External combustion engine

The fluid then, by expanding and acting on the mechanism of the engine, produces motion and usable work.

"Combustion" refers to burning fuel with an oxidizer, to supply the heat.

Engines of similar (or even identical) configuration and operation may use a supply of heat from other sources such as nuclear, solar, geothermal or exothermic reactions not involving combustion; they are not then strictly classed as external combustion engines, but as external thermal engines.

The working fluid can be of any composition and the system may be single-phase (liquid only or gas only) or dual-phase (liquid/gas).

[clarification needed] Dual-phase external combustion engines use a phase transition to convert temperature to usable work, for example from liquid to (generally much larger) gas.

Model Stirling engine , with external heat from a spirit lamp (bottom right) applied to the outside of the glass displacer cylinder.
Newcomen's engine , a precursor of the steam engine , with the boiler heated from beneath
Sectioned steam locomotive . Although the fire is within an enclosed firebox , this is still an external combustion engine, as the exhaust gas and the steam working fluid are kept separate.