Air Cars Have Major Range and Emission Challenges, Says Two New Studies

As an auto writer who specializes in "green" cars, I have been regularly chastised for ignoring the breakthrough that will completely change transportation as we know it. And that is a car capitalizing on technology last seen powering early 20th century short-hop trains in Europe and the U.S.--a zero-pollution vehicle running on nothing but compressed air.

I get queries like this a lot, even though I have written extensively about air cars, just not with the total suspension of belief that the enthusiasts would prefer. My first story on MDI, the French company that says it will put compressed-air cars on the road, was in 2000, and I updated it in 2008, when MDI showed renewed signs of life.

All my reporting has been somewhat skeptical, because engineers have been telling me for a decade that compressed air is simply not a very good energy carrier. Further, the energy expended compressing the air might be better spent on, say, charging up batteries for electric cars. Compressed air--as in those early trains--might work for short-run commuter transit but it is a challenge for any vehicle that needs to make trips of 150 miles or more.

And now two academic papers--the first known on the subject--support that impression. A paper by Andrew Papson, Felix Creutzig and Lee Schipper--all currently or formerly associated with the University of California at Berkeley--concludes that the practical compressed-air vehicle is "severely constrained due to its limited driving range."

That paper will be presented at a Transportation Research Board meeting in January. A second paper, with three of the same authors, was published this week in Environmental Research Letters. It reaches similar conclusions about the efficacy of air cars (though it concludes that a hybrid vehicle using compressed air could be effective).

MDI's business model is interesting, and global in scope. The company has ambitious plans to enter the world's major auto markets simultaneously with manufacturing partners. It also announced, in 2007, a technology partnership with Tata Motors (famous for the $2,500 Nano). A New Paltz, New York company called Zero Pollution Motors holds the U.S. franchise, and CEO Shiva Vencat told me that the new American launch date is approximately 2012. A location has not been chosen, he said, and will depend on the package of state tax breaks and other incentives available.

Air car drives have never been offered to journalists in the U.S. to my knowledge, but many scribes have driven prototypes at MDI's headquarters in Nice, France. There's no question that compressed air works, and the cars make a good impression for television cameras, but range is a major issue. Although Vencat claims the air car (which has been shown in a range of models) can under certain conditions travel 120 miles on a tank of air, Papson's calculations (based, he said, on MDI's published data) say 29 miles is a more likely estimate.

MDI's range estimates zoomed skyward after the company said it had devised an onboard heater (capable of running on many fuels) to expand the air. The use of that heater would negate zero-emission claims, but undoubtedly would produce more range. Vencat said the heater will be on the second-generation air cars, not on the vehicles that are expected to emerge from Nice as early as next year.

Papson says that compressed air, by volume, holds less than one percent of the energy of gasoline. "This is the air car's greatest limitation," he said in an email to me. "You can't drive a vehicle very far on so little fuel! In my simulation of the MDI CityFlowAIR, the vehicle's air tank holds the equivalent of a half gallon of gasoline." And when greenhouse gas emissions per mile driven are calculated, he predicts that an air car's carbon footprint would more than double a gas car and be four times more than an EV.

"Even under optimistic thermodynamic assumptions, the compressed air car is highly inefficient," says Felix Creutzig, now a postdoctoral fellow at the Technische Universitat in Berlin.

I predict that the air car will not be laid low by a couple of academic papers. The car's enthusiasts are true believers and not likely to scan the scientific journals or go to conferences. We may yet see these cars on the road, running on nothing more than the air around us.