A friend and I are working on this idea which is fairly common, but I'm having trouble starting out. The idea is that of a compressed air engine. Where we take a normal, small capacity engine and modify it such that only compressed air, stored in tanks is sufficient to move the piston. Now, I'm not too sure how to go about starting this. We thought we'll get a small scrap engine and see the amount of pressure we'll need to make the piston start moving. Based on what we see, we go forward. As for the calculations part, I'm completely lost. Suggestions anyone?