A compressed air car is a car that uses a motor powered by compressed air. The car can be powered solely by air, or combined (as in a hybrid electric vehicle) with gasoline, diesel, ethanol, or an electric plant with regenerative braking.
Compressed air cars are powered by engines fueled by compressed air, which is stored in a tank at high pressure such as 30 MPa (4500 psi or 300 bar). Rather than driving engine pistons with an ignited fuel-air mixture, compressed air cars use the expansion of compressed air, in a similar manner to the expansion of steam in a steam engine.
Storage tanks are often made of carbon-fiber for weight reduction while maintaining strength; if penetrated carbon fiber will crack but not produce shrapnel.
There have been prototype cars since the 1920s and compressed air has been used in torpedo propulsion as well.
The major manufacturers that are developing air cars have designed safety features into their containers.[In contrast to hydrogen’s issues of damage and danger involved in high-impact crashes, air, on its own, is non-flammable. It was reported on Seven Network’s Beyond Tomorrow that on its own,[carbon-fiber is brittle and can split under sufficient stress, but creates no shrapnel when it does so. Carbon-fiber tanks safely hold air at a pressure somewhere around 4500 psi, making them comparable to steel tanks.
Compressed air is also relatively space inefficient of storing energy when compared to conventional gasoline. Air at 30 MPa (4,500 psi) contains about 50 Wh of energy per liter. Gasoline contains about 9411 Wh per liter.
Compressed air cars are emission-free at the exhaust. Since a compressed air car’s source of energy is usually electricity, its total environmental impact depends on how clean the source of this electricity is. Different regions can have very different sources of power, ranging from high-emission power sources such as coal to zero-emission power sources such as wind. A given region can also update its electrical power sources over time, thereby improving or worsening total emissions.
However a study showed that even with very optimistic assumptions, air storage of energy is less efficient that chemical (battery) storage.
The principal advantages of an air powered vehicle are:
- Refueling can be done at home using an air compressor or at service stations. The energy required for compressing air is produced at large centralized plants, making it less costly and more effective to manage carbon emissions than from individual vehicles.
- Compressed air engines reduce the cost of vehicle production, because there is no need to build a cooling system, spark plugs, starter motor, or mufflers.[
- The rate of self-discharge is very low opposed to batteries that deplete their charge slowly over time. Therefore, the vehicle may be left unused for longer periods of time than electric cars.
- Expansion of the compressed air lowers its temperature; this may be exploited for use as air conditioning.
- Air turbines, closely related to steam turbines, are a technology over 50 years old.
- Reduction or elimination of hazardous chemicals such as gasoline or battery acids/metals
- Some mechanical configurations may allow energy recovery during braking by compressing and storing air.
- Analog-Digital Hybrid Modulation for improved efficiency