How does knock control work?
11-16-2009, 11:33 AM
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How does knock control work?
I was wondering if anyone has any information on how the control system in the F20C controls the spark timing? Has anyone read anything on what the control system will do when it starts to sense knock from the knock sensor? Will only spark timing be changed or a combination of spark and fuel? Will this adaption stay for a limited time or will the spark timing be integrated back to the normal setting?
Has anyone seen the algorithm used?
Has anyone seen the algorithm used?
11-16-2009, 02:59 PM
#2
Im curious as to this myself. With the OEM ECU, im my experience, when I was last on the dyno... Seems the ECU saw some knock up top on my first pull and timing was Waaayyy too high and on the second pull it seemed better, richer and lower timing and no knock. Who knows, I wanna hear what an expert says in reguards to this.
11-16-2009, 05:01 PM
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Honda ECU knock control confuses a lot of people, mainly because it does not work as they would think. You may get knock retard without any knock, and knock without any knock retard, so you would be excused thinking Honda knock control is broken.
From the K-Series ECU onwards, knock control basically means adjusting the ignition timing for different octane fuel.
In more depth, you need to know how Honda tunes their engines. The engine goes onto an engine dyno with a high octance spec fuel, and they are computer tuned to set the fuel to give a stoichiometric air fuel ratio (14.7, 14.68, 14.58, depending on how pedantic you are about the fuel here), and the ignition is tuned to MBT. MBT = mean best torque, which basically means the ignition is set to give maximum engine output at each load / rpm combination, irrespective of knock.
After the engine has been tuned to stoichiometric and MBT ignition, the WOT fuel compensation tables are used to enrich the fuel for high load / fuel throttle. For ignition, there is a variable amount of ignition retard from MBT to ensure the engine does not knock. The amount of retard varies by load and rpm, but also by the amount of knock sensor noise that the ECU sees. This knock control value is a fairly long term thing, where it may increase fairly quickly which knock sensor noise, increasing the knock retard, and then slowly decrease if they is not much noise, decreasing the knock retard.
The knock sensor processing is fairly smart. The ECU knows which cylinder is compressing, and knows that knock is most likely to occur at 18 degrees past TDC, so it can work out what cylinder is knocking. You can datalog this.
The thing that gets most people at this point is that the ECU doesn't retard / advance very quickly - it is applying a pre-programmed retard based on estimated fuel octane rather then retarding / advancing quickly when it gets a small amount of knock.
The next thing that confuses is that the ECU will retard, even without any knock. This is because it may thing that even with the highest octane fuel it can't reach MBT timing, so will retard anyhow.
Next, the ECU may detect knock, increase the knock control value, but if the knock sensitivty is low, it may not retard enough (or at all), so you get knock but no knock retard.
So how do you tune around this? In short, tune the ignition timing so you don't get any knock counts. Otherwise a small amount of knock in one place will cause knock retard everywhere. When this happens on the dyno you can get more power everywhere by retarding the ignition just where it is knocking.
Finally, allow the factory knock sensor to do its job by not giving it additional noise to worry about. Typically culprits are exhaust headers hitting something, worn engine mounts, noisy superchagers - all these can give 'false knock', which both decreases engine performance and masks real knock.
There's probably more, but that's it for now.
From the K-Series ECU onwards, knock control basically means adjusting the ignition timing for different octane fuel.
In more depth, you need to know how Honda tunes their engines. The engine goes onto an engine dyno with a high octance spec fuel, and they are computer tuned to set the fuel to give a stoichiometric air fuel ratio (14.7, 14.68, 14.58, depending on how pedantic you are about the fuel here), and the ignition is tuned to MBT. MBT = mean best torque, which basically means the ignition is set to give maximum engine output at each load / rpm combination, irrespective of knock.
After the engine has been tuned to stoichiometric and MBT ignition, the WOT fuel compensation tables are used to enrich the fuel for high load / fuel throttle. For ignition, there is a variable amount of ignition retard from MBT to ensure the engine does not knock. The amount of retard varies by load and rpm, but also by the amount of knock sensor noise that the ECU sees. This knock control value is a fairly long term thing, where it may increase fairly quickly which knock sensor noise, increasing the knock retard, and then slowly decrease if they is not much noise, decreasing the knock retard.
The knock sensor processing is fairly smart. The ECU knows which cylinder is compressing, and knows that knock is most likely to occur at 18 degrees past TDC, so it can work out what cylinder is knocking. You can datalog this.
The thing that gets most people at this point is that the ECU doesn't retard / advance very quickly - it is applying a pre-programmed retard based on estimated fuel octane rather then retarding / advancing quickly when it gets a small amount of knock.
The next thing that confuses is that the ECU will retard, even without any knock. This is because it may thing that even with the highest octane fuel it can't reach MBT timing, so will retard anyhow.
Next, the ECU may detect knock, increase the knock control value, but if the knock sensitivty is low, it may not retard enough (or at all), so you get knock but no knock retard.
So how do you tune around this? In short, tune the ignition timing so you don't get any knock counts. Otherwise a small amount of knock in one place will cause knock retard everywhere. When this happens on the dyno you can get more power everywhere by retarding the ignition just where it is knocking.
Finally, allow the factory knock sensor to do its job by not giving it additional noise to worry about. Typically culprits are exhaust headers hitting something, worn engine mounts, noisy superchagers - all these can give 'false knock', which both decreases engine performance and masks real knock.
There's probably more, but that's it for now.
11-17-2009, 11:06 AM
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Thanks for the info Hondata!
Is it correct then that fuel settings are not changed then from knock sensor input?
It makes sense that the timing is adjusted all over the rpm range, this seems to be the case for several other systems as well.
Another question: Is the F20C sensitive to knock? Usually knock appears where the volumetric filling is as large as possible, this is pretty much the same as where the torqe is at its maximum. F20C has max. torque at ~7300 rpm. Isn't this at a high enough rpm so knock starts to get less impact since the piston is only at TDC at a very short time? Or do you have to be even higher up in the rpms for this effect to be seen?
(I have my litterature about engine control from the university at work..)
Is it correct then that fuel settings are not changed then from knock sensor input?
It makes sense that the timing is adjusted all over the rpm range, this seems to be the case for several other systems as well.
Another question: Is the F20C sensitive to knock? Usually knock appears where the volumetric filling is as large as possible, this is pretty much the same as where the torqe is at its maximum. F20C has max. torque at ~7300 rpm. Isn't this at a high enough rpm so knock starts to get less impact since the piston is only at TDC at a very short time? Or do you have to be even higher up in the rpms for this effect to be seen?
(I have my litterature about engine control from the university at work..)
11-18-2009, 05:52 AM
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- Is it correct then that fuel settings are not changed then from knock sensor input?
Yes
- Is the F20C sensitive to knock
Spark advance rises at rpm to compensate for the increased burn duration, so the engine can/will knock pretty much anywhere in the rev range. Peak torque is at just over 6000 rpm.
- Do the knock counters get reset every time the car is turned off?
Yes
Yes
- Is the F20C sensitive to knock
Spark advance rises at rpm to compensate for the increased burn duration, so the engine can/will knock pretty much anywhere in the rev range. Peak torque is at just over 6000 rpm.
- Do the knock counters get reset every time the car is turned off?
Yes
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03-10-2010, 05:28 PM
#9
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This may be a basic question but do the knock tables in the flash pro determine the amount of knock % required to add to the knock count?
What is the relationship between the knock % in the logs and the numbers in the knock table?
What is the relationship between the knock % in the logs and the numbers in the knock table?
