If you place the engine on some hot surface (it could be a glass with hot water or even your hand) and wait for about half a minute, the propeller will start turning. Once it has been put in motion the propeller will go on nearly forever.

It is clear that this engine runs using the temperature difference. The bottom surface has to have different temperature than the top one. In this way, one surface provides heat and the other is the receiver (radiator). The engine changes some of the heat provided from the outside into mechanical energy. The engine works quite well if you put it on a hot glass of water as well as on a block of ice. The only thing that changes is the direction of heat flow.

This strikingly simple engine was developed in 1816 by a Scottish clergyman, Rev. Robert Stirling. It was supposed to be an alternative for the existing steam engines which often failed and exploded. In contrast to other engines, it runs on a small temperature difference.

Let’s consider heating of the lower part and cooling of the upper one. Inside the engine there are two cylinders with pistons, the big piston is not tightly fitted, the air can flow around it, the work is carried out mainly by a smaller piston. The pistons rotate on the same shaft. The bigger one is ahead of the small one by 1/4 of a full thermodynamic cycle. The chamber is filled with air and tightly sealed. The picture is a schematic diagram of Stirling engine.

As a result of heating the air increases the air pressure in the chamber and forces the smaller piston up, which movement is transmitted by the transmission gear to the bigger piston, which lowers. Once, the bigger piston reaches the lowest point, a vast majority of air gets into contact with the upper cold wall. The air becomes cooler, the pressure becomes lower and the smaller piston lowers (the bigger one moves up and the whole cycle is repeated).

In short the whole thermodynamic cycle involves three processes:

1. heating,

2. decompression,

3. cooling,

4. compression.

Due to their numerous strengths such engines have found practical application as silent propulsion in some submarines. They are also used to recover heat sources and power. Sometimes they are used in liquefying air, hydrogen or helium.