F. BADIN, Ed.
IFP Energies nouvelles
Hybrid Vehicles From Components to System
Prefac by
Olivier APPERT,
IFP Energies nouvelles Chairman and CEO
Translated from the French
by Robert Bononno and Trevor Jones (Lionbridge)
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Preface
Carriage of persons and goods depends almost exclusively on conventional fuels from petro-
leum origin. In a context of transition towards a sustainable energy system, electrification of
vehicles offers the twofold advantage of helping to reduce consumptions and of contributing
to the necessary effort to reduce greenhouse gases in the fight against climate change. In this
respect, hybridization of drivetrains represents a promising technological pathway and forms
the subject of major technological research and development studies.
By permanently offering the possibility of choosing in real time the drive mode most
adapted to traffic conditions, hybridization of vehicles plays a direct role in improving the
energy efficiency in transportation by optimizing the energy efficiency of the entire pow-
ertrain. It also offers the possibility of recovering some of the energy previously dissipated
during the braking phases and, more generally, optimizing the onboard energy management.
Furthermore, thanks to the availability of electrical power, combining electrification with
an internal combustion engine provides the possibility of installing new technologies in the
engine that significantly improve its performance.
Lastly, the development of plug-in hybrid vehicles helps reduce greenhouse gas emis-
sions, provided however that electricity is produced with reduced C 0 2 emissions.
Relying on the experience acquired in conventional drivetrains, IFP Energies nouvelles
is developing the technological bricks and setting up the methodologies which will allow car
manufacturers to commercialize the hybrid vehicle - or rather vehicles - of the future. Vari-
ous levels of hybridization are in fact possible (stop&start, energy recovery during braking,
engine assist, optimization of onboard energy management, all-electric mode, plug-in, etc.)
depending on the vehicle segment, use conditions and required performance.
Amongst these options, IFP Energies nouvelles decided to focus on the plug-in hybrid
vehicle, an approach which offers potential energy consumption savings of up to 50% over
conventional vehicles. More generally, the research studies conducted by IFP Energies nou-
velles concern optimization of the energy on board a vehicle or a fleet of vehicles. The aim, in
particular, is to consider the interaction between the vehicle and its environment through the
use of information and communication technologies. The main research axes of IFP Ener-
gies nouvelles therefore include the architecture of new hybrid drivetrains and their integra-
tion within the vehicle, numerical simulation and dimensioning of the various components
and their management, and supervision of the onboard energy.
All car manufacturers are now involved in vehicle hybridization. This book therefore
comes at the right time, proposing an update of the knowledge available in this fast-devel-
oping field. It also emphasizes the efforts still required if we are to take up the challenge of
manufacturing increasingly clean, energy-efficient and economically viable vehicles.
Olivier Appert, Chaiman and CEO
IFP Energies nouvelles
Table of Contents
Preface V
Acknowledgements VII
List of Authors XVII
Abbreviations XIX
Introduction XXV
Chapitre 1 1
2
Vehicle Use 4
Franck Vangraefschèpe 5
1.1 Modeling Vehicle Use 7
1.1.1 Balance-of-Forces Equation 7
1.1.2 Driving Cycles 9
1.1.3 Energy Balance on Cycle 11
1.2 Comparing 3 Types of Vehicle Use: Urban, Rural Road and Motorway . . . 15
1.2.1 Urban Use Conditions 15
1.2.2 Rural Road Use Conditions 16
1.2.3 Motorway Use Conditions 18
1.3 Use of the Internal Combustion Engine 19
1.3.1 Power Transmission from Engine to Wheel 19
1.3.2 Optimizing the Gearbox Ratio on Driving Cycles 21
1.3.3 Influence of Idle 25
26
1.4 Effect of Vehicle Parameters on the Energy Balance
1.4.1 Effect of Mass 28
1.4.2 Effect of Rolling Losses 28
1.4.3 Effect of Aerodynamics 30
1.4.4 Effect of Auxiliaries 30
30
1.5 Study of the Normalized European Driving Cycle 31
1.5.1 Definition of the Cycle
1.5.2 Effect of the Various Vehicle Parameters on the "Hot" NEDC 33
1.5.2.1 Optimization of the Gearbox Ratio
1.5.2.2 Effect of the Vehicle Parameters
1.5.2.3 Effect of Cold Start
1.6 Energy Recovery During Braking
X Hydrogen, the Post ofFuel ? 34
35
1.6.1 Mass Transfer 37
1.6.2 Braking Distribution Between Axles and Equal Adhesion Parabola 42
1.6.3 Energy Recovery During Braking at Constant Deceleration 42
1.6.4 Energy Recovery in Use 53
56
1.6.4.1 Definition of Braking Management Strategies
1.6.4.2 Estimation of Recoverable Energies under Real Use Conditions 56
1.6.5 Conclusion
60
1.7 Conclusion 60
60
Chapitre 2 62
63
Internal Combustion Engines 65
Pierre Leduc 68
68
2.1 Internal Combustion Engines - Characteristics and Context 69
2.1.1 Thermodynamic Principles of Internal Combustion Engines 70
2.1.1.1 Types of Engines 70
2.1.1.2 Simplified Chemistry of Combustion 71
2.1.1.3 Power, Load, and Output 72
2.1.2 Evolution of Pollution Standards and C 0 2 Emissions 73
2.1.3 Fuel
2.1.3.1 Traditional Fuels 75
2.1.3.2 Liquefied Petroleum Gas (LPG) 75
2.1.3.3 Compressed Natural Gas (CNG) 75
2.1.3.4 Biofuels 75
2.1.3.5 Hydrogen 78
2.1.3.6 Energy Comparison 79
2.1.3.7 Cost and Taxation of Fuel in France 80
80
2.2 Spark-Ignition Engines (Gasoline Engines) 84
2.2.1 General 86
2.2.2 The Standard Situation 90
2.2.2.1 The Engine 90
2.2.2.2 Emissions Control and Complete Combustion 92
2.2.2.3 Efficiency Cascade Applied to the Gasoline Engine
2.2.3 Technological Advances 93
2.2.3.1 Downsizing 93
2.2.3.2 Variable Valve Timing 94
2.2.3.3 Charge-Diluted Combustion 97
2.2.3.4 Other Refinements 98
2.2.3.5 The Future: Controlled Auto-Ignition Combustion? 98
2.2.4 Summary 98
2.3 Compression Ignition Engines (Diesel Engines)
2.3.1 Description of Existing Diesel Engines
2.3.1.1 The Engine
2.3.1.2 Pollution Control: EGR and Emissions Control System
2.3.2 Technological Improvements
2.3.2.1 Downsizing and Downspeeding
2.3.2.2 Two-Stage Turbocharging
Table of Contents XI
2.3.2.3 Low-Pressure EGR 99
2.3.2.4 Advanced Injection Systems 100
2.3.2.5 New Combustion Types for Diesel Engines 101
2.3.3 Summary 102
2.4 Use in the Vehicle 102
2.4.1 Effect of Engine Use on Energy Distribution 102
2.4.1.1 Cold Engine Operation 102
2.4.1.2 Influence of the Engine's Operating Point on Thermal Losses 105
2.4.1.3 The Recovery of Thermal Losses 106
2.4.2 Principal Impact of Hybridization on the Engine 109
2.4.2.1 Advantages 109
2.4.2.2 Limitations 110
2.4.2.3 An Opportunity for 2-Stroke and Rotary Engines? Ill
2.5 Summary and Future Outlook 113
Chapitre 3 117
117
Electric Drivetrain 117
El Hadj Miliani, YoussefTouzani 118
119
3.1 Overview of Electric Machines 119
3.1.1 Principles 119
3.1.1.1 Electrodynamic Machines 119
3.1.1.2 Variable Reluctance Machines 120
3.1.1.3 Sign Convention 120
3.1.2 Composition
3.1.2.1 Electrodynamic Machines 122
3.1.2.2 Variable Reluctance Machines 123
3.1.3 Losses in Electric Machines 124
3.1.4 Electric Machine Operating Ranges 124
125
3.2 Classification of Electric Machines Used in Automobile Drivetrains 128
3.2.1 Mechanical Commutator DC Machines (MCDCM) 131
3.2.2 Synchronous Machines 133
3.2.2.1 Electronic Commutation DC Machine (ECDCM) 134
3.2.2.2 Wound Rotor Synchronous Machine (WRSM) 134
3.2.2.3 Permanent Magnet Synchronous Machine (PMSM) 136
3.2.2.4 Variable Reluctance Synchronous Machine (VRSM) 138
3.2.3 Asynchronous Machine (ASM) 138
3.2.4 Novel Machines 139
3.2.4.1 Axial Flux Machines
3.2.4.2 Wheel Motors 140
3.2.4.3 Double Excitation Machines 141
3.2.4.4 Double Rotor Machines 144
3.2.4.5 Permanent Magnet Reluctance Machines (PRM)
3.3 Modeling of Electric Machines
3.3.1 Electrical Aspects
3.3.2 Thermal Aspects
XII Hydrogen, the Post of Fuel ? 146
147
3.4 Power Electronics 148
3.4.1 Power Components 148
3.4.1.1 Diode 149
3.4.1.2 MOSFET 150
3.4.1.3 IGBT 150
3.4.2 Commutation 150
3.4.2.1 Natural Commutation 150
3.4.2.2 Forced Commutation 151
3.4.2.3 Dead Time During Commutation 151
3.4.3 Electrical Conversion Structures 153
3.4.3.1 DC-AC Conversion Structures 157
3.4.3.2 DC-DC Conversion Structures 158
3.4.3.3 AC-DC Conversion Structures 159
3.4.3.4 Example of Implementation in a Vehicle 159
3.4.4 Losses in the Power Converters 160
3.4.5 Modeling the Power Converters 161
3.4.5.1 Topological Model
3.4.5.2 Mean Model 162
163
3.5 Controlling Electric Machines 164
3.5.1 Control 164
3.5.2 Pulse Width Modulation (PWM) 165
3.5.2.1 Principle 166
3.5.2.2 Practical Case of Modulation (Toyota THS II) 166
3.5.3 Angular Position Measurement 167
3.5.3.1 Optical Encoder
3.5.3.2 Resolver 167
169
3.6 Electric Machine and Power Electronics Integration Constraints 170
3.6.1 Integration of the Electric Machine 170
3.6.2 Integration of the Power Electronics 171
3.6.2.1 Control and Sensors 174
3.6.2.2 Power Components
3.6.2.3 Thermal Behavior 175
175
3.7 Perspectives 176
3.7.1 Power Electronics 176
3.7.2 Electric Machines
3.7.3 Control 179
Chapitre 4 180
180
On-Board Energy Storage Systems 180
Valérie Sauvant-Moynot 183
4.1 Storage Requirements
4.2 Electrical Storage
4.2.1 General
4.2.1.1 Electrochemistry
4.2.1.2 Operation and Characteristics of a Storage Cell
Table of Contents XIII
4.2.1.3 Operation and Characteristics of Supercapacitors 192
4.2.2 Lead Storage Cells 195
195
4.2.2.1 Description [Robert and Alzieu, 2004b] 195
4.2.2.2 Characteristics 197
4.2.2.3 Aging 198
4.2.2.4 Applications 199
4.2.2.5 Outlook 199
4.2.3 Ni-MH Storage Cells 199
4.2.3.1 Description [Caillon, 2001] 201
4.2.3.2 Characteristics 202
4.2.3.3 Aging 203
4.2.3.4 Application 206
4.2.3.5 Outlook 207
4.2.4 Lithium Storage Cells 207
4.2.4.1 Description 212
4.2.4.2 Applications 216
4.2.4.3 Outlook 217
4.2.5 Supercapacitors 218
4.2.5.1 Electrostatic Supercapacitors 219
4.2.5.2 Electrochemical Supercapacitors 219
4.2.5.3 Hybrid Supercapacitors 219
4.2.5.4 Summary 220
4.2.6 Comparison of Electrical Energy Storage Technologies 220
4.2.6.1 Characteristics of Energy Storage Systems (ESS) for Different Applications 224
4.2.6.2 Comparison of Energy Efficiency in a Hybrid Vehicle 225
4.2.7 Modeling 226
4.2.7.1 Modeling Batteries 239
4.2.7.2 Modeling Supercapacitors 242
4.2.8 From the Element to the Battery Pack 242
4.2.8.1 Criteria for Selecting a Battery Cell 248
4.2.8.2 Architecture of a Battery Pack 252
4.2.9 Management of Electrochemical Storage Systems 253
4.2.9.1 Battery Management Systems - BMS 255
4.2.9.2 Methods for Estimating State of Charge 260
4.2.9.3 Methods of Thermal Management
262
4.3 Other on-Board Storage Systems
Chapitre 5 275
275
Hybridization 277
François Badin 279
281
5.1 Principles 282
5.1.1 Missions and Constraints of a Drivetrain
5.1.2 Complementarity of Thermal and Alternative Drivetrains
5.1.3 Principle of Hybridization
5.1.4 Usable Components
5.1.5 Series Hybridization
XIV Hydrogen, the Post ofFuel ? 282
282
5.1.6 Parallel Hybridization 284
5.1.7 Comparison of Series and Parallel Hybridizations
5.1.8 Series-Parallel Hybridization 285
285
5.2 Architectures 285
5.2.1 Coupling Devices 286
5.2.2 Series Hybridization 286
5.2.3 Parallel Hybridization 289
5.2.3.1 Coupling by Addition of Torques 290
5.2.3.2 Coupling by Addition of Speeds 293
5.2.4 Series-Parallel Hybridization 293
5.2.5 Power-Split Hybridization 295
5.2.5.1 Notion of Power Splitting 297
5.2.5.2 Input Split Type Configuration 301
5.2.5.3 Multimode Configurations 305
5.2.5.4 Output Split Type Multimode Configuration 306
5.2.5.5 Multimode Configuration with Discrete Ratios 306
5.2.5.6 Applications 306
5.2.6 Special Architectures 308
5.2.6.1 Electric Drive 310
5.2.6.2 Electric Four-Wheel Drive
5.2.6.3 Compressed Air Hybrid 310
311
5.3 Features 311
5.3.1 Discrete Hybrids 311
5.3.1.1 Optimized Management of Onboard Electrical Energy 314
5.3.1.2 Stop-Start 318
5.3.1.3 Stop-Start with Extended Features 321
5.3.1.4 Engine Assist 323
5.3.1.5 All-Electric Mode 323
5.3.2 Functional Hybrids 325
5.3.2.1 Non-Interruption of Torque when Changing Gear 334
5.3.2.2 All-Electric Mode with Range and Charging on the Grid 337
5.3.2.3 Summary of All-Electric Autonomy and Grid Charging Features 340
5.3.2.4 Expressing the Consumptions of a Plug-in Hybrid Vehicle 340
5.3.2.5 Exchanging Energy with the Grid
5.3.2.6 Distributed Drive 343
343
5.4 Summary 343
5.4.1 Features and Gains 343
5.4.2 Implementation 346
5.4.2.1 Drive
5.4.2.2 Vehicles 348
348
5.5 Examples 352
5.5.1 Parallel Hybrid Transmission 352
5.5.2 Toyota Prius 354
5.5.2.1 History 354
5.5.2.2 Principle and Manufacture 358
5.5.2.3 Evolutions of the Toyota Hybrid System Concept
5.5.2.4 Operating Features
Table of Contents XV
Chapitre 6 369
369
Control of Hybrid Vehicles 374
Antonio Sciarretta 374
379
6.1 The Need for Energy Monitoring and Management 379
6.1.1 Hybrid Architectures and Degrees of Freedom 381
6.1.2 Energy Management Laws 381
382
6.2 Heuristic Energy Management 384
388
6.3 Optimal Energy Management 388
6.3.1 Basic Concepts of Optimal Control Applied to Hybrids 390
6.3.2 Optimal Offline Energy Management 390
6.3.2.1 Dynamic Programming 391
6.3.2.2 Pontryagin's Minimum Principle (PMP) 391
6.3.3 Optimal Online Energy Management 391
392
6.4 Modeling Hybrid Drive Systems for Optimization 393
6.4.1 Forward and Backward Models 393
6.4.2 Backward Models of Hybrid Components 395
6.4.2.1 Vehicle 401
6.4.2.2 Transmission 401
6.4.2.3 "Coupling" Node 402
6.4.2.4 Power Splitter 403
6.4.2.5 Internal Combustion Engine 404
6.4.2.6 Electric Machine 405
6.4.2.7 Battery
6.4.3 Forward Models of Hybrid Components
6.5 Outlook for a Future Generation of Hybrid Vehicles
6.5.1 Extension of Optimal Control
6.5.2 Thermal Management
6.5.3 Brake Management
6.5.4 Recharge Management for Plug-in Hybrids
6.6 Conclusion
Chapitre 7
Comparative Study of Hybrid Vehicles: Greenhouse Gas Emissions, Energy
Consumption, and Cost
Frédérique Bouvart, Lionel Thellier, Simon Vinot
7.1 Greenhouse Gas Emissions and Energy Consumption 414
7.1.1 Methodology and General Principles 414
7.1.2 Example of a Discrete Hybrid Vehicle 416
7.1.2.1 Assumptions and Data 416
7.1.2.2 Results 421
7.1.3 Plug-in Hybrid Vehicle 424
7.1.3.1 Assumptions and Data 424
7.1.3.2 Results 431
XVI Hydrogen, the Post of Fuel ?
7.1.4 Comparison of Results for Discrete Hybrid Vehicles and Plug-in Vehicles: 437
Conclusion and Perspectives
440
7.2 Economic Balance of Hybrid Electric Vehicles 440
7.2.1 Factors Included in an Economic Balance of Hybrid Vehicles 441
7.2.2 Items Composing the Vehicle Investment Cost 441
7.2.2.1 Evaluation of Direct Costs 442
7.2.2.2 Evaluation of Indirect Costs 442
7.2.2.3 Summary of the Reference Vehicle Investment Costs 443
7.2.3 Analysis of Hybrid Vehicle Cost Structure 444
7.2.3.1 Internal Combustion Engines 444
7.2.3.2 Electric Drivetrain 445
7.2.3.3 Batteries 448
7.2.3.4 Assembly Costs 448
7.2.3.5 Investment Cost Balance 449
7.2.4 Analysis of Hybrid Vehicle Use Cost Structure 450
7.2.4.1 Maintenance and Insurance Costs 450
7.2.4.2 Energy Prices 451
7.2.5 Evaluation of the Total Cost of Ownership 451
7.2.5.1 Evaluation of the Use Cost 455
7.2.5.2 Extra Cost/Feature Ratio
456
7.3 Sensitive Material Balance for Electrified Vehicles 456
7.3.1 Nickel 459
7.3.2 Lithium 462
7.3.3 Rare Earths
Appendix 1 - Summary of the Various Drivetrain Electrification Solutions . . . . 469
Appendix 2 - Equivalence Between Fuel Consumption and C 0 2 Emissions . . . . 471
Appendix 3 - Regulation ECE R83 on Measurement of Pollutant Emissions.
Regulation ECE R101 on Measurement of Fuel Consumption and C 0 2 Emissions 473
Appendix 4 - Toyota Prius 3 Collaborative Braking System 485
Appendix 5 - Power-Split Hybridization. Comparison of the Mechanical Solution
with Planetary Gear and the Electrical Solution with Dual Rotor Machine 487
Appendix 6 - Evolution of Characteristics for the Various Prius Models 489
Appendix 7 - Illustration of All-Electric Mode Phases on a European Test 491
Procedure (Warm Start) Depending on the Initial Battery State of Charge
(AMESim IFP Energies Nouvelles Simulations)
1I Vehicle Use
Franck Vangraefschèpe \
This introductory chapter describes the bases which can be used to determine the energy bal-
ance of a vehicle performing a previously defined mission.
The primary concern of hybrid vehicle designers is to reduce vehicle fuel consumption
and consequently C 0 2 emissions ^ To optimize each vehicle in this respect, a balance of
all the energy consumed on board the vehicle (mainly, but not solely, for traction) must be
drawn up. This chapter therefore describes the energy balance of a mid-range vehicle, indi-
cating the various factors affecting energy consumption, and therefore fuel consumption, on
board this vehicle.
The results obtained show that the energy required for vehicle traction varies largely
depending on the driving conditions (urban, rural road or motorway driving). They also dem-
onstrate that reducing the vehicle weight is the most important pathway to lowering energy
consumption, at least for urban and rural road use (since, in contrast, aerodynamic losses are
dominant in motorway use).
On cycles with numerous accelerations and decelerations, it should be possible to recover
and store a non-negligible proportion of the vehicle kinetic energy for future traction purposes
if the drivetrain includes a reversible energy system (5.1.3), thereby reducing the energy con-
sumption accordingly. While recovery is feasible in practice, the technical and technological
constraints considerably limit its potential. Developments are required on vehicle braking
management in order to maximize energy recovery.
The analysis carried out in this chapter is based on the mission profiles imposing the
vehicle speed every second. This definition therefore implicitly includes the planned trip
(displacement from point A to point B) and the type of driving used on the trip. However,
we must remember that C02 emissions can be significantly reduced by adopting smooth
driving, limiting in particular accelerations and braking. This human and sociological aspect
of displacement is also being investigated (development of eco-driving aid systems) but will
not be discussed here.
1.1 MODELING VEHICLE USE
The energy balance of a vehicle on a trip is calculated by modeling the vehicle use. Modeling
is based on writing the balance-of-forces equation for the vehicle and on the definition of driv-
ing cycles. These two steps, then the resulting energy balance calculation, are described below.
1. Fuel consumption and C02 emissions are directly proportional (for a given fuel) (see appendix 2).
2 Hybrid vehicles
1.1.1 Balance-of-Forces Equation
The basic equation used to determine the energy balance of a vehicle is the general equation
of dynamics applied to the vehicle system considered as indéformable, apart from rotation
of the wheels2:
(1.1)
With
f Sum of the forces applied to the vehicle
xext Acceleration of the vehicle
a This vector equation can be projected in two scalar equations, one along the road axis, the
other along the perpendicular axis (1.1). We then obtain the following equilibrium equations:
(1.2)
(1.3)
with:
Ftmct tractive force
Faero aerodynamic drag force
RR reaction of the ground on the tires, which is projected along the displacement axis in
RR χ, rolling drag and along the perpendicular axis in RR vertical load
a angle between the road and the horizontal plane. The road gradient is often given,
which is the tangent of the angle (so a gradient of 100% corresponds to an angle of 45°)
mveh vehicle mass
Jrot inertia of the rotating parts (in particular the internal combustion engine and the wheels)
^wheel w h e e l r a d i u s
Figure 1.1
Balance of forces exerted on the vehicle.
2. Deformation of the vehicle by crushing the suspensions would have to be taken into account in order
to study the vehicle attitude, either during acceleration or braking, or the roll when cornering, which is
outside the scope of this book.
Chapter 1 · Vehicle use 3
The tractive force is calculated directly from the engine torque, using the transmission
gear ratio (^trans), the transmission efficiency 0ltrans) and the wheel radius (R^eel)·
(1.4)
The aerodynamic drag is one component of the aerodynamic forces: integration of the
dynamic pressure effects around the vehicle results in a single force which is applied at
the center of thrust. For simplification purposes, we consider that the center of thrust and
the center of gravity are at the same point. The resultant of the aerodynamic forces can be
broken down into two components: the drag, which is parallel to the direction of vehicle
displacement and the lift, which is perpendicular to it. Only the drag is taken into account
in terms of energy since it is the only force doing work. The lift only has an indirect effect
since it modifies the load of the tires, but this effect is neglected (the aero lift is not included
in equation 1.3). The aerodynamic drag is proportional to the ambient air density (p), the
vehicle front surface area (S), its aerodynamic drag coefficient (Cx) and the square of the
vehicle aerodynamic speed (vehicle speed + component of the wind speed along the vehicle
displacement axis):
(1.5)
The rolling drag is due mainly to the tires. Projected along axis x'x, this force is propor
tional to their vertical load (RR χ = fR.RR y). The coefficient fR (no unit) depends mainly on the
materials in contact. For a tire-road contact, it depends on the pavement type (micro and macro
roughness), the humidity, the tire technology, its temperature, inflation and, to a lesser extent,
the speed. The order of magnitude for a correctly inflated tire on dry road is 0.006 to 0.012
for cars and 0.006 for heavy goods vehicles (the speed factor can be neglected for regular
applications). As a comparison, note that the coefficient corresponding to a steel-steel contact
on railway is much lower, about 0.001. Concerning the tires, the difficulty for manufacturers
is to manage to create a product combining a low rolling drag and good grip (especially when
braking), without making any compromise on the other criteria, especially durability.
Note that a small proportion of the rolling drag is due to residual friction of the braking
system or mechanical parts rotating during the measurement procedure (differential, gear
box, etc.).
When homologating a vehicle, its fuel consumption, emissions of carbon dioxide (C02)
and regulated pollutants - carbon monoxide (CO), unburnt hydrocarbons (HC), nitrogen
oxides (ΝΟχ) and particulate matter (PM) - are not measured on road but on a test bench in
order to reproduce the vehicle rolling (with sampling of exhaust gases) by rotating the wheels
on braked rollers. The bench must therefore be adjusted for each vehicle by modifying the
law governing the rollers simulating the road, so as to absorb the same force as that which
would be absorbed by the total losses for the vehicle considered. These losses must first be
measured on flat track with no wind, then retranscribed in a polynomial of degree 2 function
of the vehicle speed, the sum of the rolling losses (tire drag, brake rubbing, etc.) and the aero
dynamic losses measured. The effect of a gradient can also be added by calculation (using
equation 1.2), to reproduce rolling on gradient on the test bench.
4 Hybrid vehicles
Figure 1.2 shows the forces applied to the wheel for each steady speed. The rolling
resistance is virtually independent of the speed; the resistance due to penetration in the air
increases with the square of the speed; the resistance due to the weight is also independent
of the speed but increases in proportion to the gradient. As we will see below, in fact, what
matters in terms of energy balance is not so much these forces but the work they dissipate
during a complete trip (1.2).
Figure 1.2
Distribution of vehicle losses according to the speed for a vehicle of lower
medium segment Ml (weight ~ 1,360 kg).
1.1.2 Driving Cycles
Vehicles are used in a wide variety of ways. This is due to the type of traffic (urban, rural
road, motorway) and the driving mode (calm, sportive, etc.). In engine design and optimiza-
tion studies, these vehicle use conditions must be transcribed into tests that are sufficiently
reproducible to be repeated in a similar way on test benches or during simulations. Mis-
sion profiles, for which the vehicle speed and the road gradient are defined as a function of
time, are therefore used 3. These standardized mission profiles are generally called "driving
cycles", or simply "cycles".
These driving cycles are either built arbitrarily by defining steady speed phases and accel-
erations/decelerations to link these phases (as with the normalized European (NEDC) and
Japanese driving cycles - 10 to 15 modes), or from statistical data collected on vehicles
in real use, then compiled to extract an average use (normalized American cycles - FTP,
3. The cycle generally also requires other parameters: the gear change times, in case of manual gear-
box, or the vehicle load, especially for public transport and utility vehicles.
Chapter 1 · Vehicle use 5
UDDS, etc. -, Artemis cycles). This second technique offers the advantage of representing
true vehicle use more closely. The cycles obtained are more chaotic, however, and therefore
more difficult to implement and less repeatable on test bench. Figure 1.3 shows two exam-
ples of these cycles.
Figure 1.3
Examples of normalized cycles used for vehicle homologation.
(a) European cycle NEDC, (b) American cycle FTP75.
1.1.3 Energy Balance on Cycle
For a given driving cycle, it is worthwhile evaluating the energy required for vehicle traction
and the contribution of each force applied to the vehicle to the energy balance. The most
relevant technological choices can therefore be identified in order to reduce the vehicle fuel
consumption.
Figure 1.4 provides an example of calculating the instantaneous power on two different
portions of the normalized European driving cycle (NEDC).
By convention, we will assume throughout this chapter that the positive torques, forces
and energies come from the system considered and supplied to the exterior, while the nega-
tive signs correspond to quantities supplied from the exterior to the system.
On the urban driving cycle (called the ECE cycle, where the vehicle speed remains less
than or equal to 50 km/h), the power corresponding to the rolling forces and the aerodynamic
losses remains fairly low (maximum a few kW). The largest power demands are due to the
vehicle inertia during accelerations (including the inertia of the rotating masses).
6 Hybrid vehicles
On the extra-urban driving cycle (EUDC), the effects of vehicle inertia remain dom-
inant but, at 100 and 120 km/h, the power consumed by the aerodynamic drag becomes
non-negligible.
Figure 1.4
Calculation of the instantaneous power required for a vehicle of lower medium
segment (weight 1,360 kg) on the urban driving cycle (ECE (a)) and on the
extra-urban driving cycle (EUDC (b)) of the normalized European driving
cycle (NEDC).
During the deceleration phases, the sum of the external forces applied on the vehicle has
a negative projection on the road axis. Since the aerodynamic drag and the rolling drag of the
tires always have a negative projection (these two forces oppose the vehicle progress), the
sign of the tractive force is variable:
- when the deceleration requested is less than that produced by the vehicle losses alone
(rolling and aerodynamic), a positive tractive force must be maintained to mitigate the
natural vehicle deceleration;
- when the deceleration requested is more than that produced by the vehicle losses, a
negative tractive force (therefore braking) is required to follow the speed instruction.
This is obtained partly by the internal losses of the engine (engine brake), but also
by the friction losses produced by the vehicle mechanical brakes which convert the
kinetic energy into heat lost in the atmosphere. With hybrid and electric vehicles, some
of the vehicle kinetic energy can be recovered and stored in a reversible accumula-
tor (battery or supercapacitor for electrical storage, pressurized tank for hydraulic or
hydropneumatic storage, flywheel for mechanical storage, etc.).
Chapter 1 · Vehicle use 7
We must therefore make a distinction between deceleration and braking: the vehicle starts
to decelerate as soon as its speed decreases, i.e. when the positive tractive force does not
compensate for the negative external forces (rolling losses, aerodynamic losses, weight if
climbing); we only speak of braking when a negative tractive force is required to follow
the speed instruction, which occurs when the deceleration requested is more than what the
vehicle losses generate alone or when the weight acts as a positive force which must be coun-
teracted (when going down a slope).
By integrating the powers corresponding to the various forces applied to the vehicle, we
obtain an energy balance for the entire driving cycle, allowing us at the same time to quantify
the energy share consumed by each force applied to the vehicle.
1.2 COMPARING 3 TYPES OF VEHICLE USE: URBAN, RURAL ROAD
AND MOTORWAY
To indicate the various types of vehicle use, we will use cycles obtained from statistical
analyses of real records taken during the European Artemis program.
The vehicle simulated on these cycles is a car of lower medium segment (weight 1,360 kg
+ inertia of the rotating masses, especially the wheels); its rolling losses correspond to a
rolling loss coefficient fR of 0.010 (independent of the speed) and an aerodynamic loss coef-
ficient S.Cx of 0.69 m2. This gives a polynomial of the losses (expressed in newton) as a
function of the vehicle speed (in m/s) in the following formulae.
(1.6)
1.2.1 Urban Use Conditions
We will take the urban Artemis cycle as reference for urban use conditions: maximum speed
of 57 km/h and average speed of 17.7 km/h, including 259 s during which the vehicle is sta-
tionary. If we exclude these stationary phases, the average speed increases to 24.7 km/h The
greatest acceleration on this cycle is 2.86 m/s2 but the average acceleration during the 415 s
of traction phases (positive tractive force) is 0.61 m/s2. Inversely, the greatest braking4 cor-
responds to a deceleration of 3.14 m/s2 while the average deceleration during the 318 s of the
braking phases (negative tractive force) is 0.79 m/s2. The total cycle represents a duration of
992 s and a distance of 4.87 km.
Figure 1.5a shows both the speed profile imposed and the gear to be engaged at each
moment of the cycle (for a 5-speed manual gearbox). Figure 1.5b shows the position of the
operating points in a vehicle speed - acceleration plane.
4. Standard R13 on braking of road vehicles sets the emergency braking limit at 6 m/s2. As a compari-
son, on ground offering good grip, a deceleration of 8 or even 9 m/s2 is possible.
8 Hybrid vehicles
Figure 1.5
Definition of the urban Artemis cycle (a) and positions of the vehicle operating
points in a vehicle Speed-Acceleration field (b).
The iso-power hyperbolas (calculated for the vehicle studied) can be used to estimate
the driving power required depending on the speed and acceleration required for the vehicle
(on zero gradient). The same figure also shows the maximum acceleration curve possible
with the powertrain (composed of the engine, gearbox and differential) fitted on the vehicle,
considering the vehicle losses (blue curve). The discontinuous appearance of the blue curve
is due to the finite number of gears in the gearbox.
To estimate the contribution of each loss (rolling friction, aerodynamics, inertia 5) to
the global energy balance, we must distinguish between the traction phases and the braking
phases (Table 1.1):
- in traction (tractive force F t m c positive), the average energy required to propel the
vehicle considered is 150 Wh/km 6 (i.e. 540 J/m in SI units). This energy consists of
80% vehicle inertia, 15% rolling losses of the wheel and 5% aerodynamic drag. The
power required to propel the vehicle is 6.3 kW on average but peaks at 25.6 kW;
5. The weight is not shown here since the cycles are assumed to take place on level ground, which
does not correspond to actual conditions. In practice, since a vehicle eventually returns to its starting
point, its average change in altitude is zero over long periods, which is why it is not taken into account.
Instantaneously, however, this load may induce quite large requirements in terms of power necessary
at the wheel.
6. The SI (International System of units) unit for energy is the joule (J) which corresponds to 1 W dur-
ing 1 s. The Wh (= 3600 J) is generally used, however.
Chapter 1 · Vehicle use 9
- in contrast when braking, the vehicle inertia provides the energy (up to the kinetic
energy stored 7) while the wheel friction and aerodynamic drag continue to dissipate
some of this kinetic energy. On a conventional vehicle, the braking energy cannot be
recovered and must be converted into heat by the brakes. On a hybrid (or electric)
vehicle, at least some of this energy can be converted into electricity and stored in the
onboard energy storage system, for future reuse for traction or the auxiliaries, as we
will see in Chapter 6. Integration of all times when the traction power is negative (brak-
ing) leads to an average energy of - 102 Wh/km on this cycle (and for this vehicle).
The average power during braking is - 5.6 kW, while the peak power is - 40.6 kW.
Table 1.1. Energy balance on the urban Artemis cycle
Inertia Rolling Aerodynamic Total Average
point
Traction phases 120.7 Wh/km 22.1 Wh/km 7.3 Wh/km 150.1 Wh/km
(Wheel 80% 15% 5% 100% 0.61 m/s2
power > 0) 6.3 kW
- 120.7 Wh/km 14.9 Wh/km 4.2 Wh/km - 102.3 Wh/km
Braking phases 100% 12% 4% 84% - 0.79 m/s2
(Wheel - 5.6 kW
power < 0)
1.2.2 Rural Road Use Conditions
The same analysis can be conducted on the rural road Artemis cycle with a maximum speed of
110 km/h and an average speed of 57.5 km/h (59.3 km/h if we exclude the stopping phases).
The extreme accelerations are + 2.36 m/s2 and - 4.08 m/s2. The average accelerations over
the 748 s of traction (power at the wheel strictly positive) and the 305 s of braking (power
at the wheel strictly negative) are + 0.27 and - 0.67 m/s2 respectively. The cycle duration is
1,081 s for a distance traveled of 17.3 km.
Figure 1.6 shows the speed profile and the positions of the vehicle operating points in a
speed - acceleration field (black dots). In terms of speed, all values from 0 to 110 km/h are
used, with a maximum probability between 40 and 80 km/h. The accelerations are distributed
quite evenly between positive and negative values, with some significant points indicating
intense braking.
7. The energy balance related to the inertia forces is not strictly zero between traction and braking if
we take into account the fact that some of the thermal engine inertia is dissipated by the clutch and the
gearbox synchronizers when changing gear.
10 Hybrid vehicles
Figure 1.6
Definition of the rural road Artemis cycle (a) and positions of the vehicle oper-
ating points in a vehicle Speed-Acederation field (b).
When our reference vehicle follows this speed profile, the extreme powers recorded at the
wheel are 42 kW and - 61 kW. The average power during traction is 10 kW and - 8.2 kW
during braking (Table 1.2). The traction energy is 121 Wh/km, lower than that required on the
urban cycle. Its distribution depending on the various losses is also different since the share
due to inertia is lower, while that due to the aerodynamic drag has increased significantly.
Table 1.2. Energy balance on the rural road Artemis cycle
Inertia Rolling Aerodynamic Total Average
point
Traction phases 59.4 Wh/km 27.5 Wh/km 34.2 Wh/km 121.1 Wh/km
(Wheel 49% 23% 28% 100% 0.27 m/s2
power > 0) 10.1 kW
- 59.4 Wh/km
Braking phases 100% 9.6 Wh/km 9.8 Wh/km - 40.0 Wh/km - 0.67 m/s2
(Wheel 16% 17% 67% -8.17kW
power < 0)
Chapter 1 · Vehicle use 11
1.2.3 Motorway Use Conditions
The motorway Artemis cycle (in its 150 km/h version) (Figure 1.7) has an average speed of
99.5 km/h (101.1 km/h excluding stops). In the vehicle speed - acceleration graph, we see
that the point cloud is clearly shifted towards the high speeds. The extreme accelerations are
+ 1.92 m/s2 and- 3.36 m/s2. The average accelerations are + 0.17 and- 0.82 m/s2. The cycle
duration is 1,067 s (875 s during traction, 179 s during braking, 13 s stopped) and the distance
traveled 29.5 km.
Figure 1.7
Definition of the motorway Artemis cycle (a) and positions of the vehicle oper-
ating points in a vehicle Speed-Acceleration field (b).
Table 1.3 below shows the results of the energy balance calculation on our reference
vehicle. Operation at high speed generates greater aerodynamic losses, which explains the
high energy consumption (179 Wh/km). Since this cycle includes relatively little braking,
the recoverable energy is less than 13% of the energy spent on traction. The average power
required during traction is 22 kW. Note the presence of high braking powers (up to 94 kW)
corresponding to sharp deceleration (-3.36 m/s2) at high speed (80 km/h).
12 Hybrid vehicles
Table 1.3. Energy balance on the motorway Artemis cycle.
Inertia Rolling Aerodynamic Total Average
point
Traction phases 35.7 Wh/km 32.7 Wh/km 110.3 Wh/km 178.7 Wh/km
(Wheel 20% 18% 62% 100% 0.17 m/s2
power > 0) 21.7 kW
- 35.7 Wh/km 4.4 Wh/km 9.3 Wh/km - 22.0 Wh/km
Braking phases 100% 12% 26% 62% - 0.82 m/s2
(Wheel -13.1kW
power < 0)
Representative driving cycles
(M. André, IFSTTAR, LTE)
The aim of driving (for experiments on roller bench) or more generally test (for experi
ments on engine bench or for simulations, etc.) representative cycles is to reproduce
or simulate conditions as close as possible to reality in order to measure/evaluate the
performance of vehicles and transport systems. We therefore try to reproduce through
driving cycles the actual conditions of traffic and vehicle usage in order to measure on
test bench the true pollutant emissions and energy consumption resulting from these
conditions (impact calculation), or simulate and optimize the operation of alternative
engines (evaluation, design).
In this respect, the representative cycles must not be confused with the standard
cycles, carried out to validate vehicles with respect to legal emission limits. Similarly, their
respective elaboration obeys fundamentally different principles (reproducibility, universal
ity, simplicity required for a standard cycle, while the main purpose of the representative
cycles is to describe true traffic conditions, including their heterogeneity).
The most recent studies to elaborate representative cycles were carried out as part
of the European ARTEMIS 1 research project, which aimed at developing European tools
to calculate pollutant emissions for all modes of transport, based on large experimen
tal campaigns of pollutant emission measurements and the definition of representative
methods [André et al, 2008].
Within this framework, the ARTEMIS cycles for light duty vehicles [André, 2004]
as well as other cycles and test procedures (utility vehicles, buses, motorcycles and
mopeds, etc.) have been developed.
The method used to analyze and build the cycles is based on observation of actual
conditions of vehicle use and operation, by instrumenting and monitoring vehicles in actual
conditions of use (or operation). The vehicle speeds and accelerations, engine speeds,
thermal conditions, use of air conditioning, etc., which are all important emission factors,
are therefore measured continuously. Approximately 90,000 km have been recorded on
board 80 private cars in France, Germany, Great Britain and Greece during earlier studies.
Driving conditions are analyzed using these data, through the speed profiles,
recorded on kinematic segments of homogeneous size (from a few hundred meters in
urban conditions to several kilometers on motorway). The approach considered (cor
respondence analysis and classification of cross-distributions of instantaneous speeds
Chapter 1 · Vehicle use 13
and accelerations) is used to build a typology of urban, rural road and motorway traffic
conditions in 12 standard classes (E1.1), whose main purpose is to identify standard
conditions and to later reproduce the diversity of the conditions observed (rather than the
"average" conditions which would be meaningless).
The structure of the trips and their composition under these various types of condi
tion are then used to build 3 cycles representative of urban, rural road and motorway
trips. These 3 driving cycles of total duration 40 min therefore describe all driving condi
tions (E1.2). An algorithm for determining the gears to be used with these cycles has also
been developed and applied on a typology of European cars, thereby allowing laborato
ries to measure true car emissions under conditions most representative of usage and
driving behaviors, while taking into account vehicle performance.
Development of the ARTEMIS cycles led to numerous exchanges between laborato
ries, comparison of numerous approaches and finally validation of the principles through
analysis of additional databases. The 40 min cycles are characterized by a detailed
breakdown of the driving conditions into "subcycles" allowing fine, almost "localized"
analysis of the emissions. They are accompanied by a procedure specifying the test,
engine start and emission sampling conditions and are widely distributed and used in
Europe to measure the emissions of light vehicles, to simulate engine operation, and as
reference for numerous studies.
Cycles adapted to the car engine size (small/big engine) have also been developed
[Joumard etal, 2003] which demonstrate that using a single set of cycles for all vehicles
leads to overestimations of emissions from cars with small engines and of urban emis
sions. Based on similar principles, specific cycles have been proposed for light utility
vehicles [André et al, 2006] and for buses [André et al, 2005].
Figure E1.1
Standard car traffic conditions in Europe identified by automatic classification
of speed profiles recorded in actual use, and plotted as a function of the average
speed and the acceleration.
14 Hybrid vehicles
Figure E l 2
Urban and motorway cycles and their structure in standard traffic conditions.
REFERENCES
André M, Keller M, Sjodin A, Gadrat M and McCrae I (2008): The ARTEMIS European Tools for
Estimating the Pollutant Emissions from Road Transport and Their Application in Sweden
and France. 16th Symposium Transport and Air Pollution, Graz, June. Proceedings, pp 86-96,
See also: ARTEMIS: http://www.Inrets.Fr/Ur/Lte/Publi-Autresactions/Fichesresultats/
Ficheartemis/Artemis.Html
André M. (2004) The ARTEMIS European Driving Cycles for Measuring Car Pollutant Emissions.
Science of The Total Environment 334-335, 73-84. ISSN 0048-9697.
André M, Joumard R, Vidon R, Tassel P and Perret P. (2006) Real-World European Driving Cycles,
for Measuring Pollutant Emissions from High- and Low-Powered Cars. Atm. Envir. 40, 5944-
5953, ISSN 1352-2310.
André M, Garrot B, Roynard Y, Vidon R, Tassel P and Perret P (2005) Operating Conditions of
Buses in Use in the Ile-De-France Region of France for the Evaluation of Pollutants Emis-
sions. Atmospheric Environment 69, 2411-2420. ISSN 1352-2310.
Joumard R, André M, Vidon R and Tassel P (2003) Characterizing Real Unit Emissions for Light
Duty Goods Vehicles. Atmospheric Environment 37 (37), 5217-5225. ISSN 1352-2310.
1. ARTEMIS: Assessment and reliability of transport emission models and inventory systems. European Commis-
sion project, 5th PCRD, DG TREN.
Chapter 1 · Vehicle use 15
1.3 USE OF THE INTERNAL COMBUSTION ENGINE
Having specified the various vehicle driving conditions, we will now examine how the
engine is used while driving. The powertrain chosen for our study consists of a 1.6 L gaso-
line internal combustion engine, not turbocharged, developing a maximum power of 80 kW,
and a 5-speed gearbox producing speeds at 1,000 r.p.m. of respectively 7.3, 13.7, 19.3, 25.4,
32.3 km/h.
1.3.1 Power Transmission from Engine to Wheel
The engine is defined by the following characteristics (Figure 1.8.a):
- its maximum performance (torque or power) as a function of its speed of rotation,
- its efficiency mapping.
Figure 1.8
Definition of the engine characteristics and transposition of the engine power
curve as a function of the vehicle speed on the various gearbox ratios.
The power from the engine is transmitted to the wheels via the gearbox. For each gearbox
ratio, there is a proportionality relation between the engine speed and the vehicle speed. For
a 5-speed gearbox, the maximum power curve of the engine will therefore be projected in
the Vehicle speed - Power field (Figure 1.8.b) in 5 separate curves whose sawtooth shape is
characteristic of discrete transmissions (as opposed to continuously variable transmissions).
Iso-efficiencies are projected in the same way, but superimposing them would make the fig-
ure difficult to read. The power curve required to drive the vehicle forward at steady speed
(green curves) for various gradients (0%, 5% and 10% on the graph) can also be plotted on
this graph.
16 Hybrid vehicles
For a vehicle operating point identified by its speed and the power required at the wheel,
we see that several operating points are possible for the engine, depending on the gear
engaged. In the above example, to drive at a steady speed of 100 km/h on a 5% gradient, the
engine must develop 30 kW (red dot on Figure 1.8.b). In first and second gear, the engine
speeds would be too high. 3rd, 4th and 5th gears can be used, however, giving the three engine
operating points indicated in the Table 1.4 and shown by red dots on Figure 1.8.a:
Table 1.4. Example showing the choice of gearbox ratio for a given vehicle operating point
Vehicle operating point: 100 km/h - 30 kW
Gearbox ratio Engine speed Engine efficiency Power reserve
3rd 52 kW
4th 5,173 r.p.m. 27% 31 kW
17 kW
5th 3,937 r.p.m. 31%
3,099 r.p.m. 34%
Using a high gear (5th) reduces the engine speed of rotation and improves efficiency. The
drawback, however, is that there is a lower power reserve. The power reserve corresponds
to the power surplus available to the driver if he accelerates suddenly without changing
gear (therefore at the same engine speed of rotation); it is shown by a vertical arrow on Fig-
ure 1.8.a. Gear management is therefore a compromise between consumption and driving
comfort. With a manual gearbox, drivers make this compromise themselves, depending on
their driving type and their anticipated requirements in the short term. With an electronically-
controlled gearbox, however, development of management algorithms often proves difficult
due to the diversity of possible situations and the fact that the driver's intentions are not
known precisely, being interpreted from the accelerator pedal alone, with no possibility for
anticipation.
1.3.2 Optimizing the Gearbox Ratio on Driving Cycles
On driving cycles, a gearbox ratio instruction is given to the driver at the same time as the
vehicle speed instruction. Obviously, if the gearbox is not managed by the driver (automatic
gearbox, variator or CVT, etc.), the ECU manages the transmission according to control laws
which try to find a compromise between lower consumption and driving comfort (minimiz-
ing the number of gear changes, sufficient "power reserve" permanently available, etc.).
If we completely ignore these driving comfort criteria, we can calculate a potential con-
sumption gain by optimizing the gearbox ratio used at every second, in order to minimize the
consumption. However, the mathematical optimization implemented induces too many gear
changes, which would be unpleasant for the driver. An intermediate step is therefore required
to limit the number of gear changes.
On the rural road Artemis cycle, for example, by optimizing the use of gearbox ratios, the
point cloud of engine operation can be shifted to the low engine speed - high torque region,
which is at the same time the optimum efficiency region (Figure 1.9).
Chapter 1 · Vehicle use 17
Figure 1.9
Change of the engine operating region used on the rural road Artemis cycle
depending on whether gearbox management is imposed by the cycle (a) or opti-
mized for consumption (b).
Table 1.5 shows, for each Artemis cycle, the change in consumption and available power
reserve depending on how the gearbox is managed.
Table 1.5. Fuel consumption for three types of use depending on the gearbox management
URBAN Gearbox ratios imposed by Consumption Average Number
ARTEMIS the cycle on cycle power reserve of gear
L/100 km changes
CYCLE Optimized gearbox ratios with kW
limited number of changes 10.1 16.4 83
RURAL
ROAD Optimized gearbox ratios with 9.2 9.0 112
ARTEMIS no constraint (- 8.5%)
CYCLE 8.4 324
Gearbox ratios imposed by 9
MOTORWAY the cycle (- 10.4%) 25.6 37
ARTEMIS
CYCLE Optimized gearbox ratios with 6.1 15.1 69
limited number of changes
5.5 14.5 282
Optimized gearbox ratios with (-10.1%)
no constraint 24.0 24
5.45
Gearbox ratios imposed by (-11.1%) 22.5 54
the cycle
7.2 22.2 186
Optimized gearbox ratios with
limited number of changes 7.1
(- 0.7%)
Optimized gearbox ratios with
no constraint 7.1
(-1.0%)
18 Hybrid vehicles
In urban use, the average power demand during traction is low (about 6 kW). The effi-
ciency of the engine is therefore poor. Optimizing the gearbox ratio implies using the engine
at low speeds of rotation (on average 1,300 r.p.m.). It is possible to work in regions of higher
efficiency by displacing the operating points. Applying this principle to the entire cycle, we
can expect a gain in consumption of 8% to 10% at the expense of the power reserve which is
reduced by an average of 45% to 50%.
On the rural road cycle, the average power during traction is already slightly higher
(10 kW) and the engine efficiency is better. Optimizing the gearbox ratios will still reduce
the global consumption on the cycle by a similar proportion (10 to 11%). Once again, the loss
in terms of power reserve is 40%.
Lastly, in motorway driving, the vehicle uses on average of more than 20 kW of power
and its speed is high, which means that before optimization, the highest gear is already used
most of the time. Consequently, optimizing the gearbox ratio only offers a small gain (maxi-
mum 1%). Adopting a gearbox with a higher top gear would improve the consumption gain
but, in this case, gearbox designers are often faced with technological difficulties.
1.3.3 Influence of Idle
Operation at idle is a specific feature of internal combustion engines, which cannot run below
a certain speed of rotation (generally 650 to 800 r.p.m.). When no power is requested from
the engine, there are two options:
- either allow it to idle, so that it is readily available when power is once again required,
which consumes fuel,
- or switch it off, which means that the engine must be started before it can provide
power once again.
With traditional starting technologies, it takes about 1 second to start the engine, which is
too long to consider switching off the engine during brief stops (at stop signs, traffic lights,
etc.). With the starter-alternators or other hybrid technologies described in Chapter 5, it takes
0.3 s to start the engine, which becomes compatible with daily driving requirements.
Table 1.6 shows the influence of idle (vehicle stationary) in terms of time spent and
global consumption on each of the three driving cycles.
Table 1.6. Share of idling in terms of fuel consumption for each of the three cycles.
Driving cycle Share of idling in time (%) Share of idling in consumption (%)
Urban Artemis 26.1 16 to 17
Rural road Artemis 2.6 1
Motorway Artemis 1.2 0.2 to 0.3
Chapter 1 · Vehicle use 19
As can be seen, for urban use, the share of consumption related solely to idling exceeds
10%. For these types of use therefore, it is quite legitimate to consider a technical solution to
switch off the engine when the vehicle is stopped.
For other uses, the vehicle spends little time stopped and the share of consumption related
to this mode of operation becomes negligible fairly quickly.
1.4 EFFECT OF VEHICLE PARAMETERS ON THE ENERGY BALANCE
Apart from the definition of the powertrain itself (choice of engine, gearbox and differen-
tial), the vehicle parameters affect both the energy balance at the wheel and the final vehicle
consumption.
In this section, we will study the effect of the following three vehicle parameters: vehicle
mass ( m ^ ) , rolling loss coefficient (fR), aerodynamic loss coefficient (S.CX), and also the
power of onboard auxiliaries, which has increased considerably over the last few years.
1.4.1 Effect of Mass
Between 1990 and 2005, the average mass of private vehicles (or LV, for light vehicles) sold
in France increased by 30%, i.e. about 290 kg (ADEME data). This change was accompanied
by a simultaneous increase in the power produced by engines since, given the strong market
competition, manufacturers focused on maintaining the performance of their vehicles at the
same time (therefore the power to weight ratio).
Generally speaking, the high mass of vehicles is the price to pay for their versatility. The
mass increase observed between 1999 and 2005 is due mainly to the structural reinforce-
ments added to improve the passive safety of cars in case of accident, but also to the progres-
sive generalization of safety (airbags, etc.) and comfort (electric windows, air-conditioning,
etc.) accessories. The choice of lighter materials and the optimization of parts to reduce their
mass while delivering equivalent performance, but also the higher sales of small cars (bonus-
malus and scrappage premium), nevertheless halted the average mass increase of vehicles
sold in 2009, after 20 years of constant increase.
Looking at the equilibrium equation of forces on the vehicle (1.2), we see that some forces
(translation inertia, rolling drag, weight) are proportional to the vehicle mass, while the others
(rotating inertia, aerodynamics) are not. This remains true for powers, then for energies by inte-
grating the power over the cycle duration such that, for each cycle studied, the traction energy
and braking energy both vary in direct proportion to the vehicle mass. Figure 1.10 shows this
relation for the urban Artemis cycle. For the other use conditions, the gradient of the straight
lines is lower since the inertia forces have a lower effect on the global energy balance.
On our reference thermal vehicle, as with all vehicles, the higher energy required during
the traction phases necessarily induces higher fuel consumption. Nevertheless, the consump-
tion increase coefficient is slightly less than the energy coefficient, since the engine is on
average more highly charged and therefore works more efficiently.
20 Hybrid vehicles
Figure 1.10
Effect of mass on the traction and braking energies (a) and on operation of the
engine (fuel consumption and average efficiency) (b) - urban Artemis cycle.
The increase in braking energy is a direct result of the increase in vehicle kinetic energy
(l/2.m.V2). On conventional vehicles (neither electric, nor hybrid), this braking energy cannot be
recovered or stored and must therefore be converted into heat by the vehicle brakes. We can there-
fore see that increasing the vehicle mass will induce increased demand on the mechanical brakes.
With hybrid or electric vehicles, being able to recover some of the vehicle kinetic energy during
braking and then use this stored energy during a subsequent acceleration phase offsets the nega-
tive effect of mass on vehicle consumption. However, since this kinetic energy recovery is neither
perfect nor complete, the increase in energy recovered during braking phases is not sufficient to
offset the increase in energy required during traction phases. Finally therefore, any increase in
the vehicle mass increases consumption, even if this increase is attenuated with hybrid vehicles.
For the other use conditions, the changes are qualitatively the same, but the change ampli-
tudes are reduced, since the share of inertia forces in the energy balance is lower (Tables 1.1,
1.2 and 1.3).
To summarize these results, Table 1.7 gives, for each characteristic quantity (traction energy,
braking energy, consumption) the ratio between its change (as a %) and the mass variation (as
a %) causing this change. The calculations are based on mass variations in the range ± 25% 8 :
Table 1.7. Quantification of the linear effect of vehicle mass on the energy balance and engine operation,
expressed as a % per mass %
Traction energy Urban use Rural road use Motorway use
Braking energy 0.926 0.704 0.362
Consumption 1.012 1.202 1.370
0.460 0.448 0.274
8. For a medium category vehicle (like our reference vehicle), a mass difference of + 25% roughly
corresponds to the maximum payload allowed for the vehicle (gross vehicle weight - unladen weight).
Chapter 1 · Vehicle use 21
The effect of a mass increase on our 1,360 kg vehicle can therefore be rapidly quantified:
in urban use, a 10% mass increase (136 kg) leads to an increase of 9.26% in traction energy,
10.12% in braking energy and 4.60% in consumption.
1.4.2 Effect of Rolling Losses
A simple and common method to calculate rolling losses is to model them using a constant
coefficient (fR) between the vertical load and the drag force:
(1.7)
(for our reference vehicle, we chose fR = 0.010 or 10 kg/t).
Under these conditions, the energy consumed by these losses over the entire cycle (Wroll)
is directly proportional to this loss coefficient fR and the distance traveled 9:
(1.8)
During traction phases, the friction losses must be compensated by the engine to follow
the cycle. If the losses are increased, more traction energy is required.
Inversely, increasing the tire rolling resistance reduces the braking energy. During brak-
ing, these losses therefore reduce the amount of energy to be dissipated by the brakes as heat
(or that could be recovered by the electric machine in case of an electric or hybrid vehicle)
(Figure 1.11).
Figure 1.11
Effect of rolling loss coefficient on the traction and braking energies (a) and on
operation of the engine (fuel consumption and average efficiency) (b) - urban
Artemis cycle.
9. Only for a driving cycle on flat land.
22 Hybrid vehicles
As for the mass, the changes observed on the three driving modes can be summarized
(Table 1.8).
Table 1.8. Quantification of the linear effect of vehicle rolling loss coefficient on the energy balance and engine
operation, expressed as a % of fR %
Traction energy Urban use Rural road use Motorway use
Braking energy 0.148 0.226 0.184
Consumption
-0.146 - 0.240 -0.196
0.088 0.164 0.142
The values indicated in Table 1.8 are much lower than those shown in Table 1.7 (effect
of mass), which amounts to saying that the effect of rolling losses is lower than the effect of
mass. If the rolling losses are increased by 10%, the traction energy consumed by the vehi-
cle is therefore increased by just 1.48% in urban use, 2.26% in rural road use and 1.84% in
motorway use. We therefore see that, both for the energy required during traction and the
energy that can be recovered during braking, the change gradients are directly proportional
to the share of energy related to rolling losses (15%, 23% and 18% respectively) (Tables 1.1
to 1.3).
Tire of the future to answer the challenge of sustainable mobility
(P. Couasnon, Communication and Brands, Michelin Group)
Today, there are nearly 800 million vehicles on the road worldwide, a figure that could
double by 2030. Issues in areas such as health and safety, energy and raw material con
servation, environmental protection must be addressed to ensure sustainable mobility.
The tire plays a role through permanent innovations aimed at simultaneously improv
ing safety, longevity and fuel efficiency. At Michelin, this is known as the balance of
performance.
Figure E.1.3
Tire performances balencing
Source: Michelin
Chapter 1 · Vehicle use 23
Tire life cycle assessments have shown that three quarters of the tire's environmen
tal impact are due to fuel consumption during use.
Figure E.1.4
Tire life cycle
Source: Michelin
It is therefore extremely interesting to monitor the consumption improvement
obtained by reducing the rolling resistance. The resistance has been divided by a factor
of more than three since the introduction of the solid tire, with the emergence in particular
of the radial tire then the introduction of silica in rubber compounds.
Figure E.1.5
Automotive tire rolling resistance evolution
Source: Michelin
24 Hybrid vehicles
Research is continuing on this subject since the tire still represents about 20% of the
energy consumed by the vehicle, a percentage which can reach 30% for a heavy goods
vehicle or even an electric vehicle in urban use.
Several pathways can be considered for the future.
- The ultra-low rolling resistance tire of the future with the emergence of new poly
mers, latest generation silica and the progress made in tire architectures.
- Another solution consists in reducing the tire - and therefore the vehicle - mass by
developing new dimensions which, although smaller, are capable of carrying up to
25% more load. For example, the current 14 inch diameter tire can be replaced
by a solution offering equivalent performance but with a 10 inch diameter, thereby
saving several precious kilograms as well as space inside the vehicle. This solu
tions seems particulary adapted to small urban vehicles.
Figure E.1.6
Example of small tires for an urban vehicle
Source: Michelin
For larger road vehicles, tire dimensions of greater diameter and optimized width
can be considered. Large diameters help reduce rolling resistance, while narrow treads
improve aerodynamics, a non-negligible criterion, at high speed.
It is therefore clear that the tire has a fundamental role to play in tomorrow's sus
tainable mobility through its function as vehicle safety component but also by controlling
its environmental impact (fuel consumption and C 0 2 emissions, mass and use of raw
materials).
The various improvements will have to be made simultaneously The combined
contribution of technology and innovation will be essential to answer the challenge of
tomorrow's sustainable mobility.
Chapter 1 · Vehicle use Next Page
25
1.4.3 Effect of Aerodynamics
As with rolling losses, aerodynamic drag is a dissipative loss which increases the energy
required for traction and reduces the energy recoverable during braking.
Since this drag is proportional to the square of the aerodynamic speed (vehicle speed
+ component of the wind speed projected along the vehicle displacement axis), its effect is
therefore mainly noticeable at high speed.
The following figure shows the effect of a ± 25% variation in aerodynamic drag 10 on the
energy balance and vehicle fuel consumption for the motorway Artemis cycle (the effect is
strongest on this cycle).
Figure 1.12
Effect of S.CX (aerodynamic loss coefficient) on the traction and braking ener-
gies (a) and on operation of the engine (fuel consumption and average effi-
ciency) (b) - motorway Artemis cycle.
As with the mass and the rolling losses, the effect of the aerodynamics can be expressed
as a % increase in energy consumption per % increase in aerodynamic losses (Table 1.9).
Table 1.9. Quantification of the linear effect of vehicle aerodynamic drag on the energy balance and engine
operation, expressed as a % per S.CX %
Traction energy Urban use Rural road use Motorway use
Braking energy 0.048 0.282 0.618
Consumption
- 0.040 - 0.242 - 0.420
0.026 0.204 0.476
10. As a reminder, it is estimated that, on a vehicle, the outer rearview mirrors alone are responsible
for about 5 to 10% of the aerodynamic drag and that adding a package on the vehicle roof may increase
its aerodynamic drag by 40%.
Previous Page Hybrid vehicles
26
As expected, Table 1.9 shows that the effect of aerodynamics is very slight on the slowest
cycle and, on the contrary, very strong on the fast cycle: for aerodynamic losses increased by
10%, the energy required during traction is increased by 6.18% in motorway use and only
0.48% in urban use.
1.4.4 Effect of Auxiliaries
The energy produced by the engine is not used solely for vehicle traction: a certain number
of ancillary functions also consume energy, either electrical or mechanical.
These ancillary devices, which can be grouped under the generic name of "auxiliaries",
are used for various purposes:
- to run the powertrain (engine and gearbox ECU, ignition and injection control,11 etc.),
- to operate the vehicle safely (lighting/signaling, ABS braking control, ESP trajectory
control, etc.),
- or to provide a certain degree of comfort to the driver (power steering, servo braking)
and the passengers (heating, air-conditioning, etc.).
Mechanical energy consumers are generally the servo-braking vacuum pump (diesel
vehicles), the power steering hydraulic pump (electric steering is now becoming more wide-
spread 12) and air-conditioning. Consumption of the vacuum pump (which runs permanently)
remains low, but the power steering pump may consume up to 1 kW when operating and the
air-conditioning up to 2 or 3 kW (variable power depending on the vehicle application and
the temperature variation dynamic imposed by the manufacturer).
Electrical consumption ranges from a few dozen watts (ABS and ESP) to several hundred
watts (injection) for ECUs. Similarly, ignition consumes a maximum of a few dozen watts.
We may nevertheless mention some large consumers: demisting of the rear window (by
heated wires embedded in the glass, ~600 W), although this function is generally only used
for a few minutes, and the heating elements added in the passenger compartment heating cir-
cuit of some vehicles, in particular diesel13 (~1 kW), in order to heat or demist the passenger
compartment quickly, even when the engine is cold.
On board the vehicle, electricity is produced by an alternator driven by the engine via a
belt. These electric machines (generally synchronous with wound rotor mounted on claw-
shaped pole pieces) are highly constrained by the fact that they must produce powerful cur-
rents at very low speeds of rotation (engine idling) and by the low output voltage (12 V)
required. While simple and proven technologies must be used to reduce the cost of parts,
11. Oil pumps, water pump and fuel injection pump could be included in this category, but they are
generally considered as engine components.
12. With an electric system, the pump rotation speed no longer depends on the thermal engine rotation
speed. As a result, the pump does not have to be designed to provide its maximum flow rate at low
speed of rotation (maneuvers carried out with the engine idling).
13. On thermal vehicles, the passenger compartment is heated by taking some of the heat produced by
the engine, which does not consume any energy. On electric vehicles, production of heat in winter is a
real problem.
Chapter 1 · Vehicle use 27
they are not always very efficient (for example: diode rectifier to convert the alternating cur-
rent produced by the alternator into the direct current required by the battery and the vehicle
network). This has an adverse effect on the global electricity production efficiency on board
the vehicle, with efficiencies as low as 50 to 75%. To produce 1 kW of electricity on board
the vehicle, the engine may therefore have to produce up to 2 kW of mechanical power, a
non-negligible figure compared with the average powers required for traction, especially in
urban conditions.
For a constant power (Paux) consumed by the auxiliaries over time, the associated energy
(Waux) can be quickly calculated since it is proportional to the trip duration and the power
consumed:
(1.9)
Since the energy (mechanical or electrical) is sampled before the wheels, the auxiliaries
have no influence on the energy required for traction or on the energy recoverable during
braking, which are calculated at the wheels. If we consider the engine operation, however, we
observe, as with the other losses, that the total energy to be provided by the engine increases.
Logically, the engine efficiency should also improve as the load increases, although the effect
is hardly noticeable. The global vehicle consumption therefore increases with the power
requested by the auxiliaries. Figure 1.13 shows a graph of consumption against power on the
rural road Artemis cycle. Table 1.10 quantifies this effect on the three cycles (caution: this
time, the change coefficients are given for every 100 W of auxiliary power, assuming that
this power is drawn off continuously and not intermittently during the cycle).
Figure 1.13
Effect of the power of the auxiliaries on operation of the internal combustion
engine (consumption and average efficiency) - rural roads Artemis cycle.
28 Hybrid vehicles
Table 1.10. Quantification of the linear effect of the power consumed by the vehicle mechanical auxiliaries on the
energy balance, expressed as a % per 100 W of Paux
Traction energy Urban use Rural roads use Motorway use
Braking energy 0 0 0
Consumption 0 0 0
3.743 1.581 0.720
We observe that 100 W of electrical power consumed on the onboard network (which
requires up to 200 W of mechanical power from the engine, based on our efficiency assump
tion) cost about 3.74% of extra fuel, i.e. 0.34 L/100 km in urban use for our reference vehicle.
1.5 STUDY OF THE NORMALIZED EUROPEAN DRIVING CYCLE
Compared with the Artemis cycles studied above, which are representative of real use since
obtained by statistical processing of records taken on vehicles, the normalized European
driving cycle was produced by the Motor Vehicle Emissions Group, to homologate road
vehicles of mass less than 3.5 t in Europe. The purpose of this homologation is to:
- check that the emissions of regulated pollutants (HC, CO, ΝΟχ and particulates) are
below the authorized limit on the homologation date (UNECE regulation R83) (see
Appendix 3);
- calculate the vehicle fuel consumption and C 0 2 emissions (UNECE regulation R101)
(see Appendix 3).
1.5.1 Definition of the Cycle
The Normalized European Driving Cycle, formerly called MVEG after the name of the work
group which proposed it, consists of a pattern a priori based on a first urban driving cycle
(ECE cycle) repeated 4 times, followed by an extra-urban driving cycle (EUDC) reaching
120 km/h (Figure 1.14a). The pattern of speed as a function of time shows numerous test
phases during which the speed is constant ("stabilized" points), connected by constant accel
eration or deceleration phases (Figure 1.14b). Its representativeness with respect to real use is
therefore often questioned. This cycle nevertheless gives good results in terms of test repeat
ability, which is essential for vehicle homologation.
To reproduce the conditions of use of our vehicles as closely as possible, the cycle must
be performed on a cold engine (vehicle conditioned for several hours at 25 °C), which is
more severe in terms of pollutant emissions since the pollutant post-treatment system is not
active immediately after a cold start. The main change to the cycle was the removal in year
2000 of the 40 s idle period phase after starting the engine, during which the engine emissions
Chapter 1 · Vehicle use 29
were not measured. Since 2000 (standard Euro 3), engine emissions are therefore measured
from engine start. The modified cycle is generally called the NEDC (New European Driving
Cycle).
Figure 1.14
Definition of the NEDC cycle (a) and positions of the vehicle operating points
in a vehicle Speed-Acceleration field.
The energy balance can be used to quantify the share of the various forces acting on the
vehicle during traction and braking (Table 1.11). The result obtained is slightly different
from that of the Artemis cycles, firstly since the NEDC is a mixture of 4 km in urban mode
and 7 km in extra-urban road, and secondly due to the presence of steady speed segments,
rarely found on real cycles. The average energy during traction is 110 Wh/km, which is less
than all the values calculated for the various conditions of use (Table 1.1 to 1.3) for the same
vehicle. The recovery potential during braking (32 Wh/km) is also lower than for urban and
rural roads types of use.
Table 1.11. Energy balance on the NEDC
Inertia Rolling Aerodynamic Total Average
point
Traction phase 43.2 Wh/km 31.4 Wh/km 35.7 Wh/km 110.3 Wh/km
(Wheel 39% 0.21 m/s2
power > 0) 29% 32% 100% 6.13 kW
Braking phase -43.2 Wh/km 5.6 Wh/km 5.4 Wh/km - 32.4 Wh/km - 0.78 m/s2
(Wheel 12% 75% -6.81kW
power < 0) 100% 13%
30 Hybrid vehicles
1.5.2 Effect of the Various Vehicle Parameters on the "Hot" NEDC
This section is based on the same parameter variations as those studied on the Artemis cycles.
However, the effects induced on the engine by cold starting are neglected (hence the name:
"hot" NEDC). These effects are discussed specifically from vehicle test results (1.5.2.3).
1.5.2.1 Optimization of the Gearbox Ratio
As for the Artemis cycles, in case of manual gearbox the NEDC imposes the gear to be
engaged at each moment of the cycle (Figure 1.15a). With automatic or semi-automatic gear-
boxes, the transmission ECU selects the gear, allowing optimization biased towards fuel
consumption or driving comfort (Figure 1.15b).
Figure 1.15
Change of the engine operating region used on hot NEDC cycle depending
on whether gearbox management is imposed by the cycle (a) or optimized for
consumption (b).
On this cycle, by optimizing the gear change times with the sole objective of reducing fuel
consumption (disregarding driving comfort constraints), we can expect 6.6% fuel savings.
1.5.2.2 Effect of the Vehicle Parameters
As for the Artemis cycles, the parameters defining the vehicle - mass, tire rolling coefficient,
aerodynamics - can be adjusted. For the auxiliaries, the electrical power varies from 0 to
1,500 W consumed on the onboard network. In this case, the changes observed are also linear
and can therefore be included in Table 1.12 (note that these calculations were performed by
simulating a vehicle with a hot engine; the effects of cold start are not taken into account).
Chapter 1 · Vehicle use 31
The results obtained show that the mass has the greatest impact on consumption changes.
On this cycle, however, the predominance of mass is less pronounced than under the urban
use conditions studied previously. This is due to:
- firstly, definition of the cycle, which combines an urban phase and an extra-urban
phase,
- secondly, presence of steady speed segments which have a non-negligible impact on
global consumption.
Table 1.12. Quantification of the linear effect of the vehicle definition parameters on the energy balance and
engine operation on NEDC (hot conditions)
Traction energy Mass Rolling Aerodynamic Electrical
Braking energy auxiliaries
Consumption %/mass % %/fR % %/s.cx % %/100WofPaux
0.668 0.284 0.324
1.136 0
0.344 -0.176 -0.168 0
0.146 0.166
2.801
To serve as examples, we will give a few fuel consumption and C 0 2 emission figures:
- our reference vehicle consumes 6.1 L/100 km on the "hot" NEDC, which corresponds
to 145 g of C 0 2 per k m 1 4 ;
- increasing its mass by 10% (mass: 1,360 —> 1,496 kg) increases its consumption by
3.44%, bringing it to 6.3 L/100 km, i.e. 150 g of C 0 2 par km;
- increasing its rolling losses by 10% (fR: 0.010 —> 0.011) increases its consumption by
1.46%, bringing it to 6.2 L/100 km, i.e. 147.5 g of C 0 2 per km;
- increasing its aerodynamic losses by 10% (S.Cx: 0.69 —> 0.76 m2) increases its con-
sumption by 1.66%, bringing it to 6.2 L/100 km, i.e. 148 g of C 0 2 per km;
- increasing the electrical power consumption of the auxiliaries by 100 W (Paux:
400 —> 500 W) increases consumption by 2.80%, bringing it to 6.25 L/100 km, i.e.
149gofC02perkm.
1.5.2.3 Effect of Cold Start
As mentioned earlier, the NEDC for vehicle homologation imposes a cold start. More spe-
cifically, the standard stipulates that the vehicle must be conditioned at 25 °C for 12 h before
performing the test. Running the powertrain cold during the first part of the cycle has numer-
ous consequences (not taken into account in the simulations described so far) on both con-
sumption and pollutant emissions.
14. Knowing the fuel density and average chemical formula, we obtain a direct relation between the
volume fuel consumption and the C02 emissions (see Appendix 2).
32 Hybrid vehicles
In terms of pollutant emissions, various physical phenomena related to cold start must be
taken into account (most are common to gasoline and diesel engines):
- the catalyzer in the engine exhaust line is not effective on any of the three regulated
pollutants (HC, CO and ΝΟχ) until it reaches about 350 °C. The pollutants emitted by
the engine during this phase are therefore not processed and are discharged directly
into the atmosphere. On gasoline vehicles 15, and increasingly on diesel vehicles, a
specific strategy has been developed to reduce the time required by the catalyzer to
heat up. The catalyzer activation times (which vary depending on the vehicles and the
pollutants) are currently less than 60 s;
- vaporization of the liquid fuel in the engine is more difficult when cold, leading to an
increase in emissions of unburnt hydrocarbons (HC);
- on gasoline vehicles, it is more difficult to regulate the equivalence ratio (wetting of
the walls, equivalence ratio measurement unavailable for several seconds after starting
the engine for some probes, etc.). Operation at an equivalence ratio other than stoichi-
ometry increases emissions (HC and CO if the fuel/air ratio is too high; ΝΟχ if the air/
fuel ratio is too high).
In terms of consumption, cold running induces over-consumption due to:
- higher friction in the engine as well as in the gearbox, caused mainly by the greater oil
viscosity. Although the engine warms up fairly quickly (due to the use of a thermostat
in the water circuit so that only a small volume of the cooling water is heated up first),
it takes a lot longer for the gearbox which is heated solely by its internal losses and
the heat received from the engine by conduction along the shaft line and the casings;
- higher emissions of unburnt hydrocarbons, which amounts to wasted fuel;
- the catalyzer activation strategy (catalyzer initially fitted on gasoline vehicles only,
but now increasingly used on diesel vehicles): the extra heat sent to the catalyzer is
obtained at the expense of a significant drop in engine efficiency (ignition retarded
with respect to top dead center - TDC);
- to a lesser extent, the coefficient of friction of the tires is slightly higher since cold tires
are less flexible and have lower pressure.
By comparing tests performed on cold engine (vehicle conditioned at 25 °C before the
test) and hot engine (the vehicle has already traveled a few kilometers before the test) on real
vehicles, we can quantify the consumption penalty during the cold test for various gasoline
vehicles (Figure 1.16).
15. And, by extension, vehicles running on fuels whose combustion is similar to that of gasoline: LPG,
natural gas (VNG), ethanol (pure or blended with gasoline in proportions which may vary from one
country to another: for example E10 with 10% ethanol, E85 with 85% ethanol).
Chapter 1 · Vehicle use 33
Figure 1.16
Over-consumption measured for the various ECEs of a test in cold conditions
compared with the average ECEs of a test in hot conditions for various gasoline
engines.
The over-consumption (greater than 20%) during the first cold ECE (195 s) is mainly due
to the catalyzer activation strategy. This is no longer true of the other ECEs since catalyzer
activation always takes less than 195 s. The other reasons for over-consumption (friction,
mixture formation, etc.) still remain, however, but their impact decreases as the powertrain
warms up. Globally for the entire cycle, the consumption generated with cold start is 4 to
10% greater than for the hot cycle.
1.6 ENERGY RECOVERY DURING BRAKING
In the energy balances described previously, we calculated the braking energy as the integral
of the negative traction powers applied at the vehicle wheels. On conventional vehicles, this
energy is lost since dissipated as heat via the powertrain losses (engine brake) or the vehicle
brakes. In a hybrid or electric vehicle, it is important to recover this energy (which is simply
the vehicle kinetic energy after deducting the losses - rolling and aerodynamic) to store it and
then reuse it for other traction phases. The vehicle energy balance is therefore significantly
improved.
Nevertheless, not all the vehicle kinetic energy can be recovered during braking, either
because only one axle is equipped with an electric machine whereas braking occurs on both
axles for safety and efficiency reasons, or because one part of the electric chain (machine,
34 Hybrid vehicles
electronics or storage) is limiting in terms of power. In this paragraph, we will first describe
the physical phenomena which limit energy recovery during braking, then try to quantify the
share of energy which can be recovered in average use, depending on the choice of power-
train architecture or the braking management strategy implemented.
1.6.1 Mass Transfer
The notions of tire grip and mass transfer must be introduced in order to study braking (we
will restrict ourselves to braking in straight line).
Figure 1.17
External forces acting on the vehicle during braking.
In paragraph 1.1 (Figure 1.1), we analyzed the forces applied to a vehicle moving at con
stant speed. We will now consider the case of a vehicle during braking. The various forces
acting at the bottom of the wheel (reaction of tires and tractive force) can be added directly
to produce a force that can be broken down into its axial component Xi and its normal com
ponent Nj (where i is the axle index: 1 for the front, 2 for the rear). The adhesion demand is
expressed as
With these notations (Figure 1.17), we can write the following equations: (1.10)
- in projection on the horizontal axis (1.11)
- in projection on the vertical axis (1.12)
- moments about G
allowing us to calculate Nj and N2 as a function of the acceleration γ and the position of the
center of gravity (€l5 €2, h):
and (1.13)
Chapter 1 · Vehicle use 35
Nj is therefore the sum of two terms:
m h *δ·^2 which is the mass supported at the axle when acceleration is zero (static load)
——
m ,.γ.η
—— called the mass transfer induced by the acceleration γ.
And similarly for N2.
We see that the mass transfers of the front and rear axles have opposite signs: when
accelerating (γ > 0), the front axle supports a lower load than when stationary, while the rear
axle supports a higher load. When braking (γ < 0), the front axle has a higher load and the
rear axle a lower load.
1.6.2 Braking Distribution Between Axles and Equal Adhesion Parabola
This variation in the vertical force induces a difference in horizontal force that each wheel
can transmit before the wheel locks (during braking) or skids (during acceleration).
Vehicle tires are deformable bodies which can only transmit a horizontal force if the tire
locally deforms and consequently if there is a certain degree of sliding between the tire and
the road. Tire adhesion measurements reveal a region where the adhesion increases linearly
with the sliding up to a maximum μιηΗχ for 15 to 30% sliding. For greater sliding values,
adhesion gradually decreases, quickly causing the wheel to lock. For the following calcula
tions, we will use a simplified adhesion law (Coulomb's law) more suitable to rolling of
non-deformable solids:
incase of rolling (1.14)
in case of sliding (1.15)
For vehicles driving on a smooth, flat road, the values of μ are as follows:
- tire-- dry tar 0.8 to 1.0
- tire-- damp tar (0.2 mm of water) 0.5 to 0.65
- tire-- wet tar (1 mm of water) 0.3 to 0.55
- tire--ice 0.1 and less
In the remainder of this document, we will only work with the max. adhesion μιηΗχ char
acteristic of the tire-road contact under the temperature and humidity conditions considered.
This value will be simply written μ.
In other words, the horizontal force that can be transmitted by this contact is limited to
the value μ Ν. For a given coefficient of friction μ (which depends mainly on the materials in
contact), the horizontal force is therefore proportional to the vertical load of the tire.
During braking, both axles help to decelerate the vehicle. Components Xj and X2, as well
as acceleration γ, are negative.
The front axle locks when the requested adhesion μ1 reaches the maximum adhesion μ
allowed by the materials in contact, then for a load:
36 Hybrid vehicles
After development, we obtain Xl,lim (1.16)
(1.17)
and similarly X, (1.18)
2,lim
Rather than working on the forces as calculated above, we generally speak about the
braking ratio of each axle by dividing the braking force Xi by the total vehicle weight niveh-g.
To work on the distribution of braking force between the two axles, we can use a graph
with the front axle braking ratio on the x-axis and the rear axle braking ratio on the y-axis
(Figure 1.18). With this type of graph, the total iso-braking ratio straight lines (sum of the
front braking and rear braking ratios: X j / m ^ . g + X2/mveh.g) are parallel to the second
bisector. They are also vehicle iso-deceleration straight lines (if we neglect the aerodynamic
losses), according to equation 1.9.
Figure 1.18
Equal adhesion parabola for a vehicle of lower medium segment Mj
Equation 1.17 shows that, for a given value of μ, the maximum braking ratio that the front
axle can develop just before locking is an increasing linear function of the rear axle braking
ratio; since deceleration is increasing, the vertical load of the front axle increases (almost
vertical dotted lines on Figure 1.18). Similarly, the maximum braking ratio of the rear axle is
Chapter 1 · Vehicle use 37
a decreasing linear function of the front axle braking ratio (eq. 1.18) since the mass transfer
tends to take the weight off the rear axle (almost horizontal dotted lines on Figure 1.18). For
a given value of μ, the intersection of these two straight lines gives a point corresponding to
simultaneous lockup of both axles and the maximum total braking ratio that can be obtained
on the vehicle considering the adhesion μ.
When μ is varied, the simultaneous lockup point of the two axles follows a parabola
called the vehicle equal adhesion parabola (Figure 1.18). This parabola can be graduated in
adhesion values (every 0.1 on Figure 1.18).
When calibrating the vehicle braking system, the aim is to satisfy two conditions for all
values of μ:
- remain permanently below the equal adhesion parabola so that the front axle locks first
(the opposite could result in the vehicle spinning),
- remain as close as possible to the equal adhesion parabola in order to maximize the
total braking force before locking one of the axles.
Technically speaking, this involves fitting a brake limiter on the rear axle.
1.6.3 Energy Recovery During Braking at Constant Deceleration
The previous equations allow us to study braking forces theoretically.
All the numerical applications will be carried out with our reference vehicle. In addition
to the data given previously (1.4), we will consider:
- wheelbase (distance between the two axles) L = 2.61 m
- load distribution
756 kg at the front and 604 kg
- height of the center of gravity at the rear (which amounts to
€1 = 1.16mand€2=1.45m)
0.56 m
The equal adhesion parabola of this vehicle is shown on Figure 1.18.
We will now study the braking forces involved during constant decelerations from
120 km/h for various deceleration values. Since the vehicle losses (aerodynamic and rolling)
contribute to deceleration, the maximum braking forces occur at end of braking (when the
aerodynamic losses have become negligible). However, since the power is the product of
the vehicle speed by the braking force, maximum power is observed at the start of braking.
Lastly, by integrating the power with respect to time, we can calculate the energy involved
on each axle.
Figure 1.19. a shows the power to be dissipated on each axle at the start of braking, assum
ing that the distribution of braking forces between axles is that prescribed by the equal adhe
sion parabola. We can see that the power reaches 100 kW on the front axle for a deceleration
of 4 m/s2. During emergency braking on dry road (μ of 0.9, therefore deceleration of up to
9 m/s2), the powers to be dissipated on the front and rear axles will reach respectively 300
and 100 kW, very high values compared with the traction. Obviously, as the vehicle slows
down and even if deceleration is maintained (therefore at constant braking force), the power
38 Hybrid vehicles
decreases with the speed. Figure 1.19.b shows the proportion of kinetic energy dissipated as
heat by the brakes of each axle, still using the equal adhesion assumption (at 120 km/h, the
kinetic energy of our reference vehicle is 0.21 kWh). We see that the higher the requested
deceleration, the greater the transfer of braking energy from the rear axle to the front axle
caused by mass transfer (1.6.1). At the same time, we observe that the total braking energy
(both axles combined) increases since the work carried out by the vehicle losses decreases
due to the shorter braking time.
In order to extrapolate to other situations, we must bear in mind that the total braking
force is the product of vehicle mass multiplied by the requested deceleration, while the power
is the product of this force by the speed. For an 800 kg urban vehicle reaching a decelera
tion of 9 m/s2 (maximum deceleration that can be reached on ground with μ = 0.9), the force
is 5,600 N and the total power at 50 km/h is 77.8 kW. However, the kinetic energy is only
21 Wh (braking lasts only 1.5 seconds).
1. Corresponds to the automatic warning light activation according to UN ECE R13 regulation
Figure 1.19
Braking power (a) and energy (b) on each axle when braking from 120 to
0 km/h depending on deceleration on our reference vehicle (1360 kg)
On a hybrid vehicle fitted with a reversible energy storage system, it may be possible to
recover all or some of this braking energy. In this case, however, we must take into account
the power limitation of the electrical system recovering this energy (whether this limitation
be due to the electric machine or the storage): when the power required for braking exceeds
the capacities of the electric system, the mechanical brakes take over to absorb the surplus
power, which means that this amount of energy will be lost.
Chapter 1 · Vehicle use 39
Figure 1.20 shows the result obtained for both front and rear axles during braking from
120 km/h, assuming that the braking distribution between the axles corresponds to the equal
adhesion parabola shown in Figure 1.18. We observe that all curves have the same shape
for all powers of the electric system: they follow the curve with no power limitation for low
decelerations, then exhibit an inflection point before decreasing for high accelerations.
Figure 1.20
Proportion of recoverable kinetic energy on front (a) and rear (b) axles depend-
ing on the braking intensity and maximum power of the electric system when
braking from 120 to 0 km/h.
Obviously, this saturation effect increases when braking at higher vehicle speed, since
higher braking powers are involved as shown on Figure 1.19a. Figure 1.21 illustrates the
same result when braking from 60 to 0 km/h.
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Heavy Duty Truck Systems by Sean Bennett
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