Knock Detection

[Tom Cloud]

>Hi,
>
>I have a question regarding the detection of knocking from too great an
>ignition advance and too low an octane fuel.
>
>If I understand the process correctly if the ignition is advanced too far
>and the octane level is too low the fuel will burn unevenly creating uneven
>pressure wave fronts resulting in the pinging noise.  Backing off on the
>timing corrects this for a given RPM/Throttle Position.  The fundamental
>frequencies are 5KHz to 11Khz and the Knock Detection devices produce a
>voltage out based on the amount of Knock Noise detected.
>
>So here is the question.  For an over advanced engine does the knock only
>occur between the time of ignition and Top Dead Center or does it carry over
>into the Power Stroke?  Note I am curretnly not interested in Engine knock
>due to lean fuel or othe rconditions.  Only knock that shows up when the
>ignition is advanced too far. 

I'm no expert ... that said, here's what I've learned.  There's
two kinds of knock -- (and I'm not sure I'm using the correct terms).
There's the knock caused by low octane or too great an advance (pinging)
and there's that caused by the fuel exploding violently rather than burning
in a controlled manner (detonation??).  The first, it would seem, only
occurs BTDC. the second, I would think, could occur at any point in the
cycle.  Both are destructive though I've been told the detonation is
the worst.

I do know about the knock sensors.  They are just piezoelectric
microphones.  I've read that they are "tuned" to knock frequencies
but I doubt the veracity of those comment.  I've checked one and
it seems to respond quite nicely to ALL engine noises!  If you
understand the construction of microphones / accelerometers, you
know that it will have a natural resonance at some frequency just
due to the fact it has mass, shape, etc and that will cause it to
react "better" to frequencies near its resonant point -- but I'm
not sure that's what is meant when it's said to be "tuned".

I doubt you'll be able to distinguish knock due to octane/advance
from that due to detonation though it would seem they would have
a different characteristic. And I certainly can't imagine how you
could distinguish knock due to advance from that due to low octane
since they're the same thing???  Anyone ??


Tom Cloud