X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Sat, 02 Jun 2007 19:19:20 -0400 Message-ID: X-Original-Return-Path: Received: from sfa.gami.com ([68.89.254.162] verified) by logan.com (CommuniGate Pro SMTP 5.1.9) with ESMTP id 2080872 for lml@lancaironline.net; Sat, 02 Jun 2007 19:05:29 -0400 Received-SPF: none receiver=logan.com; client-ip=68.89.254.162; envelope-from=gwbraly@gami.com Received: from localhost (localhost.localdomain [127.0.0.1]) by sfa.gami.com (Postfix) with ESMTP id 9D1EB29C06E for ; Sat, 2 Jun 2007 18:04:50 -0500 (CDT) Received: from sdf1.mail.taturbo.com (unknown [10.10.10.173]) by sfa.gami.com (Postfix) with ESMTP id 3758729C05E for ; Sat, 2 Jun 2007 18:04:50 -0500 (CDT) content-class: urn:content-classes:message MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----_=_NextPart_001_01C7A56A.6BFE43EF" X-MimeOLE: Produced By Microsoft Exchange V6.0.6603.0 Subject: RE: [LML] Detonation & pre-ignition X-Original-Date: Sat, 2 Jun 2007 18:04:50 -0500 X-Original-Message-ID: X-MS-Has-Attach: X-MS-TNEF-Correlator: Thread-Topic: [LML] Detonation & pre-ignition Thread-Index: AcelVxd6HCZhC+MvQpmObnT6Wqm5pAAEPuIA References: From: "George Braly" X-Original-To: "Lancair Mailing List" X-Virus-Scanned: by amavisd-new-2.3.2 (20050629) (Debian) at gami.com This is a multi-part message in MIME format. ------_=_NextPart_001_01C7A56A.6BFE43EF Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable =20 =20 >>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. << =20 That is a generally accurate re-statement of the detonation and pre-ignition events. =20 =20 >>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."* =20 I would not try to quantify the differences in destructiveness by reference to the ratio of Hp to cubic inches. =20 =20 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..=20 =20 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. =20 =20 "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. =20 =20 =20 >>When people see broken ring lands they mistakenly blame it on on pre-ignition and overlook the hammering from detonation that caused the problem. << =20 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. =20 =20 =20 >>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."* =20 I think that is true - - but it is not consistent with the previous statement. =20 (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.)=20 =20 =20 "...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. " =20 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. =20 =20 =20 "A piston will only take a few revolutions of that distress before it fails." =20 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. =20 =20 " 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."*=20 =20 Generally that is an accurate observation. =20 =20 =20 ************************************************************************ * =20 But I'm not sure what set of messages precipitated this particular discussion ? =20 Regards, George ------_=_NextPart_001_01C7A56A.6BFE43EF Content-Type: text/html; charset="us-ascii" Content-Transfer-Encoding: quoted-printable

 

 

>>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

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