Heavy Duty Truck Systems by Sean Bennett

292 Hybrid vehicles

Figure 5.14
Implementation of TM4 MÖGEN series-parallel hybridization.
Source: from TM4, www.TM4.com

Figure 5.15
Diagram of the TM4 MÖGEN series-parallel hybridization coupling.

Table 5.4. Clutch positions corresponding to the various modes of the MOGEN transmission

Modes Operation CLU1 CLU2
1 All-electric on machine EMC2 0 0
2 All-electric on both machines 0 1
3 Series hybrid 1 0
4 Parallel hybrid 1 1

Chapter 5 · Hybridization 293

In these two configurations, the fact that there is no gearbox in the transmission would
suggest that parallel mode is used at high speeds, when the engine can be connected directly
to the wheels, series mode being more efficient at very low speeds. Also in both cases, since
there is a clutch, the engine can be disconnected during decelerations in order to optimize
energy recovery (1.6).

This transmission offers a wide choice of operating modes, but the presence of two
clutches mounted directly on the main shaft requires very precise management of their tran­
sient changes in order to guarantee optimum driving comfort throughout the vehicle lifetime.

While this configuration has not yet found any major industrial developments, it is pro­
posed by various companies:

- the Swedish company Stridsberg Powertrain AB with its Strigear system, a configura­
tion with two electric machines and a clutch, similar to that proposed by TM4 [Strids­
berg, 2006],

- the drive train manufacturer Arvin Meritor which proposes a system of this type for
trucks (Dual mode Hybrid Drive unit),

- the Chinese manufacturer BYD with its FD3M model introduced in 2008 but not yet
commercialized.

5.2.5 Power-Split Hybridization

5.2.5.1 Notion of Power Splitting

As already mentioned (5.1.8), in this architecture, use of a planetary gear which provides
coupling by addition of speeds allows the speed of rotation of the engine to be uncoupled,
as much as possible, from that of the wheels. The possibility of controlling the engine speed
provides an additional degree of freedom in the drivetrain management which can be used
to increase the energy efficiency. For the planetary gear to perform this function, however,
the power must be permanently distributed on its three shafts, whose speeds of rotation are
related by the following equations in the case of the gear shown in Figure 5.16:

Equation expressing the addition of speeds: (5.1)

Equation expressing the torque ratio: (5.2)

and (5.3)
where: R: radius of the outer ring gear

r: radius of the sun gear
coc and T c speed and torque on the planet gear carrier (C)
coR and TR speed and torque on the outer ring gear (R)
cos and T§ speed and torque on the sun gear (S)

A quantity characteristic of the use of a planetary gear in a hybrid drive train is its power-

split ratio xd. In the example shown on Figure 5.16, a power P c supplied on the planet gear
carrier (C) is distributed at all times in a power P§ on the sun (S) and a power PR on the ring
gear (R), τΗ being expressed by:

(5.4)

294 Hybrid vehicles

Figure 5.16
Schematic diagram of a planetary gear.

Figure 5.17
Graph showing the power-split ratio of a planetary gear according to the speed
of(C) and (R) shafts.

Chapter 5 · Hybridization 295

The variations in xd shown on Figure 5.17 clearly illustrate the power-split operating pos-
sibilities of the planetary gear depending on the speeds of rotation of its shafts, from xd = 1,
where all the power supplied at C is split by branch S, up to xd = 0 where all the power is
transmitted directly on R. The split ratio may nevertheless take negative values, in this case
the flow in the branch S is reversed and the power supplied at R is greater than that at C. In
this case, we speak of recirculation.

As we will see below, power-split transmissions may involve one or more planetary gears
in different configurations which will modify the drivetrain behavior.

5.2.5.2 Input Split Type Configuration

The configuration shown on Figure 5.18 has a planetary gear beside the engine which is
connected to the planet gear carrier (C); one of the two electric machines is connected to the
sun gear (S) while the other is connected directly to the transmission to the driving wheels by
a single shaft type coupling. This is known as an "input split" configuration.

Figure 5.18

Implementation of an input split type transmission.

Figure 5.19 shows that this configuration is very similar to that proposed by Lucas (Fig-
ure 5.13), but with coupling by planetary gear in MC3 (PGC) which offers an additional
degree of freedom on the choice of engine speed. This solution is mechanically very simple,
with in particular no torque interruption system, such as a clutch, an important aspect for
driving comfort and transient management. Although it includes two electric machines and
implies complex management, we will see below (5.5.2) that this solution was chosen by
Toyota in the mid 1990s for its THS system, fitted on most of its hybrid models since the
Prius.

296 Hybrid vehicles

Figure 5.19
Coupling diagram of an input split type transmission.
For a power-split transmission, the search for maximum efficiency involves limiting the
flow of energy going though the split branch, which corresponds to a series architecture of
lower efficiency, to favor the direct branch which is similar to parallel operation. This is also
true for the cost, volume and mass of the transmission which can be decreased if the power
of the electric machines is reduced.
The variation in split ratio of power-split transmissions is traditionally represented as
a function of the transmission reduction ratio [Fukino, 2009; Conlon, 2005; Schroder and
Hofmann, 2008 and Pognant et al, 2010].
The transmission ratio K, defined by equation (5.5), is a characteristic factor of a trans-
mission. We will focus more especially on the variation range of this ratio, which should be
as large as possible, in order to minimize the engine speed range for a given vehicle speed
range (notion of opening the transmission).

(5.5)
coe: speed of rotation of the transmission, engine side
cos: speed of rotation of the transmission, at gear output

Figure 5.20, representing the change in power split ratio as a function of the ratio K, is a
characteristic of the transmission.

Chapter 5 · Hybridization 297

Figure 5.20

Theoretical change in split ratio as a function of the transmission ratio in an
input split type configuration (excluding storage system).

Figure 5.20 shows the following details:
- series operation at zero speed,
- linear variation of xd as a function of K,
- power-split operation for low values of K,
- remarkable point I for which the power split is zero, corresponding to purely parallel

type operation,
- operation in recirculation for high values of K.

This configuration is also known as electric continuously variable transmission (e-CVT).
It nevertheless offers an advantage compared with mechanical CVT since it can even operate
when the vehicle speed is zero.

In operation, the management system permanently tries to adjust the engine speed, there-
fore a value of K, to obtain a split ratio offering maximum efficiency for the entire power-
train. Presence of the battery provides greater freedom regarding this choice. When moving
off and at very low speeds, the split ratio is high, but use of all-electric mode minimizes the
need for the engine. In practice, the maximum values observed for the split ratio on most
types of driving do not exceed 0.6 to 0.7 [Fukino, 2009; Villeneuve and Mendes, 2004].

For further details concerning the operation of this transmission, readers can refer to the
case of the Toyota Prius (5.5.2).

5.2.5.3 Multimode Configurations

Although the THS input split system developed by Toyota is by far the most widespread,
other configurations have been studied in order to reduce the split flow and therefore increase

298 Hybrid vehicles

the overall efficiency, while minimizing the size of the electric machines [Schroder and Hof-
mann, 2008; Villeneuve and Mendes, 2004 and Meisel, 2009]. By using several planetary
gears in a "compound" configuration, we can obtain a parabolic expression of the split ratio
with the presence of two nodes and an intermediate region with a low split ratio (Figure 5.23).
The graph of the split ratio nevertheless indicates that the compound configuration cannot
be used when starting the vehicle, due to an infinite split ratio, which is not the case with the
input split configuration, as mentioned previously. Solutions have therefore been developed
to optimize the use of configurations with one and two planetary gears. In these multimode
solutions, a set of clutches is used to change configuration depending on the operating condi-
tions, as illustrated by the example of Figure 5.21. As we can see on Figure 5.22, this com-
plex configuration has 2 couplings more than the input split shown on Figure 5.19: MCI,
which consists of the 2nd planetary gear, and MC2, which is a single shaft coupling. The link
between the 2 electric machines includes the two clutches (from [Miller, 2005; Guzzella and
Sciarretta, 2007]).

Figure 5.21

Implementation of a two-mode type transmission.

Clutch CLU1 is used to connect the ring gear (R) of gear 2 to the assembly composed
of EMC1 and the sun gear (S) of gear 1, while CLU2 locks the ring gear (R) of gear 2 by
connecting it to the transmission housing. Operation of the two clutches must therefore be
coordinated. As indicated in Table 5.5, position CLU1 open and CLU2 closed corresponds
to an input split type configuration, suitable for low speeds, with planetary gear 2 acting as
reducing gear only. Inversely, position CLU1 closed and CLU2 open corresponds to a com-
pound type configuration, more suitable for high speeds.

Chapter 5 · Hybridization 299

Figure 5.22
Coupling diagram of a two-mode type transmission.

Table 5.5. Clutch positions corresponding to the various modes of an output split multimode transmission

Modes Operation CLU1 CLU2
1 Input split, low vehicle speeds 0 1
2 Compound, high vehicle speeds 1 0

Figure 5.23 illustrates the split ratio variations that can be obtained by combining the
two operating modes; the 2-mode configuration therefore limits the split ratios over a wide
reduction ratio range. Change of mode is carried out at zero split ratio, as close as possible to
the nodes of each mode (I and Cj). To respect the constraints regarding driving comfort and
mechanical strength of components, the transmission geometry is designed so that the nodes
are as close as possible to each other (example on Figure 5.23), even at the same point. In
particular, combining compound mode and input split mode provides a way of lowering the
transmission ratio corresponding to node I, which reduces the split power and therefore the
transmission losses more rapidly (Figure 5.20 and Figure 5.23). Split ratios of about 0.3 are
observed for this transmission [Fukino, 2009].

300 Hybrid vehicles

Figure 5.23
Theoretical change in split ratio as a function of the transmission ratio in a
2-mode type configuration (excluding storage system).

Figure 5.24
Change in efficiencies depending on the modes (2-mode type transmission).
Source: from [Ahn and Cha, 2008]
Figure 5.24 illustrates the efficiency variations simulated for the 2-mode transmission
shown on Figure 5.21 [Ahn and Cha, 2008]. We see that the values obtained at low speed
are much higher for the input split configuration and that the opposite situation is observed
at high speed with the compound configuration, which offers better efficiencies. Two-mode
transmission therefore preserves high efficiencies over a wide range of vehicle speeds.

Chapter 5 · Hybridization 301

5.2.5.4 Output Split Type Multimode Configuration

This configuration is very similar to the input split configuration but, as shown on Figure 5.25,
single shaft coupling and planetary gear coupling have been inverted, like the roles of the two
electric machines. The energy flows are therefore shared or recombined as close as possible
to the wheels, hence the name of "output split" given to this transmission. Figure 5.26 shows
that coupling MC3 is single shaft type, while MC4 is planetary gear type.

Figure 5.25
Implementation of an output split type transmission.

Figure 5.26
Coupling diagram of an output split type transmission.

302 Hybrid vehicles

The position of the planetary gear is important since it governs the power split expres-
sion which, as we can see on Figure 5.27, is hyperbolic, very high at low speeds and even
becoming infinite at zero speed, as with the multimode configuration described above.
Consequently, a vehicle cannot be started in hybrid mode with this configuration, which is
not efficient at low speeds. In addition, since machine EMC2 only sees the driving wheels
through the planetary gear, it cannot provide traction alone in electric mode and some of its
power must be split in the gear.

Figure 5.27

Theoretical change in split ratio as a function of the transmission ratio in an
output split type configuration (excluding storage system).

Due to these restrictions, output split configuration is generally implemented in mul-
timode operation through the use of various clutches like those shown on Figure 5.25. In
this example, which corresponds to solution 4ET50 proposed by GM on its Volt vehicle,
the clutches can be controlled to select the 4 possible transmission modes (Table 5.6). The
operating modes are as follows:

- mode 1 : all-electric with machine EMC2 only, blocking the ring gear (R) by CLU2
converts it into a simple reducing gear,

- mode 2: all-electric with the two machines, the ring gear is released and EMC1 is con-
nected to the gear by CLU1, which reduces the speed of rotation of machine EMC2
(and therefore its constraints and losses at high vehicle speed) or increases the maxi-
mum vehicle speed,

- mode 3: series hybrid, the engine is connected by CLU3 to EMC1 which acts as gen-
erator. CLU1 being open, no link with the wheels is possible. EMC2 provides traction
via the gear which acts as a simple reducing gear,

- mode 4: power-split, closing CLU1 and CLU3 connects the engine and EMC1 to the
wheels via the gear which acts as power split through EMC2.

Chapter 5 · Hybridization 303

Table 5.6. Clutch positions corresponding to the various modes of the GM Voltec 4ET50 output split multimode
transmission

Modes Operation CLU1 CLU2 CLU3
1 All-electric, low speeds 0 1 0
2 All-electric, high speeds 1 0 0
3 Series hybrid, low speeds 0 1 1
4 Power-split hybrid, high speeds 1 0 1

This operation is very ingenious since it uses all four different transmission modes in
the operating ranges where they are the most efficient, as shown on Figure 5.28. We can
see how difficult it is to distribute the various modes in hybrid operation, which stresses the
importance of the upstream research studies which were required to determine these priority
ranges as well as the importance of the control to implement them while guaranteeing vehicle
performance and comfort. The Volt management and control software, for example, would
contain nearly 10 million lines of computer code, i.e. more than the 787 Dreamliner, the lat-
est Boeing aircraft [Ulrich, 2011].

Figure 5.28
All-electric a) and hybrid b) operating modes of the GM 4ET50 system.
Source: [Miller et al, 2011]

Although this configuration has 2 electric machines and 3 clutches to control, it was cho-
sen by GM for its Volt and Ampera vehicles, which had been initially announced as being
series hybrid only (mode 3 in Table 5.6), (Figures 5.29 and 5.30). Use of power-split con-
figuration gives this multipurpose vehicle good consumption performance, even for motor-
way trips.

304 Hybrid vehicles

Figure 5.29
Components of the output split multimode transmission on the Volt car.
Source: [www.cartech.fr, 2010]

Figure 5.30
Chevrolet Volt.
Source: GM

Chapter 5 · Hybridization 305

5.2.5.5 Multimode Configuration with Discrete Ratios

Even in multimode configurations, the power-split transmission can generate high losses and
require powerful and expensive electric machines for some applications. This is the case for
heavy vehicles, used at high speed and even capable of towing loads such as 4-wheel drive
or premium vehicles. To meet these constraints, manufacturers and suppliers have developed
multimode configurations comprising several discrete ratios in which a direct mechanical link
is provided between the engine and the wheels, without splitting by the electric machines.
Figure 5.31 shows a schematic diagram of this type of drive for a transmission proposed by
GM; this complex configuration requires no less than 3 planetary gears and 4 clutches.

Figure 5.31

Schematic diagram of a multimode type transmission with discrete ratios.
Sources: [Ahn and Cha, 2008]

As previously, the clutches are controlled to select the various possible modes, as illus-
trated in Table 5.7 which shows the 6 possible modes for this transmission, depending on the
positions of the 4 clutches. We see the 2 modes with splitting and the 4 fixed ratios which
optimize the efficiency by reducing losses in the machines, thereby improving vehicle per-
formance when towing a trailer or when starting [Grewe et al, 2007].

Table 5.7. Clutch positions corresponding to the various modes of a GM transmission

Modes Operation CLU1 CLU2 CLU3 CLU4
1 Input split hybrid, low speeds 1 0 0 0
2 Compound split hybrid, high speeds 0 1 0 0
3 Discrete ratio 1 1 0 0 1
4 Discrete ratio 2 1 1 0 0
5 Discrete ratio 3 0 1 0 1
6 Discrete ratio 4 0 1 1 0

306 Hybrid vehicles

5.2.5.6 Applications

The input split configuration, used by Toyota in its THS system, is described extensively
below (5.5.2).

The output split configuration in multimode variant was selected by GM for its Volt and
Ampera vehicles.

Configurations comprising several planetary gears and clutches have been developed
by Allison for urban buses. In 2005, GM, Daimler Chrysler and BMW created a research
center of 500 people at Troy (Michigan) called the Hybrid Development Center, where the
automakers joined forces to apply this technology to cars and launch its industrialization
[Guldner, 2009]. Created to reduce costs, the association was nevertheless dissolved in 2009.
Two-mode transmission with fixed ratios has been adapted to various models in longitudinal
(Chevrolet Tahoe, BMW X6 ActiveHybrid, Mercedes ML450 Hybrid) and transverse {Saturn
Vue and Chevrolet Malibu for 2012) versions [Hendrickson et al, 2009]. While they are
suitable for multi-purpose premium vehicles, these hybrid transmissions prove to be highly
complex and consequently very expensive. Consequently, there is no guarantee that manu-
facturers will extend it to other models in their range.

5.2.6 Special Architectures

For completeness, we will mention several special architectures derived from the previous
ones, especially by removing the reversible energy storage in order to simplify the transmis-
sion. These architectures cannot be considered as hybrid. Since there is no storage system,
they do not have all the features of hybrid vehicles (in particular, no reversible system allow-
ing energy recovery during braking).

Several variants of the architecture with no reversible onboard storage have been pro-
duced, e.g. electric drive and electric four-wheel drive.

5.2.6.1 Electric Drive

This variant uses series architecture, but without its storage. Consequently, there is no longer
any coupling but a single electric link between the two machines (EL) (Figure 5.32). Electric
drive is one of the oldest systems, used initially on diesel-electric locomotives then, more
recently, for urban public transport. While the drive is simpler and less expensive, removing
its energy storage prevents all-electric mode and recuperative braking, downsizing of the
engine and limitation of its power transients.

Chapter 5 · Hybridization 307

Figure 5.32
Coupling diagram for an electric drive.

This architecture was used, in diesel-electric configuration, for a bus application by Iris-
bus, Alstom and Michelin in the mid 1990s [Jeanneret et al, 1998]. Several vehicles have
been sold by Irisbus in the Civis 18.5 m version (Figure 5.33), equipped with optical guiding
developed by Matra Transport (now Siemens) and in the Cristalis version, without the guid-
ing system. In terms of energy, although choosing the operating point of the engine reduces
its consumption, it is difficult to achieve consumption values less than those of the conven-
tional vehicle due to loss of efficiency in the electric drive.

Mercedes also developed a diesel-electric drive on a 9 m public transport vehicle (Fig-
ure 5.34), the CITO, which has been tested in particular in the town of Nîmes (France) as part
of an ADEME project [Gauducheau, 2001]. The results demonstrated a significant drop in
ΝΟχ emissions, but no noticeable consumption improvement.

Figure 5.33
Irisbus 18.5 m Civis with electric drive (Rouen version).
Source: Irisbus

308 Hybrid vehicles

Figure 5.34
Mercedes CITO diesel-electric vehicle.
Source: [Gauducheau, 2001]

5.2.6.2 Electric Four-Wheel Drive
This system is based on the series-parallel architecture shown on Figure 5.13, still removing
the energy storage of the reversible energy system and adding a direct mechanical link from
the engine to the front wheels; the electric coupling therefore simply consists of an electric
link (EL) (Figure 5.35). For the mechanical part, couplings MCI and MC2 are not present,
MC3 is a dual shaft coupling with pulley-belt and MC4 corresponds to a mechanical cou-
pling through the road between the front and rear driving wheels.

Chapter 5 · Hybridization Next Page

309

Figure 5.35
Coupling diagram for an electric four-wheel drive.
This configuration, proposed by Nissan in its Cube concept car, offers the possibility
of 4-wheel drive whenever required while keeping a simple and inexpensive solution (Fig-
ure 5.36). Note that this architecture corresponds to that chosen by PSA for its hybrid 3008,
but keeping the battery and therefore all the hybrid features (Figure 5.10).

Figure 5.36
Implementation of an electric four-wheel drive on the Nissan prototype Cube.
Source: Nissan

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310

5.2.6.3 Compressed Air Hybrid

This configuration is based on parallel architecture, but the compressed air hybrid configu-
ration is original since, although it comprises an irreversible energy system as well as a
reversible system, coupling takes place inside the engine itself and storage uses compressed
air (Figure 5.37). Studies are being conducted on this process for various types of engine,
via an extra valve in the combustion chamber or a system fitted in the intake manifold. The
air which is compressed during the deceleration phases can then be allowed to expand in the
engine to start the vehicle and drive it at low speed [Brejaud et al, 2008], [Lee et al, 2008].

Figure 5.37

Coupling diagram for a compressed air hybrid drive.

Given the low quantity of energy contained in the compressed air tank, charging from the
outside, using an electric compressor for example, does not seem feasible.

5.3 FEATURES

This paragraph describes the various features offered by hybridization. This approach clearly
identifies the advantages of a hybrid drivetrain compared with its equivalent using a conven-
tional drivetrain.

Based on their various features, hybrid vehicles can be classified into two broad categories:
- vehicles whose features are primarily intended to reduce fuel consumption, which can

be qualified as "discrete hybrids". These solutions offer no other advantages and are
similar to those aimed at optimizing the engine and its mechanical transmission (with
possible synergies),
- vehicles offering additional features for the driver, the passengers and the local or
global environment, which can be qualified as "functional hybrids". These additional
features must in particular compensate for the initial extra cost of the vehicle during

Chapter 5 · Hybridization 311

use (driving without local emissions of nuisances for town centers with restricted
access) or further reduce its greenhouse gas emissions and fossil fuel consumption
(use of another energy vector).

The implementation and potential of the various features are described below, through
concrete examples.

5.3.1 Discrete Hybrids

5.3.1.1 Optimized Management of Onboard Electrical Energy

On conventional vehicles, electricity is produced by an alternator driven by a belt and
delivering a voltage regulated by a diode bridge to supply the vehicle 14 V network. Until
recently, these systems were mainly optimized in order to lower the cost, ignoring the global
efficiency, which is about 70%. The increase in power of the vehicle network and the search
for minimum consumption have led suppliers to improve the machine, its electronics and its
control.

In an optimized system, the energy delivered by the alternator is not only related to the
consumption on the vehicle network, but can be regulated so that it is in phase with the vehi-
cle use. The battery can therefore be mainly recharged during decelerations, when the engine
is driven by the wheels, in order to reduce consumption, by recovering a proportion - even
very small - of the energy during deceleration. Inversely, the alternator can be switched to
run at zero torque during acceleration phases while the 12 V battery supplies the accessories
with electricity. This type of system is also implemented by Valeo with a controlled alterna-
tor in the Volt Control concept.

While the effects of this approach are limited, its cost is very small and it can be distrib-
uted on massive scale in the short term: it is generally implemented in the systems described
in the following chapters.

5.3.1.2 Stop-Start

The main feature implemented in this system, also known as "micro hybrid", is the possibil-
ity of switching off the engine when it is not producing a driving force to propel the vehicle,
i.e. mainly to eliminate the idling phases. Although the principle is extremely simple, imple-
mentation of this feature under optimum conditions involves starting the engine safely (gear-
box and clutch positions), cleanly (as regards regulated pollutants), quickly (a few tenths of
a second), silently, without vibration and automatically (by the driver pressing the pedals).

As we will see, numerous improvements are currently being made to the Stop-Start sys-
tems which will enhance their performance and progressively bring them closer to the mild
hybrid feature described below (5.3.1.4). We will therefore have a continuum of technologi-
cal solutions, from the simplest to the most complex.

312 Hybrid vehicles

A. Reinforced Starter

In this solution, a minimum number of modifications are made, so that the system can with-
stand the 300,000 to 600,000 engine starts required while guaranteeing sufficiently low noise
emissions and vibrations. The starter electric motor is more powerful and its meshing mecha-
nism reinforced, but integration requires no major adaptation of the powertrain components;
its industrialization cost is therefore limited. This system also includes a specific battery
replacing the conventional 12 V battery to cope with the greater number of charge/discharge
cycles.

However, keeping the conventional starter principle makes it difficult to restart the engine
when it is still rotating ("flying start") and slightly increases the time to start the engine. As
a result, the engine can only be stopped for very low vehicle speeds (< 7 km/h), thereby
slightly reducing the possibilities of reducing the fuel consumption (gain of about 4% to 6%
on European Test Procedure).

Several suppliers have proposed this system for several years, for example Bosch on the
BMW series 1 and 3 and the Mini, or Valeo on the Volvo S40 1.6D DRIVe.

Numerous studies are conducted by the suppliers to improve the performance of rein-
forced starters. Some companies such as Denso, for example, have commercialized rein-
forced starters allowing flying start, making them more like the starter-alternators described
below. This optimization is achieved in particular by precisely synchronizing the speeds
before engaging the pinion or by permanent engagement of the starter pinion on the flywheel,
the pinion disconnecting at high speed through the use of a free wheel.

B. Starter-Alternator

This solution implements a reversible electric machine, which acts both as starter and alterna-
tor, hence its name: starter-alternator. In most systems, the machine develops a power of 2 to
3 kW and is connected to the engine by a pulley-belt system, as with conventional alterna-
tors. The system can nevertheless transmit torque in both directions, which implies reversible
control electronics and a belt tensioning system on the two strands. These modifications have
little impact on the powertrain architecture. This system is more complex that the reinforced
starter, but the engine can be started over a broader range of engine speeds, especially when
the vehicle is moving. This characteristic allows the engine to be switched off for longer
periods of time (speeds below 20 km/h), and switched on even if the car does not come to a
complete stop. In addition, by using a more efficient, better controlled machine, the electric-
ity generation function for the vehicle network can also be optimized. These systems offer a
consumption gain of 6% to 8% on the normalized cycle, but measurements have shown that
a gain potential of nearly 15% could be reached in dense urban traffic. The starter-alternator
system is capable of starting the engine more than 500,000 times.

Starter-alternator systems have been produced by Valeo since 2004 with the StARS fitted
on the Citroën Cl and C2 Stop-Start and on the Daimler-Benz Smart Fortwo MHD. Since
then, Valeo has developed a more efficient version with integrated electronics called the
i-StARS, intended for mass production applications. PSA intends to equip its diesel vehicles
of up to 1.6 L with this system, in its e-Hdi concept.

Chapter 5 · Hybridization 313

These systems retain an electrical energy storage system based on an optimized 12 V
lead-acid battery. On some systems such as the e-HDi proposed by PSA, the battery can be
associated with supercapacitors in order to supply more power to start the engine more easily
and back up the vehicle network to reduce disturbance during each engine start.

Starter-alternators are more suitable for urban vehicles where offering maximum con-
sumption gain; the extra cost is currently estimated at between €200 and €500. However,
with the development of the i-StARS system which can be used to start larger engines, the
Stop-Start function could be applied to a broader range of vehicles.

The conventional engine starter can be kept for cold start on vehicles sold in countries
with harsh weather conditions, the high gearing of the traditional system used in this case to
"loosen" the cold engine.

C. Remarks on Implementation of the Stop-Start Function

Once the decision has been taken to stop the engine when it is not producing any driving
force, we realize that its operation is not totally pointless since it also drives various comfort
or assistance auxiliaries, such as the air-conditioning compressor, vacuum pump for braking
and the oil pump of an automatic gearbox. Suppliers and car manufacturers therefore had to
consider alternative solutions, for example:

- engine restart, controlled by a setpoint on the auxiliaries: this is the simplest solution,
which was implemented on the first vehicles sold;

- electric drive of the auxiliaries: the air-conditioning compressor, for example, can have
an all-electric drive, as on the latest Prius versions, or be driven by the starter-alternator
via the pulley-belt system which is in this case disconnected from the engine, as on the
Toyota Crown shown on Figure 5.38. All these systems require a voltage higher than
12 V and are generally fitted on vehicles equipped with more functions than Stop-Start
alone. For completeness, we may also mention systems based on a high-temperature
fuel cell powered by the engine fuel, which reduces the constraints on the battery;

- use of thermal storage as sensible, latent or chemical energy, to provide the thermal
conditioning of the passenger compartment {StopStayCool and StopStay Warm systems
proposed by Valeo) [Guyonvarch et al, 2003];

- acceptance of a temporary loss of performance on the thermal conditioning of the pas-
senger compartment for economic solutions.

Moreover, when starting the vehicle from cold, i.e. with the engine and the after-treatment
system at ambient temperature, it may be better to leave the engine running for the first few
stops so that the system warms up more quickly. Manufacturers and suppliers have focused
extensively on this situation, included in the European standard test procedures (Chapter 1)
and in a large number of our applications, in order to minimize the delay before the system
starts.

314 Hybrid vehicles

Figure 5.38
Stop-Start systems with extended features of the Toyota Crown.
Source: from [Itagaki etal., 2002]

5.3.1.3 Stop-Start with Extended Features
Belt-driven systems can achieve powers far higher than those described above, up to 15 kW
for the eAssist proposed by GM [Savagian, 2011]. In this case, the machine must be con-
nected to a storage system of much higher voltage than that of the vehicle network. This
storage can be obtained by coupling supercapacitors with a 12 V lead-acid battery, as for
the StARS 14+X developed by Valeo which operates with a 4 kW machine at a voltage of
between 14 V and 28 V. Storage may also consist of a 36 V lead-acid battery, as on the
THS-M system proposed by Toyota on its Crown in 2001, or a 115 V lithium-ion battery for
the eAssist system which is proposed by GM on its LaCrosse model in 2012 (Figure 5.39).
Due to the powerful electric system, additional features can be proposed:

- energy recovery during decelerations, which simply consists in capturing some of the
energy returning to the engine. The consumption gain achieved by this type of system
can reach 8% to 12% for a passenger car on the European Test Procedure;

- vehicle "take-off, which corresponds to the first few wheel revolutions (also called
creeping). In this case, the electric machine is used to start the vehicle moving
during the first few seconds, thereby improving the driver feel when starting, in a
Stop&Go system;

- starting engines of higher displacement (e.g. the 2.4 L gasoline for the GM eAssist
system) and therefore use over a broader range of gasoline and diesel vehicles.

Chapter 5 · Hybridization 315

Figure 5.39
Stop-Start systems with extended features of the GM LaCrosse eAssist.
Source: [Savagian, 2011]

Figure 5.40 summarizes the various features offered by an extended Stop-Start system on
a large number of vehicle life situations.

Figure 5.40
Summary of the features offered by the GM eAssist system.
Source: from [Savagian, 2011]

Use of a more powerful machine and numerous components (supercapacitors, DC/DC
converters, high-voltage battery) will increase the cost of these systems by about €500 to
€900, which means that they will not be sold as rapidly as the previous ones.

316 Hybrid vehicles

In their most advanced developments, such as the system proposed by GM, the electric
machine can also provide the mild hybrid feature (5.3.1.4). These systems are therefore very
similar to those described in the next chapter; some authors use the term micro-mild hybrid
to describe them, which clearly reflects the continuity between all these features.

The regenerative braking feature is highly dependent on the vehicle architecture and its
use conditions: it is detailed in 1.6.

Evolution of the electrification solutions proposed by Valeo

For many years Valeo R&D teams have been working on the development of a
complete range of engine electrification solutions that would be easy to implement on a
wide range of vehicles since requiring few modifications to the powertrain. The systems
proposed are in fact based on a non-integrated electric machine connected to the drive
by pulley-belt (5.2.3.1 A a ) . Synchronous machines are used, with several technologies,
including optimized claw-pole machines, a solution largely adopted by Valeo for alterna­
tors. Apart from reinforced starters, these machines are associated with active power
electronics allowing control as generator and motor, which is not the case with traditional
alternators.

The developments conducted by Valeo focus on the following two main areas:
• Propose products suitable for the widest possible range of vehicles;
• Maximize the functions, and the consumption gain, offered by the various solutions.
This approach is illustrated in Table E5.1 which lists the various systems developed
by Valeo with several integration and performance levels, up to the features of a mild
hybrid for the most advanced system.
At the same time, extensive studies have been conducted on mechatronic integra­
tion of the components, which is required to achieve cost levels allowing manufacturers
to propose these systems in very large production volumes. The first generation of the
StARS system proposed by Valeo in 2004 included a machine, an electronic box and the
cables to the three phases with their connectors (Table E.5.1). This system represented a
good compromise for fast series manufacture at an attractive cost on limited production
volumes. To move to mass production, however, Valeo had to propose a component
that was as easy to implement and as reliable as a traditional alternator, at minimum cost
(targeting 40 % less than the 1st generation). Valeo therefore developed the i-StARS,
a more integrated and more standardized system comprising fewer parts. As shown
on Figure E5.1, the power electronics have been integrated in the machine, inducing
extremely harsh operating conditions for the components (max. temperature 140 °C,
max. acceleration 30g). Valeo has developed novel methods, in terms of component
assembly, soldering of connections and dedicated integrated circuits (Insulated Molded
LeadFrame technology).

Table E5.1 Evolution of the syste

Re Start Re Start StARS1
Reinforced Reinforced 1st generation

1stsat rgteenremraottioorn 2sntad rgteernemraottioorn

Nominal voltage 12 V 12 V 12V

Associated storage Lead Acid Lead Acid Lead Acid

system AGM battery AGM battery AGM battery

Performance 2.2 kW, 40 Nm 2.2 kW, 40 Nm 1.5 kW, 50 Nm2
in starter mode
4.0 L Gasoline 4.0 L Gasoline ~ 2.5 kW
Performance in 2.5 L Diesel 2.5 L Diesel
generator mode 2.0 L Gasoline
High-mid High-mid range 1.6 L Diesel
Internal range and and premium
combustion engine premium Economy and
low-mid range
Vehicle market
segment

Features I.C. En£*ine start I.C. Engine start
onboa

Marketing year 2009 2014 2004

1. Starter Alternator Reversible System.
2. @50 rpm.

tems developped by Valeo

i-StARS i-StARS+ StARS + X Mild

12 V 12 to 28 V 48 V

Lead Acid battery and Lead Acid Supercapacitors Chapter 5 · Hybridization
supercapacitors (5V) battery and or Li-Ion battery
or DC/DC converter supercapacitors

28 V

1.5 kW, 2.7 kW, 4kW, 80 Nm2 8tol6kW
56 Nm2 75 Nm2

2.2 kW ~3kW 6kW 8tol6kW

2.0 L Gasoline 4.0 L Gasoline 4.0 L Gasoline
1.6 L Diesel 3.0 L Diesel 3.0 L Diesel

Economy and High-mid High-mid range and premium
low-mid range range and
premium

and optimized management of I.C. Engine start, onboard electrical
ard electrical energy energy generation, regenerative
braking, engine assist (boost)

2010 2011 Prototype 2015?

317

318 Hybrid vehicles

Figure E5.1
Integrated electronics Valeo i-StARS system.

Alongside these studies, Valeo is continuing its research work on solutions imple­
menting a more powerful electric machine (up to 16 kW) capable of assisting the engine,
in a mild hybrid concept (5.3.1.4). The objective of this type of «mild» hybridization is to
achieve consumption gains of 15% to 20% with limited additional cost, which could allow
massive distribution on the market.

Studies on this subject are mainly conducted at the Créteil (France) R&D center,
industrialization of the i-StARS and its electronics being carried out in Valeo plants at
Etaple (Nord - Pas de Calais) and Sablé sur Sarthe (Sarthe) in France.

5.3.1.4 Engine Assist
The mild hybrid solution uses a more powerful electric machine (10 to 20 kW) which, in
addition to the previous functions, can assist the engine over a very wide range of engine
speeds. Assistance is provided during the driving phases by supplying additional torque,
positive or negative, which is defined by the management and control system according to
one or more of the following criteria (also known as "boost" mode):

- over long times, share the torque between the engine and the electric machine to opti-
mize the overall energy consumption and control the battery state of charge. The meth-
ods implemented to provide these functions are described in Chapter 6;

- over short times, supply additional torque to provide the vehicle dynamics, thereby
allowing the use of a smaller engine (downsizing) less optimized in terms of response
time and possibly operating over a longer gearbox ratio. As shown on Figure 5.41,
the torque curve obtained by combining engine and electric machine delivers good
performance at low engine speeds, like a turbocharged diesel engine, while using a
considerably downsized gasoline engine;

- over extremely short times, filter the engine acyclisms to reduce the vibrations, espe-
cially on a downsized engine with a smaller number of cylinders, or during temporary
phases such as starting and stopping.

Chapter 5 · Hybridization 319

Figure 5.41

Static torque characteristic with and without electric machine, Honda Civic
IMA 2003.
Source: [Ogawa et al, 2003]

Honda and Daimler propose this type of hybridization on their Insight and S400 vehicles
(configuration known as PI). The electric machine, characterized by a very high diameter/
length ratio, is attached to the engine, fitted between the engine and the gearbox in single
shaft position (Figure 5.42). The machine may also act as flywheel for the engine. Note that
the engine and the electric machine are connected together; consequently, in some operating
phases such as energy recovery during braking, the friction and pumping losses on the engine
reduce the possible gain achievable by the electric machine. Manufacturers such as Honda
have therefore worked extensively on reducing these losses by minimizing, for example, all
sources of friction in the engine and on the valve control, in order to significantly lower its
drag and increase the proportion of recoverable energy [Iijima, 2006], [Takemoto, 2009].
According to Honda, the car manufacturer most involved in the mild hybrid solution, some
operating phases, like stabilized phases at very low speed, could be carried out without the
help of the engine, with the electric machine supplying the torque for traction and the residual
losses of the engine.

One solution to address the problem of the overall length of the powertrain, which is
highly restricting in transverse configuration, is to reduce the size of the engine (by reducing
the number of cylinders or suppressing the belt-driven auxiliaries) or the gearbox (number
of gears).

For this power level, the storage voltage is generally 120 to 150 V with a total energy of
less than 1 kWh. Implementing this features offers consumption savings of about 30% under
urban conditions, 15% to 25% on the European Test Procedure, and very little under extra-
urban conditions.

320 Hybrid vehicles

Figure 5.42
Honda Insight 2008 engine and electric machine.
Source: Honda

The extra cost of this system is higher, with a current bracket of €1,000 to €2,200.
As seen previously with the GM LaCrosse, less complex solutions are proposed with a
non-integrated machine connected by belt. As part of the French cost shared Prédit3 VEH-
GAN program, in 2008, IFP Energies nouvelles built a Smart-based demonstrator equipped
with a Valeo StARS machine powered by supercapacitors [Venturi et al, 2008].
With the mild hybrid function, the power and displacement of the engine can be reduced
while guaranteeing the vehicle dynamic performance through the extra torque provided by
the electric machine. However, this machine is powered by a battery which can only provide
a very limited quantity of energy for these applications and which may also exhibit ther-
mal limitations. The drivetrain sizing must therefore take this feature into account to avoid
situations where the vehicle dynamic performance would be reduced. In these downgraded
modes, the vehicle dynamic performance would be temporarily reduced, which could pos-
sibly lead to safety problems. We can therefore understand that car manufacturers would be
reluctant to commercialize this type of vehicle. The design specifications for hybrid driv-
etrains are therefore complex and must take into account numerous life situations to ensure
that the driving comfort remains acceptable and that the vehicle is not potentially dangerous
under certain circumstances.
An example of specifications is given below (5.5.1). In addition, concerning the engine,
the case of the Prius (5.5.2) illustrates the fact that Toyota has increased the power of its
engine on each new version commercialized, to - amongst other things - limit the possibil-
ity of downgraded modes which may have been observed on the first model and which were
indicated to the driver by a "tortoise" light on the dashboard.

Chapter 5 · Hybridization 321

5.3.1.5 All-Electric Mode

In all-electric mode, the vehicle is powered by the electric machine alone, with the engine dis-
connected from the drive and switched off during these phases (configuration known as P2).
This type of "full hybrid" solution offers far more possibilities for optimizing the operation of
the engine since most of the unfavorable situations, such as vehicle take-off or very low speeds,
can be carried out in all-electric mode. In this case, the electric drive is much more powerful,
with a 20 to 50 kW machine, storage at 200 to300 V and a total energy of 1 to 2 kWh.

Figure 5.43 illustrates the arrangement of components on the prototype Hybrid-HDI
unveiled by PSA in 2007. Note in particular the clutch (10) used to isolate the engine from
the transmission in order to switch into all-electric mode. Energy recovery during braking
with this type of configuration is also much more efficient since the engine friction losses are
eliminated, which is not possible with the mild hybrid solution.

1. Diesel engine 1,6L HDi (66 kW).
2. FAP: Particulate filter (patent PSA).
3. Stop and start system.
4. Electric machine 16 kW.
5. 6 speed automated transmission.
6. Power electronics.
7. 12 V battery.
8. Contrôler (PTMU).
9. High voltage wires.
10. Clutch.

Figure 5.43

PSA Hybrid HDI engine.
Source: [Beretta et al, 2009]

The very high degree of optimization that can be obtained on the engine use offers
extremely high consumption savings, up to 40% under urban conditions where conventional
drivetrains are inefficient. These relative savings will drop to between 10% and 20% under

322 Hybrid vehicles

rural road conditions and remain low on motorway. The complexity of this system and the
design of its components nevertheless induce a substantial extra cost, currently estimated
at between €2,500 and €5,000. It is quite incredible to see that the first hybrid vehicle com-
mercialized and the most widely sold today, the Toyota Prius, implements all the features
described in the previous chapters.

During operation in all-electric mode, the auxiliaries normally driven by the engine will
be stopped and alternative solutions must therefore be found, such as those described above
(5.3.1.2.C) with the Stop-Start system.

Although the vehicle can be driven in all-electric mode with this feature, it is important
to point out that the type of driving is not controlled by the driver himself but by the driv-
etrain management system which automatically decides when to start the engine if necessary.
Under these conditions, this feature can be qualified as "supervisor all-electric mode". The
conditions for switching on the engine are represented on Figure 5.44 for the case of the
hybrid transmission installed on the Prius 2. The criteria mainly concern 3 aspects:

- vehicle speed too high (mechanical constraints on the transmission),
- tractive force too high (maximum power constraint for the NiMH battery),
- battery energy consumption (protection of the SOC window).
Note that other criteria such as battery temperature too high or engine and/or after-treat-
ment system temperature too low may also cause the system to switch back into hybrid
mode. Even though, in the case of this Prius, the vehicle electric mode can be extended by
the driver pressing the "EV button" on the dashboard, the system nevertheless switches back
into hybrid mode automatically depending on the criteria present. In practice, this type or
vehicle can only travel a few hundred meters, possibly a maximum of one or two kilometers,
in electric mode under favorable dynamic conditions (stabilized at low speed on flat ground).
This situation must be born in mind as regards the all-electric autonomy feature (5.3.2.2).

Figure 5.44
Conditions for switching on the engine of a Prius.
Source: from [Yaegashi, 2009]

Chapter 5 · Hybridization 323

When the management system decides to switch from all-electric mode to hybrid mode,
the engine restart must be managed very carefully so that it is imperceptible for the vehicle
occupants. Restart can be carried out by a belt drive Stop-Start type system on the engine
(PSA Hybrid 4, Hyundai Sonata, IFP Energies nouvelles demonstrator Flex Hybrid), by
one of the electric machines in power split (Toyota Prius) or directly by the traction elec-
tric machine. In the latter case, to avoid any torque interruption to the wheels, a second
clutch after the gearbox can be used (Nissan Infiniti m35h) or even a torque converter (VW
Touareg).

5.3.2 Functional Hybrids

5.3.2.1 Non-Interruption of Torque when Changing Gear

Manual gearboxes represent a significant proportion of vehicles on the world market and
offer potential for improvement, especially through the use of robotized control which can
reduce consumption by implementing optimized gear shifting laws.

Although they offer good energy efficiency, the down side is the torque-interruption
effect that accompanies ratio shifts, which may be uncomfortable, especially for robotized
gearboxes where the decision to change gear is not taken by the driver.

Solutions have been proposed for many years to eliminate or attenuate this problem, for
instance:

- continuous transmissions, which totally eliminate gear changes but whose lower effi-
ciency increases consumption, while the engine running at high speed might represent
a handicap. This type of transmission should find limited development in the future
(no more than 2% sales expected in Europe, for example, according to Global Insight
[Global Insight, 2007]);

- dual clutch transmissions (DCT) which effectively consist of two parallel (or coaxial)
gearboxes, one for even gears, one for uneven gears. By engaging one clutch and
disengaging the other clutch at the same time it is possible to change gear without
any torque interruption. Although this technology is complex, some manufacturers
propose models such as VW DSG, BMW DKG and the Ford PowerShift;

- ingenious solutions, based on a conventional gearbox, where the synchro on 5th gear
has been replaced by a multiplate wet clutch to form a bypass of the engine torque
when changing gear, in order to maintain a driving force, even limited, during the
operation [Yamasaki et al, 2005].

In some configurations, hybrid transmissions can implement this non-interruption of
torque feature:

- series hybrid architecture in which the electric machine, the only one connected to
the driving wheels, exhibits highly favorable torque at low speeds of rotation, thereby
removing the need for a gearbox in most applications for passenger cars. Obviously,
there is no torque interruption with this type of architecture. A gearbox may neverthe-
less be used to reduce stresses on the electric machine, for applications on trucks or
very fast vehicles in particular;

324 Hybrid vehicles

parallel hybrid architectures allow the use of a gearbox with no torque interruption,
provided that the machine is located at a position where it can operate independently
of the gearbox. We may mention:
• the machine connected to the secondary shaft or located between the gearbox and

the wheels. It does not benefit from the gearing but can supply torque to the wheels
when changing gear (post transmission configuration);
• through-the-road coupling. One or more electric machines are located on the second
axle and can provide drive when the torque is interrupted. This function remains
active as long the machine(s) is/are connected to the wheels; a decoupling system
may nevertheless be used at high speed to reduce stresses on the machine(s);
• two-way transmission In this configuration patented by IFP Energies nouvelles, the
electric machine is located on a parallel path in order to bypass the gearbox when
changing gear through the use of a set of clutches (Figure 5.45).

Figure 5.45

Schematic diagram of a two-way transmission.
Source: [Venturi, 2007]

For these solutions, when using an electric machine to supply some or all of the engine
torque when changing gear, highly accurate synchronization with the mechanical compo-
nents is essential to avoid impairing passenger comfort. Figure 5.46 illustrates a sequence
of gear changes simulated in real time on the IFP Energies nouvelles HIL test bench at
Rueil-Malmaison [Chasse, 2009], [Del Mastro, 2009]. The curve of Figure 5.46a shows the
torque of the electric machine taking over from the engine while changing gear. Figure 5.46b
shows the effect of the assistance by the electric machine on the vehicle speed, which can
be maintained while changing gear. The graph of accelerations (Figure 5.46c) illustrates the
fact that with electrical assistance, the gear change should be hardly noticeable by the driver
and passengers.

Chapter 5 · Hybridization 325

Figure 5.46
Effect of electrical assistance on the gear change of an automated mechanical
gearbox.

5.3.2.2 All-Electric Mode with Range and Charging on the Grid

We combined these two features since they both induce very high energy and power con-
straints on the electric drive, in particular storage.

A. Description of the Problem
a. All Electric Mode with Range
The all-electric range of a vehicle is studied since it offers an advantage with respect to local
nuisances, atmospheric pollution and noise, which can be either eliminated or significantly
reduced. Implementing this feature would enable vehicles to enter dense town centers, pro-
hibited to vehicles emitting local pollution. These zones already exist in some very dense

326 Hybrid vehicles

towns in northern Italy and the concept of priority action zones for air (Zone d'Actions Prior-
itaires pour l'Air - ZAPÄ) launched recently by the French public authorities takes a similar
approach. Other ways of using utility vehicles could also be considered, such as night deliv-
eries, for vehicles with very low noise emissions. Lastly, this feature is required for vehicles
which need to travel through closed environments (warehouses, tunnels, etc.).

The electric autonomy may also be considered since it favors the consumption of electri-
cal energy, especially at the start of a trip, thereby increasing the possible transfer of con-
sumption from oil to other primary energies for a plug-in vehicle (see below).

b. Charging on the Grid

Charging hybrid vehicles on the grid has been studied for many years as a way of transferring
some of the hydrocarbon consumption of road vehicles to other primary energies via electric-
ity. In the mid 1980s, after the second oil crisis, the American DOE launched a research pro-
gram on plug-in hybrid vehicles [Trummel et al, 1984], [Badin, 1986]. The demonstration
vehicle, manufactured by General Electric, was based on parallel architecture with a 34 kW
electric machine, combined with a 13 kWh lead-acid battery. This vehicle could already
transfer more than half of its energy consumption to electricity over the first 60 km traveled.
The technologies used at the time for the components and for the management and control
system nevertheless prevented industrialization of this solution.

More recently, in the 1990s in France, as part of the national VERT program (Véhicule
Électrique Routier à Turbine), PSA and Renault developed series hybrid prototype plug-in
vehicles with an all-electric range of 20 to 40 km, with 6 to 10 kWh of NiCd and NiMH bat-
teries [Badin et al, 1994], [Jeanneret et Badin, 1996].

This concept was then abandoned with the commercialization of non plug-in vehicles,
such as the Toyota Prius, whose total battery energy does not exceed 1.3 kWh, and the
Honda IMA system, which has no all-electric mode. These choices were justified as a result
of the difficulties in implementing high-energy battery packs (safety, costs, ageing, etc.) and
the advantage obtained by remaining independent from a charging grid. Despite this highly
pragmatic approach with respect to onboard storage and all-electric capabilities, more than
2 million hybrid vehicles have been sold since 1997.

In recent years, further developments have been made to the concept of the plug-in hybrid
electric vehicle (PHEV), mostly in the United States, due in particular to the research program
set up by the Department of Energy in 2007 [Wall, 2007] and at the same time by an amend-
ment to the California ZEV (zero emission vehicles) mandate, to include plug-in hybrids in
the quotas from 2008. The objectives are still to reduce oil consumption - and imports - but
also atmospheric pollution in cities and greenhouse gas emissions from road transport. Since
the battery represented a major obstacle to the massive development of PHEVs due to its par-
ticular working conditions, the DOE launched research studies on high-energy battery packs
(useful energy 3.5 to 11.5 kWh) aimed in particular at improving their compromise between
cost and lifetime [Howell, 2007], [Howell, 2009].

Chapter 5 · Hybridization 327

B. Implementation

In non plug-in hybrid vehicles, the battery state of charge is constantly maintained around
an average value, generally 50% to 60%, which is a compromise taking into account ageing
and the discharge and charge performance (Chapter 4). In this type of Charge Sustaining
(CS) operation, the battery is never fully recharged, since the vehicle is not equipped with an
external charger.

With plug-in vehicles, the energy management system will allow the battery state of
charge to deplete, in Charge Depleting (CD) operation, down to a minimum value imposed
by the battery lifetime or the dynamic performance constraints. On reaching these conditions,
the PHEV driver has a choice not available with all-electric vehicles, i.e. continue to drive in
CS mode, with no range limit, or charge the battery.

The range that the vehicle can travel in CD (Charge Depleting Range - CDR) is a charac-
teristic of a plug-in hybrid. In the United States, it has even been included in vehicle names,
with for example the notions of PHEV 10 and PHEV40, which have a range of 10 miles and
40 miles respectively in CD mode. As pointed out by J. Axsen from Davis University, how-
ever, [Axsen et al, 2010], this notion is ambiguous since it does not specify the conditions
under which this distance was traveled, especially as regards use of the engine. Several situ-
ations can be identified, both for operation in CD mode and in CS mode:

1. Blended: the engine is started frequently, irrespective of the battery state of charge,
since it is required to provide the vehicle dynamics in most driving conditions, even urban.
Electric mode is very limited and will only be used for take-off and for moving at very low
speeds and over very short distances. In this type of operation known as Blended mode, the
vehicle therefore does not have true all-electric range. We are back to operation in "supervi-
sor all-electric mode" (5.3.1.5), but with a battery state of charge which is not sustained;

2. Urban Capable: the vehicle can operate in all-electric mode, but only for urban trips
corresponding to moderate demands for power. The UDDS cycle in the United States or
the ECE and urban ARTEMIS cycles in Europe are generally taken as reference for this
type of use (Chapter 1). With all-electric range under restricted conditions, this vehicle will
therefore be qualified as an Urban Capable PHEV in North America [Täte et al, 2008].
However, if the driver presses the accelerator to obtain more dynamic performance, there are
two possibilities:

• 2.1 the engine starts automatically and the vehicle switches back into the previous situ-
ation, in blended mode, which is the general case,

• 2.2 use of the engine is prohibited in a protected area (city center, ZEV area or future
ZAPA area for example) and the driver must therefore accept reduced dynamic performance
in this particular situation. This scenario could occur in the future for highly dense urban
areas prohibited to locally polluting vehicles,

3. Range Extended: the vehicle has all-electric range with full dynamic capabilities, max-
imum acceleration and speed, as long as the battery has not reached its minimum state of
charge. While this type of operation is the most efficient with respect to electric mode, it nev-
ertheless imposes severe constraints in terms of maximum power for the electric drive and
its battery in particular. This is therefore a dominant electric vehicle, known as the extended

328 Hybrid vehicles

range electric vehicle (E-REV). Depending on the operating conditions in CS phase, we can
identify two situations:

• 3.1 No compromise E-REV: the engine is used to maintain the dynamic performance,
which means that it must be sufficiently powerful and the battery must always be able to sup-
ply power at its minimum SOC whenever necessary. With its 53 kW engine, the Chevrolet
Volt corresponds to this type of vehicle; it can be termed a no compromise E-REV;

• 3.2 E-REV: the engine is unable to maintain the dynamic performance under all driving
conditions. The vehicle could use series architecture with a small engine acting as on-board
generator, for example (5.3.2.3).

We pointed out that the types of operation described in 1 and 2.1 correspond to "supervi-
sor all-electric mode" (5.3.1.5). The types of operation described in 2.2 and 3.2 correspond
to all-electric mode with autonomy, which could be qualified as "driver all-electric mode"
since it can be imposed by the driver himself, or possibly by an external infrastructure. The
distance that the vehicle can travel under these conditions is called the All-Electric Range
(AER). The distance may be short, about 5 to 20 km for Europe, where city centers are more
dense, but may reach up to 60 km in the United States as announced by GM, especially for
its Volt which uses a lithium battery pack of 16 kWh total energy.

Note, however, that the vehicle dynamic performance will differ in cases 2.2 and 3.2
(summary in 5.3.2.3).

These cases are illustrated on Figure 5.47 according to their performance in terms of total
power and range. The figure shows the relative importance of the various operating modes.

The example on Figure 5.48 shows the change in the battery state of charge over one day
for a plug-in hybrid vehicle. All possible operating modes are illustrated, i.e.:

all-electric operation allowing use without local nuisance emissions, for the Urban Capa-
ble vehicle, in urban area, or the E-REV, below their AER (area No. 1 on Figure 5.47),

operation in charge depleting mode, for the blended case below its CDR, or the Urban
Capable outside urban conditions and above its AER and below its CDR (area No. 2 on
Figure 5.47),

operation in charge sustaining mode to avoid a range limitation for all cases above the
CDR or the AER (area No. 3 on Figure 5.47),

battery charge on the grid, to transfer consumption to electricity.

We can see that the plug-in hybrid concept is highly complex. Its results depend on the
choices made when defining the vehicle (CD mode, AER, CDR) and its use (type of use,
distance between charging). Good matching between the vehicle definition, with respect to
its assumed use, and its actual use, is necessary and involves numerous system simulations.
Chapter 5.5.1 provides an example of evaluating the consumptions of a plug-in hybrid vehi-
cle. We will see that an extremely large transfer from fuel to electricity can be obtained on
daily trips. However, estimating the gain in greenhouse gas of the plug-in hybrid involves a
global approach taking into account, in particular, the performance of the electricity produc-
tion plants (Chapter 7).

Chapter 5 · Hybridization 329

Figure 5.47

Comparison of the electric capabilities of four types of plug-in hybrid vehicle
under nominal conditions.

330 Hybrid vehicles
The example shown on Figure 5.48 corresponds to a type of operation quite similar to that

of an electric vehicle, with a battery charge at end of day. Other solutions are being devel-
oped, however, especially for industrial vehicles, with the onboard storage system being
charged on the grid, possibly partially, in general at a high power level, and throughout the
period the vehicle is in use (Figure 5.49).

(1) Precise determination of the moment the mode is switched from CD to CS, from the SOC signal
recorded on a vehicle, may be difficult.

Figure 5.48
Diagrammatic representation of the battery state of charge for a daily mission
of a plug-in hybrid vehicle.
This type of operation, known as opportunity charging, is used to either reduce the quan-
tity of energy carried in the vehicle, therefore the storage cost and mass, or increase the range,
but requires an extensive infrastructure. This type of operation has already been implemented
for urban electric buses at their terminus, such as the PVI Montmartrobus. Other urban bus
projects based on the same principle are also being investigated in France (PVI Watt project
with opportunity charging at the bus stops and the Iribus EILISup project connected to the
terminus). Demonstrations of this concept have been conducted in China in particular, dur-
ing the 2010 Shanghai universal exhibition. We may also imagine electric or hybrid urban
delivery vehicles, which could be connected at each stop for deliveries or even, in the future,
sections of road equipped with inductive systems to allow partial recharging of the onboard
storage system.

Chapter 5 · Hybridization 331

Figure 5.49

Example of electric vehicle with opportunity charging, changes in battery
charge during use.
Source: IFP Energies nouvelles

C. Achievements

Recent achievements of plug-in hybrid vehicles have been commercialized, based on modi-
fied Toyota Prius cars. Due to the technological success of these vehicles and their wide-
spread distribution on certain geographical areas (especially California), some enthusiasts
have even developed and commercialized PHEV kits. These systems substantially increase
the battery capacity and allow grid recharging via onboard electronics. Developed and com-
mercialized without the manufacturer's cooperation, these solutions involve limited techni-
cal modifications in the drivetrain. One ingenious method is to "fool" the drive management
laws so that the battery discharges progressively. The information indicating the battery state
of charge to the drive management system can therefore be permanently fooled and over-
evaluated. The management system will therefore permanently draw energy from the bat-
tery and use the engine less. Due to the highly limited dynamic performance of the original
vehicle in electric mode, this PHEV will be classified as blended (Figure 5.47). Obviously
this modification means that the battery useful energy must be significantly increased, which
can be achieved by replacing the original NiMH battery by a lithium battery of higher capac-
ity. Several American companies propose this type of equipment, such as Energy CS with a
Valence Saphion battery of total energy 9 kWh (Figure 5.50) [Hanssen, 2005] or EEtrex with
its HybridPlus system which uses an A123 9 kWh lithium phosphate battery kit (7.5 kWh
usable energy). Other companies, such as Hymotion (Figure 5.51), propose keeping the orig-
inal NiMH battery and adding a second A123 5 kWh lithium phosphate battery (4.3 kWh
usable energy).

332 Hybrid vehicles

Figure 5.50
Prius with EnergyC S rechargeable battery kit (lithium battery).
Source: [Hanssen, 2005b]

Figure 5.51
Prius with Hymotion rechargeable battery kit (original battery and lithium battery).
Source: [Carlson et al, 2008]

Chapter 5 · Hybridization 333

These vehicles have been extensively bench tested by the Argonne National Laboratory
during an exhaustive project to evaluate plug-in hybrid vehicles [Carlson et al, 2008]. The
results obtained demonstrated that, in urban use, a range of 55 km in depleting mode could
be obtained with the EEtrex system and 20 km with the Hymotion system. These vehicles
are therefore able to transfer a significant proportion of fuel to electricity over the first kil-
ometers traveled. According to the measurements taken, the fuel consumption of a plug-in
Prius traveling 50 km under urban conditions will drop to 1.5 L/100 km or 36 gC02/km, bet-
ter than 170 mpg, which is quite an outstanding achievement. We must not forget, however,
to add the 100 Wh/km taken from the electric grid when charging the battery (useful energy
5 kWh) and point out that these figures are valid for a distance of 50 km between charges.
This result corresponds to an energy transfer rate of nearly 60%. At greater distances, this
rate will obviously drop, as shown on Figure 5.56.

The public interest in plug-in hybrid vehicles has led manufacturers to study versions
derived from their models, which can offer this feature. Toyota has developed a plug-in Prius
equipped with a nominal 5.2 kWh Li-ion battery (reduced to 4.4 kWh on the model sold in
2012) which can be used in electric mode up to a speed of 100 km/h on flat ground [Fran-
cisco, 2011]. The vehicle can travel a distance of between 10 and 20 km, depending on the
driving conditions, in electric mode. Figures 5.52 and 5.53 stress the impact of weather and
traffic conditions on the range measured in use [Chammas et al, 2011].

Figure 5.52

Impact of temperature on the range in electric mode ofthe Toyota plug-in Prius.
Source: from [Chammas et a/., 2011]

334 Hybrid vehicles

Figure 5.53

Impact of temperature and use on the range in electric mode of the Toyota
plug-in Prius.
Source: from [Chammas et al, 2011]

In view of this performance, this PHEV is classified as Urban Capable. This model was
commercialized in 2012, previously 600 vehicles have undergone real-life tests in various coun-
tries, including France, for several years. Ford has developped a plug-in version of its hybrid
Escape equipped with a Saft 10 kWh Li-ion battery. The modifications made to the current
hybrid version would apparently be limited and concern the battery (higher energy), addition
of a charger and modifications to the lubrication and cooling systems to cope with the extended
electric mode. Ford has marketed the PHEV concept in 2013 on its C-Max Energi model.

Since 2011, GM has sold its highly publicized plug-in hybrid, on the basis of the Volt in
the United States and the Ampera in Europe (5.2.5.4). PSA is announcing a plug-in version
of its hybrid 3008 for the near future. The Chinese manufacturer BYD is also proposing its
FD3Mmodel, sold in small numbers since 2008 (5.2.4).

These vehicles are equipped with large lithium batteries, which are difficult to design
since the storage system must offer good performance both in terms of power and energy
over a large battery SOC range. The additional purchase cost of the vehicle will therefore
be high, estimated at between €5,000 and €20,000 for a passenger car according to current
data. While plug-in hybrid vehicles are still in the early stages of commercialization, they
represent a challenge which is both technical and economic (7.3).

5.3.2.3 Summary of All-Electric Autonomy and Grid Charging Features
As mentioned earlier, these two features are closely related and we felt that it would be
worthwhile showing the various situations that can be encountered on the same diagram,
comparing the criteria of each one and identifying the following situations:

- can the vehicle be charged on the grid?

Chapter 5 · Hybridization 335

- does the vehicle have a range on all-electric mode? If yes, we must distinguish
between the following two cases:

• electric mode is not controlled directly by the driver but by the vehicle management
system which decides to switch to hybrid mode if necessary (mode that we called
supervisor mode),

• electric mode is selected by the driver, or by an external infrastructure, and main-
tained irrespective of the driver's action on the accelerator pedal (mode that we
called driver mode),

- for a vehicle equipped with all-electric mode with range, the question of maintaining
the dynamic performance in electric mode may be examined,

- lastly, as mentioned earlier (5.3.2.2.B), for a plug-in hybrid, the question of maintain-
ing the dynamic performance in hybrid mode may also be examined.

Based on the answers made to these questions on the features, Figure 5.54 illustrates the
various types of hybridization that can be identified.

Figure 5.54

Comparison of hybrid vehicle features with respect to their grid charging capac-
ity and their all-electric range.

336 Hybrid vehicles

1. No grid charging or electric mode, therefore no all-electric range. These are the micro
and mild hybrid vehicles currently sold, thermal dominant solutions (e.g. Stop-Start
systems and Honda Civic, Insight).

2. No grid charging with extremely limited all-electric range, highly restricted dynamic
capacities and no driver control ("supervisor" mode). These are the full hybrid vehi-
cles currently sold (e.g. Toyota Prius, Lexus RX400h, Ford Escape HEV, etc.).

3. Grid charging with no electric mode. A priori, this solution is of little interest since,
due to the low power of the electric machine, it will be difficult to make this type of
system profitable.

4. Plug-in vehicle but with limited electric capacities which, even under urban condi-
tions, will require use of the engine in blended mode to achieve acceptable dynamic
performance, e.g. plug-in vehicles obtained from a discrete hybrid by implementing a
conversion kit (5.3.2.2.C).

5. Non plug-in vehicle with an electric mode offering only limited performance. This
type could be considered for a vehicle only having to drive a few kilometers in electric
mode at low speed in an urban center, the small quantity of energy consumed in this
case not justifying a grid charge with its specific battery, charger and infrastructure.
A vehicle such as the PSA 3008 Hybrid4, with its 27 kW electric drive capable of
traveling 2 to 4 km, could be included in this category.

6. Non plug-in vehicle but with a powerful electric mode capable of achieving the neces-
sary dynamic performance. It would seem that there is little interest in not recharging
this type of vehicle.

7. Plug-in vehicle with all electric range of a few tens of kilometers whose dynamic
performance, although limited, is sufficient for urban requirements (Urban Capa-
ble PHEV). Examples include the PHEV10 to PHEV40 vehicles being studied in
the United States and the plug-in version of the Prius manufactured by Toyota. The
dynamic performance of this vehicle is quite compatible with urban use, with a range
ofupto20km(5.3.2.2.C).

8. Plug-in vehicle with high range offering sustained dynamic performance in both elec-
tric and hybrid mode. This can be obtained with a series architecture by keeping an
engine and a powerful generator. This solution, which requires the use of three power-
ful machines (so impact on the mass, volume and cost), will not offer good consump-
tion performance for extra urban driving. To obtain all these features, GM proposes
its power-split solution on the Volt"avAAmpera(5.2.5.4).

9. There is no application for this case, since the dynamic performance must be main-
tained either in electric mode or in hybrid mode.

10. Plug-in vehicle with high range offering sustained dynamic performance in electric
mode. These are electric dominant architectures, generally series type, although some
solutions with parallel coupling are proposed. The onboard generator is not powerful
enough to maintain the dynamic performance when the battery has reached its mini-
mum permitted charge level. This range extender configuration therefore increases
the range of an electric vehicle, which may help to promote sales. The performance of

Chapter 5 · Hybridization 337

the vehicle with its battery discharged depends on the power of the onboard generator.
The driver will at least be able to reach a charging point, even at low speed, thereby
eliminating the strong psychological effect due to worrying about having a "flat bat-
tery". Numerous manufacturers have proposed this concept or are still working on it,
for example the Kangoo Elect'Road vehicle with its 12 kW onboard generator com-
mercialized by Renault in the years 2000, the Al e-Tron recently released by Audi
with its 15 kW Wankel engine and Lotus which sells a generator capable of working
at 15 or 35 kW to equip a future Jaguar PHEV. Lastly, note that the range extender
function is perfectly suitable for a fuel cell in a series hybrid architecture, for which
low power will limit the cost and constraints on onboard hydrogen storage. As part
of the French GENEPAC research program, in 2007 PSA proposed a hybrid concept
car with a 20 kW fuel cell giving a range of 350 km, with 3 kg of onboard hydro-
gen. More recently, the French manufacturer FAM Automobile released a version
of its F-City electric vehicle equipped with a low-power fuel cell offering a range of
150 km.

As we can see, there are numerous possible solutions. Figure 5.54 illustrates the distance
separating the hybrid models currently sold, which are close to conventionnal vehicles, from
the E-REV concept, which is more like an electric vehicle using an engine only to extend
the range beyond the battery maximum allowed discharge. There are nevertheless a large
number of intermediate solutions which will allow the drives to become progressively more
complex, adapting to the various market situations and constraints.

5.3.2.4 Expressing the Consumptions of a Plug-in Hybrid Vehicle

By grid charging, the vehicle consumption can be transferred from a hydrocarbon to other
primary energy sources through the use of electricity. The vehicle is therefore characterized
by its two consumptions, which can be varied by modifying the parameters of the energy
management laws. This notion of transfer was tackled during the simulation studies con-
ducted in the framework of the Renault and PSA plug-in hybrid vehicle projects in France in
the 1990s (5.3.2.2.A). The results showed that there was an almost linear relation between the
two consumptions calculated over a cycle of a few kilometers with warm start and a normal
battery operating range [Jeanneret and Badin, 1996]. These observations have been con-
firmed by recent projects carried out by manufacturers and the results of American laborato-
ries deeply involved in the tests and simulations of plug-in hybrids, for example the Argonne
National Laboratory (www.transportation.anl.gov). Figure 5.55 illustrates the relation for a
Ford vehicle.

338 Hybrid vehicles

Figure 5.55

Graph of fuel and electricity consumptions for a plug-in hybrid vehicle on vari-
ous usage cycles.
Source: [MARAKBY, 2010]

It is interesting to observe that the same type of linear relation can be observed for non
plug-in vehicles over driving cycles, with however a much more limited amplitude on the
variations of the two consumptions due to the very limited quantity of energy provided by
the battery.

This type of representation cannot be generalized for vehicles traveling distances exceed-
ing the few kilometers of the reference cycles (Chapter 1), however, due to the small quan-
tity of energy contained in the battery. Charge depleting mode cannot be maintained for
more than a few tens of kilometers, in fact, depending on the battery pack energy capacity,
the authorized SOC range, the type of driving and lastly the management laws. As already
mentioned (5.3.2.2.B), the vehicle must switch back into charge sustaining mode when its
battery reaches the minimum authorized charge level. This situation is illustrated on Fig-
ure 5.56 which shows the consumption transfer rate from fuel to electricity for various types
of hybrid vehicle. The transfer rate can be defined as the ratio of electricity consumption to
the total consumption of fuel plus electricity. We note in this example that the electric domi-
nant hybrid vehicle with all-electric range can cover the first 20 kilometers completely in this
mode, with a transfer rate of 100%. In the two other cases of engine dominant vehicles used
in blended mode, the engine is switched on from the very first few kilometers to provide the
dynamic performance, which initially reduces the transfer rate, the Prius equipped with a
PHEV kit exhibiting the lowest values. In contrast, vehicles used in blended mode retain a
higher transfer rate over longer distances.

These examples illustrate the extreme versatility in the design and use of a plug-in hybrid
vehicle, depending on its capacities in electric mode, its range and its energy management
laws. Definition of the architecture, components and sizing most suitable for a given set of

Chapter 5 · Hybridization Next Page
339

specifications therefore becomes highly complex, requiring intensive simulation of the vehi-
cle system in use.

Hybrid with AER
Blended mode hybrid
Prius with PHEV kit

Figure 5.56

Graphs showing the rate of transfer to electricity for three types of plug-in
hybrid vehicle depending on the distance traveled between two charges.
Sources: [Badin, 1997; Badin et al, 2006a and Smart et al, 2009]

The plug-in hybrid is therefore characterized by its two consumption values. Conse-
quently, a specific procedure is necessary to compare the energy performance between dif-
ferent vehicles and assess the impact on the environment. We can either:

- express the C 0 2 emission and electricity consumption values separately. Hybrid
vehicle test procedures have been modified and a specific protocol to measure these
two consumptions has been set up, especially in Europe (regulation R101) and in the
United States (standard J1711) (appendix 3). The methodology implements specific
tests on driving schedule to combine operation with battery discharge and operation
with sustained SOC or charging. It is interesting to note that the procedures developed
in Europe and the United States do not apply the same method to combine the dis-
tances traveled in the two battery operating modes,

- add together the greenhouse gas emissions from the fuel and those from the electricity
used. For this procedure, we must consider the energy production chain right back to
the primary energies involved, using a well to wheel approach (Chapter 7). Labora-
tories are specialized in these procedures which, in particular, take into account the
greenhouse gases emitted by electricity generation (e.g. the Argonne GREET model
or EMPA ECOINVENT database). The problem becomes even more complex when,
in addition to the numerous variables already involved in the vehicle environmental
balance, the geographical area where the vehicle will be used and the type of electricity
consumed, depending on the charge time for example, are considered.

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340

5.3.2.5 Exchanging Energy with the Grid

This feature is deduced from the previous one, but in this case, the system will be able to sup-
ply energy compatible with that of the electricity distribution grid, i.e.:

- on the domestic network of a house to complement production obtained from renew-
able energy(ies) which are difficult to control (wind, sun), to make up for a grid failure
or optimize costs (Vehicle to Home (V2H) concept),

- on the grid to simplify regulation by the operators, especially for the supply of peak
demand, or optimize costs (Vehicle to Grid (V2G) concept),

- on the vehicle itself, from 110 or 220 V AC sockets to power comfort or work
auxiliaries.

The V2H and V2G approaches will require the implementation of bidirectional grids
capable of managing in particular the safety devices, pricing or charge control depending on
the grid constraints or even the greenhouse gas content of the electricity used. Major invest-
ments will be necessary to create smart grids. A considerable number of laboratories are
conducting R&D studies on these highly innovating subjects (EPRI, Argonne, CEA, EDF,
CSTB, G2ELAB, etc.).

5.3.2.6 Distributed Drive

Innovating distributed drive configurations can be set up by installing electric machines in
the powertrain. Several complexity levels can be identified:

- electric drive on the axle not connected to the engine, as illustrated on Figure 5.59 for
an architecture based on a front wheel drive vehicle. Fitting the electric drive on the
rear gives a four-wheel drive vehicle without the mechanical shaft connecting the front
and rear axles and therefore without the losses inherent to a viscous coupling unit. Due
to the almost instantaneous operation of the rear electric machine, four-wheel drive
is available only when required (e.g. in case of loss of grip on the front wheels) or if
demanded by the driver to obtain additional dynamic performance, within the limits
of the operating constraints. This type of configuration was implemented by PSA in
its Prologue HYmotion4 concept car, unveiled in autumn 2008, and is proposed on the
European market in the 3008 Hybrid4. To start 4WD at any time, irrespective of the
battery SOC, the rear machine must be fed with electricity from the front powertrain
(serial flow). This is carried out on the 3008 Hybrid4 by a high power generator con-
nected to the engine (Figure 5.10). In more complex solutions, the rear drive is used in
conjunction with a hybrid transmission on the front wheels, as with the Lexus RX450h
for example. This car therefore has a maximum braking energy recovery potential,
at the expense however of a highly complex system (Figure 5.57). As we can see
on Figure 5.58, this configuration involves 4 energy systems. We find the input split
drivetrain shown on Figure 5.19 as well as a third reversible energy system R3 with its
connection to the rear wheels;