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.

 

[patprimmer]

Charge reaching auto ignition temperature.

Regards
Pat
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[mattsooty]

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

MS

 

[pontiacjack]

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

 

[NormPeterson]

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

The existence of detonation might indicate otherwise.


Norm

 

[mattsooty]

?

Ms

 

[BrianPetersen]

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

 

[patprimmer]

A light weight off the cuff answer befits a simplistic question.

Regards
Pat
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[MikeHalloran]

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

 

[moon161]

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.

 

[dicer]

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.

 

[patprimmer]

I think that is kinda what Matty said as well, but he mentioned the process rather than the underlying cause.

Regards
Pat
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[LostHippie]

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

 

[patprimmer]

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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[Tmoose]

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

 

[FahlinRacing]

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.

 

[dicer]

" 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?

 

[FahlinRacing]

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.

 

[Lou Scannon]

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).

 

[dicer]

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.