Conventional cars work by exploding fuel and air in a chamber to exert force on the pistons, but what if rather than using an explosion to provide that pressure on the piston, you just stored your ‘fuel’ at a constant high pressure. That’s the concept behind the air car, and it looks like a pretty promising technology. Like Hydrogen the air is not actually the source of energy, it is the storage medium, but unlike Hydrogen the production of an air powered car is surprisingly cheap. According to Wikipedia, without the need for a cooling, ignition or starter system these vehicles can be produced for approximately 20% cheaper than conventional car engines. That factor should not be underestimated since up until now, greener options have always seemed to come at a premium. Unlike battery electric vehicles a full tank of air will stay more or less full for a long time, they have very low discharge rates. Again compared to battery electrics the storage medium (tank) is not limited by a shelf life or number of cycles and therefore reduces waste. As the air expands in the engine it actually cools it, which can be used for passenger comfort. The car itself has no emissions since it just uses normal air (the air might actually end up cleaner because of filtration) and the compression can be done using renewable energy to make the entire system emission free. It can even run underwater if need be! This technology is honestly quite promising, but like the other ‘green’ options will this fail to stack up when it is looked at more closely?
Well the answer is yes and no. The cars certainly have their limitations. Range and power are obvious hurdles since the air-tank stores the equivalent of about a gallon of gasoline, the advertised range for an MDI air car is 125km (78 miles, possibly optimistic?), though the addition of conventional fuels may change this dramatically to over nearly 1000 miles (I'd be very surprised if that were true but time will tell) and apparently boosting the top speed from 35mph (55km/h) to 100mph (160km/h). According to this interview it uses the liquid fuel to heat the air 'increasing the volume' to extend the range. Having air stored at 300 Bar (4350psi) under your seat is a little disconcerting but the experts assure us that carbon fibre will do the job, air isn't flammable on its own so there is no fire risk. Compressed air cars have the same chicken and egg infrastructure problem as Hydrogen cars, since standard compressors at service stations don't go anywhere near the required psi and the on-board compressor takes approximately 4 hours to charge. When the air is compressed at the compressing station the process also heats the air as a byproduct. This can effect range since when the air cools in the tank the pressure will also drop. More energy can either be used to cool the air before it goes into the car or the heat can be recycled eg. for domestic hot water. The cooling effect of the engine can cause ice formation especially at lower ambient temperatures and requires addition fuel for heating the engine. But efficiency wise compressed air cars perform better than any of the alternatives we have seen in this series. According to the efcf, the cars can have a theoretical compression efficiency of 46% and a tank to wheel efficiency of up to 84%, the report concludes that given 100MJ of energy a Hydrogen FCEV could travel 42km, an air car 46 and a Li-ion BEV an impressive 133km.
Compared to the other alternatives we have looked at, the compressed air car certainly has some impressive advantages. It is obviously superior to liquid fossil fuels since it can be cheaper to build and has the potential to be renewable and emission free. They don't have the wider environmental and humanitarian consequences of biofuel vehicles. Provided the range and power issues are resolved, it betters Hydrogen on both efficiency and production cost. Its only rival in the alternative fuel game is the battery electric car, which trumps it on efficiency but not on production costs and battery life. The widespread adoption of compressed air cars I think will depend a great deal on public preference rather than the technology itself (unless it manages to exceed battery electric efficiency with clever thermodynamic tricks, or battery cost and life can be drastically reduced). Will consumers choose lower vehicle production and maintenance costs despite the increase in infrastructure and energy usage? Or perhaps will they decide that the higher vehicle costs of battery electrics are worth the price for the higher efficiency?