What Is Regenerative Braking?

brakesIf you've been paying attention to the development of hybrid electric vehicles, you've noted that a number of these cars are designed to use "regenerative braking." If you're not an engineer, you may not know what regenerative braking is or how it works.

About Regenerative Braking

Simply, regenerative braking enables the vehicle to capture some of the energy that would otherwise be lost when the vehicle slows to a stop. In conventional brake designs, the forward momentum of the vehicle is lost when the brakes are applied. This energy is converted to heat via friction and is dissipated into the surrounding air.

In hybrid cars with regenerative braking, some of the forward momentum of the car is captured and stored in batteries that provide power for accessories or locomotion. Regenerative brakes will slow the car and capture some of the "lost" energy, but they will not reliably bring the car to a complete halt.

Regenerative braking systems use the energy that would ordinarily be dissipated as heat to power a motor that is also wired to function as a generator. When the vehicle is in motion, the wheels turn the motor, generating current that aids the forward momentum of the vehicle. When the brakes are applied, the current flow in the motor reverses and causes the motor to oppose the vehicle's forward motion.

The applied braking force is proportional to the current that opposes forward motion. The current produces a force in the motor called torque, which slows the vehicle and allows the motor to charge the battery pack. The regenerative braking process slows the vehicle but not enough to bring the car to a precise stop.

For safety reasons, a regenerative braking system is typically used in parallel with a traditional friction brake design. Under most normal driving conditions, regenerative braking would be desirable in a hybrid electric vehicle. Under emergency braking conditions, however, the vehicle requires immediate maximum braking force, which can be delivered only by conventional friction brakes.

There are a few other reasons to use dual brake designs on hybrid vehicles. Most passenger vehicles are classified as two-wheel drive. Since regenerative braking systems are employed only on the vehicle's drive wheels, a set of friction brakes on the vehicle's unpowered wheels is also needed. Conventional brake systems, which are used on all of the vehicle's wheels, also act as an active back up in case the regenerative brakes fail under normal driving conditions.

Having two brake systems sounds like a good idea from a safety perspective, but it also means that the actions of these two independent systems need to be coordinated to balance energy recovery with the need for braking precision. Conventional designs accomplish braking via mechanical means. Brake systems are hydraulic, and use fluid power to generate and control the required braking forces. The brakes themselves are attached to the wheels. Most commonly, drum brakes or disc brakes are used along with highly heat-resistant material called brake pads or "shoes." The shoes, (or calipers in disc brakes) are mechanically forced against the drum (or discs) to slow the wheels to a stop. The force that presses the shoes or calipers against the wheel's brake system can also be generated hydraulically or pneumatically ("air brakes"). Electromagnetism can also be used to generate braking forces.

In hybrid electric vehicles, coordinating the actions of two braking systems requires extremely precise control. Mechanical components don't react fast enough to provide precise braking controls. A network of electronic controllers, sensors and actuators distributed throughout the vehicle is increasingly used in automotive braking designs. Electronic braking control is a major departure from the largely mechanical process of braking. Inclusion of these "brake-by-wire" technologies also enables other braking and safety technologies like collision detection and corrective braking.

Within an electromechanical braking system, a bevy of sensors track the wheel speed, the positions of the brake pedal and the emergency brakes, the actions of the throttle and steering systems, and the position and movement of the vehicle. Electromechanical braking employs other sensors to control how forcefully the brake should be applied and to measure the build-up of heat within the brakes.

The brake system must know all of this information at all times. In addition, the brake system must determine when to deploy safety mechanisms like anti-lock brakes, and must also track and work with other systems that are designed to improve the stability of the vehicle in sudden driving emergencies.

Regenerative brakes are neither new nor limited to automobiles. The technology was developed more than 40 years ago and is often found on other transports, like electric trains and large trucks where fuel economy is critical. Regenerative braking can save wear and tear on mechanical braking components and improve fuel economy in a vehicle by using more of the power train's output and providing a ready source of power to recharge batteries that would otherwise be depleted in the early stages of a trip.

Regenerative braking is not used on conventional vehicles. In a conventional vehicle, the battery is used to start the engine and to power the electrical accessories and control components. The alternator serves as a highly efficient recharger once the engine is started. The design of a conventional vehicle gives little reason to use regenerative braking.

Other "hybrid" technologies, like stop-start, can be used on conventional vehicles. Stop-start cuts the internal combustion engine when the car is idling and immediately restarts the motor when the gas pedal is pressed. Like regenerative braking, stop-start is not new, nor is it used exclusively on hybrids. It has come into vogue with hybrids as a way to improve fuel economy and conserve energy when the car is idling. Stop-start could reduce fuel consumption by 7-10% in a vehicle (hybrid or otherwise) outfitted with the technology.

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