By Michael J Coren

Hydrogen. Lithium. Solar. Flywheels. Ethanol. Now add compressed air to the list of possible energy sources that will propel future vehicles. French automaker PSA Peugeot announced this month it will build a hybrid gasoline vehicle that can store energy as compressed air. The air-hybrid car, the AFP reports, should achieve 81 miles per gallon, while emitting just a fraction of the carbon dioxide per mile of conventional vehicles. "This breakthrough technology...represents a key step towards the two-liter-per-hundred-kilometer car by 2020," said Chief Executive Philippe Varin at a press conference.

The "breakthrough," however, has been a long time coming. Like many auto technologies, the first versions were tested in the mid-1800s without ever gaining much traction. Although materials and engineering are better now, today's engines work in a similar way. Fresh air is pumped into a chamber under high pressure, and then released into "combustion" chambers where the air forces down pistons and turns the wheels.

It would seem that compressed air has plenty going for it: It's compact, emissions-free (except for the energy to compress the air) and dirt cheap compared to the typical $11,000 lithium battery pack. But that has not inspired many carmakers to jump on the technology.

It turns out it's hard to match the efficiency or the energy density of rival batteries or liquid fuels: "Air compressors are one of the least-efficient machines to convert electricity to work," said Harold Kung, professor of chemical and biological engineering at Northwestern University, to the AP. "Why not use the electricity directly, as in electric cars? From an energy utilization point of view, the compressed (air) car does not make sense." And Indian carmaker Tata has reported they've struggled to get enough compressed air energy into their Airpod (although the buglike three-seater should go on sale soon).

Despite their drawbacks, compressed-air cars (you can see a line developed by the firm MDI) are cheap, straightforward to design, and demand little in the way of exotic technology or infrastructure. That, at least, has been enough to convince Peugeot that an affordable compressed-air car offers another alternative to today's hybrids.

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