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An external combustion engine (EC engine) is a heat engine where an internal working fluid is heated, often from an external source, through the engine wall or a heat exchanger. The fluid then by expanding and acting on the mechanism of the engine provides usable motion and usable work. The fluid is then cooled and reused (closed cycle), or dumped, and cool fluid pulled in (open cycle).

Burning fuel with an oxidizer, or any other heat source can supply the external heat, hence "external combustion". The internal fluid is quite often an inert gas or very commonly steam. The fluid can be of any composition and can be liquid or more commonly, any gas, and may change state.

In the case of the steam engine, the fluid changes phases between liquid and gas.

EC engines require at least one heat exchanger and often have two, for supplying the heat into and removing the heat from the engine. The necessary temperature difference is then available to the engine. In contrast, IC engines have a heat exchanger, commonly called a cooling system, to keep the combustion chamber from overheating. Thus, the IC engine must remove heat as waste. A device called a regenerator in the EC engine saves some of that heat and increases efficiency. The EC engine's heat transfer technology requires greater care during the engineering process than a cooling system. EC engines often have cooling systems, however, they may or may not be smaller than a corresponding IC engine's.

Since the combustion is external to the mechanism, they are much less particular about the type of fuel they burn. They also tend to be cleaner because the combustion is continuous and more easily regulated for temperature, oxidizers and fuel amount. If lower combustion temperatures and pressures are used it will create less exotic exhaust gases, such as nitrogen oxides (NO, NO₂ etc.).

Examples: A steam turbine is a good example of an external-combustion engine. Heat from a burning fuel/oxidizer mix, or from a nuclear reactor changes water in a device called a boiler to steam. Pipes carry the steam into the turbine, which has a series of bladed wheels attached to a shaft. The high-temperature steam expands as it rushes through the turbine and so pushes on the blades and causes them to turn the shaft. Steam leaving the turbine has a much lower temperature. The spinning shaft can drive an electric generator, move a ship's propeller, or do other useful work.

Some other externally heated cycles are Stirling cycle, Ericsson Cycle, Papin and Brayton cycle.

• steam engines
• Stirling engines
• Trochilic engine
• internal combustion engine
• Nuclear power
• Solar thermal rocket (an externally heated rocket)