Diesel multiple-unit

A diesel multiple unit or DMU is a multiple-unit train powered by on-board diesel engines. A DMU requires no separate locomotive, as the engines are incorporated into one or more of the carriages. They may also be referred to as a railcar or railmotor, depending on country. Diesel-powered units may be further classified by their transmission type: diesel-electric (DEMU), diesel-mechanical (DMMU) or diesel-hydraulic (DHMU).

The diesel engine may be located above the frame in an engine bay or under the floor. Driving controls can be at both ends, on one end, or in a separate car.

DMUs are usually classified by the method of transmitting motive power to their wheels.

In a diesel-mechanical multiple unit (DMMU) the rotating energy of the engine is transmitted via a gearbox and driveshaft directly to the wheels of the train, like a car. The transmissions can be shifted manually by the driver, as in the great majority of first-generation British Rail DMUs, but in most applications gears are changed automatically.

In a diesel-hydraulic multiple unit, a hydraulic torque converter, a type of fluid coupling, acts as the transmission medium for the motive power of the diesel engine to turn the wheels. Some units feature a hybrid mix of hydraulic and mechanical transmissions, usually reverting to the latter at higher operating speeds as this decreases engine RPM and noise.

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