How do Teslas friction brakes work

Recover braking energy

If you brake, you win: that is the motto for braking energy recovery. As a rule, all hybrid and electric vehicles have the ability to recuperate - this is the technical term. You can even recuperate without applying the brakes. But how does recuperation actually work and what does it bring?

Answer by Frank Ewerhart, an expert in regenerative braking at the automotive supplier Bosch: With regenerative braking, electric and hybrid vehicles convert kinetic energy back into electrical energy. Unlike conventional cars, which release a large part of the kinetic energy into the environment via the conventional friction brake in the form of unused heat, this enables electric vehicles to increase their range by up to 20 percent. In the case of hybrid vehicles, consumption and CO2 emissions are reduced by up to 20 percent.

Electric motor acts as a generator

When a hybrid or electric car drives down a mountain or the driver actively steps on the brake pedal, the electric motor is used as a generator. The wheels transmit the kinetic energy via the drive train to the electric motor, which then works in a similar way to the dynamo on a bicycle: it brakes the car by absorbing energy and converting it into electrical energy. This is called recuperation. The recovered energy is stored in the high-voltage battery and made available to the electric or hybrid vehicle when it starts or accelerates.

If a car has to be decelerated heavily, more braking power is required than the electric motor can generate. In this case, the conventional brake also intervenes. The friction braking torque adds up with the deceleration power of the generator to the actually required braking torque - the process is called torque blending. The driver does not notice whether a vehicle is being decelerated by the electric motor, the friction brake or both at the same time. He steps on the brake pedal and the control software does the rest. The aim is to use the regenerative braking torque as much as possible so that as much energy as possible can be recovered.

Greatest effect at low speeds

The braking potential of the electric motor depends on its size, the driving speed or the rotational speed. The braking torque and energy recovery are greatest at low speed. The battery's state of charge also plays a role. The electric motor can only provide braking torque and feed back energy if it is not fully charged. For example, an electric vehicle with a 100 kW electric motor generates up to 0.03 kWh per second through regenerative braking. In the event of emergency braking or in the event of an unstable driving situation, however, the car is almost exclusively decelerated using the friction brake, since individual interventions using the conventional braking system are necessary.