Si Engines and detonation
Si Engines and detonation
(OP)
What is and the cause of Si engine detonation?
And before answering think multiple spark plugs, and flame fronts.
And before answering think multiple spark plugs, and flame fronts.





RE: Si Engines and detonation
Regards
Pat
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RE: Si Engines and detonation
MS
RE: Si Engines and detonation
RE: Si Engines and detonation
The existence of detonation might indicate otherwise.
Norm
RE: Si Engines and detonation
Ms
RE: Si Engines and detonation
RE: Si Engines and detonation
Regards
Pat
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RE: Si Engines and detonation
Not applicable.
As Pat said, in detonation, the whole charge goes BOOM all at once, because of chamber conditions. No spark is required.
It can happen in a Diesel too; the pressure rise due to detonation is much faster than that due to a normal Diesel event.
Mike Halloran
Pembroke Pines, FL, USA
RE: Si Engines and detonation
I don't know if the new HCCI engines detonate or what.
RE: Si Engines and detonation
RE: Si Engines and detonation
Regards
Pat
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RE: Si Engines and detonation
Also, just thinking about the propagation of multiple flame fronts, I wonder if there could be some constructive interference going on where the two fronts meet, I guess from a transient standpoint you could have a line around the edge of the combustion chamber where the 2 flame fronts collide and the pressure wave associated with each one constructively interferes to cause uneven and extreme loading along that line...just thinking outside the box, feel free to correct anything that could not or would not happen ^_^
peace,
LostHippie
RE: Si Engines and detonation
1) Avoid the chamber pressure building to the point of autoignition in an uncontrolled fashion across the chamber.
2) Avoid chamber pressure building up from combustion while the piston is still moving upward on the compression stroke.
3) Still achieve maximum cylinder pressure by about 12 deg ATDC while the piston can still extract maximum power.
Regards
Pat
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RE: Si Engines and detonation
Those would describe >>one<< useful squish/quench effect as cooling the end gas, thus creating greater chemical reaction "delay" (first steps in combustion) than would happen with more open chamber.
http://www.factorypipe.com/t_deto.php
RE: Si Engines and detonation
Lets clarify what we are after, Auto-igniton or Detonation?
Two different flame speeds especially with squish and quench employed. Detonation can happen almost anywhere in the chamber and in rich or lean circumstances. Majority lean lean of stoich.
If you have multiple plugs, proper chamber qualities, the burn will not be accelerated it will just have two or whatever amount of ignition sources consuming the working fluid in shorter time due two flame fronts present. Similar to MSD, you don't burn faster, you just have a smaller chance of misfire.
When detonation is present we have two compressions taking place, first our obvious compression stroke and second the expanding charge within the chamber. The area that may detonate, like has been already mentioned, is where there may be fuel available and become vaporized quickly before the flame front actually reachs it where ever it may be.
Without any expansion happening how can we produce a detonation? Our flame front is proceeding to the next molecule to consume as is our heat propagation across the chamber. The flame fronts are not totally out of the mix here. I too have read the collision that is supposedly at fault for the knock. Which it isn't, its the power robbing explosion. The frequency is not within ours to hear it, the ringing/sounds comes from our engine parts.
RE: Si Engines and detonation
Isn't power robbing only a concern if the detonation occures before completion of the compression stroke?
RE: Si Engines and detonation
RE: Si Engines and detonation
RE: Si Engines and detonation
The detonation I'm familiar with arrives BTDC, and does rob power.
RE: Si Engines and detonation
That would be interesting to see Hemi. I am sure that would be very complex in calculating too. But a real time test would be more fun anyhow. Probably photography and pressure traces etc.
I would test a non-softened chamber to a softened and then compared to a polished chamber of each. Of course it would have to be the same type of engine configuration.
Adiabatic is a pretty interesting topic Dicer. Haven't studied very much on that specifically though. I've been digging at Smokey's phase 1 adiabatic enngine when I can, nobody could get it to detonate. Albeit there is berely any information on what makes it work, the answers are there, I believe things could have been overlooked or of the like.
RE: Si Engines and detonation
Quench does cool by increasing the surface area to volume of the highly compressed gas in the quench zone when at or near TDC. The surface temperature of the head and piston are below the temperature of the compressed and burning charge and the thinner the layer of charge, the quicker and more completely it cools.
Regards
Pat
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RE: Si Engines and detonation
Quench has multiple meanings but if you think about it, we do not need anything to be extinguished, cooled or destroyed. We aim for satisfaction of our flame front. That is what we need to achieve with our squish action in the quench region.
RE: Si Engines and detonation
Sophisticated scientific style of language might work to bluff your way through in some places, but if it is unsound it will always be challenged here.
Regards
Pat
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RE: Si Engines and detonation
A mixture that auto-ignites still just burns - true detonation is explosive - meaning the reaction moves faster than the speed of sound and produces shock waves.
If you have ever witnessed really severe detonation (especially at low RPM when the peroxides have more time to form) you will notice that the exhaust has the typical acrid, eye-watering "tear-gas" odour of organic peroxides.
It is also notable that the exhaust turns very dark - indicating a lot of partly-combusted fuel - this is probably the cause of the drop off in power due to detonation. Possibly the unburnt fuel is caused by the shock waves actually "putting the fire out".
RE: Si Engines and detonation
First thought to mind is an increase in mean presure due to detonations resulting high pressure, whether or not it "puts the fire out". And that would not cause power drop off. Well again depending on what side of TDC it occured.
RE: Si Engines and detonation
I think the National Advisory Committee for Aviation (NACA) now NASA has done a lot of work on this and even has lots of high speed photographs of it in their archives if anyone cares to search.
Regards
Pat
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RE: Si Engines and detonation
Dicer - I don't see your point about hydrogen fuel. Do you mean H2 doesn't detonate in engines because H2O2 doesn't form during detonation or something else?
Like you (I think) I have wondered why detonation causes loss of power. If the fuel/air mixture is being burnt and heat relaesed then the power should be unaffected even if the heat release is explosive.
It is just a suggestion that the possible cause of the power loss when detonation occurs is due to the "fire being put out" and not allowing full combustion of the fuel.
I have read in an Aviation Week article from the 1940's about engineers setting the maximum boost level on aero engines by looking for the onset of dark smoke (indicating detonation) from the exhaust.
I remember seeing in the 1960's high compression Cooper S's blowing clouds of brown smoke while undergoing fairly mild detonation.
RE: Si Engines and detonation
If its BTDC the explosion consumes fuel among building cylinder pressure, ATDC it consumes fuel but we are closer to peak cylinder pressure.
Is it more detrimental when the detonation happens before or after TDC?
RE: Si Engines and detonation
Mike Halloran
Pembroke Pines, FL, USA
RE: Si Engines and detonation
RE: Si Engines and detonation
RE: Si Engines and detonation
A quick Google gives:
http://r
http://e
http://www.jstor.org/pss/91560
You can also read about peroxides/detonation in Colin Campbell's book "The Sports Car - Its Design and Performance".
Pages 22 to 26.
RE: Si Engines and detonation
RE: Si Engines and detonation
RE: Si Engines and detonation
What has carbon build up got to do with octane rating.
Brand new engines with virtually zero carbon will knock if the cylinder pressure and temperature gets to high for the octane rating of the fuel. There are a large number of variables that impact on this.
Regards
Pat
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RE: Si Engines and detonation
Thats why I mentioned it.
Heat released? I would say there is an abundant amount released, I've seen a few pistons melted from it.
The presence of dark smoke does not mean the fuel charge is not for simpler terms doing its job.
Agreed its misslabled,
http://www.youtube.com/watch?v=0noRVIHQigE
RE: Si Engines and detonation
MS
RE: Si Engines and detonation
RE: Si Engines and detonation
Very often the two phenomena go hand in hand - progressing to 'run away' knocking.
Ms
RE: Si Engines and 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.
RE: Si Engines and detonation
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
RE: Si Engines and detonation
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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RE: Si Engines and detonation
- Steve
RE: Si Engines and detonation
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.
RE: Si Engines and 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.
RE: Si Engines and detonation
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
RE: Si Engines and detonation
Ms
RE: Si Engines and detonation
I though you said the boundary layer was broken down during.
Did you see the question in the earlier post?
I ask if a combustable mixture can be called a boundary layer??
RE: Si Engines and detonation
'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
RE: Si Engines and detonation
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
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RE: Si Engines and detonation
Now enter HCCI. Detonation ? Boundary layer quench? End gas?
RE: Si Engines and detonation
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