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Re: Detonation vs battlin' flamefronts

To: mgs@autox.team.net
Subject: Re: Detonation vs battlin' flamefronts
From: russ@scubed.com (Russ Wilson)
Date: Thu, 7 Nov 1996 10:36:03 -0800
Bill Eastman wrote:

> Since flame
>fronts don't really have any mass and since many engines (the aircraft
>industry is full of them) use twin spark plugs and, by nature, have flame
>fronts colliding with every combustion cycle, I am skeptical that you could
>hear the collision of flame fronts.

There is much truth in this and Bill's skepticism points out a distinction
that is often overlooked when discussing engine knock: the normal burning
of fuel in an engine and the burning that causes knock involve two
distinctly different processes.

Normal burning occurs by a strickly thermal process called deflagration.
Heat generated by the reaction at one point is transferred to adjacent
unreacted fuel, raising its temperature and causing it to react.  This
process is orderly and predictable.  It occurs on a time scale of
milliseconds.  On the other hand, engine knock occurs by a process more
akin to a true detonation - albiet at lower pressures - wherein the
reaction propagates mechanically.  In a detonation, unreacted fuel is
compressively heated to its reaction temperature by a shock wave.
Detonations occur on a time scale of microseconds - essentially
instananeous compared to other engine processes.

Normally the reaction front in an engine propagates smoothly to consume all
the fuel in the combustion chamber.  Pressure measurements made on research
engines show a correspondingly smooth increase in pressure during the burn,
both behind AND ahead of the flame front.  Even in engines having multiple
spark plugs, the flame fronts combine smoothly and relatively slowly.  I
have a research report that shows photos of well-behaved flame fronts
merging in the combustion chamber of an engine having four spark plugs per
cylinder (and a transparent head - wouldn't that look nice on your XPAG!).


However, under the right conditions, the pressure increase ahead of the
flame front will produce enough additional heating of the unreacted gasses
to cause them to detonate simultaneously.  Measurements show that the
additional pressure these gasses would have contributed more gradually if
the deflagration process had consumed them is instead produced
instantaneously.  It is this abrupt increase in pressure, coupled directly
to the combustion chamber walls, that causes the knock we hear.  It really
has nothing to do with colliding flame fronts.

Score one for healthy skepticism.

Russ Wilson





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