I know a guy that I used to work with, had a version of this
( http://www.youtube.com/watch?v=VIZLCOw1MVs ) but it used a Hydrogen gas flame, to power it.
Because the Hydrogen was slow burning, it sucked the flame inside where it burnt it, thus pushing the piston out.
It was a very old unit, say early 1900's type. It was initially cooled by air, but you put some water at the base of the unit to absorb some heat, to stop it seizing.