Air Cars Revisited

Last week I wrote a post about cars powered by compressed air. I was fairly critical of the claims made by MDI, one company developing an air-powered car. Don’t get me wrong; it’s not the technology or innovative efforts that bother me. I think the idea of an air-powered car is pretty cool. Some of the claims made by MDI, on the other hand, do get me a little fired up. I watched the MDI video that I included in my last air car post again, and I am astounded at what it contains. First, MDI neglects (initially) the cost of compressing air and glosses over the energy loss of a compressed air system, but more importantly, they propose a ‘perpetual motion’ system for their car. I called this a red flag earlier. I want to explain why perpetual motion claims are a red flag. This drawing illustrates what they are talking about at about the 2:45 point in the video:

MDI suggests that they should use a compressed-air powered electric generator to power an electric air compressor to refill the air tank. They claim such a car will refuel itself and result in perpetual motion (perpetual motion violates the laws of physics). What do they think this system is going to accomplish? At every stage in the process energy is lost. Electric generators are not 100% efficient; energy is lost to heat and electromagnetic radiation. Air compressors put off a lot of heat, so again there is more energy loss. This system will recharge the air tank with far less compressed air than was used to compress it. The only purpose a system like this serves is to create waste heat! It will not extend the range of an air car, it will reduce it. It will increase overall energy consumption. Whoever proposed this does not understand basic thermodynamics. Does MDI not have any engineers on staff? 

This reminds me of an April Fools post on Autobloggreen that describes a car that uses a miniature wind turbine to power the headlights. This is similarly impractical idea since the energy to turn the wind turbine comes indirectly from the engine that moves the car. Because wind turbines are not 100% efficient, this system would result in worse fuel economy, not better. At least the post about the wind turbine car was a satirical April Fools article, in contrast to MDI’s video. 

I aim to keep most of my writing positive, but claims like this really need to be addressed and exposed for what they are. Either MDI really doesn’t know what it’s doing, or they intentionally are using a deceptive marketing approach. 

I do want to point out that there are some cases where air-powered cars make a lot of sense. There is no local pollution with these powertrains, and the technology is very simple (and low cost). The video below describes another vehicle intended for use indoors, where zero pollution is important. Air cars on factory or warehouse floors may be an especially practical alternative since many of these buildings already have compressed air infrastructure.

The rotory air motor here is a more sophisticated design. It has smoother, more efficient operation than a reciprocating piston motor, like the one used by MDI. Keep in mind, however, that using compressed air to power a car involves a lot of energy loss. When the air is compressed to fill the tank, the air gets hot. This waste heat is lost energy, and is significant. Battery electric vehicles are a better alternative in terms of energy efficiency, but would likely cost more than air cars.

What do you think about air cars? Where else might they be practical transportation alternatives?