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 Post subject: Traction control
PostPosted: Sat Nov 26, 2005 11:07 
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From: http://www.safespeed.org.uk/forum/viewt ... 4615#54615

Roger wrote:
Not sure how relevant this is, but a quick bit of praise for the traction control/stability system on the latest gen Accord: About a year ago I had to visit a farm in Lincolnshire somewhere. I had followed the Satnav which - correctly as it turned out - took me on a road that I thought was metalled with a bit of mud on it. not so. It was a clay road - and it was raining.

I half expected to have to be applying steering corrections to take account of the odd bit of slippage. What I did notice was what I at first thought was the engine misfiring. however I soon realised it was traction control - and I had not had to apply ANY corrective steering. I tentatively pushed the "disable button". Engine started behaving, but car was going everywhere. I pushed the button again!


This leads me to wonder about the power reduction strategies used by current traction control electronics. There are two problems that must be avoided. If the mixture becomes too weak then there's a risk of engine damage. If the mixture becomes too rich (especially unburnt) there's a risk of damage to the cat.

Ok so in the first instance retarding the ignition is going to be helpful. But if you retard too far then unburn mixture will damage the cat.

If you turn off fuel to some cylinders that might be OK, but I can see potential problems with weak mixture.

If you shut the throttle with a drive by wire system then you won't get the misfire that Roger describes.

So how does modern traction control work?

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 Post subject:
PostPosted: Sun Nov 27, 2005 01:13 
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I'm confident that there were two factors at work in my case that I described under the umbrella as a misfire. In fact, my particular car is "fly by wire" throttle, so I'm certain that the power was regulated by controlling the butterfly, noty ignition timing or mixture strength per se. Snapping the throttle shut and then opening it again to the level I had set it at with my foot simulates a misfire. The other effect is to brake the wheel that is slipping at the same time, giving further misfire impression and the feel of a limited slip diff.


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 Post subject:
PostPosted: Fri Dec 16, 2005 20:55 
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Got this from a major manufacturer. It wasn't Honda but I guess that many of them are similar in principle. On cars with "throttle-by-wire" they tend to use the brakes and the throttle to help you out depending on the conditions.

"Acceleration Slip Regulation (ASR)
If the drive wheels tend to slip when the vehicle is starting off or accelerating, the ASR intervenes by means of a brake and/or an engine control circuit. The ASR also detects slippage at one or both wheels by evaluating signals input from the wheel rpm (ABS) sensors.
If the ASR system detects a slipping wheel while the vehicle is starting off, the wheel is braked as required until no more slippage is detected at that wheel. This intervention increases the vehicle traction.
If the ASR system detects a slipping wheel while the vehicle is en route, this wheel is braked as required and the engine torque is reduced until no more slippage is detected at that wheel. This intervention increases the vehicle's directional stability.
If both wheels continue to slip even after the brake system has intervened, the ASR system then escalates the intervention by communicating with the engine control system via the CAN bus, causing the control system to reduce the engine torque. The engine torque is reduced even if the accelerator pedal is, in the extreme case, pressed all the way to the floor.
The operation of the ASR system is canceled whenever the brake pedal is operated.
To prevent the system from intervening in the engine control system, the driver can press the "ASR Off" switch.

Engine Braking Regulation (EBR)
When the accelerator pedal is released and the vehicle is decelerating, deceleration slip may occur at the drive wheels. In this case, the ASR/EBR systems
intervene by communicating with the engine control system via the CAN bus, causing the control system to increase the engine torque."


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 Post subject:
PostPosted: Sat Jan 14, 2006 19:53 
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ok well i don't know a great deal about the EMS side of things, just that often the input to it is a torque demand.. or demand for a certain reduction in torque.

a few generic variants:
engine only - as described above, retard ignition (etc?) to reduce torque to drven wheels & reduce slip.

brake only - probably not practically used. use the hydraulics usually associated with ESP to brake the driven axle & reduce slip. nasty. but can also operate as below.

brake & engine
- for straight line traction launch usually just demands engine torque reduction
- if the driven axle is on different surfaces (ice patch?) the diff means that wheel will spin up and you go nowhere. in this case the brake is applied to that wheel to allow some drive to go to the one that has some traction.
- what order of precedence these come in probably depends on the system & vehicle tuning.


EBR as defined in mole's post can just be an add on to ABS (i.e. not necessarily full ESP & brake based TC).. as it is aiming for the same purpose - preventing wheel lock under braking. not much point having a superb ABS system if the driver brakes in a low gear and the engine drag is sufficient to lock the wheels. in this case as stated a increase in engine torque is demanded! ... disturbing.. but fortunately only usually required in extreme circumstances (i.e. braking/downshifting in a low gear on ice).

thats my take on it anyway.

not done much tuning of it myself but the ultimate test is usually a split tarmac / polished ice traction hill..... where a bad tune will see you going backwards :lol:


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 Post subject:
PostPosted: Sat Jan 14, 2006 22:31 
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well i can't promise this kind of service all the time... but i was talking to our TC boys over dinner!

the torqe reduction is indeed (usually) by a combination of spark retardation & throttle. some mention of reducing turbo pressure too but i dont think that's currently used much.

spark retardation is usually used first as it takes immediate effect on that ignition cycle whilst cutting off the fuel takes longer (a few ignition cycles i'm guessing) in which time the wheels are still spinning and the car not going anywhere.

the spark retardation should still burn all the fuel just not at the optimum point, whilst yes throttle intervention has emission & CAT issues. (i think)


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 Post subject:
PostPosted: Mon Jan 16, 2006 11:55 
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[meant to add, if you can't post in here and have a qvestion, pm me]


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 Post subject:
PostPosted: Thu Jan 19, 2006 13:41 
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one of my outline descriptions for completeness...


Purpose
Prevent spinning/excess slip of driven wheels during driving, maintain traction when driven wheels are on surfaces with differing grip levels.

Basic Function
Often integrated in ESP algorithms (brake control requires extra valves to ABS alone), monitors wheel slip (in the same way as ABS) and demands reduction in engine torque (requires interface to engine management) and/or applies brake if required.

See above posts on this thread for more detail.

Hardware
(minimum-engine only)
Wheel speed sensors + Engine Interface
(common-engine+brake)
see ESP (coming shortly-ish)

For further info & how to comment on this post please refer to active vehicle systems


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