Cars that don't use internal- or external-combustion engines certainly are hot news items. The Wall Street Journal reported this week that General Motors is looking into who "leaked" photos of their upcoming Chevrolet Volt battery-powered car. Some say GM did it, to keep the PR rumor mill stoked (I'm shocked, shocked!), though GM denies it. Who knows? It doesn't really matter: the reality is that a) the Volt is "stylish" (no surprise there), b) the final car released for production may look quite different, and c) most important, what we really want to know about is its internal design and BOM, not the flowing body lines!
Also this week, an article in the New York Times reported that a European company called Motor Development International (operating as Zero Pollution Motors in the U.S.) had developed 2-cylinder car that could run on tanks of compressed air (coming from where?). The article was skeptical and cited several engineering professors who pointed out that when you go through the numbers, you find the energy storage capacity and density of a reasonably sized compressed-air tank isn't very high, even at fairly high pressures. Also, there also are numerous operating inefficiencies in such as setup, as well as issues with pressurizing the tank. Potential range and speed would be low, while fixed and operating costs would be high.
Interestingly, compressed-air is a viable vehicle-power source in some applications; it is used today in some specialty vehicles and off-road trucks where relatively short bursts of substantial peak power are needed. These "hydraulic-powered" vehicles, though, carry an on-board compressor to charge the tank, which in turn is powered by a conventional internal-combustion engine. The tank acts as an accumulator, as the hydraulic analog to a capacitor.
But a compressed-air car does have that "something for nothing" and "it's air, so it's free" sound to it, for sure!