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.Several companies including Tata Motors have signed a deal with MDI to produce the air cars by the summer of 2008.
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 under high pressure such as 30 MPa (4500 psi or 300 bar), following ISO 11439. 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. It has been used since 1930s in cars and in Formula 1 as a start-up engine, as well as for the propulsion of torpedoes. While gasoline or diesel fuel tanks have the same amount of energy per litre of fuel from the first to the last litre, compressed air motors rely on the pressure within the tank, which decreases as air is drawn off. Because the air flows through a pressure regulator before going to the engine, no reduction in power would be noticed until the tank was nearly exhausted. This is the case for electric vehicles, too. This process of pressure regulation will reduce the energy available due to the Second Law of Thermodynamics.
Compressed air is a heavy way of storing fuel, 300l air at 30 MPa (300 Bar) about 16 kWh  (the equivalent of 1.7 liters (0.44 gallons) 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. Though no company has yet demonstrated the effectiveness of an imploding engine (ZAP) vs. a quick release (MDI) standard, and other safety designs, it is expected that large-scale production may lead specific governments to set their own standards. It was reported on Discovery's Beyond Tomorrow that on its own, carbon-fiber is brittle and splits, but creates no shrapnel.
Compressed air cars are emission-free at the 'tailpipe', but the energy, usually electric, required to recharge the compressed air tanks may come from the combustion of fossil fuels. Unless all of the energy comes from non-combustion sources, like nuclear power, wind power, solar power, or hydroelectricity, compressed air cars will still cause net emissions of greenhouse gas, however not as much as from fossil fuels . Also, emissions would be relocated from city streets to where power generators reside. CO2 can potentially be sequestered from these large plants. Of course a solar and/or wind recharging system can be installed in one's home, in the car or power plants, thereby eliminating the dependence on the consumption of nuclear-fossil electricity. When energy from biomass is used as the energy source with CO2 removal, neutral emissions of CO2 would -- in theory -- be the result. The same is true of traditional internal combustion vehicles run on biofuels.
An air motor also releases air cleaner than its intake, due to the presence of an air filter to keep contaminants out of the mechanism. Consequently, this can be viewed as air purification instead of air pollution.
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 principle 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 lbs. is a reasonable design goal.
- One manufacturer promises a range of 200 kilometers 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.
- Compressed air is not subject to fuel tax.
- 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 only $3.00 for the cost of electricity for filling a tank..
Just like the modern car and most household appliances, the principle disadvantage is that of indirect energy use. Energy is used to compress air, which - in turn - provides the energy to run the motor. Any indirect step in energy usage 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.
- According to thermodynamics, when air is expanded in the engine, it cools via adiabatic cooling and thereby loses pressure, reducing the amount of power passed the engine at lower temperatures. Furthermore, it is difficult to maintain or restore the temperature of the compressed or compressing air using a heat exchanger due to the high rate of flow. The ideal isothermic energy capacity of the tank will therefore not be realized. Low temperatures may also encourage the engine to ice up.
- 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. 
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.
Air Car Factories SA is proposing to develop and build a compressed air engine.
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), aiming to escape the constraints from thermodynamics. To do so, 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.
The technical concept of the K'Air pneumatic engines returns to direct conversion of what makes the fundamental characteristic of compressed air, namely:
- the pushing force of compressed air is exclusively exploited for conversion into kinetic energy of translation,
- itself is simultaneously converted into induced power of rotation of the axis and
- thus gives to the engine a particularly imposing torque while needing only a very low “fuel” consumption.
To simplify, one can compare the principle to that of the rotary jacks:
- the energy of the fluid (compressed air) is directly transformed into rotational movement;
- the double-acting jacks involve a pinion-toothed rack system;
- the cyclic angle of rotation can vary between 90 and 360°;
- it supports hydraulic supercharging systems.
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..
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.
Team Psycho-Active (TPA) has competed in the Automotive X PRIZE. TPA is working to develop the EPOCH: a highly customizable and serviceable pneumatic / multi-fuel hybrid. At its core is the DBRE engine. The DBRE's unique characteristics allow the EPOCH to use most commercially available fuels and/or compressed air.
The Quasiturbine is a prototype for a hybrid engine which, according to its originators, could function with an explosive fuel and with compressed air alike.
Hinged Rotor Internal Combustion Engine
Despite its name, this patent (US 6,718,938) describes an engine very similar to the Quasiturbine, and equally applicable to compressed air technology. The main difference is the much simpler method used to transfer rotor power to the central driveshaft.