Are compressed air / gasoline hybrid
vehicles (CAV)
the future or urban transportation as reported
in MIT's Technology Review? These cars use isothermal
compression of regular air in carbon-fiber tanks at 4500 psi.
From high school chemistry, you know that PV = nRT.
As the air is compressed into the tank, the temperature will tend to
increase. With the air car, this heat is given off as the pressure
increases, keeping PV = nRT =
constant. When the air is later released, and allowed to
expand to power the engine, the temperature drops dramatically (known
as adiabatic expansion). This cools the air inside the engine, and
reduces performance by lowering the pressure that would otherwise
be achieved. Therefore, the cold air is heated before entering the engine, and while in the engine, by exchanging heat with the outside air (extracts energy from the environement on a hot summer day). To further increase performance and boost range, the compressed air energy storage tanks can be supplemented with regular petrol/gasoline. A gasoline burner can be used to heat the compressed air before it enteres the engine to boost the pressure. In theory, you can burn gasoline with very high efficiency this way since much of the heat is captured in the Carnot cycle. In a regular internal combustion engine, much of the energy is converted to heat and lost.
Compressed air vehicles do have some
drawbacks, however. Firstly, isothermal compression stores energy as
the logarithim
of pressure.
Energy =
nRT·ln(P)
In other words, if you double the pressure in your tank, you won't
double the amount of energy, or the range of your vehicle. Consider the
plot below of the natural logarithim, ln(P),
function:
Clearly, after the
pressure reaches a few hundred times atmospheric pressure, (~300 bar),
you would have to increase the pressure tremendously to see any
noticable increase in the amount of energy stored. Therefore, the only
way to actually increase the performance of an air car is to add more
air tanks (to increase the volume, not the pressure of the compressed
air). Due to material limitations, safety regulations, and the
logarithimic nature of energy storage, existing tanks are limited to
about 4500
psi.
Despite these difficulties, CAVs have some tremendous advantages. They are inexpensive to build, are extremely light weight, contain no toxic batteries, and can be recharged within minutes. Tata Motors, in conjuction with MDI, which recently announced the world's cheapest car, claim their OneCAT CAV will achieve 106 mpg when it goes on sale, hopefully by 2009. With a range of nearly 900 miles on a single 8 gallon tank of gas, and a top speed of up to 96 mph, this technology will give hydrogen, electric and hybrid vehicles a serious run for their money in developing nations and urban environments. Tata Motors, India's largest car manufacture, is listed on the NYSE, symbol TTM.
Despite these difficulties, CAVs have some tremendous advantages. They are inexpensive to build, are extremely light weight, contain no toxic batteries, and can be recharged within minutes. Tata Motors, in conjuction with MDI, which recently announced the world's cheapest car, claim their OneCAT CAV will achieve 106 mpg when it goes on sale, hopefully by 2009. With a range of nearly 900 miles on a single 8 gallon tank of gas, and a top speed of up to 96 mph, this technology will give hydrogen, electric and hybrid vehicles a serious run for their money in developing nations and urban environments. Tata Motors, India's largest car manufacture, is listed on the NYSE, symbol TTM.