On 21 Jun 2005 11:44:26 -0700, "nrp" wrote:

As the piston approaches TDC, the pressures and temperatures in the
cylinder rise due to adiabatic compression. If the
pressure-temperature combination ever reaches the limit supposedly
defined in the above map, the whole air-fuel charge detonates
violently. The ignition is not controlled by the magneto anymore as
the whole charge expolodes at once. With detonation, the turbulence
inside the cylinder is so violent that the combustion boundary layer
next to the cylinder head is scrubbed away, causing the cylinder head
to receive even more heat - which quickly leads to a runaway situation.

I doubt octane has anything to do with the rate of flame propagation
except that if detonation should occur, there really isn't a flame
front.

Pinging is different. With it there is charge ignition from
non-sparkplug sources that are usually the glowing (or at least extra
hot) embers inside the combustion chamber. These multiple ignition
sources cause the pressures to rise more rapidly than simply the spark
ignition, and things could deteriorate into detonation if it persists
long enough. With pinging there is still a flame front, although it is
from more sources so the pressures will rise more quickly.

It has taken a long time for me to understand the difference between
detonation and pinging. Detonation quickly wrecks engines and grossly
feeds on itself.

Pinging, which is heard only in automobiles, not aircraft engines (too
noisy to hear it), is the sound of detonation. What's happening
inside the combustion chamber is that once the spark plug starts the
flame front, it spontaneously ignites in other areas (due to heat and
pressure) and the flame fronts collide together causing turbulence
which bangs off the combustion chamber walls and gives you that
"pinging" sound.

When the fuel/air mixture ignites as the piston is rising up, that is
something wholy different and FAR more dangerous than detonation.
That's called "Pre-ignition". Pre-ignition will destroy an engine in
a matter of a few seconds. Detonation results in overheating (and
eventual damage if not corrected) which is detectible and can be
countered.

Pre-ignition pulverizes pistons, turns them to pieces of their former
selves.

John Deakin did a series of hugely informative articles on engine
mixture and what happens inside the combustion chamber on AVWeb in the
columns section. The articles are all still there.

Corky Scott