ever play with an originial air hogs plane ?
i had one of those, i wish i still did :-\, but anyway, you pump the tank up with air, and spin the prop, and a custom "engine" with spring-loaded valves with little bumpers on the pistons was how it worked.
piston comes down towards bottom valve by you spinning the prop, pitson bumper opens valve, air from the tank forces piston back up, causing the prop to continue rotating, this causes the piston to come back towards the valve again, valve opens again from the force of the piston, and gets shoved back up while the prop is still rotating....this turns into a cycle, and the prop continues to turn, under power from the plane itself, you throw plane into the air, plane keeps going, you get lots of enjoyment.
i seriously wish i still had that thing, because later downt he road i wanted to (still do) replace the tank with something else, like a pop bottle or something, and pressurize it with an air compressor to like 80 PSI and then flick the prop and let it go haha.
anyway, there is a way to make an engine that uses air pressure to run, as i described above, but there really is no point, there is no feasable reason to do so. other than just to prove you can do it lol.