Stirling engine

Every Stirling engine has a sealed cylinder with one part hot and the other cold.

The working gas inside the engine (which is often air, helium, or hydrogen) is moved
by a mechanism from the hot side to the cold side.

When the gas is on the hot side it expands and pushes up on a piston. 

When it moves back to the cold side it contracts. Properly designed Stirling engines have two power pulses per revolution, which can make them very smooth running.

Two of the more common types are two piston Stirling engines and displacer-type Stirling engines. The two piston type Stirling engine has two power pistons. The displacer type Stirling engine has one power piston and a displacer piston.