Detonation - Pre-ignition, further discussion
Charles K. Scott
for the events Detonation and Pre-Ignition. He said that detonation
was always after ignition and accompanied by a sharp and very narrow
pressure spike while pre-ignition was somethign that always occured
prior to the ignition by the spark plug.
What I didn't detail was what the result of such events was. In the
case of detonation, if it's prolonged, it can damage the lands of the
piston allowing blow-by which can cause further damage to the engine;
like melting the piston below the rings, causing scuffing and sometimes
seizure. Cline said that detonation tended to require long term and
severe cases to cause actual damage. Pre-ignition on the other hand
was extremely harmful to the engine and could damage it in a very short
period.
Cline said pre-ignition could occur with the piston at BDC on the
compression stroke all the way to just prior to ignition. Because this
event occurs while the piston is still going up, heat and stress of the
piston are dramatically accelerated. The typical result of even a
short period of pre-ignition is often a hole melted right in the middle
of the piston. This is because the piston is on the way up and the
heat stays on top of the piston that much longer, instead of being the
brief event just as the piston passes TDC. Pre-ignition events rob the
piston of the quenching coolness (relatively speaking) of the incoming
fuel/air mixture. Pistons have the least ability to deal with excess
heat right in the middle of their surface so that's what melts.
In an interesting aside, Cline mentioned that while the legendary
"Hemi-head" Chrysler engine did very well on the street, that very
design mitigated against it's abilities in competition form.
Detonation is avoided through good chamber design that promotes
complete burning of the F/A mixture. The street engines had flat
pistons and domed combustions chambers - good for full burning of the
mixture. The competition engines though needed to bump the compression
ratio way up. They couldn't do this by changing the shape of the
chamber, that was fixed. The usual manner of bumping up compression
ratio's is by doming the piston and that's what they did in this case.
The problem is, the highly domed piston of the engines built with 14 to
1 compression ratio's compromised the complete combustion that was the
hallmark of the street engines. Because the spark plug was in the
middle of the dome, when it sparked and burning began, the flame front
had to squeeze down to the very narrow corners of the camber to get all
the F/A mixture lit. Because it took so much time (again speaking
relatively here) for the mixture to completely burn, tuners had to
advance the spark event way up to 40 and even 45 degrees. This made
the engines REAL sensitive to fuel and conditions. Tuners would
advance the spark for more and more power and suddenly found detonation
and broken piston lands. Cline says people didn't really understand
what was happening to the engines then, but they do now.
Today's typical "high performance" engine is a "pent roof" design that
is relatively flat. This allows a high compression ratio without
needing domed pistons.
Corky Scott
Cl...@snyder.on.ca
(Charles K. Scott) wrote:
>In the previous post I showed the definitions of engineer Allen Cline
>for the events Detonation and Pre-Ignition. He said that detonation
>was always after ignition and accompanied by a sharp and very narrow
>pressure spike while pre-ignition was somethign that always occured
>prior to the ignition by the spark plug.
>
I would mostly agree with Cline. However, I don't believe he is 100%
accurate in his assessment.
Detonation CAN be caused by preignition - in which case it happens
BTDC.
Detonation ATDC is definitely less harmful, and is what you hear at
part throttle in second gear in the majority of late '80s GM engines.
The dealers all say don't worry, it's normal.
I would say detonation can happen with or without pre-ignition, but
pre-ignition USUALLY also causes detonation. The pre-ignition in this
case is the cause, and you must eliminate it - which eliminates the
resultant detonation.
Detonation, on the other hand, can, and very often does, occur without
the presence of pre-ignition.
They are "kissin' cousins" , not "twins"
David M Parrish
>I would say detonation can happen with or without pre-ignition, but
>pre-ignition USUALLY also causes detonation. The pre-ignition in this
I can see how it can cause it, and how the reverse would also be true.
If we have pre-ignition, the cylinder pressures _have_ to go through the roof
(Somtimes literally.) and that pressure could easily push the fuel past it's
detonation limit. Of course, this is more like adding insult to injury than
anything else.
The reverse is can also true, though I'd think it's less likely. Detonation
blasts away that thin layer of insulating gases next to surfaces, causing
localize heating. If what's getting heated can't get rid of the heat quickly
enough, like carbon deposits or exposed gasket edges(?), the spot could still
be at ignition temperatures when the intake charge comes in.
---
David Parrish