14-06-2013, 12:22 PM
Compressed air car
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Technology
Engines
Compressed air cars are powered by motors driven by compressed air, which is stored in a tank at high pressure such as 30 MPa (4500 psi or 310 bar). Rather than driving engine pistons with an ignited fuel-air mixture, compressed air cars use the expansion of compressed air, in a similar manner to the expansion of steam in a steam engine.
There have been prototype cars since the 1920s, with compressed air used in torpedo propulsion.
Storage tanks
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 Seven Network's Beyond Tomorrow that on its own,[clarification needed] carbon-fiber is brittle and can split under sufficient stress, but creates no shrapnel when it does so. Carbon-fiber tanks safely hold air at a pressure somewhere around 4500 psi, making them comparable to steel tanks. The cars are designed to be filled up at a high-pressure pump.
Energy density
Compressed air has relatively low energy density. Air at 30 MPa (4,500 psi) contains about 50 Wh of energy per liter (and normally weighs 372g per liter). For comparison, a lead–acid battery contains 60-75 Wh/l. A lithium-ion battery contains about 250-620 Wh/l. Gasoline contains about 9411 Wh per liter;[1] however, a typical gasoline engine with 18% efficiency can only recover the equivalent of 1694 Wh/l. The energy density of a compressed air system can be more than doubled if the air is heated prior to expansion.
In order to increase energy density, some systems may use gases that can be liquified or solidified. "CO2 offers far greater compressibility than air when it transitions from gaseous to supercritical form."[2]
Emissions
Compressed air cars are emission-free at the exhaust. 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. 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 over time, thereby improving or worsening total emissions.
However a study showed that even with very optimistic assumptions, air storage of energy is less efficient than chemical (battery) storage.[3]
Advantages
The principal advantages of an air powered vehicle are:
• Refueling could be done at home using an air compressor[4] 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. However, compressors for 250-300 bars are not normally available for home standard utilization, considering the danger inheritant to these pressure levels.
• Compressed air engines reduce the cost of vehicle production, because there is no need to build a cooling system, spark plugs, starter motor, or mufflers.[5]
• 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.
• Expansion of the compressed air lowers its temperature; this may be exploited for use as air conditioning.
• Reduction or elimination of hazardous chemicals such as gasoline or battery acids/metals
• Some mechanical configurations may allow energy recovery during braking by compressing and storing air.
• Sweden’s Lund University reports that buses could see an improvement in fuel efficiency of up to 60 percent using an air-hybrid system[6] But this only refers to hybrid air concepts (due to recuperation of energy during braking), not compressed air-only vehicles.
Disadvantages
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. Any conversion of energy between forms results in loss. For conventional combustion motor cars, the energy is lost when chemical energy in fossil fuels is converted to mechanical energy, most of which goes to waste as lost heat. For compressed-air cars, energy is lost when chemical energy is converted to electrical energy, when electrical energy is converted to compressed air, and then when the compressed air is converted into mechanical energy.
Crash safety
Safety claims for light weight vehicle air tanks in severe collisions have not been verified. North American crash testing has not yet been conducted, and skeptics question the ability of an ultralight vehicle assembled with adhesives to produce acceptable crash safety results. Shiva Vencat, vice president of MDI and CEO of Zero Pollution Motors, claims the vehicle would pass crash testing and meet U.S. safety standards. He insists that the millions of dollars invested in the AirCar would not be in vain. To date, there has never been a lightweight, 100-plus mpg car which passed North American crash testing. Technological advances may soon make this possible, but the AirCar has yet to prove itself, and collision safety questions remain.[14]
The key to achieving an acceptable range with an air car is reducing the power required to drive the car, so far as is practical. This pushes the design towards minimizing weight.
According to a report by the U.S. Government's National Highway Traffic Safety Administration, among 10 different classes of passenger vehicles, "very small cars" have the highest fatality rate per mile driven. For instance, a person driving 12,000 miles per year for 55 years would have a 1% chance of being involved in a fatal accident. This is twice the fatality rate of the safest vehicle class, a "large car". According to the data in this report, the number of fatal crashes per mile is only weakly correlated with the vehicle weight, having a correlation coefficient of just (-0.45). A stronger correlation is seen with the vehicle size within its class; for example, "large" cars, pickups and SUVs, have lower fatality rates than "small" cars, pickups and SUVs. This is the case in 7 of the 10 classes, with the exception of mid-size vehicles, where minivans and mid-size cars are among the safest classes, while mid-size SUVs are the second most fatal after very small cars. Even though heavier vehicles sometimes are statistically safer, it is not necessarily the extra weight that causes them to be safer. The NHTSA report states: "Heavier vehicles have historically done a better job cushioning their occupants in crashes. Their longer hoods and extra space in the occupant compartment provide an opportunity for a more gradual deceleration of the vehicle, and of the occupant within the vehicle... While it is conceivable that light vehicles could be built with similarly long hoods and mild deceleration pulses, it would probably require major changes in materials and design and/or taking weight out of their engines, accessories, etc." [15]