I’ve several various engine designs for compressed air engine vehicles.  I really believe this has merit as some of them are proving to be a viable alternative to fuel based engines.  They probably won’t see mass acceptance for general use but they have promise for specifc applications.

Air Cars: A New Wind for America’s Roads?

A new carmaker has a plan for cheap, environmentally friendly cars to be built all over the country

An air-powered car? It may be available sooner than you think at a price tag that will hardly be a budget buster. The vehicle may not run like a speed racer on back road highways, but developer Zero Pollution Motors is betting consumers will be willing to fork over $20,000 for a vehicle that can motor around all day on nothing but air and a splash of salad oil, alcohol or possibly a pint of gasoline.

I particularly like this rotary engine design

Significant new rotary engine design runs on compressed air

There is no other motor as efficient as the Di Pietro Rotary Air Engine. It is 100% more efficient than any other air powered engine built to date and its high torque makes it the first air engine suitable for mobile applications. The invention http://en.wikipedia.org/wiki/Invention has the capacity to revolutionise transportation, plus offer a multitude of energy-saving benefits in stationary applications.