External combustion engine
An external combustion engine is a reciprocating heat engine where a working fluid, contained internally, is heated by combustion in an external source, through the engine wall or a heat exchanger. The fluid then, by expanding and acting on the mechanism of the engine, produces motion and usable work. The fluid is then dumped, or cooled, compressed and reused.
In these types of engines, the combustion is primarily used as a heat source, and the engine can work equally well with other types of heat sources.
Combustion
"Combustion" refers to burning fuel with an oxidizer, to supply the heat. Engines of similar 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.Working fluid
The working fluid can be of any composition and the system may be single-phase or dual-phase.Working fluids for external combustion engines include air, hot water, pressurized water or even boiler-heated liquid sodium.