LEARNING ABOUT KNOCK SENSORS
I captured more data over the weekend. I have been flummoxed a bit by what I'm seeing on the knock sensor, perhaps needlessly so.
The knock sensor is just a piezo microphone. It has a stub attached which appears to make it resonant around 14 kHz, and appears to help make it easier for the ECM to detect pings, somehow. You can see its resonance when you zoom into the signal. Perhaps detonation includes a wave that reflects inside the cylinder space around this frequency??? and maybe this is why it's not a good idea to use a sensor from another car? If a resonant sensor is used, perhaps this mechanically filters noise... could the ringing signal be rectified and the envelope used to detect the knock? Just guessing here; I'm probably wrong. (see the TI article linked to at the bottom of the post.)
This first screenshot was taken while at full open throttle as the car was passing through 6000 RPM in first, just before the downshift to second:
There is a LOT of analysis still to do (more on that in a second) but what was concerning me was that the knock sensor signal was not even. There is a ton of noise surrounding ignition TDC of cylinder two, while it is maybe 6 dB quieter around ignition of cylinders 1 and 3. You can see this at idle, too:
One possible explanation is that the knock sensor is directly beneath the intake port for cylinder 2. But see the annotations above. Why don't we see anything in the same position under cylinder 1 and just a little thing under cylinder two? (note that the knock sensor amplitude is scaled way up from the high RPM screenshot before; this is not a huge signal).
Now I'm wishing I could see some examples from other Insights.
Below we see something similar in lean burn under significant load at around 1700 RPM:
I don't know how the knock sensor is positioned relative to the cylinder wall (maybe someone who has a block and cylinder head separated can tell me???) but perhaps the knock sensor is set up to be a stethescope into cylinder 2, and it is just relatively deaf for cylinders 1 and 3.
But note also there are a number of pulses, and they don't all line up with the ignition signals. (Well, there is a delay from spark ignition to maximum pressure...) But still, there are more pulses, and at different places.
This is an area that I'm unfamiliar with, but it seems we could learn a lot from, and perhaps even use the knock sensor signal as an indicator as to the quality of engine operation.
Trying to learn more about the operation of knock sensors and how these patterns may appear, I found
this article providing useful background on the causes of knock and some factors that influence its onset.
This
TI article talks about different knock sensor types, as does the above article. From the ringing characteristic seen in the Insight sensor, my guess is that it is a tuned or resonant sensor, not a broadband sensor; tuned for resonance with the knock. Thus it should be easy to tell if an arbitrary sensor is compatible with the Insight: rap it (gently!!!!) against the bench, and see if the frequency of the vibrating element inside matches that of a genuine Insight sensor?
The TI article notes that background noise that increases as engine speed increases can mask knocking, and that some systems stop detecting knock above 4000 RPM.
Or is the omission of signal a sign of misfiring? The TI article mentions this too.
Thoughts are welcome.