What is (HBA) Hydraulic Brake Assist:

Hydraulic Brake Assist (HBA): Research on braking behavior in critical situations indicates that most drivers exert insufficient pressure on the brake pedal. Consequently, precious stopping distance is “lost” because the maximum potential braking force is not utilized.

Sequence of an HBA-controlled braking manoeuvre

 In a critical braking scenario, the operator presses the brake pedal at a much faster rate and with significantly greater force compared to a predictive braking maneuver (smooth braking). The result is a rapid increase in brake pressure. During a smooth braking maneuver, the rate of pressure escalation is within a range of roughly 30–60 bar/second, whereas in a critical braking scenario, this figure is several times greater. The control mechanism identifies the emergency braking scenario by analyzing the rate of pressure escalation. The pressure sensors embedded within the ESC hydraulic module provide this data. The next control process consists of three stages.

The purpose of HBA is to amplify the brake pressure during an emergency braking maneuver to achieve the shortest possible stopping distance while maintaining the vehicle’s maneuverability. HBA is an enhanced version of ESC and is integrated into all current Audi models. It cannot be disabled by the operator for safety purposes.

Hydraulic Brake Assist (HBA) Stage 1

A predetermined threshold for the rate of brake pressure escalation initiated by the driver is surpassed, and the emergency braking situation is identified.

 Active pressure buildup begins. For this, the high-pressure control valves and the changeover valves are engaged. The recirculation pump is triggered and draws brake fluid through the opened high-pressure control valves. The brake pressure applied by the operator is further augmented by the active buildup of pressure across all four-wheel brakes. The goal is to activate ABS-controlled braking as swiftly as possible.

Hydraulic Brake Assist (HBA) Stage 2

Once the ABS engages, the vehicle decelerates at the maximum theoretically achievable rate of brake deceleration while maintaining full maneuverability. ABS-controlled braking operates through the “pressure buildup,” “pressure holding,” and “pressure reduction” functions.

 In each braking circuit, the wheel with lower brake pressure is controlled by the corresponding intake valve, while the wheel with higher brake pressure is managed by the assigned changeover valve.

For instance, if the brake pressure in the front right wheel brake is higher than that in the rear left wheel brake, the front right pressure is prevented from increasing further by the “pressure holding” function. Simultaneously, the rear left brake pressure can continue to increase. The respective intake valve for the front right wheel is activated, disconnecting the pump from the wheel brake. Pressure can still increase at the rear left wheel via the open intake valve. To maintain consistent brake pressure, the corresponding intake valve is also closed.

Hydraulic Brake Assist (HBA) Stage 3

When the operator decreases pressure on the brake pedal, the system detects this by assessing the pressure signal from the brake pressure sensor, and the HBA control process is deactivated.

HBA in Vehicles with Adaptive Cruise Control (ACC)

In vehicles equipped with ACC, the surrounding traffic conditions are continually “monitored” by radar sensors, even when ACC has not been activated by the driver. If a situation with a higher risk of collision is identified, the criteria for activating HBA are changed.

In such cases, the function is triggered earlier, i.e., at a lower rate of brake pressure buildup. This more responsive deployment strategy ensures even shorter stopping distances in critical braking scenarios.