MDI's Air-Powered Prototype, the "Airpod"

It might sound too good to be true, refilling your car with air instead of gas, but it is possible. As long as it’s compressed air we’re talking about, with cars like the “Airpod” from Motor Development International’s and the “OneCAT” from Tata making for very interesting prototypes.

Compressed air cars operate by engines fueled by compressed air, with the air being stored in carbon fiber tanks at very high pressure (around 4500 PSI or 300 bar).
Rather than powering engine pistons with ignited fuel, compressed air cars use the expansion of compressed air, not unlike steam expansion in a steam engine. While the storage tanks are made of carbon-fiber for weight reduction, they’re also very strong, and if penetrated carbon fiber will crack but not produce shrapnel.

The main downsides to the compressed air powered car the lack of compressed air storage (meaning a lack of range) and the problems with keeping the engine warm enough to help the air expand to it’s full potential.
This is because when air expands in the engine it cools dramatically, therefore you need an energy-draining heat exchanger to keep the engine from operating at a cold temperature.

That problem falls into the background however, when you look at the main issue to overcome: a drastically low overall inefficiency. This includes the energy wasted from the compressing of the air, right down to the wheels. This means the overall efficiency of a compressed air vehicle can’t normally get past 14%. That’s even less efficient than a gasoline powered car.

What this equates to in the real world is that compressed air powered vehicles often can’t achieve more than 40 mph, and struggle to get past 80 kilometers of range per fill up.

This is a vital when comparing compressed air cars to electric cars with efficiency levels of anywhere from 60% to 80%. Stanford University recently compared compressed air energy storages to electrically storage energy in batteries, concluding that even under highly optimistic assumptions the compressed air car is less efficient than a battery electric vehicle, and even produces more greenhouse gas emissions than a conventional gas powered car, if the air was compressed by a coal intensive power mix.

Personally I like the compressed air car idea & believe it offers great food for the imagination, but unless compressed air cars increase in efficiency by at least 40%, they’ll never be a threat to the current real-world future of motoring: lithium powered electric vehicles.