Every Stirling engine has a sealed cylinder with one part cold and the other hot. The working gas inside the engine, often air, helium, or hydrogen, is moved by a mechanism from the hot side to the cold. When the air/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.
Antique Marklin toy Stirling engine:
Though it had been suggested as early as 1884 that all closed cycle air engine should be generically called Stirling engines after the inventor of the first practical example, the idea found little favour and the various types on the market continued to be known by the name of their individual designer or manufacturer. Then, in the 1940s, the Philips company was searching for a suitable name for its version of the 'air' engine which by that time had already been tested with other gases. Rejecting many suggestions, including 'hot gas engine' ('gas engine' was already in general use for internal combustion engines running on gaseous fuels) and 'external combustion engine' (did not differentiate between open and closed cycles), Philips eventually settled on 'Stirling engine' in April 1945, though general acceptance of the term seems to have lagged a few years behind.
What's it worth? Take a look at this Stirling Engine price guide: sold listings for a value indication.