Left Coast Mogas (Ping! Jim Weir)

Allen wrote:

I was always told the higher octane burns faster, and is consumed in a
shorter duration, thus a lower temperature.

Other way 'round on the burn rate. From a motorcycle performance site -

"The higher the octane, the slower the speed of the burn; the lower the octane
the faster the burn. If the burn is too fast, uncontrolled combustion can
occur. This most often is heard as “pinging” and is commonly referred to as
detonation. This is because instead of burning through top dead center of the
combustion stroke, the entire charge is ignited too early and explodes in the
chamber and acts as a bomb. Conversely, if the speed of the burn is too slow,
it continues after the useful work can be done in the motor and manifests itself
as poor throttle response, reduced power output and increased emissions and fuel
consumption. An engineering fact: THE MOST HORSEPOWER IS MADE AT THE THRESHOLD
OF DETONATION. We have often gained horsepower on the dyno and felt improved
starting and driveability going from Premium grade gas to Regular. This change
was recently quantified in a customer’s Ducati M900 by reducing the cranking
time to start-up from 15 to 3 revolutions, although part of the improvement is
explained below."

George Patterson
Why do men's hearts beat faster, knees get weak, throats become dry,
and they think irrationally when a woman wears leather clothing?
Because she smells like a new truck.

I understand that octane is really a measure of the resistance to self
ignition in the pressure/temperature map. High temperatures or high
pressures will cause self ignition (actually this is detonation), but
octane gives us a way to measure the combinations of pressure and
temperature that also causes self ignition. It isn't measured directly
but instead octane is measured as an equivalent fuel mixture based
evaluation using an extra rugged variable compression ratio lab engine
under various conditions.

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.

nrp wrote:

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 is the knocking sound you hear when the air fuel mixture
ignites or explodes prematurely when the piston is still compressing
the mixture. I think the effect you describe as pinging is commonly
called pre-ignition. It differs from detonation in that there is an
alternate ignition source in the cylinder. Both pre-ignition and
detonation can cause an engine to ping or knock.

In an aircraft engine, detonation usually comes from using gasoline
with a lower than required octane rating (a.k.a. anti-knock index).
Pre-ignition is usually the results of cylinder contamination by bits
of carbon, or ash residue from using automotive type oils.

John Galban=====N4BQ (PA28-180)

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