http://peswiki.com/index.php/Directory:Bob_Rohner%27s_Noble_Gas_Engine#Plasma_Cycle_is_Not_Air_Driven
A 1-cylinder Papp is assembled from scratch, charged with gas and operated.
http://www.youtube.com/watch?v=0fd-coQ84XU
The cylinder fires against a large spring : it looks to me as if it moves at least an inch. On the recovery stroke it generates current through a capacitor, driving an electric motor. This was just set up to prove it's not air-driven (feuding Rohners) -- there's no way of calculating the power balance.
He has a cylinder of pre-mixed gas (which he says costs $1700).
