How Does Brake Energy Regeneration Work?

January 27, 2012

A hot topic among both the professional racing community and hybrid owners community is brake energy regeneration. These systems function by storing the energy normally lost during braking and storing it for later use. Prius brakes use this system as does the F1 oriented KERS system, but how exactly they go about it can vary depending on the specific type and application.

Conventional Braking

Conventional braking systems function by converting a vehicle‘s kinetic energy into heat energy via friction. When you press down the brake pedal, the brake calipers clamp your brake pads against the brake rotor, and the friction between the two slows the rotation of the brake rotor and the wheel attached to it. Combined with the traction your tires have on the road surface, your car’s momentum is slowed, and the resulting heat energy is left to dissipate into the air as the braking system cools down.

Regenerative Braking

Unlike traditional braking systems, far less energy is wasted during braking with a regenerative braking system. While traditional braking is still required as a safety precaution as well as for lower speeds where regenerative yields begin to diminish, in the case of the Toyota Prius, the bulk of braking duties are handled primarily by the electric motors mounted on the drive wheels. As the brake pedal is depressed, these electric motors reverse their direction of rotation, and the resulting torque moving counter to the wheel’s motion helps to slow the car. When these motors reverse direction, they become in essence, a generator, and the surplus energy they create is stored in a battery for later use. This helps keep the battery packs in electric hybrids like the Prius fully charged without having to resort to older, outdated methods such as physically plugging them into a power socket. This entire regenerative braking system fits behind the wheels and resembles a large brake drum. These systems are not normally serviceable by the owner, and require no extra attention.

KERS Systems

KERS stands for Kinetic Energy Recovery System. This system tends to come in an electrical and mechanical variety. Electrical based systems tend to work similarly to those in the Prius, storing the energy normally lost in braking in a battery for use. Mechanical systems work similarly, but store this energy in the form of a flywheel, which can spin up to 80,000 rpm. When additional power is needed, this flywheel is connected to the rear wheels. Additional power can reach as high as 80hp from these systems, usually for a duration of up to a maximum of 6.67 seconds.

While regenerative braking systems are still just starting to become used in automotive street applications, they are showing a large amount of promise in both passenger car and racing applications alike. With exception to additional weight, there are almost no downsides to regenerative braking. With F1 KERS systems offering as much as an 80hp increase on demand, it is not unlikely to expect to see these systems taking on a more performance-oriented role in future performance street cars.

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