This may seem like old news now but I figured I’d do this analysis anyway because it helps me understand these things, and hopefully it helps you too!
In 2014 the FIA allowed F1 teams to use a “powered control system” on the rear brake system as the additional regenerative capabilities of the MGU-K under the current regulations resulted in inconsistent retardation of the rear axle. Not that the teams had a choice as to whether they used such a system, as a conventional hydraulic setup would be incredibly inconsistent and – more importantly – unnatural to the driver. This system is more commonly known as brake-by-wire (BBW)
So what exactly happens when the driver hits the brake pedal?
Above is a diagram of how the BBW system is implemented within the car. Red lines indicate hydraulic lines and yellow lines are electrical wiring.
The brake pedal is connected to the front and rear brake master cylinders, which are in turn connected to their respective reservoirs which can be seen in the front bulkhead when the nose of the car is removed.
A hydraulic brake line runs from the rear master cylinder (red) along the side of the car and into a simulator. A simulator turns hydraulic pressure into an electrical signal. The brake pedal experiences loads from 0kg to the 85-100kg region, so the simulator must have assigned a signal for the pressure of the brake fluid at any of those loads on the pedal.
These signals are then constantly sent to the ECU, as are signals from the MGU-K (cylindrical shape) which tell the ECU how much energy harvesting is occurring in real time. The ECU has to process all of this information to decipher just how much brake pressure is needed on the rear axle to slow the car down in the manner in which the driver wants.
A series of signals are then sent back up towards an actuator (yellow box) at the rear of the car, which is responsible for opening a control valve in a hydraulic block. The block is composed of three paths for hydraulic fluid to pass through – the left and right brakes and a high-pressure reservoir. When the actuator opens the control valve, a pump on the reservoir drives fluid up the brake lines and into the rear brakes. The control valve is always varying as the signals from the ECU to the actuator vary given that the MGU-K is also harvesting variably, i.e. as the MGU-K harvest rate decreases/increases, rear brake pressure increases/decreases.
In some cases the driver may press the brake pedal and the brakes themselves won’t actually be needed – the car slows down just under the regenerative effect of the MGU-K, if that’s the deceleration rate that the ECU thinks the driver is demanding at the brake pedal.
In case of emergency (i.e. when the ECU detects a problem with any signal from the simulator or MGU-K), the braking system bypasses the simulator and continues to act as a normal hydraulic system, hence why there is a red line running off the simulator and round to the rear hydraulic block. This ensures that the driver is always able to apply the brakes even if an electronics failure occurs.
Just to clarify, Jules Bianchi’s crash in Japan had nothing to do with this emergency bypass. The FIA Accident Panel confirmed that Bianchi attempted to cut the engine as he went off the circuit by pressing both brake and throttle pedals at the same time. This is called the FailSafe algorithm which failed because of a sensor in Bianchi’s BBW system, whose system is unique to then-teammate Max Chilton. More on this here.