Compressed-air vehicle

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

Compressed air propulsion may also be incorporated in hybrid systems, 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 with this system.

Technology

Engine

Main article: Compressed air 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

Main article: Compressed air tank

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

The storage tank may be made of:

• steel,
• aluminium,
• carbon fiber,
• Kevlar,
• other materials, or combinations of the above.

The fiber materials are considerably lighter than metals but generally more expensive. Metal tanks can withstand a large number of pressure cycles, but must be checked for corrosion periodically.

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.

The tanks may be refilled at a service station equipped with heat exchangers, 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 a 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

Main article: Compressed air

Compressed air has a low energy density. In 300 bar containers, about 0.1 MJ/L and 0.1 MJ/kg is achievable, comparable to the values of electrochemical lead-acid batteries. While batteries can somewhat maintain their voltage throughout their discharge and chemical fuel tanks provide the same power densities from the first to the last litre, the pressure of compressed air tanks falls as air is drawn off. A consumer-automobile of conventional size and shape typically consumes 0.3-0.5 kWh at the drive shaft per mile of use, though unconventional sizes and may perform with significantly less.

Emission output

Like other non-combustion energy storage technologies, an air vehicle displaces the emission source from the vehicle's 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.

In 1903, the Liquid Air Company located in London England manufactured a number of compressed air and liquified air cars. The major problem with these cars and all compressed air cars is the lack of torque produced by the "engines" and the cost of compressing the air. Reference: http://www.didik.com/ev_hist.htm

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

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

Some Disadvantages could include:

• Compressed air vehicles likely will be less robust than typical vehicles of today. Which poses a danger to users of compressed air vehicles sharing the road with larger, heavier and more rigid vehicles.

• When the air is expanded in the engine, it will cool down via adiabatic cooling and lose pressure thus its ability to do work at colder temperatures. It is difficult to maintain or restore the air temperature by simply using a heat exchanger with ambient heat at the high flow rates used in a vehicle, thus the ideal isothermic energy capacity of the tank will not be realised. Cold temperatures will also encourage the engine to ice up.

• Very little is known about air powered vehicles thus far.

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. Their potential advantages over other vehicles include:

• Much like electrical vehicles, air powered vehicles would ultimately be powered through the electrical grid. Which makes it easier to focus on reducing pollution from one source, as opposed to the millions of vehicles on the road.
• Transportation of the fuel would not be required due to drawing power off the electrical grid. This presents significant cost benefits. Pollution created during fuel transportation would be eliminated.
• 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.
• Air, on its own, is non-flammable.
• High torque for minimum volume.
• The mechanical design of the engine is simple and robust.
• Low manufacture and maintenance costs as well as easy maintenance.
• Compressed-air tanks 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.
• Lighter vehicles would mean less abuse on roads. Resulting in longer lasting roads.
• The price of fueling air powered vehicles will be significantly cheaper than current fuels.

Other effects

• Many entities would suffer from the adoption of air powered vehicles. Federal, state and local governments possibly would lose significant income which was gained from taxes on high priced fuel. Fueling stations would have to invest in new dispensers. The oil industry would suffer the effects of less demand.

• Adoption of a new fuel would have a turbulent effect. Jobs would be lost and gained.

Possible Improvements

• 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 might be needed to increase efficiency. Some designs obtain this heat from the ambient air and from the passenger compartment for air conditioning.

• 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 could be reclaimed for space heating or water heating, or used in a stirling engine.

Vehicles

Mopeds

Jem Stansfield, an English inventor has been able to convert a regular scooter to a compressed air moped. This has been done by equipping the scooter with a compressed air engine and air tank.

Cars

Main article: Compressed air car

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 locomotives have been historically used as mining locomotives.

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.

See also

• Alternative propulsion
• Airbus Industries, compressed air tanks maker.
• Air engine
• Charging station
• Compressed air energy storage
• Compression ratio
• Eolocompressor
• Global warming
• Plug-in hybrid
• Pneumatics

External links

• cair, an external wiki
• Studies: Aluminum Frames, "Air Hybrids" Yield Fuel Savings (EERE).
• Hydraulic Hybrid Research (EPA)
• OSEN page about Compressed Air Technology
• Compressed air vehicles history
• Photograph of the 1903 Liquid Air Company car