Hill-start Assist Control (HAC)

Hill-start Assist Control (HAC) is an active safety feature that prevents a vehicle from rolling backwards when moving off on an incline by automatically maintaining brake pressure. It provides a brief buffer while the driver switches their right foot from the brake pedal to the accelerator pedal, serving as an essential extension of the Electronic Stability Programme (ESP) system.

Operating Principle and Key Benefits

HAC is built upon the vehicle's Electronic Stability Programme (ESP) system. Using onboard inclination sensors, the system detects when the car is stationary on an incline after the driver applies the brakes, placing HAC into standby mode.

Its operating mechanism is straightforward:

1. Brake Pressure Retention: Once the driver releases the brake pedal, the HAC system temporarily holds hydraulic pressure in the brake lines, maintaining braking force on all four wheels for approximately 1 to 3 seconds.

2. Smooth Drive-off: During this brief window, the driver can smoothly move their foot to the accelerator pedal. The system continuously monitors engine or electric motor torque output, and once sufficient drive force is detected to overcome rollback, it linearly and smoothly releases the brakes, allowing the vehicle to pull away seamlessly.

This automated process eliminates hill-start anxiety and reduces the risk of rollback collisions caused by delayed pedal transitions.

⚙️ Activation Criteria

To engage properly, the HAC system typically requires the following preconditions:

• The gear selector is not in P (Park) or N (Neutral) (for automatic transmission variants).

• The accelerator pedal is not depressed.

• The vehicle is brought to a complete stop.

• The handbrake or Electronic Parking Brake (EPB) is released.

• The vehicle is on a slope that meets the system's gradient threshold.

Limitations and Safety Precautions

HAC is strictly a driver assistance feature and cannot substitute a proper parking brake. Drivers should be mindful of its operational limits:

• Short Hold Duration: HAC holds brake pressure only momentarily (typically around 2 seconds). It is designed solely for smooth take-offs and must never be used for prolonged holding or parking. On steep inclines or slippery surfaces, do not rely on HAC alone; always engage the Electronic Parking Brake (EPB) or manual handbrake to secure the vehicle.

• Dependent on System Health: HAC relies entirely on the proper operation of the ESP and ABS braking hardware. If the ESP or ABS warning light illuminates on the instrument cluster, hill start assistance will be disabled, and drivers should avoid taking risks on inclines.