Why aren't we using compressed air cars?

Question

Today cars are predominantly powered by an internal combustion engine burning liquid fuels, electricity from a battery or both. Niche cars may use compressed methane or compressed hydrogen.

Electric cars require an expensive battery and expensive power electronics. They are different from internal combustion engine cars in a very major way, but if the battery costs can be reduced, can prove to be a giant leap in mobility.

However, there's a way that allows cars to be powered by grid electricity in a way that doesn't require as major changes to the drivetrain as electric cars require. The internal "combustion" engine can be run with compressed air. The fuel tank is replaced by a compressed air tank, and compressed air then runs the engine that doesn't require any spark plugs or exhaust system parts.

As far as I understand, compressed air cars have been tested. The technology is not rocket science. But yet we can't see any commercial cars that would utilize the technology. Why not? Why is a compressed air car such a bad idea that it isn't available in the way niche cars like hydrogen fuel cell vehicles are?

Answer 1

There are two reasons I know why compressed air isn't a useful source of motive power.

Loss of pressure.
When air is released from the tank, the reservoir cools, and less pressure is available. So performance drops, even without the loss of pressure due to the tank being depleted. Whereas the performance of an ICE is constant right up until 'empty'.

Energy density.
The energy density of compressed air is only a tiny fraction of petroleum. The link states

• Petroleum (gas): 34.2 MJ/L
• Compressed air: 0.2 MJ/L

Answer 2

Compressed air cars are not generally available, because the technology suffers from a fatal flaw. Compressed air cars are charged simply by compressing air, but the charging is done many hours or even days before the car is driven.

When air is compressed fast, it undergoes adiabatic compression. Part of the energy is stored in the compressed air and part of the energy is stored into heating the compressed air.

Because the charging is done many hours or even days before the car is driven, the heat in the compressed air has time to dissipate. So although the fast charging was done with adiabatic compression, the end result is similar to isothermal compression, in that the temperature at start and at end (after heat dissipation) is the same. I'm not experienced enough in thermodynamics to say whether it's only similar to isothermal compression, or actually equivalent to isothermal compression.

Then when the car is driven, the energy in the compressed air is utilized very fast. It undergoes adiabatic expansion where it cools. The problem is, this cooling robs the efficiency of the compressed air engine. Also because the expansion is adiabatic and not isothermal, the part of the energy that was lost into the heating of the surrounding environment is lost forever.

If a car is driven very slow, let's say at walking speed in level ground, theoretically the energy usage rate could be so low that a very big heat exchanger could change the adiabatic expansion into isothermal expansion, where the expansion is able to utilize the heat that was lost into the surroundings again.

But that's not how cars are driven. Cars are driven often very fast, often up hills too. Acceleration of the big >1000 kg object is fast, so lots of energy is used in a very short timeframe. There is no big enough heat exchanger that could allow isothermal expansion to happen quickly enough. So it's adiabatic expansion only, and adiabatic expansion doesn't utilize the part of the energy that was lost to surroundings as heat.

Compressed air systems have been used in grid energy storage. According to Wikipedia, the McIntosh, Alabama compressed air grid energy storage system requires 2.5 MJ of electricity and 1.2 MJ of natural gas used for reheating for 1 MJ of energy output. That's a terribly poor energy efficiency, even lower than hydrogen vehicles. Note the natural gas for reheating is necessary, since some way to heat the cooling expanding air is needed.

Because of this need for heating, a compressed air energy storage system should be viewed only as a more efficient way to utilize fuels, not as some fuel-free way like what electric batteries are. And if you already have fuels on board, it's just simpler to use those fuels for all of the propelling energy.