Stirling engine, an external combustion reciprocating engine having an enclosed working fluid that is alternately compressed and expanded to operate a piston, thus converting heat from a variety of sources into mechanical energy. A Stirling engine can use any type of fuel as well as solar energy and heat from the waters of a hot spring. The engine was invented in 1816 by a Scottish minister, Robert Stirling, before the gasoline and diesel engines appeared. Stirling engines are unique heat engines because their theoretical efficiency is nearly equal to their theoretical maximum efficiency, known as the Carnot cycle efficiency.

Stirling engines have two pistons that create a 90° phase angle and two different temperature spaces, and the working fluid is sealed within the engine. The engines can be classified as two pistons type or displacer type. The two pistons type engine has two power pistons, and the displacer type has one power piston and a displacer piston, which serves to control when the gas chamber is heated and when it is cooled. When the fluid in the cylinder is heated it expands, forcing the power piston to move and transfer the fluid to a cold region for cooling. It is then recompressed and transferred to the hot region to start the cycle again.

Because the fluids used inside a Stirling engine never leave the engine, and because the engine is not powered by explosive combustion, as in a gasoline or diesel engine, there are no exhaust valves that vent high-pressure fluids. As a result, Stirling engines are very quiet and can be used in specialized applications, such as submarines or auxiliary power generators, where quiet operation is important.