Si Engines and detonation 6

[dicer]

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

 

[FahlinRacing]

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.

 

[patprimmer]

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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[FahlinRacing]

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.

 

[patprimmer]

Complete bunkum.

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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[BigClive]

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".

 

[dicer]

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.

 

[patprimmer]

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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[BigClive]

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.

 

[FahlinRacing]

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?

 

[MikeHalloran]

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

 

[hansforum]

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.

 

[Lou Scannon]

Well played, hansforum.

 

[BigClive]

"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.

 

[FahlinRacing]

Thank you for the links Clive.

 

[matrauto]

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.

 

[patprimmer]

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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[dicer]

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

 

[mattsooty]

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

 

[LostHippie]

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.

 

[mattsooty]

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

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

Ms