Could Liquid Nitrogen Work in a Compressed-Air Motor?

Automobile researchers have been looking into many alternatives to the petroleum-powered internal combustion engine [ICE], including electric, hydrogen, and even compressed air. What is an ICE, in its simplest terms, but a air pump? Compressed-air motor [CAM] vehicles, though, are light, slow, and not very practical for the average user.

Liquid Nitrogen [LN2] could be a viable replacement in a compressed-air motor because of some excellent properties. First, LN2 is not reactive at all, making it very safe. Secondly, under pressure, LN2 takes up about 1/700^th the space of the uncompressed gas. The stored energy in LN2 has to do with it’s boiling point, which makes it an excellent “fuel” for a CAM vehicle.

Because LN2 boils at -196°C [-321°F], after compression and cooling, it must be kept cold and pressurized. On exposure to the atmosphere and ambient temperatures, LN2 instantly expands 700x its original volume, and if in a confined space, with catastrophic results. In 2006 at Texas A&M University, a LN2 tank pressure-relief device failed. The tank failed and blew through the ceiling above it.

Harnessing the gaseous expansion of Liquid Nitrogen, much like the expansion of fuel when ignited, could be an ideal situation in a compressed-air motor, realizing a number of benefits. First, LN2 is cold, so a CAM running LN2 would require no cooling system. Second, a CAM running LN2 could be made from high-strength plastics, reducing weight and increasing efficiency.

Thirdly, LN2 has a much higher energy potential than traditional compressed air. 20MPa of compressed-air occupies about 5 liters, while 20MPa of Liquid Nitrogen only occupies about 1.4 liters, meaning that the range of a CAM vehicle running LN2 could be 3.5x greater.

Finally, since the expansion occurs in the cylinder, the pressure is controlled by the injection of Liquid Nitrogen, not by the pressure in the compressed-air tanks. Thus, the compressed-air motor running on LN2 could be designed to run on a constant pressure, instead of the variable pressure as traditional compressed-air bleeds off.