How Does Regenerative Braking Work in Formula E?

Learn about the nuances and the complexity of braking in Formula E… 

Formula E braking systems have changed extensively since the championship’s inaugural season, but throughout every campaign, the complexity has been maintained.

With regenerative capabilities, braking is one of the key aspects of the championship and it not only informs but defines the way a driver takes every corner.

To secure even a glimpse of the chequered flag, regen is a must but to understand how it works, it’s important to first understand how the car works.

Simply, the energy stored in the battery powers the vehicle’s motor which, in turn, rotates the wheels to make the car move. Regen is the opposite of this process. 

When the brakes are applied, further power is generated which spins the motor in reverse, forcing the current to flow back into the battery, therefore, regenerating used energy.

This process increases the car’s range over the course of a race but all of this takes place inside a system called brake-by-wire which was introduced to Formula E in 2018.

Despite using an electric system instead of a fully hydraulic system, brake-by-wire still shares similarities to conventional braking systems found on regular road cars.

The brake pedal is still connected to two master cylinders, and when pressure is applied, a rod pushes onto the master cylinder, forcing fluid from a reservoir through hydraulic pipes that are connected to the brakes.

This fluid engages the brakes by forcing pistons in the brake calliper to apply pressure onto the brake pads, increasing friction on the brake disc and therefore, slowing the car down.

That’s for the front brakes at least. The big differences are at the rear of the car.

In a brake-by-wire system, the pressure that is applied to the brake pedal is measured by the ECU [Engine Control Unit]. This sends a message to the actuator which is connected to the rear brakes via hydraulic pipes.

At the same time, the MGU [Motor Generator Unit] also measures the pressure applied while calculating how much of the braking demand it will be supplying itself when harvesting energy.

Because the MGU provides the majority of this stopping force by slowing the car aggressively for this harvesting, the rear brakes have a reduced responsibility to slow the car down.

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The MGU still, however, informs the actuator as to how much hydraulic fluid will be needed to compensate for the remaining braking demand.

The rear brakes then supply the rest of the stopping force that is required. In short, slowing down is a joint effort between the MGU and the rear brakes.

This subsequently impacts brake temperatures, however, and because less friction is being applied to the rear brakes, it is more difficult for drivers to keep them in their operational window.

This challenge is further enhanced by the need to keep the brakes cool in general because if they get too hot, they will fail.

Brembo’s carbon brakes on the Gen2 Formula E car are capable of withstanding temperatures of up to 800ºC, but to ensure this boundary is not surpassed, the discs feature adequate ventilation.

With all of this combined and working together, the Gen2 can regenerate 250kW of energy in the heaviest of braking zones.