This is an idea I had on 3D printing a compressed air engine. It has not been tested and there are no dimensions yet.

The principle: when the piston is at the top position (as in the image) the piston holder wil overlap with the air inlet, the air wil push the piston further.

when the piston then passes its maximum position the holder wil then overlap with the outlet so the air can escape and the piston can be pushed back.

Momentum of the shaft will have to bring the piston back to the top position so the entire process can start again.

Step 1: Piston Holder

This is the component that contains the Piston. It is rotatable around the axle. the hole in the side wil let compressed air in or out, depending on its position relative to the air inlet of outlet.

The contact surface should be reduced as much as possible, but the outlet should always be fully covered when the piston is at the maximum position.

Step 2: The Piston

The piston wil fit in the piston holder. The contact surfaces will have to be lubricated to reduce friction. Extra support should be added to the bar that fits in the piston holder to reduce bending when it is halfway out.

Step 3: The Shaft

The shaft will have to be lubricated or bearings could be used. the wheel will act as a flywheel and give the momentum to bring the piston back to its starting position.

Step 4: The Frame

I tried to design a frame that wil hold all the components together. On the left of the picture we have the part that wil hold the shaft. For a real prototype lubrication should be added, or you can include bearings in the model.

On the right we have the part to hold the shaft. the shaft wil be able to rotate. This wil cause the most friction because of the large surface area between the holder and the frame.

The tube on this part of the frame is the air inlet. connect this to a compressed air source to power the engine.

Step 5: Assembling the Components

When all the components are assembled, you have something like this. the idea is very basic and friction is a problem for actual use, but it shows the easy concept and is easy to design.