 Some varieties of HEV use an internal combustion engine to turn an electrical generator, which either recharges the vehicle's batteries or directly powers its electric drive motors; this combination is known as a motor–generator.Many HEVs reduce idle emissions by shutting down the engine at idle and restarting it when needed; this is known as a start-stop system.A hybrid-electric produces fewer tailpipe emissions than a comparably sized gasoline car since the hybrid's gasoline engine is usually smaller than that of a gasoline-powered vehicle.