General Antilock Brake Operation

WARNING: This page is about a different variant/trim than selected.

General Operation

The Bosch ESPhev Hydraulic Unit and iBooster work in conjunction to provide both friction and regenerative braking. The iBooster Brake Booster assembly works very similarly to a standard vacuum booster, except brake boost is provided by an electric motor instead of engine vacuum. When the driver applies the brake pedal the iBooster and master cylinder delivers fluid to the ESPhev unit. The ESPhev unit then balances the friction and regenerative braking via a series of valves and a low pressure fluid line that is connected to the brake fluid reservoir. The ESPhev unit can move fluid to and from the reservoir via this low pressure line to meet the drivers braking demand with friction that cannot be achieved via regenerative braking. In order to balance the brake pedal feel of the driver, the iBooster braking support force is changed to match the brake fluid pressure in the system. When there is no regenerative braking possible (i.e. standstill), the separation valve inside the ESPhev is closed and the master cylinder only supplies brake fluid to the front axle. The rear axle braking demand is supplied by the ESP pump pulling fluid from the reservoir.

A pressure transducer on the primary master cylinder circuit (Master Cylinder Pressure), a pedal travel sensor on the input piston assembly and the brake switch are inputs into the electronic control unit. These inputs are used to determine the driver's braking intent which is used to control the torque applied to the wheels.

Base Brakes (Friction Braking)

The braking torque applied to the wheels in proportion to the driver's braking intent is achieved with a combination of regenerative braking from the powertrain and hydraulically applied friction braking controlled by the electro-hydraulic control unit. The friction braking achieved by the electro-hydraulic control unit controlling a combination of valves and ESP Pump Motor Duty in a way that moves fluid in and out of the wheels via the low pressure line connected to the brake fluid reservoir. The Bosch ESPhev system is designed to be used with a Bosch iBooster in order to achieve seamless driver brake pedal feel.

Base Brake Valves (No Valve Action)

The Separation Valve (SV) and Pressure Control Relief (PCR) Valves are open and closed respectively when they are not being controlled. In this state the brake fluid delivered by the master cylinder is sent directly to the wheels. This mode is used during a failsafe state to ensure proper friction braking and during the first apply of a new vehicle power cycle for diagnostics.

Base Brake Valves (Regenerative Braking)

When regenerative braking is available from the electric powertrain, the ESPhev unit will try to utilize this as much as possible. It does this by leaving the SV valve open and opening the PCR valve to return the brake fluid in the system to the brake fluid reservoir. If the regenerative braking is not able to met the driver brake demand, the fluid can be reintroduced into the system by the ABS Pump pulling fluid from the reservoir. In order to match the driver brake pedal feel with the amount of fluid in the brake system, the iBooster support force is adjusted to match the expected feel.

Base Brake Valves (No Regenerative Braking)

When there is no regenerative braking available, i.e. vehicle is at standstill or very low speed, the SV valve will close with a driver brake apply. The SV valve closing diverts all the fluid from the master cylinder to the front brakes. The rear brake fluid will be pulled from the brake fluid reservoir via the low pressure line by the ABS Pump.