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Author Topic: Acceptable limits for knock voltage  (Read 92200 times)
ElementalVoid
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« on: March 09, 2011, 12:56:05 AM »

When measuring knock voltage via blocks 26 and 27 what is an acceptable limit?
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carlossus
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« Reply #1 on: March 10, 2011, 02:02:30 PM »

I too would appreciate some wisdom on this.
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julex
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« Reply #2 on: March 10, 2011, 04:41:37 PM »

When measuring knock voltage via blocks 26 and 27 what is an acceptable limit?

That's a good question for seasoned stage 3+ tuners/users. You can find this information indirectly on EcuX plots where knock voltage is usually graphed. If my memory serves me well, it is not uncommon to get 40-50v with significant timing corrections:

http://forums.quattroworld.com/s4/msgs/13648.phtml
http://forums.quattroworld.com/s4/msgs/11637.phtml
http://forums.quattroworld.com/s4/msgs/10458.phtml
http://forums.quattroworld.com/s4/msgs/98949.phtml

and so on...
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carlossus
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« Reply #3 on: March 10, 2011, 05:12:35 PM »

There's a gulf between these guys and the casual stage 1/2 hobbyist.
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julex
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« Reply #4 on: March 10, 2011, 05:19:22 PM »

There's a gulf between these guys and the casual stage 1/2 hobbyist.

The question was about acceptable levels... so here you go.

Keep in mind that knock sensor voltage gets taken consideration and correction factor CF is used to indicate timing retard ECU is instituting to lower the knock... Too much knock, ECU retards timing until the knock is acceptable. You can control aggressiveness of this behavior by one of the maps... But keep in mind that the correction goes down by maximum 12deg so if you put in some stupid timing values you will still kill your engine if timing causes massive knock.
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carlossus
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« Reply #5 on: March 10, 2011, 05:23:18 PM »

You're right, thanks for the links.
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ElementalVoid
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« Reply #6 on: March 11, 2011, 12:36:43 AM »

Very helpful guys. Thanks!
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silentbob
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« Reply #7 on: March 11, 2011, 06:16:17 AM »


I would forget about the Knock voltage as an indicator for how close you are to the knock limit.
It's way to complicated to explain here when you have no background on that, but if you are interested in how knock detection works you can read and try to understand what is written in KRKE in the ME7 Funktionsrahmen.

By far more important is to keep the CF under control. Always make small steps when you ajust timing and log your CFs. Although there is also a load limitation when continous knock occurs that can't get under control with ignition retard you shouldn't push the limits here.

« Last Edit: March 11, 2011, 08:46:54 AM by silentbob » Logged
silentbob
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« Reply #8 on: March 11, 2011, 10:29:02 AM »

As I bored over the weekend anyway I can do a short writeup on how knock control works on ME7 if there is demand for it.
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julex
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« Reply #9 on: March 11, 2011, 11:22:27 AM »

As I bored over the weekend anyway I can do a short writeup on how knock control works on ME7 if there is demand for it.

Please Smiley
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silentbob
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« Reply #10 on: March 13, 2011, 05:56:38 AM »

OK first of all I strongly advise you to leave all the maps I will mention alone because you can cause serious damage to your engine if you change something and you don t know what you are doing !!!!
What I have read so far on Audi forums implies that hardly anyone knows how knock control works in a modern ECU so I will start with some basics first and then describe it based on a calibration workflow.  
For a better understanding I will do some simplifications but if anyone has some deeper questions feel free to ask.
What is knock: When talking about knocking combustion you refer to the phenomenon that parts of the fuel/air mixture, that has not been reached by the flame front that was initialized by the spark plug, self-ignites due to the rising pressure/temperature in the combustion chamber.  This causes pressure vibrations in the combustion chamber which leads to the characteristic pinging noise. The shockwaves (as well as normal engine noise) induce structure-borne acoustic oscillations that can be measured with a knock sensor.
How does knock detection work:
Main problem is to distinguish normal engine noise from knocking noise.  
To do that the signal from the knock sensor is amplified, filtered and then integrated over a certain range after tdc (knock window) for each cylinder on each combustion cycle.  
For calibration the engine is equipped with cylinder pressure sensors and an indication system to monitor knock while operated on an engine dyno.



As a first step you usually determine the knock windows which are KFMAKR and KEMLN in ME7 by letting the engine knock and log the cylinder pressures and Knock sensor signal.



The next step is to find a proper frequency for filtering the knock sensor signal that is least impacted by general engine noise. You identify this by a frequency analysis of knocking combustion cycles with 3D graphs that have the knock intensity and frequency as x and y and the knock sensor voltage as z (sorry can t upload an example for that). You can calibrate 4 frequencies (FMFKRBx) depending on rpm (NKRFMx) on ME7. On S4/RS4 files only one is used (12kHz).
When you have your knock windows and frequencies you can calibrate the noise levels that define the threshold between normal noise and knock. The values are calibrated for each cylinder separately in KFKEx depending on RPM and LOAD. They represent the quotient of the current integrator value to the reference level (ikr(x)/rkrmx).



The reference level for each cylinder is limited to the values in RKRMX1N and RKRMX2N. This two maps group the cylinders in quiet and noisy cylinders by a bitmask. The limit is used to ensure a good recognition under noisy engine conditions.



What is also taken into consideration is rate of change in load (KFELDY) and rpms (KFENDY) which is multiplied with the values out of KFKEx.
When Knock is detected the ECU retards the ignition angle with every knock even by the values in KRFKN/KRFKLN on the affected cylinder.  When knock isn t present any more the ignition angle is phased back to normal with KRVFN/KRVFSN.
As you can see there is no easy way to link the knock voltage to how close you are to the knock limit as first of all it is dependent on the general noise level of every individual engine and second highly relative to the engine operation point and transient condition.
« Last Edit: March 13, 2011, 10:35:45 AM by silentbob » Logged
carlossus
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« Reply #11 on: March 13, 2011, 07:10:53 AM »

This is excellent. Perhaps you should add this to the Wiki?
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jibberjive
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« Reply #12 on: March 13, 2011, 01:22:33 PM »

Really good post!
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Rick
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« Reply #13 on: March 14, 2011, 05:31:33 AM »

I defined these maps in my xdf some time ago.  I'm looking at tuning them soon, as I think the ECU is too sensitive to knock from what i have found listening through det cans.

Rick
« Last Edit: March 21, 2011, 01:54:43 PM by Tony@NefMoto » Logged
silentbob
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« Reply #14 on: March 14, 2011, 02:31:23 PM »

I defined these maps in my xdf some time ago.  I'm looking at tuning them soon, as I think the ECU is too sensitive to knock from what i have found listening through det cans.

Rick

I have never used det cans, but how do you seperate normal engine noise on 6000+ RPM from knocking noise without cylinder pressure sensors? Sure the OEM calibration is always a compromise to cover the deviations in series production. It's more on the safe side but you have knock endurance tests with statistics to validate the calibration which don't tolerate a great number of false knock recognitions. I can hardly see how you can improve this with the method you will use without a high risk to blow up your engine.
« Last Edit: September 13, 2011, 05:27:32 PM by silentbob » Logged
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