>>I'm having a little problem here in how the words
"detonation" and "pre-ignition" are sometimes used. I am of
the understanding that detonation refers to a spark-triggered event in which
the air-fuel had a more-than-even or explosive combustion take place
resulting in the "ping" we hear in our cars. Pre-ignition, as the
name implies, is where combustion is triggered by some source earlier than the
normal spark event. <<
That is a generally accurate re-statement
of the detonation and pre-ignition events.
>>As I have read, "detonation can cause mechanical
damage such as broken ring lands, abrasive pitting of the piston crown, and
overheating.* "Detonation is not necessarily destructive"*, and
"An engine that is making O.5 HP/in^3 or less can sustain moderate levels
of of detonation without any damage; but an engine that is making 1.5HP/in^3,
if it detonates, it will probably be damaged fairly quickly...within
minutes."* "Detonation will actually cause EGTs to drop."*
"...most engines will live with a fairly high level of detonation for some
period of time. It is not an instantaneous type of failure."*
I would not try to quantify the
differences in destructiveness by reference to the ratio of Hp to cubic
inches.
However, it is true that an aircraft
aircooled engine that has peak internal combustion pressures of 800 PSI can
certainly tolerate an additional 400 PSI of peak pressure on top of the
baseline 800 PSI and those kinds of additional peak pressure increments superimposed
on routine 75% power operation are unlikely to directly cause mechanical damage
in a short to moderate period of time. One can get extended periods of light
detonation superimposed on top of even rather high peak internal cylinder
pressures associated with very high power without doing any damage at all to
the engine. In fact, the CW -3350 engine had a “book” acceptable operating
mode where it would operate at high power in light detonation for hours at a
time..
Detonation DOES cause EGTs to drop. But
only slightly. And by an amount that is likely to NOT be noticed by the
pilot. Pre-ignition will cause rapid and substantial drops in EGT that could
be noticed by the pilot. Note: This valid statement is contrary to several
published (and inaccurate and misleading) statements printed in a couple of
widely read and widely relied upon engine monitor manuals.
"There are no engines that will live for any period of
time when pre-ignition occurs. <
Generally true for spark ignition
engines. However, I have seen engines survive with no damage after a
pre-ignition event that lasted a few seconds. Not minutes.
>>When people see broken ring lands they mistakenly
blame it on on pre-ignition and overlook the hammering from detonation that
caused the problem. <<
I think holes in pistons are most likely (ie,
most often) due to pre-ignition. There may well have been detonation that
precipitated pre-ignition - - - but the holes we see are most likely and most
often due to pre-ignition that resulted from the detonation which caused a
spark plug ceramic (or a helicoil tang, or some other source) to become
excessively hot and initiate pre-ignition.
>>A hole in the middle of the piston, particularly a
melted hole in the middle of the piston, is due to the extreme heat and
pressure of pre-ignition."*
I think that is true - - but it is not
consistent with the previous statement.
(Note, exception - - there was a period
when some TCM pistons were ending up with holes. The holes were due to a crack
that started at the stamped number on the edge of the piston and propagated to
the center of the piston where gas leakage through the crack then caused a hole
that was often erroneously blamed on improper leaning.)
"...the most likely point for pre-ignition to occur is
180 BTDC, some 160 degrees before the spark plug would have fired because
that's the point (if there is a glowing ember in the chamber) when it's most
likely ignited. We are talking some 160-180 degrees of burn being compressed
that would normally be relatively cool. “
I don’t
think this is true at ALL. I have documented combustion events logged at
50KPS on the test stand showing pre-ignition. It started at only something
like 40 to 60 degrees BTDC - - NOT 160 or 180.
“A piston will only take a few revolutions of that
distress before it fails.”
Generally that is a true statement.
Although it may be a spark plug that self ejects from the cylinder, or a head
that cracks, or rings that fail - - rather than the piston.
“ As for detonation, it can get hammered-on for
seconds, minutes, or hours depending on the output of the engine and load,
before any damage occurs. Pre-ignition damage is almost instantaneous."*
Generally that is an accurate
observation.
*************************************************************************
But I’m not sure what set of messages
precipitated this particular discussion ?
Regards, George