Prime mover (locomotive)

Prime mover (locomotive)

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In engineering, a prime mover is an engine that converts fuel
Fuel
Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...

 to useful work. In locomotive
Locomotive
A locomotive is a railway vehicle that provides the motive power for a train. The word originates from the Latin loco – "from a place", ablative of locus, "place" + Medieval Latin motivus, "causing motion", and is a shortened form of the term locomotive engine, first used in the early 19th...

s, the prime mover is thus the source of power for its propulsion
Ground propulsion
Ground propulsion is a different term than transport, because it refers to solid bodies being propelled. Those bodies may be mounted on vats or using wheels while the latter dominates for standard applications....

. The term is generally used when discussing any locomotive powered by an internal combustion engine
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...

. The term is also applied to engine-generator set
Engine-generator
An engine-generator is the combination of an electrical generator and an engine mounted together to form a single piece of equipment. This combination is also called an engine-generator set or a gen-set...

s, 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
Diesel engine
A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...

 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
Electrical generator
In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge to flow through an external electrical circuit. It is analogous to a water pump, which causes water to flow...

 responsible for producing electricity to power the traction motor
Traction motor
Traction motor refers to an electric motor providing the primary rotational torque of a machine, usually for conversion into linear motion ....

s that are geared to the drivers. The prime mover can also be a gas turbine
Gas turbine
A gas turbine, also called a combustion turbine, is a type of internal combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between....

 instead of a diesel engine. In either case, the generator, traction motors and interconnecting apparatus
Apparatus
Apparatus may refer to:*Technical term for body of the Soviet and post-Soviet governments *Machine*Equipment*Critical apparatus, the critical and primary source material that accompanies an edition of a text-See also:*Fire apparatus...

 are considered to be the power transmission system
Transmission (mechanics)
A machine consists of a power source and a power transmission system, which provides controlled application of the power. Merriam-Webster defines transmission as: an assembly of parts including the speed-changing gears and the propeller shaft by which the power is transmitted from an engine to a...

 and not part of the prime mover. An electric
Electric locomotive
An electric locomotive is a locomotive powered by electricity from overhead lines, a third rail or an on-board energy storage device...

 or battery-electric locomotive has no on-board prime mover, instead relying on an external power station
Power station
A power station is an industrial facility for the generation of electric energy....

.

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 only applied to the diesel engine.