Compressed Air Vehicles can be a potential mode of urban transport in India

The environmental challenges posed by passenger cars are significant and require a broad range of evolutionary and revolutionary solutions. Gasoline (petrol), which is the most-used fuel in such cars, is responsible for vehicular pollution as it produces carbon dioxide, nitrogen oxide, and unburned hydrocarbons.

A possible alternative to the gasoline-power car is one which runs on air power. Air, which is abundantly available and is free from pollution, can be compressed to higher pressure at a low cost. Compressed Air-powered Vehicles (CAVs) are environment-friendly and provide an effective method of power production and transmission.

Compressed air has been used since the 19th century to power mine locomotives and trams in cities such as Paris and was previously the basis of naval torpedo propulsion. Compressed air was also used in some vehicles for boosting the initial torque or rotary motion. In the 1970s, Willard Truitt invented CAV but sold the design to the US Army & NASA because of financial constraints. In 1979, Terry Miller invented the air car and patented it.

In 2007, Tata Motors signed an agreement with the Motor Development International (MDI), a French firm, to roll out a car that would run on compressed air. The AIRPod is one of five derivative vehicles designed by the MDI based on compressed air engines. The MDI developed two versions: a single fuel engine that relies solely upon compressed air, designed only for urban areas (like the AIRPod) and a duel fuel version that uses compressed air and a combustible fuel. The MDI has claimed that an air car would be able to travel 140 kiometres (km) in urban driving, and have a range of 80 km with a top speed of 110 km per hour on highways when operating on compressed air alone, besides promoting 6 CAV models ranging from single passenger cars to 6-seater urban minibuses.

According to reports, Tata Motors' new car that is powered by compressed air technology could be launched in India in three years time. The manufacturer has also successfully completed the first phase of the project. The second stage of the detailed development started a few years ago. Air-powered cars will weigh below 907 kg, which will make them more fuel-efficient. The AIRPod concept can be driven with the help of a joystick and only costs Rs 70 per 200 km. The production model of the AIRPod will have a top speed of more than 65km/hr. 

The CAV fuel cycle is conceptually simple: air is compressed to high pressure at a stationary compressor station, transferred to an on-board storage tank, and slowly released to power a pneumatic motor. The motor converts air power to mechanical power, which is transferred to the wheels and is used to operate the vehicle. In this way, compressed air acts not as an energy source like gasoline but as an energy storage medium similar to an electric battery.

There are advantages as well as disadvantages of the CAV technology.

CAV technology reduces the cost of vehicle production by about 20 per cent, because there is no need to build a cooling system, fuel tank, ignition systems or silencers. The engine can be reduced in size. The engine runs on cold or warm air, so can be made of lower strength light weight material such as aluminium, plastic, low friction teflon or a combination. Compressed air tanks can be disposed of or recycled with less pollution than batteries. The air tank may be refilled more often and in less time than batteries can be recharged, with re-filling rates comparable to liquid fuels. Lighter vehicles cause less damage to roads, resulting in lower maintenance cost.

The major problem with all compressed air cars is the lack of torque produced by the "engines" and the cost of compressing the air. The principal disadvantage is the indirect use of energy. Energy is used to compress air, which in turn, provides the energy to run the motor. For compressed air cars, energy is lost when electrical energy is converted to compressed air, and when fuel is burned to drive the electrical generators. Refueling the compressed air container using a home or low-end conventional air compressor may take as long as 4 hours, while the specialised equipment at service stations may fill the tanks in only 3 minutes. Tanks get very hot when filled rapidly. However, if well-insulated, the heat would not be lost but put to use when the car is running.

Proponents of this technology claim that CAVs are greener and cheaper to operate since they do not consume fossil fuels and produce zero tailpipe emissions while offering the power and performance needed for light-duty vehicle use. A research report (P Saiprasanna Kumar et al) suggests that CAVs are the best options which provide the most comprehensive answer to the present urban pollution problems. They are clean, easy to drive, comparatively low cost and do not take a lifetime payoff. Their speed, range and power are limited now, so further research could provide more effective results.

Researchers Pepson, Felix & Schipper observe that although the concept of CAVs has received great attention in the popular press, there have been few studies evaluating the potential of air cars as an alternative to conventional vehicles. Today, CAVs take the form of lightweight passenger cars designed for slow speed city driving. However, unlike those fuels, the efficiency of a CAV is largely dictated by the thermodynamic properties of gases with accompanying inefficiencies of compression and expansion.

It is pertinent to mention that passenger transportation faces very strong challenges including emission of greenhouse gases, health hazards and high import dependence of petroleum products. Several evolutionary solutions are being developed to reduce the impact of motor vehicles, such as increased fuel economy standards and the accelerated adoption of hybrid vehicles. One new approach is found in CAVs which addresses acute urban pollution problems. CAVs will likely become the potential mode of urban transportation in the future. However, further research on their speed, range and power would be needed to make that possible.

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