60 Electric and Hybrid Vehicles 3.2.3 h ybrids b ased on d egree of h ybridizaTion The classification based on the degree of hybridization corresponds to the mission that the vehicle is designed to achieve [5]. This is a more consumer-oriented classification method used by the automotive industry. There are three ‘mission-based’ classes: mild hybrids, power hybrids and energy hybrids. The ‘mild’ hybrids have the lowest degree of hybridization with a moderate effect on fuel economy and emissions. Typical electrical rating of a mild hybrid would be in the range of 5–10 kW, with energy capacity in the range of 1–3 kWh.
The ‘power’ hybrids have a larger electric propulsion component, with an electrical rating as high as 40 kW; these hybrids allow significant amount of power transfer between battery and motor drive system, although the battery storage is designed with relatively low energy capacity (3–4 kWh). The power hybrids have a greater poten- tial to provide fuel economy improvements. These also have better engine-out emissions due to a more focused engine duty schedule. Power hybrids, like the mild hybrids, are charge-sustaining type receiving all motive energy from the on-board combustion of the fossil fuel.
The ‘energy’ hybrid employs a high-energy battery system capable of propelling the vehicle for a significant range without engine operation. The electrical rating and battery capacity are typically in the ranges of 70–100 kW and 15–20 kWh. A zero emission range of 50 miles would meet the daily commute range of the majority of population based on average driving habits of about 12,000 miles/year in the United States. The energy hybrids are obviously charge-depleting type with pro- visions to recharge the batteries at home electrically. These vehicles are the subject of treatment in the following section.
3.3 PLUG-IN HYBRID ELECTRIC VEHICLE The PHEVs are similar to the charge-sustaining hybrids except that they have a higher capacity energy storage system with a power electronic interface for connection to the grid. The energy stor- age system in a PHEV can be charged on-board and also from an electrical outlet. The vehicle can operate in a battery-only mode for a much longer period than the charge-sustaining hybrid vehicles. The PHEV is intended to operate as a pure BEV for the design-specified distances during the daily commute. The IC engine is used to provide additional power and range for long-distance driving. This type of vehicle is also sometimes known as a ‘range extender’.
The energy obtained from the external power grid in the PHEV displaces the energy that would otherwise be obtained by burning fuel in the vehicle’s IC engine. This has the potential of higher usage of alternative fuels in compari- son to other hybrid vehicles where all of the energy comes from the fossil fuel. The architectural choices for PHEVs are the same as those charge-sustaining hybrids. The series architecture is the simplest architecture and is highly suitable for PHEVs. The IC engine and elec- tric generator need not be sized to match the peak rating of the traction electric motor due to the availability of high-energy capacity battery-pack.
An on-board power electronic interface similar to that of a BEV is necessary for grid connection. The architecture of the series PHEV is shown in Figure 3.9. Parallel and series–parallel architectures similar to the regular hybrids but with the grid connectivity are also possible with PHEVs. A plug-in hybrid is generally rated based on the zero-emission distance traveled; it is designated as PHEV‘X’ where ‘X’ is the distance traveled in miles using off-board electrical energy. This range of travel where the IC engine is not used is known as the zero-emission vehicle (ZEV) range.
A PHEV40 is a plug-in hybrid with a useable energy storage capacity equivalent to 40 miles of driv- ing energy on a reference driving cycle. The PHEV40 can displace petroleum energy equivalent to 40 miles of driving on the reference cycle with off-board electricity. The PHEV uses the battery-pack storage energy for most of its driving, thereby reducing the vehicle emissions, and air and noise pollution. The maintenance cost of a PHEV is low since the vehicle is primarily electric. A PHEV can also be used to even out electricity demands during peak load demand on the grid. Excess battery charge from the plug-in hybrid can be used to send power