This engine was the first
development of an external
 combustion machine
built by Reverend Robert Stirling
in 1816 ...

in Scotland!
 

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and here you will find an
engine's animation

Reverend Robert Stirling designed the first hot air machine that could perform useful work. His invention was to add a regenerator, which he called "economizer", that helped to increase the power output by widening the delta between the temperatures of the cold and the hot side of the engine. Hence, the working gas could deliver more power respectively torque.

Stirling engines are external combustion devices; the only other external engine is the steam engine where the performing engine is not in contact with the combustion of any fuels.

The advantage is that there is no contamination of any internal parts and combustion and lubrication are not influencing each other. Also, there are much less moving parts which again directly relates to maintenance issues. This and the fact that there is a multiple choice of fuels to be used for heating up the head of the Stirling engine makes the technology very attractive for alternative energy schemes.

Most if not all of the disadvantages of the Stirling principle are now-adays overcome by the use of most modern materials and compre-hensive electronic regulations.