Hybrid vehicles

Summary
Hybrid vehicles combine an internal combustion engine with an electric motor, producing significantly lower emissions and increased fuel economy.

Hybrid vehicles use a combination of electric power and an internal combustion engine. They combine the different advantages of internal combustion engines and electric motors to provide a vehicle that operates more efficiently. Usually, the engine drives a generator to provide the electricity which is stored in batteries which then drive an electric motor. The engine may also drive the wheels directly.
Hybrid vehicles are designed to drive like conventionally powered vehicles, while producing significantly lower emissions and increased fuel economy.
In vehicles that only have an internal combustion engine, the engine must be large enough and powerful enough to produce sufficient torque for rapid acceleration at low speeds or from a stationary position. Such an engine will then be larger than necessary for highway cruising. On the other hand, a vehicle that only has an electric motor would need to carry a large volume of heavy batteries to give it sufficient range to be useful, and these would need to be recharged or replaced when flat.
Electric motors can produce maximum torque at very low speeds, whereas a gasoline engine can be most efficient at higher rpm. Combining a small, but efficient gasoline engine that is designed to operate at its optimum rpm, with an electric motor that can double as a generator that can recapture braking energy when the vehicle slows down, is both effective and efficient.
There are three basic types of gasoline/electric hybrid vehicle:
The series hybrid
In this configuration, the gasoline engine is used only to drive a generator, which produces power to drive an electric motor. The electric motor then drives the wheels of the car. The gasoline engine is really only a battery charger inside an electric vehicle.
The parallel hybrid
In the parallel hybrid, the engine always drives the wheels as well as keeping the batteries charged. The electric motor acts like a large starter motor, helping the engine to crank over when more power is needed, particularly when starting from rest or when accelerating rapidly. This allows the engine to be smaller and lighter and more fuel efficient than if it was the sole source of power in the vehicle.
The series-parallel hybrid
This later design allows both the engine and the motor to drive the vehicle, or either, under the control of an onboard computer which determines the optimum combination of power delivered to the wheels at any time. The engine can shut down completely when the batteries are charged and the electric motor is all that is needed, firing up again to assist in driving the wheels or recharging the battery whenever needed. The engine can be optimized for efficiency within a narrow rpm range, and the overall combination of both power sources takes up little more room than a conventional all-purpose gasoline engine, and yet can deliver lower emissions and much greater fuel economy. This is design used by the very successful Toyota Prius.