Brake-by-wire: How it works

Bahrain GP Thursday 16/04/15

A new ERS system introduced in 2014, which now goes by the name "MGU-K" (for kinetic motor generator unit), doubled the energy harvesting potential, with power transmission now amounting to 120 kW, while a maximum of 2MJ per lap can be transferred to the energy store (or battery).

Because of this considerable increase in performance, and the subsequent increase and variations in reverse torque in the rear axle (what is commonly known as engine braking), it was essential that the engineers install a system to compensate for the powerful effect the MGU-K has on brake balance and braking stability.

Ultimately, this was the reason for the FIA allowing the adoption of an electronically controlled brake balance in order to ensure the system did not destabilise a car with a sudden balance shift, and help it remain as consistent and stable as possible.

How it works

The rear brake system includes sensors, two hydraulic pumps (instead of one) and an ECU, a self-contained engine management system and data logger manufactured by McLaren Electronics.

Basically, when the brake pedal is pressed, the control unit uses information from a sensor fitted to the rear master-cylinder to determine how much braking force each rear caliper needs. Relevant information such as the pressure a driver is applying to the brake pedal, and the level of energy harvest the driver is requesting under braking to charge the battery are also sent to and processed by the ECU. The unit will then combine the two signals, and the rear-brake pressure needed to achieve the front-to-rear brake balance requested by the driver will be defined.

Pressure is generated by a hydraulic pump and managed by an ERS brake pressure reducing valve mounted near the rear of the car on the pipework that would normally feed the brake fluid to the rear calipers. In short, the additional braking capacity of the MGU-K produced by the reverse torque effect on the drive shaft (in order to recover a certain amount of energy) will be compensated, in a split second, by bleeding excess hydraulic fluid back to the fluid tank which will subsequently decrease the amount of pressure applied to the rear calipers, thus appropriately balancing the conventional brakes with the ERS braking.