Traction Control is the art of preventing the loss of traction of a vehice under any circumstances, overriding the driver's inputs if necessary.
A very common example is ABS, or anti-block braking system. If it detects the wheels are about to lock up during braking it will intervene and pulsate the brakes in order to keep the wheels as close to locking up as possible without actually locking them up. This is the most efficient way to stop the car, and in general an ABS system will outperform the avarage driver, is always alert, and will always react in a split second. It is however not without flaws: when driving on snow you usually have a better chance at stopping if you just let the car slide with snow piling up in front of the wheels, than the ABS alternative of rolling along, or on top of the snow.
Under low traction acceleration, some ABS systems are able to apply the brakes to wheels that are losing traction. This has the effect of transferring torque across the differential to the opposite wheel, which may have more traction. It can also be used for stability control to balance and adjust the car's attitude. This technique is used by Mercedes Benz M-series, Range Rover and Subaru, among others.
It is not recommended to interfere in any way with a vehicle's braking systems, including modifying existing ABS systems. The type of traction control which is described here does not interfere with the braking system, and only has the ability to limit engine power, not increase it
Another form of traction control is during normal driving where power is delivered to the wheels (most pronounced during accelerations). If it detects the wheels are about to lose grip (do a "wheelie") it will limit the power output as much as necessary, but no more, to maintain traction.
Question: How do detect loss of traction?
Answer: sensors. Wheel sensors, the more sensitive the better. This means continuously monitoring the angular velocity of the wheels, not just a magnet-and-hall-sensor kind of setup. If there is a disrepancy between front/rear, right/left wheels it means (usually) the faster wheel has lost traction.
Note that this kind of extreme accuracy is required only for ABS.
Question: Ok, so we're about to lose traction, what can we do?
Answer: It depends (hah!)
- Some cars have a drive-by-wire system where the InTake/ThrottlePlates are controlled electronically based on a ThrottlePositionSensor?, i.e. how far the accelerator is depressed. A perfect drive-by-wire system has no latency (lag) and behaves exactly like a direct mechanical connection between the accelerator and the InTake/ThrottlePlates. What's the point then, some might ask. With drive-by-wire the ECU can limit what the driver can do, it can limit the rate of acceleration (for example when cold), and smooth out the driver's Right Foot movements. With a mechanical throttle, if the driver floors the pedal, the engine has to go along with it, it has no choice. Drive-by-wire can turn this into a smooth gentle acceleration if it thinks you're being harsh on the engine. And it can also limit the throttle if it thinks traction is about to be lost.
- Cut the spark. Easy enough, no spark means no power. The problem with this is that the fuel keeps flowing, potentially wetting the plugs, and pumping raw fuel down the exhaust. All of these things are very bad. A catalyst converter will certainly be destroyed if it has to swallow raw fuel, never mind the potential of reenabling the ignition and have the new hot exhaust ignite the fuel in the exhaust system. Not good!
- Cut the fuel. The only concern here is in systems such as TBI where the fuel travels through a wet manifold, if the fuel is suddenly cut there might be enough mixture in the manifold for a few lean combustions, and again when the fuel is turned back on. Someone who has experience with this type of traction control, please add your experiences here. Then again fuel-cut-on-deceleration seems to work alright also. I will try this someday and write more about it! Anyway, if you are not in control of the ignition system, this might be the only option you have to implement traction control.
- Retard the ignition timing. The most elegant solution (IMHO), reduce power output by retarding the ignition timing. No drawbacks, except perhaps that this can not reduce the power as much as cutting the fuel can, i.e. it compensates for a sloppy Right Foot, but if the driver floors it, it might not be enough. I will try this also someday.
Others please post your experiences and thoughts
- Decrease Boost(Electrical boost controller) This should be easy if you are using bleeder system. Just let the wastegate see the real boost instead of bleeded boost.
From racelogic traction control FAQ:
The Racelogic system works on individual injectors, and is designed only to cut out one whole injection pulse. The amount of fuel is not just decreased, it is completely removed for one stroke. This means the engine either receives a 100% complete amount of fuel or none at all. If it receives none at all, the spark has no effect, the mixture will not ignite, and therefore the temperature does not go up. We have succesfully fitted the system to Formula One engines, 750 BHP Turbo Engines (in our own Supra), motorbike engines and kart engines.
A feature that we must implement: notice the loss of tire-pressure (in one or more wheels but not if all wheels lose pressure at the same rate) from the rotation counts/speed of the 4 wheels. Periods when 1 or more wheels are slipping or off-the ground must be neglected. Maybe it must be called "wheel-speed difference counter" instead of tire-pressure thingy, if there are related patents. (don't sell products that violate patents, unless the patents are trivial to nullify; anything that collides with patents can be published, but not sold or licensed for money, so anything such must fall under the public-license part of the firmware - as most part of the firmware anyway).
Would this really work? Speed difference between the two wheels should be negitable as the angular velocity shouldn't change. Not anymore than say when you're turning a corner? --FredrikJ?
Yes, this would work. Definitely some smarties are needed to eliminate fake alarm because of cornering or blocking/slipping wheels.