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A traction control system (TCS), in German known as Antriebsschlupfregelung (ASR), is typically (but not necessarily) a secondary function of the anti-lock braking system (ABS) on production motor vehicles, designed to prevent loss of traction of driven road wheels. When invoked it therefore enhances driver control as throttle input applied is mis-matched to road surface conditions (due to varying factors) being unable to manage applied torque.
Intervention consists of one or more of the following:
Typically, traction control systems share the electro-hydraulic brake actuator (but does not use the conventional master cylinder and servo), and wheel speed sensors with ABS.
The predecessor of modern electronic traction control systems can be found in high-torque, high-power rear-wheel drive cars as a limited 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 introduced MaxTrac, which used an early computer system to detect rear wheel spin and modulate engine power to those wheels to provide the most traction. A Buick exclusive item at the time, it was an option on all full-size models, including the Riviera, Estate Wagon, Electra 225, Centurion, and LeSabre.
Cadillac introduced the Traction Monitoring System(TMS) in 1979 on the redesigned Eldorado.
The basic idea behind the need of a traction control system is the difference between traction of different wheels evidencing apparent loss of road grip that compromise steering control and stability of vehicles. Difference in slip may occur due to turning of a vehicle or differently varying road conditions for different wheels. At high speeds, when a car tends to turn, its outer and inner wheels are subjected to different speed of rotation, that is conventionally controlled by using a differential. A further enhancement of the differential is to employ an active differential that can vary the amount of power being delivered to outer and inner wheels according to the need (for example, if, while turning right, outward slip (equivalently saying, 'yaw') is sensed, active differential may deliver more power to the outer wheel, so as to minimize the yaw (that is basically 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.
When the traction control computer (often incorporated into another control unit, like the anti-lock braking system module) detects one or more driven wheels spinning significantly faster than another, it invokes the ABS electronic 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 a differential. All-wheel drive (AWD) vehicles often have an electronically controlled coupling system in the transfer case or transaxle 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 (more) 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 can't get the same effect. Many vehicles have a traction control shut off switch for just such circumstances.
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 like Ford’s four-wheel electronic traction control (ETC) which is included with AdvanceTrac, and Porsche’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 the Expedition and Cayenne to keep moving, even with two wheels (one front, one rear) completely off the ground.
Very effective yet small units are available that allow the driver to remove the traction control system after an event if desired. In Formula One, an effort to ban traction control has led to the change of rules for 2008: every car must have a standard (but custom mappable) ECU, issued by FIA, which is relatively basic and does not have traction control capabilities. NASCAR suspended a Whelen Modified Tour driver, crew chief, and car owner for one race and disqualified the team after crossing the finish line first in a September 20, 2008 race at Martinsville Speedway after finding questionable wiring in the ignition system, which can often be used to implement traction control.
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 drive wheels will lose traction and slide sideways. This occurs as understeer in front wheel drive vehicles and oversteer in rear wheel drive vehicles. Traction control can prevent this from happening by limiting power to the wheels. It cannot increase the limits of 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 drivers' control.