External combustion engine

An external combustion engine is a reciprocating heat engine in which a working fluid, held inside the engine, is heated by combustion occurring outside it, through the engine wall or a separate heat exchanger, rather than inside a cylinder. The heated fluid then expands to produce motion and mechanical work. This is the defining contrast with internal combustion engines, where fuel ignites directly inside the cylinder. The Stirling engine uses a gas as its working fluid heated externally, while the steam engine, the most common example, relies on the liquid-to-gas phase change of water; Thomas Newcomen's early steam engine is considered a precursor to the type. Because the heat source sits outside the working fluid, external combustion engines can in principle run on heat from sources other than burning fuel, including nuclear, solar or geothermal heat, though the label strictly applies when fuel combustion supplies that heat.