The Air Car

A while ago I stumbled across the Air Car. It’s one of the alternative power vehicles currently gaining recognition. It uses high pressure compressed air stored in a tank to drive the engine. Its makers claim it can run 200-300 km on one tank. I had a look at the description and specs on the website and it’s pretty interesting. And it’s a fairly simple design. The air-engine weighs less than a similar Internal Combustion Engine (ICE) as it’s made of aluminum rather than higher density alloys which are better able to withstand the high heat of combustion. The air car engine doesn’t have this concern since it runs at ambient temperature.

The way it works is, the high pressure air from the tank is slowly throttled into the engine where it drives pistons. The piston motion powers the vehicle just like in ICE vehicles. The design is simple and apparently maintenance is minimal. The compressed air tank has to be very strong, as the pressure at full capacity is around 4500 psi. The tank is made of carbon fibre which splits open in the case of failure, avoiding the safety concern present in metal tanks which, in the case of a collision, could burst open and send dangerous shrapnel flying.

The benefit of such a vehicle is obvious. There are no emissions. However, it can be a bit noisy. But it can be combined with a hybrid gasoline/diesel or an electric plant and regenerative braking (ref: http://en.wikipedia.org/wiki/Compressed_air_car).

Currently, these cars are mostly suitable for city and in-town driving where the speeds are lower. For highway driving a gas/diesel engine can be switched on, in the case of hybrid models.

You can read more about it here.

Efficiency Of Air Car Compared To Electric Car

We can estimate the efficiency of an Air Car using Thermodynamics, as follows:

First, calculate the amount of energy it takes to fill the tank with compressed air. Assume an ideal isothermal (constant temperature) process where the pressurized air is kept as close as possible to ambient temperature before it enters the tank. This is done by cooling the air. If the air is not simultaneously cooled as it is pumped into the tank it will reach a very high temperature. This is a fundamental property of air (which we can model as an ideal gas – PV = mRT). If you compress air into a smaller volume, it will tend to heat up.

Furthermore, cooling the air (by a passive means, such as by air cooling with a radiator) will actually reduce the amount of energy it takes to pump the air into the tank. By cooling the air beforehand, the pumping energy is a lot less than it would be if the air were not cooled before entering the tank.

It is important to mention, however, that (in the absence of cooling) the extra input (pumping) energy would result in extra output energy as you drive the car, but only if the heated air in the tank could maintain its temperature. But in reality, the heated air would cool down to the ambient temperature. And that extra pumping energy would be for nothing (i.e. it wouldn’t be recovered in the driving phase). So this is one reason why we have to cool the air as close as possible (within practical limits) to ambient temperature, as it enters the tank. This is achieved with multi-stage cooling.

With isothermal pumping of the air into the tank, the (ideal) input energy is, E_in = P₁V₁ln(P₂/P₁), where P₁ and V₁ is the initial pressure and initial volume of the air, respectively, and P₂ is the pressure of the air after it’s pumped into the tank. From the ideal gas equation given above, and given an isothermal process, P₁V₁ = P₂V₂, where V₂ is the volume of air after it’s pumped into the tank (V₂ is equal to the volume of the tank).

Substituting standard values of P₁ = 0.1 MPa (atmospheric pressure), V₁ = 90 m³, V₂ = 0.3 m³ into the two equations above we calculate E_in = 51.3 MJ (mega-joules).

Now, in reality the air heats up somewhat between cooling stages so this “forces” the compressor to use more than 51.3 MJ to pump air into the tank. Based on data from the European Fuel Cell Forum, a realistic efficiency factor for multi-stage cooling is 48% (ref: http://www.efcf.com/reports/E14.pdf). So the actual input energy is E_in = 51.3/0.48 = 107 MJ.

Once in the tank, the air will cool until its temperature reaches the ambient outdoor temperature. The energy of the air at this point will be 51.3 MJ.

Note that 51.3 MJ is greater than the actual energy that will be extracted from the tank to power the car. This is due to thermodynamic (physical) losses. To keep these losses as low as possible it is necessary that the air is heated up as it expands inside the engine, after exiting the tank. Since air has a tendency to cool down upon expanding it is best to keep its temperature as high as possible using the ambient air as a heating source. In other words, we wish to maintain the air temperature as close as possible to its temperature inside the tank. If this could be accomplished perfectly then the extracted energy will exactly equal 51.3 MJ. But in reality this is not the case. The air will unavoidably cool, so to help counteract this, multi-stage heating is used to heat the expanding air as it exits the tank and goes into the engine.

Based on data from the European Fuel Cell Forum, a realistic efficiency factor for multi-stage heating is 84% (ref: http://www.efcf.com/reports/E14.pdf). So the actual output energy is E_out = 51.3×0.84 = 43 MJ.

Therefore, the total efficiency of the Air Car is E_out/E_in = 43/107 = 0.40 = 40%.

One last point to consider is that the air compressor used to fill the tank has to get its energy from somewhere. Ideally, such a compressor would run on electricity using “green” power from the grid. Electric motors are at least 80% efficient. So, the total efficiency from the “wall outlet” electrical energy to the available compressed air energy is 32% (0.40×0.80 = 0.32). Compare this to the efficiency of an electric vehicle. The total efficiency from the wall outlet electrical energy to the available electric motor energy is 68% (assuming a battery storage efficiency of 85% and a motor efficiency of 80% – 0.85×0.80 = 0.68). So, the Air Car is less than half as efficient as an electric car.

Another potential disadvantage of the Air Car is that there is a loss of engine power over time as air pressure in the tank drops. It would be interesting to see the torque curve for the car as the tank discharges.

Practicality Of Air Car Vs. Electric Car

Whether you prefer the Air Car or an electric vehicle will depend on a variety of factors such as personal preference, application, and convenience. For instance, in certain situations water infiltration may be a concern (e.g. flooding) so an air powered vehicle may be more suitable than an electric powered vehicle. Electric vehicles are also a few thousand dollars more expensive than Air Cars so economics is a factor. There’s also a matter of convenience where replacement parts are concerned. In those areas where electronic and electrical components are not as readily available, an air car may be the better choice. But for situations that require driving over long distances, with greater economy, an electric vehicle may be the better choice.

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Cost Of Electric Vs. Gas Powered Vehicles