This is a closed cycle external heat engine of a type invented by Robert Stirling in 1816. The working fluid is air, and the working cycle is equivalent to the Carnot cycle that has the highest possible efficiency of any heat engine. Like the Carnot cycle it is reversible and can run as a 'cold engine' or as a heat pump or refrigerator. Heat is transferred between the upper and lower metal plates as the air between is displaced by the loose-fitting black foam plastic piston at the same time as it is compressed and expanded by the glass piston on top. With a large displacer piston and small power cylinder/piston, this example was designed to work with small temperature differences eg it will work when sitting on a hot cup of coffee or on a few ice cubes. The loss-free efficiency depends on the temperature difference and is about 10% for a 30 degree difference. Losses reduce this significantly.
