Only hot air ....
If you think I am trying to describe the function of hot air engines here you
For those who prefer reading all this in a book, there is a new German edition of the authoritative work "Stirling-Maschinen Grundlagen Technik Anwendungen" by Martin Werdich and Kuno Kübler (ISBN 3-992 964-35-4), published by "ökobuch Verlag", 1999.
You can also find an adequate historical survey of stirling engine development and many good books at this website: http://www.stirlingmotor.com/buecherd.htm
... but it works