Stirling Engines

Have a look.  Have a read.

http://auto.howstuffworks.com/stirling-engine.htm

http://www.animatedengines.com/vstirling.html

http://www.animatedengines.com/stirling.html

http://www.animatedengines.com/ross.html

http://www.animatedengines.com/ltdstirling.html

Also, if you have time - look on youtube for some Stirling engines.