Theory of Operation
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The basic principal of traction control is reduce wheel spin by comparing the driven and un-driven wheel speeds, then sending the engine computer (ECU) a signal to reduce engine output if the wheel speed differential exceeds a certain value.
When a force is applied to a tire, it produces a frictional force from the interaction of the tire and road surface. In our case, we're interested in the longitudinal force on the driven wheels which is being used to accelerate the vehicle and so the tire force is from torque generated from the engine. A tire cannot produce a frictional force to accelerate the vehicle without any wheel torque, so the frictional force can be expressed as a ratio of the wheel torque to frictional force, often called the coefficient of friction.
A driven tire has a slip ratio, which is the wheel speed divided by the actual speed of the vehicle. We can also measure the slip ratio as a percentage. e.g. a slip percentage of 10% means the tire is moving 10% faster than the road surface. One important thing to remember is that the slip ratio is not the same as wheel spin. When torque is applied to a tire the tire distorts, and the tire surface tends to 'creep' along the road without actually slipping, so a wheel can be moving faster than the road without any wheel spin. In practice street tires give 1-3% wheel slip (without wheel spin) under moderate acceleration, and drag tires have much more wheel slip, depending on construction.
The tire coefficient of friction depends on many factors, such as tire construction, road surface, tire loading, temperature, moisture etc, but generally it increases with increasing wheel slip, up to a point, and then decreases. For best acceleration if we wish to maximize the tire coefficient of friction we need to keep the wheel slip at the point of tire maximum coefficient of friction.
The above graph shows the friction vs wheel slip for a typical tire in the dry, with optimum wheel slip of around 6-7%. In the wet the optimum wheel slip tends is be much lower.
Calculating wheel slip involves using one or more wheel speed sensors on the driven wheels, and one or more wheel speed sensors on the un-driven wheels (the calculations are the same for both front and rear wheel drive). The wheel slip is how much faster the driven wheels are moving than the un-driven wheels.
The target wheel slip is the ideal wheel slip (which gives the greatest acceleration). Because this varies depending on conditions (the greatest is the difference between a wet and dry road surface), we need to adjust the target wheel slip. The traction control switch module has a six position switch which allows the target slip to be set on the fly. The exact wheel slip settings for each switch position are user-definable.
Over slip is the difference between the target wheel slip and the actual wheel slip.
When the wheel slip is greater than the target wheel slip, the ECU needs to reduce engine output. There are various methods of doing this (changing the throttle plate, cutting spark) but the most effective methods are retarding the ignition and selectively not firing cylinders.