Stopping Power: Kinetic Energy Recovery

Engineerblogger
Jan 11, 2012

Slamming on the brakes often comes with a good amount of cursing. The car that just cut you off, the jaywalking pedestrian, the exit you missed, all deserve a few foul words. For Alberto Boretti, University of Ballarat, Australia, the cursing is directed at the brakes themselves. The energy they use to slow his vehicle, or anyone else's, is gone, never to be recovered—a source of endless frustration.

There's no good reason why it shouldn't be otherwise. "A simple mechanical device that all the kids love in their toy cars is still not applied in production vehicles because of the lack of funding," says Boretti, who spent 20 years in the automotive industry before joining the faculty at the University of Ballarat as a professor of engineering.

Kinetic Energy Recovery

In 2009, the Formula One Team Association decided to allow this device for kids' toys to be used in the speedier toys for bigger kids. The "Kinetic Energy Recovery Systems" (KERS) came in many varieties for the race cars, with the most promising storing the mechanical energy in a flywheel. The systems gave drivers a bit more acceleration, cut a theoretical fifth of a second off their lap time, and saved some energy. KERS were dropped from Formula 1 during the 2010 season (due to development costs) but are back again this year.

So far, though, the technology has not made it onto a car for the rest of us. To help prove that such a move would be a good idea, Boretti ran a simulation of a car with a flywheel KERS. In a paper he published in the International Journal of Vehicle Design, he showed that fuel consumption would be reduced by 25–33% for a typical sedan and 20% for a diesel-powered engine.

An example of a KERS device used in Formula 1 racing.

With mechanical energy being stored as mechanical energy before it's used again, 70% of the braking energy is recovered. Compare that to the regenerative braking system that a Prius currently uses: Braking is converted to electrical energy then stored as chemical energy before making the reverse journey back to help power the car. After all the losses along the way, the Prius system recovers only 30% of braking power.

Boretti's computational experiments were made with and without the continually variable transmission used with the Formula 1 flywheel system. In addition to this potential simplification, Boretti's virtual design also used a vacuum pump, where other flywheels do their rotation in a sealed vacuum. In both cases, the idea was not to increase efficiency, but to cut costs—in the hopes of luring future investors in research, as well as eventual customers. A stripped-down system like the one Boretti used would add only $400 to $500 to the price of a car. The extra cash could easily be recouped in fuel savings after just a year or so of driving.

A KERS device system.

In the current economic climate, industries and governments aren't looking to shell out the money needed to give drivers this choice. "People are looking for a device that doesn't use carbon at all—there's a lot of pressure, but no money to develop a new idea. It's quite frustrating," says Boretti. "The technology is already established. It just needs to be transformed to production."



Source: ASME