A hybrid electric vehicle (HEV) combines a conventional internal combustion engine propulsion system with an electric propulsion system. The presence of the electric powertrain is intended to achieve either better fuel economy than a conventional vehicle, or better performance. 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, although hybrid electric trucks (pickups and tractors) also exist.
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 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 to directly power the electric drive motors. 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.
The hybrid-electric vehicle did not become widely available until the release of the Toyota Prius in Japan in 1997, followed by the Honda Insight in 1999. While initially perceived as unnecessary due to the low cost of gasoline, worldwide increases in the price of petroleum caused many automakers to release hybrids in the late 2000s; they are now perceived as a core segment of the automotive market of the future. Worldwide sales of hybrid vehicles produced by Toyota reached 1.0 million vehicles by May 31, 2007, and the 2.0 million mark was reached by August 31, 2009, with hybrids sold in 50 countries. Worldwide sales are led by the Prius, with cumulative sales of 1.43 million by August 2009. The second-generation Honda Insight was the top-selling vehicle in Japan in April 2009, marking the first occasion that an HEV has received the distinction. American automakers have made development of hybrid cars a top priority.