Well, as suspected, others have thought of this and even have prototypes. I guess the biggest hurtle to implementation is perfecting camless valvetrains.
http://www.treehugger.com/files/2009/02/compressed-air-car-pneumatic-hybrid-system.php
I was thinking the other day about a possible hybrid conversion vehicle that could store braking energy as compressed air. The setup is like this:
The engine valves are all electric solenoid controlled. When the motor is running on gasoline it run like any other 4-stroke IC engine. There is...