Only hot air ....
If you think I am trying to describe the function of hot air engines here you
are mistaken.
I do not regard it as efficient to copy whole paragraphs from books or even
other websites, only to repeat what is described correctly and comprehensively
there in the same way.
I recommend the website of
Peter Fette to you; it is very informative, containig information from the
description of phenomena to the basic thermodynamic principles.
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 on this website: http://www.solnet.ch/stirling/story.html