Compressed air engine help?

My friend and i are making a compressed air engine strong enough to work lawnmowers or better yet, a small car. Does anybody know any equations for calculating torque or RPM's? We are somewhat basing it off of the Airhogs air engine but better. Also, does anybody think making it out of PVC would be a good decision? or aluminum? I don't know if it will make lots of heat since there is no igniting of gases so i think it will be safe to use PVC for a lighter engine so more power can be used for moving. any thing wrong/bad about that?

PKM7 years ago
You can theoretically calculate torque using

torque = force * distance
force = pressure * area

Subtract atmospheric air pressure from your supply air pressure to get the pressure acting on your piston, multiply that by the area of your piston (pi * radius * radius for circular piston) to get force and multiply that by the length of your crankshaft cranks to get the maximum possible torque.

Example- 40PSI air supply, 3 inch piston diameter, 2 inch crank

40PSI - 15PSI(atmospheric pressure) = 25 PSI pressure
3.14 * 1.5 * 1.5 = 7 square inches
25PSI * 7sq.in. = 177 pounds of pressure
177 pounds * (1/6 foot) = 30 pound-feet of torque.

Note that this is assuming completely ideal conditions, don't expect to make even half of this from a home-made PVC engine. If you want hundreds of pound-feet, you'll have to scale up the engine or use a different design. I say that rather than the obvious answer of "increase the pressure" because working with very high-pressure air tanks is very dangerous, particularly with a home-made engine.

The other consideration is that the Air Hog engines work well because propellers require high revs but don't need much torque. Air engines are good at this requirement, but not so great for the low-speed high torque a ground vehicle requires. To get decent performance under those conditions you need a combination of a very tight air seal but low friction in the cylinder.

Not trying to be negative- just being realistic.