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** Tried it and seems pretty useless. Check this record: https://quasar.dynaweb.hu/~lezsi/bmw/knock-hit-on-block-test.mp3
I couldn't get my resonance frequency in spectral view or freq analysis either.
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* This is very hard without WideBand closed-loop EGO control, because the engine will want to stop before any knocking, and adjusting the fuel manually at the same time ignadv is adjusted is hard.
* note that the MAP should be low, 25..40 kPa (max 55kPa), otherwise the engine is endangered. RPM also low (<1500 RPM). * play back the sound after the engine stopped. You'll be surprised how well the ping sound can be heard without the loud exhaust sounds. * just read the characteristic frequency from the FFT diagram. |
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* This is very hard without WideBand closed-loop EGO control, because the engine will want to stop before any knocking, and adjusting the fuel manually at the same time ignadv is adjusted is hard.
* note that the MAP should be low, 25..40 kPa (max 55kPa), otherwise the engine is endangered. RPM also low (<1500 RPM). * play back the sound after the engine stopped. You'll be surprised how well the ping sound can be heard without the loud exhaust sounds. * just read the characteristic frequency from the FFT diagram. ** I was going to play high and raised ignition advance gradually on boost to get signals just like real life situations. A snippet from slight knock:
I think frequency analysis didn't justify expectations rather amplitude changes shows quite a difference! I've marked the knocking area with gray background. The long "needles" in the waveform view are the "pings". This is a shot of a bit heavier knock (still couldn't be heard by naked ear):
Unfortunately it is clipped because of excessive volume. This is a compilation of three records, one with no knock, second with slight knock and third with heavier knock: https://quasar.dynaweb.hu/~lezsi/bmw/knock_record.mp3 After listening to it a few times I can recognize very slight knock (high pitch "pings") even where amplitude or spectral analysis doesn't show. Interestingly only left channel (knock sensor 2) hears slight knock -it should be a rear cylinder- and it is not reported by sensor 1. - It is clear that distinguishing sensors with cam-sync is a must for large engine blocks. (BMW uses two sensors even on 4cyl engines) - Second observation is HUGE amplitude changes over rev-range. I think calibration of noise (in best case by one-time learning) is necessary on several (4-8?) points of revolutions. It seems much more consequent to base sensing on amplitude deviances from a set of tight (noise amplitude) reference values. Even frequency calibration seems unrelevant compared to those big amplitude differences. I have to point out that all these experiments were slight knock without making any damage to my engine, and were -in fact- undetectable from the cabin. |
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