The ICE is intrinsically inefficient at converting the fuel’s chemical energy to mechanical energy, losing energy to engine friction, pumping air into and out of the engine, and wasted heat. In EVs, the electric motor does not consume energy at standstill and it has higher peak and average efficiency than the ICEs. The electric motor also has maximum torque at standstill and low speeds and it allows bidir- ectional energy flow such that mechanical energy can be converted into electrical energy and stored in the traction batteries during regenerative braking.
Comparing vehicular efficiencies of the ICEVs and EVs, the ICEV has low thermal and mechanical efficiencies such that most of the fuel is dissipated as heat in the engine. As illustrated in Figure 4.2, the vehicular efficiency of a typical ICEV is about 15%. Around 62% of the fuel’s energy is lost in the engine and 17%
and 6% of the energy are lost in idling and in the driveline, such as acceleration and braking (USDOE, 2011a; Taylor, 1998). On the other hand, the vehicular efficiency
Driveline Normal driving
High-energy energy source
High-power energy source
Driveline Acceleration
Driveline Braking
Primary power flow Secondary power flow High-energy
energy source
High-power energy source
High-energy energy source
High-power energy source
Motive power
Recharging power
Motive power
Motive power
Regenerative power Regenerative
power
Figure 4.1 Principle of energy hybridization in the EV
62%
17 % 6%
15%
14% 100%
6%
80%
Input energy
Engine loss
Idle
loss Driveline loss
Effective energy
Driveline loss Effective
energy
Electrical loss
Input energy
ICEV powertrain EV powertrain
100%
Figure 4.2 Vehicular efficiencies of ICEVs and EVs
Hybridization of energy sources for electric and hybrid vehicles 99
of a BEV is around 80%. The vehicular efficiency of the HEV is between the ICEVs and the EVs, depending on the drivetrain topology and control strategies.
4.2.1 Energy efficiency improvement in HEVs
In an HEV, the motor can supplement motive power to the ICE or be the sole source of the traction power. The engine can charge up the battery in light load. The kinetic energy during braking and down-slope driving can be captured by the motor and stored in the vehicle batteries or ultracapacitors in regenerative braking. In the power boost mode, the electric motor provides supplementary power to the engine such that the engine can operate in the most efficient point or region. Figure 4.3 shows the system architecture of a series–parallel HEV.
The electrical drivetrain boosts an HEVs fuel economy in the following ways:
● Assist vehicle launch and acceleration
● Assist load enhancement for the ICE for faster warm-up and better efficiency
● Undertake regenerative braking
● Deliver pure electric driving for certain period of time
The energy saving potential of HEVs achieved by the electrical powertrain is shown in Figure 4.4. The HEV can boost the fuel economy from its ICE counterpart by about 6% by reducing the energy losses during idling and stop-and-start in city driving. An additional 6% of energy savings can be achieved by regenerative braking. By optimizing efficiency of the ICE, an HEV can achieve an aggregated
Front transmission Motor
Power converter
Battery
Engine Fuel tank
Generator
Figure 4.3 System architecture of a series–parallel HEV
savings potential of 15–30% in fuel consumption relative to the ICE vehicle, subject to the vehicle design and energy management strategies. The HEVs can achieve a reduction of 3.9–20.3% in fuel consumption in the highway driving cycle and a higher reduction of 8.4–46.7% in the urban driving cycle (Wonget al., 2014).
4.2.2 Drivetrain design of BEVs and HEVs
The key considerations in designing the HEV drivetrain are summarized below:
● Optimal engine operating point – the optimal operating point on the torque–
speed plane of the engine can be based on the maximization of fuel economy, the minimization of emissions, or even a compromise between fuel economy and emissions.
● Optimal engine operating line – in case the engine needs to deliver different power demands, the corresponding optimal operating points constitute an optimal operating line.
● Optimal engine operating region – the engine has a preferred operating region on the torque–speed plane, in which the fuel efficiency remains optimum.
● Minimum engine dynamics – the engine operating speed needs to be regulated in such a way that any fast fluctuations are avoided, hence minimizing the engine dynamics.
● Minimum engine speed – when the engine operates at low speeds, the fuel efficiency is very low. The engine should be cut off when its speed is below a threshold value.
● Minimum engine turn-on time – the engine should not be turned on and off frequently; otherwise, it results in additional fuel consumption and emissions.
A minimum turn-on time should be set to avoid such drawbacks.
Power boost
Regen.
braking Stop and
start
Energy saving potential
~6%
~12%
~15–30%
Stop and start
Regen.
braking Stop and
start
Figure 4.4 Energy savings potential in HEVs
Hybridization of energy sources for electric and hybrid vehicles 101
● Proper battery capacity – the battery capacity needs to be kept at a proper level so that it can provide sufficient power for acceleration and can accept regen- erative power during braking or going downhill. When the battery capacity is too high, the engine should be turned off or operated idly. When this capacity is too low, the engine should increase its output to charge the battery.
● Safe battery voltage – the battery voltage may be significantly altered during discharging, generator charging or regenerative charging. This battery voltage should not be too high or too low; otherwise, the battery may be permanently damaged.
● Relative distribution – the distribution of power demand between the engine and battery should be proportionally divided during the driving cycle.
● Geographical policy – in certain cities or areas, the HEV needs to be operated in the pure electric mode. The changeover should be controlled manually or automatically.
The energy source has to be intrinsically tolerant of abusive conditions such as overcharge, short circuit, crush, fire exposure, mechanical shock and vibration.
Energy sources are connected in series and/or parallel in a vehicle’s energy system.
The key requirements for onboard energy source are high specific energy, high specific power, long cycle life, high efficiency, wide operating temperature and low cost for commercialization.