Re[3]: Who can that be knocking at my...piston?

[Toby Erkson <Toby_Erkson@ccm2.hf.intel.com>]

The book I was referencing is "Car Secrets Revealed" and its ISBN is 
-09698083-1-3.  This was a special mail-order book so I don't know if you 
can get it just anywhere (?).  I don't think there were many secrets 
revealed, but then I'm not a complete automotive idiot.

I was wrong about the 1000ft/sec.  Apparently, when the heat and pressure 
in the chamber become too high for the fuel being used the gas breaks down 
at a molecular level and becomes more volatile.  At this level it burns at 
7000ft/sec (yes Vern, seven thousand feet per second)!

A higher octane fuel can still pre-ignite but it will have a lesser chance 
of detonating.  Gasoline looses its octane rating as it ages, by the way.

The book says detonation is worst when the piston is traveling up or at the 
top of the stroke and this is where one can hear the knocking.  Pinging is 
light to medium detonation (so I guess knocking is the worst level of 
pinging) and can also occur when the piston is traveling *down* (it doesn't 
cause as much damage).

     Toby "maybe I should've become a Physics major, Mech engineer minor" Erkson
     
______________________________ Reply Separator _________________________________
Subject: Re[2]: Who can that be knocking at my...piston?
Author:  type-3-errors@umich.edu at SMTPGATE
Date:    4/23/97 7:04 PM

...
>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.
...