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Si Engines and detonation
6

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.
 

RE: Si Engines and detonation

.....with the cessation of laminar combustion (deflagration) & the prevalence of combustion >=sonic velocities.

MS

 

RE: Si Engines and detonation

Umm... "Si" perhaps denotes "Spark ignition"?

RE: Si Engines and detonation

When everything in the combustion process is proceeding as intended, yes.  

The existence of detonation might indicate otherwise.


Norm

RE: Si Engines and detonation

?

Ms

RE: Si Engines and detonation

Vague question, dodgy answers. Clarify the question and add more detail and then maybe more helpful answers may be possible.

RE: Si Engines and detonation

Okay, Dicer, I thought about multiple plugs and flame fronts.
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 usually hear about detonation in a diesel in the context of ether starting fluid- the whole charge autoignites, supposed to crack rings and blow heads, except for engines designed to tolerate it. WD40 is supposed to be kinder.

I don't know if the new HCCI engines detonate or what.

RE: Si Engines and detonation

(OP)
Reason for the question is I seem to always hear or read the rubbish of flame fronts colliding theory. I think Pat hit the nail on the head. Its got nothing to do with flame fronts, ie multiple ignitors. Everything to do with heat and pressure.
 

RE: Si Engines and detonation

By definition, detonation is when the flame fronts or propagation of a chemical reaction through the medium is faster than sonic velocities...causing unusually fast increases in temperature and pressure. I haven't done any work with Internal Combustion Engines in a while but I think that in the case of combustion, you get a relatively gradual increase in pressure while the piston begins moving past TDC, allowing a little expansion as the gases continue to combust. With detonation, almost all of the combustion happens right at TDC, greatly increasing the overall temperature and pressure inside the combustion chamber for a short time, putting greater cyclic stresses on all associated pressure boundaries, as well as exposing them to higher temperatures. In the case of multiple flame fronts you aren't quite detonating but you are at least doubling the combustion rate, thus halving total combustion duration, causing the same issues that combustion causes.

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

Two controlled flame fronts can e quite useful as it can increase combustion rate and therefore requires less advance to:-
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

Some descriptions of detonation describe rapid reaction in the end gases as a result of normal combustion progressively raising pressure and temp out there, ahead of normal "flame" progression.

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

Quote:

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

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

(OP)
" Which it isn't, its the power robbing explosion."

Isn't power robbing only a concern if the detonation occures before completion of the compression stroke?  

RE: Si Engines and detonation

Detonation typically happens soon ATDC still during our usuable burn. Any fuel/air that is consumed by the detonation occurrance 'uses up heat' we could have contributed to the burn cycle by creating more heat transfer due to the boundary layer disruption. Regardless of the time and place the detonation happens.

RE: Si Engines and detonation

I think I get the gist but I'd like to see some quantitative analysis that supports the postulate that the rapid energy release near the ideal TDC point is wasted due to excessive loss of heat from the combustion chamber (due to resulting turbulence/convective heat transfer).

RE: Si Engines and detonation

(OP)
My first thought would be if it occurs ATC, then we have an ideal HCCI combustion process. And since it is a very rapid and early event, there would be more energy available than normal combustion process. And actually the process would transfer less heat and more energy in the form of pressure. Adiabatic expansion.

The detonation I'm familiar with arrives BTDC, and does rob power.  

RE: Si Engines and detonation

We should remember that with our squish action in our quench region, we are not cooling anything really, we are supressing detonation by satisfying our flame front by feeding air and fuel at an adequte rate that allows the majority of our charge to be consumed in the chamber, then as the rest or our power stroke, we have a 'reverse' squish you could say (opening up) taking place allowing the rest of our charge to be consumed and have a evenly spread pressure throughout the cylinder. Many, many variables.

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

Ummmm

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

It may cool physically, but, we don't need our fuel to lose its heat too much due to what you have stated. It just happens by the nature of heat transfer. It also loses heat through the squish action from the velocities our charge travels in the quench area, the higher speeds within our region is bad. We cool our fuel too much we create areas prone to detonating. We need to satisfy our flame front in the chamber area for complete thorough combustion, we don't like cooling our charge once the flame propagation has begun, we want it readily to burn.

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

Nobody seems to have mentioned what is the basic cause of detonation - the formation of explosive organic peroxides (and some organic peroxides are frighteningly explosive).  
 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

(OP)
If hydrogen is the fuel there are no peroxides formed.

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

If you have a mixture of fuel and oxygen already well mixed, then compress it until the vast majority of its mass is at or above auto ignition temperature it combusts spontaneously across most of its volume instead of spreading in a "relativly"  slow flame front from a small point or points of ignition.

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

Could the loss in power have been caused by the detonation occurrance cutting off the rest of the charge, if the rest has not been moved by in-cylinder mixture motion?

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

I think an anomalous pressure rise before TDC causes the flywheel to do extra work, and lose extra energy, compressing the charge.

Mike Halloran
Pembroke Pines, FL, USA

RE: Si Engines and detonation

BigClive this peroxide theory sounds very interesting because books and web pages about internal combustion engines doesn't mention it. Can you tell more about it or maybe recommend some sources to read more about it.

RE: Si Engines and detonation

Well played, hansforum.

RE: Si Engines and detonation

  "Well played"?  I presume the implication here is that the involvement of organic peroxides in detonation is just some odd theory I have dreamed up.  I am a little surprised at this as, as far as I know, this is mainstream automotive (chemical) engineering. Organic peroxides etc. is really the only generally accepted mechanism for detonation.
 A quick Google gives:
 
   http://rsta.royalsocietypublishing.org/content/243/870/463

      http://encyclopedia2.thefreedictionary.com/Detonation+of+Motor+Fuels

    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

Thank you for the links Clive.  

RE: Si Engines and detonation

most detonations in the engine are because there is excessive carbon build up in the cylinders due to not running the engine with the right octane fuel. That is the main cause of spark knock.

RE: Si Engines and detonation

Ummm

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

(OP)
Hydrogen evidently does detonate, but it is not caused by peroxides that is why I mentioned it. The key here is not caused by....
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

The reasons why pistons melt is because the boundary layer, present over the combustion chamber walls, is broken down during the uncontrolled combustion. This allows the 'full heat' of the combustion to be transmitted to the piston which is further compounded by the shorter duration of the heat release.

MS

RE: Si Engines and detonation

So are we talking about detonation or pre-ignition? My understanding was that pre-ignition was the phenomenon that causes pistons to melt...but that has more to do with the high pressures, temperatures, and time of exposure to said conditions coupled with the concept of thermal inertia on the piston. Basically you are transferring too much heat to the piston too fast. Spark knock typically results in force-related component damage rather than heat-related damage.

RE: Si Engines and detonation

........which is where black & white become grey.

Very often the two phenomena go hand in hand - progressing to 'run away' knocking.

Ms

RE: Si Engines and detonation

(OP)
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.

RE: Si Engines and detonation

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  

RE: Si Engines and detonation

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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RE: Si Engines and detonation

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
 

RE: Si Engines and detonation

Quote:

It that layer is stagnent then maybe that is the start point for the 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

(OP)
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.  

RE: Si Engines and detonation

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
 

RE: Si Engines and detonation

No puzzles involved, the area that is being described is known as the 'end gases' and it is in these aaeas that detonation generally occurs.

Ms  

RE: Si Engines and detonation

(OP)
So then you are saying the detonation begins in the boundary layer.
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

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

RE: Si Engines and detonation

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
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RE: Si Engines and detonation

(OP)
I've been through that info some years back.

Now enter HCCI. Detonation ? Boundary layer quench? End gas?
 

RE: Si Engines and detonation

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

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