Simple air engine, can be run off of as low as 1 psi.

Materials needed: 3mm wooden skewer, 16 gauge (or 1.3mm) wire

Assembly:

cut 2  20cm long piece of the wooden skewer, and glue one into the center of one of the piston crankshafts. Then glue a 8mm long 16 gauge piece of wire into the smaller hole of the crankshaft, which is further away from the center. After gluing the wire, slide in the piston conrod into it. Then you can place another piece of the piston crankshaft the same way you did with the first one. 

After putting it together you can fit the wooden skewers into the both pieces of the crankcase and it should be closed into one box. after that, put the cylinder onto the piston and press fit it into the crankcase, making sure the hole on the side of the cylinder points to one of the skewers. Try spinning it, and if it spins easily, you can continue on. Glue the valve holder onto the hole of the cylinder, making sure the valve opening points down to the engine.

After that, put the sliding valve into the hole of the sliding valve holder, making sure the little hole of the sliding element points down to the engine, not up.

Then put a 7mm piece of 16 gauge wire into the sliding valve hole and glue it. Next put the valve crankshaft on the skewer, which is under the valve. IMPORTANT: the crankshaft should be 90° compared to the piston crankshaft. This is very important because this basically times when the air will go to the cylinder. After that you can put another 7mm piece of wire into the smaller hole of the crankshaft. Slide the valve conrod into the wire of the valve crankshaft and into the sliding valve piece of wire. After that you can glue a flywheel onto the other skewer and the engine is done.

You can run it on your own breath or a few balloons. Thats why i left a balloon adapter which you can use. But you will have to have at least 3 balloons inside of each other for this to run. Have fun!