Prime mover (locomotive)
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. Generally it is any locomotive powered by an internal combustion engine. In an engine-generator set, the engine is the prime mover, as distinct from the generator.
In a diesel-mechanical locomotive, the prime mover is the diesel engine that is mechanically coupled to the driving wheels (drivers). In a diesel-electric locomotive, the prime mover is 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. A wired-electric or battery-electric locomotive has no on-board prime mover, instead relying on an external power station.
The engine (prime mover) and generator set of a diesel-electric locomotive are sometimes coupled as a removable unit called "the power unit".