Rough Combustion

[Tom Walker]

Anybody want to share some thoughts about rough combustion or "Rumble" as sometimes referred to.
I have always assumed normal detonation was the auto ingnition of all of the end gas remaining once detonation started in that specific combustion event, but could rough combustion events actually have several pockets of end gas auto ingniting totally separate from each other and even occurring in a slightly different time?
In other words, all the detonation may not consume all the remaining fuel mixture at once, or even in one pocket of mixture, could there be multiple pockets of mixture that have their own "personality"?
Not sure if I have asked my questions in a way that communicates clearly, but if find the study of combustion and some of its abnormalitys interesting.

[Tom Walker]

Meant to say in my last sentence , "I find the study of combustion and some of its abnormalitys interesting"

[Tom68]

How is rumble measured ?

[Tom Walker]

Don't know if it could be defined as "measured", but maybe the term detected would fit. A knock sensor of some type seems to be the equipment detecting this abnormal combustion.
The phenomenon I am referring to as rough combustion or rumble as some call it, seems to define detonation on a low level.
As far as normal detonation we are accustomed to discussing,
1.What is the normal percentage of the mixture that ends up auto ingniting that leads to engine damage?
2.Does a smaller percentage of end gas auto ingniting go unnoticed and unheard?
Is this rough combustion just a very small percentage of the fuel mixture that detonates and doesn't really cause a concern, just a little roughness heard by a knock sensor?
I do believe there are varying degrees of detonation due to the various amounts of end gases that are left unburned when the conditions for auto ingnition are met in that particular combustion cycle .
Believe me, I have way more questions than answers, and that is the purpose of my posts.

[barnym17]

I can see where it is possible there can be more than one pocket of mixture could detonate or autoignite seperately.But I would wonder what the effect would really be once the "damage" to the combustion event would be after the first pocket has already occured?

[Tom68]

Don't know if it could be defined as "measured", but maybe the term detected would fit. A knock sensor of some type seems to be the equipment detecting this abnormal combustion.
The phenomenon I am referring to as rough combustion or rumble as some call it, seems to define detonation on a low level.
As far as normal detonation we are accustomed to discussing,
1.What is the normal percentage of the mixture that ends up auto ingniting that leads to engine damage?
2.Does a smaller percentage of end gas auto ingniting go unnoticed and unheard?
Is this rough combustion just a very small percentage of the fuel mixture that detonates and doesn't really cause a concern, just a little roughness heard by a knock sensor?
I do believe there are varying degrees of detonation due to the various amounts of end gases that are left unburned when the conditions for auto ingnition are met in that particular combustion cycle .
Believe me, I have way more questions than answers, and that is the purpose of my posts.

I don't think of detonation "pinging" as being auto ignition, so we're on different wave lengths already, "mild detonation" would be detonation under lower loads and or lowest speeds.

[Tom Walker]

Appreciate the thoughts there barnym 17, yeah, just curiosity on my part contemplating the various degrees of detonation.
Am still curious of what percentage of the end gas remaining that it takes to evolve into detonation before we are able to actually hear it?
Would 10% remaining end gas detonating be audible or damaging? Have no clue myself, just questions in my head when I can't sleep, but inquiring minds would like to know.

[Tom68]

Whilst you think detonation and the associated noise is just from colliding flame fronts you won't see the answer.

[Warp Speed]

Each event is either happening in the "optimum" burn range or outside. When recording combustion, events happen on both sides of the optimal range. You look for the average, while trying to minimize the outliers. That is as long as minimizing them doesn't hurt overall output........

[Tom Walker]

Warp, I am assuming, optimal range means most power in your line of work, but are there ever some outliers on a particular cycle that exhibit some form of abnormal burn that can produce equal or even better power than the normal expected cycle?
Or is it always a power loser with any abnormal type of combustion?
Interesting stuff and appreciate the insight.
Did see Dr. Randolph of NASCAR fame discussing some of which you were referencing.

Tom68, not sure if you are including me in the collding frame fronts club,but no, I renounced my membership years ago.

[Tom68]

So 10% end gas auto igniting before the spark ignited gas mix has burnt could most likely tip an engine running on the edge into detonation.

[Tom Walker]

Yeah, I call that 10% detonation. That was easy, what's next?

[Roundybout]

Is peak cylinder pressure, power and torque always best just before the onset of detonation for the fuel used? Far from detonation? A little detonation here and there?

I think that and a lot more is happening. I asked once is a detonation event ever a power producer. It isn’t. A/F mixture is mostly burned. Piston is farther down the cylinder. Pressure is dropping. Now the last 10% detonates. It’s the uncontrolled heat release that’s the killer.

Monitoring my knock sensor. I see a bit here and there. Sometimes significant. But I’ve never heard it audibly. It is an interesting question. How much is audible. How much is happening cycle to cycle to notice. The best engines have the least cycle to cycle variation. I’d imagine those engines are far from the knock threshold for the fuel used. Just a guess lol.

[Tom68]

Is peak cylinder pressure, power and torque always best just before the onset of detonation for the fuel used? Far from detonation? A little detonation here and there?

I think that and a lot more is happening. I asked once is a detonation event ever a power producer. It isn’t. A/F mixture is mostly burned. Piston is farther down the cylinder. Pressure is dropping. Now the last 10% detonates. It’s the uncontrolled heat release that’s the killer.

Monitoring my knock sensor. I see a bit here and there. Sometimes significant. But I’ve never heard it audibly. It is an interesting question. How much is audible. How much is happening cycle to cycle to notice. The best engines have the least cycle to cycle variation. I’d imagine those engines are far from the knock threshold for the fuel used. Just a guess lol.

Detonation to power threshold depends on the engine.

Detonation happens pretty much at TDC, not when the piston is rocketing away.

That is a good question. If it's not audible is one thing, at the same time it's possibly not then hurting power and hopefully not parts ?

[Tom Walker]

Roundybout, fantastic comments and observations.
I believe also, that it seems any abnormal burn is a loser, but how close to the onset of detonation is an engine running when it is running at its most effiecent and powerful tune?
The cycle to cycle variations also seem to be a killer in so many ways.

Tom68, if we are looking for peak combustion pressure 14 to 18 degrees after TDC, would not any detonation activity be happening after that?
Appreciate your thoughts on these topics.