Compressed Air Hybrid Vehicles?

by Emil Morhardt (An early compressed air vehicle is shown in the photo above. Not what this paper is talking about though.)

The usual candidate power supplies for the non-fossil-fuel part of hybrid vehicles are chemical batteries, supercapacitors, and flywheels, all powered up using electricity, and generating electricity when their power can usefully replace or supplant the main power source, the internal combustion engine. But these types of electrical storage and the motor/generators they utilize are complex, sophisticated, and expensive, and have barely appeared at all in the developing parts of the world where fossil fuel use is growing fastest. Maybe there is a simpler, cheaper option. One possibility is compressed air energy storage. All you need is a tank (cheap), a reversible compressor (fairly cheap), and a way to link it to the engine. That last part is tricky because the general run of such systems work optimally at a specific pressure, but their performance falls off dramatically as pressures in the tank exceed or fall below optimum as would be expected the tank is being pressurized or depressurized. The simple solution, according to Brown et al. (2014), is to use inexpensive check valves on the tank to prevent over-compression and over-expansion, and an infinitely variable transmission between the compressor and the engine that can operate efficiently at a range of tank pressures. The transmission adjusts by changing the number of thermodynamic cycles of the compressor executed per driveshaft rotation. Continue reading