Guy Negre, an engineer from the little town of Carros, France, discovered a breath of fresh air, both literally and figuratively. During his career designing formula one engines he became familiar with isotherm dynamics, a process that creates power by expanding air at an almost constant temperature. Negre theorized that by heating and expanding super-cooled compressed air he could power a nonpolluting car. Six years and four prototypes later, it would appear he's done it.

Negre's company, Motor Development International (MDI), created what it calls the Compressed Air Technology (CAT) car by combining a lightweight automobile body with a new type of small rear-mounted engine. The 1,500-pound frame is made from aluminum and fiberglass with four very light, steel-reinforced thermoplastic air tanks attached to the undercarriage of the car. The engine measures only one-foot square and weighs just 70 pounds, but because it propels a relatively light vehicle, it can run at 55 mph. 

Negre, who was interviewed through an interpreter, explains that, in the tanks, the air is both cooled to minus 100 degrees Centigrade and compressed to 4,500 pounds per square inch. Then it's injected into a small chamber between the tanks and pistons, where it's heated up by ambient outside air that forces it to expand into a larger chamber situated between the small chamber and the pistons. That heat exchange between the two chambers, he continues, creates the propulsion that drives the up-and-down strokes of the engine's four pistons. Finally, the air is passed through carbon filters like those in scuba diving tanks and expelled as pollutant-free exhaust. The dynamic is not unlike that of a spring that takes in energy when it's compressed and gives it back when it expands.

The big plusses of the air-powered car, according to Negre, are super-efficient energy consumption as well as minimum pollution and maximum affordability. Though the car seats five, it will go from zero to 50 mph in seven seconds-certainly adequate acceleration for an urban vehicle like a taxi. What's more, with fully loaded air tanks, it will take passengers about 120 miles at an average of 30 mph-again, about the right capacity for urban drivers who don't want to fill up too often. 

Charging the car with air is fairly easy-it takes four hours using a household electric outlet or three minutes using special compressed air stations that MDI sells for about $100,000. Obviously, the vehicle also drastically reduces pollution-it takes in polluted outside air, filters it, and expels cleaner air as exhaust. All that for a price tag of between $10,000 and $14,000. 

According to Michael Baltierra, a reporter for ABC News, "we tested the car and it ran quite well. The only major problem that we noticed," he continues, "was that it was quite noisy-[but Negre] said this was something that would be fixed in later models." According to Shiva Vencat, vice president of the U.S. wholly owned subsidiary of MDI, Baltierra tested the car in June, 2000. At the time, Vencat explains, the car "was not a finished productour engine was attached to the car but did not have the body shell all the way around it to muffle the noise." Since then, he says they have encased the engine to make it run more quietly.