BOX 1-5: Series and Parallel HybridsIn a series hybrid, the engine is used only to drive a generator, while the wheels are powered exclusively by an
electric motor. The motor is fed directly by the generator or by electricity from a storage device such as a battery (or
flywheel or ultracapacitor)--or by both simultaneously when high power is needed. The storage device obtains some
energy input from regenerative braking, and most of the input from the engine/generator; in some configurations, it
could also be charged externally like an EV. Decisions about how well the vehicle must perform, whether the battery
should be recharged externally or only by the engine, and when to use the battery or the motor/generator can lead
to very different configurations, such as large engine/small battery and small engine/large battery.
In a parallel hybrid, both the engine and the motor can drive the wheels. This type of hybrid is generally
acknowledged to be more difficult to develop than a series hybrid. U.S. automakers appear to be focusing their
attention on series hybrids, although some European automakers do appear to favor parallel hybrids. Conceptually,
however, the general strategy of a parallel hybrid is to downsize the engine, so that the maximum power
requirement of the vehicle is satisfied by having both engine and motor operate simultaneously. The electric motor
size required in a parallel hybrid is much smaller than that required in a series hybrid, because in the latter, the
motor is the only source of power driving the wheels. If the vehicle is powered only by the electric motor when
power demand is low, the engine will be needed only at higher loads, where it is most efficient.