> pre-ignition occurs when fuel ignites prior to the
>proper spark timing. This is usually caused by carbon deposits in the
This often-repeated explanation of spark knock is actually not true. I,
like nearly everyone else assumed it was true but always wondered how
adjusting the spark timing could affect knocking if the pre-ignition indeed
occurred first - ie, how can the effect precede the cause?
I stumbled onto the answer several years ago when reading a report from the
Lawrence Livermore Lab (which is looking for more useful things to do now
that the bottom has fallen out of the nuclear bomb making business). They
wanted to learn why all the octane-boosting additives (of which tetraethyl
lead was only one) worked. Incredibly, after more than half a century of
adding lead to gasoline, no one knew why it worked.
Livermore built an internal-combustion engine with a transparent cylinder
head, permitting high-speed photography and spectroscopy of the combustion
process. Spark knock was found to be caused by "end gasses" far from the
spark plug location igniting prematurely, but still *after* the spark plug
fired. The additional compression and resulting heating of the end gasses
by the expansion of the shock front away from the spark plug is what causes
the premature ignition. The knock is created by the colliding shock fronts
from the two sources.
The end gasses are intermediate products that result from thermal reactions
of the gasoline in the heat created by the compression of the fuel mix as
the piston is rising - its no longer gasoline by the time the spark fires.
It was found that the lead worked by surpressing the creation of certain
readily-ignitable compounds in the end gasses.
No, I don't understand it either. It was amazingly complex considering how
well understood it is *thought* to be.
Russ Wilson
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