Electrical braking (energy recovery)

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Electric braking enables braking energy recuperation. Here, the electrical machine works as an alternator and brakes the drive wheels. The energy that is generated is used to charge the high-voltage battery unit via the Electrical Machine Electronics (EME).

The decisive input variables for electric braking are the accelerator pedal angle and the brake pedal path.

The brake pedal path is measured by the brake pedal path sensor and read in by the Dynamic Stability Control (DSC).
The accelerator pedal angle is measured by the accelerator pedal module and read in by the Digital Engine Electronics (DME).

With the brake pedal not operated but an accelerator pedal angle of 0°, the electrical machine is operated as an alternator. The Electrical Machine Electronics (EME) activate the electrical machine in such a way that brake force results for the complete vehicle that corresponds to a conventional vehicle in the over-run mode. The extent of energy recuperation is still at a low level here.

When the brake pedal is operated, a response threshold must first be overcome during which there is no hydraulic braking. However, the brake pedal path is already being evaluated and with the help of the electrical machine a greater brake force than in the purely over-run mode is created.

When the brake pedal is operated beyond the response threshold, both brake interventions are simultaneously active, because now hydraulic braking is added for braking energy recuperation. The braking force generated by the electrical machine is increased as the brake pedal path increases, until the maximum is reached.

Electric braking with the help of the drive train only affects the rear axle. The brake force on the rear axle must not exceed a certain level relative to that on the front axle. Otherwise, driving stability would be impaired. This is also why the maximum deceleration that can be achieved through braking energy recuperation is limited. The maximum permitted brake force by electric braking depends on monitoring of the slip, lateral acceleration and control interventions in the driving dynamics system. This ensures that the vehicle in also remains in a stable driving state during braking energy recuperation at all times. Here, the Dynamic Stability Control (DSC) contributes to the amount of fuel saved from the point of view of active safety.

If the high-voltage battery unit is already fully charged, no more electrical energy can be stored. This special state can be a reason why braking energy recuperation cannot be possible, but it rarely occurs. The operating strategy means that during normal driving an adequately large reserve is kept in the state of charge of the high-voltage battery unit. It is deliberate that energy is drawn from the high-voltage battery unit at regular intervals. This keeps the state of charge within a range that also allows for space to store electrical energy generated by electrical braking on long hill descents.