compressed air

compressed air

compressed air, air whose volume has been decreased by the application of pressure. Air is compressed by various devices, including the simple hand pump and the reciprocating, rotary, centrifugal, and axial-flow compressors. Compressed air exerts an expansive force that can be controlled and used in various devices including tires, air brakes, caissons, and diving suits. As a source of power it is used to operate pneumatic tools, e.g., pneumatic hammers and drills and spraying equipment. It is widely employed for cleaning dust and dirt out of mechanical equipment. It is used also in mining, tunneling, and the manufacture of explosives, since it is not a fire hazard. Compressed air is in readily available supply and is easily stored and transported.

A Compressed air car is an alternative fuel 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 electric plant and regenerative braking.

Tata Motors signed an agreement with MDI to continue research on air powered motors and cars. The time line for bringing their compressed air car, the Tata OneCAT, to market has been said to be in 2008 or 2009. Other companies are also working on air cars.


The compressed air car has been mentioned in Popular Mechanics as being the true car of tomorrow, with a range comparable to an electric vehicle or fuel-cell car. It claims to offer zero emissions at the tailpipe without batteries or hydrogen fuel.




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). The storage tank is likely to be made of carbon-fiber in order to reduce its weight while achieving the necessary strength. Instead of mixing fuel with air and burning it to drive pistons with hot expanding gases; compressed air cars use the expansion of compressed air to drive their pistons.

The idea is not new. There have been prototypes cars since the 1920s, and compressed air has been used in torpedo propulsion.

Storage tanks

Compressed air is a heavy way of storing fuel, air at 30 MPa (4,500 psi) contains about 16kWh of energy (the equivalent of 1.7 liters [0.44 US gal, 0.37 imp gal] of gasoline, assuming a 100% efficiency of the engine). During rupture testing, the tank cracks, but does not break up, producing no splinters or fragments.

All four 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 Discovery's Beyond Tomorrow that on its own, carbon-fiber is brittle and splits, but creates no shrapnel.

Emission output

Compressed air cars are emission-free at the 'tailpipe'. 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.

Due to this reason, a compressed air car's emission output can vary both with location and time. 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 with time, thereby improving or worsening emissions output.


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.
  • Reduced vehicle weight is the principal efficiency factor of compressed-air cars. Furthermore, they are mechanically more rudimentary than traditional vehicles as many conventional parts of the engine may be omitted. Some plans include motors built into the hubs of each wheel, thereby removing the necessity of a transmission, drive axles and differentials. A four passenger vehicle weighing less than 800 pounds (360 kg) is a reasonable design goal.
  • One manufacturer promises a range of by the end of the year at a cost of € 1.50 per fill-up.
  • Compressed air engines reduce the cost of vehicle production by about 20%, because there is no need to build a cooling system, spark plugs, transmission, axles, starter motor, or mufflers.
  • Most compressed air engines do not need a transmission, only a flow control.
  • 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.
  • Lower initial cost than battery electric vehicles when mass produced. One estimate is €3,000 less.
  • Expansion of the compressed air lowers in temperature; this may be exploited for use as air conditioning.
  • Compressed-air vehicles emit no pollutants.
  • Air turbines, closely related to steam turbines, is a technology over 50 years old. It is simple to achieve with low tech materials. This would mean that developing countries, and rapidly growing countries like China and India, could easily implement a less polluting means of personal transportation than an internal combustion engine automobile.
  • Possibility to refill air tank at home (using domestic power socket).
  • Lighter vehicles would result in less wear on roads.
  • The price of fueling air powered vehicles may be significantly cheaper than current fuels. Some estimates project $3.00 for the cost of electricity for filling a tank.
  • Reduction or elimination of hazardous chemicals such as gasoline or battery acids/metals
  • With appropriate equipment it is possible to fill compressed air tanks rapidly. However, this requires some means of avoiding the tank heating up due to Adiabatic heating, which would otherwise reduce the amount of gas that can be added, or even cause damage. This either requires active cooling of the tank, which is difficult and will add weight to the vehicle, or cooling the refilling gas. Both of these solutions waste energy.


Like the modern car and most household appliances, the principle disadvantage is the indirect use of energy. Energy is used to compress air, which - in turn - provides the energy to run the motor. Any conversion of energy between forms results in loss. For conventional combustion motor cars, the energy is lost when oil is converted to usable fuel - including drilling, refinement, labor and storage. For compressed-air cars, energy is lost when electrical energy is converted to compressed air.

Further disadvantages:

  • When air is expanded in the engine it cools dramatically (Charles law), and needs to be heated to ambient temperature using a heat exchanger, similar to the Intercooler in internal combustion engines, to obtain a significant fraction of the theoretical energy output. The heat exchanger is practically difficult, as while it performs a similar task to the Intercooler, the temperature difference between the incoming air, and the working gas is smaller and in heating the stored air, it gets very cold, being prone to icing in cold climates. However this does not affect countries with hot temperatures.
  • Refueling the compressed air container using a home or low-end conventional air compressor may take as long as 4 hours. Service stations may have specialized equipment that may take only 3 minutes.
  • Early tests have demonstrated the limited storage capacity of the tanks; the only published test of a vehicle running on compressed air alone was limited to a range of 7.22 km.
  • The key to attaining acceptable range with an air car is to reduce the power required to drive the car, so far as is practical. This pushes the design towards low weight. In a collision between cars of different masses the occupants of a heavy vehicle will, on average, suffer fewer and less serious injuries than the occupants of a lighter vehicle. An accident in a 2000 lb (900 kg) vehicle will on average cause about 50% more injuries to its occupants than a 3000 lb (1350 kg) vehicle. Air cars may use low rolling resistance tires, which typically offer less grip than normal tires. The weight (and price) of safety systems such as airbags, ABS and ESC may encourage manufacturers not to include them.
  • A 2005 study demonstrated that cars running on lithium-ion batteries out-perform both compressed air and fuel cell vehicles more than three-fold at same speeds. MDI has recently claimed that an air car will be able to travel 140km in urban driving , and have a range of 80 km with a top speed of 110km/h on highways, when operating on compressed air alone.

Developers and manufacturers

Various companies are investing in the research, development and deployment of Compressed air cars. Overoptimistic reports of impending production date back to at least May 1999. For instance, the MDI Air Car made its public debut in South Africa in 2002, and was predicted to be in production "within six months" in January 2004. Most of the cars under development also rely on using similar technology to Low-energy vehicles in order to increase the range and performance of their cars.

The AirCar

Air Car Factories SA is proposing to develop and build a compressed air engine. This Spanish based company was founded by Miguel Celades. Currently there is a bitter dispute between Motor Development International, another firm which developed compressed-air vehicles, and Mr. Celades, who was once associated with that firm.


The Energine Corporation was a South Korean company that claimed to deliver fully-assembled cars running on a hybrid compressed air and electric engine. These cars are more precisely named pneumatic-hybrid electric vehicles. Engineers from this company made, starting from a Daewoo Matiz, a prototype of a hybrid electric/compressed-air engine (Pne-PHEV, pneumatic plug-in hybrid electric vehicle). The compressed-air engine is used to activate an alternator, which extends the autonomous operating capacity of the car.

The CEO is the first compressed air car promoter to be arrested for fraud.

A similar (but only for braking energy recovery) concept using a pneumatic accumulator in a largely hydraulic system has been developed by U.S. government research laboratories and industry, and is now being introduced for certain heavy vehicle applications such as refuse trucks.


K'Airmobiles has presented two running prototypes of VPA (Vehicles with Pneumatic Assistance). Their leaders now seek to gain the means of developing several projects of urban or leisure VPP (Vehicles with Pneumatic Propulsion). K'Airmobiles propose a different technology with their VPP , which may allow a reasonable range, generally with compressed air tanks of about 50L-100L/3000 psi capacity only.

These ecological vehicles use the technology of the compressed-air engine K'Air, developed in France by a small group of researchers, which thus proposes a range of projects around an idea: that of the urban or leisure compressed-air vehicles.

K'Airmobiles is the name given to a set of projects relating to "VPA" (Vehicles with Pneumatic Assistance) and "VPP" (Vehicles with Pneumatic Propulsion), These models are conceived like ultra light vehicles (limited to 250 kg max.), and their consumption of compressed air was calculated to remain lower than 120 L/min., although developing a dynamic push able to reach 4kN.

Two VPA prototypes are operational today, the "K'AirBike" and the K'AirKart. Two new VPP prototypes, the one-seater "K'AirTrike" and the three-seater "K'AirMobile Max" are intended for public presentation in October and November 2007 respectively.

MDI and Tata Motors

MDI proposes a range of vehicles developed on an identical concept, made up of MiniCATs and CityCATs.

The MDI MiniCat has a range of up to 1000 miles when fitted with an internal combustion engine, but that drops to 30 miles when used in ZEV mode at low speeds in cities. OneCAT, priced in a range ($5,100 to $7,800) within reach of consumers in a developing economy, such as India.

The ultralight bodies of the vehicles would be made of glued-together fiberglass and injected foam, and the aluminum chassis would also be glued, not welded, to simplify manufacturing.

The engine is available in two versions. The Mono Energy air engine is a true air engine. The Dual Energy engines are Internal combustion engines, which use hydrocarbon fuels.

They have licensed 12 factories : 5 in Mexico, 3 in Australia and New Zealand, 1 in South Africa and 3 in France. MDI Andina S.A is going to sell the car in Colombia, Peru, Ecuador and Panama. MDI has entered into an agreement with Tata Motors, to produce air cars in India. Zero Pollution Motors will be the first company to manufacture and sell a car in the U.S. called the "Air Car" using MDI technology. ZPM will begin taking reservations in early 2009 for US deliveries in early 2010.


As of May 2007 Tata Motors of India planned to launch an MDI air car in 2008.

Electro-Tech Enterprises

Electro-Tech Enterprises is a small company that specializes in electric air vehicles using a new technology discovered by themselves that is currently in the proto-type phase.

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