Try this on for size! This was built by one of my 7th grade students. It's powered by a Lego Mindstorms NXT and actually runs pretty darn smoothley!
Square piston and crankshaft.
Interesting spoked flywheel and the "electric start".
Intake / exhaust port at the bottom of the cylinder.
Video of it in action!
He said he just kinda started putting stuff together and this is what came out of it. Very impressive!
Now, I could have sworn I posted this one somewhere but can't seem to find it... So here it is (again?).
This was built by one of my 8th grade students that was also building my wobbler air engine. Same story- he was playing with legos and this sort of emerged from the chaos. This one even RUNS on compressed air. It takes a LOT of it though- lots of leaks through all the gaps.
Kind of hard to see but if you look to the right of the little square lego piece you can see the air intake port.
And of course a video of it in action!