Compressed air has been used for decades in the power tool industry. Ironically, a typical auto assembly line in Detroit uses compressed air to bolt together our American gas-guzzlers. But French engine designer Guy Negre, has developed a way to actually run the vehicle itself off of compressed air.
The key is carbon fiber. Typically, highly compressed air need a very thick steel tank to contain the pressure without causing an explosion. But carbon fiber now allows for the same pressurization without the weight. MDI uses carbon fiber tanks in the chasis, to create a standard-sized minivan that holds enough air to go about 125 miles.
The CAT has no emissions whatsoever, making it carbon-free. But of course the compression of the air takes energy. I calculate -- very roughly based on a standard $1.75 cost of "airing up" -- 1.5 cents per mile or 0.14 kWh per mile, based on 11 cents per kWh. In the US that equates to less than 1/5 of a pound of CO2 per mile (based on 1.35 lbs CO2 per kWh for standard electricity). That is about 1/5 of the CO2 from a 20 MPG gasoline car, plus there are no particulate emissions. Also, since the compressor is electric, it can be run off solar panels, makings for virtually carbon-free transportation.
But there are a few problems. The fiberglass body, means the car will not hold up to the tough safety standards of the American consumer. Also the car can only go 65 miles per hour at top speeds, and higher speeds severely diminishes the range of the vehicle.
The solution is a hybrid which uses the compressed air much like a battery to supplement a gasoline engine, but without the cost, deterioration and environmental impacts of a standard lithium ion battery. MDI is now working on a hybrid version, as is the Swiss Federal Institute of Technology, whose new 750 ml gas engine developed by engineer Lino Guzzella, uses CAT to improve fuel efficiency by 80%, making the Prius look like a gas-guzzler in comparison.