Overview

In engineering the term "prime mover" denotes the machine that converts a fuel or energy source into useful mechanical work. In the context of locomotives, the prime mover is the principal source of tractive power. It may be an onboard internal combustion unit or another device that ultimately drives the wheels via mechanical linkages, hydraulic transmission, or electrical transmission.

Types and arrangements

Several prime mover types have been used in railway practice. Common arrangements include:

  • Diesel-electric: a diesel engine drives a generator or alternator; that electrical output feeds traction motors on the axles.
  • Diesel-hydraulic: the engine connects to hydraulic torque converters and gearboxes that turn the wheels.
  • Gas turbine: turbines have been tested and used in some high-speed or experimental designs, typically driving generators for electric traction.
  • Steam: historically the dominant prime mover, converting heat from burning fuel into reciprocating or turbine motion.

How it is integrated

In most modern diesel locomotives the prime mover is coupled to auxiliary systems: cooling, air intake and exhaust treatment, fuel supply, and electrical generation. The prime mover provides a continuous source of power that is regulated by governors and power electronics. For more on the broader engineering concept see fuel-to-work conversion and for a background on the common engine type see internal combustion engine.

History and development

Early rail traction relied almost entirely on steam prime movers. The twentieth century saw a gradual shift toward internal combustion and electric systems as engines, transmissions and electrical controls improved. Diesel-electric designs became widespread because they balance fuel efficiency, reliability and ease of control, while electric locomotives obtain power from an external supply and do not carry a conventional onboard prime mover.

Operational considerations and significance

The choice of prime mover affects fuel type, maintenance regime, emissions, route suitability and operational cost. Diesel prime movers require periodic overhauls and emission control equipment; hydraulic systems favor different maintenance skills than electrical ones. Railway planners select prime movers based on traffic density, fuel availability, environmental rules and compatibility with existing infrastructure.