It must store sufficient energy (k Wh) to provide the all-electric range of the vehicle or to permit the engine or fuel cell to operate near the average power required by the vehicle (in a load-leveled mode).
It also must have sufficient power capability to meet on demand the peak power of the motor/electronics for vehicle acceleration or regenerative braking.
The engine would be used only on those days when the vehicle is driven long distances.
Hybrid vehicles designed to maximize fuel economy in an all-purpose vehicle could use the series, parallel, or dual configurations depending on the characteristics of the engine to be used and acceptable complexity of the driveline and its control.
As shown in Figure 1b, the engine output can be split to drive the wheel (parallel mode) and to power a generator to produce electricity (series mode).
This configuration is the most flexible and efficient, but it is also likely to be the most complex and costly.
This is the driveline system used in diesel-electric locomotives.
In the parallel configuration (Figure 1, Bottom), the engine and the electric motor act in parallel to provide torque to the wheel of the vehicle.
In most cases, the energy storage unit in a hybrid vehicle is sized by the peak power requirement.
Because the size (weight and volume) of the energy storage unit (often a battery) in the hybrid vehicle is smaller than the battery in a battery-powered electric vehicle (EV), the power density (W/kg and W/liter) requirements of the energy storage unit in the hybrid vehicle are greater than for the battery in an electric vehicle.