54 Electric and Hybrid Vehicles Unidirectional power flow FIGURE 3.3 Parallel HEV powertrain. Series HEV is the simpler type where only the electric motor provides all the propulsion power to the wheels. A downsized IC engine drives a generator, which supplements the batteries and can charge them when they fall below a certain state of charge (SoC). The power required to propel the vehicle is provided solely by the electric motor. Beyond the IC engine and the generator, the propul- sion system is exactly the same as that in an EV; the electric motor power requirements are exactly the same as that in an EV.
The drawback of the series configuration is the size of the electrical traction motor, which has to be rated for the maximum power requirement of the vehicle. The drawback can be removed if the engine is used in parallel with the electric machine to supply power to the wheels. This is called a parallel architecture where both the IC engine and the electric motor are connected to the driveshaft through transmission and clutch. In the parallel HEV, the power requirements of the electric motor in the parallel hybrid are lower than that of an EV or series hybrid, since the IC engine complements for the total power demand of the vehicle.
The propulsion power may be supplied by the IC engine alone, by the battery-motor set, or by the two systems in combination. Both series and parallel hybrids come in a variety of types. The mission of the vehicle and the optimum design for that mission dictates the choice. If the HEV is to be basically an EV with an IC engine assist for achieving acceptable range, then the choice could be series hybrids with the IC engine ensuring that the batteries remian charged all the time.
On the other hand, if the hybrid vehicle is to be basically a vehicle with almost all the performance characteristics and comforts of an IC engine vehicle but with lower emission and fuel usage standards, then the choice could be a parallel configuration. Parallel HEVs have been built with performance that is equal, in all aspects of normal operation, to a conventional car. However, some series hybrid vehicles have also been built that perform as well as the IC engine vehicles. The advantages of a series hybrid architecture can be summarized as follows: • Flexibility of location of engine-generator set. • Simplicity of drivetrain. • Suitable for short trips with stop and go traffic.
The disadvantages of a series hybrid architecture are as follows: • It needs three propulsion components: IC engine, generator and motor. • The motor must be designed for the maximum sustained power that the vehicle may require, such as when climbing a high grade. However, the vehicle operates below the maximum power for most of the time. • All three drivetrain components need to be sized for maximum power for long-distance sustained, high-speed driving. This is required since the batteries will exhaust fairly quickly leaving IC engine to supply all the power through the generator. Wheel Engine Clutch Trans- Drive- Diff.
mission shaft Motor Battery and Inverter Bi-directional power flow Wheel Wheel Battery Motor and Inverter Wheel