[automotivebreath]

Originally Posted by tommurphy73

I am still not 100% on why exactly an increase in turbulence prevents detonation. Tom

Tom to get a better vision of the role turbulence plays in reducing detonation
you must consider pre flame reactions, formation of “hot spots” and the role
of free radicals. As the fuel/air mixture is heated and compressed in front of
the flame front “hot spots” begin to form, very small centers of auto ignition.
These are considered areas of locally high reactivity due to imperfect mixing of
residual exhaust gas with the fresh charge. This is very complex may be
better explained with this quote.

In a running engine, air is drawn in at some ambient temperature, and this
air then begins to pick up heat from the hot internal engine surfaces it
contacts. Finally it enters the actual cylinder, where is it heated by even
hotter surfaces. Trapped there, it is heated again by the process of
compression.

In this heating process, some little discussed chemical reactions begin to
occur in the fuel. Called preflame reactions, these take the form of slow,
partial breakdown of the least durable types of fuel molecule. Fuel
hydrocarbons have three basic forms; straight carbon chains, branched
chains, and ring structures. Temperature is a measure of average
molecular activity, but there are always some gas molecules moving
significantly faster than the others. These faster moving molecules hit and
break some of the less durable fuel molecules, dislodging some of their
more weakly bonded hydrogen atoms. This released hydrogen is very
reactive (normally hydrogenous travel in bonded pairs, but his is atomic
hydrogen) and instantly pairs with an oxygen from the air to form what is
called a radical, a chemical fragment that is highly reactive because if
contains and unpaired electron. Its attraction for the missing electron is so
great that it can snap one out of other chemical species it happens to
collide with, thereby breaking it down as well.

At some point in the compression stroke, the spark ignites the mixture and
combustion begins. The burned gases, being very hot, expand against the
still unburned charge, compressing it outward into the squish band. This
compression rapidly heats the unburned charge even more, accelerating
the preflame reactions in it. As a rule of thumb, the rate of chemical
reaction doubles every seventeen degrees F. All this while, the population
of reactive molecular fragments radicals is increasing in the unburned
endgas. If this process of heating takes long enough, and reaches a
temperature high enough, this population of radicals becomes great
enough that its own reaction rate one radical creating more and more
through further reactions accelerates into outright combustion. This is
autoignition.

Now why does this heated, chemically changed endgas detonate instead of
simply burning? The fuel in the endgas is no longer ordinary gasoline. The
preflame reaction that have taken place in it have changed it into a
violent explosive much like a mixture of hydrogen and air, or acetylene
and oxygen. It is in a hair-trigger state, being filled with reactive
fragments from preflame reactions. When it autoignites spontaneously,
combustion accelerates almost instantly because the material is so easily
ignited now. The combustion front accelerates to the local speed of sound,
igniting the material it passes through simply by suddenly raising its
temperature, through the shock wave it has now become.