: Theory for Bbob: Fuel grade AGAIN ;)



JSMeloche
05-09-05, 10:22 AM
I know this as been discussed over and over again but with the fuel price so high these days I think im a bit crazy and thinking too much, but i tought of something.

We can use regular grade fuel in the N* no problem, but it will knock, knock sensor will detect it and adjust timing, loosing performance.

Knocking is pre-ignition i.e. the fuel ignites before the cylinder has a TDC, before it is light by the spark plug. Pre-ignition occurs because low octane fuel auto-ignite at a lower pressure. When you compress a gaz (fuel-air mixture) it get hotter, until it auto-ignites. So as you can see there is pressure AND tempereture in this equation.

Theoraticly if the mixture is COLDER it should be able to take more PRESSURE. Colder thermostat can achieve this, so in theory could a colder thermostat allow the N* engine to run on regular grade fuel more efficiently/performance???

The N* engine was designed to run and be efficient at a specific tempereture, so maybee messing with the temp wont do much for efficiency.

But im just curious and im pretty sure Bbob will come up with a nice explanation ;) hhehehe

dkozloski
05-09-05, 11:13 AM
Auto ignition like a diesel is not necessarily the problem. It more likely is detonation which is the instantaneous explosion of the mixture rather than a controlled burn. Reducing the temperature of the intake charge by use of an intercooler or water/alcohol injection helps this condition as a substitute for increased octane rating of the fuel. Reducing the cooling temperatue by changing the thermostsat is not likely to do much for you.

EcSTSatic
05-09-05, 12:07 PM
You are close. That is what water injection is used for. Google on "water injection detonation" and learn more about it.

powerglide
05-09-05, 04:03 PM
Yup, lowering coolant temps by 20 degrees isn't likely to solve this problem.

However you're reasoning and thought process is in the right direction (in tems of thermodynamics)!

BeelzeBob
05-09-05, 04:18 PM
I know this as been discussed over and over again but with the fuel price so high these days I think im a bit crazy and thinking too much, but i tought of something.

We can use regular grade fuel in the N* no problem, but it will knock, knock sensor will detect it and adjust timing, loosing performance.

Knocking is pre-ignition i.e. the fuel ignites before the cylinder has a TDC, before it is light by the spark plug. Pre-ignition occurs because low octane fuel auto-ignite at a lower pressure. When you compress a gaz (fuel-air mixture) it get hotter, until it auto-ignites. So as you can see there is pressure AND tempereture in this equation.

Theoraticly if the mixture is COLDER it should be able to take more PRESSURE. Colder thermostat can achieve this, so in theory could a colder thermostat allow the N* engine to run on regular grade fuel more efficiently/performance???

The N* engine was designed to run and be efficient at a specific tempereture, so maybee messing with the temp wont do much for efficiency.

But im just curious and im pretty sure Bbob will come up with a nice explanation ;) hhehehe



The fuel octane rating is merely it's ability to prevent detonation.

Detonation is NOT pre-ignition as you have stated. Detonation and pre-ignition are similar and related abnormal combustion phenomenon but they are NOT the same thing and spark knock or detonation from lower octane fuel is not preignition.

Octane rating of the fuel has nothing to do directly with preignition.

Detonation is the spontaneous combustion of the end gas in the chamber that occurs AFTER the spark plug has ignited the burn. As the mixture burns the heat and pressure builds rapidly and if the end gas in the chamber is not of sufficient octane rating it spontaneously combusts. Key here is that detonation happens AFTER the spark plug initiates the burn normally. The characteristic spark "knock" or ping of detonation is the severe pressure spike caused by the auto-ignition of the end gas mechanicallly exciting the structure of the cylinder head and engine.

Pre-ignition is the ignition of the mixture BEFORE the spark plug ignites it normally. If there is an ignition source in the chamber such as an overheated spark plug tip or glowing carbon ember or other hot spot it will serve to ignite the mixture BEFORE the plug can arc causing the mixture to burn well in advance of the normal ignition event. Additional or higher octane rated fuel will not have any effect on preignition as the mixture is being ignited by an anciliary ignition source....not just the heat and pressure of combustion. There is no sound associated with preignition.


Detonation is rarely harmfull or fatal to an engine. Engines can run for hundreds of hours, even at full throttle, with light to moderate levels of detonation. Heavy detonation at full throttle will cause cracked ring lands, crack valves, cracked spark plug porceleans, etc. and will drive tremendous heat into the combustion chamber parts likely overheating them. But this is a concern only at max output and on high specific output engines.

Pre-ignition is always fatal to the engine and fatal very rapidly. Since it makes no noise the engine fails by melting a hole in the piston. When it looses power and starts to smoke like crazy from the tailpipe you then know of the preignition. No warning otherwise.

There is such a thing as detonation induced preignition. In that case the engine detonates due to low octane fuel and that causes the spark plug to overheat. If the plug is of too hot a heat range the plug tip can overheat to the point of glowing and becoming and ignition source. The glowing plug tip casues pre-ignition and engine failure. The lower than required octane fuel caused the detonation which indirectly casued the preignition...but putting a colder heat range plug in the combustion chamber would have prevented the preignition even with the lower octane fuel.

There is some correlation , yes, to the operating temperature of the engine and detonation. Generically speaking, running the engine colder helps with detonation. However, running the engine colder is not advantageous for a number of other reasons. Friction being one, longevity being another. If you run the engine colder it might gain a tiny bit in the detonation area yet loose a lot in the friction area resulting in a net loss of fuel economy.

The engine is optimized to run at the normal operating temperature with compression ratio, burn rate, EGR fraction, etc... all optimized for that operating temperature. You are throwing all that out of whack by lowering the operating temperature and not re-optimizing all the other factors...so it would likely result in an additional loss in efficiency/fuel economy in addition to the increase in friction at colder temps.

Kev
05-09-05, 04:27 PM
The engine is optimized to run at the normal operating temperature with compression ratio, burn rate, EGR fraction, etc... all optimized for that operating temperature. You are throwing all that out of whack by lowering the operating temperature and not re-optimizing all the other factors...so it would likely result in an additional loss in efficiency/fuel economy in addition to the increase in friction at colder temps.That does not sound good.

dkozloski
05-09-05, 05:44 PM
One of the startling differences between auto and aircraft engines is that an aircraft engine at high power setings can be destroyed in less than a minute of detonation. An aircooled engine becomes heat saturated so quickly that holes through the head of the piston or the cylinder head opening up like a melon are the result. That is why the FAA is so goosey about approving spark advance mechanisms and the use of auto gas in airplanes.
One evening I stood and watched a DC-6 that had been refueled with the wrong fuel go over Fairbanks at a low altitude shedding engine parts that were tearing the cowlings off. The flames were a sight to behold. Detonation was destroying the powerplants as it left the ground but it was too late to abort the takeoff. The plane was unable to return to the airfield and plowed into the ground behind a school house. The cylinder heads were opened up and split in half. It took all night to pry the pilot and co-pilot out of the wreckage and they survived but the loadmaster was not so lucky.

JSMeloche
05-10-05, 10:59 AM
The fuel octane rating is merely it's ability to prevent detonation.

Detonation is NOT pre-ignition as you have stated. Detonation and pre-ignition are similar and related abnormal combustion phenomenon but they are NOT the same thing and spark knock or detonation from lower octane fuel is not preignition.

Octane rating of the fuel has nothing to do directly with preignition.

Detonation is the spontaneous combustion of the end gas in the chamber that occurs AFTER the spark plug has ignited the burn. As the mixture burns the heat and pressure builds rapidly and if the end gas in the chamber is not of sufficient octane rating it spontaneously combusts. Key here is that detonation happens AFTER the spark plug initiates the burn normally. The characteristic spark "knock" or ping of detonation is the severe pressure spike caused by the auto-ignition of the end gas mechanicallly exciting the structure of the cylinder head and engine.

Pre-ignition is the ignition of the mixture BEFORE the spark plug ignites it normally. If there is an ignition source in the chamber such as an overheated spark plug tip or glowing carbon ember or other hot spot it will serve to ignite the mixture BEFORE the plug can arc causing the mixture to burn well in advance of the normal ignition event. Additional or higher octane rated fuel will not have any effect on preignition as the mixture is being ignited by an anciliary ignition source....not just the heat and pressure of combustion. There is no sound associated with preignition.


Detonation is rarely harmfull or fatal to an engine. Engines can run for hundreds of hours, even at full throttle, with light to moderate levels of detonation. Heavy detonation at full throttle will cause cracked ring lands, crack valves, cracked spark plug porceleans, etc. and will drive tremendous heat into the combustion chamber parts likely overheating them. But this is a concern only at max output and on high specific output engines.

Pre-ignition is always fatal to the engine and fatal very rapidly. Since it makes no noise the engine fails by melting a hole in the piston. When it looses power and starts to smoke like crazy from the tailpipe you then know of the preignition. No warning otherwise.

There is such a thing as detonation induced preignition. In that case the engine detonates due to low octane fuel and that causes the spark plug to overheat. If the plug is of too hot a heat range the plug tip can overheat to the point of glowing and becoming and ignition source. The glowing plug tip casues pre-ignition and engine failure. The lower than required octane fuel caused the detonation which indirectly casued the preignition...but putting a colder heat range plug in the combustion chamber would have prevented the preignition even with the lower octane fuel.

There is some correlation , yes, to the operating temperature of the engine and detonation. Generically speaking, running the engine colder helps with detonation. However, running the engine colder is not advantageous for a number of other reasons. Friction being one, longevity being another. If you run the engine colder it might gain a tiny bit in the detonation area yet loose a lot in the friction area resulting in a net loss of fuel economy.

The engine is optimized to run at the normal operating temperature with compression ratio, burn rate, EGR fraction, etc... all optimized for that operating temperature. You are throwing all that out of whack by lowering the operating temperature and not re-optimizing all the other factors...so it would likely result in an additional loss in efficiency/fuel economy in addition to the increase in friction at colder temps.

I know an engineer could come up with a nice explanation ;) thanks bbob!

When my STS will be fixed (today hopefully) i should be able to experience a nice controled ignition during a healty dose of WOT ;)

Katshot
05-10-05, 01:24 PM
The fuel octane rating is merely it's ability to prevent detonation.

Detonation is NOT pre-ignition as you have stated. Detonation and pre-ignition are similar and related abnormal combustion phenomenon but they are NOT the same thing and spark knock or detonation from lower octane fuel is not preignition.

Octane rating of the fuel has nothing to do directly with preignition.

Detonation is the spontaneous combustion of the end gas in the chamber that occurs AFTER the spark plug has ignited the burn. As the mixture burns the heat and pressure builds rapidly and if the end gas in the chamber is not of sufficient octane rating it spontaneously combusts. Key here is that detonation happens AFTER the spark plug initiates the burn normally. The characteristic spark "knock" or ping of detonation is the severe pressure spike caused by the auto-ignition of the end gas mechanicallly exciting the structure of the cylinder head and engine.

Pre-ignition is the ignition of the mixture BEFORE the spark plug ignites it normally. If there is an ignition source in the chamber such as an overheated spark plug tip or glowing carbon ember or other hot spot it will serve to ignite the mixture BEFORE the plug can arc causing the mixture to burn well in advance of the normal ignition event. Additional or higher octane rated fuel will not have any effect on preignition as the mixture is being ignited by an anciliary ignition source....not just the heat and pressure of combustion. There is no sound associated with preignition.


Detonation is rarely harmfull or fatal to an engine. Engines can run for hundreds of hours, even at full throttle, with light to moderate levels of detonation. Heavy detonation at full throttle will cause cracked ring lands, crack valves, cracked spark plug porceleans, etc. and will drive tremendous heat into the combustion chamber parts likely overheating them. But this is a concern only at max output and on high specific output engines.

Pre-ignition is always fatal to the engine and fatal very rapidly. Since it makes no noise the engine fails by melting a hole in the piston. When it looses power and starts to smoke like crazy from the tailpipe you then know of the preignition. No warning otherwise.

There is such a thing as detonation induced preignition. In that case the engine detonates due to low octane fuel and that causes the spark plug to overheat. If the plug is of too hot a heat range the plug tip can overheat to the point of glowing and becoming and ignition source. The glowing plug tip casues pre-ignition and engine failure. The lower than required octane fuel caused the detonation which indirectly casued the preignition...but putting a colder heat range plug in the combustion chamber would have prevented the preignition even with the lower octane fuel.

There is some correlation , yes, to the operating temperature of the engine and detonation. Generically speaking, running the engine colder helps with detonation. However, running the engine colder is not advantageous for a number of other reasons. Friction being one, longevity being another. If you run the engine colder it might gain a tiny bit in the detonation area yet loose a lot in the friction area resulting in a net loss of fuel economy.

The engine is optimized to run at the normal operating temperature with compression ratio, burn rate, EGR fraction, etc... all optimized for that operating temperature. You are throwing all that out of whack by lowering the operating temperature and not re-optimizing all the other factors...so it would likely result in an additional loss in efficiency/fuel economy in addition to the increase in friction at colder temps.

I think that's a great explaination and I agree with everything except where you say: "Detonation is rarely harmfull or fatal to an engine."
You're absolutely the first qualified professional that's ever said that, that I'm aware of. Every piece of GM literature I have claims it to be damaging to the engine. You yourself describe detonation as a "severe pressure spike" within the combustion chamber yet you claim it is rarely harmful to an engine? Sorry but something doesn't wash here.

Katshot
05-10-05, 01:41 PM
Hey, wait a second here. Bbob's post sure looks an awful lot like this technical article. Except the part about where detonation is rarely harmful to the engine. What's going on here? Dude, are you plagerizing?

http://www.streetrodstuff.com/Articles/Engine/Detonation/Page_2.php

dkozloski
05-10-05, 01:50 PM
Bbob, is right about detonation in a car engine. Liquid cooling controls the heat saturation and the engines are built strongly enough that structural problems are controlled. I suppose there are exceptions to everything to where prolonged heavy detonation could be problematic. Most people when they hear the pinging sound that sounds like someone throwing pea gravel at a piece of pipe, take action, like letting off the foot feed or shifting to a lower gear. The pinging that you hear climbing a hill in a too-high gear is not as destructive as the pinging you don't hear at higher revs that produces more heat quicker. When you tear down an engine that has operated regularly in detonation it is very clean inside with virtually no carbon deposits and the top corner of the piston will look like a rodent has been gnawing on it.

dkozloski
05-10-05, 01:54 PM
Hey Katshot, how do you know that Bbob is not the original author? He is published you know!!

BeelzeBob
05-10-05, 02:13 PM
Well....

Without writing a book on combustion....let me repeat that most engines can tolerate light to moderate rates of detonation for long periods of time with no damage. Having witnessed many engines running like that with obvious detontation I can attest to that fact.

Taking that statement out of context and applying it to aircraft engines and dirt bikes and everything except liguid cooled car engines is a bit dangerous...and why I hate putting statements on the internet sometimes. It is impossible to explain yourself fully and cover ALL the bases. The chapter covering ALL the nuances of detonation takes up half the text book on combustion that I could write...so bear with me.

This is a Cadillac forum and specifically a NORTHSTAR engine topic so my comments were slanted in that direction and I did not cover all the other 10,000 engine types out there for this abnormal combustion capability....LOL

Katshot, I'll ignore you anyway as you will go to great lengths to make me out a lier or say something negative so I understand. At least you granted me the title of "a qualified professional"....wow. I feel uplifted and better now....just kidding. LOL LOL


The statements to avoid detonation and that it can be harmfull are armour plated statements. Detonation "can" be harmfull and "can" damage an engine if it is heavy and continuous. AS I STATED detonation can also frequently induce preignition so if THAT happens the engine will fail. Since it is impossible to explain different levels of detonation to customers they are advised to avoid detonation period. That is the nice thing about internet discussions.....you can get the whole story instead of the Cliff Notes summary.

The fact is that detonation does not put holes in the pistons. Pre-ignition does that. If things look melted in the chamber and piston then suspect preignition happened. Detonation causes cracks and broken edges but I have never seen it hole a piston in the center. If that aircraft engine had holes in the pistons then the lower octane fuel caused detonation that overheated something in the chamber causing preignition which is what put holes in the pistons. I'd bet money on it. My explaintion took that into account in detail. When I say detonation doesn't usually hurt an engine I mean detonation alone...not detonation leading to preignition.

Observing cylinder pressure traces it is very obvious why an engine can live with detonation and not live with preignition. The sharp, high pressure spike of detonation is VERY brief and it happens AFTER TDC when the piston is traveling downward anyway. On most traces it looks like an upward straight line lasting just long enough to register on the pressure transducer. And, since detonation occurs well AFTER the spark plug has initiated the burn in the chamber the normal combustion process has well commenced, the piston is past TDC and on the down stroke. The end gas was going to burn anyway and create pressure as the piston is moving down so the spike of pressure from detonation is just superimposed onto the normal cylinder pressure. It is abnormal for THAT point in the combustion cycle but it is not that much higher than the peak pressures encountered many times. Remember that if the spark timing is correct for optimum power the spark will ignite the mixture and the burn will commence and the location of peak pressure in the chamber is at 14 degrees AFTER top dead center. It stands to reason that any end gas that is going to detonate (under any sort of "normal" conditions) is going to spontaneously combust after the pressure reaches it's peak....so....logically the piston is at at least 14 degrees after top dead center and moving downward.

Just like the "brief" combustion temperatures of 1800 degrees do not melt aluminum pistons and head the "brief" spike of pressure from detonation is not as likely to hurt anything.

Preignition is a totally different animal. Think about the intake stroke and when the piston starts to compress the mixture as the piston starts upward well BEFORE top dead center. At some point in there the mixture is homogenous enough to light from a ancilliary ignition source such as a glowing spark plug tip, ground electrode, carbon bit, etc.... The mixture is likely to ignite well BEFORE TDC and well BEFORE the spark plug ever arcs. So, the piston is now being forced to TDC against the burning mixutre pressure. This results in extreme heat and pressure against the piston crown as the pre-ignition casues combustion to commence so early that the pressure is there well before TDC and, in fact, the piston is actually trying to compresses this burning mixture. The pressure/heat from pre-ignition lasts for many degrees before top dead center until well after. The detonation pressure spike was there for a fraction of a degree of crank rotation and well after TDC. Big difference. The extra heat and pressure from preignition might be said to "exist" for 40 or 50 degrees or more of crank rotation PLUS it is much higher pressure since the whole chamber is likely to have burned before the piston reached TDC so it will be compressing that hot, burned , expanded mixutre back to the combustion chamber volume. This creates pressure MUCH higher than the normal combustion pressures since normally, even with detonation, most of the chamber has NOT burned before TDC...most of it burns by 14 degrees AFTER TDC.

With the very high pressures of preignition the piston cannot hold the load. Heat pours into it and it gets soft. The piston head is like a drumhead... The center of the crown cannot reject the heat fast enough and it gets soft. The immense pressure collapses it where it is weakest and it blows a hole thru. End of story.

We were recently running spark plug heat range tests on a supercharged 4.4 liter Northstar running at over 470 HP. Spark plugs are sized for heat range so as to be "cool" enough to tolerate light/moderate levels of detonation without overheating. The engine in question ran for well over 25 hours at full throttle , max power, peak RPM with light to moderate levels of detonation artificially induced by overriding the knock sensor feedback. The engine lasted fine. NO damage. Proof that an engine can tolerate reasonable amounts of detonation with no damage. And this is an engine that is running at 107 HP/liter. VERY high specific output. Believe me, listening to the engine rattle it was so bad that you expected parts thru the bottom any instant. Yet, it ran 25 hours with no problem.

The end of the story validates the "concern" over detonation. The engine exploded big time. Parts on the floor exploded. Post mortem reveiled that the extended operation in detonation had acutally fatigued the ground electrode on one of the spark plugs off. It was literally fatigue fractured off of the plug shell. The small bit of ground electrode trapped an exhaust valve open a crack. The next cylinder event leaked burning, high pressure gases past the partially opened exhaust valve torching a hole in the cylinder head passage. Water entered the port from the hole into the coolant passage and the next compression event hydrostatically locked the cylinder and blew the engine to bits. At 470 Hp and 6400 RPM these things happen FAST.

So....WITHIN reason.....engines can tolerate light to moderate levels of detonation for long periods of time without any damage. If the detonation leads to preignition, the engine will fail.

BeelzeBob
05-10-05, 02:14 PM
Hey, wait a second here. Bbob's post sure looks an awful lot like this technical article. Except the part about where detonation is rarely harmful to the engine. What's going on here? Dude, are you plagerizing?

http://www.streetrodstuff.com/Articles/Engine/Detonation/Page_2.php




uhh....NO...not plagerizing...I wrote that article. Don't give my real name away. The article was first published in the magazine Contact! and is categorized in the Library of Congress in a book on alternate aero powerplants.

I was thinking of referenceing that article and saying that he was a bud of mine.....he IS a bud of mine, that guy that wrote that article. It wasn't me. LOL

BeelzeBob
05-10-05, 02:22 PM
I have noticed different responses to detonation in two stroke engines. Air cooled two strokes will tolerate detonation for long periods. They will ping like crazy due to the cooling fins radiating the noise but the detonation doesn't seem to hurt them too badly...if you have cold plugs in it. I have an ice racer that detonates like mad if I don't have good gas in it or bog the engine in a corner and it never seems to hurt it....but I have a very cold plug in it. Start it on the lake with ether and a hot plug, warm it up, switch plugs to a cold plug and hammmer on it.

On the other hand, liguid cooled two strokes can fail from continuous detonation. The air cooled engines get globally hot....the piston heats up as well as the bore ....and the clearances go UP as the detonation drives heat into the parts. The liquid cooled engines, however, get the piston hot with detonation and the bore stays the same size as the liquid cooling can carry the extra heat away easily. So....the piston "swells up" and scuffs and carries material into the rings cuasing a loss of compression.

Two strokes are far more likely to do this than four strokes because the two stroke fires every revolution thus the piston gets heated every revolution instead of every other revolution. The piston soaks up so much heat so fast with a two stroke that when it detonates the piston gets very hot very fast and expands. It won't put a hole in it due to the very brief spike of pressure but it will make it hot. Then the spark plug overheats and causes preignition and the piston suddenly has a hole in it.

BeelzeBob
05-10-05, 02:23 PM
Hey Katshot, how do you know that Bbob is not the original author? He is published you know!!


Gosh....don't let gmt2u find out....

BeelzeBob
05-10-05, 02:34 PM
Really, there are SO many idiosyncracies with detonation and combustion that it is hard to make hard and fast statements that cover ALL engines all the time. The safe thing to do is to avoid detonation.

But...the calibrators refer to detonation cruising down the road as the "sound of fuel economy"....LOL....or, they say you tune the engine "to top dead ping.." Most auto engines most of the time are running at their optimum tuning for fuel economy if they are cruising onlong just on the borderline of detonation. At part throttel cruising along like that an engine will run forever with borderline detonation.

Obviously your results may vary as the specific output goes up. I would not recommend tuning your supercharged Northstar to top dead ping due to the high specific output...unless it is under controlled conditions and it is not YOUR engine...LOL.

Most people get confused by the appearance of the failed parts after the fact as to what happened....??...detonation or preignition...or BOTH. Most likely scenario is the detonation induced preignition but that is not likely except at very high speed and high load.

Realize, also, that you (or the engine) can get into preigntion for other reasons...such as running too hot of a spark plug. I know of a Northstar in an aeroplane that crashed at the end of the runway due to power failure. I have the spark plug from that engine on my desk mounted on a wood block. It has severe preignition damage. After hundreds of hours of operation on the OEM plugs he decided, for what reason I cannot determine, to put a set of aftermarket replacement Autolite plugs in the motor and took off. They were too hot, overheated at full throttle and induced preignition that took out the motor.

Same thing happens when you run a 200 shot of nitrous using OEM plugs. The specific output just went WAY up, the OEM plug cannot reject enough heat and it overheats, glows and becomes and ignition source. You MUST go to colder plugs when upping the output to avoid preignition...nothing to do with detonation or the octane rating of the fuel.

On nitrous engines the plug ground electrode becomes an issue with overheating and glowing. That is why most nitrous kits tell you to go to colder plugs and to clip the ground electrode in half so that it is just the side wire of the ground electrode. Less distance for the heat to travel and the cooler it will run.

BeelzeBob
05-10-05, 02:46 PM
Ok...so someone PM'ed me and asked about preignition warnings and how is it measured/detected.

Answer: no warning and very carefully.

Detonation occurs so suddenly that there is such a rapid pressure spike that it rings the structure of the engine making the pinging sound that has a characteristic frequency of about 6400 Hz.

Preignition occurs so early in the compression/combustoin cycle that the pressure comes from compressing the burning/expanding mixture over quite a few crankshaft degrees of rotation. There is a hugh, broad pressure buildup...not a sharp, intense pressure spike so there is no "sudden" event to ring the chamber. Preigntion is silent death.

It is measured or detected in development engines by feeding a low voltage current to the spark plug tip thru the high tension lead. A large diode prevents the high ignition voltage from feeding up the instrumentation lead. The low voltage current from the instrumentation goes thru an amp meter (milli-amp meter, actually) so that any current flow across the spark plug gap to ground will show up on the meter. There is a separate meter and diode for each cylinder. When the plug tip starts to overheat sufficiently to act as an anciliary ignition source it starts to form a plasma cloud at the gap...or the gap and the material in that area is heavily ionized due to the temperature. That plasma or ion cloud conducts electricity...so...the low voltage from the meters will start to leak across that path and show up as a reading in millli-amps on the meters. You will watch the meters jiggle and move a bit under normal combustion and get more "active" as the load and temp go up. Typically with 15 volts across the gap you might see as much as 20-30 milliamps of current during high load "normal" combustion. When detonation is deliberately casued the spark plug starts to get hotter and the milli-amp reading goes up....40...50....and it will stabilize.....unless the plug is too hot and then it will suddenly peg the milli-amp meter. It will peg instantly. If the engine is making near 1 HP per cubic inch it is too late. Preigntion likely already damaged something. With low specific output engines like at 1/2 HP per cubic inch you might be able to close the throttle quick enough to avoid major damage....maybe if you are really quick and were looking directly at the meter and didn't blink.....LOL I was not usually fast enough so they didn't let me watch the meters.....

Katshot
05-10-05, 02:48 PM
Dude, if you think I spend my time trying to make you look bad, you certainly don't know me. If I wanted to do that, you look like a TOTAL ASS by now. This particular issue just popped up as I was reading an article I found online. It struck me like a case of dejavu. Then I re-read your post a what do you know? It was the same info. The problem was that you happened to leave out the part about how Detonation DOES cause damage. Remember writing this?

One thing to understand is that detonation is not necessarily destructive. Many engines run under light levels of detonation, even moderate levels. Some engines can sustain very long periods of heavy detonation without incurring any damage. If you've driven a car that has a lot of spark advance on the freeway, you'll hear it pinging. It can run that way for thousands and thousands of miles. Detonation is not necessarily destructive. It's not an optimum situation but it is not a guaranteed instant failure. The higher the specific output (HP/in3) of the engine, the greater the sensitivity to detonation. An engine that is making 0.5 HP/in3 or less can sustain moderate levels of detonation without any damage; but an engine that is making 1.5 HP/in3, if it detonates, it will probably be damaged fairly quickly, here I mean within minutes.

Detonation causes three types of failure:

Mechanical damage (broken ring lands)
Abrasion (pitting of the piston crown)
Overheating (scuffed piston skirts due to excess heat input or high coolant temperatures)
The high impact nature of the spike can cause fractures; it can break the spark plug electrodes, the porcelain around the plug, cause a clean fracture of the ring land and can actually cause fracture of valves-intake or exhaust. The piston ring land, either top or second depending on the piston design, is susceptible to fracture type failures. If I were to look at a piston with a second broken ring land, my immediate suspicion would be detonation.

Another thing detonation can cause is a sandblasted appearance to the top of the piston. The piston near the perimeter will typically have that kind of look if detonation occurs. It is a swiss-cheesy look on a microscopic basis. The detonation, the mechanical pounding, actually mechanically erodes or fatigues material out of the piston. You can typically expect to see that sanded look in the part of the chamber most distant from the spark plug, because if you think about it, you would ignite the flame front at the plug, it would travel across the chamber before it got to the farthest reaches of the chamber where the end gas spontaneously combusted. That's where you will see the effects of the detonation; you might see it at the hottest part of the chamber in some engines, possibly by the exhaust valves. In that case the end gas was heated to detonation by the residual heat in the valve.

BeelzeBob
05-10-05, 02:53 PM
Katshot...I think you really do try....you just haven't been able to...yet....LOL

Like I said above....I didn't try to rewrite the whole book, just answer the question about Northstar engines.

That article is actually a transcription from an improptu, extemporaneous "speech" to a group of individuals that do a lot of auto engine conversions into aircraft. One of the individuals was a reporter from Contact! magazine and recorded the whole thing, got it transcribed and published it. It is not written as well as I would like but at least he left out the "ahhhs" and "ands" and curse words that I uttered.

BeelzeBob
05-10-05, 02:58 PM
One additional item.

Exhaust gas temperatures.

People with exhaust temp probes watch the exhaust temps carefully when tuning the engine to avoid too high of an exhaust temp so as to avoid damageing the engine. Most gasoline engines will run around 1400-1700 F exhaust temps at the port. Depending on the type of engine, specific output, number of valves, etc...there are different "allowable" ranges...the main thing is to watch for change. Typically, when leaning or tuning the temps will go higher and higher as the engine is making more power.

Interstingly enough, detonation causes the temps to drop suddenly. So, you have to be carefull watching the exhaust temps as they can be running "cool" due to detonation...!!! and that can drive the engine into preigniton or cause other damage....like the scuffed pistons in a two stroke.

dkozloski
05-10-05, 03:30 PM
Bbob, FAA Advisory CircularAC 65-12A on page 444: "The explosive burning during detonation results in an extremely rapid pressure rise. This rapid pressure rise and the high instantaneous temperature, combined with the high turbulence generated, cause a "scrubbing" action on the cylinder and the piston. This can burn a hole completely through the piston." Pratt & Whitney in their literature says it can happen in less than a minute. I'm with you though in that it is probably combined with a pre-ignition event.

dkozloski
05-10-05, 03:53 PM
Regarding exhaust temperatures; 1650 degF. is usually regarded as the upper limit for extended operation. Erosion of exhaust valves even sodium cooled valves made from Inconel is excessive above that. When leaning the mixture while watching an EGT gauge, max power occurs 100 deg.F on the rich side of the peak reading noted. Best economy occurs at 25deg F. on the rich side. On the lean side of the peak, power falls off pretty rapidly and unless the engine is designed for the erosive atmosphere of the exhaust, damage can be done to the engine. Turbo-charged engines are a whole different animal. Here you can be playing with a hand grenade at high power settings. Unless you have specific guidelines it is best to stay below 1650F on the rich side.

BeelzeBob
05-10-05, 04:04 PM
What usually happens is that the piston swells up so much due to the heat input that it scuffs. It will usually scuff on the "corners" of the piston just across from the two pin bosses. The pin bosses are solid aluminum across the bore so there is no way for the piston to deflect in those four areas as it expands and it scuffs badly there. The upset aluminum from the scuffing transfers to the cylinder wall and the rings and gets into the ring lands. This sticks the rings. Between the stuck rings and rough ring bearing surface of the aluminum built up on the cylinder wall the seal of the rings is lost and hot combustion gases escape past the piston. This flow of hot combustion gases erodes the aluminum of the piston crown at that area and breaks down the isolating boundary layer of gases protecting the piston from the temperature. The flow of combustion gases leaking past the piston top land and rings will "torch" a hole in the piston and actually torch away the piston skirt in that area.

The other thing that happens with continuous heavy detonation under high load is that the top or second piston ring land can eventually fatigue and fail. In an engine that as endured continuous heavy detonation (the kind that makes you cring....not the pinging or light rattling) you will take the pistons out and the second ring land will be broken off due to the force against the top ring bearing downward on the second ring land. If operation continues, this will also cause a loss of sealing and propogate into the torched out area due to the original leakage from the cracked ring land.

True, heavy continuous detonation can leave behind a "hole" in the piston....but it is usually collateral damage from the initial detonation induced failure. The original failure was destroyed by the torching effect of the hot combustion gases after the failure cause a loss of combustion seal. A "hole" like this is always on the circumference of the piston NOT in the center as it happens due to preignition. I have seen the hole on the circumference torch away enough of the ring land and skirt so as to break thru the piston crown just inside the skirt of the piston under the crown. A "hole" yes....but not a "hole" as in punch a perfect, circular hole in the middle of the piston. That is the clue for preignition.


Many of these failure modes and examples depend on when the engine was shut down when detonating and how long it had run like that..or how far the failure had grown. Since the detonation can occur for a fairly long period of time there is a good chance of catching damage before it goes completely catostrophic. So you might see blue smoke, shut down and find broken ring lands or just scuffed pistons. If it kept running you would have found torched holes. Big variability in detonation failures.

The FAA words are pretty much iron cladding statements about the worst that can happen...not that it WILL happen...but what could happen.

If the engine goes into preignition it is all over. I have never seen an engine apart that someone said "that engine was preigniting" without it being the reason for the teardown due to a catostrophic failure.

eldorado1
05-10-05, 04:17 PM
uhh....NO...not plagerizing...I wrote that article.

So you ARE _______! I think you've had a few conversations with Will Lucke? And all this time I've been under the impression you were someone else! What's with the pseudonym?

dkozloski
05-10-05, 04:19 PM
I have seen several failures through the rings like you describe that were all traced to contaminated fuel leaning out the mixture or jet fuel in a recip. Thanks for the elaboration on the failure mode. I have also seen the holes in the center of the piston that appeared to be more mechanical than temperature failures.

BeelzeBob
05-10-05, 04:22 PM
Regarding exhaust temperatures; 1650 degF. is usually regarded as the upper limit for extended operation. Erosion of exhaust valves even sodium cooled valves made from Inconel is excessive above that. When leaning the mixture while watching an EGT gauge, max power occurs 100 deg.F on the rich side of the peak reading noted. Best economy occurs at 25deg F. on the rich side. On the lean side of the peak, power falls off pretty rapidly and unless the engine is designed for the erosive atmosphere of the exhaust, damage can be done to the engine. Turbo-charged engines are a whole different animal. Here you can be playing with a hand grenade at high power settings. Unless you have specific guidelines it is best to stay below 1650F on the rich side.


That is for a relatively low specific output engine (here we go again) that has sodium cooled or inconel valves.

In the automotive world that is generally considered to be a very high temperature if it is at full throttle. An LS7 would NOT live very long with 1675 EGT.....

A Northstar can live at 1400-1500 EGT because of the smaller diameter valves which can reject the heat easier.

A two stroke snowmobile engine is going to get into trouble if the EGT starts getting above 1250-1300 .... sooo....it depends totally on the engine design and type.

dkozloski
05-10-05, 04:23 PM
eldorado1, he probably doesn't want to find parcels of dog poop in his mailbox from gtm2u.

BeelzeBob
05-10-05, 04:24 PM
So you ARE _______! I think you've had a few conversations with Will Lucke? And all this time I've been under the impression you were someone else! What's with the pseudonym?


Too much $hit from people that find out who I am.

eldorado1
05-10-05, 04:30 PM
Too much $hit from people that find out who I am.

That's too bad. I would've gone with a cooler name though... like Max Power. <back to your regularly scheduled program> :drinker

dkozloski
05-10-05, 04:49 PM
Lycoming engines have 1/2" sodium cooled Inconel valves and I used to have a five gallon bucket full of failed ones that were burned, had pie shaped chunks broken out, and snapped stems. The same class of Teledyne Continental engines has 7/16" solid stem valves that are much less exotic material and you never see a failed one. I was trying to say that 1650F was the point above which valve erosion is the problem. Fracture failures seemed to occur at higher temps. I'm very surprised at the low temperature tolerance of the North* valves. They must be made of cheap stuff. Of course the revs are a lot higher. Sleeve valves over all.

BeelzeBob
05-10-05, 04:58 PM
Lycoming engines have 1/2" sodium cooled Inconel valves and I used to have a five gallon bucket full of failed ones that were burned, had pie shaped chunks broken out, and snapped stems. The same class of Teledyne Continental engines has 7/16" solid stem valves that are much less exotic material and you never see a failed one. I was trying to say that 1650F was the point above which valve erosion is the problem. Fracture failures seemed to occur at higher temps. I'm very surprised at the low temperature tolerance of the North* valves. They must be made of cheap stuff. Of course the revs are a lot higher. Sleeve valves over all.


It isn't the material as they are quite good stuff but the specific output. At the lower operating RPM of the aircraft engines they sit on the seat longer and can cool better so they can stand higher exhaust gas temps as a result. That is only an issue, really, when you start leaning the engine as aircraft engines do. I am constantly amazed at how those engines are leaned...I would expect a lot more failures. With car engines they can run richer at full throttle and control the temps. The RPM range has a large effect as a Northstar engine valve is on the seat for much shorter intervals to cool. It "sees" the seat much more often, of course, but the time interval between heat cycles is much less. Same is true of many automotive engines. Even the 4.9 engine has inconel exhaust valves to live at the full throttle heat loads that they see at 200 HP due to the larger valve diameter. The larger valves soak up more heat and it has further to travel from the center of the valve to the seat to cool.

The thing that makes 4 valve engines so much more efficient air pumps makes the valves cool better. If you look at the total valve circumference (of two exhaust valves vs. one) compared to the valve area (of two exhaust valves vs. one) the four valve engines have much more circumference. This equates to more area for heat rejection for exhaust temperature and it also equates to the "curtain area" of the valves for flow at low lifts...where the 4 valve engines excel.

ljklaiber
05-10-05, 04:58 PM
Am always amazed at how 'PART's are blamed all the time. The knowledge and skill of the machinest who does the valve job is the most important factor. Seat widths, and(lapping to true), consistant true valve guide clearances, and accuracy with valve springs, from seat pressure to gain rate to pressure consistency at max lift....are so important to engine stability,

Every piece that moves must be part of the dance. Otherwise, just blame the parts for failure,

dkozloski
05-10-05, 05:12 PM
ljklaiber, how true you are. Most of the Lycoming valve failures I saw were from one big operator and were the result of operator abuse. The saving grace was that he was the first to admit it and considered it a cost of doing business. All he would have had to do to eliminate the problem was to reduce the loads carried and pull less power from the engines. It didn't help much that the planes he flew were called a Trilander and had three recip engines, one of which was nearly inaccessible up on the tail. The only way to work on that beast was wirh a bucket truck or scaffolding, neither of which he ever owned. I would overhaul the same engines for a different operator and he could run them at least 700hr beyond the reccommended overhaul period. Every overhaul I did for him we would submit paper work for a further extension.

dkozloski
05-10-05, 05:32 PM
ljklaiber, what a lot of assemblers miss is that the valves, guides, and seats have to be absolutely concentric. I've torn down engines with leaky valves and found the seats off to the side like the handle on a piss pot.

BeelzeBob
05-10-05, 09:59 PM
I feel that it is a real testament to the guys that designed the Northstar valve train (not me...) that folks are pulling heads off the engine with 150K or much more on them and they are in perfect shape, do not need any major work at all and in most cases can be simply cleaned up and bolted back on. The valve seats look good for any used head much less for ones with over 100K on them. Even the deposits on the seats are very light and not really worth messing with as they will reappear when the heads are run again. The springs do not loose any tension over the life of the engine. All this indicates that the premium seats, valves, seals and springs used were worth the money in the long run. It also indicates that the valve train is very stable, even at the high RPMs that the engine operates at. If the cam is lifting and closing the valve in control all the time the seats and valves last a great deal longer then when things crash together.

This and the fact that the timing chains and sprockets also live the life of the engine indicatest that the timing drive is stable and of premium materials. Anyone ever think that they would tear an engine down, especially a DOHC V8 with 3 chains, and be able to reuse the timing chains after 150K..???

Some of the nice features of the engine really get overlooked with all the talk of head gaskets and timeserts and such.....LOL.

Stoneage_Caddy
05-10-05, 10:03 PM
thats what surpirses me , the heads are perfect , i havent seen anyone gripeing about a cracked head or whatever .....swap and gasket and go (maybe sert it) but really , thats pretty good comapred to what im used to (throw away the head)

eldorado1
05-10-05, 10:12 PM
Just think... as soon as the headgasket problem is fixed (I think you mentioned it was in the y2k4+ editions).... you'll have yourself a winner. Well, assuming this variable cam timing thing doesn't explode... Can't say that I'm a big fan of it, more things to break.

dkozloski
05-11-05, 02:26 AM
eldorado1, don't worry about the VVT. Mercedes had it back in the 1930's. The only thing new about it is how Cadillac came up with a great implementation. I fooled around with a VVT deal on small block Chevies 35 years ago. The kit converted the engine from a chain cam drive to gears and had a movable idler shifted by engine oil pressure to change the timing. The fly in the ointment was that the spark timing changed with it unless you relocated the distributor drive. I saw another deal that used helical gears to drive the cam and the cam shaft was slid back and forth to change the timing. The pitch of the helical drive gears complimented that of the distributor drive gear so the ignition timing didn't change. The amount you could shift the timing with those old Chevy setups was miniscule compared to the Cadillac. The Mercedes had a full blown governor driven deal with centrifugal weights and a lot of other stuff.

JSMeloche
05-11-05, 08:43 AM
eldorado1, don't worry about the VVT. Mercedes had it back in the 1930's. The only thing new about it is how Cadillac came up with a great implementation. I fooled around with a VVT deal on small block Chevies 35 years ago. The kit converted the engine from a chain cam drive to gears and had a movable idler shifted by engine oil pressure to change the timing. The fly in the ointment was that the spark timing changed with it unless you relocated the distributor drive. I saw another deal that used helical gears to drive the cam and the cam shaft was slid back and forth to change the timing. The pitch of the helical drive gears complimented that of the distributor drive gear so the ignition timing didn't change. The amount you could shift the timing with those old Chevy setups was miniscule compared to the Cadillac. The Mercedes had a full blown governor driven deal with centrifugal weights and a lot of other stuff.

So now we can put V-tec stickers on our N* to get 20HP more WOOHOO!!! Dont forget the Type-R sticker, this one is for ground effet :drinker:deadhorse:banghead2:gungrin:

BeelzeBob
05-11-05, 11:04 AM
The cam phasers used on the Northstar are actually sourced thru DC....they are Merc parts........there are many strange bedfellows in the auto industry...

There are several different styles of cam phaser actuation devices and the Merc phasers provide the most authority and the fastest response time...so that is what was used.