Introduction to Regenerative Braking

Hybrid vehicles are very different from traditional vehicles. The two main differences are the engine does not run at all times, and the high voltage batteries need to be recharged on the drive.

The control side of a hybrid brake system contains differences when compared to its conventional cousins in order to accommodate these conditions.

Regenerative Brake Booster

Regenerative Braking

Energy cannot be created or destroyed. It can only be transformed from one form to another. Conventional friction brakes do this by transforming the energy of motion (kinetic energy) into heat energy, via friction, and then dissipating it into the air.

With regenerative braking, instead of just wasting that energy by releasing it into the air, the vehicle stores the energy and later converts it back into kinetic energy.

Regenerative braking

On a hybrid vehicle, the brake pedal acts more as an interface for the ABS module than as a request for braking. The brake pedal and ABS module pressure sensors “tell” the modules of the hybrid system that a request for braking has been made and how much braking is desired — the driver’s pedal efforts are calculated by the modules in order to produce the correct pedal feel and stopping power. The modules that control the hybrid system then apply regenerative braking via the traction motor inside the transmission, as needed. More regenerative braking increases the amount of electrical charge generated for the hybrid battery while it produces more drag to slow down the vehicle. Of course, at a stop or near stop, the friction is applied to stop the vehicle from rolling. Some hybrid systems apply the hydraulic brakes at lower speeds.

It is normal for a hybrid to wear the rear brake pads two and three times faster than the front pads because, during many light-braking events, the rear friction brakes will be lightly applied and the front may not be applied at all until the vehicle is at a stop or near stop.

On most hybrids, the pressure generated for the calipers is not the direct result of the driver’s foot pedal efforts. The pedal-generated pressure is used to stimulate a pressure sensor in the system that makes up the brake request. Normally, the fluid pressure for the wheel hydraulics is generated by the pump and/or accumulator in the ABS HCU or a stroke simulator and is regulated by the teamed efforts of the ABS and the PCM’s logic.

The higher the request for braking, the system electronically calculates and decides how to apply friction and regenerative braking to meet the demand. All brake applications, from feather-light braking to a panic stop, are electronically calculated and delivered from a normally operating electrohydraulic brake system.

In the event that the ABS HCU can’t function well enough to build pressure, the driver’s pedal pressure will directly provide the hydraulic pressure for the calipers. There are valves inside the HCU that allow direct passage from the pedal feel simulator, through the HCU and to the wheels. This is called “manual mode.” In manual mode, there will be no boost so the driver will have a very hard/stiff brake pedal. The driver will need so much braking effort in this situation that it may prompt them to tow the vehicle for fear of having no ability to stop at all. Complaints of “no brakes” or “barely stops” is the often how customers will describe a brake system that has entered manual mode.

hybrid brake system

Friction Considerations

Replacement of friction material is the same for a hybrid as it is with a conventional system, however, the ABS module will test the brake hydraulic system’s integrity by pressurizing it during key-off events (door opening, dome light activation, etc.) and again when the key is switched on.

After four minutes have passed since the key was switched off, the accumulator discharges its stored fluid pressure back into the master cylinder reservoir. Before attempting friction material replacement, you must follow the procedures to depressurize the system.

It’s common to come across a hybrid with 70,000 or 100,000 miles with the original set of front

Always use a high-quality pad for hybrid applications to avoid problems. It’s not a question of better performance, but more a question of quality and engineering.

Since it’s impossible to perform a conventional break-in/bedding procedure on the test drive, make sure the manufacturer promises excellent performance right out of the box. Also, applying a non-direction finish with a ball hone will help the new pads evenly deposit a layer of friction material to the new rotor.

Customer Complaints

The two-mode hybrid system has a unique brake pedal because the hydraulic system is not the lone provider of vehicle deceleration. How the system blends these forces can cause a customer to notice the changes and become concerned.

Over several braking events, the customer may perceive that he or she is applying a consistent brake force and yet the vehicle may decelerate at slightly different rates. This is a normal operating characteristic. The driver may think it is the same type of stop, but to the system, a little extra speed or pressure can change braking performance or the blending of the brake mechanisms.

Another common complaint for used hybrids is noise and pedal feel. Most two-mode hybrid vehicles use a braking system that employs a high-speed electric motor/pump. Pump operation sometimes can be felt in the brake pedal as the system builds pressure. This is most noticeable to customers who start the vehicle with their foot on the brake pedal. Also, the noise can be more noticeable if the engine is off and the customer is braking.