What is Regenerative Braking?

Written by Bryan Johannsen

Regenerative braking is a feature on most hybrid and electric cars. While not the mythical perpetual motion machine, regenerative braking can restore energy to the power plant when the vehicle is stopping.

Conventional brakes use a pad that creates friction against the rotor or the inside of a drum. There is energy generated in this process but it is heat from the friction and it escapes into the air.

In a regenerative braking system when the brake pedal is depressed the electric motor switches to reverse and slows the vehicle. When the motor switches to reverse it actually changes from a motor to generator. Now when it turns it is producing power instead of using it. The power produced by the motor turned generator under braking is stored back into the car's batteries to be used again when the car starts accelerating again.

In that simple explanation it sounds like the car could keep going and going as long as it stopped every now and then to recharge by braking but due to energy lost in friction with the road and heat generation the amount of energy returned to the car is never as much as is spent. So while it never generates as much energy as it uses it does greatly increase the effeicency of a vehicle equipped with a regenerative braking system.

KERS

If you're a Formula 1 fan you haven't been able to escape the coverage of the Kinetic Energy Recovery System or KERS that is in use by some of the teams on the F1 grid. KERS is another sort of regenerative braking system in that it uses the kinetic energy of braking to create potential energy in the form of an 80 horsepower boost activated by a button on the driver's steering wheel.

There are two form of Kinetic Energy Recovery System, the first is electrical, very similar to the braking system discussed above. It uses the a generator to create resistance and slow the wheels while charging a battery pack that can be used later for extra power to overtake or defend a position. There is also a mechanical KERS, it uses a heavy flywheel to store the energy. Turning the heavy flywheel creates a resistance on the wheel to slow the vehicle then when the steering wheel button is pressed the rapidly spinning flywheel is connected to the engine's crankshaft to give the power boost.

There aren't any on the road now but the term for a gas/flywheel hybrid is a "flybrid", watch out for them in the future.

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