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.
Nobody is doing this yet in road vehicles, the authors believe, because the standard approach uses the engine as a compressor and requires the addition of variable valve timing with expensive actuators, rather than the relatively inexpensive addition of a dedicated external compressor and transmission. Although the efficiency of the proof of concept system they cobbled together is not very high—only about 10% of the energy converted to compressed air comes back to drive the engine—the authors figure that if the exhaust heat of the engine were used to keep the air tank hot (which would cost nothing in terms of energy) a round trip efficiency of 47% might be realized at a cost lower than that of battery hybrids, and in a package that would last a long time and could be repaired using mechanical capabilities common in the developing world.
Brown, T., Atluri, V., Schmiedeler, J., 2014. A low-cost hybrid drivetrain concept based on compressed air energy storage. Applied Energy 134, 477-489.