Definition
An engine in which fuel is burned outside the working cylinder or chamber, and the heat produced is transferred to a separate working fluid (typically water turned to steam) that drives the pistons or turbine.
Plain English
An engine where the fuel is burned in one place and the heat is then used to push something else, like steam, that actually moves the engine.
Context Anchor
Seen in powerplant theory and maintenance glossary material, usually to contrast older steam-type engines with the internal-combustion engines used in most aircraft.
Derivation
External means 'on the outside,' and combustion means 'burning.' So the burning happens outside the part of the engine that does the work — the opposite of an internal-combustion engine, where the burning happens inside the cylinder.
Why Pilots Care
Knowing the difference helps pilots and technicians understand why aircraft piston engines (which are internal-combustion) work the way they do — fuel burns directly inside the cylinder to push the piston, with no separate steam or working fluid involved.
Analogy
Think of a steam locomotive: coal burns in the firebox, heating water in a separate boiler, and the steam then drives the pistons. The burning and the working part are in two different places.
Intuition Check
External does not mean the engine is located outside the aircraft. Here it means the fuel burns outside the engine’s main power-producing chamber.
Example Sentence 1
A steam engine is an external-combustion engine because the fuel burns in the firebox, not inside the cylinder.
Example Sentence 2
Early experimental aircraft sometimes tested external-combustion engines before gasoline piston engines became standard.