Heavy Duty Truck Systems by Sean Bennett

Chapter 2 · Internal Combustion engines 91

In some engines of this type, at partial load combustion is no longer controlled by the
spark generated at the plug, it is the heat transmitted to fresh gases by the residual burned
gases that generates auto-ignition of the fresh gases. This auto-ignition, which is found almost
throughout the entire volume of the chamber, could be extremely harmful to the engine if the
very high dilution caused by burned gases does not, at the same time, lead to a clear reduc­
tion in the release of heat, which may ultimately be acceptable although it occurs much more
rapidly than during normal combustion. The principal difficulty is that there is no longer
any direct control of starting combustion because the ignition spark no longer participates
in combustion. Combustion is then sensitive to the engine's operating conditions (thermal
state and history, outside conditions). Nevertheless, this type of combustion has been used
in series on some two-stroke engines, primarily from Honda, which refers to them as Acti­
vated Radical Combustion (ARC) engines. Some of the leading advantages claimed for such
engines are more stable engine operation and reduced ΝΟχ emissions.

This method of combustion can be transposed to four-stroke engines (in the form of
controlled auto-ignition, CAI), providing the poor elimination of exhaust gases can be dupli­
cated. To do this, the engine's valve timing must be modified. Various solutions are possible
but the simplest is to reduce the duration of the exhaust phase: the exhaust valves are opened
at the very end of expansion (when the piston begins its return to TDC) but the process is
interrupted long before the piston reaches the top of the cylinder. Because the valves are
closed before the end of the exhaust stroke, the burned gas trapped in the chamber is com­
pressed. To avoid the complete loss of the energy expended for this compression sequence,
the intake valves are not opened until the piston has returned to the position it had assumed
when the exhaust valves were closed. However, the amount of time the intake valves are
closed is the same as it is during conventional combustion (near BDC), which results in - for
intake as well as exhaust - a reduction of the time the valves remain open. In this way the
timing diagram used to implement CAI combustion is characterized by shorter opening dura­
tions for both exhaust and intake valves. In the case of the exhaust valves, this process limits
exhaust; in the case of the intake valves, it prevents loss of the recompression energy from
residual gases. In practice, it is necessary to make use of a very flexible valve train system,
otherwise the engine can only operate over a very limited range.

In analyzing the resulting combustion process, we find, at the end of the compression of
the air-fuel-residual gas mixture, spontaneous auto-ignition at some point in the combustion
chamber. This auto-ignition results from the heat supplied by the residual gases and com­
pression of the mixture. Its spatial selectivity arises from the heterogeneity of the mixture
and the temperature in the chamber, so that, from cycle to cycle, the initiation of combustion
will vary (even if there are preferential zones). Once combustion has started, auto-ignition
spreads quickly throughout the chamber because nearly the entire mixture experiences sim­
ilar thermal conditions. However, because of the very strong dilution caused by residual
burned gases, the chemical kinetics of combustion are considerably slower. Combustion
occurs without a flame front or, more precisely, with a flame front that, rather than measuring
1/10 of a millimeter in thickness, is dispersed throughout the entire chamber. Even if chemi­
cal reactions are locally slow, the fact that they occur simultaneously throughout the chamber
results in a release of energy that is faster overall than it is during normal combustion: with
a few degrees of rotation of the crankshaft, the entire combustion process is complete. This

92 Hybrid vehicles

generates noise. However, the temperatures obtained within the chamber itself are lower,
which greatly limits the formation of nitrogen oxides.

This type of combustion can only take place at low load for two reasons: the noise gener­
ated increases rapidly as the load increases, and it is impossible to introduce large amounts of
fresh mixture and large amounts of residual burned gases into the same volume at the same
time. Therefore, before it can be used in a vehicle, a dual-mode engine must be designed that
can operate with CAI at low loads and spark ignition at high loads.

In terms of consumption, this type of combustion can lead to gains of 30% at certain
engine operating points compared to standard spark ignition, providing a lean mixture is
used. However, unlike stratified combustion, operating with a lean mixture does not require
the use of a dedicated after-treatment system for nitrogen oxides, given that raw ΝΟχ emis­
sions are very low. Controlling this type of combustion on the vehicle is difficult, however,
because it is the valve timing settings (when the valves open and close) that control combus­
tion, including transient periods when the engine changes operating points, and because we
must control the transitions between CAI and spark ignition modes at the border between the
two zones.

2.2.4 Summary

Table 2.5 shows the fuel consumption gains that can be expected from conventional gasoline
powered vehicles in 10 to 15 years by implementing the various technological solutions
described above. If different solutions can be combined in the same engine, their respec­
tive gains are not additive, however. To see why, consider that downsizing, variable valve
train, charge-diluted combustion, and cylinder deactivation all reduce pumping losses. It is
estimated that, over the long term, potential fuel consumption gains in gasoline-powered
vehicles will be on the order of 25 to 30%.

Table 2.5. Potential fuel consumption gains in gasoline engines

Technology used Expected consumption gain
10-20%
Downsizing 5-15%
5-15%
Variable valve systems 10-15%
15-25%
Charge-diluted combustion 10-20%

Cylinder deactivation 5-10%

Downsizing coupled with variable compression ratio

CAI (controlled auto-ignition)

Other advances in engine design that do not directly involve
the combustion process (better use of the engine with optimized
transmissions, reduced engine friction, improvement of the
efficiency of auxiliary components, thermal management)

Chapter 2 · Internal Combustion engines 93

2.3 COMPRESSION IGNITION ENGINES (DIESEL ENGINES)

In comparison with the gasoline engine, which is in widespread use around the world, diesel
engines for passenger cars are primarily found in Europe and India, and to a lesser extent in
China. It is uncommon in Japan or the Americas.

In this type of engine, combustion is initiated, at the end of the compression phase, by
auto-ignition of the charge under the effect of increased temperature and pressure in the
combustion chamber. During the 1990s, diesel engines with indirect injection were largely
abandoned for direct injection engines equipped with turbochargers to boost pressure. Con-
siderable progress in diesel technology was made subsequently, with improvements to fuel
consumption, performance, and noise reduction. Over the past few years, in Europe, on aver-
age as many diesel as gasoline engines have been sold. Below, we describe combustion in
diesel engines and current development trends.

2.3.1 Description of Existing Diesel Engines

Modern diesel engines today make use of common rail direct-injection systems, turbocharg-
ing, and a system to recirculate high-pressure exhaust gas (HP-EGR) (Figure 2.18). After-
treatment of exhaust gas is provided by an oxidation catalyst, and, since the introduction of
the Euro 5 standard in Europe (2.1.2), a particulate filter.

Figure 2.18

Schematic operation of a diesel engine with a turbocharger and HP-EGR cir-
cuit. Turbocharger: TC, T: turbine stage, C: compressor stage.

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2.3.1.1 The Engine

A. Combustion
Unlike spark ignition engines, the conventional diesel engine operates with a lean mixture,
that is, with an excess of air compared to the stoichiometry. The load is controlled by the
amount of diesel fuel injected. Strictly speaking, there is no throttle on a diesel 8. Therefore,
diesels do not experience the pumping loss at partial loads found in gasoline engines, which
results in improved efficiency.

While gasoline engines most often use homogeneous combustion of the premixture with
propagation of the flame front, diesel engines use heterogeneous combustion propagated
by diffusion, that is, controlled by the encounter of fuel and air in the chamber. A modern
diesel combustion system is a successful merger of an injection system, efficient in-cylinder
air motion, and complex design of a bowl-in-piston geometry. The goal is to optimize the
coupling between chemical reactions and the very rapid flow of air, combustion phenomena
here responding to the laws of aerothermochemistry. Air is admitted into the combustion
chamber alone and rolls around the bowl in the piston head in a rotational movement known
as a "swirl." Injection of diesel fuel into the bowl occurs later, at the conclusion of the com-
pression phase. The jets of fuel emitted by the injector nozzle are dispersed into droplets and
vaporized by the high temperatures, mixing locally with air. This is followed by local auto-
ignition of fuel in the combustion chamber.

More precisely, there are two main phases in diesel combustion (Figure 2.19). Firstly, the
fuel accumulated during the self-ignition delay - duration between introducing the first drop
of fuel and the time when self-ignition starts - burns as a pre-mixture flame. This generates
the characteristic knocking sound of diesel engines, due to a rapid release of energy. A diffu-
sion flame then appears, i.e. the reaction rate is controlled by the mixing rate. During injec-
tion, combustion occurs around the fuel jet and is maintained by the introduction of diesel.
When injection is finished, any remaining unburnt fuel continues to oxidize as a diffusion
flame, depending on its location and internal aerodynamics.

8. Some recent engines do have a throttle in the air-intake circuit. It is used primarily to force the recir-
culation of burned gases by increasing the pressure differential between exhaust and intake, or, during
regeneration of the particulate filter, to promote a rise in temperature of the exhaust gas or to muffle the
engine when it is stopped.

Chapter 2 · Internal Combustion engines 95

Figure 2.19

The various phases of diesel combustion.

B. Injection System, Sensors, and Actuators

To better manage these two phases, we make use of the ability of modern injection systems
to perform multiple injections in very short periods of time. An initial injection, known as a
"pilot" injection, sometimes broken down into two small "double pilot" injections, initiates
combustion of a limited amount of fuel during the premix phase. This generates little noise
but can increase the temperature and pressure in the chamber. Then, during the next injection
- known as the main injection, and which can also be broken down into separate injections -
the auto-ignition delay of this new charge and, therefore, of the accumulated quantity burned
during the premix phase, is reduced. This limits combustion noise. In some cases, there is a
final injection ("post-injection") that promotes the oxidation of soot at the end of combustion
phase to reduce particulate emissions.

The great majority (up to 95%) of particulates produced are oxidized in the combustion
chamber. Most of the time, the exhaust gas contains no more than a miniscule amount of
particulates, which will have been produced throughout the combustion process.

To establish orders of magnitude, a "small" injection at low load involves no more than a
few milligrams of diesel fuel injected during an approximately one millisecond period. Injec­
tors have undergone extensive development over the years. The number of holes (each hole
generates a jet of diesel) and their shape (diameter, profile) are optimized together with the
shape of the piston bowl and internal air-motion of the chamber. Typically, an injector has 6
to 8 holes, each of which is about 100-200 μιη in diameter.

The piston bowl is circular with an "omega" shape and a central pin (Figure 2.20). This
shape promotes swirl and the distribution of diesel droplets throughout the entire combus­
tion chamber. The swirl is generated by the shape of the air intake ducts and its intensity is
sometimes controlled by an aerodynamic throttle inside one of the ducts.

96 Hybrid vehicles

Figure 2.20

Injector, swirl, and piston bowl in a diesel combustion system.

The actuators needed for operation of a diesel engine include the injection system, the
turbocharger, and the EGR valve; the sensors include an airflow meter, and sensors for injec­
tion pressure and intake air temperature.

C. Efficiency Breakdown

When applied to a diesel engine, the efficiency breakdown provides:
- higher combustion efficiency than gasoline engines (typically greater than 0.98), HC
and CO emissions are only a few g/kWh
- thermodynamic efficiency on the order of approximately 0.6, taking into account com­
pression ratios of between 16 and 18, and a thermodynamic coefficient, γ, equivalent
to lean-mixture combustion (γ equal to approximately 1.33)
- the values of cycle efficiency vary less widely than for a gasoline engine since there is
no penalty from pumping losses; measurement of experimental values range from 0.6
(low load) to 0.7 (high load)
- mechanical efficiency is highly variable with charge. This efficiency can be zero at
idling speed, when combustion merely compensates for friction and energy consump­
tion by auxiliary equipment; it exceeds 0.85 at full load. It is important to note that
friction losses in a diesel engine are greater than those for a gasoline engine because of
the greater size of the moving parts (piston-connecting rod-crankshaft) and the energy
consumed by the injection pump, which operates at very high pressure.
Figure 2.21 is an example of an efficiency map of a modern diesel engine. The maximum

efficiency reaches 40% at 2,500 rpm and high load, which results in specific fuel consump­
tion of approximately 215 g/kWh.

Chapter 2 · Internal Combustion engines 97

Figure 2.21

Example of an efficiency map for a 1.5 L diesel engine. The map shows the
iso-efficiency (black) and iso-power curves (brown) as a function of engine
rpm and torque.

2.3.1.2 Pollution Control: EGR and Emissions Control System

Regulated pollutants in diesel exhaust gas consist of ΝΟχ, particulates, HC, and CO. The
last two are post-treated on all modern diesel engines in an oxidation catalyst that transforms
them into C 0 2 and H20, using residual oxygen present in the exhaust gases. Increasingly, a
dedicated filter stores particulates and burns them periodically by increasing the temperature
of the exhaust gases. This temperature increase is generated by modifying engine settings
(late injection in the combustion chamber) or by using a supplementary injector located in
the exhaust circuit.

Until now, NOx gases have been processed at the source, that is, during the combustion
process itself, using EGR, which consists in reintroducing some of the burned gases from the
previous combustion into the combustion chamber. A pipe connects the exhaust manifold
to the intake air circuit and a valve controls the rate of recirculation in the pipe. In modern
engines, EGR gases are cooled by a heat exchanger that uses the engine's coolant.

Over the long term, EGR may be incapable of accommodating the growing strictness
of pollution standards and it may be necessary to add a system for after-treatment of ΝΟχ
effluents, at least in some vehicles. Considerable efforts have been invested in developing
two types of systems: ΝΟχ traps (similar to those used in lean burn gasoline engines) and
urea systems (selective catalytic reduction, or SCR). With SCR, a chemical reducer (urea) is
injected into the exhaust system ahead of a dedicated catalytic converter to convert nitrogen
oxides into nitrogen and oxygen.

Eventually, diesel engines will contain three emissions control systems in series: an oxi­
dation catalyst (for HC and CO), a particulate filter, and an SCR catalyzer (for ΝΟχ). Con­
siderable research efforts are underway to optimize and combine these systems in order to
reduce their complexity, size, and cost.

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2.3.2 Technological Improvements

2.3.2.1 Downsizing and Downspeeding

Over the past ten years, significant effort has been invested in reducing the displacement of
diesel engines. As with gasoline engines, this technique is known as downsizing. In addition
to the reduced losses from friction associated with the reduction in engine size, downsizing
helps reduce diesel consumption through the use of operating ranges that provide improved
efficiency (see Figure 2.12). To maintain the same level of performance possible with a larger
engine, the reduction in displacement is accompanied by an increase in boost pressure, and
turbochargers for diesel engines are undergoing constant improvement to meet these goals.

There has also been a tendency to reduce engine speed (rpm) in diesels during use, a
technique known as downspeeding. For a given power output, an engine that rotates at a
lower speed must operate with a greater load; the engine operates with greater efficiency,
as with downsizing. In addition, friction losses are reduced when engine speed decreases,
which further improves fuel consumption. Here too, downspeeding is made possible only
because of the improvements made to turbochargers, which deliver greater amounts of boost
pressure even when engine speed is low. These improvements have also been accompanied
by progress in transmission systems, which is necessary when using engines that function at
reduced operating speeds.

2.3.2.2 Two-Stage Turbocharging

Historically, in a diesel engine, the "matching" of a turbocharger - that is, the choice of a
specific model of turbocharger - was a compromise between the production of high torque
at low engine rpm (a constraint that requires the use of a "small" turbocharger) and the need
to generate adequate boost up to the maximum power running condition (which requires a
"large" turbocharger). More recently, turbocharger matching had to account for its ability
to ensure high rates of burned gas recirculation (EGR) at partial loads. Here, recirculation
of a non-negligible portion of the exhaust gases to the air intake circuit reduces the flow of
exhaust gas to the turbocharger turbine, which does nothing to promote energy recovery for
air compression. This new constraint as well leads to the use of a small turbocharger, to the
detriment of maximum power.

Two-stage turbochargers avoid the need to make this compromise, even if it is to the det-
riment of size and cost. Two turbochargers are used for this purpose (Figure 2.22).

Chapter 2 · Internal Combustion engines 99

Figure 2.22
Two-stage turbocharger (R2S™ system from BorgWarner). The diagram illus-
trates two turbochargers placed in the air intake and exhaust gas lines of a diesel
engine. Depending on the engine's operating point, the second turbocharger
(center) is activated through the action of a bypass valve (in the air-intake cir-
cuit, in blue) and a wastegate valve (in the exhaust gas circuit, in red).
Source: BorgWarner

2.3.2.3 Low-Pressure EGR

The EGR we have been discussing in this chapter is an HP-EGR (high-pressure exhaust gas
recirculation) system. To avoid interfering with the rate of exhaust gas flow when recirculat-
ing burned gases, a different arrangement of the EGR circuit is possible: low-pressure EGR,
or LP-EGR. Exhaust gas, after it exits the turbocharger's turbine stage, enters the circuit and
is returned ahead of the inlet to the turbocharger's compressor stage (Figure 2.23). The use
of LP-EGR does not reduce exhaust gas flow through the turbine, thereby promoting energy
recovery by the turbine, unlike HP-EGR. The turbo lag is reduced. In the air circuit, the
compressor has a greater flow of air to compress, allowing it to operate in a more favorable
region, far from the surge limit9.

EGR gas cooling is improved as well. The gases are cooled as they pass through the tur-
bine, then again as they mix with air before reaching the compressor, and as they pass through
the heat exchanger for EGR gas, and once again as they cross the air intercooler. Because the
gas is denser, the mass of recirculated exhaust gas introduced into the combustion chamber

9. When the turbo compressor delivers pressure but the flow through the compressor is too low, we
observe instability in the flow of air over the vanes of the compressor. This unwanted phenomenon is
known as surging. Conversely, if the flow of air through the compressor is too great, there is a risk of
choking.

100 Hybrid vehicles

is greater, promoting a reduction in ΝΟχ. We also observe greater uniformity of the mixture
between EGR gas and intake air.

Unfortunately, this architecture is more difficult to integrate under the hood of contem­
porary vehicles because of the length of the circuit. Additionally, without the addition of a
check valve in the exhaust line, the amount of EGR gas will be limited, because the pressure
difference between the burned gas intake downstream of the turbine and the intake circuit is
too low. However, this check valve creates a pressure drop that has a deleterious effect on
engine efficiency. At some engine operating points, recirculated gases can be too cold, with
a negative impact on HC and CO emissions. Finally, separation of the EGR valve in relation
to the intake valves results in diminished response by the system to a request for a variation
in the rate of EGR. Modern engines are equipped with two EGR units that incorporate both a
low-pressure and a high-pressure circuit.

Figure 2.23
Diesel engine with LP-EGR system. Turbocharger: TC, T: turbine stage,
C: compressor stage.

2.3.2.4 Advanced Injection Systems
The ability of an injection system to manage several controlled injections of diesel fuel in a
very limited amount of time is critical. Even though current systems are already highly effi­
cient, further developments are ongoing.

One current area of development seeks to increase injection pressure, which is dependent
on the engine operating point. This pressure increases with the load. The value targeted by
the manufacturer when fine tuning the engine is recorded in a map stored in the computer that
controls the engine in the vehicle. It is typically approximately 400 bar at idling speed, 1,600
to 2,000 bar at full load, depending on the engine, and can reach 2,500 bar in some cases.
The benefit of high injection pressure is that it can help increase specific power by injecting

Chapter 2 · Internal Combustion engines 101

large amounts of diesel fuel in a very short time, which is the only way to simultaneously
control the beginning and end of combustion. Concerning maximum power operating point,
it is important to ensure that all the fuel injected is burned while engine speed is high and that
it is dependent on the auto-ignition delay.

Other research has been directed at increasingly fine control of the amounts of fuel
injected. Engine operation at partial load requires very small amounts of diesel, sometimes
divided into five successive injections. This means that injection systems must be able to
meter small quantities repeatedly. Work is also being done to reduce the influence of one
injection on subsequent injections, keeping the idle time between two successive injections
as short as possible.

Using prototype injection systems, we have also been able to vary the instantaneous flow
of injection even during an injection. This is known as rate shaping and will eventually
enable engineers to further optimize the interaction between air and fuel in the engine.

2.3.2.5 New Combustion Types for Diesel Engines

New antipollution standards that are now coming into force are pushing development toward
forms of combustion whose goal is the continued reduction of nitrogen oxide and particulate
emissions at the source. This may be obtained by avoiding combustion in locally rich zones
at high temperature. Several approaches are possible.

Once, is the use of homogeneous charge compression ignition (HCCI) combustion. Die-
sel fuel is injected very early in the cycle in order that the fuel is completely premixed with
air. There is no combustion in rich zones. In this case, combustion resembles gasoline com-
bustion but the diesel burns as a mass, without a flame front. This strategy is reserved for
operation at very low loads, while the amount of diesel fuel remains low, because of the noise
generated by combustion.

Other approaches emphasize premixed combustion, without interfering with diffusion.
Low-temperature combustion (LTC) is designed to reduce combustion temperature, primar-
ily through the massive introduction of recirculated burned gases (EGR). Auto-ignition is
delayed, leaving time for the fuel to mix. This results in less uniform combustion than with
HCCI combustion, but at a lower temperature. The operating range covered is also extended.

Engines using these methods of combustion are bimodal: they use premix combustion at
low and partial loads, and switch to conventional combustion for high loads.

Although no vehicle is sold today with an HCCI engine, diesel combustion in the latest
generation of engines greatly favors premixed combustion with high rates of EGR through-
out the operating points of the certification cycle. At present, this is the only way to make the
transition to the Euro 5 standard without having to add a dedicated after-treatment system for
nitrogen oxides.

Such methods of combustion have one intrinsic drawback: because of the low tempera-
tures obtained in the combustion chamber, oxidation of HC and CO is not promoted and
emissions of these two pollutants are higher at the source than in conventional engines. Of
course, these pollutants are treated by the oxidation catalyst, but the delay until the catalytic

102 Hybrid vehicles

converter "lights off once the engine has started needs to be controlled, which is to say that
its temperature must rise before it can become active.

2.3.3 Summary

By making use of the various solutions described above, the fuel consumption gains that can
be expected from diesel engines in the next 10 to 15 years are shown in Table 2.6. By com-
bining several technological advances, the potential for improving diesel consumption over
the long term is on the order of 15 to 20%.

Table 2.6. Potential fuel consumption gains in diesel engines

Technology used Expected consumption gain
5-10%
Downsizing and downspeeding 3-5%
3-5%
Improvement of supercharger systems 1-3%

Low-pressure EGR 5-10%

Improvement of injection systems

Other changes to engines that do not directly involve
the combustion process (better use of the engine through
optimized transmission systems, reduction of friction in
the engine, improved efficiency of auxiliary components,
thermal management)

2.4 USE IN THE VEHICLE

2.4.1 Effect of Engine Use on Energy Distribution

2.4.1.1 Cold Engine Operation

As we have seen, operation of an engine is highly dependent on load and speed. The engine's
operating temperature is another important factor that affects efficiency. Operating tempera-
ture is the temperature of the engine's coolant; it is approximately 90 °C when the engine is
"hot," that is, once the temperature has stabilized. The operating temperature is regulated by
controlling the flow of coolant, which circulates through a heat exchanger, commonly known
as a "radiator," that directs heat to the surrounding air in the front of the vehicle. This coolant
is also used to heat the vehicle's passenger compartment in cold weather. To do this, another
heat exchanger, located in the passenger compartment's ventilation system is used.

Several minutes are needed to stabilize the temperature of the coolant when the engine
is cold. Additional time is needed to raise the temperature of the engine's lubricating oil and
the oil in the transmission system.

Chapter 2 · Internal Combustion engines 103

As the temperature of the engine rises, the vehicle's energy consumption increases. For
example, the change in overconsumption during the European standard cycle is shown in
Figure 1.16 for different vehicles. Oil viscosity is greater, mechanical friction is higher, and
thermal losses along the combustion chamber walls are greater. Additionally, during the
period shortly after the cold engine is started, combustion control tends to promote increased
temperature in exhaust gas after-treatment systems so they become operational as quickly as
possible. Emissions control systems based on chemical reactions (such as three-way catalytic
converters for gasoline engines and oxidation catalytic converters for diesel engines) must
reach a temperature threshold - generally about 200 °C - to become active. Heat is directed
toward the emissions control system by exhaust gas, which is why, from the point of view of
combustion, engine settings temporarily promote high exhaust gas temperature to the detri-
ment of efficiency.

In diesel engines with good efficiency, the temperature of the engine rises slowly. There
is little energy in the form of heat to heat the passenger compartment. That is why these
vehicles are often equipped with electrical heaters, which are turned on when the engine is
cold to compensate for the lack of heat in the engine's coolant. These electrical heaters can
be placed in the engine's cooling circuit, in which case they participate in helping to raise the
temperature of the water, or directly into the passenger compartment's ventilation system.

Modelling engines with the simulation platform
LMS Imagine. Lab AMESim

(F. Le Berr, IFP Energies nouvelles)

For the multidomain simulation platform developed by LMS, IFP Energies nouvelles
created four libraries to model, amongst other things, engines and their exhaust gas post-
treatment systems. Various model levels can be used, in order to cover a broad range of
timescales and characteristic phenomena: from modelling of engines at the scale of the
thermodynamic cycle to taking into account phenomena related to turbulence.

The IFP-Engine library is used to model the engine, taking into account its various
components: combustion chambers, timing system, air loop architecture including the
supercharging system, etc. The characteristic timescale is the crankshaft degree. When­
ever possible, the models of the various components are based on a phenomenological
model to represent the physics involved. The combustion and pollutant models devel­
oped in this library are based on the experience of IFP Energies nouvelles regarding its
understanding of engine phenomena and simplification of multidimensional digital mod­
els. This level of modelling is especially well suited to design of the engine, its optimisation
and calibration, development of control laws, testing of the engine ECU and definition of
boundary conditions for the transmission system or the thermal components, for exam­
ple. The computation times for this type of simulation are generally greater than "real
time" for the reference models but may drop below "real time" through the use of suitable
model reduction or simplification methodologies or parallelisation in order to benefit from
the new architectures of multicore PCs.

104 Hybrid vehicles
The IFP-C3D library was developed for 3D simulation of reactive and two-phase

flows in piston engines, taking into account fuel injection jets. It involves numerous digital
methods and the latest physical models. The simulators obtained are especially suited
to highly detailed analysis of the phenomena inside the combustion chambers, such as
scavenging of residual burnt gases, formation of liquid fuel films on the walls, combustion
initiation and the formation of pollutants. Piston bowls, for example, can be optimised
with this tool.

The IFP-Exhaust library is used for detailed simulation of post-treatment systems.
The simulators are capable of handling 12 chemical species, transient thermal phenom­
ena and the main chemical kinetic mechanisms; they can therefore be used to analyse
and optimise the design and operation of the components forming the depollution sys­
tem: management of catalyst light-off phases, management of a particle filter and, in
particular, its regeneration phases, optimisation of catalytic efficiency, injection of urea
in an SCR system, etc. Simulators developed with this library can be coupled with the
engine simulators developed with the IFP-Engine library in order to deal with the themes
on interactions between the engine and the post-treatment system.

Figure E2.1
4-cylinder supercharged diesel engine simulator in the LMS ImagineXab
AMESim environment

Lastly, the IFP-Drive library is available, dedicated to global simulation of complete
vehicles. It is detailed in paragraph 6.4.3.

Chapter 2 · Internal Combustion engines 105

2.4.1.2 Influence of the Engine's Operating Point on Thermal Losses

In the first part of this chapter, we saw how the efficiency of an engine for an automobile
doesn't exceed 40% at most. However, the engine is frequently operated at operating points
whose efficiency is closer to 20 to 30%. This efficiency measures the ratio between the
mechanical work delivered by the engine and the amount of energy introduced into the com-
bustion chambers, given by the fuel's lower heating value. In addition to calculating the value
of basic efficiency (2.1.1.3), energy balances are used to analyze the distribution of losses
between the heat energy introduced into the combustion chambers and the work recovered
from the engine at the end of the crankshaft. Figures 2.24 and 2.25 show that these energy
losses consist primarily (more than 50%) of heat evacuated by exhaust gas and heat lost to the
walls of the combustion chamber, which is primarily evacuated by the engine's coolant. The
heat energy lost with exhaust gas increases at a greater load and engine speed (rpm) increase.
Conversely, proportional to the energy introduced into the combustion chambers, the heat
losses evacuated by the coolant increase as the engine load decreases.

Figure 2.24

Energy content of exhaust gas (as a percentage of the lower heating value intro-
duced into the combustion chambers). The example shown is for a 1 L gasoline
engine.

106 Hybrid vehicles

Figure 2.25
Energy content dissipated in the engine's coolant (as a percentage of the lower
heating value introduced into the combustion chambers). The example shown
is for a 1 L gasoline engine.

2.4.1.3 The Recovery of Thermal Losses
The recovery of heat given off by engines has been a subject of interest for many years,
primarily with the goal of improving engine efficiency. Recently, there has been renewed
interest in the field and a number of research projects are underway. Four approaches to the
recovery of heat are currently being studied:

- the direct use of heat to heat a component after starting the vehicle (using heat exchang-
ers in the exhaust line, for example);

- the direct recovery of mechanical energy to generate work (turbocompound systems,
for example, make use of a turbine located in the exhaust system);

- the conversion of heat to work (Rankine cycle);
- the direct conversion of heat to electricity (thermoelectric generators).
For the first case (the direct use of heat), two types of systems are possible: heat storage
with deferred recovery (for example, the use of a thermos in some models of the Toyota
Prius to keep engine coolant hot from day to day) and heat transfer (use of a heat exchanger
between the exhaust gas line and the engine coolant circuit to accelerate the increase in tem-
perature of the coolant). These techniques are useful only when the engine is cold, during
which time they can be used to improve efficiency. However, once the engine has reached its
normal operating temperature, they are no longer useful. The first approach has already been
employed in the automobile industry, but the remaining three are still in the experimental
stage.
To recover available energy from the exhaust gases, several types of systems make use
of a turbine in line with the gas stream. This turbine can be mechanically coupled to the

Chapter 2 · Internal Combustion engines 107

transmission or drive an alternator. The difficulty stems from the high rotational speeds of
the turbine (tens of thousands οίφηι). The drive mechanism - in the case of a mechanical
connection - or electric machine - in the second case - must be designed accordingly, which
is a problem with current technologies. Experiments that have been conducted with trucks
show that fuel consumption gains are no more than 5-10% on average. The efficiency of such
systems falls very quickly once the load on the engine is small.

Concerning the conversion of heat to work, the system that has been most extensively
studied for automobile use is the Rankine cycle (Figure 2.26).

Figure 2.26

Operating principle of the Rankine cycle.

Other systems are also being tested, such as the Stirling engine.

In the system based on the Rankine cycle, a pump circulates a working fluid to the heat
source (often exhaust gases). A heat exchanger is used to vaporize this working fluid. The
vaporized and pressurized working fluid then gives up part of its energy to a receiving
machine (turbine, expansion piston engine, scroll-type expander, etc.) before being recon-
densed at another heat exchanger connected to a cold sink, which in this case is ambient air.
In most cases an alternator is driven by the receiving machine during expansion to produce
electricity in the vehicle [Endo et al, 2007]. These devices could produce a few hundred
watts to a few kilowatts. For the vehicle, the potential fuel consumption gain would be about
5-10% but, here as well, the gain is low when the engine is operating at low loads. At present,
these systems are too complex, heavy, and bulky to be used inside an automobile.

Thermoelectricity can be used to create electrical voltage at the terminals of certain mate­
rials to which a temperature difference has been applied. This phenomenon is known as the
Seebeck effect. This material must combine at least two elements (chemical elements) that
have thermoelectric properties. It is placed in contact with a heat source (exhaust gas or engine
coolant). The cold sink is the atmosphere, either directly, or indirectly if an intermediate

108 Hybrid vehicles

cooling fluid is used to cool the thermoelectric generator. In this field, the improvement of
materials (choice of couples, "doping" with additional elements, increasing the operating
temperature range, work on the structure of the material itself, the objective being to obtain
materials that are good electrical conductors but poor heat conductors, and so on) and manu-
facturing methods is still necessary [Aixala and Monnet, 2010]. The production of several
hundred watts of electrical power is possible and fuel consumption gains could reach 5-10%.

These techniques all provide efficiency gains that are comparable but low when com-
pared to the amount of energy lost by the engine in the form of heat (Figures 2.24 and 2.25).
In fact, the balances are deceiving because they do not distinguish between two forms of
energy: work and heat. Combustion within the engine releases heat primarily, and we often
refer to a fuel's lower heating value. But in order to move, the vehicle needs a source of
work. The laws of thermodynamics show that the conversion of heat to work can never be
complete. The amount of energy theoretically convertible to work in a given quantity of heat
is known as "exergy." To evaluate the potential value of heat recovery in the automobile, we
must make use of exergy balances rather than energy balances [El Habchi et al, 2010]. Fig-
ure 2.27 shows that the maximum potential work that can be extracted from heat losses must
take into account irreversible phenomena (in the thermodynamic sense of the term). The fact
remains, however, that the recovery of thermal losses would represent a highly significant
source of improvement for engines.

Figure 2.27
Examples of energy and exergy balances.
Reference: Lower heating value. Two liter gasoline engine. Operating point at
2,000 rpm, MEP 4 bar, or approximately 13 kW of mechanicalpower produced.

Chapter 2 · Internal Combustion engines 109

2.4.2 Principal Impact of Hybridization on the Engine

2.4.2.1 Advantages

The environment of a hybrid vehicle provides more rational possibilities for the use of an
engine in terms of energy consumption. The goal is to obtain as much as possible from the
large amount of energy available in the fuel tank (compared to that available in the same
volume of battery, for example). We have seen how the engine is not efficient except at inter-
mediate and high loads. It makes sense, then, to avoid operating at low loads or idling speed.
In a hybrid vehicle context, these operating ranges at low power will be preferentially handed
over to the electrical portion of the drive system, depending on its capacity.

Thus, the first impact of hybridization is the elimination of the majority of the operating
phases of an engine at idling speed, which already provides a significant gain in consump-
tion. This is especially true in urban areas and even more so when driving in dense traffic,
where the vehicle is frequently not moving at all. Above these low speeds, if the electrical
portion of the hybrid drive system is sufficiently capable, some movement at slow speed
can be carried out with electricity alone, thereby avoiding the need to use the engine at low
power. This results in additional consumption gains.

One alternative is the continued use of engines under conditions where vehicle traction
doesn't require much power while also making use of the electrical portion of the drive
system. Two solutions are then possible. The first consists in operating the engine at loads
greater than what is required to move the vehicle and to redirect the excess power to the bat-
tery through the electrical machine. This can be used to recharge the battery and enable the
engine to operate in a zone of improved efficiency. The second solution is to combine the
torque from the engine and the electric drive system. Electrical machines deliver maximum
torque at low rotational speed and they can then be used to assist the engine. It then becomes
possible to reduce the displacement of the engine without penalizing the performance of the
vehicle compared to a conventional engine. The gain in consumption is obtained through the
effect of downsizing (2.2.3.1).

Similarly, the assistance of the electrical drive system when starting (after coming to a
complete stop, for example) and accelerating allows us to reconsider the approach to the
engine. The specifications of the engine can then be refocused on efficiency and set aside
the compromises necessary for managing operation at slow driving speeds and torque per-
formance at low engine rpm. If the electric machine assists the engine, it then becomes pos-
sible, in a gasoline engine, for example, to increase the compression ratio because the risk
of knock is lower at low speed, which will have a positive effect on efficiency. If the engine
is turbocharged, we can also reduce the operating range of the turbocharger system because
the torque requirement at low engine speeds is not as great. We can then focus on adapting
the turbocharger system to engine operation at higher power levels, always with the goal of
improving efficiency.

In a conventional vehicle, the engine supplies energy for a number of auxiliary compo-
nents, which it drives mechanically by means of gears, belts, or chains (Chapter 1). For exam-
ple, it drives an alternator, which produces electricity for the vehicle. In current systems, the

110 Hybrid vehicles

alternator delivers an average of a few hundred watts. At maximum output, power can exceed
2 kW. Engines also drive the pump that circulates coolant for the engine, the oil pump that
lubricates the engine, and sometimes the compressor for the air conditioner or the pump for
the power steering. Most forms of energy consumption cannot be disconnected and remain
operational even when the engine is at idle speed, which often severely limits their design.
Even when not needed - for example, the power steering pump when the vehicle is traveling
in a straight line - they consume some energy to compensate for internal friction. Even when
they can be disconnected, like the air-conditioner pump, the simple presence of their clutch
system in the belt path dissipates energy through friction.

Electrically powered auxiliary components may provide an efficiency gain in vehicles.
Electrical power steering systems are becoming increasingly common in conventional vehi-
cles. In hybrids, which are highly electrified by their very nature, electrically powered aux-
iliaries may even help improve efficiency. Take the example of power steering. It would
make little sense to keep the engine running simply to provide energy to the power steering
pump when the hybrid vehicle is traveling at slow speed, under exclusively electric trac-
tion. Another area of possible improvement is the electrification of the engine's water pump
[Yaguchi et al, 2009]. In terms of pure efficiency, the mechanical drive of a water pump is
better than the electric drive on a hybrid vehicle. However, the electric drive can be more
finely controlled than the mechanical pump, where the flow of cooling water in the engine is
directly proportional to its rotational speed. Electrical control of the water pump can be used
to adjust the flow of water more precisely. It even becomes possible to stop water circulation
in the engine until it has reached its normal operating temperature. Therefore, throughout the
entire period when the temperature of the engine is rising, energy consumption by the water
pump is zero. In terms of operating efficiency, the electric water pump is, in many cases, an
improvement.

2.4.2.2 Limitations

In city driving, a hybrid vehicle may be required to stop and start its engine once a minute
on average. This places limitations on the engine, especially its lubrication, because today's
oil pumps are mechanically driven. There is a lapse of time before nominal oil pressure is
restored when restarting, during which time the engine is poorly lubricated. Such frequent
stops and starts can also be difficult for the vehicle's occupants because of noise and vibra-
tion. Before stop and start phases are accepted, therefore, additional development work may
be required. The significant electrical power available on the hybrid vehicle can be used to
start the engine much more rapidly than in a conventional vehicle. Assuming the engine is
properly equipped, we can also use the cam phaser to limit the effective compression ratio of
the engine when stopping and starting, thus smoothing rpm descent (or rise). More compli-
cated technologies involve use of the electric machine to rotate the crankshaft to an angular
position that is favorable for rapidly restarting the engine.

When stopped, there is a risk of rendering exhaust gas after-treatment systems inoperable,
that is, a drop in temperature to levels that prevent emissions control of exhaust gases in the
seconds following a restart of the engine. In some cases, we may have to restart the engine
when the vehicle is stopped to ensure that the temperature of the vehicle's after-treatment

Chapter 2 · Internal Combustion engines 111

systems is maintained. It is also true that the potential availability of electrical traction means
that the engine can be used at dedicated operating points that provide sufficient heat to the
catalytic converter, thereby promoting its continued operation.

For engines in electric vehicles with a range extender, these limitations are especially
severe. Because vehicle occupants are accustomed to a quiet environment when no engine is
present, when it is present and functioning, noise and vibration in the engine must be limited.
The majority of time, the space reserved for the range extender is extremely limited. There­
fore, the engine must be very compact and operate infrequently. But we must also make sure
that during long periods of inactivity, the engine remains fully operational. It is essential that
mechanical parts do not oxidize and deposits not solidify, forcing actuators to seize up, which
could happen to the throttle or the injector needles. When the engine is needed, it must often
reach an operating point at high load without having the time needed for the temperature
to rise because the vehicle is already in motion. In this case, the thermomechanical design
becomes especially complex.

In some simple hybrid architectures, where the engine and electric machine cannot be
uncoupled (this is true of "ultra-thin" electric machine located between the engine and the
transmission), the recovery of braking energy is penalized by pumping losses and friction in
the engine. To reduce pumping losses when fuel injection is cut (when the vehicle deceler­
ates), the engine can be equipped with specific technologies. In the Honda IMA hybrids, for
example, there is a feature on the valve system in the engine that prevents the valves from
opening when requested. Consequently, there is no air intake phase and no release of air, and
pumping losses in the engine are sharply reduced.

2.4.2.3 An Opportunity for 2-Stroke and Rotary Engines?

For the specific case of an electric vehicle equipped with a range extender, research and
development efforts have begun to reconsider drive systems that are rarely found in modern
automobiles, primarily the use of 2-stroke (Figure 2.28) and rotary engines.

The 2-stroke engine is commonly used in two- or three-wheel vehicles, for outboard
motors on small boats, and portable power tools, but it has failed to make inroads in the
automobile sector in technologically advanced countries, primarily because of exhaust emis­
sions concerns. It's method of operation requires a very rapid "scavenging" of the combus­
tion chamber between two successive combustions. The exhaust contains a mix of fresh and
burned gases (which often results in unburned hydrocarbon emissions), making simultane­
ous after-treatment of HC, CO, and ΝΟχ difficult. Aside from this significant drawback,
the 2-stroke engine has a number of benefits compared to a 4-stroke engine with the same
maximum power: it is lighter and more compact, efficiency is good, losses from friction and
pumping are reduced, the torque it produces is uniform (it is less acyclical), and, depending
on the technology involved, it requires little maintenance and cost is low - obvious advan­
tages for use as a range extender.

112 Hybrid vehicles

Figure 2.28

Principle of the 2-stroke engine.
Source: [Guibet, 1997]
The rotary engine - the best known and most technologically developed being the design
by F. Wankel - could also be used as a range extender. Here too, the engine is light and
compact compared to a 4-stroke engine, it is uniform in operation and naturally balanced,
resulting in low acyclicity and low vibration. While the Wankel engine is compact overall,
this is not true of its combustion chamber, whose shape is flat. This results in high thermal
losses and long combustion periods, which reduce efficiency.
Here too, further research is needed in order to overcome the inherent drawbacks in these
engines.

Chapter 2 · Internal Combustion engines 113

2.5 SUMMARY AND FUTURE OUTLOOK

After more than a hundred years of development, the combustion engine, whether spark-
ignition or diesel, continues to make progress. Recent developments involve the increasingly
greater capabilities of computers, control-command algorithms, and mechatronics: techno-
logical improvements, development of innovative systems, new types of sensors. The latest
developments include:

- engine's displacement reduction, including reduction in the number of cylinders; some
new vehicles have appeared on the market with two-cylinder engines [Sacco et al,
2010];

- the use of increasingly advanced supercharging technologies;

- the use of increasingly flexible variable valve train systems, which will some day be
applied even to diesel engines;

- high-pressure injection systems;
- friction reduction;

- optimized thermal management.

In addition to these technologies, which are in current use but continue to undergo further
development, the future will see the appearance, on the commercial market, of systems being
developed today. They include mechanisms that will have the ability to alter the engine's
compression ratio while in operation or recover the heat lost by the engine.

In terms of combustion, progress in the research technologies available to engineers will
result in continuous improvements in engine efficiency and the reduction of pollutants at the
source. Such advanced diagnostic methods are becoming increasingly sophisticated: trans-
parent combustion chambers for in situ visualization of combustion, software for 3D calcula-
tion of combustion cycles, digital tools such as large eddy simulation, which is capable of
incorporating stochastic phenomena such as combustion instability.

Better understanding of processes based on aerothermochemistry has led to considerable
progress in diesel engine technology. Who could have predicted, 15 years ago, that today
most luxury vehicles in Europe would be equipped with diesel engines? That is currently a
reality. Regular progress has also been made in understanding combustion at very high loads
or with very dilute mixtures. The recirculation of very high amounts of burned gas (EGR),
while ensuring stable, complete, and relatively clean combustion, is now commonplace. We
are capable of filling as much as half the combustion chamber with burned gas from the
previous combustion step to help reduce pollution emissions. Eventually, it is likely that new
combustion methods will also be understood, opening the way to new designs: homogene-
ous HCCI combustion for diesel engines or controlled auto-ignition (CAI) in spark-ignition
engines. Dual-fuel engines that can simultaneously take advantage of gasoline and diesel
could one day come on the market. In addition, great steps have been made in engine optimi-
zation to make better use of nonconventional fuels such as biofuels and gas fuels, primarily
natural gas.

Finally, engineers have begun to take advantage of the numerous opportunities offered
by the gradual electrification of vehicles. For example, many mechanical components have

114 Hybrid vehicles

been replaced with electrical components, along with improved control. The complemen-
tarity between the capabilities of engines and electric machines has been exploited in order
to reduce the performance requirements of engines and take advantage of this freedom to
improve their operating efficiency. Obviously, these developments are far from being com-
plete and a comprehensive vehicle system approach should lead to additional progress. Nor
is there any doubt that manufacturers will eventually be able to address the challenges pre-
sented by the development of fully integrated range extenders to turn electric vehicles into
multifunction vehicles that can be operated under a number of different driving conditions.

Compared to current drive systems, in 10 to 15 years, engines will see considerable pro-
gress in terms of energy efficiency, with gains of about 15-20% for diesels and 25-30% for
gasoline engines. We can reasonably expect that the most promising approach will involve a
reduction in engine displacement and rpm, in both diesel and gasoline engines. This technol-
ogy has been used over the past several years, of course, but further development is possible.
Moreover, it is highly suitable for use in hybrid vehicles, which will eventually result in the
production of fairly generic engines, which can be used in both conventional and electric
vehicles.

REFERENCES

Aixala L and Monnet V (2010) RENOTER Project: Waste Heat Recovery on Passenger Car and Heavy-
Duty Truck Diesel Engine Thanks to Thermoelectricity, 2nd Conference Thermoelectric s Goes
Automotive, Munich, Germany. In: Jänsch D., Thermoelectric s Goes Automotive Thermoelek-
trik, IIISBN-13: 978-3-8169-3064-8, Editions Expert Verlag, Renningen.

Anselmi P, Gautrot X, Pagot A and Leduc P (2007) A New Approach to GDI Turbo Engine: Combin-
ing New Technology to Low Cost Applications. Internationales Stuttgarter Symposium Automo-
bil Und Motorentechnik, Stuttgart, Germany.

Degobert P (1992) Automobile et pollution. Éditions Technip, Paris.
El Habchi A, Ternel C, Leduc P and Hetet JF (2010) Potential of Waste Heat Recovery for Automotive

Engines Using Detailed Simulation, Congress ASME-ATI-UIT, Conference on Thermal and
Environmental Issues in Energy Systems. Sorrento, Italy.
Endo T, Kawajiri S, Kojima Y, Takahashi K, Baba T, Ibaraki S, Takahashi T and Shinohara M (2007)
Study on Maximizing Exergy in Automotive Engines, SAE Technical Paper 2007-01-0257, DOI:
10.4271/2007-01-0257.
Fujiwara M, Kumagai K, Segawa M, Sato R and Tamura Y (2008) Development of a 6-Cylinder
Gasoline Engine with New Variable Cylinder Management Technology, SAE Technical Paper
2008-01-0610, DOI: 10.4271/2008-01-0610.
Guibet JC (1997) Carburants et moteurs - Technologies, énergies, environnement. Éditions Technip,
Paris.
Heywood JB (1988) Internai Combustion Engine Fundamentals. McGraw-Hill.
Kling R (1980) Thermodynamique générale et applications. Éditions Technip, Paris.
Ministère de l'Écologie, de l'Énergie, du Développement Durable et de la Mer - Direction de l'Énergie
et du Climat (DGEC):
http://www.developpement-durable.gouv.fr/La-fiscalite-des-hydrocarbures, 11221 .html

Chapter 2 · Internal Combustion engines 115

Rapport de la Commission au Parlement européen, au Conseil et au Comité Économique et Social euro-
péen. Surveillance des émissions de C02 des voitures particulières neuves dans l'UE : données
pour l'année 2009, COM (2010) 655 final, November 2010.
http://www.nhtsa.gov/DOT/NHTSA/Rulemaking/Rules/Associated%20Files/CAFE_Updated_
Final_Rule_MY2011 .pdf

Sacco D, Mastrangelo G and Micelli D (2010) TwinAir: The Extreme Downsized Engine Solution
for Future Urban Mobility, 19. Aachener Kolloquium Fahrzeug- Und Motorentechnik, Aix La
Chapelle, Germany.

Ternel C, Pagot A, Anselmi P and Gautrot X (2009) Development and Results of a New Cylinder Deac-
tivation Approach: the OVaLiD® Concept. SIA Congress - Spark Ignition Engine of the Future,
Facing the C 0 2 and Electrification Challenges, Strasbourg, France.

Yaguchi H, Uehara T, Watanabe K and Tokieda J (2009) Development of New Hybrid System for
Compact Class Vehicles. Congress EVS24, Stavanger, Norway.

I Electric Drivetrain

El Hadj Miliani, Youssef Touzani \

This chapter, which deals with the electric drivetrain and its applications to hybrid and elec­
tric road vehicles, describes the three main components of an electric drivetrain, i.e.:

- the electric machine itself,
- its power electronics, which permanently manage and adjust the voltage, current and

frequency of the electric energy required or supplied by the machine, depending on the
use requirements; in this respect, they are an essential part of all applications on road
vehicles,
- and lastly its control, to optimize its operation.
The mechanical link between the machine and the other parts of the drive (engine, trans­
mission, driving wheels) is discussed in Chapter 5 (5.4.2).
The electric machines are described here in the context of their applications to hybrid or
electric vehicles and are therefore not discussed in detail. For further information, readers can
refer to specialized books [Grellet and Clerc, 1999; Grenier et αί, 2001; Ehsani et αί, 2005;
Multon, 2001].

3.1 OVERVIEW OF ELECTRIC MACHINES

3.1.1 Principles

All rotating electric machines comprise a stator (fixed part) and a rotor (rotating part), sepa­
rated by an air gap. Traditionally, their operation is based on one of the two principles used
to obtain electromagnetic torque [Grellet and Clerc, 1999; Grenier et αί, 2001]:

- either by the interaction of two magnetic fields created by stator and rotor currents or
magnets (on the rotor or the stator); these are known are electrodynamic machines,

- or by the variation in energy depending on the rotor position for variable reluctance
machines (VRM).

3.1.1.1 Electrodynamic Machines

Electrodynamic machines apply the left-hand rule (Figure 3.1) according to which a wire
carrying an electric current I in a magnetic field B is subjected to an electromagnetic force,
called the Laplace force.

118 Hybrid vehicles

Figure 3.1

Principle of the left-hand rule.

The elementary force is expressed by the relation (3.1):

(3.1)

The modulus of this force is proportional to the magnitude of the current I and the modu­
lus of the magnetic field B. Maximum force is obtained when the magnetic field is perpen­
dicular to the current. This force creates the electromagnetic torque of an electric machine
operating according to the principle of interaction of two magnetic fields.

3.1.1.2 Variable Reluctance Machines

In variable reluctance machines, the expression of electromagnetic torque Tem involves the
variations in specific 1 and mutual2 inductances of the coils depending on the rotor position,
as expressed in equation (3.2):

(3.2)

where Wem represents the electromagnetic energy and Θ the rotor position.
As we will see below, these two effects can be combined, as is the case in particular for

salient-pole synchronous machines.

1. The specific inductance is the self-induction coefficient of a circuit, it is calculated by the ratio of the
flux to the current generating it and represents the capacity of the circuit to store the field.
2. The mutual inductance expresses the induction produced in a circuit induced by the variation in cur­
rent in another circuit. It characterizes the coupling between two circuits.

Chapter 3 · Electric drivetrain 119

3.1.1.3 Sign Convention

Faraday's law of induction governs the electric equations of all electric machines, irrespec­
tive of the operating principle. This law indicates that an electric circuit subjected to a vari­
able magnetic flux Φ generates an electromotive force (EMF) E, expressed according to the
generator law, by equation (3.3):

(3.3)

As soon as the rotor of an electric machine is rotating, a voltage called the electromotive
force (EMF) is induced in the stator windings, of value proportional to the speed of rotation.
When the machine is used as a motor (traction phase for a vehicle), the "-" sign in equation
(3.3) disappears and the EMF becomes a counter-electromotive force (CEMF).

As described in Chapter 2, in the road transport applications, electric machines are used
in both motor and generator modes.

3.1.2 Composition

From the technological point of view, an electric machine consists of a field frame, made
from a stack of plates, which channels the magnetic flux and supports the copper wire wind­
ings. While the air gap must be as small as possible to improve the magnetic performance,
mechanical and thermal constraints impose a minimum value.

3.1.2.1 Electrodynamic Machines

Electrodynamic machines, known as traditional machines, consist of an inductor and an
armature separated by a constant air gap. The inductor and the armature are made from cop­
per wires wound on field frames. On some machines, the inductor is replaced by permanent
magnets, generally made from rare earths (e.g. neodymium-iron-boron "NdFeB", samarium-
cobalt "SmCo", etc.). The notion of inductor 3 and armature 4 depends on the machine type
and will be specified in the chapter below, Classification of electric machines.

3.1.2.2 Variable Reluctance Machines

Variable reluctance machines are also composed of field frames, but in this case the air gap
can be varied mechanically depending on the rotor position. These machines have coils on
the stator only.

For both types of machine, the stator coil can be distributed or concentrated (tooth wind­
ing; for further details, readers can refer to [Soon et al., 2006]).

3. Source creating the magnetic field.
4. Site of magnetic induction.

120 Hybrid vehicles

3.1.3 Losses in Electric Machines

Energy cannot be converted in electric machines without losses. These losses reduce the
energy efficiency of the machine and cause heating which may eventually damage it. The
main losses in an electric machine can be divided into two categories: mechanical and
electrical.

Mechanical losses are caused by:
- friction in the bearings and, possibly, friction of the brushes on the commutator or the

rings (in case of wound rotor machines; these losses increase with the rotor speed);
- viscous friction of the rotor in the air gap, proportional to the square of the speed.

For high-speed machines used in automotive applications (about 10,000 r.p.m.), these
aerodynamic losses may be non negligible.

The electrical losses can also be divided into two categories:
- losses in the conductors, due to the Joule effect; they are expressed by the general for-

mula P = RI2, where I is the magnitude of the current flowing through the winding of
resistance R. These losses generate heat in the wires;
- losses in the plates, generally known as core losses, which occur in the ferromagnetic
parts where the magnitude or direction of the magnetic flux varies; these losses are
difficult to quantify and intensive studies are being conducted on this subject. They are
traditionally broken down into hysteresis losses and eddy current losses:
• hysteresis losses are proportional to the speed of rotation and depend on the qual-

ity of the plates; to a first approximation, they are proportional to the square of the
magnetic induction 5;
• eddy current losses (currents created due to flux variations in the magnetic materi-
als which heat the iron by Joule effect) are proportional to the square of the speed
of rotation and the square of the magnetic induction. To minimize these losses, the
field frame is made from a stack of thin plates electrically insulated from each other
with varnish to minimize the circulation of current from one plate to another (we
speak of laminated frame). Thinner plates are used at high flux frequencies to reduce
eddy currents even further; one technique consists in using thinner plates of lower
conductivity, such as silicon alloy steel plates (1 to 5% Si).

3.1.4 Electric Machine Operating Ranges

Operation of an electric machine is described by its torque-speed characteristic, a curve
which shows the change in torque as a function of the machine speed of rotation.

Figure 3.2 shows a typical torque-speed characteristic of a traction electric motor pow-
ered by a limited battery voltage.

5. Magnetic induction expressed in Tesla is defined by a magnetic induction field tending to displace
the electric charges, which creates a potential difference across the ends of the wire.

Chapter 3 · Electric drivetrain 121

Figure 3.2

Example torque-speed characteristic of a traction machine.

If we consider the machine speed range, we can identify two characteristic areas:
- a first area corresponds to operation at constant torque and therefore to power increas-

ing with speed. Within this range, the counter-electromotive force increases in direct
proportion to the speed; consequently, the supply voltage delivered to the machine by
the power electronics must increase up to the maximum possible with the existing bat-
tery voltage. The corresponding speed is a characteristic of the machine and its power
supply, it is known as the base speed (Sbase);
- above this speed, a second machine operating area is possible by limiting the CEMF
to its maximum value, by reducing the magnetic flux in the machine. In this area, also
known as the "defluxing" area, the power supplied by the machine is practically con-
stant, with the torque therefore dropping hyperbolically.

The ratio between the machine maximum speed Smax and base speed Sbase is called the
machine "flexibility"; this is an important characteristic of the machine and its control.

If we now consider the torque range, we can also identify two different operating areas:
- a maximum torque range in which the torque can only be maintained for a very short

period of time imposed by the thermal limit of the machine (or the physical limit of its
power electronics),
- a permanent speed range at "nominal" torque in which the thermal state of the machine
is stabilized, with a nominal torque that can be maintained without damaging the
machine.

122 Hybrid vehicles

The permanent operating area corresponds to the nominal use conditions of the machine,
generally found in industrial applications. For road transport applications, since the machine
frequently operates in the maximum torque area, it must be very carefully designed. Numerous
studies simulating the thermal behavior of the machine must therefore be carried out to predict
the temperature changes under given vehicle use conditions, as illustrated on Figure 3.22. In
addition, the machine temperature is often checked, even when the vehicle is running.

3.2 CLASSIFICATION OF ELECTRIC MACHINES USED
IN AUTOMOBILE DRIVETRAINS

Electric machines can be classified into two main categories, direct current (DC) and alter-
nating current (AC), depending on their structure defined by the combination of the machine
architecture and power supply.

DC machines have undergone no major changes in recent years and are used less and
less for electric traction. In contrast, AC machines are being used increasingly in automotive
applications.

Amongst the AC machines, self-controlled permanent magnet synchronous machines are
currently found most frequently in the hybrid drives of private vehicles, due to their high
power-to-weight and power-to-volume ratios, combined with the progress made in power
and control electronics.

This paragraph provides a brief description of the various electric machine structures
(Figure 3.3), together with their advantages and disadvantages.

Figure 3.3

Diagram of the main structures of electric machines used in automobile
drivetrains.

Chapter 3 · Electric drivetrain 123

We stress the importance of the machine power electronics/control/architecture associa-
tion, since the torque-speed external characteristics are related to these three aspects. All syn-
chronous machines must be self-controlled according to the rotor position; the control signals
from the power electronics are generated to guarantee the synchronism of the machine which,
in this case, exhibits external characteristics similar to those of the mechanical commutator
DC machine.

3.2.1 Mechanical Commutator DC Machines (MCDCM)

In mechanical commutator DC machines, the electromagnetic torque is the result of interac-
tion between the magnetic field produced by the inductor windings on the stator and that
produced by the current flowing through the rotor coils. To obtain an electromagnetic torque
of non-zero average value, current must flow in the same direction under each pole (Fig-
ure 3.4.a). This is achieved mechanically in the DC machine, in which the rotor currents are
switched by a mechanical system (mechanical commutator with copper strips and brushes).
For completeness, we may mention that for the MCDCM the stator and rotor windings can
be connected in series (series excitation, MCSDCM) or in parallel (MCPDCM). In this case,
the windings can be connected directly in parallel (shunt machine) or with two independent
power supplies (separate excitation). Note that shunt excitation is rarely used in automobile
applications, which generally implement separate excitation.

This machine was the first to be used in electric transport, since it is quite compatible with
direct current and requires no, or very little, power electronics. The direct current machine
was fitted in particular on the PSA electric vehicles sold in the 1990s, e.g. AX, 106, Saxo
(Figure 3.4.b); it is still used on some electric vehicles and to drive the electric auxiliaries of
conventional vehicles.

Figure 3.4

Mechanical commutator DC machine (MCDCM).
Source: (b) Citroën document, 1996.

124 Hybrid vehicles

The wound inductor can be replaced by permanent magnets. This is generally the case for
the auxiliary motors. For a traction motor, however, defluxing is impossible with this solu-
tion, a factor which considerably limits its speed range and therefore its use.

Concerning the drive, due to the progress made in recent years in the field of power elec-
tronics, the mechanical commutator DC solution has been virtually abandoned in all recent
projects.

Table 3.1 summarizes some of the advantages and disadvantages of mechanical commu-
tator DC machines. The disadvantage related to use of the DC machine is mainly due to the
presence of the brush/mechanical commutator system which represents a serious handicap
compared with alternating current machines.

Table 3.1. Advantages and disadvantages of mechanical commutator DC machines

Advantages • Simple and inexpensive control electronics
Disadvantages • Defluxing easy to implement for DC machines with wound inductor

• presence of the brush-mechanical commutator system; which represents an
additional volume
• Wear of the brush-mechanical commutator system (sliding contacts) which
requires periodic maintenance
• Presence of compensation windings, further increasing the volume
• Operation of the rotating armature which prevents proper cooling and limits
the possibilities of improving performance
• Relatively low power-to-weight ratio
• Relatively high manufacturing cost

3.2.2 Synchronous Machines

In synchronous machines, the electromagnetic torque is the result of interaction between the
rotating magnetic field, produced in the stator windings supplied by alternating current, and
that produced in the rotor by a current or magnets. As detailed in the next chapter; the stator
windings can be powered by square wave current for the special case of the brushless DC
machine. Synchronous machines are characterized by the fact that the speed of rotation of the
rotor (machine shaft) is equal to the speed of rotation of the stator rotating field. Several types
of synchronous machine can be identified, depending in particular on how the rotor field or
the electromagnetic torque is produced; the main types are described below.

3.2.2.1 Electronic Commutation DC Machine (ECDCM)

In an electronic commutation DC machine, the brush-mechanical commutator system is
replaced by control electronics which perform commutation functionally by supplying the
stator with square wave current. This type of machine therefore operates like an AC machine.
The term Brushless-DC (BLDC) indicates that this motor behaves like a traditional DC
motor, apart from the fact that commutation is carried out electronically.

Chapter 3 · Electric drivetrain 125

3.2.2.2 Wound Rotor Synchronous Machine (WRSM)

In a wound rotor synchronous machine, the rotor windings are supplied with direct current
which creates an excitation flux (magnetic field). With their normal structure, they are only
rarely used in transport applications except for the alternator which charges the battery via a
diode bridge. Wound rotor synchronous machines are generally used in stationary applica-
tions, for energy production and constant speed drives.

In transport applications, this machine is associated with power electronics and is very
similar, in terms of external behavior, to the direct current machine, the difference lying in
the way the current is switched: mechanical commutation for the MCDCM and electronic
commutation for the self-controlled wound rotor synchronous machine or ECDCM motor
(BLDC). In addition, like the MCDCM, the sliding contacts limit high-speed operation due
to brush wear, but at much higher values due to the absence of commutations.

Wound rotor synchronous machines are generally less efficient than permanent mag-
net synchronous machines due to Joule effect losses in the rotor, except at high speeds due
to defluxing. In addition, for a given power range, wound rotor machines are bulkier and
heavier than permanent magnet machines. Table 3.2 summarizes some of the advantages and
disadvantages of the wound rotor synchronous machine.

Table 3.2. Advantages and disadvantages of wound rotor synchronous machines

Advantages • Easy defluxing by reducing the rotor current
Disadvantages • Greater operating range than permanent magnet machines, control laws
optimizing efficiency
• Little loss of efficiency at high speeds

• Lower torque/mass ratio than for permanent magnet synchronous machines
• Sliding contacts limiting the speeds of rotation
• Lower efficiency than permanent magnet synchronous machines due to losses
in the rotor
• Limited number of poles
• Relatively complex control electronics (need for a position sensor)

The WRSM is rarely found in hybrid drives for road vehicles, being penalized by its
higher specific mass and volume; these machines are nevertheless used in all-electric vehi-
cles for which these criteria are less restricting. In addition, the WRSM requires no magnets,
thereby removing any constraints related to the availability of sensitive materials such as the
rare earths necessary for their manufacture (refer to paragraph 7.3 for an update on sensitive
materials). Remember that a 60 kW peak permanent magnet synchronous machine like that
fitted on the Toyota Prius 3 contains nearly 800 g of magnets. The electric vehicles commer-
cialized by Renault in 2011 are therefore fitted with a wound synchronous machine supplied
by Continental, as illustrated on Figure 3.5.

126 Hybrid vehicles

Interconnecting box
Charger
DC/AC converter
Electric machine and gear

Figure 3.5
Electric drive with wound rotor synchronous machine proposed by Renault on
the Fluence.
Source: Renault document

The Renault range of electric vehicles
The automaker Renault is firmly committed to the commercialization of electric
vehicles, proposing a complete range of 4 models, as described below. With its part­
ner Nissan, Renault has launched a program costing the two manufacturers the total of
nearly 4 billion Euros, with the following objectives:
• Actively contribute to lowering the C 0 2 emissions of its range of vehicles, based on

the assumption of electricity produced with European, and in particular in French,
resources;
• Propose vehicles emitting no local nuisances, atmospheric pollution or noise,
which can therefore be used in the large urban centers and especially in the future
zones reserved for vehicles emitting no local pollutants;
• Propose private and professional users all-electric mobility solutions that are tech­
nically reliable, economically viable and best adapted to their requirements;
Renault has therefore implemented the technological advances achieved in the field
of Lithium-Ion batteries and electric drives to propose a range of vehicles offering attrac­
tive dynamic performance and autonomy, as shown in Table E3.1.

Chapter 3 · Electric drivetrain 127

Figure E3.1

Renault electric vehicles models
Source: Renault document

Table E3.1 Renault electric vehicles characteristics

Renault Renault ZOE Renault Renault Twizy
(45 or 80 km/h
Fluence Z.E. Kangoo Z.E.
version)
Vehicle type Family Sedan Compact Mini Van Two seaters
Sedan
4213 in tandem
Lenght (mm) 4748 4086 1829 2338
1410 1237
Width (mm) 1813 1730
44 446 or 474
Dry weight (kg) 1543 NC
226 7 or 13
Electric machine peak 70 65
power (kW) 130 33 or 57

Electric machine peak 226 220 170 45 or 80
torque (Nm)
Lithium-ion 100
Maximum speed (km/h) 135 135
22 Lithium-ion
Nominal range under 185 210
standard cycle (km) 6 to 9 6.1
From
Battery type Lithium-ion Lithium-ion 13 000 EOT 4 3.5
From 72 EOT 6 990 or
Battery nominal capacity 22 22 (10 000 km/ 7 690 IOT
(kWh) 6to92 6to93 36 months) From 50 IOT
(7 500 km /
Battery charge time (h)1 36 months)

Retail price (€) From From
18 900IOT4 13 700 IOT4

Battery rental (€/month)5 From 82 IOT From 79 IOT
(10 000 km/ (12 500 km/
36 months) 36 months)

1. On standard plug
2. Quick Drop option available for battery pack change in 3 minutes
3. 30 minutes for 80% in quick charge or 1 hour in accelerated charge
4. Battery excluded and 7000 € incentive included
5. Full covering assistance

128 Hybrid vehicles
Figure E3.2 shows the layout of components in the Fluence ZE vehicle. The all-elec­

tric model is 13 cm longer than the internal combustion engine version, to accommodate
the battery pack. The vehicle is equipped with the Quick-Drop system to exchange the
battery pack.

Figure E3.2
Fluence ZE components
Source: Renault document

3.2.2.3 Permanent Magnet Synchronous Machine (PMSM)
The permanent magnet synchronous machine is a synchronous machine whose excitation
is produced by permanent magnets. The stator is similar to that of all alternating current
machines, but there are different types of PMSM depending on the rotor geometry, in par-
ticular the positions of the magnets. Figure 3.6 shows 5 examples of the most common types
of rotor geometry.

Chapter 3 · Electric drivetrain 129

Figure 3.6

Examples of rotors in permanent magnet synchronous machines.
Sources: [Saint-Michel, 2006] and [Jannot, 2010]

The position of the magnets in the rotor is extremely important since it governs the varia-
tion in reluctance 6 as the rotor rotates (notion of saliency). Reluctance of the magnetic circuit
varies depending on whether the magnets are separated by air or a ferromagnetic material,
air and magnets exhibiting similar permeabilities 7 while ferromagnetic materials are charac-
terized by higher values. This variation in reluctance is important since it affects the torque
produced; in this type of machine, most of the torque is produced by interaction of the stator
and rotor fluxes, but extra torque can be obtained due to the variation in reluctance (we speak
of reluctance torque or saliency torque). For further details, readers can refer to paragraph 3.3
on modeling of machines.

Figure 3.6.a shows the "smooth pole" architecture, the magnets being separated by air
(similar permeabilities); consequently, there is no variation in rotor reluctance seen by the
stator. Since the magnets are bonded on the surface, a sleeve ring may be required to hold
them, which may impair the qualities of the magnetic circuit. This architecture is therefore
especially suited for low speeds, when the centrifugal force on the magnets is low. In addi-
tion, there is a risk of the magnets becoming demagnetized, although very slight with rare
earth magnets (unlike ferritic magnets).

In contrast, architectures 3.6.b to 3.6.e which show a variation in rotor reluctance seen
by the stator, are said to be "salient-pole". Machines equipped with rotors c, d and e, known
as interior magnets, are more suitable for high-speed operation since the added parts - the
magnets - are maintained by the magnetic part in which they are inserted. In addition, this
magnetic part defluxes the machine, to increase the achievable speed range without risk of
demagnetizing the magnets, since they are less exposed to the armature magnetic reaction.
Magnets with high induction may be prove difficult to insert in the housing, however.

Architectures d and e are known as flux concentration architectures; since the magnets
are arranged in radial and V-shaped layout, the sum of the facing areas of the magnets (2.Sa),

6. Reluctance is the quantity which opposes the passage of flux in a magnetic circuit. It is defined by
the quotient of the magnetomotive force in the magnetic circuit divided by the induction flux crossing it.
7. Magnetic permeability is a constant characterizing the ability of a material to channel the magnetic
field lines.

130 Hybrid vehicles
which create the flux of a pole, is greater than the area of the pole (Sp). The flux crossing the
area Sp is the sum of the fluxes crossing the two magnets (the two areas S ). Since area Sp is
less than 2.S , induction in the air gap, with respect to area Sp, is therefore greater than that
in the magnets (Figure 3.6.d). This geometry provides high induction levels in the air gap,
greater than those of the magnets used.

While the reluctance torque is almost negligible for smooth pole machines, it may reach high
values: up to 20% of the machine total torque in some architectures with saliency [Seong, 2009].

As shown on Figure 3.7, for Toyota, manufacturers and suppliers have developed over time
different architectures combining the solutions illustrated on Figure 3.6, always with the same
objective of maximizing dynamic and energetic performance while minimizing cost and size.

Lexus LS 600h
(2008)

Camry (2007)

Prius (2003)

Prius (1997)

Figure 3.7

Change in the position and geometry of magnets in the PMSMs of vehicles sold
by Toyota and Lexus.
Sources: [Olszewski, 2009] and [Hsu etaL, 2004]
Table 3.3 lists the advantages and disadvantages of permanent magnet machines. The
functional disadvantage related to the use of permanent magnet machines lies mainly in the
problem of controlling the magnetic flux:

Chapter 3 · Electric drivetrain 131

Table 3.3. Advantages and disadvantages of permanent magnet synchronous machines

Advantages • High torque/mass ratio
Disadvantages • High power/mass ratio
• Good efficiency

• High cost due to the price of magnets
• Problem of magnet temperature resistance
• Manufacture more complicated than squirrel cage and variable reluctance
asynchronous machines
• Risk of demagnetization for high currents at very high temperature > 200 °C
• Difficulty of defluxing
• Presence of rare earths for the magnets

- at speeds above the base speed, the current in the stator must be significantly increased
due to defluxing of the permanent magnets (see paragraph 6.4.2.6),

- in case of fault on the power electronics, since the magnetic flux cannot be stopped,
there is a possibility of very high voltage across the phase terminals of these machines
if the phase is open, or a high current if it is short-circuited. The machine itself, or more
generally the inverter, could be seriously damaged.

3.2.2.4 Variable Reluctance Synchronous Machine (VRSM)

The rotor of a variable reluctance synchronous machine has neither magnets nor excitation
winding; the rotor flux is therefore zero and all the torque is created due to the effect of
variable reluctance. Unlike permanent magnet machines, the effect of variable reluctance is
produced in this case by a variation of the air gap due to the presence of teeth on the rotor
(Figure 3.8). The rotor is designed to have maximum saliency, making the machine highly
flexible but inducing manufacturing constraints, which have a negative impact on the cost.
The stator of this machine is similar to that of most alternating current machines.

Figure 3.8

Rotor of a variable reluctance synchronous machine.
Source: [Moghaddam, 2007]

132 Hybrid vehicles

Due to the inductive nature of the variable reluctance machine, its power factor is less
than that of induction or brushless motors, which means that a larger converter is required.

Table 3.4. Advantages and disadvantages of variable reluctance synchronous machines

Advantages • Passive rotor allowing high-speed operation
Disadvantages • Efficiency relatively better than asynchronous machines

• Need for a high saliency ratio, making large series manufacture difficult
• Relatively low power factor

One variant of this type of machine, the doubly-salient variable reluctance machine
(DSVRM), has saliency on both stator and rotor. Composed of stacked laminated steel plates,
it is by far the easiest to manufacture.

Figure 3.9 shows the stator and its concentric windings. This type of winding offers a
good winding coefficient8 and reduces the size of the end windings. Figure 3.8 also shows
the rotor with neither magnets nor winding ("passive" rotor), which allows very high speeds
of rotation.

Figure 3.9
Doubly-salient variable reluctance machine
Source: [Multon et al, 1995]

Although this type of machine offers numerous advantages, the pulsating nature of the
torque generated and the acoustic noise have prevented it from breaking into the market
(Table 3.5). Over the last few years, however, scientists have shown increasing interest in the
DSVRM, with more and more laboratories studying its performance.

8. The winding coefficient takes into account the distribution of the winding wires, the wire inclination
and other manufacturing techniques used to make the flux as sinusoidal as possible.

Chapter 3 · Electric drivetrain 133

Table 3.5. Advantages and disadvantages of doubly-salient variable reluctance synchronous machines

Advantages • Easy manufacture
Disadvantages • Robust machine
• Possibility of operating over a broad speed range
• Good torque/mass ratio
• Good winding coefficient

• Low power factor
• Relatively complex control electronics (need for a position sensor)
• Non-negligible presence of noise and vibrations

3.2.3 Asynchronous Machine (ASM)

The asynchronous machine (ASM), or induction machine, runs on alternating current like the
synchronous machine, but has a different rotor.

Two types of rotor are used depending on the electromagnetic characteristics required:
wound rotor or cage rotor (Figure 3.10.a).

Figure 3.10
Asynchronous machine used in the automobile.
Source: (b) [Badin et al, 2000]

In cage rotors, the electric part consists of copper conducting bars inserted in notches and
permanently short-circuited by two end rings crimped each side of the rotor.

In wound rotors, the electric part consists of a three-phase winding wired in star configu-
ration and inserted in notches; the rotor is short-circuited on the outside by three terminals

134 Hybrid vehicles

connected electrically to the three coils by sliding contacts (rings). High purchase and main-
tenance costs and the emergence of frequency variators have made this type of machine
obsolete.

The asynchronous machine owes its name to the "sliding" between the speed of the rotat-
ing field created by the inductor winding and the rotor speed. Currents are induced in the
windings or bars at the rotor; they generate a second field rotating at the same speed as the
inductor rotating field created by the stator. The torque is caused by the interaction between
these two fields.

The asynchronous machine has the advantage of being well-known since it is widely
used in industry and its manufacturing process is therefore thoroughly understood. The major
drawback with asynchronous machines is due to the Joule effect losses in the rotor which
are essential for their operation (Table 3.6). Reactive power is also required to magnetize the
iron, hence a power factor of about 0.9 and additional Joule effect losses in the stator. Finally,
asynchronous machines, as regards pure performance, lose out compared with synchronous
machines and more especially permanent magnet synchronous machines. They are therefore
rarely used in automobile applications. Asynchronous machines are nevertheless much less
expensive and contain no sensitive materials, a strong argument for automobile manufactur-
ers which, in the future, could tip the scales in their favor.

Table 3.6. Advantages and disadvantages of asynchronous machines

Advantages • Relatively easy manufacture
Disadvantages • Robust machine

• Less efficient than the synchronous machine (need for losses in the rotor
to produce torque)
• Difficulty of evacuating the heat caused by Joule effect losses in the rotor
(difficult cooling)
• Low power factor

3.2.4 Novel Machines

Other concepts have been developed, alongside the traditional machines. We may mention
in particular axial flux machines, wheel motors, double excitation machines, double rotor
machines and permanent magnet reluctance machines (PRM).

3.2.4.1 Axial Flux Machines

Axial flux machines differ from radial flux machines in the direction of the flux lines (Fig-
ure 3.11), which as the name implies are axial and not radial. There are permanent magnet
machines (Figure 3.12) or simply variable reluctance machines. They are known for their
high torque-to-mass ratio and are therefore well adapted for solutions in wheel motor (Gen-
eral Motors). Being more compact than radial flux machines, these machines are suitable for
low speed and high torque applications.

Chapter 3 · Electric drivetrain 135

Another advantage is that several of these machines can be assembled on the same axis to
increase the total torque. They have a number of major faults, however, due to their complex
structure, the mechanical strength of the various parts subjected to high axial forces and the
possible presence of torque pulses at low speeds.

Figure 3.11
Directions of flux lines in the axial flux machine.
Source: [Bommé, 2008]

Figure 3.12
Double air gap axial flux permanent magnet synchronous machines.
Source: [Woolmer and McCulloch, 2006]

136 Hybrid vehicles

3.2.4.2 Wheel Motors
In the wheel motor concept, the electric machine is incorporated directly in the wheel,
whereas in most configurations it is connected to the vehicle powertrain or body.

By fitting the machine in the wheel the vehicle architecture can be completely rede-
signed, due to the volume gained. Numerous configurations are therefore possible (for fur-
ther details, refer to paragraph 5.4.2). In particular, a 4WD architecture can be produced with
four machines, ensuring precise control of the vehicle trajectory (ABS, ESP, etc.). However,
fitting the machines in the wheels increases the unsprung mass and may cause problems of
vehicle dynamics, especially at high speed.

The link between the machine and the wheel may include a reduction gear. A priori,
although the noise, mass, volume and efficiency are improved without a reduction gear, the
last three points must be considered with caution: since the machine is rotating at the same
speed as the wheel, it must deliver a very high torque and therefore be designed and cooled
accordingly. The decision to use a reduction gear therefore be taken after conducting a global
study based on the vehicle specifications and the technologies used.

From a technological point of view, an external rotor machine can be used, i.e. with the
rotor located outside the stator (Figure 3.13). The immobile stator is connected to the vehi-
cle suspension, the rotor being connected to the wheel. With permanent magnet machines,
excellent mechanical strength is obtained since the magnets are located on the inner surface
of the rotor.

Figure 3.13
Permanent magnet external rotor synchronous motor.
Source: [Espanet, 2009]

Chapter 3 · Electric drivetrain 137

This concept was extensively studied by Hydro-Québec in the 1990s, especially by its
subsidiary TM4, but one of the most advanced examples is the Active Wheel developed by
the tire manufacturer Michelin (Figure 3.14). This system incorporates in a wheel:

- a high-speed electric machine (18,000 r.p.m.) providing traction and recuperative
braking,

- a mechanical reduction gear,
- an active suspension system using a second machine and controlling the vehicle body

movements (some models even propose energy recovery),
- a conventional braking system.

Figure 3.14

Michelin Active Wheel design (version with active electric suspension).
Source: [Bernard, 2011]

Another example is the wheel machine manufactured jointly by companies Irisbus,
Alstom and Michelin which has been running on trolleybuses for many years (Figures 3.lO.b
and 5.33).

These solutions offer considerable potential for technological disruption in vehicle design
architecture; they nevertheless remain very expensive and may be vulnerable under certain
conditions of use.

138 Hybrid vehicles

3.2.4.3 Double Excitation Machines
In double excitation machines, wound and permanent magnet excitations coexist, combin-
ing the advantages of wound excitation offering flexibility and those of permanent magnet
excitation offering good performance.

They seem well suited to pulsed operation, as is the case in an electric machine on board
a hybrid vehicle. These machines offer several possibilities. The coil and magnet can be
combined to create high flux if high torque is required; the magnet can also work alone if the
torque demanded is not too high and, lastly, the coil can create a flux of sign opposite to that
of the magnet to allow defluxing and operation at high speeds.

3.2.4.4 Double Rotor Machines
Double rotor machines can be designed with various architectures. Two examples are given
below:

- brushless machines, with two rotors including permanent magnets and a stator con-
nected to the external electrical circuit (5.2.3.2). This type of structure can be used
to produce a continuous electromagnetic transmission; it is proposed in particular by
Sheffield University, as shown on Figure 3.15 [Atallah et al., 2008],

Figure 3.15
Structure based on a double rotor brushless machine architecture.
Source: [Atallah et al, 2008]

- wound rotor machines, equipped with two connections to the external electrical cir-
cuit, to produce power-split hybridization comparable to the Toyota THS 9 (5.2.5.2
and Appendix 5). This system is proposed in particular by Ravello as shown on

9. Toyota Hybrid System.

Chapter 3 · Electric drivetrain 139

Figure 3.16 [Ravello, 2005] (Electromagnetic Split Powertrain, EMCVT), by Xin-
hua (Dual Mechanical Ports Electric Machine, DMPM), [Xinhua et al, 2008] and by
Cheng Yuan [Cheng et al, 2010].

Figure 3.16

Structure based on a double rotor machine architecture.
Source: [Ravello, 2005]

This configuration results in a solution equivalent to that produced by Toyota, but using
only one complex double rotor machine. Unlike the solution proposed by Toyota, this struc-
ture does not implement a planetary gear; the power distribution function is produced entirely
by the double rotor "set": the greater the speed difference between the engine and the wheels,
the greater the quantity of energy transiting through the electrical branch.

These electric machines are nevertheless complex and pose numerous challenges in terms
of their mechanical strength, the electricity supply of the rotors, the thermal aspects and their
cost.

3.2.4.5 Permanent Magnet Reluctance Machines (PRM)

Toshiba uses a permanent magnet reluctance machine: the permanent magnets used in the
rotor are positioned so that it induces variable reluctance. High torques can be achieved
with this construction, even with a short electric machine of large diameter. This solution
is therefore well suited for assembly for example between the engine and the transmission
[Takahashi, 2004].

140 Hybrid vehicles

3.3 MODELING OF ELECTRIC MACHINES

Electric machines must be modeled during two different steps in the design of the electri-
fied drive: firstly when sizing and designing the component, then when defining the control
algorithms.

The starting point when sizing is the machine specifications and modeling is carried out
to define the geometry and electrical characteristics (if possible optimum) meeting these
specifications.

Concerning the control, however, the geometry and electrical characteristics are known
and modeling will be carried out in order to represent the machine behavior. The aim in this
case is to be able to calibrate the machine control laws (or the vehicle energy management
laws); the same equations are implemented, but the opposite approach is used with creation
of a behavior model.

Electric machines are energy converters which use the forces resulting from the interac-
tion of magnetic fields and currents flowing in electrical conductors. These interactions are
governed by Maxwell's equations. Since the machine is not isolated from the external envi-
ronment, however, equations describing its relations with the outside must also be taken into
account: electrical, mechanical and thermal equations.

The complexity of the phenomena to considered and the desire to obtain highly efficient
machines in the smallest possible volume generate local problems of magnetic saturation and
thermal heating inside the machines.

Figure 3.17

Example of calculating the induction flux density (G) and the induction lines (D).

In this case, a finite element approach, based on fine discretization of the machine geom-
etry and on solving electromagnetic as well as thermal equations in each mesh proves highly
efficient. However, solving systems of several hundred thousand meshes requires extensive
computation time.

Chapter 3 · Electric drivetrain 141

Finite element codes are now an essential feature during the fine design of electric
machines; they are nevertheless too time-consuming if the aim is to perform a complete
simulation of an entire vehicle. In this case, a simplified model must be found that offers a
good compromise between precision of results and resolution complexity.

3.3.1 Electrical Aspects

A simplified analytical approach can be adopted to model the behavior of electric machines.
The equations and, in some cases the method, depend on the type of machine to be modeled.
We decided in the remainder of this chapter to describe a traditional approach concerning a
permanent magnet (or wound rotor) synchronous machine, used in most automobile appli-
cations. Simplified modeling of the machine therefore involves modeling various aspects:
electrical, magnetic, mechanical and thermal.

The electrical equations can be simplified by changing coordinate system: rather than
working with a stator three-phase coordinate system (a, b, c), the Park transform is used to
produce a rotating two-phase coordinate system (d, q), whose d-axis is aligned on the rotor
field (Figure 3.18).

Figure 3.18

Change from a three-phase representation in a coordinate system (a, b, c) to a
two-phase coordinate system (d, q) (Park's transformation).

In this new coordinate system, the machine operation is governed by equations involving
continuous quantities in case of a sinusoidal power supply or if we make the assumption of
only considering the fundamental. This change of coordinate system applied to the stator cur-
rents and voltages results in the definition of currents and voltages in a two-phase coordinate