A compressed air car is a compressed air vehicle that uses a motor powered by compressed air. The car can only be powered by air, or combined (as in a hybrid electric vehicle) with gasoline, diesel, ethanol, or a power plant with regenerative braking. Compressed air cars are driven by compressed air motors, which are stored in a high pressure tank as 30 MPa (4500 psi or 310 bar). Instead of driving the engine pistons with a fuel-air mixture on, the compressed air cars use the expansion of compressed air, similar to the expansion of steam in a steam engine. There have been prototypes of cars since the 1920s, with compressed air used in torpedo propulsion.
Compressed air has a relatively low energy density. Air at 30 MPa (4,500 psi) contains approximately 50 Wh of energy per liter (and normally weighs 372 g per liter). For comparison, a lead-acid battery contains 60-75 Wh / l. A lithium-ion battery contains approximately 250-620 Wh / l. The EPA estimates that one gallon of gasoline equals 33.7 kWh however, a typical gasoline engine with 18% efficiency can only recover the equivalent of 1694 Wh / l. The energy density of a compressed air system can be more than doubled if the air is heated prior to expansion. In order to increase energy density, some systems may use gases that can be liquefied or solidified. "CO2 offers a much greater compressibility than air when it goes from gaseous form to supercritical".