These are no ordinary cars. Power comes from fresh air stored in reinforced carbon-fiber tanks beneath the chassis. Air is compressed to 4,500 pounds per square inch — about 150 times the pressure of the typical car tire. The air is fed into four cylinders where it expands, driving specially designed pistons. About 25 horsepower is generated. Though technical problems are being worked out, company officials say the car is capable of 70 mph and a 120-mile range under normal city conditions, performance that is comparable to electric cars. Critics say the car has had trouble living up to its...