[aligncare]

A couple of companies are pursuing a car engine that operates on compressed air...seems like a good solution for congested cities to run their fleet of buses and city cars. I read somewhere that Mexico city has ordered thousands.

http://auto.howstuffworks.com/air-car1.htm

The car is in production and is seen as solution to localized pollution problems (such as in doors in industrial settings, fork lifts and such.)

http://www.theaircar.com/faq.html

[Gorjus]

Haven't had a chance to look at this yet, but the energy efficiency of cold air is just horrible. We can't even use it to start a jet engine once, not with reasonable size bottles.

I suppose with a zillion psi or so you could store enough energy to be useful - but that would make one heck of a bomb, or cut through just about anything if it ever developed a leak.

My take on it is that cold air (meaning compressed, but not combusted for power) becomes dangerous before it becomes useful as a power source. Of course, with a low enough horsepower requirement, you can make a lot of things work. But I wonder if an equivalently sized gasoline/diesel engine might not be better overall for pollution and fuel use, once you figure in the need to have some way to compress that air in the first place.