A diesel locomotive is a type of railway locomotive in which the prime mover is a diesel engine. Several types of diesel locomotives have been developed, differing mainly in the means by which mechanical power is conveyed to the driving wheels. A diesel–mechanical locomotive uses mechanical transmission to transfer power to the wheels. This type of transmission is generally limited to low-powered, low speed shunting (switching) locomotives, lightweight multiple units and self-propelled railcars. Diesel–electric locomotives are diesel locomotives using electric transmission. The diesel engine drives either an electrical DC generator (generally, less than 3,000 horsepower (2,200 kW) net for traction), or an electrical AC alternator-rectifier (generally 3,000 horsepower (2,200 kW) net or more for traction), the output of which provides power to the traction motors that drive the locomotive. There is no mechanical connection between the diesel engine and the wheels. The vast majority of diesel locomotives today are diesel-electric. Diesel–hydraulic locomotives are diesel locomotives using hydraulic transmission. In this arrangement, they use one or more torque converters, in combination with gears, with a mechanical final drive to convey the power from the diesel engine to the wheels.