Detonation and EGT questions

[jake98gst]

I read an entire article today outlining and defining detonation and pre-ignition. It explained the causes, effects, and possible courses of action to avoid them. The article was written by an automotive engineer.

A few questions arose when I discover some of the things written contradicted what I've read on this and other boards.

My main question is as follows: detonation is defined as the spontaneous combustion of the leftover gasses (after combustion) due to intense heat and/or pressure in the chamber. He goes on to explain that exhaust gas temperatures will actually be lower when an engine is detonating. Herein lies the paradox. This "expert" is saying knock lowers EGT when the entire DSM community tells me EGT will rise when detonating. To me, you all make sense. From the definition, intense hear and/or pressure cause the problem yet the effect is a cooler temperature than normal??? Can someone clarify?

Thanks,
Jake

 

[Blk_99gst]

My main question is as follows: detonation is defined as the spontaneous combustion of the leftover gasses (after combustion) due to intense heat and/or pressure in the chamber.

This would be wrong. Detonation is the spontaneous combustion BEFORE combustion. Combustion happens when everything is good to go and the spark plug sets it off. Detonation happens when combustion is caused not by the spark plug, but because of excessive heat or pressure.

He goes on to explain that exhaust gas temperatures will actually be lower when an engine is detonating. Herein lies the paradox. This "expert" is saying knock lowers EGT when the entire DSM community tells me EGT will rise when detonating. To me, you all make sense. From the definition, intense hear and/or pressure cause the problem yet the effect is a cooler temperature than normal??? Can someone clarify?

The key here is exhaust gas temperatures; i.e. you have your egt probe sitting right outside the runner of cylinder #1. When detonation starts happening, the knock sensor picks this and tell the ecu, the ecu then starts pulling timing due to the detonation. When your timing gets pulled then real combustion happens very late in the cycle, therefore you have very hot (sometimes still burning) exhaust gases leaving the cylinder and rapidly heating up the egt probe.

So really, the detonation gases are not causing high temps. It's the fact that timing is getting pulled, and the regular combustion gases are excessively heating up the egt probe. This is bad power wise, because you are not getting the most power from the combustion, rather it's happening too late and not all the energy from the explosion is transfered to mechanical energy of moving the cylinder.

(back to egt gauge) This is why people say the only thing an egt gauge is for is to tell you when your timing is getting severly pulled; i.e the needle on the egt gauge rises very quickly.

 

[jake98gst]

This would be wrong. Detonation is the spontaneous combustion BEFORE combustion. Combustion happens when everything is good to go and the spark plug sets it off. Detonation happens when combustion is caused not by the spark plug, but because of excessive heat or pressure.

According to the article, your first definition is a description of pre-ignition, not detonation and your second definition of detonation is actually a generalization describing both detonation and pre-ignition. "Detonation is the spontaneous combustion of the end-gas (remaining fuel/air mixture) in the chamber. It always occurs after normal combustion is initiated by the spark plug. The initial combustion at the spark plug is followed by a normal combustion burn. For some reason, likely heat and pressure, the end gas in the chamber spontaneously combusts. The key point here is that detonation occurs after you have initiated the normal combustion with the spark plug....Pre-ignition is defined as the ignition of the mixture prior to the spark plug firing. Anytime something causes the mixture in the chamber to ignite prior to the spark plug event it is classified as pre-ignition. The two are completely different and abnormal phenomenon. "

The key here is exhaust gas temperatures; i.e. you have your egt probe sitting right outside the runner of cylinder #1. When detonation starts happening, the knock sensor picks this and tell the ecu, the ecu then starts pulling timing due to the detonation. When your timing gets pulled then real combustion happens very late in the cycle, therefore you have very hot (sometimes still burning) exhaust gases leaving the cylinder and rapidly heating up the egt probe.

So really, the detonation gases are not causing high temps. It's the fact that timing is getting pulled, and the regular combustion gases are excessively heating up the egt probe. This is bad power wise, because you are not getting the most power from the combustion, rather it's happening too late and not all the energy from the explosion is transfered to mechanical energy of moving the cylinder.

(back to egt gauge) This is why people say the only thing an egt gauge is for is to tell you when your timing is getting severly pulled; i.e the needle on the egt gauge rises very quickly.

Thank you for clarifying the activity of EGT in a DSM-specific situation. In a DSM, the pulled timing, while caused by detonation, directly attributes to the rise in EGT, and detonation is not the direct source of the rise in EGT. But my question is: how is EGT lowered by detonation? Also, the article provides no support for this argument, which is the reason I ask you.

 

[yo2001]

detonation breaks the coating on the top of the pistons created by the gas mixture. When that heppens, the heat penetrates the pistons which leads to drop on EGT.

 

[jake98gst]

OK. The pistons, being aluminum and having a high thermal inertia, soak up the heat of detonation, after first having the piston's coating deteriorated, causing a lower EGT.

To diverge a bit, I did some more looking into the articles and found myself another question concerning the coating. What makes the compressed air/fuel mixture in the chamber any different from the air/fuel mixture in contact with the piston? In other words, how is the coating created?

Thanks again fellas for fielding all of my questions,
Jake

 

[kpt4321]

According to the article, your first definition is a description of pre-ignition, not detonation and your second definition of detonation is actually a generalization describing both detonation and pre-ignition. "Detonation is the spontaneous combustion of the end-gas (remaining fuel/air mixture) in the chamber. It always occurs after normal combustion is initiated by the spark plug. The initial combustion at the spark plug is followed by a normal combustion burn. For some reason, likely heat and pressure, the end gas in the chamber spontaneously combusts. The key point here is that detonation occurs after you have initiated the normal combustion with the spark plug....Pre-ignition is defined as the ignition of the mixture prior to the spark plug firing. Anytime something causes the mixture in the chamber to ignite prior to the spark plug event it is classified as pre-ignition. The two are completely different and abnormal phenomenon. "

Think about it this way: If detonation happens after the combustion event is totally finished, then it would never hurt anything.

Detonation happens after the BEGINNING of the combustion event, but that does not mean it happens after the normal combustion ends. Detonation occurs when, instead of a nice smooth burn, you have multiple flame fronts (with a generally faster flame front) that collide with one another and create pressure reverbations.

Pre-Ignition happens when something ignites before the spark plug fires. This is always bad because you don't get the nice controlled and even burn that you do with a plug-caused ignition, but the degree to which it is damaging can vary quite a bit. It could act almost like a little bit more timing advance, which is not necessarily going to be noticable. On the other hand, in a lo of situations it can cause detonation when the preignition flame fronts collide with one another, or collide with the flame front created by the spark plug firing.

I hope that clears up some questions!

 

[Defiant]

This isn't to argue, it's just the way I was taught back in '71: "Detonation" is the spontaneous exploding of the fuel/air mixture, and will occur with force enough to bend rods, crack pistons, snap head studs and fracture water jackets. In my high school auto shop, it was taught that this is what "knock" is. Of course, the current "knock sensor" isn't listening for the too-late destruction of your motor, so either the terms have shifted or someone was simply wrong.

"Pre-ignition" was the phenomenon of multiple flame fronts in the air/fuel mixture, caused by hot spots or compression ignition of part of the charge by the main burning's increasing of the pressure in the combustion chamber because the engine is under too great a load for the octane of the fuel being used. When the multiple flame fronts collide, it causes the "ping" that the current "knock sensor" is listening for.

I guess none of this really matters much, so long as the phenomenons are recognized as being distinct and different. The rest of it is semantics.

 

[jake98gst]

Detonation occurs when, instead of a nice smooth burn, you have multiple flame fronts (with a generally faster flame front) that collide with one another and create pressure reverbations.

When the multiple flame fronts collide, it causes the "ping" that the current "knock sensor" is listening for.

From the article, detonation causes a sharp pressure spike that creates a "force in the combustion chamber. It causes the structure of the engine to ring, or resonate, much as if it were hit by a hammer. Resonance, which is characteristic of combustion detonation, occurs at about 6400 Hertz. So the pinging you hear is actually the structure of the engine reacting to the pressure spikes. ...Incidentally, the knocking or pinging sound is not the result of 'two flame fronts meeting' as is often stated." Also, the article points out that pre-ignition actually makes no sound, and evidence of pre-ignition is most often white excessive smoke out of the tailpipe and engine failure. In pre-ignition, there is no sharp pressure spike and thus no resonance, and thus no sound. However, the article never mentions why there is no sharp pressure spike in pre-ignition. Can someone clarify why this is the case?

[Pre-ignition] is always bad because you don't get the nice controlled and even burn that you do with a plug-caused ignition, but the degree to which it is damaging can vary quite a bit. It could act almost like a little bit more timing advance, which is not necessarily going to be noticable. On the other hand, in a lo of situations it can cause detonation when the preignition flame fronts collide with one another, or collide with the flame front created by the spark plug firing.

In the article, the author states that in every occurrence of pre-ignition, shit breaks, and the engine fails. Every time, no exceptions. Can you explain how it would advance timing and not cause total engine failure? My logic leads me towards the article's explanation. Since, in the case of pre-ignition, combustion occurs sometime before TDC, the pressure created from pre-ignited fuel would force the crank to turn in the opposite direction the other cylinders and the crank in general are wanting to turn. Also, I can logically comprehend detonation-induced pre-ignition. But I cannot do so with pre-ignition-induced detonation. Can you explain? Again, your definition of pre-ignition obviously does not entail complete engine failure.

Thanks again,
Jake

 

[kpt4321]

From the article, detonation causes a sharp pressure spike that creates a "force in the combustion chamber. It causes the structure of the engine to ring, or resonate, much as if it were hit by a hammer. Resonance, which is characteristic of combustion detonation, occurs at about 6400 Hertz. So the pinging you hear is actually the structure of the engine reacting to the pressure spikes. ...Incidentally, the knocking or pinging sound is not the result of 'two flame fronts meeting' as is often stated." Also, the article points out that pre-ignition actually makes no sound, and evidence of pre-ignition is most often white excessive smoke out of the tailpipe and engine failure. In pre-ignition, there is no sharp pressure spike and thus no resonance, and thus no sound. However, the article never mentions why there is no sharp pressure spike in pre-ignition. Can someone clarify why this is the case?

There are a couple of things to mention here.

First of all, the detonation frequency is really not that constant. It depends on a couple of things, one of them being the bore of the motor. In a 4G63, IIRC, it's going to be closer to 4k or 5k than 6400Hz.

Second of all, I have heard (or read) that thing about the block creating the noise before. It's right, and wrong. Yes, the block is vibrating, and that is what you hear (and the knock sensor hears). However, that vibration doesn't just come from nowhere. It comes form the pressure reverbations inside the cylinder, without those pressure waves the block wouldn't oscillate. So, the primary source of those sounds is still the pressure waves, because without them the block wouldn't be acting as a "speaker."

The claim that preignition doesn't create a pressure spike is absurd. It is not necessarily going to create one, but it is very possible that it would.

In the article, the author states that in every occurrence of pre-ignition, shit breaks, and the engine fails. Every time, no exceptions. Can you explain how it would advance timing and not cause total engine failure? My logic leads me towards the article's explanation. Since, in the case of pre-ignition, combustion occurs sometime before TDC, the pressure created from pre-ignited fuel would force the crank to turn in the opposite direction the other cylinders and the crank in general are wanting to turn. Also, I can logically comprehend detonation-induced pre-ignition. But I cannot do so with pre-ignition-induced detonation. Can you explain? Again, your definition of pre-ignition obviously does not entail complete engine failure.

That's another untrue claim.

Just like detonation does not necessarily destroy parts every time, there is no logical reason why preignition would be sure to hurt the motor.

Another thing that realy bothers me is your claim that the author claims that there is no pressure spike created, but motor parts are guaranteed to fail. Either you are reading it wrong, or the author is wrong. Without a pressure spike, how the heck is the motor going to a-splode? Magic?

I'm not quite sure where you are going with detonation induced preignition, and preignition induced detonation.

Preignition can induce detonation; detonation is the collision of some flame fronts, so if you have one flame front from a preignition event and one flame front from the spark plug (or from another preignition event, even), you can get detonation.

Detonation can probably cause preignition too. Detonation can heat up the chamber, which can cause preignition on later cycles.

It's important to note that detonation is MUCH more common on a modern motor like the 4G63.

 

[jake98gst]

The claim that preignition doesn't create a pressure spike is absurd. It is not necessarily going to create one, but it is very possible that it would.

It's not impossible but very unlikely. If pressure spikes are caused by the meeting of two flame fronts, then I don't understand how there would be a pressure spike in the case of pre-ignition. The chance of pre-ignition is greatest when the cylinder pressure is at its lowest, at BDC. So it would be hard for the pre-ignition flame front to meet the flame front of the spark since there is 180* between the two fronts. Therefore, unless the pre-ignition occurred very late in compression, somewhere near TDC, you wouldn't hear a sound and there wouldn't be a pressure spike. Also, pre-ignition can break shit without causing a pressure spike because the pressure built from the pre-ignited mixture will force the crank in the opposite direction its going; pushing it down when it's on the upstroke.

Preignition can induce detonation; detonation is the collision of some flame fronts, so if you have one flame front from a preignition event and one flame front from the spark plug (or from another preignition event, even), you can get detonation.

I don't see how pre-ignition could cause detonation unless the flame front of pre-ignition (assuming of course that pre-ignition is less likely the closer you get to TDC) lasted all the way through TDC and met the flame front of spontaneous end-gas combustion (defined as detonation by the author). This is where your definitions and the author's contridict. Is it true that pre-ignition, for you, is any act of spontaneous combustion of the mixture sometime between BDC and TDC, and detonation is any meeting of two flame fronts?

Thanks again. I know it may seem like I'm just telling you how the article says your wrong, but I'm trying to form a definition for myself.

Jake

 

[kpt4321]

It's not impossible but very unlikely. If pressure spikes are caused by the meeting of two flame fronts, then I don't understand how there would be a pressure spike in the case of pre-ignition. The chance of pre-ignition is greatest when the cylinder pressure is at its lowest, at BDC. So it would be hard for the pre-ignition flame front to meet the flame front of the spark since there is 180* between the two fronts. Therefore, unless the pre-ignition occurred very late in compression, somewhere near TDC, you wouldn't hear a sound and there wouldn't be a pressure spike.

I think that the presumption that preignition is only going to happen at BDC is false. Yes, on one hand the pressure is lowest then.

However, as you compress the mixture, several things happen. One of the most important of these is the fact that a lot of heat is created. As you pressurize and heat the mixture, you are generally more likely to light it up with a hot spot.

Also, pre-ignition can break shit without causing a pressure spike because the pressure built from the pre-ignited mixture will force the crank in the opposite direction its going; pushing it down when it's on the upstroke

You're contradicting yourself. You're saying that the preignition can break stuff without creating excess pressure, because ti create excess pressure.

I don't see how pre-ignition could cause detonation unless the flame front of pre-ignition (assuming of course that pre-ignition is less likely the closer you get to TDC) lasted all the way through TDC and met the flame front of spontaneous end-gas combustion (defined as detonation by the author). This is where your definitions and the author's contridict. Is it true that pre-ignition, for you, is any act of spontaneous combustion of the mixture sometime between BDC and TDC, and detonation is any meeting of two flame fronts?

For me, preignition is any ignition that occurs before the spark plug fires. That I am sure of.

I need to think more before I decide how I feel about ignition events that occur when the plug fires, or slightly after. However, I am sure that a combustion event before the plug fires is preignition.

Yes, detonation is the meeting of two flame fronts, which basically happens when you have a burn that isn't happening in the controlled manner it is supposed to.

Pre-ignition can cause detonation, if you consider that it can happen closer to the spark plug firing than BDC.

Thanks again. I know it may seem like I'm just telling you how the article says your wrong, but I'm trying to form a definition for myself.

Thank you! This discussion has been very enjoyable, really.

If the article says I am wrong, I might be. I think that right now it's more communication errors than anything else, but it's good to think about this stuff.

 

[jake98gst]

However, as you compress the mixture, several things happen. One of the most important of these is the fact that a lot of heat is created. As you pressurize and heat the mixture, you are generally more likely to light it up with a hot spot.

Right, but the voltage required to light the mixture increases proportionally with the amount of mixture compression. So almost anything can light the mixture at BDC including glowing embers, hot spots, etc.

You're contradicting yourself. You're saying that the preignition can break stuff without creating excess pressure, because ti create excess pressure.

No, no no. Aren't a pressure spike and excess pressure are two different things? A pressure spike, caused by the meeting of two flame fronts, would not necessarily be caused since the pre-ignition flame front and the spark are up to 180* away from each other. Excess pressure is exactly what is caused every instance of abnormal combustion.

Sooner or later we're just going to have to agree to disagree. But until then, Thanks again!

Jake

 

[Defiant]

Was this an online article? Or someplace I can find it? He's saying a few things differently than I'm used to hearing them.

Hell, he might even be wrong, engineering be damned.

 

[kpt4321]

Right, but the voltage required to light the mixture increases proportionally with the amount of mixture compression. So almost anything can light the mixture at BDC including glowing embers, hot spots, etc.

That's true. On the other hand, as the mixture gets compressed, everything heats up, which makes it more likely to ignite.

No, no no. Aren't a pressure spike and excess pressure are two different things? A pressure spike, caused by the meeting of two flame fronts, would not necessarily be caused since the pre-ignition flame front and the spark are up to 180* away from each other. Excess pressure is exactly what is caused every instance of abnormal combustion.

I guess it's just a matter of definition. I don't see why you ned to have two flame fronts meeting to cause a pressure spike. If you had a gauge that showed you cylinder pressure, you would definately see it go higher than normal when there was a preignition event. As such, I decided that preignition causes a pressure spike/excess pressure. Perhaps the author of this article didn't have the same definition.

 

[kpt4321]

Hell, he might even be wrong, engineering be damned.

Engineers don't always agree with each other.

 

[jake98gst]

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

That's the article. I originally saw it in a magazine but did a search and found it.

Well, I think we've covered everything we possibly can cover in this discussion. Like you said, it's a matter of definitions, and engineers don't always agree.

Thank you again for a very interesting and pleasant discussion.

Jake

 

[UnderPrssureDSM]

So would you suggest adding or removing fuel to tame the EGTs? My temps increase to well over 1700F when I'm WOT in 5th gear.

 

[Violater101]

Well, to be honest, most of us don't get to the point where we have to go WOT in 5th gear on the highway, let alone the track. If your seeing EGT's that high, this typically means your pulling timing aka Knock. You can do a couple of things here. 1) would be to turn down the boost or 2) give the car more fuel. Have you logged your car?

 

[UnderPrssureDSM]

Still adjusting the SAFC to set the LTFTs. I'm only running 12psi, so I'll try adding more fuel on the top RPMs.