A Hybrid Air Car  

Technology Review has an article on a variant of the air car idea - a hybrid vehicle using compressed air for energy storage - Hybrids Powered by Air.

A new kind of hybrid vehicle being developed at the Swiss Federal Institute of Technology in Zurich could save almost as much fuel as today's gas-electric hybrids, but at a fraction of the cost. Swiss researchers will present the results of experiments with a test version of the new system at the Society for Automotive Engineer's Congress in April.

Conventional gas-electric hybrids use batteries to store energy recovered during braking, which would otherwise be wasted as heat. They later use that energy to drive an electric motor that assists the car's gas engine. But the high-cost of batteries, and the added cost of including two forms of propulsion -- an electric motor and a gasoline engine -- make such hybrids expensive. This has slowed their adoption and limited their impact on overall greenhouse gas emissions from vehicles.

Lino Guzzella, a professor of mechanical engineering at the Swiss Institute, is developing a hybrid that requires no battery or electric motor. Instead, it stores energy by using the engine's pistons to compress air. That air can later be released to drive the pistons and propel the vehicle along. Guzzella says that the system will add only about 20 percent to the cost of a conventional engine, whereas the extra components required in hybrid electric vehicles can add 200 percent to the cost. Computer simulations suggest that the design should reduce fuel consumption by 32 percent, which is about 80 percent of the fuel-savings of gas-electric hybrids, he says. Initial experiments have demonstrated that the design can be built.

The overall idea of air (or pneumatic) hybrids isn't new, but making them efficient has been challenging. "It's difficult to keep the [energy] losses involved in moving air around small enough that it looks attractive," says John Heywood, a professor of mechanical engineering at MIT who has also worked on developing air hybrids. What's more, tanks of compressed air store far less energy than batteries, severely limiting the fuel savings in typical air-hybrid designs, says Doug Nelson, a professor of mechanical engineering at Virginia Tech. This is one of the major drawbacks of cars designed to run solely on compressed air.

Guzzella's new air-hybrid design makes use of advanced control systems to more precisely control the flow of air, improving overall efficiency. To overcome limited storage capacity, the design relies less on capturing energy from braking than other hybrids, and more on another approach to saving energy: using pneumatic power to boost the performance of smaller, more efficient gasoline engines.

Conventional vehicles use engines that can provide far more power than is needed for cruising--this excess power is used during acceleration and for sustaining very high speeds. But these engines are inefficient, especially since most of the time they operate at far lower loads than they were designed for.