Series and parallel configurations

However, since the vehicle has on board generator, one could opt for a design with fewer

batteries, which saves cost and weight as compared to an electric vehicle. In such a case,

engine power is required during peak load conditions. The biggest disadvantage to the

series configuration is that all of the engine’s power must be transmitted through the

generator and drive motor. Because of the inefficiencies of these two components, some

power is lost that is not lost in vehicle designs where the mechanical power of the engine

is directly coupled to the wheels. Another disadvantage to the series configuration is that

both an electric motor and generator are required, which usually results in a heavier and

more costly vehicle as compared to the parallel configuration.

In the parallel hybrid configuration, both the internal combustion engine and the electric

motor are mechanically coupled to the drive wheels. Both the engine and the electric

motor can supply power to the drive wheels simultaneously, or the electric motor can be

used as a generator to charge the batteries. Since the internal combustion engine must be

capable of charging the batteries as well as propelling the vehicle at cruise speeds, the

engine is larger, and the electric motor is smaller as compared to a series configuration

design for a similarly sized vehicle. The internal combustion engine is sized for medium

and high speed cruise loads and usually provides slightly better highway fuel economy

compared to the series configuration due to optimal loading at those speeds without the

added inefficiencies of the generator and electric drive motor. The parallel configuration

can also be used ZEV for a limited period of time. However, since the internal

combustion engine is required to be on for full vehicle power, full vehicle power is not

attainable as a ZEV for a parallel hybrid. The disadvantage of the parallel configuration

is that the direct coupling of the internal combustion engine, electric motor and drive

wheels often requires an expensive and complex transmission. Also, since the internal

combustion engine must operate over a wide range of speed and loads, it can’t be run at

optimum efficiency or emissions points all of the time like in the series configuration.