Here is the not-very informative website for the MTI Air Car:http://www.theaircar.com/acf/
Powered by a tank of compressed air, the MTI Air Car has been "ready for production" for the past 6 years now. It's as bad as the Moller SkyCar. Not much in the way of real life performance data is available, only a lot of hot air from the inventor of the car. One test had the car sputtering along for only 4 miles before running out of air.
What we do know is this: The car stores 300 liters of air at 4500 psi (i hope the tanks have been well engineered). When fully charged, the air in the tank weighs 220 lbs. If the tank and associated hardware weighs another 100 lbs, that would mean the "battery" weighs 320 lbs. Assuming a perfect isothermal compression and expansion process, the tank would contain 16 Kwhr of energy.
However, the perfect compressor and motor are hard to achieve in real life. Extremely sophisticated engineering is required*. The compressor will consume over 20 Kwhr to fill the tank, and the motor will extract a fraction of that very theoretical 16 Kwhr. It would be a miracle if even 8 kwhr were actually available to drive the car, after friction and thermodynamic losses take their toll.
One version of the air car uses heat from a gasoline to extend its range...hardly a pollution free "electric car" anymore.
In my former life I was a mechanical engineer, so I think the concept is intriguing...but the numbers don't work out so well, alas.
*To efficiently use the energy of the compressed air you would have to use a multi-stage expansion engine. The compressed air would be first expanded a bit, using a piston or some such device. The air would get colder as it expands. To rewarm the air between each expansioin stage, the air would have to be sent through a heat exchanger like a car radiator to extract energy from the ambient air. Many stages of expansion and reheat would be required to approach the maximum efficiency.