Regenerative Braking

Basic Description

Regenerative braking systems recapture some of the vehicle's kinetic energy when the brakes are applied and store this energy so that it can be used to reduce the engine load when the vehicle accelerates. It is widely used in electric and hybrid electric vehicles that already have batteries to store the recaptured energy. Regenerative braking has minimal impact on fuel economy during highway driving, but it can significantly improve the fuel economy of vehicles that are driven primarily in city traffic. In heavy vehicles that make frequent stops (e.g. garbage trucks), regenerative braking systems can improve fuel economy substantially.

Electric and hybrid electric vehicles typically employ motor-generators that can convert electric current into torque (like a motor) or torque into electric current (like a generator). When the brakes are applied, the motor-generator provides the resistance necessary to slow the vehicle as it supplies current to the battery. In the event that the motor-generator cannot slow the vehicle fast enough, a torque coordinator module will apply traditional friction brakes to the extent necessary.

Some regenerative braking systems store the recaptured energy mechanically, typically by pumping hydraulic fluid into an accumulator where the energy is stored in a compressed gas. These systems still require a significant amount of electronics to regulate the energy transfer and to apply the friction brakes as needed.

A mild form of regenerative braking can be achieved in non-electric vehicles by adding a clutch to the alternator. The clutch engages when the vehicle is coasting or braking causing the alternator to help slow the engine as it charges the battery that powers the starter motor and other electric devices in the vehicle. This type of regenerative braking is relatively inexpensive and easy to implement in existing vehicles. It can improve overall fuel economy by as much as 4% [7].

Sensors

Brake pedal position, vehicle speed, energy storage (e.g. battery) status

Actuators

Brakes, energy storage device (e.g. battery)

Data Communications

Control Unit Communication: Typically Control Area Network (CAN) Bus System

Manufacturers

Bosch, Continental, Denso, TRW

For More Information

[1] How Regenerative Braking Works, Christopher Lampton, HowStuffWorks.com, Jan. 23, 2009..

[2] How does the regenerative braking process work?, Yahoo Answers.

[3] What is Regenerative Braking?, Sebastion Blanco, Autobloggreen, Apr. 16, 2009.

[4] BMW Brake Energy Regeneration, YouTube.com, Feb. 2007.

[5] Hydrostatic Regenerative Braking System HRB (parallel), YouTube.com, June 2008.

[6] Hydrostatic Regenerative Braking System HRB (series), YouTube.com, June 2008.

[7] BMW Introduces Intelligent Alternator Control with Regenerative Braking; Reduces Fuel Consumption by About 4%, Green Car Congress, Sep. 29, 2006.

[8] The Magic of Tesla Roadster Regenerative Braking, Greg Solberg, Tesla Motors website, June 29, 2007.