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Atraction control system (TCS), is typically (but not necessarily) a secondary function of theelectronic stability control (ESC) on productionmotor vehicles, designed to prevent loss oftraction (i.e.,wheelspin) of the driven road wheels. TCS is activated whenthrottle input,engine power andtorque transfer are mismatched to the road surface conditions.
The intervention consists of one or more of the following:
Typically, traction control systems share theelectrohydraulic brakeactuator (which does not use the conventional master cylinder and servo) and wheel-speedsensors withABS.
The basic idea behind the need for a traction control system is the loss of road grip can compromise steering control and stability of vehicles. This is the result of the difference in traction of the drive wheels. The difference in slip may occur due to the turning of a vehicle or varying road conditions for different wheels. When a car turns, its outer and inner wheels rotate at different speeds; this is conventionally controlled by using adifferential. A further enhancement of the differential is to employ anactive differential that can vary the amount of power being delivered to outer and inner wheels as needed. For example, if outward slip is sensed while turning, the active differential may deliver more power to the outer wheel in order to minimize theyaw (essentially the degree to which the front and rear wheels of a car are out of line.)Active differential, in turn, is controlled by an assembly of electromechanical sensors collaborating with a traction control unit.
The predecessor of modernelectronic traction control systems can be found in high-torque, high-power rear-wheel-drive cars as alimited-slip differential. A limited-slip differential is a purely mechanical system that transfers a relatively small amount of power to the non-slipping wheel, while still allowing some wheel spin to occur.
In 1971,Buick introducedMaxTrac, which used an early computer system to detect rear wheel spin and modulate engine power to those wheels to provide the most traction.[1] A Buick exclusive item at the time, it was an option on all full-size models, including theRiviera,Estate Wagon,Electra 225,Centurion, andLeSabre.
Cadillac introduced theTraction Monitoring System (TMS) in 1979 on the redesigned Eldorado.
When the traction control computer (often incorporated into another control unit, such as the ABS module) detects one or more driven wheels spinning significantly faster than another, it invokes the ABSelectronic control unit to apply brake friction to wheels spinning with lessened traction. Braking action on slipping wheel(s) will cause power transfer to wheel axle(s) with traction due to the mechanical action within the differential.All-wheel-drive (AWD) vehicles often have an electronically controlled coupling system in thetransfer case ortransaxle engaged (active part-time AWD), or locked-up tighter (in a true full-time set up driving all wheels with some power all the time) to supply non-slipping wheels with torque.
This often occurs in conjunction with the powertrain computer reducing available engine torque by electronically limiting throttle application and/or fuel delivery, retarding ignition spark, completely shutting down engine cylinders, and a number of other methods, depending on the vehicle and how much technology is used to control the engine and transmission. There are instances when traction control is undesirable, such as trying to get a vehicle unstuck in snow or mud. Allowing one wheel to spin can propel a vehicle forward enough to get it unstuck, whereas both wheels applying a limited amount of power will not produce the same effect. Many vehicles have a traction control shut-off switch for such circumstances.
Generally, the main hardware for traction control and ABS are mostly the same. In many vehicles, traction control is provided as an additional option for ABS.
In all vehicles, traction control is automatically started when the sensors detect loss of traction at any of the wheels.
When programmed or calibrated for off-road use, traction control systems likeFord’s four-wheel electronic traction control (ETC) which is included withAdvanceTrac, andPorsche’s four-wheel automatic brake differential (ABD), can send 100 percent of torque to any one wheel or wheels, via an aggressive brake strategy or "brake locking", allowing vehicles like theExpedition andCayenne to keep moving, even with two wheels (one front, one rear) completely off the ground.[4][3][5][6][7]
Very effective yet small units are available that allow the driver to remove the traction control system after an event if desired. InFormula One, an effort to ban traction control led to a change of rules for 2008: every car must have a standard (but custom mappable) ECU, issued by theFIA, which is relatively basic and does not have traction control capabilities. In 2003,Paul Tracy admitted thatCART teams used traction control in the nineties, a device that was not formally legal until 2002 (although the switch to single engine supplier for 2003 reverted the legalization).[8] In 2008, NASCAR suspended aWhelen Modified Tour driver, crew chief, and car owner for one race and disqualified the team after finding questionable wiring in the ignition system, which could have been used to implement traction control.[9]
Traction control is not just used for improving acceleration under slippery conditions. It can also help a driver to corner more safely. If too much throttle is applied during cornering, the driven wheels will lose traction and slide sideways. This occurs asundersteer in front-wheel-drive vehicles andoversteer in rear-wheel-drive vehicles. Traction control can mitigate and possibly even correct understeer or oversteer from happening by limiting power to the overdriven wheel or wheels. However, it cannot increase the limits of frictional grip available and is used only to decrease the effect of driver error or compensate for a driver's inability to react quickly enough to wheel slip.
Automobile manufacturers state in vehicle manuals that traction control systems should not encourage dangerous driving or encourage driving in conditions beyond the driver's control.
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