Air power takes off

Apply the brakes in your car, and you generate friction energy. At the minute, there’s no easy way that a regular vehicle can save that energy, so that it can be used to power the motor when you want to go forward again. Most electric hybrid cars use braking energy to recharge their batteries. But the process is expensive and not very efficient. Researchers in Sweden are working on an alternative idea that would harness the energy more effectively and make it available to any vehicle.

Their plan is to store the braking energy in the form of compressed air. The air is kept in tanks onboard the vehicle and released to give the engine a boost when it is time to accelerate again, or to keep the motor ticking over when the car is at a standstill.

There are no air-hybrid engines in production yet, but they would be much cheaper to build than electric-hybrid motors, says Per Tunestål, a researcher in combustion engines at Lund University in Sweden. “The technology is fully realistic,” he says.

Air-hybrid engines would be particularly suited to the kind of stop-start and slow driving that plagues urban road users and sucks up fuel, says Tunestål. A doctoral student at the university, Sasa Trajkovic, has run simulations that show buses in cities could reduce their fuel consumption by 60 percent if their engines had air hybrid support. About half of the energy from braking could be stored in a small air tank for later reuse, Trajkovic estimates.

The engine would not require any expensive materials so would be cheap to manufacture. It would take up much less space than an electric hybrid engine and could be combined with petrol, natural gas and diesel motors.

Ford started experimenting with air hybrid engines in the 1990s, but shelved its plans when it hit technological barriers. The researchers in Lund hope to convert their research results from a single cylinder to a complete, functioning, multi-cylinder engine. They would then be able to move this intriguing concept one step closer to a real vehicle. “The research so far has only been theoretical,” says Trajkovic, “This is the first time anyone has done experiments in an actual engine.”