When it comes to cars, everyone brakes. The kinectic energy from braking is captured by an electric or hybrid vehicle and used to power a generator that recharges its battery. But researchers at Lund University in Sweden think the kinectic energy of braking could be captured in the form of compressed air and used to power the engine. This type of engine could be cheaper to produce and more energy efficient than battery-electric hybrids.

The pneumatic engine of a so-called air hybrid vehicle collects compressed air in a small storage container during braking and then provides power during acceleration or idling. And because air hybrids depend on starting and stopping for their “recharge,” they are ideal in jerky-driving situations like the stop-n-go motions characteristic of city driving. Calculations show that the a compressed-air hybrid could could reduce the fuel consumption of city buses by 60 percent. That's promising, for an experiment full of hot air.

So far the Swedes have only experimented on a single-cylinder engine, but they are already seeking contracts and getting requests to study multi-cylinder engines. Though designs for pneumatic engines have been around for decades, the Lund team claims they are the first to actually experiment with an engine.

Photo: Lund University