Parallel hybrids, in which the engine is the main power source with the electric motor providing assistance as needed.

The engine serves as the main power source. The motor, which delivers maximum torque from zero rpm, is called upon to assist the engine during standing starts and during acceleration, when engine fuel consumption is high. This type of hybrid is also simple: a parallel hybrid can be created simply by adding an electric motor and batteries to an existing vehicle.

Combined hybrids share characteristics of series and parallel designs. The electric motor alone is used at low speeds, and the engine and electric motor work together at higher speeds.

The electric motor powers the vehicle from a standing start and at low speeds. As speed increases, engine and electric motor work in combination to efficiently provide the power required.

The system is more complex, featuring a power split device and a generator. The engine is also required to power the generator.

Series hybrids, like electric vehicles, run solely on power provided by electric motors.

The vehicle runs on power from the electric motor. The engine only powers the generator that charges the batteries.

The system powering the vehicle is the same as that of an electric vehicle, but because the vehicle also has an engine, it is considered a hybrid.