pre-ignition / detonation(post ignition)

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pre-ignition / detonation(post ignition)

Post by jrcentllc » Thu Jul 06, 2006 5:23 pm

I have done a search thru the archives and have come to the conclusion some things (and maybe me) need to be clarified, defined, about preignition and detonation. Way to many people say one thing and mean the other.

in short:

Preignition- The a air/fuel mixture that ignites prior to the spark plug. Some causes maybe octane, carbon, high cylinder temps,etc. Two flame fronts than collide and make a AUDIBLE noise.

Detonation- Or better yet and more accurately "post ignition". A uncontrol explosion after the ignition event.? A extreme amount of pressure is exserted on the pistons, rings, bearings, etc. An AUDIBLE noise is NOT associated with this, as that the piston is on the way down already. This is why detonation is also refered to as the "Silent Killer".

I would like to get some feed back and see if anyone else has something to add or change about these statements.

Thanks in advance for your responses.
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Re: pre-ignition / detonation(post ignition)

Post by SWR » Thu Jul 06, 2006 7:06 pm

jrcentllc wrote: Two flame fronts than collide and make a AUDIBLE noise.
If this is so,then all twin plug heads would ping every cycle...
Detonation- Or better yet and more accurately "post ignition". A uncontrol explosion after the ignition event.? A extreme amount of pressure is exserted on the pistons, rings, bearings, etc. An AUDIBLE noise is NOT associated with this, as that the piston is on the way down already.
Detonation is just what it says...a spontaneous combustion happening due to excessive pressure/heat igniting the charge all at once. Does usually happen at or just before TDC,if it happened quite a bit after TDC the pressures needed to set it off would already be much lower,wouldn't they?
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Post by BCjohnny » Thu Jul 06, 2006 7:29 pm

Pre-ignition defines itself...................

Detonation......."post ignition"....................?

Detonation can be a sympton of the former.

Detonation to me is a rise in pressure that can't be usefully harnessed, so is often (mostly) destructive.

I may have this wrong but I thought "High speed detonation" was the "silent killer", not detonation per se.

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Post by beth » Thu Jul 06, 2006 8:52 pm

Detonation is audible, in fact that is what a "Ping" is when the timing is overly advanced. At high speeds detonation is not usually audible. Preignition leads to detonation exactly the way over advanced timing does. It is possible to have an event similar to preignition that is not detonation that occurs after ignition has happened. Preignition that is timed closely with the normal ignition is sometimes called autoignition. Many racing engines under high loads will auto ignite.


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Post by MadBill » Thu Jul 06, 2006 9:07 pm

I'm with SWR and Beth. Preignition is the silent killer and there is no such issue as "colliding flame fronts"; it's a frequently stated myth, disproved as per SWR by the value of dual plugs in some applications (which would of course create "collisions" every firing stroke)

Detonation is usually audible (in muffled applications) at any RPM, but is far, far more common at low RPM, since a certain time must pass after sufficient temperature and pressure conditions are reached for detonation before it actually happens. At high RPM, it is almost certain the regular flame front will pass through before it occurs.
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Post by BCjohnny » Fri Jul 07, 2006 4:06 am

The above post was an attempt to put the answers in their very simplest froms, to which the causes/consequences could be added.

In the nineties, as oe's made attempts to make engines operate more efficiently they came across a new phenomenon. Referred to as "roughness" or "rumble", where the pressure rise per crank degree (rate of burn) in what was considered "normal" combustion was becoming an issue. The mixture was on the edge of becoming unstable, but not quite detonating.

Two things:

It shows how controlled modern engines can be made to operate.

Detonation is not the "on/off" switch that many presume.

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Post by BritishTurbo » Fri Jul 07, 2006 9:36 am

I have to agree that detonation, or "knock" is one of the most misunderstood things we come across when dealing with an internal combustion engine.

I do a lot of work with Turbo engines, so I come across the problem of "knock" quite a lot because of the increased combustion chamber pressures that we run at high boost levels...

The biggest misconception is that adding more fuel is always better, or that removing more timing is always better as far avoiding detonation... when in reality if you add too much fuel, or remove too much timing, you can raise combustion chamber pressures, and increase the chance of preigntion... how ironic!

I feel there is much yet to be learned as far as detonation... perhaps the answer is to run at extreme high rpms... thus lowering the chance of "knock"... ;) No wonder so many of those high strung turbo Honda engines manage to survive for so long!
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Post by JBrady » Fri Jul 07, 2006 1:47 pm

Pre-ignition happens when fuel begins to burn before spark ignition. Interestingly this often correlates to relative cylinder pressure. Just as an increase in octane improves the RESISTANCE of combustion so does cylinder pressure prior to ignition. IOW, just after the intake valve(s) close and the piston is relatively near BDC the in cylinder pressure is relatively low and EASIEST to ignite by say a hot spot on a piston or spark plug electrode. Once a burn has begun the pressure rises quickly, coupled to the crankshaft compressing things further the temperature rise is huge and piston melting can occur.

The thing about pre-ignition is that it is a "burn" where-as detonation is an explosion. Detonation occurs when the air/fuel mixture involved reaches its critical tempurature and explodes. Usually (and luckily) detonation is isolated to a small amount of the total charge. As the explosion releases all of the energy at once vs. over time. This instantaneous release causes broken rings and cracked pistons. If the detonation is large enough it can be catastrophic.

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Post by stealth » Fri Jul 07, 2006 2:13 pm

Check this link.......

http://www.3si.org/forum/showthread.php?t=281822

article by Allen W. Cline (designer of the Northstar engine): Engine Basics: Detonation and Pre-Ignition.

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Detonation

Post by cleverlever » Fri Jul 07, 2006 3:30 pm

High compression contributes to detonation and all other combustion abnormalities.

What would be desireable is to slow down the burn rate at heavy loads and increase the burn rate at light loads.

Any one interested in this subject might be interested in reading patent 4,961,406

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Post by mike_belben » Sat Jul 08, 2006 1:32 am

there are some pressure graphs in this one..

http://www.avweb.com/news/columns/182132-1.html

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Post by BCjohnny » Sat Jul 08, 2006 8:35 am

As has come out in the posts, people now realise that "detonation" occurs on many levels.

These are opinions, as I have no facts, and have not yet read the links, but are based on twenty years of building engines. Please feel free to disagree.

Detonation probably occurs in a lot more engines than people are aware. Not the catastrophic detonation that instantly wrecks engines, but a niggling, hidden detonation, say less than 5% of the charge, that over a period of time lessens an engines reliability/longevity.

This "end gas" detonation results from the pressure rise in the chamber, and the flame front not being able to burn the mixture before it explodes. Chamber design, and to a lesser extent piston design are the usual suspects.

The "cleaner" a chamber is the better. Not clean from the point of view of emissions, quite the contrary, but compact. I'd rather have the last 2% of the charge go down the exhaust than detonate in the chamber. With race engines we are allowed this luxury, and although it looks like throwing away power it is just another compromise that has to be made.

The old Clevelands are the obvious comparisons here. You'd even today be hard pushed to find many cleaner designs than the quench Cleveland head in this respect, especially in comparison to the later open chamber "emissions" design, which is more knock sensitive. Most modern four valves are also excellent in this. Chamber design (its ability to resist knock) I believe is more important than most credit it in creating a successful powerplant. A commonly (and successfully) tuned engine over here, the 2.0l "Pinto", has a very "Clevelandesque" chamber. Porting, quite rightly, is the number one priority. But the chamber is left to "control" what's let in.

There are obviously other factors regarding knock/detonation, such as mixture stratification. But when all other things are tuned as correctly as they can be, you've still got the chamber...............

Like I said, just opinions.

John.
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Post by JBV-HEADS » Sat Jul 08, 2006 11:02 pm

Jr,

I always love this debate. A lot of examples and theories, many valid, are always presented. To answer your question, pre-ignition is any ignition of the charge prior to the planned ignition of the charge. Many examples were given. Detonation is the creation of another or multiple ignition points that were unplanned for. Again many examples were given. Diesels are sometimes referred to as detonation engines. I think this is a misuse of the word. While the pressures produce the heat needed to ignite the charge, it is a planned event. Pre-ignition and detonation are just unplanned for and uncontrolled ignitions of the charge. One before and one after the planned event. Good luck :wink: ,

Joe

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Post by BCjohnny » Sun Jul 09, 2006 5:51 am

If you'll indulge me a shade further, then there are a couple of things that, although not answering the original post, dovetail neatly into it. Then I'll leave the thread to those better informed.

This may be obvious to most, but we've all come across engines that "like" to run rich. They produce a tad more power, but live longer than if they are tuned close to correct a/f. I think in most cases that the bulk of the charge (the combusted bit) is not that far off correct a/f (for max power), but seem to be running richer due to the uncombusted "quench" portions. Leaning them out can lead to problems, and detonation.

Secondly, the whole debate about " grooves" in the quench area I find fascinating. The thrust of the argument, and correct me if I'm wrong, seems to be the visualisation of "jets" of mixture being forced into the main chamber cavity, creating swirl and allowing more complete combustion. I don't discount this but I think there maybe more going on. I think that the channels created may be allowing a combustion path into the quench area that might not otherwise exist, a dendritic structure, allowing general combustion in the quench area, that might not otherwise take place. Somenders testing seems to indicate that the greatest improvements (mainly bsfc) occur in modern side valve engines, which now have some of the largest quench areas of modern engines.

Also the translation to other engines has thrown up interesting results; no real power gains but other improvements. If I've read right then the best results are gained by not having too tight a quench, and not too small a groove (automotivebreath I think), when if you wanted maximum charge transfer/swirl you'd want the opposite. Also if you look at the chambers of engines so modified there are virtually no carbon deposits (in the quench area, where you'd expect them) so you're getting a complete burn. The relative size of the groove/quench area must be allowing this to occur. Idle quality (especially) with bigger cams is also improved, so the exhaust gas diluted mixture can now support combustion. Reports of civlised idle rpms with some fairly hairy cams are common. Why I'm not quite sure, but maybe you have a localised richness in the grooves allowing peripheral combustion, like the concept of the Honda CVCC. Maybe it's simply a critical mass situation at work; theres now enough mixture to support a burn.

Supression of detonation, by allowing a more complete burn, is also probably occuring. I think even quench heads may be subject, on occasion, to a dgree of ping.

Sorry about the length, and if any of this has been covered before, but I havn't read it. They are just opinions/theories so as always, feel free to disagree.

John
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Post by automotive breath » Mon Jul 10, 2006 1:28 pm

BCjohnny wrote:...The "cleaner" a chamber is the better. Not clean from the point of view of emissions, quite the contrary, but compact. I'd rather have the last 2% of the charge go down the exhaust than detonate in the chamber. With race engines we are allowed this luxury, and although it looks like throwing away power it is just another compromise that has to be made....
John.
John,
I'm not understanding what you are saying about throwing away the last 2% of the charge, can you explain further?

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