Once automakers and suppliers expanded the availability of anti-lock brake systems from high-end luxury cars to more mainstream models in the late-1980s, they began looking for additional ways to use the hardware. With wheel speed sensors in place and the ability to control the amount of brake pressure going to individual wheels, traction control (TCS) was soon born.
When a tire is sliding across a road surface either because it is locked or spinning, the phenomenon is known as slip. As traction control was added to ABS, the combined systems came to be known as slip control. Just as trying to slow a vehicle on a slippery road surface can be difficult to do while maintaining control, the same applies to getting the car going.
Pressing the accelerator too hard can get the drive wheels spinning to the point that the driver cannot control the direction. In a front wheel drive car, if the front wheels are spinning, the car can just slide right off the road in whatever direction it has momentum. With a rear wheel drive car, wheel spin can cause the back end to swing out and put the whole car into a spin. Even with four or all-wheel-drive any or all of the wheels can spin and the results can be unpredictable.
The earliest TCS systems relied mainly on brake control to manage wheel spin. The ABS hydraulic unit was modified with extra control valves allowed brake fluid to be drawn in from the master cylinder with out letting it go back. As a result the pump that normally returned fluid back to the master cylinder during ABS could be turned on and used to build pressure for the brakes without the driver applying the pedal.
As soon as the TCS control determined that braking was needed to stop wheel spin, the same isolation valves used for ABS control would be closed on each wheel while pressure is built. The isolation valves for the drive wheels could then be pulsed to provide a controlled brake apply. Because each wheel is controlled independently, the pressure could be tailored to how much that wheel was spinning. Once the wheel begins to recover back toward vehicle speed, the pressure is reduced so that the car could still accelerate.
The speeds from the undriven wheels could be used as an accurate reference of the true vehicle speed. Based on how fast the vehicle is accelerating the type of road surface can be estimated and the amount of wheel slip allowed can be tailored for the best combination of performance and stability.
Doing traction control with just the brakes is typically somewhat rough and load and many early TCS equipped vehicles were unpleasant to drive. Engineers soon incorporated engine torque control as part of TCS. The earliest such systems used a device known as a throttle relaxer that actually pushed back against the gas pedal to help cut engine output. These were mechanically complex and slow to respond. Before long, spark control and fuel cutoff were added to the mix for faster and more precise torque control.
By the late-1990s many cars were starting to incorporate electronic throttle control (ETC). Unlike earlier systems that used a metal cable between the gas pedal and the throttle to physically control engine speed, ETC has no physical connection. Instead a position sensor on the pedal sends a signal the engine control computer which then controls a small motor to open the throttle. The engine control system can modify the amount of throttle opening directly based on requests from the traction control. The combination of throttle, spark and fuel control allows for quick, precise torque management and the brakes don't have to work as hard.
On modern traction control systems, the brakes are primarily used to manage side to side wheel slip. If the car is starting with one wheel on a patch of ice or snow or the shoulder of the road, just that wheel may spin up and cause the car to pull to one side or the other. Because the engine control manages the total drive torque sent to all of the wheels, the brakes are used to manage the slip at individual wheels. By applying the brakes to single wheel that is spinning, the torque will automatically be sent to the wheel on the other side of the car so that it can still accelerate.
Determining the optimum amount of wheel slip for a given scenario is still difficult because the system relies mainly on the wheel speeds to guess. Based on the way the wheel speeds respond to brake and engine inputs, the control software is generally very reliable but there are a few scenarios when it can be better to actually disable TCS. Most cars equipped with traction control have a disable switch and other than smokey burnouts, the main time when this is useful is on deep fresh snow. In this condition, if the car starts to move and the traction control kicks in to cut torque, the car can drop back into the snow and dig itself in deeper. The best thing to deep under these conditions is turn off the TCS and gently apply the gas to try to get the car going.
For the most part, modern traction control systems work pretty seamlessly and do an excellent job of keeping vehicles from losing control if the driver is to aggressive on the accelerator. Next time we'll look at electronic stability control.