Compressed air vehicle

A compressed air vehicle is powered by an air engines, using compressed air, which is stored in a tank. Instead of mixing fuel with air and burning in the engine it to drive pistons with hot expanding gases, compressed air vehicles (CAV) use the expansion of compressed air to drive their pistons. The use of that air in the engine is 90 percent efficient.

Compressed air propulsion may also be incorporated in hybrid system, e.g., battery electric propulsion and fuel tanks to recharge the batteries. This kind of system is called a hybrid-pneumatic electric propulsion. Additionally, regenerative braking can also be used in conjunction to this system.

Engine
One can buy the vehicle with the engine or buy an engine to be installed in the vehicle. Typical air engines use one or more expander pistons. In some applications it is advantageous to heat the air, or the engine, to increase the range or power.

Tanks
The tanks must be designed to safety standards appropriate for a pressure vessel, such as ISO 11439.

The storage tanks, usually circular, are commonly made from made of:
 * steel
 * carbon fiber in order to reduce its weight while achieving the necessary strength (a thermoplastic container surrounded by a carbon fiber shell)
 * Kevlar&trade;.
 * Aluminium

One company stores air in tanks at 4,500 pounds per square inch (about 30 MPa) and hold nearly 3,200 cubic feet( around 90 cubic metres) of air.

All four major manufacturers who are developing air cars have designed safety features into their containers as opposed to hydrogen's issues of damage and danger involved in high-impact crashes. Carbon fiber is brittle and splits; but creates no large flying fragments when the tank fails.

The tanks may be refilled at a service station (using volume transfer), or in a few hours at home or in parking lots, plugging the car into the electric grid via an on-board compressor. The cost of driving such car is typically projected to be around €0.75 per 100 km, with a complete refill at the "tank-station" at about US$3.

Compressed air
Compressed air has a low energy density. At 0.14 MJ/L, 300 litre air at 300 bar only amounts to about 12 kWh (the equivalent of 1.4 litre (0.37 gallons) of gasoline). While gasoline or diesel fuel tank have the same amount of energy per litre of fuel from the first to the last litre, compressed air tanks rely on the pressure in the tank, which falls as air is drawn off.

Emission output
Like other non-combustion energy storage technologies, an air vehicle displaces the emission source from the vehicle tail pipe to the central electrical generating plant. Where emissions-free sources are available, net production of pollutants can be reduced. Emission control measures at a central generating plant may be more effective and less costly than treating the emissions of widely-dispersed vehicles.

Since the compressed air is filtered to protect the compressor machinery, the air discharged has less suspended dust in it, though there may be carry-over of lubricants used in the engine.

History
Compressed air has been used since the 19th century to power mine locomotives, and was previously the basis of naval torpedo propulsion.

In 1863, Jules Verne wrote a novel called Paris in the 20th Century about a world of glass skyscrapers, high-speed trains, and air-powered automobiles.

Recently several companies have started to develop Compressed air cars although none have been released to the public, or have been tested by third parties.

Disadvantages
The disadvantages are less well known since the vehicles are currently at the pre-production stage, and have not been extensively tested by independent observers.


 * When the air expands in the engine it will cool down. This will encourage the engine to ice up. However, by the use of a sealed system charged with dry nitrogen instead of nitrogen+oxygen with moisture (common air), this problem, along with corrosion and filtration, may be avoided. The high pressure air (nitrogen) is discharged into a low pressure receiver that starts at a high vacuum.


 * When the air is being compressed at reasonable speeds it heats up. This heat needs to be retained in the air, otherwise the overall efficiency of the cycle is reduced. The heat given off during compression both by the compressed air and by the compressor's electric motor, could be reclaimed for space heating or water heating, or used in a stirling engine.


 * While other common users of compressed gas fuels, such as a propane barbecue or a liquid oxygen rocket engine, use some of the heat of combustion to reheat the gas prior to its use, compressed air offers no reheating. Some auxiliary reheating energy input be needed to increase efficiency. Some designs obtain this heat from the ambient air and from the passenger compartment for air conditioning.


 * The claimed range for many of the cars has not been demonstrated, and seems unlikely from the known laws of physics. Higher charged pressures increase efficiency but require a stronger (heavier or more expensive) pressure reservoir.


 * In the event of an accident the compressed air tanks can pose a hazard.

Advantages
The advantages are well publicised since the developers need to make their machines attractive to investors. Compressed-air vehicles are comparable in many ways to electric vehicles, but use compressed air to store the energy instead of batteries. The principal advantages for an air powered vehicle areTheir potential advantages over other vehicles include:


 * Compressed air technology reduces the cost of vehicle production by about 20%, because there is no need to build a cooling system, fuel tank, spark plugs or silencers (mufflers - USA).
 * Air, on its own, is non-flammable
 * High torque for minimum volume.
 * The mechanical design of the engine is simple and robust, similar to the reciprocating steam engine in use for over 200 years. Vane, wobble plate, and other types of motor-compressors are possible.
 * They do not suffer from the effect of corrosion of batteries in hot, humid climates.
 * Low manufacture and maintenance costs as well as easy maintenance.
 * Compressed-air bottles can be disposed of or recycled with less pollution than batteries.
 * Compressed air vehicles are unconstrained by the degradation problems associated with current battery systems.
 * The tank may be able to be refilled more often and in less time than batteries can be recharged, with re-fueling rates comparable to liquid fuels.

Cars
Several companies are investigating and producing prototypes, and others, plan to offer cars in the beginning of 2008. .

Buses
MDI makes MultiCATs vehicle that can be used as buses or trucks. RATP has also already expressed an interest in the compressed air pollution-free bus.

Locomotives
Compressed air locomotive s historically have been used in mining.

Trams
Various compressed air powered trams were trialled, starting in 1876.

Watercraft and aircraft
Currently, no water or air vehicles exist that make use of the air engine. Historically certain torpedoes were propelled by compressed air engines.