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Si Engines and detonation 6
- Thread starter dicer
- Start date
- Thread starter
- #41
LostHippie, thank you I forgot the pressure component damage deal, like con rod bearing distress etc. But is that related more to BTDC detonation?
Boundary layer? Can a combustable mixture of fuel and oxidizer be called a boundary layer? It that layer is stagnent then maybe that is the start point for the detonation.
Boundary layer? Can a combustable mixture of fuel and oxidizer be called a boundary layer? It that layer is stagnent then maybe that is the start point for the detonation.
LostHippie
Mechanical
Detonation typically occurs near TDC...preignition is when the combustion process begins well before TDC...resulting in the engine having to compress the hot gasses to the clearance volume after all of the combustion energy has been released...resulting in much higher temperatures and pressures than would normally be subjected to the components of the combustion chamber, but that also means that the components are exposed to these increasingly hostile conditions for a longer period of time (most of the compression and power stroke). Because detonation typically occurs near TDC...and because detonation is a completely different phenomena, the high pressures and temperatures generated are at their worst only during the period near TDC and the powerstroke.
The difference b/w detonation and preignition is that preignition is usually still a single ignition source, controlled combustion process, releasing energy relatively slowly compared to detonation. Detonation, by definition, is an irregular combustion in which the normal ignition source and/or multiple autoignition points ignite the compressed air-fuel mixture, resulting in multiple flame fronts converting the air-fuel mix into a mixture of combustion byproducts and heat. Because there are more than one flame front, the release of byproducts and energy happen much more quickly, resulting in a pressure spike occurring faster than the engine was designed for.
In contrast...normal combustion is timed such that the bulk of combustion begins aTDC and continues to progress in a relatively slow, controlled manner as the piston begins to move into the power stroke, that way you actually use all of the energy released. The spark ignition source can be timed bTDC, but only to account for ignition lag, the bulk of the energy release comes aTDC. In comparison, detonation can occur very very quickly in an uncontrolled manner, such that the bulk of the energy released is done so bTDC, at TDC, or very slightly afterwards, which is not what the combustion chamber is designed for. This sudden violent release of combustion energy imparts much higher stress on related components, even though the total amount of energy released is more or less the same as normal combustion.
While preignition and detonation (in the scope of automotive applications) are semi related due to the fact that they are both undesirable and uncontrollable combustion processes, as well as some of their potential causes, they are completely different beasts in terms of where they happen, what damage they cause, and how they cause said damage. I do admit that detonation can eventually become the cause of preignition. And to dicer, yes the boundary layer in terms combustion chambers refers to the protective layer of low-velocity gas that surrounds the inner surfaces of the combustion chamber.
I hope this helps,
LostHippie
The difference b/w detonation and preignition is that preignition is usually still a single ignition source, controlled combustion process, releasing energy relatively slowly compared to detonation. Detonation, by definition, is an irregular combustion in which the normal ignition source and/or multiple autoignition points ignite the compressed air-fuel mixture, resulting in multiple flame fronts converting the air-fuel mix into a mixture of combustion byproducts and heat. Because there are more than one flame front, the release of byproducts and energy happen much more quickly, resulting in a pressure spike occurring faster than the engine was designed for.
In contrast...normal combustion is timed such that the bulk of combustion begins aTDC and continues to progress in a relatively slow, controlled manner as the piston begins to move into the power stroke, that way you actually use all of the energy released. The spark ignition source can be timed bTDC, but only to account for ignition lag, the bulk of the energy release comes aTDC. In comparison, detonation can occur very very quickly in an uncontrolled manner, such that the bulk of the energy released is done so bTDC, at TDC, or very slightly afterwards, which is not what the combustion chamber is designed for. This sudden violent release of combustion energy imparts much higher stress on related components, even though the total amount of energy released is more or less the same as normal combustion.
While preignition and detonation (in the scope of automotive applications) are semi related due to the fact that they are both undesirable and uncontrollable combustion processes, as well as some of their potential causes, they are completely different beasts in terms of where they happen, what damage they cause, and how they cause said damage. I do admit that detonation can eventually become the cause of preignition. And to dicer, yes the boundary layer in terms combustion chambers refers to the protective layer of low-velocity gas that surrounds the inner surfaces of the combustion chamber.
I hope this helps,
LostHippie
patprimmer
New member
Nice in theory, but in practice the pressure rise as approaching TDC after pre-ignition almost always results in detonation anyway, so the end result is the same.
You can certainly have detonation without pre-ignition, but when you have pre-ignition, you almost certainly also get detonation as a consequence.
I have certainly pulled pistons with big thumb print size depressions in the crown as a result of detonation. I am pretty sure the deformation was a result of the aluminium having softened combined with pressure. Pressure alone would have cracked it, probably in the ring land area.
Regards
Pat
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You can certainly have detonation without pre-ignition, but when you have pre-ignition, you almost certainly also get detonation as a consequence.
I have certainly pulled pistons with big thumb print size depressions in the crown as a result of detonation. I am pretty sure the deformation was a result of the aluminium having softened combined with pressure. Pressure alone would have cracked it, probably in the ring land area.
Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &
for site rules
SomptingGuy
Automotive
Nostalgia is not as good as it used to be. New grads here used to go through a 2 week course in engine running/testing when they started (as I did 20+ years ago). The first week was with a variable CR E6 research engine. Loads of fun and very educational.
- Steve
- Steve
FahlinRacing
Automotive
If a certain area collects fuel, where ever it may be, in the center, more towards the bore wall or in a crevice volume, and becomes soaked with enough heat and pressure it will ignite.
This could be influenced by how well our low-lift reatomization of our fuel and/or the strength of the mixture motion. I agree that if there is some point at which our A/F become stagnant at the chamber surfaces, it is a part of the puzzle we call a root cause.
- Thread starter
- #46
Losthippie, all you mentioned is pretty much common knowledge stuff. I guess I should have asked the question more like this. Do you think any mechanical damage is caused by pre TDC or ATDC detonation?
SomptingGuy, Did you mean a CFR engine? New grads in what?
I think most only know what a picture of an engine is. I've recently been around 2 ME's that didn't know much about the real thing.
FahlingRacing, I was waiting for Mattsooty to answer, since he mentioned it.
SomptingGuy, Did you mean a CFR engine? New grads in what?
I think most only know what a picture of an engine is. I've recently been around 2 ME's that didn't know much about the real thing.
FahlingRacing, I was waiting for Mattsooty to answer, since he mentioned it.
SomptingGuy
Automotive
dicer,
A variable compression ratio engine (Ricardo E6, single cylinder). The one where you can dynamically vary the CR by some kind of mechanism that moved the head up and down.
My employer generally recruits mechanical engineering graduates and rotates them around the various departments. The first posting for all used to be this engine running/testing course. Seriously hands-on, using simple but educational equipment (no computers or controllers anywhere).
- Steve
A variable compression ratio engine (Ricardo E6, single cylinder). The one where you can dynamically vary the CR by some kind of mechanism that moved the head up and down.
My employer generally recruits mechanical engineering graduates and rotates them around the various departments. The first posting for all used to be this engine running/testing course. Seriously hands-on, using simple but educational equipment (no computers or controllers anywhere).
- Steve
- Thread starter
- #49
No, I am saying that the Deflagration Detonation Transition occurs in the end gases. Which may or may not be in the boundary layer, there is a difference.
'End gases' is simply a term for that part of the charge that is not currently involved in the oxidization of the flame front not as commonly thought, next to the combustion chamber. That is the 'boundary layer'.
During normal deflagration combustion, in an SI engine, there is a laminar and controlled oxidization of the charge, with a succinct flame front. (Combustion in a CI engine combustion is via a diffusion flame, which is another matter for another time).
In an experimental bomb this flame front moves away from the spark initiation kernel, in a controlled laminar manner, and the charge adjacent to the combustion chamber wall (the boundary layer) will quench & not burn, due to cooling. Which is not good for carboniferous emissions.
In practice, to improve the development of the kernel and subsequent oxidation of the charge, in cylinder charge motion can be utilised - Such as squish, tumble or swirl - this serves to increase the speed of combustion and improve the 'mixing'. Meaning more of the combustible charge is indeed combusted. Though there will still be quench at the combustion chamber walls, due to cooling.
When the DDT threshold is surpassed and detonation combustion occurs, high energy shockwaves are present within the combustion chamber. Which, as I said, break down this boundary layer and, I am quite sure, involve the charge that would usually quench in the detonation combustion.
So, in a roundabout way and considering the modern and accepted models of detonation, the gist of which Pat summed up in the very first reply to the OP - I would not expect detonation to be initiated in the boundary layer.
Dicer, There is copious amounts of text regarding topics such as this, you couldn't go far wrong with the following: -
Automotive fuels and emissions - Barry Hollembeak
The Internal-combustion Engine in Theory and Practice - Charles Fayette Taylor
Internal combustion engine fundamentals - John Heywood
Does that answer your question?
MS
'End gases' is simply a term for that part of the charge that is not currently involved in the oxidization of the flame front not as commonly thought, next to the combustion chamber. That is the 'boundary layer'.
During normal deflagration combustion, in an SI engine, there is a laminar and controlled oxidization of the charge, with a succinct flame front. (Combustion in a CI engine combustion is via a diffusion flame, which is another matter for another time).
In an experimental bomb this flame front moves away from the spark initiation kernel, in a controlled laminar manner, and the charge adjacent to the combustion chamber wall (the boundary layer) will quench & not burn, due to cooling. Which is not good for carboniferous emissions.
In practice, to improve the development of the kernel and subsequent oxidation of the charge, in cylinder charge motion can be utilised - Such as squish, tumble or swirl - this serves to increase the speed of combustion and improve the 'mixing'. Meaning more of the combustible charge is indeed combusted. Though there will still be quench at the combustion chamber walls, due to cooling.
When the DDT threshold is surpassed and detonation combustion occurs, high energy shockwaves are present within the combustion chamber. Which, as I said, break down this boundary layer and, I am quite sure, involve the charge that would usually quench in the detonation combustion.
So, in a roundabout way and considering the modern and accepted models of detonation, the gist of which Pat summed up in the very first reply to the OP - I would not expect detonation to be initiated in the boundary layer.
Dicer, There is copious amounts of text regarding topics such as this, you couldn't go far wrong with the following: -
Automotive fuels and emissions - Barry Hollembeak
The Internal-combustion Engine in Theory and Practice - Charles Fayette Taylor
Internal combustion engine fundamentals - John Heywood
Does that answer your question?
MS
patprimmer
New member
Matty.
I would add the NACA archive to that list. They have quite a few good papers on it including one with a bunch of combustion chamber photos of the actual combustion event. I was really amazed that in the erea of WW11 when the NACA was really active in increasing performance of military aircraft piston engines that they could get reasonable photos at the speed required to stop a combustion event in a running engine.
Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &
for site rules
I would add the NACA archive to that list. They have quite a few good papers on it including one with a bunch of combustion chamber photos of the actual combustion event. I was really amazed that in the erea of WW11 when the NACA was really active in increasing performance of military aircraft piston engines that they could get reasonable photos at the speed required to stop a combustion event in a running engine.
Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &
for site rules
- Thread starter
- #52
Pat,
yes indeed, totally agree with you. Some very interesting and informative material contained therein.
Dicer,
I'm pleased that you are fully au fait with all the information mentioned....
So then, HCCI: -
I don't believe it a fair comparison to make - knocking combustion in an SI engine (which will be at something approaching full load) - and combustion in an equivalent displacement HCCI engine (which is pretty much impossible to run at an equivalent load).
The reason for the low specific torque/power of HCCI engines is because the combustion employed is akin to detonation and, as such, all of the damage caused in an SI engine due to knocking will also be present.
In terms of the boundary layer, ie that portion of gas adjacent to the combustion chamber walls.
The temperature gradient across this layer will have a large influence. If the region reaches sufficient temperature then it will autoignite, if it doesnt then it wont. Not only that, at part load and with a very dilute charge the heat transfer to the combustion chamber during the HCCI combustion cannot be likened to the degradation of the boundary layer and subsequent heat transfered during full load knocking, in an SI engine.
As no explicit flame front exists then by definition there are no 'end gases'.
HCCI is all very nice, in theory, yet in practice it is not so good. The very nature of its combustion process makes it unrefined and noisy at anything other than low load - a rate of in cylinder pressure rise of ~3 bar/degCrank is about the limit and with almost immediate heat release in HCCI this will easily be surpassed.
Other factors that currently make its use very difficult 'real world' is its lack of controllability during transients, interfacing with current controllers & vehicle systems and cold starting.
MS
yes indeed, totally agree with you. Some very interesting and informative material contained therein.
Dicer,
I'm pleased that you are fully au fait with all the information mentioned....
So then, HCCI: -
I don't believe it a fair comparison to make - knocking combustion in an SI engine (which will be at something approaching full load) - and combustion in an equivalent displacement HCCI engine (which is pretty much impossible to run at an equivalent load).
The reason for the low specific torque/power of HCCI engines is because the combustion employed is akin to detonation and, as such, all of the damage caused in an SI engine due to knocking will also be present.
In terms of the boundary layer, ie that portion of gas adjacent to the combustion chamber walls.
The temperature gradient across this layer will have a large influence. If the region reaches sufficient temperature then it will autoignite, if it doesnt then it wont. Not only that, at part load and with a very dilute charge the heat transfer to the combustion chamber during the HCCI combustion cannot be likened to the degradation of the boundary layer and subsequent heat transfered during full load knocking, in an SI engine.
As no explicit flame front exists then by definition there are no 'end gases'.
HCCI is all very nice, in theory, yet in practice it is not so good. The very nature of its combustion process makes it unrefined and noisy at anything other than low load - a rate of in cylinder pressure rise of ~3 bar/degCrank is about the limit and with almost immediate heat release in HCCI this will easily be surpassed.
Other factors that currently make its use very difficult 'real world' is its lack of controllability during transients, interfacing with current controllers & vehicle systems and cold starting.
MS
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