The hill-start control system consists of a number of different sensors (though the specific sensors may vary according to the implementation), an electronic control unit (ECU) and a brake actuator under the control of the ECU that can apply the brakes as needed to prevent the car from rolling backward. We'll look at these safety and regulatory devices individually; bear in mind those specific hill-start control systems may not necessarily require every one of the following components:
Angle sensors: These detect the angle of the car on an incline, which corresponds to the slope of the hill the car is on.
Pressure sensors: These are part of the suspension system of the car and can detect the vehicle's weight, including the weight of passengers and cargo. This can also be done by piezoelectric sensors or strain gauges. These sensors produce an electrical signal proportional to the weight of the vehicle.
Torque sensor: Torque is the rotational force from the engine that eventually accelerates the vehicle from a complete stop. The torque sensor can detect how much torque is being transmitted to the wheels via the drivetrain.
Wheel-speed sensors:These detectors, usually placed on the axles, can determine the speed and direction the wheels are turning.
Electronic Control Unit (ECU): This is the vehicle's embedded computer system that receives signals from the various sensors. The ECU decides when the brakes need to be applied based on that input. The ECU can also calculate the traveling resistance, which is a function of the car's weight (determined by the pressure sensors) and the slope of the hill that the car is on (determined by the angle sensors). Traveling resistance is used to calculate how much engine torque will be necessary to move the vehicle uphill.
Brake actuator: An actuator is a device that converts an electrical signal into a physical movement. The brake actuator receives a signal from the ECU telling it to trigger the brakes. It then activates brake valves, sending brake fluid to the brakes to hold the vehicle in place, which keeps it from rolling back down the hill. In the case of a hybrid vehicle, the electric motor may be used in place of the brake to apply sufficient forward motion to the vehicle to keep it from rolling backward.
Once the driver starts to accelerate, the torque sensors help the ECU to determine if the engine's torque is sufficient to overcome the traveling resistance (already calculated by the ECU). If it is, then the ECU sends a signal to the brake actuator telling it to turn off the brakes and let the car move.
Ideally, the driver should be aware of none of this. The release of the brakes should be so smooth that the driver is unaware that brake force was still being applied after the brake had been released. Only later will the driver realize that he or she never once had to worry that the vehicle was going to slide back down the hill and collide with the car behind it. Driving safety is rarely this painless.
Is the safety advantage supplied by hill-start control worth the extra cost of buying a car that comes equipped with it? We'll consider that question on the next page.