Pre-ignition vs. Detonation

Sushidot wrote:

> As far as I can explain it, P.I. happens on the ignition stroke but
> before the stroke is completed and is caused by a carbon spot or a
> plug fouling.
>
> Detonation occurs due to pressure in the Cylinder being high or the
> engine/cyl temps are high and cause an explosion..

Yeah. Detonation -is- the explosion.

Pre-ignition is simply the fuel-air mixture igniting prematurely, ie,
before the spark plug fires. Combustion as such is otherwise relatively
even.

Detonation is when it explosived spontaneously or instantaneously and
can happen after the spark. Of the two, detonation is the most destructive.

-c

<snip>
> Detonation is when it explosived spontaneously or instantaneously and
> can happen after the spark. Of the two, detonation is the most destructive.
>
> -c

Actually it is the other way around. Pre-ignition is the most
destructive in that the pressure become extremely high due to the fuel
igniting and burning while the piston is still on the compression
stroke. There is some evidence that mild detonation can be normal and
may even be good for the engine.

The Pelican Perch Articals on the Avweb website discuss this in depth
and have the research done by Gami to back it up.

Brian

On Jun 2, 5:33 pm, Brian <brianfc...@msn.com> wrote:
> <snip>
>
> > Detonation is when it explosived spontaneously or instantaneously and
> > can happen after the spark. Of the two, detonation is the most destructive.
>
> > -c
>
> Actually it is the other way around. Pre-ignition is the most
> destructive in that the pressure become extremely high due to the fuel
> igniting and burning while the piston is still on the compression
> stroke. There is some evidence that mild detonation can be normal and
> may even be good for the engine.
>
> The Pelican Perch Articals on the Avweb website discuss this in depth
> and have the research done by Gami to back it up.
>
> Brian

Pre-ignition can cause detonation, so either one is bad.
Normal combustion has a flame front speed of around 100 feet per
second; detonation might have a flame front speed of 5000 fpm or more.
Detonation involves the fuel molecules breaking apart into simpler
structures that will auto-ignite much more easily in the pressure wave
ahead of the flame front, given the time to do so. Slow combustion
times are pro-detonation, giving the molecules time to break up. Lower-
octane fuels have simpler structures and will detonate more easily.
Lean mixtures burn more slowly. Large cylinders take longer to
burn across. Low RPM allows more time. So lean mixtures with low RPM
and high manifold pressures are to be avoided, hence the POH limits on
MP vs. RPM, and minimum requirements for octane ratings.

Dan

Brian wrote:
> <snip>
>
>>Detonation is when it explosived spontaneously or instantaneously and
>>can happen after the spark. Of the two, detonation is the most destructive.

> Actually it is the other way around. Pre-ignition is the most
> destructive in that the pressure become extremely high due to the fuel
> igniting and burning while the piston is still on the compression
> stroke. There is some evidence that mild detonation can be normal and
> may even be good for the engine.

Some evidence? Now I'm curious...

-c