Re: Who can that be knocking at my...piston?

[jadney@vwtype3.org (Melissa & Jim)]

>From: t.hughes@chem.csiro.au (Tim Hughes)

>My understanding of preignition is that it occurs before the spark has
>occurred hence the "pre" in the name.  It occurs when the piston is
>travelling up while compressing the air fuel mixture, and as you said the
>mixture is prematurely ignited by something hot in the combustion camber
>(typically sharp edges).  The knocking noise comes from the explosion
>occurring while the piston is coming up (wrong direction- it is trying to
>compress the explosion) hence causing damage. My understanding is that the
>use of higher octane fuel and/or tetraethyl lead reduces the tendency of
>preignition.  I understand pinging to be similar (a milder form?) but due
>to the ignition timing being set to advanced for the octane rating and/or
>rpm; ie the explosion of fuel air mixture is occurring at the wrong time,
>too early.  One normally hears pinging when accelerating away from a
>standing stop.

I think your understanding of preignition is the usual one, the most logical 
one, the best one, but, unfortunately, not the only one that is used.

The problem is that ping/knock is the spontaneous ignition of part of the 
mixture before the flame front reached it.  As such, it is a condition where 
burning occurred PREmaturely in that part of the combustion chamber, 
although after the spark.  So I sometimes see ping/knock referred to as 
preignition.

I would be happy if this confusion never occurred, but I guess we are just 
stuck with the weaknesses of language.

The noise is not due to the fact that combustion is occurring while the 
piston is coming up; that is quite normal, ignition almost always happens 
before TDC, but the difference is that normal burning happens smoothly, 
burning outward from the spark plug via the advancing flame front.  
Ping/knock produces distinctive VERY large pressure spikes that can be 
destructive to the engine.  Ping/knock is a low RPM, full throttle phenomena 
which does occur, as you say, when pulling vigorously away from a stop.  TEL 
will prevent ping/knock, but will not do anything for (pre-spark) preignition.

If you have access to a good library you should be able to find a good 
textbook on automotive engines.  These usually have pictures of combustion 
which show the progression of the flame front and then a few bright little 
"islands" where knock is occuring.

Jim
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       Melissa Kepner                                    Jim Adney
       jadney@vwtype3.org              jadney@vwtype3.org
                             Laura Kepner-Adney
                             Madison, Wisconsin
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