How does traction control work?

[gijim]

Okay, wheel sensor detects rear wheels are slipping on acceleration. (You floored it from a stop in slick conditions) On the newer engines with electronic throttles, they can just close the throttle a little bit... but if you have a mechanical throttle, how do you reduce engine torque enough to regain traction?

The common answer seems to be "retard the timing", but can that reduce the torque enough?

 

[Fabrico]

Many systems control loss of traction while accelerating, by quickly applying brake pressure to slow down the spinning wheel. Similar to ABS, but backwards.

 

[GregLocock]

Retarding (or even cutting ) the spark is faster than controlling the throttle.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[gijim]

I've heard cutting the spark is bad on the engine/cat/muffler... especially if it backfires when the raw fuel hits the exhaust.

But I don't think cutting fuel on a WOT run is a good idea either.

 

[GregLocock]

Well I guess that's why calibrators get the big bucks (haha), they tune the response so as to minimise the damage and maximise the benefit. The brakes are neat and fast, but down the track I think you'll see more done in the diffs themselves.

Bear in mind that they are way out of any concerns about fuel consumption and emissions when calibrating this stuff, and also bear in mind that at WOT they are probably running very rich to cool the cat and valves anyway, so running the cat feedgas even richer is no great disaster.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[NickE]

One thing that using the brakes provides is some sort of troque transfer across the diff (open).

In fact I think that mercedes uses a sort of logic to braking across the three open diffs to provide an active torque control in their awd (?4-matic?) system.


Nick
I love materials science!

 

[GregLocock]

Yes, there's a sort of battle going on between those who want to fit open diffs throughout the driveline, and then control the torque split via the brakes, and those who want to make the diffs more complex and keep the brakes for braking.

A cynic might argue that the second proposal would be better for a real off roader, and all the soft roaders would be happy enough with the first.

When you add in a vehicle stability system the argument gets even more complex. Personally I like the idea of steering the car via left to right torque split, rather than the brakes.


Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[NickE]

I agree with the second camp, but I'm more of a performance and technology type of guy.

As far as hardware cost I can easily see why open diffs and a brake controlling system to modulate torque at each wheel is better for the sales guys.

But I also think that the really awesome electro hydraulic diffs that the WRC uses are the cats pyjamas...

Imagine a car that can accelerate as fast as an F1 car, on dirt, snow, and tarmac.

 

[dgallup]

The brake solution also sells lots of replacement pads and rotors if you abuse the system. Ignition retard and cylinder cutout are quite effective at controlling power. In some racing series you can hear the engines popping and rasping as the power is cut in low traction situations.

 

[Warpspeed]

Another effective way to reduce engine torque is to place a second throttle body in the induction path in series with the main throttle body.

That can then be operated quite independently from a stepper motor, very smooth, and with a very wide range of control.

 

[harrisj]

NickE says: "Imagine a car that can accelerate as fast as an F1 car, on dirt, snow, and tarmac."

Sorry - no I can't. Not if propulsive forces go through tyres anyway. Rubber on snow just won't do it, however much traction control you've got.

 

[LordMalaK]

Traction control- another system designed for idiots who really don't know how to drive properly.

I've never been a fan of 'drive by wire' and am constantly amazed how some peeps survive their daily commute whilst glued to the cell phone, or tinkering with their radio or nav system. I'm absolutely floored by the new Lexus coming out soon that will parellel park ITSELF.

No freekin wonder todays techs need a degree in computers just to fix the brakes. The cars price goes up- the drivers I.Q. goes down. Thankfully my car still has a throttle cable and not a computer network controlling my throttle plate, brakes, ignition timing, and fuel mixture. So what if I can vaporize the tires at 50MPH and only get 17 MPG. At least when an alternator diode dies, my car doesn't immediately shut down.

 

[mattsooty]

LordMalak,

Ha, Ha, Ha - your starting to sound a bit like a luddite there!

Surely progress is the name of the game: fuel economy, safety, customer satisfaction etc etc....

MS

 

[gijim]

LordMalak -

Well at least I don't have to change points! hahaha!

So without sophisticated brake actuators, or extra parts (i.e. electronic throttle) is the timing retard the best way?

How much can you safely retard the timing at WOT? What are the dangers of doing so? (Burnt valves? Overheating exhaust?)

 

[patprimmer]

I would think a large drop off in power might be the biggest problem with pulling out to much spark advance.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[mattsooty]

Modern EMS systems use a 'torque down' system; as opposed to just cutting the spark x number of degrees the EMS will reduce the torque by x %. There is a big difference because the torque management system will have been calibrated so that the correct amount of spark (fast path torque) is safely removed before the throttle (slow path torque) must be moved - with out burning valves etc. Infact there will probably be an exhaust temp limit function that works all that out as well - if it has been calibrated correctly.

The thing that can often get overlooked is that during large spark retards the boost increases and the ECU starts to worry that something is going wrong causing all sorts of problems.

MS

 

[HLC]

Read ....... (search for ["traction control" "generic manual" site:uk] in google, and download that PDF file. I am not allowed to promote according to the rules)to understand how these systems work, including selective injector cutting or selective ignition cutting..

Since almost all race and rally cars work with multiple throttle bodies controlled by cable still this will apply for those cases.

Of course with almost all road cars in Europe now with ETC you may as well close the throttle a bit as already explained above. Stability control uses the selective brake method..

Hope that answers your question and that you may install many traction controls..

 

[LordMalaK]

gijim-

Well at least I don't have to change points! hahaha!

No, no points- but the first thing I did when I bought my car was rip out the fuel management computer and replace the distributer with a stock GM HEI unit with a vacuum advance unit, and I recurved the mechanical advance to match that of the 72 vette (which is where I got the engine).

Anyhow, I'm not familiar with the traction controls that apply brakes to slow the 'gripless' wheels to regain traction, but I've worked on countless others whose engine management system momenterily changes the 'performance map' to lower engine output until traction is regained.

But as the markets work, applying brakes to control traction makes absolutely NO sense, so thats probable where the market is heading (so you buy more pads/rotors). I still chuckle when I remember a certain mercedes that came into our shop with a trac light on, ABS light on, and the trans/engine in 'limp mode' because a brake lamp bulb burned out. About a 200 dollar repair bill to clear all the computer codes, and replace a 2 dollar bulb.

 

[Surestick]

Well, it does make sense if you are using an open differential which will transfer torque to the wheel with less traction.
Applying the brakes on the slipping wheel will make it look like it has traction & the differential will transfer torque back to the wheel with traction.

If you are doing it on both/all wheels though it seems like it would just be easier to reduce torque at the engine by closing the throttle, retarding timing, cutting fuel or cutting boost.

 

[Warpspeed]

Makes sense to me as well, where the car has 4WD and three open diffs, especially if the car already has ABS fitted.

Just about all of the brake control hardware and wheel speed sensors would already there, so all it needs is additional software to make the system work.

I don't see that increased pad and disk wear is really going to be a serious issue either. At least not on an ordinary road car.