A traction control system (TCS), also known as ASR (from German Antriebsschlupfregelung, engine slippage regulation), is typically (but not necessarily) a secondary function of the electronic stability control (ESC) on production motor vehicles, designed to prevent loss of traction of driven road wheels. TCS is activated when throttle input and engine torque are mismatched to road surface conditions.
Intervention consists of one or more of the following:
Typically, traction control systems share the electrohydraulic brake actuator (which does not use the conventional master cylinder and servo) and wheel speed sensors with ABS.
Perhaps pulsated power delivery from a 1,2,3 or four IC cylinder engine could be considered the original traction control. IC engine power pulses are usually considered to be from 45 crankshaft degrees (high compression plus aggressive camshaft) to as much as 100 degrees (low compression, mild valve timing, often supercharged). A single cylinder vehicle (e.g. motorcycle) will have a power pulse and the driven tire momentarily will slip more than the optimum necessary for maximum thrust (3-5 % pavement, 15-250 % on dirt track). The engine flywheel will tend to maintain this speed but friction of many types will cause greater or lesser slowing down over the subsequent exhaust and intake strokes totaling about a full 360 degrees crankshaft revolution. Analogous to modern active systems is when the following compression stroke causes a large decrease in drivetrain speed including the tire. Often this effect will cause the tire to "grab" the road or track surface momentarily so that the immediately following power pulse can have more thrust-effect than otherwise. A dirt or pavement racing vehicle can thus be more effecively driven with rough power delivery than with perfectly smooth "dial-a-torque" power (electric motor, gas/steam turbine, 12+cylinder IC engine). Dirt track racing motorcycles have long experimented with flywheel inertia and compression ratios to get a desirable balance of roughness and smoothness. Triumph 360 degree vertical twin engines were often modified to be "Twingles" i.e. - both cylinders firing simultaneously. Harley Davidson V-Twin engines were modified so that the front cylinder fired 45 degrees after the rear one. These practices were AMA-outlawed in the 1950s. This effect is also readily apparent if one experiments with a 4-cylinder car with a perfectly flexible and willing engine coupled to a manual transmission. Very slow engine operation from 125 rpm down to 25 rpm (usually in 2nd, 3rd or 4th gears) often will allow climbing polished icy surfaces without resorting to sand or chains. (caution! - NO OIL PRESSURE available at those rpms). In the 1990s Grand Prix roadracing motorcycles once more experiemnted with rough power delivery. Commonly all 4 cylinders of a 2 or 4-cycle engine were fired in 180 degrees (2- stroke) or 360 degrees (4-stroke) in order to enhance the riders ability to modulate engine power coming out of corners. 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 for a traction control system is the loss of road grip that compromises steering control and stability of vehicles because of the difference in traction of the drive wheels. Difference in slip may occur due to 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 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 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 the yaw (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.
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 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 the 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 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 to 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 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 led to a 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 driver's control.