The first EV was a tricycle invented by Thomas Davenport in 1834, which was powered by an electric motor with a non-rechargeable battery and operated on a short track (Wakefield, 1994). By the 1880, road EVs began to appear on both sides of the Atlantic Ocean. For instance, the first EV in England was built by William Edward Ayrton and John Perry in 1882, and the first EV in America was built by William Morrison in 1890 (Georgano, 1996). By 1912, nearly 34,000 EVs were registered in America. Nevertheless, their cost was equivalent to a Rolls Royce of today which could only be afforded by the wealthy elite. In 1925, Ford produced the low-cost Model T which was only a fraction of the cost of EVs but could offer a much longer driving range, which eventually finished off EVs by the 1930s. The resurrection of EVs started with the outbreak of energy crisis in the 1970s. Then, the development of EVs was accelerated due to the concern over global warming in the 1980s. Currently, the impetus for the advancement of EVs is the new business opportunities.
The first hybrid EV (HEV) was a series hybrid invented by Ferdinand Porsche in 1898. Then, in 1905, Henri Pieper filed a patent of a parallel hybrid which used an electric motor to boost the acceleration of its heat engine (Wouk, 1995). From 1906 to 1910, some HEVs using this hybrid system were built. By the time this hybrid
1Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong, China
went into production, Henry Ford built the first assembly lines to mass-produce the Ford Model T that was powerful enough to accelerate the vehicle unaided.
Consequently, by the 1930s, similar to EVs, HEVs had screeched to a halt. Rather than waiting for the breakthrough of EVs, the development of HEVs became widespread beginning in the late 1990s. The first mass-produced HEV was the Toyota Prius which was launched in 1997 (Hermance and Sasaki, 1998). Recently, Toyota has announced that its global hybrid sales surpassed the 7 million unit mark.
There have been various definitions of EVs and HEVs. Some definitions are academic in nature while some are based on general perception, whereas some classifications are quite ad-hoc while some are too general. For instance, EVs are generally classified as the pure EV (PEV) and HEV based on their propulsion systems, whereas they are also classified as the battery EV (BEV), HEV and fuel-cell EV (FEV) based on their energy sources (Chan and Chau, 2001). Sometimes, the BEV is loosely called the EV so that they are also named as EV, HEV and FEV. The HEV may even be extended to embrace the vehicle using any two different energy sources such as the fuel-cell-and-battery hybrid or battery-and-ultracapacitor hybrid (Chau and Wong, 2001). In recent years, there has been a trend that EVs should first be classified by their propulsion devices, and then be further classified by their energy carriers and energy sources (Chau, 2014). So, as shown in Figure 1.1, EVs are first classified as the PEV and HEV families based on their propulsion devices, namely, the PEV solely adopts the electric motor for electric propulsion and the HEV uses both the electric motor and heat engine for hybrid propulsion. It should be noted that the general public prefers to loosely name the PEV as the EV and the HEV as the HV, leading to form the general term of EVs and HVs.
Based on the variation of energy carriers and energy sources, the PEV family can be split into the BEV and FEV due to the use of batteries and fuel cells as their main energy sources, respectively. Taking into account the latest energy sources of capacitors (specifically dubbed ultracapacitors (UCs) or supercapacitors) and
Motor Engine HEV
(HV)
PEV (EV)
Vehicle families
Propulsion devices
Figure 1.1 Definition of various EVs
flywheels (specifically dubbed ultraflywheels (UFs) or ultrahigh-speed flywheels), the ultracapacitor EV (UCEV) and ultraflywheel EV (UFEV) are also members of the PEV family. On the other hand, based on the hybridization level between the electric motor and heat engine, the HEV family consists of five members: the micro hybrid, mild hybrid, full hybrid, plug-in HEV (PHEV) and range-extended EV (REV) in accordance with their increasing contribution from the electric motor for hybrid propulsion. Among them, the micro, mild and full hybrids are termed con- ventional HEVs which are solely refuelled with liquid fuel in filling stations (Chau and Chan, 2007), whereas the PHEV and REV are called griddable HEVs (GHEVs) which can be recharged by electricity via charging ports or refuelled with liquid fuel in filling stations (Chau, 2010). This classification is depicted in Figure 1.2.
In the PEV family, the BEV is almost exclusively used in real life and is the unique one commercially available. Essentially, all members of the PEV utilize the battery as the sole energy source or one of the energy sources. For instance, the FEV has to incorporate the battery to store the regenerative braking energy, while the UCEV and UFEV should incorporate the battery to provide sufficient energy storage for normal operation because the UC and UF possess relatively low specific energy. From the general public perspective, both the UCEV and UFEC are called an EV using the hybrid energy source of battery and UC and the hybrid energy source of battery and UF, respectively. It should be noted that there are some attempts on using the UC and UF as the sole energy source, the resulting UCEV and UFEV inevitably suffer from the problems of heavy weight, bulky size and very short driving range per charge.
Among those conventional HEVs, the micro, mild and full hybrids offer dif- ferent capabilities of hybrid features. For the micro hybrid, it is equipped with an
Vehicle types Micro hybrid
Propulsion devices
Energy carriers
Energy sources
Mild hybrid Engine Liquid fuel Liquid fuel
Full hybrid PHEV
REV
BEV Motor Electricity
Hydrogen
UC UF Fuel cell Battery UCEV
UFEV FEV
Figure 1.2 Classification of various EVs
Overview of energy systems for electric and hybrid vehicles 3
integrated-starter-generator (ISG) which is typically 3 5 kW with the system voltage of 14 42 V. Instead of providing power assist to help the heat engine, it offers two important hybrid features: the idle stop-start which shuts down the heat engine whenever the vehicle is at rest so as to reduce the fuel consumption, and regenerative braking which recovers the braking energy during deceleration to regenerate electricity for battery charging. For the mild hybrid, the ISG is increased to 7 15 kW with the system voltage of 100 150 V. It not only provides the hybrid features of idle stop-start and regenerative braking, but also assists the heat engine to propel the vehicle. Thus, it enables to adopt a downsized engine. Differing from the micro and mild hybrids, the full hybrid can offer versatile operations by using the heat engine alone, the electric motor alone or a combination of both. Instead of using the ISG, it adopts the electronic-continuously variable transmission (EVT) system which is typically 50 60 kW with the system voltage of 500 600 V. Using EVT, the full hybrid can offer all hybrid features, including the idle stop-start, regenerative braking, power assist and electric launch. Increasingly, it can enable the heat engine working at its optimal operation line (OOL) to achieve efficiency optimization.
Among those GHEVs, the PHEV and REV are quite similar in terms of their system configurations although the former is derived from the full hybrid while the latter is deduced from the BEV. For the PHEV, it provides all features of the full hybrid, while having an additional feature of plug-in rechargeable. In general, it is equipped with more battery packs than the full hybrid, typically 4 5 kWh, so that it can offer a decent electric range at the pure electric mode. It normally operates at the blended mode in which the electric motor and heat engine complement one another to maximize the fuel economy. Compared with the PHEV, the REV usually installs a smaller heat engine but a larger battery pack. As reflected by its name, it prefers to work as a BEV until the battery capacity drops to a predefined threshold.
Then, it works as a series hybrid so that the heat engine drives the generator to charge the battery pack and provide power to the electric motor. At higher speeds and loads, the REV generally operates at the blended mode to maximize the fuel economy. In order to provide the desired electric range, the battery requirement is typically over 16 kWh.
Table 1.1 summarizes the hybrid features of various HEVs, including the conventional HEVs and GHEVs.
Table 1.1 Hybrid features of HEVs
Micro Mild Full PHEV REV
Idle stop-start ü ü ü ü ü
Regenerative braking ü ü ü ü ü
Power assist ü ü ü ü
Electric launch ü ü ü
Efficiency optimization ü ü ü
Decent electric range ü ü