Principles of Operation

The hydraulic control unit (HCU) controls the brake assist function as well as the anti-lock functions. The HCU contains an accumulator, driven by an electric motor, to supply pressurized brake fluid to the disc brake calipers during brake application. The electric motor begins to build pressure in the accumulator whenever the vehicle doors are opened or when the dome light is activated. The electric motor also charges the accumulator and initiates the brake system self-test whenever the ignition switch is turned to the ON position or the brake pedal is applied. During this time, 12 mm (0.47 in) of brake fluid is drawn from the master cylinder reservoir into the accumulator. The accumulator discharges 4 minutes after the ignition switch is turned to the OFF position and the vehicle dome light turns off, returning the brake fluid used during the charging of the accumulator to the master cylinder reservoir.

The regenerative braking system recovers the vehicle kinetic energy during deceleration to replace the high voltage traction battery energy. A certain level of regenerative braking is commanded by releasing the accelerator pedal. Further regenerative braking is commanded by applying the brake pedal. Whenever the anti-lock brake system (ABS) is activated, the affects of regenerative braking are reduced until the ABS event is over and the accelerator is applied.

The series of regenerative braking feature provides isolation of the driver brake demand from the friction brakes in order to provide a portion of the braking through negative torque request to the electric drive motor. In addition, this torque request results in a change in the flow of energy. The energy flows into, rather than out of, the battery. The motor's change from drive motor to generator is referred to as regeneration.

Regenerative braking logic is as follows:

• The ABS module determines the total torque request from accelerator pedal and brake pedal input.
• The ABS module compares the drivers total request for a reduction of speed to the powertrain control module's (PCM) ability to slow the vehicle or negative "torque limit".
• The ABS module requests the reduction in speed up to the torque limit from the PCM, via a torque modification request message sent to the PCM.
• The ABS module applies the friction brakes beyond the torque limit.