Anyone looking at the Volt, electric delivery trucks, and other stored energy vehicles should keep an eye on MDI (privately owned) and their air storage technique. This could be formidable competition in the field. Performance of this PHAV is close to the better known PHEV designs, but since there are no batteries, the retail price looks like about half that of the more ballyhooed machines. The quickest way to get current news is to search on ("zero pollution motors" AND (minicat or citicat)). I've gone through the basic physics, which is good as far as I can tell. They are using a new business model, a variation on franchising. I picked up one share a few years ago as a speculative investment.

Most people thinking of air power think in terms of isentropic compression, which wastes energy by heating the air, and isentropic expansion, which wastes energy by cooling the air. They are using a triple compression/expansion design (with reheat and recool) approximation to isothermal. A pure isothermal would waste no stored energy but seems to be impossible. The best I've read of is 95% of isothermal and is not practical for transportation. MDi seems to approach 80%.

Basically, they have lower first cost, and lower storage efficiency. They've solved the safety issues also. Production is expected around '10.