The basic principle of energy hybridization in EVs and HEVs is the integration of both high specific energy and high specific power sources in the drivetrain, as illustrated in Figure 4.1. First, during the normal driving condition, the high-energy energy source supplies the propulsion energy to the driveline. To enable the system ready for sudden power demand, this source can charge the high-power power source in the light-load period. Second, during the acceleration or hill-climbing condition, both energy sources supply the propulsion energy simultaneously. Third, during the braking or downhill condition, the braking energy can be used to recharge the high-power energy source with regenerative braking. When the high- power source is fully charged, braking energy can be diverted to recharge the high- energy energy source provided that it is energy receptive.
The principle of HEV design is the integration of the EVs electric propulsion system into the ICEVs mechanical propulsion system. It is the hybridization of drivetrains in HEVs. In EVs, the whole vehicle is driven by electric motor(s) but the electric motor is powered by dual energy sources in the HES. It is the hybri- dization of energy sources in EVs.
4.4.1 Hybridization of drivetrains in HEVs
The HEV can be divided into series, parallel, series–parallel and complex hybrids with reference to their drivetrain topologies. Figure 4.7 shows the system archi- tecture, in which the electrical link is bidirectional; the hydraulic link is unidirec- tional and the mechanical link (including brakes, clutches and gears) is also Hybridization of energy sources for electric and hybrid vehicles 113
bidirectional. The series hybrid couples the engine with the generator to produce electricity for pure electric propulsion, whereas the parallel hybrid couples both the engine and electric motor with the transmission via the same drive shaft to propel the wheels. The series–parallel hybrid is a direct combination of both the series and parallel hybrids. On top of the series–parallel hybrid operation, the complex hybrid can offer additional and versatile operating modes by adding one more electric motor to deliver four-wheel drive (4WD) operations.
The series hybrid is the simplest kind of HEV. The engine’s mechanical output is first converted into electricity using a generator. The converted electricity either charges the battery or bypasses the battery to propel the wheels via the same electric motor and mechanical transmission. Conceptually, it is an engine-assisted EV,
Transmission Motor Power converter
Battery Generator
Engine Fuel tank
Transmission Motor
Power converter
Battery
Engine Fuel tank
(a) (b)
Front transmission Motor
Power converter
Battery
Engine Fuel tank
Generator
Front transmission Motor
Power converter
Battery
Engine Fuel tank
Generator Rear transmission
Motor
(c) (d)
Figure 4.7 System architecture of HEVs: (a) series hybrid, (b) parallel hybrid, (c) series-parallel hybrid, and (d) complex hybrid
aiming to extend the driving range comparable with that of the ICEV. Due to the absence of clutches throughout the mechanical link, it has the definite advantage of flexibility for locating the engine-generator set. Although it has an added advantage of simplicity of its drivetrain, it needs three propulsion devices – the engine, the generator and the electric motor. Another disadvantage is that all these propulsion devices must be sized for maximum sustained power if the series HEV is designed to climb a long grade. The Chevrolet Volt has a series hybrid drivetrain. The Chevrolet Volt is propelled by a 111 kW electric motor and 16 kWh Li-Ion batteries.
The driving range is extended by a 1.4 L ICE and a 53 kW generator.
In the series hybrid system, the energy flow can be illustrated in Figure 4.8.
During start-up, normal driving or acceleration of the series HEV, both the engine (via the generator) and battery deliver electrical energy to the power converter which then drives the electric motor and then the wheels via the transmission. At light loads, the engine output is greater than the output required to drive the vehicle so that the generated electrical energy is also used to charge the battery until the battery capacity reaches a proper level. During braking or deceleration, the electric motor acts as a generator which transforms the kinetic energy of the wheels into electricity, hence charging the battery via the power converter. The battery can also be charged by the engine via the generator and power converter, even when the vehicle is standstill.
The parallel HEV allows both the engine and electric motor to deliver power in parallel to drive the vehicle. Both the engine and electric motor are generally coupled to the drive shaft of the wheels via two clutches such that the propulsion power may be supplied by the engine, the electric motor or both. Conceptually, it is an electrically assisted ICEV for achieving lower emissions and fuel consumption.
Transmission Motor Power converter
Battery
Motive power
Motive power Generator
Engine
Fuel tank Motive power
Regenerative power Recharging power
Figure 4.8 Operating models of series hybrids
Hybridization of energy sources for electric and hybrid vehicles 115
The electric motor can be used as a generator to charge the battery by regenerative braking or absorbing power from the engine when its output is greater than that vehicle demand. The parallel hybrid needs only two propulsion devices – the engine and the electric motor. Another advantage over the series case is that a smaller engine and a smaller electric motor can be used to get the same perfor- mance until the battery is depleted. The Honda Civic Hybrid is a parallel HEV, which is propelled by a 1.3 L ICE and a 12 kW electric motor integrated into the powertrain to boost efficiency of the engine.
The energy flow of a parallel HEV is illustrated in Figure 4.9. During start-up or full-throttle acceleration, both the engine and electric motor proportionally share the required power to propel the vehicle. Typically, the power distribution between the engine and electric motor is 80–20%. During normal driving, the engine solely supplies the necessary power to propel the vehicle while the electric motor remains in the off mode. During braking or deceleration, the electric motor acts as a gen- erator to charge the battery via the power converter. Also, since both the engine and electric motor are coupled to the same drive shaft, the battery can be charged by the engine via the electric motor when the vehicle is at light load.
In the series–parallel hybrid, the configuration incorporates the features of both the series and parallel hybrids. It has advantageous features of both the series and parallel hybrids, but it is relatively more complicated and costly. The Toyota Prius is the first series–parallel HEV on the market. The series–parallel hybrid has more freedom to boost the system efficiency of the vehicle. Series–parallel hybrids can be further divided into engine-heavy and electric-heavy hybrids. Engine-heavy denotes that the engine is more active than the electric motor, whereas electric- heavy indicates that the electric motor is more active. Figure 4.10 shows an engine- heavy series–parallel hybrid system and six operating modes. At start-up, the bat- tery solely provides the necessary power to propel the vehicle while the engine is in the off mode. During full-throttle acceleration, both the engine and electric motor
Acceleration power
Regenerative power Recharging power
Transmission Motor
Power converter
Battery
Engine Fuel tank
Motive power
Figure 4.9 Operating modes of parallel hybrids
proportionally share the required power to propel the vehicle. During normal driving, the engine solely provides the necessary power to propel the vehicle while the electric motor remains in the off mode. During braking or deceleration, the electric motor acts as a generator to charge the battery via the power converter. For battery charging during driving, the engine drives not only the vehicle but also the generator to charge the battery via the power converter. When the vehicle is at a standstill, the engine can drive the generator to charge the battery.
The complex hybrid seems to be similar to the series–parallel hybrid; however, the key difference is the bidirectional power flow of the electric motor in the complex hybrid, comparing to the unidirectional power flow of the generator in the series–parallel hybrid. This bidirectional power flow can allow for versatile oper- ating modes, especially the three propulsion power (due to the engine and two electric motors) operating mode which cannot be offered by the series–parallel hybrid. Similar to the series–parallel HEV, the complex hybrid suffers from higher complexity and costliness. The Lexus RX 400 h was built with this topology. The front wheels of the RX 400 h are propelled by both the 3.3 L ICE and the 123 kW electric motor in the series–parallel hybrid mode while the rear wheels are pro- pelled by a 50 kW electric motor. There is no direct mechanical coupling between the ICE and the 50 kW rear motor, but they are electrically connected by the 82 kW generator.
The energy management system of the complex hybrid is focused on the dual- axle propulsion system. In this system, the front-wheel axle and rear-wheel axle are separately driven. There is no propeller shaft to connect the front and rear wheels, so it enables a more lightweight propulsion system and increases the vehicle packaging flexibility. Moreover, regenerative braking on all four wheels can sig- nificantly improve the vehicle fuel efficiency. Figure 4.11 shows a dual-axle complex hybrid system, where the front-wheel axle is propelled by a hybrid drivetrain and the rear-wheel axle is driven by an electric motor. During start-up,
Front transmission Motor
Power converter
Battery
Engine Fuel tank
Motive power Generator
Recharging power
Regenerative power Regenerative power Recharging power Acceleration
power Motive power
Figure 4.10 Operating modes of engine-heavy series–parallel hybrids Hybridization of energy sources for electric and hybrid vehicles 117
the battery delivers electrical energy to drive both the front and rear electric motors to individually power the front and rear axles of the vehicle while the engine is off.
For full-throttle acceleration, both the engine and front electric motor power the front axle while the rear electric motor drives the rear axle. Notice that this oper- ating mode involves three propulsion devices (one engine and two electric motors) to simultaneously propel the vehicle. During normal driving and/or battery char- ging, the engine output is split to power the front axle and to drive the electric motor, which works as a generator to charge the battery. The engine, front electric motor and front axle can be mechanically coupled by planetary gear sets. When driving at light load, the battery delivers electrical energy to the front electric motor only to drive the front axle whereas both the engine and rear electric motor are off.
During braking or deceleration, both the front and rear electric motors act as gen- erators to simultaneously charge the battery. A unique feature of this dual-axle system is the capability of axle balancing. In case the front wheels slip, the front electric motor works as a generator to absorb the change of engine output power.
Through the battery, this power difference is then used to drive the rear wheels to achieve axle balancing.
According to the level of electric power contribution and functionalities of the electrical powertrain, HEVs can be classified into micro hybrid vehicle (MHV), mild HEV (MHEV), full HEV (FHEV) and plug-in HEV (PHEV). On the other hand, EVs are classified into BEV and FCEV.
Acceleration power
Regenerative power
Front transmission Motor
Power converter
Battery
Engine Fuel tank
Motive power Generator
Recharging power Rear transmission
Motor Motive power
Regenerative power Regenerative
power Recharging power
Figure 4.11 Operating modes of dual-axle (front-hybrid rear-electric) complex hybrids
The battery is still the most important component of these vehicles but the requirements on power, energy, cycle life and system voltage are different. Key requirements for vehicle batteries are high specific energy, high specific power, long cycle life, high efficiency, wide operating temperature and low cost for commercialization. Figure 4.12 shows the power and energy requirements of bat- tery for various EVs and HEVs. Functionality of the electrical powertrain and the favourable battery voltages in these vehicles are shown in Figure 4.13.
The United States Council for Automotive Research LLC (USCAR) and the USABC have set technical targets for vehicle batteries. The HEV needs batteries with long cycle life, high specific power for power boost and regenerative braking.
Table 4.6 shows some USABC key goals for batteries in HEV and FCEV appli- cations (Wong and Chan, 2013).
4.4.1.1 MHV
The MHV has an electric motor with peak power of about 2.5 kW. The electrical powertrain is driven by a battery system at 12–42 V. The motor is small and simple in structure and serves a function similar to the starter and alternator in an ICE vehicle. The electrical and engine powertrains in an MHV are governed by an automatic stop–start mechanism, in which, the engine shuts down under vehicle braking and rest. The MHV is favourable for city driving, where there are frequent stops and starts. An MHV’s fuel economy can be 5–10% higher than that of an ICE vehicle in city driving. The Citroen C3 is an MHV using the Valeo motor system.
The battery discharges frequently in cranking the engine in MHVs. Thus, there is a demand for high cycle life for batteries in MHVs. Table 4.7 lists some key technical data of batteries for MHVs.
MHV MHEV
FCEV
FHEV PHEV
10 100 1000
1
1
0 10 100
Energy (kWh) (in logarithmic scale)
Power (kW) (in logarithmic scale)
BEV
Figure 4.12 Power and energy requirements of batteries for various EVs and HEVs
Hybridization of energy sources for electric and hybrid vehicles 119
4.4.1.2 Mild HEV
The MHEV has a more powerful electrical powertrain than an MHV. The typical electric motor power of a sedan MHEV is about 10–20 kW at 100–200 V. The motor is directly coupled with the engine. The motor has a large inertia such that it can replace the original flywheel of the engine. The motor and the engine are generally coupled in parallel. The electrical powertrain is designed to crank the engine and
ICE power
Motor power
MHV MHEV FHEV PHEV BEV FCEV(1)
Regen.
braking Motor assist
Regen.
braking Motor assist
Regen.
braking Stop–
start
EV drive
12–42
>42
>220 >220
MHV MHEV FHEV PHEV BEV FCEV
>300
Battery voltage (V)
Stop–
start
Stop–
start
Stop–
start
EV drive
>300
Remarks:
(1) : Assume the FCEV adopts a fuel cell and battery hybrid system
Figure 4.13 Battery operating voltages in EVs and HEVs
Table 4.6 Typical USABC goals for batteries in HEV and FCEV applications
Parameters Unit MHV MHEV FHEV FCEV
Discharge pulse power kW 6 13 to 25 40 20
Regenerative pulse power kW NA 8 to 20 35 25
Energy capacity kWh 0.25 0.30 0.50 0.25
Calendar life year 15 15 15 15
Cycle life cycle 150,000 300,000 300,000 NA
Maximum operating voltage V 48 400 400 440
Operating temperature C 30 toþ52 30 toþ52 30 toþ52 30 toþ52 NA: not applicable
offer regenerative braking during braking. There are demands of high specific power and long service life for batteries in MHEVs. Table 4.8 shows some technical data of batteries for MHEVs. The battery’s charge and discharge power depend on its SoC.
The battery’s discharge power decreases with its SoC. The minimum operating SoC is around 40–50% to uphold sufficient power for launch and acceleration support. On the other hand, the battery’s recharging power drops when the SoC is high; thus, the maximum operating SoC is regulated at around 70–80% to maintain sufficient recharge power for regenerative braking. Typically, the batteries operate in a SoC window between 40% and 70%. Comparing with an ICE vehicle, the MHEV can boost the fuel economy by 20–30% in city driving. Examples of MHEVs are Honda Insight Hybrid, Honda Civic Hybrid and Ford Escape Hybrid.
4.4.1.3 Full HEV
The FHEV has a high power electrical powertrain to drive the vehicle purely by electricity in a short driving range. The typical electric motor power for sedan FHEV is about 50 kW at 200–350 V. Generally, the motor, generator and engine are cou- pled in series–parallel configuration. With the aid of power split devices, which are mainly built by planetary gear sets and clutches, the energy management system of the engine, motor and generator is designed to maximize energy efficiency and minimize emissions. Table 4.9 shows the technical data of batteries for FHEVs.
Table 4.7 Technical data of batteries for MHVs
Parameters Unit MHV
Voltage V 12–42
Discharge power kW 4.2–6
Low temperature (28C) discharge power kW >3
Energy capacity kWh 0.2–1
Operating temperature C 30 toþ52
Calendar life year >3
Table 4.8 Technical data of batteries for MHEVs
Parameters Unit MHEV
Voltage V 42–200
Discharge power kW >15
Low temperature (28C) discharge power kW >4
Recharge power kW >15
SoC Window % 40–70
Recharge pulse power kW >20
Energy capacity kWh 0.8–1
Operating temperature C 30 toþ52
Calendar life Year >10
Hybridization of energy sources for electric and hybrid vehicles 121
The FHEV can be driven in pure EV mode and hybrid mode. The electrical powertrain assists the engine, not only at the starting, but also during acceleration in the hybrid mode, which is also called charge sustaining mode. In the charge sus- taining mode, the battery is recharged not only by regenerative braking but also by the engine to maintain the SoC in a high and narrow window. The FHEV can achieve higher fuel economy than that of the ICE vehicle by 30–50% in city driving.
Examples of FHEVs are the Toyota Prius, Toyota Highlander and Lexus RX 400 h.
4.4.1.4 Plug-in HEV
The PHEV is similar to that of an FHEV. The key differences are the additional battery pack and the functionality of grid recharging. In addition to the charge sustaining mode, the PHEV can operate in the charge depletion mode, in which the PHEV operates in pure EV mode. Thus, the battery SoC drops in the charge depletion mode.
The electrical drivetrain of a PHEV works in a high voltage at 220–350 V. The battery energy capacity in PHEVs is the largest among all HEVs and it is determined by the designated pure electric driving range. The PHEV operates in the charge depletion mode first and then the charge sustaining mode. In the charge depletion mode, the battery SoC decreases from 100% to a threshold SoC (typically 20–30%), which triggers the operation mode change. In the charge sustaining mode, the battery SoC oscillates around the threshold SoC. The battery is recharged from the grid at the end of the trip. Similar to the EV, the PHEV suffers from complexity and costliness.
However, the PHEV delivers longer driving range than the EVs that is comparable to conventional ICEVs. Table 4.10 shows technical data of batteries for PHEVs.
The BYD F3DM is the world’s first mass production PHEV, which went on sale to the government agencies and corporations in China in December, 2008.
Toyota also worked on a plug-in version of the Prius. The plug-in Prius was con- verted from the Prius by adding additional 1.3 kWh battery pack into the car and a charging unit. A PHEV can also be implemented in a series hybrid topology. The GM Chevrolet Volt is a series hybrid PHEV, which is also called extended-range EV (EREV). The EREV is driven by one sole electrical powertrain, powered by the battery and a small engine.
Table 4.9 Technical data of batteries for FHEVs
Parameters Unit FHEV
Voltage V 220–350
Discharge power kW >35
Low temperature (28C) discharge power kW >4
Recharge power kW >30
SoC Window % 40–80
Recharge pulse power kW >40
Energy capacity kWh 1–2
Operating temperature C 30 toþ52
Calendar life year >10