molten carbonate fuel cell MCMB mesocarbon microbead MEA membrane electrode assembly MFC microbial fuel cell

MOF metal-organic framework

NFPA National Fire Protection Association

ORR oxygen reduction reaction PAFC phosphoric acid fuel cell PEM proton exchange membrane

PEMFC proton exchange membrane fuel cell

PGM platinum group metal PPS peak power source PRD pressure relief device

RCS regulations, codes and standards RTIL room temperature ionic liquid SHE standard hydrogen electrode

SoC state of charge SOFC solid oxide fuel cell

SRP standard reduction potential STP standard temperature and pressure TPRD thermal pressure relief device

WTT well-to-tank WTW well-to-wheel

absorption, 102–3	Advanced Technology Powertrains for
accumulator model, 518	Light-Duty Vehicles (ATP-LD),
active balancing, 624	249–50
active cooling, 620	advanced tire design
active materials, 612	materials, 471–4
active regeneration, 341	contact area, 472
actual engine cycles, 209–11	pressure control, 473–4
compression ignition (CI) engines, 210–11	tire structure, 471
spark ignition (SI) engines, 209–10	advanced transmission technologies
additive manufacture, 454	vehicle performance improvement,
adsorption, 103	393–432
advanced alloys	automated commercial vehicle
ceramics for manufacturing valves and	transmission of 16-speed G260-16,
other components, 377–81	431–2
chemical composition of NIMONIC	automatic transmission of seven-speed
alloy 80A, 378	7G-Tronic Plus, 399–411
physical and mechanical properties of	continuously variable transmission
ceramics, 380	of front-wheel-drive CVT
physical and mechanical properties of	AUTOTRONIC, 411–18
materials used for manufacturing,	dual-clutch transmission of seven
380	speed front-wheel-drive 7G-DCT,
advanced internal combustion engine materials compact graphite iron (CGI), 371–3	396–9
CGI microstructure containing 10%	manual transmission of six-speed front
nodularity, 372	wheel-drive SG6-310, 395–6
deep-etched SEM micrographs	Mercedes-Benz six decades of experience in developing and
showing 3-D coral-like graphite	producing transmissions, 394
morphology, 372	P2 hybrid transmission, 418–22
Ford PSA Jaguar V6 high performance	two-mode hybrid transmission
2.7L diesel engine, 374	advanced hybrid system-cars
mechanical and physical properties of	(AHS-C), 422–31
vs conventional grey cast iron and	advanced vehicle design
aluminium, 373	drag reduction, 468–71
series production programs, 374	heavy vehicles, 470–1
advanced low-temperature combustion	passenger car, 468–70
systems, 285–8	ADVISOR, 548
illustration, 286	aerodynamic drag reduction, 4
advanced polymers	aerodynamics
polyamides for manufacturing intake	loss, 483
manifolds, 375–7	vehicle rolling resistance improvement,
plastic air intake manifold (PAIM) with	462–74
integrated adjacent components,	advanced tire design and materials,
377	471–4

advanced vehicle design for drag	angular position control (APC), 663
reduction, 468–71	anti-idling technologies, 268
future trends, 474	anti-oxidants, 144
overview, 464–6	anti-wear agents, 144
rolling resistance in vehicles, 466–8	antioxidants, 166–8
affordability, 446, 457–8	aquifers, 130
aftertreatment technologies	ASTM D665, 173
emissions control, 339–46	ASTM D975, 54
diesel oxygen catalysts (DOC)	ASTM D2699, 60
performance, 339–40	ASTM D2700, 60
LNT technology, 343–4	ASTM D5797, 28, 33
N2O emissions, 344–5	ASTM D5798, 33, 60, 74
particulate matter (PM) control,	ASTM D6751, 54
340–2	ASTM D7467, 54
selective catalytic reduction (SCR)	ASTM D7589, 142
systems, 342–3	Atkinson cycle, 206
integrated aftertreatment systems, 346	autoignition, 62
effects of air–fuel ratio and exhaust	automated commercial vehicle transmission
restriction on BSFC for line haul	16-speed G260-16, 431–2
tractor, 346	advantages, 431–2
air–fuel ratio (AFR), 74, 78, 281, 291–2	technical concept and design, 431
air-hybrid see pneumatic hybrids	automatic mechanical transmissions, 243
air system capability analysis, 252	automatic transmission
alcohol fuels, 27–36	seven-speed 7G-Tronic Plus, 399–411
compression-ignition engines, 72–3	conclusion and advantages, 410–11
physico-chemical properties, 55–65	technical concept and design, 399–403
octane numbers, 60–5	transmission control, 403–10
vapour pressure, 57–60	automatic transmission fluid (ATF), 400
volumetric energy density and	Autonomie, 548 auxiliary load, 652–3
stoichiometry, 56–7 spark-ignition engines, 65–9	auxiliary power module (APM), 427–8
efficiency, 66–8	auxiliary power unit (APU), 525
efficiency of dedicated alcohol engines,	auxiliary system, 708
68–9
performance, 65–6	Babbitt bearings, 382
pollutant emissions, deposits and	backbone chassis, 450–1
lubricant dilution, 69–72	base oil effect, 156
aldehyde emissions, 71	batteries, 583, 667–8, 687–93
alkaline electrolysers, 123	characteristics of various type of battery,
alloys, 695	688
Altamont landfill, 105	current issues, 691, 693
alternative engine operating cycles, 206–11	lithium-ion (Li-ion) batteries, 690–1 battery technologies and Ragone plot
actual engine cycles, 209–11 free piston engine, 208–9	of specific power density vs energy
over-expanded cycle, 206–7	density, Plate XX
split cycle engines, 207–8	operational principle, Plate XIX
Wankel engine, 208	Mitsubishi IMiev, 689
alternative fuels	nickel metal hydride (NiMH) batteries,
usage, 228–33	688–90
general characteristics and major	schematic diagram of electrochemical
vehicle applications, 233	reaction process, 690
Alternative Fuels Data Centre, 228	various secondary batteries and their
Alternative Motor Fuels Act (AMFA), 33	reactions, 692
aluminium, 3–4, 371	battery capacity, 649–50
aluminium alloys, 391	battery chargers, 675–7
AMESim, 522	conductive chargers, 675–6
amine process, 102	standard power levels of conductive
anaerobic digestion (AD), 98, 101	chargers, 675

inductive chargers, 676	biogas, 98–101
wireless chargers, 676–7	production, distribution and storage, 101–5
battery charging mode, 637	bulk storage, 105
battery electric powertrains	distribution of gaseous biomethane,
flywheel, 497–9	104–5
Garrett electrochemical transmission,	distribution of liquid biomethane, 105
499	European biogas feed-in projects as of
illustration, 498	May 2010, 105
battery electric vehicle (BEV), 22, 708–9	generic anaerobic digestion process,
current status, 708–9	101
future developments, 709	purification to biomethane, 102–4
history, 708	typical production by feedstock, 102
battery functionality	biological hydrogen production, 698
chemistries for vehicle applications,	biomass-based diesel, 29–30
598–610	biomass limit, 36–7
cell functionality, 599–601	biomass-to-liquids (BTL) fuels, 54
chemistries, 602–8	biomethane, 98–101
performance metrics, 601–2	purification, 102–4
battery management systems, 621–5	supply chains, 100
cell voltage measurement and control,	black sludge, 154
621–2	bladder type accumulator, 517
communication, 625	boat tail approach, 471
contractor control, 622–3	body in white (BIW), 447
isolation monitoring, 623	body spoiler, 460
state of charge and health calculation,	boil-off, 110
624–5	boosting, 213
summary, 625	boron steel, 3
temperature control, 623	bottom dead centre (BDC), 198–9, 297
battery pack, 583	brake energy recovery (BER) mode, 573
battery power	brake mean effective pressure (BMEP), 66,
rating, 644–5	68, 206–7, 246–7, 283–4
requirements, 597	brake specific fuel consumption (BSFC),
battery swapping stations, 674–5	280–1, 525
battery system, 708	braking blending, 549–50
battery technology	braking control, 548–50
CO2 reduction, 582–628	braking fundamentals, 550–5
battery functionality and chemistries	braking stability, 551–3
for vehicle applications, 598–610	free-body diagram of vehicle under
battery management systems, 621–5	braking, 552
future trends, 625–8	optimum braking distribution is
high voltage battery pack design,	governed solely by geometry of
616–21	car, 553
lithium ion cells, 610–16	primary design data for mid-size vehicle,
opportunities of using batteries, 583–98	551
definitions, 583	regenerative braking, 550–1
other potential battery benefits, 582	regenerative systems, 553–4
beam structures, 454–5	simplified representation of braking
bearing bushes, 385	algorithm satisfying requirements
belt, 466–7	of Category B, 554
belt design, 467	response to driver input, 555
belt drives, 487–8	stability and skid control, 554–5
Benedict–Webb–Rubin (BWR) equation, 518	braking rate, 549
bi-fuel vehicles, 77–8	braking stability, 551–3
Bio-CNG, 98	Brayton cycle, 347–8
biodiesel, 26–7, 166, 169, 180, 214–15, 232	Brown Electric car, 12
alternative road transport fuels, 26	buses, 9–10
internal combustion engines, 53–5	Business Enterprise and Regulatory Reform
bioethanol, 28	(BERR), 711

charge valve (CV), 536 cam-driven mechanism, 334 charging infrastructure, 673–7 battery chargers, 675–7 camless VVA system, 335 conductive chargers, 675–6 cams, 389–90 camshafts, 389–90 inductive chargers, 676 carbon black, 472 wireless chargers, 676–7 carbon capture and sequestration, 112 home charging, 673 carbon capture and storage, 697 public charging, 673–5 carbon dioxide, 7–8, 119–20 battery swapping stations, 674–5 fast charging stations, 674 carbon dioxide (CO2) reduction, 230–1 opportunity charging stations, 673–4 battery technology, 582–628 battery functionality and chemistries chemistries, 602–8 for vehicle applications, 598–610 cost, 608–9 battery management systems, 621–5 lead acid (PbA), 602–4 future trends, 625–8 nickel metal hydride (NiMH), 604–6 high voltage battery pack design, 2010 Honda Insight NiMH battery pack, 604 616–21 Toyota Prius NiMH battery pack, 604 lithium ion cells, 610–16 sodium nickel chloride, 606–7 opportunities of using batteries, 583–98 review of passenger car driving cycles, summary, 609–10 584–97 comparison of performance metrics summary, 597–8 across cell chemistries, 611 US annual energy flow in 2010 in Chevrolet Volt, 220 China Greentech Initiative, 242 quadrillions of BTUs, 598 closed-loop combustion feedback control, carbon fibre-reinforced polymer composites 284 (CFRPs), 373, 375 conrod of RA engine, 375 clutch control mechanism, 424 mechanical properties of carbon fibreglass coal, 697 material, 376 coal gasification, 121–2 mechanical properties of graphite-carbon cold filter plugging point (CFPP), 169 fibre composite material, 376 cold flow improvers, 168–70 average CFPP response of 5 base fuels, carbon monoxide, 218 169 carbon nanotubes (CNT), 694 carbon steels, 383 combined gas turbine (CGT), 128 carcass, 467 COMMAND SYSTEM, 429 case study, 555–7 commercial vehicles, 9–10 actual breaking events, Plate XIV Commission Regulation (EU) No. 406/2010, characteristics of typical 100km.h-zero 722 communication, 625 stop at constant 0.4g-regen and compact graphite iron (CGI), 371–3 foundation blend, 557 composite flywheels, 485 operating range of the electric motor, Plate XV composites, 4 cast iron crankshafts, 382 compressed biogas (CBG), 98 casting modified cast iron, 382 compressed gaseous hydrogen, 720 catalytic hydrogenation reaction, 119 compressed hydrogen, 131–2, 694, 695 compressed natural gas (CNG), 91, 215, 355 catalytic reforming, 109 distribution, 94 cell, 583 compression ignition (CI) engines, 210–11 cell balancing, 624 cell functionality, 599–601 compression-ignition dual-fuel engines, 97 battery process during charge, 600 compression ratio, 198–9, 205–6 battery process during discharge, 600 compromised battery design, 657 cell voltage measurement, 621–2 concentrated solar heat, 698	butadiene rubber (BR), 472	charge maintaining, 649



















































	cellulosic biofuel, 30–1	conceptual detailed design, 458
conductive chargers, 675–6 centralised electrolysis systems, 697–8
conductivity improvers, 175 ceramics, 377–81 cetane number, 53, 72, 176, 177 conductivity as a function of biodiesel
charge-coupled device (CCD), 555 concentration for SME, 176

decreasing conductivity with fuel	engine performance simulation of
temperature, 175	Miller cycle with early IVC timing
conformal joining, 456	vs VGT, 336–7
conformal interference joined 6061-	engine performance simulation of
T6 tube electromagnetically	valve timing effects at constant
compressed in steel mandrel,	fuelling rate, 333
457	illustration of valvetrain design
connecting rods, 382–6	considerations, 332
engine connecting rod, 382	cool exhaust gas, 326
some materials used for production of	copper chlorine cycle, 125
engine connecting rods in Russia,	copper strip test, 173
386	corporate average fuel economy (CAFE), 235
conrod, 373, 375, 384	standards, 76, 81
conrod bolts, 385	corrosion inhibitors, 144–5, 172
constant-power downsizing, 297–9	cost, 608–9
constant pressure ideal heat addition, 203–4	crankshafts, 381–2
cycle efficiency, 204	crashworthiness, 444–5
constant volume ideal heat addition, 203	crude oil consumption, 239
contact area, 472	cryo-compressed hydrogen, 132
contactors, 617	cryogenic distillation, 103–4
continuously variable transmission (CVT),	upgrading of technologies in European
236, 486, 572–3	biomethane projects, 104
front-wheel-drive CVT AUTOTRONIC,	cryogenic hydrogen, 129–30, 132
411–18	cryogenic tanks, 134
conclusion and advantages, 418	Cummins L10 test, 180
technical concept and design, 411–14	current chopping control (CCC), 663
transmission components, 414–17	customer appeal, 457–8
transmission control, 417–18	cycle life, 595
continuously variable unit (CVU), 491	requirements, 597
contractor control, 622–3	cylinder blocks, 390–1
algorithm, 622
controlled auto-ignition (CAI), 287–8	DC drives, 661–2
controller area network (CAN), 404	degradation pathway, 616
conventional combustion technologies	density calc ulation, 727
engine performance, 281–5	Department of Energy, 458
effect of peak cylinder pressure	depth of discharge (DoD), 716
on engine thermal efficiency at	desulphurisation, 167
constant fuel injection timing,	detergents, 144
282	dexos, 152
illustration of lifted flame combustion,	die casting, 387, 454
283	diesel engine, 635–6
conventional diesel combustion, 280	diesel ignition improving additives, 176–8
conventional fuel	reduced ignition delay for diesel fuel
hybrid electric vehicles (HEV), 632–54	with 10 cetane number, 177
architectures of hybrid electric drive	lubrication challenges, 145–7
trains, 633–6	dynamic viscosity reduction due to fuel
basic components, 633	dilution, 146
control and performance, 647–53	fuel dilution due to post-injection, 146
future trends, 653–4	increased oil degradation, 14
parallel hybrid electric drive trains,	turbocharging, 289
641–6	diesel fuels, 167–8, 172, 183
series hybrid electric drive trains,	diesel injector internal deposits, 182–3
636–41	effect of internal deposits on injector cold
series-parallel hybrid electric drive	start function, 184
trains and plug-in hybrids, 646–7	sodium carboxylate internal deposits, 183
conventional valvetrains	diesel oxidation catalyst (DOC), 287
improvement and variable valve actuation	performance, 339–40
(VVA) usage, 327, 332–8	diesel particulate filter (DPF), 142, 284–5, 313

diesel particulate matters, 339–40	longitudinal and transverse
diesel performance additive packages, 183–4	arrangement configurations, 658
diesel-to-gasoline ratio, 288	mechanical and electronic differential
DieselNet Brazil, 242	EV configurations, 659
DieselNet China, 242	electric vehicle (EV), 118, 586–7
DieselNet European Union, 241	electrical coupling see series hybrid electric
DieselNet India, 242	drive trains
DieselNet Japan, 241–2	electrical flywheel hybrid technology
dimethyl ether (DME), 210–11	mechanical flywheel hybrid technology to
direct clutch control, 409	store energy in vehicles, 476–502
direct injection diesel engines, 248–9	flywheel technology development,
direct injection fuel system, 64	479–80
direct injection gasoline (DIG) engines,	future trends, 501–2
187–90	performance evaluation, 493–501
direct injection nozzle deposits	technical challenges in flywheel
additive use, 189	development, 501
base fuel, 189	transmission for flywheels, 486–93
direct-injection spark-ignited engine see	types and properties of flywheels,
	480–6
direct-injection technology
direct-injection technology, 215–16	electrical regeneration properties, 543–4
direct select module (DSM), 400	electrical supercharging systems, 312–13
dispersants, 144	electrical transmissions, 490–1
distributed on-site production, 697	electrically driven accessories, 351
doubly-salient PM (DPSM), 666	electricity, 127–9
down-breathing, 294–312	electrification, 235
down-speeding, 294–312	electro-hydraulic transmission control, 417
downsizing, 270	AUTOTRONIC illustration, 417
downstream DNR set, 415–16	electrochemical capacitors (ECs), 704–5
drag reduction, 468–71	specific power against specific energy
drive neutral reverse (DNR), 411	called Ragone plot for energy
drive train topology, 576–7	storage devices, 706
dual-clutch transmission	electrochemical cell, 687
seven-speed front-wheel-drive 7G-DCT,	electrochemical double layer capacitors
396–9, Plate III	(EDLCs), 704
advantages, 398–9	electrofuels, 36, 38
technical concept and design, 396–8	electrolysers, 123
dual-stage vane pump, 416–17	electrolysis, 123–4
DW10, 180–1	methods, 13–14
dynamic programming (DP), 524–5, 578	electrolyte, 613–14
	improvements, 626–7
E85 fuel, 66–7, 166	electromechanical battery (EMB), 490–1
E100 fuel, 66, 166	electromotive force (EMF), 663
EC 443/2009, 140	electronic controls, 352–5
EcoBoost, 151, 160	electronic transmission control, 417–18
economic incentives, 442	control units comparison, 418
edge radii, 471	electronically controlled common-rail fuel
electric auxiliary oil pump, 398	injection systems, 280
electric motor power rating, 644–5	embodied energy, 435
electric motor propulsion system, 708	emission reduction, 652
electric motor pump (EMP), 424	emissions control, 339–46
electric oil pump, 410	emissions regulations, 7–8
electric pathway, 601	EN 228, 57
electric propulsion, 661–6	EN 14214, 54
advanced EV drives, 665–6	energy benefits, 680
strator-PM drive topologies, 665	energy efficiencies of EV vs ICEV, 681
existing electric vehicle (RV) drives,	energy blocks, 573–4
661–5	energy capacity, 644–5
variation, 658–9	energy conversion, 633

energy density, 716–18	engine air flow
energy-flow, 574	turbocharging systems, 288–94
energy hybridisation, 669–71	effect of air–fuel ratio and EGR rate on
energy management, 500–1, 648–9	engine delta at rates power at high
energy metrics, 601–2	EGR rate, 290
improvements, 625–6	effect of air–fuel ratio on engine delta,
Energy Policy Act (2005), 29	291
energy recovery management, 672	effects of EGR rate and air–fuel ratio
distribution of regenerative and hydraulic	on engine delta at rated power, 289
braking torques, 672	engine cost, 356–7
energy recovery technologies	engine cylinder valves, 389
hybridisation design and performance	recommendation on application of
issues, 547–50	different steels for certain
braking control, 548–50	temperature levels, 389
powertrain design, 547–8	engine downsizing, 294–312
integration and performance of	competitive benchmarking analysis of
regenerative braking in vehicles,	diesel engine power density
541–62	cylinder diameter, 296
advantages and limitations, 557–8	mean piston speed, 298
design integration and operational	design changes requirements, 300
optimisation, 550–7	effect of BMEP on cylinder pressure, 309
future trends, 559–61	effect of engine down-speeding on
types and properties, 542–7	divertrain matching, 310
energy requirements, 592	effects of diesel engine power up-rating
energy storage	and down-speeding with VGT,
management, 666–72	307–8
devices, 666–71	effects on BMEP, engine delta, BSFC
energy recovery management, 672	change, peak cylinder pressure,
state-of-charge (SOC) indicators, 671	301–6
mechanical and electrical flywheel hybrid	unified theory of down-speeding and
technology in vehicles, 476–502	down-breathing in air system
flow of liquid between tanks energy	capability charts, 295
analogy, 478	engine durability, 139
flywheel technology development,	engine geometric compression ratio, 281
479–80	engine-level strategy, 530
future trends, 501–2	engine mechanical friction reduction
performance evaluation, 493–501	technologies, 350–2
technical challenges in flywheel	engine oils
development, 501	internal combustion engines, 138–62
transmission for flywheels, 486–93	composition of modern engine
types and properties of flywheels,	lubricant, 143–5
480–6	diesel engine lubrication challenges,
variation, 657–8	145–7
configurations of battery only, two	future trends, 157–62
different batteries, battery and	gasoline engine lubrication challenges,
ultracapacitor, 657	147–51
energy storage devices (ESD), 667–71	industry and OEM specifications for
batteries, 667–8	engine oils, 151–2
application of various batteries to	lubricating modern engines in
flagship EVs, 668	developing markets, 152–7
energy hybridisation, 669–71	role of lubricant, 139–43
operation of hybrid energy system, 670	engine operating points, 293
transport sector, 686–7	engine-out emission, 245
electrochemical ESD characteristics,	engine performance, 280–1
688	model, 353–4
ultra-high-speed flywheels, 669	engine power design, 643–4
ultracapacitors, 668	engine speed-load domain, 297–9
energy storage solutions, 558	engine thermal efficiency, 264

engine traction, 637
engine volumetric efficiency, 292–3
engines
   environmental performance improvement 
          of heavy-duty vehicles, 225–70
       challenges in advanced engine and 
          vehicle technologies, 251–64
       developments in light-duty diesel 
          engine technologies, 248–50
       electric, hydraulic and flywheel hybrid 
          powertrains for improved fuel 
          economy, 234–7
       engine design to meet environmental 
          regulations, 244–8
       key issues and system design 
          approaches, 225–70
       summary of next-generation 
          technologies, 264–70
       usage of alternative fuels, 228–33
       vehicle design to meet environmental 
          regulations, 242–4
       vehicle emissions and fuel economy 
          regulations, 237–42
entrained flow gasifier, 122
environmental benefits, 680–2
   comparison of overall harmful emissions 
          between EV and ICEV, 681
environmental enclosure, 620–1
environmental performance improvement
   heavy-duty vehicles and engines and 
          particular technologies, 279–357
       advanced low-temperature combustion 
          systems, 285–8
       aftertreatment technologies for 
          emissions control, 339–46
       conventional combustion technologies 
          and engine performance, 281–5
       conventional valvetrains improvement 
          and variable valve actuation (VVA) 
          usage, 327, 332–8
       electronic controls and on-board 
          diagnostic (OBD) systems to 
          optimise engine performance, 
          352–5
       engine air flow and turbocharging 
          systems, 288–94
       engine downsizing, down-speeding and 
          down-breathing, 294–312
       engine mechanical friction reduction 
          technologies, 350–2
       exhaust gas recirculation (EGR) 
          systems, 326–7
       fuel injection systems and engine 
          performance, 280–1
       future trends, 356–7
       heavy-duty diesel engine cooling and 
          thermal management systems, 
          338–9
                                                         mechanical and electrical 
                                                            supercharging systems for 
                                                            improved emissions control and 
                                                            performance, 312–13
                                                         natural gas engines development, 
                                                            355–6
                                                         turbocompounding to improve engine 
                                                            performance, 313–26
                                                         waste heat recovery (WHR) systems, 
                                                            347–50
                                                     heavy-duty vehicles and engines key issues 
                                                            and system design approaches, 
                                                            225–70
                                                         challenges in advanced engine and 
                                                            vehicle technologies, 251–64
                                                         developments in light-duty diesel 
                                                            engine technologies, 248–50
                                                         electric, hydraulic and flywheel hybrid 
                                                            powertrains for improved fuel 
                                                            economy, 234–7
                                                         engine design to meet environmental 
                                                            regulations, 244–8
                                                         relationship between CO2 emission, 
                                                            BSFC, engine thermal and vehicle 
                                                            freight efficiency, 229–30
                                                         summary of next-generation 
                                                            technologies, 264–70
                                                         usage of alternative fuels, 228–33
                                                         vehicle design to meet environmental 
                                                            regulations, 242–4
                                                         vehicle emissions and fuel economy 
                                                            regulations, 237–42
                                                  Environmental Protection Agency (EPA), 219, 
                                                            238, 458
                                                  environmental regulations
                                                     engine design improvement, 244–8
                                                         effect and potential of fuel injection 
                                                            timing on BSFC reduction, 247
                                                     vehicle design improvement, 242–4
                                                  environmental resistance, 445
                                                  environmentally sustainable transport
                                                     role of alternative and renewable liquid 
                                                            fuels, 19–51
                                                         biomass limit, 36–7
                                                         competing fuels and energy carriers, 
                                                            19–26
                                                         market penetration of alcohol fuels, 
                                                            27–36
                                                         market penetration of biodiesel, 26–7
                                                         renewable fuels within an integrated 
                                                            renewable energy system, 40–3
                                                         sustainable organic fuels for transport, 
                                                            37–40
                                                  estimated power density, 519
                                                  ethanol, 26–7, 29, 35, 68–9, 153–4, 165–6, 
                                                            172
                                                     effects on deposit formation, 190–2
                                                         cleanliness of additives, 191

gasoline detergent solubility in E85,	flux reversal PM (FRSM), 666
191	Flybrid–Torotrak system, 495
ethylene vinyl acetates (EVA), 170	Flybus, 236, 495
European Integrated Hydrogen Project	flywheel battery (FWB), 490–1
(EIHP), 695	flywheel energy storage system (FESS), 486
European SuperLIGHT-CAR program, 458	flywheel hybrid powertrains, 234–7
European Union's criteria emissions	flywheel hybrid vehicles
regulations, 241	performance evaluation, 493–501
exhaust gas recirculation (EGR), 69, 210	energy management, 500–1
rates, 159	flywheel and internal combustion
systems, 326–7	engine-based powertrain, 493–7
diesel engine subsystem interactions at peak torque and system	flywheel in battery electric powertrains, 497–9
optimisation, 328–9	flywheel in hybrid electric powertrains,
diesel engine subsystem interactions	499–500
at rated power and system	pure flywheel-based powertrain, 493
optimisation, 330–1	flywheels
tolerance at a range of engine speeds and	transmissions, 486–93
load of methanol and gasoline, 70	electrical transmissions, 490–1
existing electric vehicle (RV) drives, 661–5	hydrostatic transmissions, 486–7
comparison, 664–5	mechanical transmissions, 491
application of existing drives to	power split principle, 491–3
flagship EVs, 664	traction continuously variable
evaluation of existing EV drives, 664	transmissions, 487–90
DC drives, 661–2	types and properties, 480–6
induction drives, 662	aerodynamic loss and vacuum system,
permanent magnet (PM) brushless drives,	483
663	low-loss bearings, 483–4
switched reluctance (SR) drives, 662–3 topologies, 661	potential effects of gyroscopic moment on vehicle dynamics, 485–6
extended range electric vehicles (EREV), 22	rotor design and materials for
	maximum ratio of stored energy to
fabricating technologies, 453–5	weight, 481–3
fast charging stations, 674	safety in event of rotor failure, 484–5
fatty acid methyl esters (FAMEs), 53–5	fossil alternative fuels, 233
Federal Motor Vehicle Safety Standard 135,	fossil natural gas, 91–2
546	estimated technically recoverable shale gas
Federal Test Procedure (FTP), 239–40, 284–5	reserves, 92
field-oriented control (FOC), 662	production, transmission and distribution,
fine rust particles, 172	92–5
finished fuel additives, 165	distribution of compressed natural gas
Fischer–Tropsch (FT)	(CNG), 94
approach, 233	distribution of liquified natural gas
diesel, 214–15	(LNG), 94–5
process, 55, 166, 170	selected properties at the wellhead, in
synthesis, 39	pipeline systems and CNG, 93
fixed camshaft (FCS), 537–8	four-stroke cycle, 200
fixed ignition angle, 67	free piston engine, 208–9
flat seat valve, 406–7	free water, 172
flexible-fuel vehicles (FFVs), 28, 32, 34	friction, 211–12
vehicle and blending technologies for	friction drag, 466
alternative liquid fuels, 73–6	friction modifier (FM), 170–1
physical sensor system, 76	additives, 141, 144
virtual sensor system, 75	HFRR test, 158
flexible printed circuit board (FPCB), 404	M111test, 159
flow field (F) plates, 700	friction stir welding, 455
fluid-based regeneration properties, 545–6	front-wheel-drive CVT AUTOTRONIC,
fluidised bed gasifier, 122	411–18

technical concept and design, 411–14	current status of low-carbon vehicle
adaptations to driving style, 413	technologies, 705–7
manual program activated via one	efficiency, 718–19
touch function, 413–14	energy consumption measured from
mid-section and with power flow	real-world testing of 23 hydrogen
through sub-assemblies of front	fuel cell vehicles, 720

wheel drive, 412	typical well-to-tank. tank-to-wheel
reduced fuel consumption and	and well-to-wheel for each vehicle
emissions, 413	technology, 719
transmission components, 414–17	electrochemical capacitors (ECs), 704–5
transmission control, 417–18	energy storage devices (ESD) for transport
electro-hydraulic transmission control,	sector, 686–7
417	fuel cell electric vehicles (FCEV), 709–14
electronic transmission control, 417–18	hydrogen and fuel cells, 693–704
fuel additives	challenges, 703–4
additive types and impact on conventional	fuel cells, 698–703

and advanced fuels, 166–75	hydrogen energy, 693–8
antioxidants and stabilisers, 166–8	safety improvement of hydrogen-powered
cold flow improvers, 168–70	vehicles, 719–29
conductivity improvers, 175	technical aspects and barriers, 714–19
effect of sulphur on gasoline silver	cost, 714–15
corrosion ratings, 174	durability and degradation, 716
lubricity improvers and friction	energy and power density, 716–18
modifiers, 170–1	fuel cell electric vehicles (FCEV), 108,
	709–14
other corrosion inhibitors, 173–4
pipeline corrosion inhibitors, 172–3	comparisons with BEV, 714
treat rate of corrosion inhibitor	vehicle specifications comparison from
additives, 174	consumer's perspective, 715
diesel performance and deposit control	example of hybrid battery, 711–14
additives, 178–84	microcab hydrogen hybrid fuel cell
diesel injector internal deposits, 182–3	vehicle 2nd and new generation,
diesel performance additive packages,	712–13
183–4	future developments, 714
injector nozzle coking, 178–82	hydrogen as fuel for fuel cell hybrids,
gasoline performance and deposit control	710–11
additives, 184–92	proton exchange membrane (PEM) fuel

cleanliness and performance of port	cell stack, 710
fuel injected gasoline engines,	fuel cell hybrids, 710–11
186–7	fuel cells, 698–703
direct injection gasoline (DIG) engines	cost, performance and scarcity, 700–3
and injector plugging, 187–90	evolution of total platinum loadings
effects of ethanol on deposit formation,	and power densities in PEMFC
190–2	stack, 702
gasoline engine deposits, 184–5	PEMFCs, 699–700
gasoline performance additive (GPA)	layout, Plate XXI
packages, 185–6	schematic representing all available and

impact on combustion characteristics,	existing fuel cells, 699
175–8	fuel consumption, 429–30
diesel ignition improving additives,	fuel cost, 243–4, 574
176–8	fuel displacement, 596–7
octane improving additives, 178	fuel economy improvement
internal combustion engines, 165–92	hydraulic and pneumatic hybrid
future trends, 192	powertrains in vehicles, 505–38
improved fuel economy and potential	design and control, 522–32
cleanliness, 167	hydraulic component design and

fuel-cell electric hybrid vehicles, 685–731	modelling, 511–20
batteries, 687–93	integrated hydraulic hybrid vehicle
battery electric vehicles (BEVs), 708–9	simulation, 520–2

pneumatic hybrids, 535–8	intake valve deposit clean-up, 186
practical applications examples, 532–4	lubrication challenges, 147–51
principle of operation and system	engine dynamometer tests, 150
architectures, 507–11	oil pickup deposits before and after
fuel economy regulations, 230, 237–42	solvent wash, 149
vehicle emissions, 237–42	power density comparison, 148
emissions and greenhouse gas	turbocharger shaft/bearing deposits,
regulations in other countries,	149
240–2	gasoline particulate filter (GPF), 160
emissions and greenhouse gas	gasoline performance additive (GPA),
regulations in USA, 237–40	185–6
fuel injection systems, 280–1	gear changes, 409–10
Fuel Quality Directive (FQD), 33–4, 57	gear level, 395–6
fuel savings, 574	geartrain, 320–1
potential, 575–6	global car manufacturing, 8–9
fuel technology, 228	global emissions control journey, 237–8
Fukushima nuclear accident, 128	globe-wide emissions, 230
fully flexible valve actuation (FFVA) system,	graphite/carbon fibre, 373, 375
537	gravity casting, 387
fuse, 618	greenhouse gas (GHG), 24
	emissions, 219
galvanic cell see electrochemical cell	model, 239–40
gas separation unit, 119–20	regulations, 287
gaseous fuels	grey cast iron, 371
alternative and renewable, to improve	Gyrobus, 544
vehicle environmental performance,	gyroscopic torque, 485–6
90–114
biogas production, distribution and	HD-10, 106
storage, 101–5	health calculation, 624–5
biomethane/biogas, 98–101	heat rejection, 338–9
fossil natural gas, 91–2	heat transfer, 212
future trends, 113–14	heavy-duty diesel engine cooling
hydrogen, 108–9	thermal management systems, 338–9
hydrogen production, distribution,	heavy-duty engines
storage and use in vehicles, 109–11	environmental performance improvement
life-cycle analysis, 111–13	of vehicles and particular
liquid petroleum gas (LPG), 106	technologies, 279–357
LPG production, distribution, storage	advanced low-temperature combustion
and use in vehicles, 106–8	systems, 285–8
natural gas engines and vehicles,	aftertreatment technologies for
95–8	emissions control, 339–46
production, transmission and	conventional combustion technologies
distribution of fossil natural gas,	and engine performance, 281–5
92–5	conventional valvetrains improvement
properties and constituents of biogas,	and variable valve actuation (VVA)
biomethane and fossil natural gas,	usage, 327, 332–8
99	electronic controls and on-board
gasoline, 635–6	diagnostic (OBD) systems to
gasoline direct-injection (GDI) engine,	optimise engine performance,
159–60, 249	352–5
gasoline engine	engine air flow and turbocharging
cleanliness and performance of port fuel	systems, 288–94
injected, 186–7	engine downsizing, down-speeding and
average emission and IVD reduction,	down-breathing, 294–312
187	engine mechanical friction reduction
improvement in power and acceleration	technologies, 350–2
on cleaning up fuel system	exhaust gas recirculation (EGR)
deposits, 188	systems, 326–7

fuel injection systems and engine	waste heat recovery (WHR) systems,
performance, 280–1	347–50
future trends, 356–7	key issues and system design approaches,
heavy-duty diesel engine cooling and	225–70
thermal management systems,	challenges in advanced engine and
338–9	vehicle technologies, 251–64
mechanical and electrical	developments in light-duty diesel
supercharging systems for	engine technologies, 248–50
improved emissions control and	electric, hydraulic and flywheel hybrid
performance, 312–13	powertrains for improved fuel
natural gas engines development,	economy, 234–7
355–6	engine design to meet environmental
turbocompounding to improve engine	regulations, 244–8
performance, 313–26	usage of alternative fuels, 228–33
waste heat recovery (WHR) systems,	vehicle design to meet environmental
347–50	regulations, 242–4
heavy-duty Supertruck research program,	vehicle emissions and fuel economy
249–50	regulations, 237–42
heavy-duty vehicles	summary of next-generation technologies,
environmental performance improvement	264–70
of engines and particular	diesel engine technology improvement
technologies, 279–357	options, 267
advanced low-temperature combustion	North America on highway diesel
systems, 285–8	engine historical technology
aftertreatment technologies for	development, 265
emissions control, 339–46	heavy vehicles, 470–1
conventional combustion	helium, 125
technologies and engine	high efficiency durable gasoline engine
performance, 281–5	(HEDGE), 217
conventional valvetrains improvement	high frequency reciprocating rig (HFRR), 158
and variable valve actuation (VVA)	high-integrity casting, 454
usage, 327, 332–8	high pressure common rail (HPCR), 180
electronic controls and on-board	high-temperature thermo-chemical process,
diagnostic (OBD) systems to	698
optimise engine performance,	high voltage battery pack design, 616–21
352–5	environmental enclosure, 620–1
engine air flow and turbocharging	high voltage system, 617–18
systems, 288–94	summary, 621
engine downsizing, down-speeding and	thermal management system, 618–20
down-breathing, 294–312	Chevrolet Volt battery pack using
engine mechanical friction reduction	liquid cooling, Plate XVIII
technologies, 350–2	Chevrolet Volt cell assembly showing
exhaust gas recirculation (EGR)	liquid cooling plate, 619
systems, 326–7	Mitsubishi i-MIEV battery pack
fuel injection systems and engine	showing air cooling ducts, 620
performance, 280–1	Nissan Leaf battery pack without active
future trends, 356–7	cooling, Plate XVII
heavy-duty diesel engine cooling and	high voltage system, 617–18
thermal management systems,	contactors, 617
338–9	fuse, 618
mechanical and electrical	HVIL, 618
supercharging systems for	manual service disconnect (MSD), 618
improved emissions control and	pre-charge circuit, 617–18
performance, 312–13	home charging, 673
natural gas engines development,	homogenous charge compression ignition
355–6	(HCCI), 283–4
turbocompounding to improve engine	technology, 216
performance, 313–26	hot die forging, 387

hot stamping, 453	model at high level of abstraction for
hybrid battery charging mode, 637	hybrid vehicle propulsion system,
hybrid drive train technologies	573
vehicles, 567–79	HybridAir vehicle, 534
configurations and classification,	configuration of HHV system, Plate XII
569–72	hybridisation degree, 644–5

design challenges, 572–7	hydrated vegetable oil (HVO), 53–5
EU emission limits and GHG emission	hydraulic control, 407–8
targets, 568	hydraulic hybrid powertrains, 234–7
solutions to design problem, 577–9	component design and modelling, 511–20
hybrid electric drive trains	hydraulic-pneumatic accumulator,
architectures, 633–6	516–20
classification of HEVs, 634	pump and motors, 512–16
usage of gasoline or diesel engines,	design and control, 522–32
635–6	parallel hydraulic hybrid, 523–8
hybrid electric powertrains	series hydraulic hybrid, 528–32
flywheel, 499–500	pneumatic hybrid powertrains for
ETH-III powetrain, 500	improved fuel economy in vehicles,
series of configuration, 500	505–38
hybrid electric vehicle (HEV), 220, 234,	integrated hydraulic hybrid vehicle
586–7	simulation, 520–2
control, 648–51	pneumatic hybrids, 535–8

simulation for different control	practical applications examples, 532–4
strategies charge sustaining and	hydraulic regeneration system designed
charge depletion, 650–1	by Bosch Rexroth for integration,
control and performance, 647–53	533
auxiliary load, 652–3	hydraulic sub-system designed by
emission reduction, 652	Eaton for integration in parallel
performance, 651	hybrid powertrain, 534
conventional fuel, 632–54	schematic view of the Bosch Rexroth
architectures of hybrid electric drive	HRB, Plate XI
trains, 633–6	series of hydraulic architecture
basic components, 633	developed by EPA and Eaton, 534
future trends, 653–4	principle of operation and system
parallel hybrid electric drive trains,	architectures, 507–11
641–6	parallel hydraulic hybrid illustration,
series hybrid electric drive trains,	508
636–41	power split hydraulic hybrid
series-parallel hybrid electric drive	illustration, 510

trains and plug-in hybrids, 646–7	series of hydraulic hybrid illustration,
hybrid powertrains, 234	509
hybrid traction mode, 637	hydraulic hybrid vehicles (HHV), 235
hybrid vehicles	hydraulic launch assist (HLA), 533
configurations and classification, 569–72	hydraulic motors, 512–16
different hybrid drive train	hydraulic-pneumatic accumulator, 516–20
configurations, 570	schematic drawings of piston type
energy management strategy of plug-in	accumulator and bladder type, 517
hybrid vehicle, 572	thermodynamic model, 518–20
hybrid and electric vehicle	accumulator efficiency for full charging
classification, 571	and discharging event, 519
design challenges, 572–7	specific energy vs power density of
drive train topologies, 577	different energy storage systems,
energy balance for secondary power	520
source, 574	hydraulic pump, 512–16
energy-flows over whole drive cycle in	bent-axis type axial-piston, 513
propulsion system, 575	mechanical model, 514–16
interlinked design problem areas,	effect of pressure, relative displacement

576	and shaft speed, Plate V

predicted variation of axial-piston	hydrogenated vegetable oil (HVO), 166
motor volumetric and torque	hydrogenation, 39, 170
efficiency with displacement, 515	hydroprocessing, 168
predicted variation of axial-piston	hydrostatic regenerative braking system, 533
motor volumetric and torque	hydrostatic transmissions, 486–7
efficiency with speed, 516	illustration, 487
photograph of axial-piston installed in-line	hysteresis behaviour, 468
with propeller shaft, 513
radial piston pump with digitally	i-ELOOP, 653
controlled with solenoid valves,	ideal cycle analysis limitations, 213–14
Plate IV	ideal cycle efficiency, 205–6
swash-plate type and inline axial-piston, 512	ideal engine operation cycles, 198–206 crank slider mechanism and volume vs
hydraulic regeneration systems, 545–6	engine position, 198
hydraulic torque converter, 414–15	four-stroke cycle, 200
requirement comparison of torque	illustration, 201
converters, 415	ideal cycle analysis, 200, 202–6
hydrogen, 21, 108–9, 215, 685–731	constant pressure ideal heat addition,
cost, 697	203–4
economy, 13–14	constant volume ideal heat addition,
hydrogen specifications, 109	203
overview of production, 119–27	illustration of three ideal heat-addition
by-product and industrial hydrogen,	methods vs actual SI engine, 202
126–7	limited pressure ideal heat addition,
coal gasification, 121–2	204–6
electrolysis, 123–4	two-stroke cycle, 199–200
high-temperature conversion from	ideal frictionless torque, 514
nuclear energy, 124–6	ideal leak-free volumetric flow rate, 514
steam methane reformation, 119–20	idle reduction, 590–2
production, distribution, storage and use in	impact welding, 455
vehicles, 109–11	impulse forming, 454
storage, 693–4	in-wheel transmission
transportation and storage, 129–34	variation, 659–60
large-scale storage, 129–31	EV configurations of two-motor geared
small-scale storage, 131–3	and gearless transmission, 660
transportation, 133–4	high-speed inner-rotor topology and
volumetric fraction, 727	low-speed outer-rotor topology,
hydrogen energy, 693–8	660
current hydrogen distribution methods,	inclination point, 469
696–8	incremental cost, 509
hydrogen facts, production, storage and	induction drives, 662
usage, 693–6	inductive chargers, 676
density vs energy densities of various	Infinite Power Solutions, 627
fuels, 694	infinitely variable transmission (IVT), 486–7
hydrogen jet flame length, 724	injector nozzle coking, 178–82
nomogram, 725	deposits on the tip of light duty diesel
hydrogen oxygen reaction (HOR), 700	injector, 179
hydrogen-powered vehicles	power loss due to injector coking in high
car in garage scenario, 722–4	speed direct injection engine test
pressure peaking phenomenon for	B10, 181
release of hydrogen with mass flow	mineral base fuel, 181
rate, 723	SEM of deposits around injector nozzle
safety improvement, 719–29	hole, 179
current safety issues, 720	injector plugging, 187–90
hydrogen jet flame length, 724	innovative shift actuators, 397
separation distances, 724–9	intake valve closing (IVC), 285
some large-scale experimental results,	intake valve closing-variable valve actuation
721–2	(IVC-VVA), 252

intake valve deposits (IVD), 185, 189	ideal operation, 198–206
differences in forming tendencies of E-85	fuel additives, 165–92
fuel from two fuel grade ethanol	additive types and impact on
sources and commercial E-85, 190	conventional and advanced fuels,
variation in different ethanol grades, 190	166–75
integrated aftertreatment systems, 346	diesel performance and deposit control
integrated gasification combined cycle	additives, 178–84
(IGCC), 122	future trends, 192
integrated hydraulic hybrid vehicle simulation, 520–2	gasoline performance and deposit control additives, 184–92
SIMULINK illustration series, 521	impact on combustion characteristics,
intelligent transport systems (ITS), 8	175–8
inter-metallic titanium, 379	future trends, 157–62
internal combustion engine-based powertrain,	economy challenges, 157–8
493–7	future durability challenges, 160–1
Eindhoven concept, 495	future emissions challenges, 158–60
Imperial College concept of brake-only	new materials, 161–2
hybrid version and CVT-brake	key challenges and blending technologies,
hybrid version, 496	52–89
Torotrak design, 497	alcohol fuels, 55–65
internal combustion engine (ICE), 22, 400,	compression-ignition engines, 72–3
507–8, 633, 687–8	ethanol- and methanol-gasoline bi-fuel
advanced and conventional materials,	vehicles, 77–8
370–91	fatty acid methyl esters (FAMEs) and
advanced alloys and ceramics for	hydrated vegetable oil (HVO), 53–5
manufacturing valves and other	flexible-fuel vehicles, 73–6
components, 377–81	pollutant emissions, deposits and
advanced internal combustion engine	lubricant dilution, 69–72
materials and compact graphite iron	spark-ignition engines, 65–9
(CGI), 371–3	tri-flex-fuel vehicles and iso
advanced polymers and polyamides for	stoichiometric ternary blends,
manufacturing intake manifolds,	78–82
375–7	lubricating modern engines in developing
graphite/carbon and carbon/	markets, 152–7
carbon fibre-reinforced polymer	ethanol, 153–4
composites (CFRPs), 373, 375	methanol, 154–7 sulphur levels, 153
materials for particular components in IC engines, 381–91	materials for particular components,
advanced engine oils to improve	381–91
performance, 138–62	camshafts and cams, 389–90
composition of modern engine	connecting rods, 382–6
lubricant, 143–5	crankshafts, 381–2
diesel engine lubrication challenges,	cylinder blocks, 390–1
145–7	engine cylinder valves, 389
gasoline engine lubrication challenges,	piston pins, 388
147–51	piston rings, 388
industry and OEM specifications for	pistons, 386–7
engine oils, 151–2	tappets and pushrods, 390
comparison of engine cycle performance,	valve springs, 390
211–17	role of lubricant, 139–43
convergence of SI and Ci engines,	action of friction modifiers in engine
215–17	oils, 141
fuel type impact, 214–15	contribution to fuel economy, 140–2
limitations, 211–14	engine durability, 139
concepts, 197–221	HTHS viscosity vs M111 test fuel
advantages and limitations, 217–18	economy, 140
alternative operation, 206–11	maintenance of low level emissions,
future trends, 219–21	142–3

internal combustion engine vehicle (ICEV),	limited pressure ideal heat addition, 204–6
112, 680, 706–7	ideal cycle analysis efficiency as function
International Fire Code (IFC), 727–8	of compression ratio, 205
International Lubricant Standardisation and	line pressure control valve, 408
Approval Committee (ILSAC),	liquid cooling, 619
151–2	liquid fuels
ISO 2011, 623	biomass limit, 36–7
ISO/IEC Guide 73:2002, 721	improve environmental performance of
iso-stoichiometric ternary blends, 52, 78–82	internal combustion engines, 52–89
isolation monitoring, 623	alcohol fuels, 55–65
	compression-ignition engines, 72–3
Japanese emissions standards, 241–2	ethanol- and methanol-gasoline bi-fuel
joining technologies, 455–6	vehicles, 77–8
	fatty acid methyl esters (FAMEs) and
kinetic energy recovery system (KERS), 480	hydrated vegetable oil (HVO), 53–5
	flexible-fuel vehicles, 73–6
ladder frame, 448–9	pollutant emissions, deposits and
landfill gas (LFG), 98	lubricant dilution, 69–72
	spark-ignition engines, 65–9
Latin-Hypercube Sampling (LHS), 578
lead acid (PbA), 602–4	tri-flex-fuel vehicles and iso
battery, 667	stoichiometric ternary blends,
lean burn natural gas engines, 355–6	78–82
levelised cost of energy (LCOE), 41	market penetration of alcohol fuels, 27–36
life cycle analysis, 652	alternative fuel consumption in the
gaseous fuels, 111–13	US, 30
ratios of petroleum fuel use and GHG	Brazil, 28
emissions, 113	China, 35–6
schematic supply chains of gas-based	EU, 33–5
alternative fuel pathways, 111	USA, 28–33
life cycle cost analysis, 442	market penetration of biodiesel, 26–7
life cycle economic benefit mass reduction,	renewable fuels within an integrated
439	renewable energy system, 40–3
lifted-flame combustion (LTC), 283–4	role in environmentally sustainable
light alloys, 454	transport, 19–51
light-duty diesel engine technologies, 248–50	cost comparison of alternative energy
light vehicle brake systems, 546	vehicles, 23
light weight vehicle structures manufacture	environmental benefits, 24–6
general challenges and opportunities,	net system volumetric and gravimetric
443–6	energy densities, 20
affordability and manufacturability,	on-board energy density, 20–2
446	vehicle cost, 22–4
crashworthiness, 444–5	well-to-wheel CO2 emissions, 25
environmental resistance, 445	liquid hydrogen, 694
stiffness, 443–4	liquid natural gas (LNG), 355
strength, 444	liquid petroleum gas (LPG), 106
possible architectures of next-generation	distribution, 94–5
vehicle, 446–52	production, distribution, storage and use in
backbone chassis, 450–1	vehicles, 106–8
ladder frame, 448–9	liquid injection systems, 108
monocoque, 451–2	LPG vehicles and fuel delivery
multi-material construction, 452	systems, 107
space frame and truss frame, 449–50	vapour pressure systems, 107–8
unibody construction, 447–8	selected properties, 107
sustainable design, 433–59	lithium-air batteries, 559–60
future trends, 456–9	lithium ion battery, 667–8
specific lightweighting technologies,	lithium ion cells, 607, 610–16
452–6	active materials, 612
value of mass reduction, 435–42	cell life, 615–16

construction, 610	Mechanical Kinetic Energy Recovery System
illustration, 613	(M-KERS), 544
cost, 609	mechanical model, 514–16
safety, 614–15	mechanical regeneration properties, 544–5
separator and electrolyte, 613–14	mechanical supercharging systems, 312–13
lithium ion separator, 613	mechanical transmissions, 491
tri-layer lithium ion separator cross	membrane electrode assembly (MEA), 700
section, 614	membrane separation, 103
summary, 616	metal hydride, 21, 132–3, 694
lithium-ion (Li-ion) batteries, 687–8, 690–1	metal organic frameworks, 694
Lotus Engineering, 452–3	metals, 695
low carbon fuel, 231–2	methanol, 29, 35, 37–8, 68–9, 134, 154–7
standard, 24	micro coking test results, 156
low-carbon vehicle technologies	oil pickup from TGDI engined car, 155
current status, 705–7	methanol-to-gasoline (MTG) process, 42
advantages of HEVs, 707	methyl-tert-butyl ether (MTBE), 28
conventional internal combustion	microcontroller software, 406
engine (ICE) vehicles, 706–7	Miller cycle, 206–7, 297
low flammability limit (LFL), 729	model-based controls (MBC), 353
low-loss bearings, 483–4	model fidelity, 522
low viscosity lubricant, 351	modulated kinetics (MK), 285
lubricant, 151	module, 583
composition, 143–5	molybdenum dithiocarbamate, 157–8
role in internal combustion engine, 139–43	monocoque, 451–2
lubricity improvers, 170–1	motored octane number (MON), 209–10
effect of palm oil fatty acid biodiesel, 171	moving bed gasifier, 122
mono-acid lubricity vs non-acidic lubricity	multi-material construction, 452
additives, 171
	Nanoslide, 161
manual program activated via one-touch function, 413–14	National Electro-Mobility Development Plan, 118
comparison of acceleration process in	National Fire Protection Association (NFPA),
S, C and M modes and kick down,	722
414	National Highway Traffic Safety
manual service disconnect (MSD), 618	Administration (NHTSA), 238
manual transmission	natural gas, 697
advantages, 396	engines and vehicles, 95–8
six-speed front-wheel-drive SG6-310,	compression-ignition dual-fuel engines,
395–6, Plate II	97
manufacturability, 446	off-road vehicles, 97
material costs, 439–40	on board fuel storage, 97–8
materials maturation, 458–9	spark-ignition lean burn engines, 96
Matlab-SIMULINK, 520	spark-ignition stoichiometric engines,
M102E, 185, 186	96
mechanical coupling see parallel hybrid	engines development, 355–6
electric drive trains	natural gas vehicle (NGV), 232
mechanical flywheel hybrid technology	natural rubber, 473
electrical flywheel hybrid technology to	negative electrodes, 626
store energy in vehicles, 476–502	Nelder–Mead simplex algorithm, 578–9
flywheel technology development,	neodymium catalysed (Nd-BR), 473
479–80	new European driving cycle (NEDC), 311,
future trends, 501–2	398–9, 572–3
performance evaluation, 493–501	new generation of compact cars (NGCCs), 396
technical challenges in flywheel	next-generation vehicle
development, 501	backbone chassis, 450–1
transmission for flywheels, 486–93	schematic diagram of backbone
types and properties of flywheels,	structure that is used in Lotus Elan, 450
480–6

uni-chassis design, 451	oxygen, 615
ladder frame, 448–9	oxygen reduction reaction (ORR), 700
schematic drawing, 449
monocoque, 451–2	P2 hybrid transmission, 418–22
schematic example, 452	advantages, 422
multi-material construction, 452	technical concept and design, 419–21
possible architectures, 446–52	expanded view of NAG2, 421
space frame and truss frame, 449–50	hybrid power train for inline
typical configuration of welded tubular	applications at Mercedes-Benz,
space frame to create small racing	420
vehicle, 450	NAG2-P2 power head with E-motor,
unibody construction, 447–8	wet clutch, torsion damper and
schematic drawing of construction of	positioning sensor, 420
stamped and spot welded, 447	technical data, 421
nickel metal hydride (NiMH), 604–6, 608–9	transmission hybrid functions, 422
battery, 667, 687–90	power flow through power head
NIMONIC 80A alloy, 377–8	E-motor stator, 423
Nissan Leaf, 220	pack cost, 609
nitrogen oxides, 96	paraffins, 168
nitrous oxide (N2O) emissions, 344–5	parallel hybrid configurations, 569–70
nodular graphite particles, 371–2	parallel hybrid electric drive trains, 641–6
noise, vibration and harshness (NVH), 561	configuration, 641
noise reduction, 582	design principles, 643
nuclear energy, 124–6	engine power design, 643–4
Nuclear Waste Policy Act (1982), 126	transmission design, 644–6
	parallel hydraulic hybrid vehicle (P-HHV),
octane improving additives, 178	508–9, 523–8
octane numbers, 60–5	design and control, 523–8
mixtures of alcohols with gasoline, 64	methodology for sequential
properties of gasoline, methanol, ethanol	optimisation of design and power
and iso-butanol, 61	management, 524
variation of RON with ethanol	schematic configuration, Plate VI
concentration for various gasoline	supervisory control methods and
blendstocks, 63	applications to P-HHV, 525–8
off-road vehicles, 97	park-by-wire (P-B-W) system, 398
on-board diagnostic (OBD) systems, 79, 241,	particulate matter (PM), 71, 218
	control, 340–2
354–5 electronic controls to optimise engine	passenger car, 468–70
performance, 352–5	passenger car driving cycles, 584–97
model-based controls and predictive	fuel displacement by plugging-in, 596–7
dynamic on-board controller,	average propulsion energy required
352–4	per km by drive cycle and vehicle
on-road diesel engines, 266	size, 597
operability additives, 169	idle reduction, 587, 590–2
operation strategy, 429–30	illustration of idle time during US FTP
opportunity charging stations, 673–4	72 driving cycle, 591
optimal control theory, 578	start stop matrices for US and select
	international driving cycles, 590
organic Rankine cycle (ORC), 347–8	separation of engine response from driver
original equipment manufacturers (OEMs),	demand, 592–6
140	per cent of US 06 completed versus
specifications for engine oils, 151–2
over-expanded cycle, 206–7	power delivered to road, 593
Atkinson cycle, 206	per cent of US FTP 72 completed
Miller cycle, 206–7	versus power delivered to road,
overcharge avoidance, 622	593
Ovonics, 689	maximum energy content of single
oxidation, 141	braking event by drive cycle and
reduction, 340	vehicle size, 595

peak acceleration power requirements	examples and key findings, 537–8
by drive cycle and vehicle size, 594	schematic of two-cylinder pneumatic
peak braking power requirements by	hybrid engine with turbocharging
drive cycle and vehicle size, 594	and intercooling, 538
theoretical braking energy available by	principle of operation, 536–7
vehicle speed and vehicle size, 596	cross section through cylinder head
summary of key statistics of common	illustrating a pneumatic hybrid
global driving cycles, 585	engine concept, 536
US 06 driving cycle, 587	illustrates possibility of using
US Federal test procedure (FTP) 72	compressed air in tank for
driving cycle, 586	supercharging, 536
US highway driving cycle, 586	schematic representation of the valve
USABC battery goals, 588–9	events, Plate XIII valve events associated with various
passive balancing, 624 peak power sources (PPS), 710	pneumatic hybrid engine modes,
Peltier cooling, 620	536
performance fuel additives, 165	schematic illustration of concept, 535
performance metrics, 601–2	polyamides, 375–7
peripheral velocity, 482	polyether amine (PEA), 192
permanent magnet (PM) brushless drives,	polynuclear aromatic hydrocarbons (PAH),
663	339–40
permanent magnetic linear contactless	Pontryagin's Maximum Principle, 578
displacement (PLCD) sensor,	port fuel system, 64
405–6	positive electrodes, 626
Peugeot XUD-9 engine, 180	positive ions, 599–600
phosphorus, 142–3	post New Long Term Emission (PNLT),
photoelectrochemical hydrogen production,	241–2
698	pour point depressants, 144, 168–9
physical sensor, 75	power absorption rates, 558
pilot injection, 280–1	power circulation, 492
pipeline corrosion inhibitors, 172–3	power density
corrosion protection, 173	energy density, 716–18
piston pins, 388	petrol vs hydrogen vs electrical storage
piston rings, 388	systems in four leading vehicles,
pistons, 375, 386–7	717
planetary gear set (PGS), 491	power distribution box (PDB), 427–8
plastic air intake manifolds (PAIM), 376	power electronics box (PEB), 427–8
plastics, 4	power-split hydraulic hybrid vehicle (PS
platinum group metal (PGM), 703	HHV), 511
platinum (Pt), 701	power split principle, 491–3
plug-in hybrid electric vehicle (PHEV), 118,	different PS CVT configurations, 492
220, 571–2, 609, 650–1 plug-in hybrids, 646–7	general PS CVT, 492 power steering pump, 429
plug-in vehicle, 596	power system components, 427–9
plugging-in, 596–7	hybrid powertrain with essential
pneumatic hybrid powertrains	components, 428
hydraulic hybrid powertrains for improved	powering auxiliary systems, 235
fuel economy in vehicles, 505–38	powertrain design, 547–8
design and control, 522–32	traditional powertrain design criteria, 547
hydraulic component design and	powertrain technology
modelling, 511–20	low carbon targets, 5–7
integrated hydraulic hybrid vehicle	timeline based on a D-class car, 6
simulation, 520–2	pre-charge circuit, 617–18
pneumatic hybrids, 535–8	premixed charge compression ignition (PCCI),
practical applications examples, 532–4	216–17, 283–4
principle of operation and system	pressure control, 473–4
architectures, 507–11	pressure peaking phenomenon, 724
pneumatic hybrids, 535–8	pressure relief devices (PRD), 722

pressure swing adsorption (PSA), 103,	electrical regeneration properties
119–20	four-wheel drive hybrid electric vehicle
primary reference paraffins, 62	in regenerative mode, 543
ProAlcool, 28	energy recovery types and properties,
process maturation, 458–9	542–7
PROCONVE P5, 242	composed of only two basic and
PROCONVE P6, 242	interconnected elements, 542
PROCONVE P7, 242	electrical regeneration properties,
proton exchange membrane fuel cell	543–4
(PEMFC), 22, 110, 697, 699–700	fluid-based regeneration properties, 545–6
proton exchange membrane (PEM)	hydraulic accumulators are used for
electrolyser, 123–4	storage of energy in fluid form, 545
fuel cell stack, 710	fraction of regenerative braking used
pseudo-capacitors, 704	varies with speed and deceleration
public charging, 673–5	rate, Plate XVI
battery swapping stations, 674–5	governance and regulation, 546–7
fast charging stations, 674	integration and performance and energy
opportunity charging stations, 673–4 pumping loss, 294	recovery technologies in vehicles, 541–62
pure electric traction mode, 637	advantages and limitations, 557–8
pure electric vehicles, 655–83	design integration and operational
benefits and limitations of EVs, 680–2	optimisation, 550–7
energy benefits, 680	future trends, 559–61
environmental benefits, 680–2	hybridisation design and performance
limitations, 682	issues, 547–50
charging infrastructure, 673–7	mechanical regeneration properties, 544–5
electric propulsion, 661–6	energy storage involves physical
energy storage and management, 666–72	connection between flywheel and
future trends, 682–3	driven wheels, 544
system configurations, 656–60	regenerative braking mode, 637
illustration, 656	regenerative systems, 553–4
variations in electric propulsion,	Regulation 13H, 546
658–9	Reid vapour pressure (RVP), 57, 58–9
variations in energy storage, 657–8	renewable diesel, 170
variations in in-wheel transmission, 659–60	Renewable Energy Directive (RED), 24, 33–4
vehicle-to-grid (V2G) technology,	renewable energy system, 40–3
677–80	integrated power, heat and transport
pure engine traction mode, 637	system, 42
pure flywheel-based powertrain, 493	Renewable Fuel Standard, 24, 29
Gyrobus, 494	fuel volume requirements for RFS2, 31
pushrods, 390	renewable natural gas (RNG), 232
	renewable power methane, 41
Ragone plot, 705	Research and Development Project, 458
ramped mode cycle (RMC), 313	research octane number (RON), 60, 68, 81,
range extended electrical vehicle, 641	209–10
Rankine cycle, 270	reservoir design, 517
rapeseed feedstock, 27	rolling element bearings, 484
rapeseed methyl ester (RME), 146–7	rolling resistance, 5, 466–8
Ravigneaux planetary gear sets, 399–400	rolling speed, 467
reactivity controlled compression ignition	rolling traction drives, 488–90
(RCCI), 216–17, 285	room temperature ionic liquids (RTIL), 705
rear spoiler, 460	rotatable liner design, 501
recyclability, 437	rotor design
recycling, 437	materials for maximum ratio of stored
redox2 shuttle, 627	energy to weight, 481–3
reforming, 109	different materials vs two basic
regenerative braking, 550–1, 592	flywheel shapes, 482

rotor failure	technical concept and design, 399–403
safety, 484–5	seven-speed automatic transmission of
laminated steel flywheel designed at	Mercedes-Benz 7G-Tronic Plus,
City University, 485	399
round trip efficiency, 519	torque converter, 401–3
rubber v-belt, 487–8	transmission components, 399–400
rule-based control, 529–31	technical data of automatic
strategy, 578–9	transmission, 400
	transmission control, 403–10
S-I cycle, 125–6	electro-hydraulic control unit, 404
safety regulations, 8	shaping technologies, 453–5
scarcity, 693	six-speed front-wheel-drive SG6-310, 395–6
Scuderi split cycle, 207–8	16-speed G260-16, 431–2
selective catalytic reduction (SCR), 238–9	advantages, 431–2
systems, 159, 342–3	technical concept and design, 431
self-piercing rivets, 455–6	automated commercial vehicle
sensors, 405–6	transmission, 432
	skid control, 554–5
separation distances, 724–9
separation from unignited release, 726–8	smooth drum, 466–7
nomogram for concentration decay	sodium carbonate, 120
calculation in unignited jets, 726	sodium carboxylate, 182, 183
three separation distances from jet fire,	sodium nickel chloride, 606–7
728–9	software, 409–10
measured axial temperature as function	solar energy, 13–14
of distance expressed in flame	solenoids, 406–7
calibres, 728	solid oxide electrolyser cells (SOECs), 124
separator, 613–14	solid state batteries, 627
series hybrid configurations, 569	soluble organic fraction (SOF), 339–40
series hybrid electric drive trains, 636–41	soot, 218
configuration, 636	soy methyl ester (SME), 169
design principles, 638–41	space frame, 449–50
series hydraulic hybrid vehicle (S-HHV), 510	spark-ignition lean burn engines, 96
advanced algorithms for control of series	spark ignition (SI) engines, 209–10
hydraulic hybrid, 531–2	spark-ignition stoichiometric engines, 96
engine visitation points on the BSFC	specific energy values, 519
map, Plate IX	specific lightweighting technologies, 452–6
packaging of the series hydraulic	joining, 455–6
hybrid system in the passenger car,	conformal joining, 456
Plate X	friction stir welding, 455
stochastic dynamic programming	impact welding, 455
framework, Plate VIII	self-piercing rivets, 455–6
design and control, 528–32	structural adhesives, 456
schematic configuration, Plate VII	shaping and fabricating, 453–5
rule-based control, 529–31	additive manufacture, 454
schematic illustration of thermostatic	beam structures, 454–5
SOC power management concept,	high-integrity casting of light alloys,
530	454
vehicle fuel improvements as function	hot stamping, 453
of S-HHV threshold charging	impulse forming, 454
power, 531	warm forming, 453
series-parallel hybrid electric drive trains,	speed control, 662
646–7	speed coupling, 642
servo module, 410	split cycle engines, 207–8
seven-speed front-wheel-drive 7G-DCT,	Scuderi split cycle, 207–8
396–9	Stirling split cycle, 208
seven-speed 7G-Tronic Plus, 399–411	stabilisers, 166–8
mechanics, 400–1	stagnation point, 469
layout and shift pattern, 401	standard temperature and pressure (STP), 695

start/stop technology, 603–4	diesel engine subsystem interactions
state of charge (SOC), 510–11, 624–5, 639–40	at peak torque and system
indicators, 671	optimisation, 257–8
state of health (SOH) algorithm, 624–5	diesel engine subsystem interactions
steam methane reforming (SMR), 109, 119–20	at rated power and system
steels, 381	optimisation, 259–61
stiffness, 443–4	effect of air–fuel ratio on engine delta,
Stirling split cycle, 208	262
stochastic dynamic programming (SDP),	engine air system capabilities at peak
531–2	torque, 253
stoichiometric combustion, 284–5	engine air system capability simulation
stoichiometric conditions, 218	at rated power, 254
stoichiometry	four quadrants of engine system and
alcohol fuels, 56–7	need to balance all trade-offs, 256
air–fuel ratio of blends of alcohol	simulation models and EPSI analysis in
gasoline blends, 58	diesel system design, 255
stranded energy, 43	environmental performance improvement
strength, 444	of heavy-duty vehicles and engines
structural adhesives, 456	key issues, 225–70
styrene butadiene rubber (SBR), 472	developments in light-duty diesel
sulphated ash, 142	engine technologies, 248–50
section of ash-blocked DPF, 143	electric, hydraulic and flywheel hybrid
sulphur, 153	powertrains for improved fuel
supercapacitors, 560	economy, 234–7
SuperTruck research program, 266	engine design to meet environmental
supervisory control methods	regulations, 244–8
applications to P-HHV, 525–8	summary of next-generation
definition of hybrid powertrain control	technologies, 264–70
rules on engine BSFC map, 526	usage of alternative fuels, 228–33

parallel HHV system behaviour with	vehicle design to meet environmental
different power management	regulations, 242–4
strategies, 527	vehicle emissions and fuel economy
Supplemental Emissions Test (SET), 239–40	regulations, 237–42
surface adsorption, 133	systems engineering approach, 252
sustainable design
light weight vehicle structures	tappets, 390
manufacture, 433–59	technical path selection, 252
future trends, 456–9	technology roadmaps, 2
general challenges and opportunities,	schematic diagram, 3
443–6	temperature control, 623
possible architectures of next	stage, 342
generation vehicle, 446–52	Tesla Roadster, 220
specific lightweighting technologies,	thermal management methods, 343
452–6	thermal management system, 338–9, 618–20
value of mass reduction, 435–42	thermal pressure relief device (TPRD), 720
sustainable mobility, 118	thermal time-constant formula, 519
sustainable organic fuels for transport (SOFT),	thermodynamic cycle, 350
37–40	thermodynamic model, 518–20
evolution of ethanol production by	thiadiazole, 173
country, 31	thought leadership, 458
fuel synthesis, 39–40	three-way catalyst (TWC), 284–5
recycling CO2, 38–9	throttling, 212–13
switched reluctance (SR), 661	thrust link belt, 415
drives, 662–3	tidal power, 128
syngas, 121	tire pressure monitoring, 473
system design approaches	tire structure, 471
challenges in advanced engine and vehicle	tires, 468
technologies, 251–64	titanium alloys, 384–5

titanium aluminide, 378–9	stoichiometric ternary blend, 78–82
top dead centre (TDC), 198–9, 286	blend proportions of gasoline, ethanol
torque converter, 401–3	and methanol, 79
centrifugation pendulum damper, 403	response of production of alcohol fuel
illustration, 402	sensor to ethanol and methanol, 80
torque coupling, 642	triactive force, 552
torque-speed coupling, 642	tribological design options, 351
total base number, 157	truss frame, 449–50
total vehicle freight efficiency, 270	tube trailers, 110, 134
traction continuously variable transmissions,	turbine design, 293–4
487–90	turbocharger, 148
belt drives, 487–8	efficiency, 290
Kumm belt design, 489	turbocharging gasoline direct injection
V-belt CVT, 488	(TGDI), 149–50, 155
rolling traction drives, 488–90	turbocharging systems, 288–94
Milner CVT, 490	turbocompounding
Torotrak design, 489	improve engine performance, 313–26
trade-offs, 251	engine performance simulation of
traffic management, 8	electrical turbocompounding, 322
	engine performance simulation of
trailing edge air, 471
transfer-case gear ratio, 523	mechanical turbocompounding,
transmission components, 414–17	317–18, 321
downstream DNR set, 415–16	mechanical and electrical
dual-stage vane pump, 416–17	turbocompounding using power and
illustration, 416	turbocharger turbine, 314–16
hydraulic torque converter, 414–15	sensitivity simulation of BSFC type 1
thrust link belt, 415	mechanical vs type 3 electrical,
variator, 415	319
transmission control, 403–10	summary of steady-state performance
electric oil pump, 410	on BSFC reduction benefit, 325
electric set and transmission control unit,	two-mode hybrid transmission advanced
404–5	hybrid system-cars (AHS-C),
hydraulic control, 407–8	422–31
clutch control schematic, 408	advantages, 430–1
sensors, 405–6	operation strategy and fuel consumption,
electro-hydraulic control unit with	429–30
speed sensors and target wheels,	displayed on COMAND system, 430
405	technical concept and design, 422, 424–7
servo module for shift by wire, 410	clutch control mechanism, 426
software, 409–10	illustration, 425
possible direct gear shifts, 409	transmission components, 427–9
solenoids, 406–7	e-motors, 427
illustration, 407	power system components, 427–9
transmission control unit (TCU), 404–5	two-stroke cycle, 199–200
transmission pipeline, 94
transport sector, 686–7	ultra-high-speed flywheels, 669
Transportation Research Board, 472	ultra light steel auto body (ULSAB), 447–8
transportation vehicles	ultra low sulphur diesel (ULSD) fuel, 146–7,
electricity and hydrogen as energy vectors,	218, 240–1
117–35	ultracapacitors, 654, 668
hydrogen storage and transportation,	ultrahigh-speed fly wheel, 658
129–34	ultramid, 376–7
overview of electricity production,	unburned hydrocarbon emissions (uHC), 70
127–9	unburnt hydrocarbons (UHC), 218
overview of hydrogen production,	underground hydrogen storage, 130–1
119–27	unibody construction, 447–8
tread design, 468	United Nations Economic Commission for
tri-flex-fuel vehicle, 66	Europe (UNECE), 546

United States Advanced Battery Consortium	dual-clutch transmission of seven
(USABC), 584–5	speed front-wheel-drive 7G-DCT,
United States Council for Automotive	396–9
Research (USCAR), 584–5	manual transmission of six-speed front
up-rating, 297–9	wheel-drive SG6-310, 395–6
upstream fuel injection system, 64	P2 hybrid transmission, 418–22
US 2014-2017 GHG regulations, 242–3	two-mode hybrid transmission
US 2010 emission regulations, 244	advanced hybrid system-cars
US Department of Energy, 228	(AHS-C), 422–31
US Environmental Protection Agency (EPA),	vehicle rolling resistance improvement
533–4	aerodynamics, 462–74
	advanced tire design and materials,
vacuum system, 483	471–4
value of mass reduction, 435–42	advanced vehicle design for drag
embodied energy and cost of several	reduction, 468–71
important light structural materials,	development of frontal area, 465
436	development of resistance as
logarithmic scale of cost per mass, 441	combination of frontal area and
marginal mass-based energy cost and CO2	aerodynamic coefficient, 465
production, 438	development of resistance coefficient
plot of various vehicle types in terms of	based on publicly available values,
approximate marginal costs, 440	464
valve breakages, 379	effect of 10% of mass, aerodynamic
valve springs, 390	resistance and rolling resistance,
valvetrain system design, 332	463
vapour pressure	effect of speed on influence of mass,
alcohol fuels, 57–60	aerodynamic resistance and rolling
variation in methanol, ethanol and	resistance, 463
iso-butanol with alcohol volume	future trends, 474
fraction, 59	overview, 464–6
vapour pressure systems	rolling resistance in vehicles, 466–8
liquid petroleum gas (LPG), 107–8	vehicle technology
variable geometry turbine (VGT), 252, 299	affordability and consumer appeal, 12–13
variable-power delivery options, 351	commercial vehicles and buses, 9–10
variable valve actuation (VVA), 327, 332–8	electrification of transport technology, 10
variable valve timing (VVT), 219–20	Nissan Leaf, 11
variable-voltage variable-frequency (VVVF),	Vauxhall Ampera, 11
662	future trends, 10–12
variator, 415	hybrid market demand, 12 global manufacturing and consumer trends,
vegetable oils, 170 vehicle costs, 440–1	8–9
vehicle crash, 621	low carbon targets, 2–5
assessment, 485	Ford Focus active grille shutter, Plate I
vehicle electrification, 582	tailor rolled boron steel B-pillar, 4
vehicle emissions, 237–42	regulatory requirements and consumer
vehicle length, 470	trends, 7–8
vehicle load, 467	solar energy/hydrogen economy, 13–14
vehicle performance improvement	vehicle-to-grid (V2G) technology, 677–80
advanced transmission technologies,	application, 679–80
393–432	load levelling and load shaving, 679
automated commercial vehicle	concept, 677–8
transmission of 16-speed G260-16,	operation, 678–9
431–2	aggregated dual-grid V2G framework,
automatic transmission of seven-speed	678
7G-Tronic Plus, 399–411	vehicles
continuously variable transmission	hybrid drive train technologies, 567–79
of front-wheel-drive CVT	configurations and classification,

569–72

AUTOTRONIC, 411–18

design challenges, 572–7	viscosity modifiers (VMs), 144
solutions to design problem, 577–9	voltage monitoring, 621
hydraulic and pneumatic hybrid	volumetric energy density
powertrains for fuel economy	alcohol fuels, 56–7
improvement, 505–38	variation of alcohol-gasoline blends, 57
design and control, 522–32	vortices, 469
hydraulic component design and
modelling, 511–20	Wankel engine, 208
integrated hydraulic hybrid vehicle	warm forming, 453
simulation, 520–2	warm-up stage, 341–2
pneumatic hybrids, 535–8	waste heat recovery (WHR) systems, 347–50
practical applications examples, 532–4	illustration of heavy-duty diesel
principle of operation and system	engine energy distribution by
architectures, 507–11	thermodynamic first law analysis,
integration and performance of	349
regenerative braking and energy	illustration of organic Rankine cycle
recovery technologies, 541–62	schematic, 347
advantages and limitations, 557–8	waste heat recovery (WHR) technology, 235
design integration and operational	wastegate, 289–90
optimisation, 550–7	wastegating elimination variable valve
future trends, 559–61	actuation (WE VVA), 335–6
hybridisation design and performance	wastewater treatment (WWT) plants, 99, 112
issues, 547–50	water gas shift reactor, 121
types and properties, 542–7	wave power, 128
mechanical and electrical flywheel	wax antisettling additives (WASA), 170
hybrid technology to store energy,	Weibull modulus, 378
476–502	weight reduction, 3
flywheel technology development,	welding cast, 391
479–80	well-to-tank (WTT), 719
future trends, 501–2	well-to-wheel (WTW)
performance evaluation, 493–501	analysis, 652
technical challenges in flywheel	pathway, 719
development, 501	Williams Hybrid Power Flywheel, 545
transmission for flywheels, 486–93	Wilson's P/M theory, 514
types and properties of flywheels,	wind power, 128
480–6	wireless chargers, 676–7
VEHLIB, 548	wireless power transfer technique, 677
very high temperature reactor (VHTR), 125	zinc/air battery, 667
virtual sensors, 74 viscosity index, 156	zinc dialkyldithiophosphate (ZDDP), 142, 144

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British Library Cataloguing-in-Publication Data

Contents

Editor and Chapter 1

Chapters 2 and 3

Chapter 4

Chapter 5

Chapter 6

Chapter 7

Chapter 8

Chapters 9 and 10

Chapter 11

Chapter 12

Chapter 13

Chapter 14

Chapter 15

Chapter 16

Chapter 17

Chapter 18

Chapter 19

Chapter 20

Chapter 21

Chapter 22

Woodhead Publishing Series in Energy

R. Folkson, University of Hertfordshire, UK

1.1 Introduction

1.2 Technology roadmaps to deliver low carbon targets

1.3 Vehicle technology contributions to low carbon targets

1.4 Powertrain technology contributions to low carbon targets

1.5 Regulatory requirements and consumer trends

1.6 Traffic management factors

1.7 Global manufacturing and consumer trends

1.8 Commercial vehicles and buses

1.9 Electrification of transport technology

1.10 Current and future trends

1.11 Affordability and consumer appeal

1.12 Long-term vision: solar energy/hydrogen economy

1.13 Conclusion

1.14 Sources of further information and advice

1.15 Acknowledgements

1.16 References and further reading

Part I

The role of alternative and renewable liquid fuels in environmentally sustainable transport

2.1 Introduction: competing fuels and energy carriers

2.1.1 On-board energy density

2.1.2 Vehicle cost

2.1.3 Environmental benefits

2.2 Market penetration of biodiesel

2.3 Market penetration of alcohol fuels

2.3.1 Brazil

2.3.2 USA

2.3.3 EU

2.3.4 China

2.4 Future provision of alternative liquid fuels: the biomass limit

2.5 Beyond the biomass limit: sustainable organic fuels for transport (SOFT)

2.5.1 Recycling CO2

2.5.2 Fuel synthesis

2.6 Renewable fuels within an integrated renewable energy system

2.7 Conclusions

2.8 Acknowledgements

2.9 References

2.10 Appendix: abbreviations

3.1 Introduction

3.2 The use of biodiesel in internal combustion engines: fatty acid methyl esters (FAMEs) and hydrogenated vegetable oil (HVO)

3.3 Alcohol fuels: physico-chemical properties

3.3.1 Volumetric energy density and stoichiometry

3.3.2 Vapour pressure

3.3.3 Octane numbers

3.4 Alcohol fuels for spark-ignition engines: effects on performance and efficiency

3.4.1 Performance

3.4.2 Efficiency

3.4.3 The efficiency of dedicated alcohol engines

3.5 Alcohol fuels for spark-ignition engines: pollutant emissions, deposits and lubricant dilution

3.6 Alcohol fuels for compression-ignition engines

3.7 Vehicle and blending technologies for alternative liquid fuels: flexible-fuel vehicles

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