Re[2]: Who can that be knocking at my...piston?

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

>From: Toby Erkson <Toby_Erkson@ccm2.hf.intel.com>

>I think I'll add to the foray...  This was my understanding of preignition and 
>detonation:
>
>Preignition:  The fuel mixture is ignited at a location within the cylinder 
>other than the spark plug.  A common preignition point would be a sharp point 
>within the combustion chamber, such as exposed threads of a spark plug or the 
>exposed threads of the spark plug HOLE.  Bits of carbon deposit that are 
jagged 
>and very hot can be a preignition source.  It's as the name implies; the fuel 
>mixture begins burning before proper (spark plug) ignition.  This condition 
will
>create a loss of power for sure but I don't have a clue if it makes any 
>particular sound.
>
>Detonation:  This is due to the fuel mixture EXPLODING and not BURNING like it 
>should be.  For those who don't know, the fuel mixture actually burns in the 
>combustion chamber.  I'm going from my (weak) memory here, but I believe I 
read 
>that gasoline burns at about 100 feet per second; when gasoline explodes it is 
>"burning" at about 1000 feet per second (roughly three American football 
fields 
>in length).  Now, imagine that 1000ft/sec occurring inside the cylinder -- 
>that's the knocking you hear!  Little explosions going off inside your 
engine.  
>The fuel mixture is expanding SO FAST that it is placing tremendous forces 
>against the piston, connecting rods and crank (well, and the little guys that 
>hold them all together).  Hmmm, from what I can remember the difference 
between 
>a low octane and high octane fuel is it's ability to resist dieseling (the 
>ability to ignite a fuel mixture by compression, without a spark or glow 
plug). 
>The more the fuel mixture becomes compressed the greater it's chance to 
>spontaneously ignite.  This is why most high compression engines require 
higher 
>octane fuel.

Ahhh.  Detonation, that was the other synonym for knock and ping that I was 
trying to think of.  Yes, I think this explanation is correct.  I probably 
muddied the waters unnecesarily by bringing up the occasional (incorrect) 
use of the word preignition to refer to detonation/knock/ping.

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