This is a video of someone actually running an air powered trike.
But this video below shows more promise.
In this video he is sort of on the right track. Forget the selenoid at the top where the spark plug goes. They are only good for so many cycles and will wear out pretty quickly. Besides, the 4 stroke engine already has great valves in it. And he makes the intake and exhaust work together for air power by adding the lobes on the cam shaft. I can't see it getting more efficient than that. You just need a cheap air compressor and a scuba tank or better, a carbon fiber tank. I like the idea of using compressed air instead of batteries. And imagine doing conversions on cars. You take the camshafts out, add the lobes and grind them smooth. Take the gas tank out and replace with a 10,000 psi carbon fiber tank. Have a double reducer on that that goes down to 100 to 150 psi. You could fill up at a welding shop or at home if you have a high pressure compressor. And once you pressurize a tank, it doesn't lose its power like batteries do even just sitting there. Gas stations could start adding high pressure tanks and compressors so they could fill up the new vehicles.