Friday, September 11, 2009

A hybrid electric vehicle (HEV) is a hybrid vehicle that combines a conventional internal combustion engine propulsion system with an electric propulsion system. The presence of the electric power train is intended to achieve better fuel economy than a conventional vehicle. A hybrid electric vehicle is also a form of electric vehicle; a variety of types of HEV exist, and the degree to which they function as EVs varies as well. The most common form of HEV is the hybrid electric car, an automobile driven by a gasoline internal combustion engine (ICE) and electric motors powered by batteries.

Modern HEVs make use of efficiency-improving technologies such as regenerative braking, which converts the vehicle's kinetic energy into battery-replenishing electric energy, rather than wasting it as heat energy as vehicles equipped with conventional brakes do. Some varieties of HEVs use their internal combustion engine to generate electricity by spinning an electrical generator (this combination is known as a motor-generator), to either recharge their batteries or directly feed power to the electric motors that drive the vehicle.

Many HEVs reduce idle emissions by shutting down the ICE at idle and restarting it when needed; this is known as a start-stop system. A hybrid-electric produces less emissions from its ICE than a comparably-sized gasoline car, as an HEV's gasoline engine is usually smaller than a pure fossil-fuel vehicle, and if not used to directly drive the car, can be geared to run at maximum efficiency, further improving fuel economy.