A Stirling engine is a kind of heat engine that converts heat into useful mechanical energy by the movement of a piston inside a cylinder within the engine. Unlike other commonly found heat engines such as the internal combustion engine used in cars and the steam engine used on railways, it re-uses the same gas for each stroke of the piston so there is no noisy exhaust. The same gas is repeatedly heated and cooled within the engine’s cylinder.

To run it needs a supply of heat to heat its hot parts. This can come from a fire, but also from the sun’s rays, from hot rocks near a volcano or from nuclear energy. It also has cold parts which cool the gas inside it and these are kept cold by a stream of air or water flowing over them.

It was invented by a Scottish minister, Reverend Dr. Robert Stirling in 1816.