In engineering, a prime mover is an engine that converts fuel to useful work. In locomotives, the prime mover is thus the source of power for its propulsion. The term is generally used when discussing any locomotive powered by an internal combustion engine. The term is also applied to engine-generator sets, where the engine is termed the prime mover, as distinct from the generator.
In a diesel-mechanical locomotive, prime mover refers to the diesel engine that is mechanically coupled to the driving wheels (drivers). In a diesel-electric locomotive, prime mover refers to the diesel engine that rotates the main generator responsible for producing electricity to power the traction motors that are geared to the drivers. The prime mover can also be a gas turbine instead of a diesel engine. In either case, the generator, traction motors and interconnecting apparatus are considered to be the power transmission system and not part of the prime mover. An electric or battery-electric locomotive has no on-board prime mover, instead relying on an external power station.
The term power unit is also sometimes used in application to diesel locomotives, with a similar meaning. Where the engine and generator set of a diesel-electric locomotive are removable as a unit, it is usual to term the coupled pair of them as "the power unit", but "prime mover" is applied to the diesel engine alone.
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