Detonation and Pre-ignition

[Walter Atkinson]

There is much confusion over these two phenomena. They are not the same and do not always go together.

First, Detonation is the explosive auto-ignition of the end gasses AFTER the planned spark ignition event. Small pockets of F:A charge auto ignite ahead of the flame front and create a shock wave in the combustion chamber which travels at the speed of sound in that medium. The shock wave bounces back and forth at the specific frequency that matches that speed.

The cause of detonation is the combination of IAT, CHT, and pressure within the combustion chamber. When those factors combine to initate combustion above the retarding effects of the latency period of the fuel, detonation is the result. Contolling CHTs and pressure are the BIG factors in NA engines. Contolling IAT in a TC'd engine thru the use of intercoolers is a major factor in widening the detonation margin. Also, the octane of the fuel and the presence of lead is a major factor.

Surprisingly, detonation, in and of itself, is not terribly destructive. We have run an engine for 30 HOURS of light detonation, 3-4 hours of medium detonation and 30 minutes of heavy detonation with NO ill effects whatsoever. As a matter of fact, the engine was sent to be inspected by an independent third party and the response was, "Why did you want a tear-down inspection, this looks like a normal, healthy engine."

As it turns out, a little, occasional light detonation can actually be good for an engine as it tends to clean out the debries and carbon build up!

The issue is the effect that detonation has on spark plug ceramic integrity.

That leads us to pre-ignition. Pre-ignition is the initiation of the flame front BEFORE the planned spark ignition event. It is a normal flame front, just started too soon. As opposed to detonation where EGT is little changed, pre-ignition results in rapidly falling EGTs and rapidly rising CHTs. The Peak Internal Cylinder Pressures can get very high, very quickly and this can destroy an engine in seconds.

The most common casues of pre-ignition are a failed spark plug ceramic which over heats and acts like a glow plug, a helicoil tang sticking in tothe combustion chamber and on very. very rare occasions, a particle of glowing carbon deposit. The later is exceedingly rare, but is the source most often cited! Go figure?

The most common progression is for a detonation event (from inadequate octane or high CHT or high IAT) to progress to the point where a spark plug ceramic is damaged resulting in pre-ignition which destoys the engine. Detonation is frequently mis-identified as the cause of the destruction.

The comments above are a sumary of the information learned from detonation studies at the most advanced engine test facility in the world, The Carl Goulet Memorial engine Test Facility. FAA experts have come to see this first hand and are adjusting their understanding of these events as a result.

Walter Atkinson
Advanced Pilot Seminars

[Allen Barrett]

In response to Walters post, I strongly encourage folks to attend Walter, George and Johns Advance Pilot Seminar. It is a very interestng course held a couple of times a year and you will definitly learn a ton of useful information on engine operating techniques. Plus, if you attend the seminar in Ada, Ok., they feed you like KINGS. Some of the best Bar-B-Que around.

Allen

[gmcjetpilot]

Originally Posted by Walter Atkinson

"The cause of detonation is the combination of IAT, CHT, and pressure within the combustion chamber..............................Also, the octane of the fuel and the presence of lead is a major factor."

"Surprisingly, detonation, in and of itself, is not terribly destructive. We have run an engine for 30 HOURS of light detonation, 3-4 hours of medium detonation and 30 minutes of heavy detonation with NO ill effects whatsoever." (how did you cause detonation and how did you tell how much detonation was occurring?)

"As it turns out, a little, occasional light detonation can actually be good for
an engine as it tends to clean out the debris and carbon build up! Walter Atkinson"

Thanks Walter, another interesting provocitive post. Detonation and Pre-
ignition are indeed two different animals, but related. Just there names say it
all, pre ignition is just that, early ignition, detonation is abrupt uncontrolled
instantaneous ignition.

A big reason for detonation you did not mention is TOO LEAN OF A
MIXTURE! You mention CHT or IAT(Intake Air Temperature ?) and fuel
octane causing detonation, which is also true.

I disagree with, "detonation is not terribly destructive". I beg to differ with all
do respect, not withstanding you "test". I would love to hear how you did
this test, what engine and how you determined it was light, medium or heavy
detonation. My question is why would you do this? For what reason?


From John Schwaner at Sacramento Sky Ranch:
Pre ignition - "heat" and "meltdown"
Detonation - "violence, explosiveness and destruction"

Detonation (can) causes pre ignition
"A dangerously lean air/fuel mixture burns with most efficiency, so much that
the insulating boundary layer also gets consumed and the flame front touches
the metal walls. At those locations, there is a dramatic rise in temperature,
high enough to cause subsequent charges of air and fuel to spontaneously
ignite resulting in multiple flame fronts. This is pre-ignition;"
John Schwaner, Sacramento Sky Ranch.

(Ref: http://www.sacskyranch.com/deton.htm)


If you are going to be a proponent of LOP (lean of peak) operations I think its
important to mention this.

You are a proponent of using Carb heat to "help" carburated engines uneven
fuel distribution. Carb heat affects (IAT) and can cause detonation.
Therefore engine manufactures advise not to use carb heat at high power.
From what you are saying you agree.

Detonation (if left alone) will cause piston and ring damage, top
ring groove wear, scoring, sticking rings, piston hole to complete engine
failure. It can cause rod bearing failure leading to lubrication failure. The
damage can be hidden.

(picture of detonation pistons damage:http://www.hastingsmfg.com/Service%2...mages/p25b.gif)

You MIGHT get away with a little, but to say it is beneficial, I don't get,
especially for an air cooled aircraft engine.

Detonation is OK, not that bad? Could you explain that. Why would you
want to even play around with this? What agenda does that serve. I know
your opinion is to lean aggressively. Pilot's following your procedures of
aggressive leaning and LOP operations, should know too lean causes
detonation!

Under 75% power detonation margins increase, but many are running high
Comp pistons and electronic ignition with 42 degrees BTDC timing. I would
caution anyone taking your advice and comments about an engine test
detonation test to be careful how that relates to them. A Lycoming with HC
pistons and EI may not be happy with lean mixtures. Your test (on a
Continental IO520?) may not relate. It's our engine and soft fleshy hind-end,
so I just suggest caution and conservatism in pursuit of a better gas milage.

Thanks George

[Joey]

Originally Posted by gmcjetpilot

A big reason for detonation you did not mention is TOO LEAN OF A MIXTURE!

I don't speak for Mr. Atkinson, but he probably didn't mention it because it isn't true. When operating lean of peak EGT, the detonation margin increases as the mixture is leaned.

I think the notion that detonation is caused by too lean of a mixture stems from when we couldn't really operate lean of peak EGT, therefore peak EGT was widely considered our lean limit. The range between peak EGT and about 30-40 degrees rich of peak provide the least amount of detonation margin, leading to the popular notion that the more you lean, the closer you are to detonation. The truth of the matter is as you lean towards peak, you do get closer to detonation, BUT if you continue to lean past peak your risk starts heading back down again.

I also agree with Mr. Atkinson that pre-ignition is worse. It can move your point of peak cylinder pressure from its ideal location of around 15 degrees ATDC back to around TDC, causing damage that many people would diagnose as detonation.

Not that he needs me to agree with him, of course. But I am becoming a fan based on what I hear of the Advanced Pilot seminars.

[gmcjetpilot]

Originally Posted by Joey

I don't speak for Mr. Atkinson, but he probably didn't mention it (lean mixture correlation to detonation) because it isn't true.

The truth of the matter is as you lean towards peak, you do get closer to detonation, BUT if you continue to lean past peak your risk starts heading back down again.

You seem to contradict your self? If you read my post and Sacramento Sky Ranch refrence I think it says it all. My question was rhetorical, but thank you. From you quote you do agree and the "TRUTH IS" leaning toward peak EGT (rich side) can cause detonation. There was no mention of LOP operations in his post. I thought it important to point it out.

I understand if you (can) get an over lean condition the temperatures are lowered, as well detonation is not possible or likely. There is just not enough fuel. More important is if you are below 75% power the pressures are lowered, which also gives more detonation margin.

This thread was put forth as a general discussion of detonation/pre-igntion, but now I see the agenda is promotion of LOP operations. Dumb me

My main concern was the "spin" on how detonation is not bad. IN FACT you agree with me, mixture can cause detonation from your quote. Walter did not mention this. It should be clear. A poorly done LOP operation can result in peak or near peak operations on the rich side. Seems obvious but to make it sound like you can't cause detonation with the mixture is of course wrong. The other part where detonation is OK, is just well, I disagree with.

I am not arguing with LOP operations at all and never said other wise. Walter does think wide spread LOP operations is possible with any carburetor 4-cylinder Lycs. It's not a matter should you, it's a matter of can you? (and still have smooth operations.) If you can than great, just so long all cylinders are on the LOP side with some delta from peak.

Again Detonation = Good, is not in my dictionary.

Thanks for the input, I think we agree, really; my poor wording may have messed you up. I AM NOT ANTI LOP! I am just saying its NOT practical for ALL, and if done poorly you can screw yourself and engine. Will Advanced Pilot seminars replace your engine if you blow a hole in your piston? I blew a hole in a piston, in an airplane over water. The metal goes into the crankcase and makes a mess. I was successful making a forced landing on a grass strip, thankfully. Stuff happens.

To say you did a 5 hour test and how detonation is OK, is misleading and could be dangerous if misunderstood. Also almost all Walters examples are on large Continental or radial engines; they do NOT relate to a small 4-banger Lycoming's. An engine is an engine, but be careful before you assume too much, that's all. A big lumbering radial can blow a JUG clean off the plane and keep running pretty smoothly. Go up in a 4-banger lyc and shut the ignition off to one cylinder. If you have glasses and fillings in your mouth, you will loose them. The airplane will shake violently.

I am very conservative and tend to stay with established procedures that have been laid down for 50 years, but again I GET LOP. That is not my point. KNOW WHAT you are getting into, you better, because operations on the edge has draw backs. Draw backs are much higher pilot attention, knowledge and more instrumentation. Any one can be trained to do it, but you know some pilots are lucky to remember the cowl flap and lean at ALL. I know I was and am still a CFI with lots of dual given.

Cheers George

PS Important document: http://www.lycoming.textron.com/supp...ps/SSP700A.pdf.

[rv6ejguy]

Other combustion studies carried out by Tim White, University of New South Wales in 1999 found that mixtures around 5% rich of stoichiometric caused the highest incidence of knock and that leaner mixtures than stoich (peak EGT) lessened knock. Detonation is highly unlikely at power settings corresponding to 75% or less unless ridiculous amounts of ignition advance are used.

While I agree that detonation CAN cause less serious damage than pre-ignition, this depends entirely on the magnitude of detonation present. Certainly many engines have operated for hundreds of hours in light detonation with little or no ill effects. Severe detonation where PCP exceeds 2500 psi will break rings and ring lands within a few seconds irregardless of the quality of the components. Detonation brings a reduction in power and is an abnomal state of engine operation. There is no good reason to operate the engine here.

Both detonation and pre-ignition are to be avoided if engine life is an important consideration and both can exist simultaneously. One can instigate the other under certain conditions. Pre-ignition will damage or destroy most engines in only a few seconds when operating at high rpm/ high power settings and is potentially more destructive than detonation.

As a builder of turbocharged racing engines for over 25 years, I have seen many instances of the damage caused by both phenomena.

[Joey]

Originally Posted by gmcjetpilot

I think we agree, really; my poor wording may have messed you up.

Yes, I do think the confusion was in the wording. If I should interpret you as meaning "Detonation may be caused by improper leaning" then I can certainly agree.

I don't envision a "stop" at peak EGT, so when you say "too lean" I assume you are at the point where the engine is about to quit, not peak EGT. Hence the confusion.

[Stephen Lindberg]

I am willing to believe that "light" detonation for a limited time is not harmful. My high mileage Chev PU V8 pings going up hills if I burn cheap gas, to which I have reverted because of high prices. I can control the detonation to "light" levels because I can hear it and back off the throttle, downshift, or both. But you can't hear detonation in an airplane - too much noise - so how would you know? We need detonation detectors such as are found on cars. I understand that our air-cooled engines with their generous clearances between moving parts make too much of a racket for these sonic detectors to work. On the window sill in my shop is a piston from a P&W 1830 (DC-3) that suffered detonation and/or preignition. It looks just like the piston pictured in George's post, with melted ring lands and smeared skirts. It wasn't my doing but I understand it was caused by high power settings, low (climb) airspeed, and auto lean when they should have been running auto rich. This engine flew home but the same level of damage to a 4 cyl. Lyc would probably make it quit running. Something to think about when twiddling the red knob. Comments? Steve

[Yukon]

This thread was put forth as a general discussion of detonation/pre-igntion, but now I see the agenda is promotion of LOP operations. Dumb me

Cheers George

PS Important document: http://www.lycoming.textron.com/supp...ps/SSP700A.pdf.[/quote]

George,
I think you hit the nail right on the head. Advocacy of LOP is what all these threads are about. That becomes pretty obvious when GAMI sends one of their guys over to post on our forum.

You know what though, they are only claiming .9 gph to 1.5 gph savings from their injectors utilizing LOP. That's not unreasonable, and a far cry from some of the claims heard here on the forum.

Your right, though, one of the mod guys with 40 degree advance and 10 to 1 pistons is going to blow a piston clear into next Wednesday, and wonder why it happened. Good thing these engine monitors are download capable,
so we can all see why it happened.

John

[zav6a]

Go back and look at Walter's original post. The words "lean of peak" were not used once. Then take a look at Georges's response. I don't see the basis for asserting that GAMI sent someone over to advocate their product. I was looking forward to the dialog that could be provided in this forum by real experts but I'm sure they, and others like them, will opt out.

I'm done with this business.

Duane Zavadil
N678BT
RV6A