In that picture, I think BDC means before dead-center. Really, before top dead center. So the bottom row is all happening very close to the top of the compression stroke (not the bottom).

Detonation in the bottom row spreads out very quickly. Not just picture-to-picture, but look at the crank positions... it's all within a couple degrees of crank, while regular combustion happens over about 20 degrees.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by mmuller &raquo;</TD></TR><TR><TD CLASS="quote">... as we all know detonation is the spontaneous combustion of end gasses after the spark plug fired its round. because of heat, pressure.. whatever.</TD></TR></TABLE>I though that was post-ignition. I don't work in I.C. engines, but I've always known the term detonation to mean supersonic combustion. In automotive engines is the term used differently than other fields?

I always figured supersonic combustion was either caused or prevented by the conditions in the CC - temperature, pressure, mixture, pre-flame reactions, etc. It was kinda separate from WHAT initiated combustion (spark vs. hot-spot).

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by JimBlake &raquo;</TD></TR><TR><TD CLASS="quote">In that picture, I think BDC means before dead-center. Really, before top dead center. So the bottom row is all happening very close to the top of the compression stroke (not the bottom).

Detonation in the bottom row spreads out very quickly. Not just picture-to-picture, but look at the crank positions... it's all within a couple degrees of crank, while regular combustion happens over about 20 degrees.

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Im sorry for the misunderstanding; what i ment by BDC is bottom dead center.

The picture that i posted is detonation, not pre- ignition wich i was talking about.But yeah they mean before top dead center
As far as the picture; yeah it happens pretty fast dont it?


<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by JimBlake &raquo;</TD></TR><TR><TD CLASS="quote">

I though that was post-ignition. I don't work in I.C. engines, but I've always known the term detonation to mean supersonic combustion. In automotive engines is the term used differently than other fields?

I always figured supersonic combustion was either caused or prevented by the conditions in the CC - temperature, pressure, mixture, pre-flame reactions, etc. It was kinda separate from WHAT initiated combustion (spark vs. hot-spot).</TD></TR></TABLE>

well, i just like to keep terms simple; detonation and pre-ignition.
detonation as abnormal cobustion, starting as a normal cycle; spark plug firing and then crap going wrong by end gasses starting their own flame and creating multiple flame fronts.
as far as your 2nd paragraph; arent we saying the same thing?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by mmuller &raquo;</TD></TR><TR><TD CLASS="quote">Im sorry for the misunderstanding; what i ment by BDC is bottom dead center.

The picture that i posted is detonation, not pre- ignition wich i was talking about.But yeah they mean before top dead center
As far as the picture; yeah it happens pretty fast dont it?</TD></TR></TABLE>What seemed wrong to me, is the top row is also labeled BDC, but you see the flame kernel coming from the spark plug. That doesn't happen at 20 before bottom dead center, it happens at 20 before TOP dead center.

Yeah, detonation is almost an order of magnitude faster. It's pretty neat how they can actually photograph that.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by mmuller &raquo;</TD></TR><TR><TD CLASS="quote">as far as your 2nd paragraph; arent we saying the same thing?</TD></TR></TABLE>Yeah, I think so, mostly. But detonation is only one kind of abnormal combustion, it's not the only kind.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by JimBlake &raquo;</TD></TR><TR><TD CLASS="quote">But detonation is only one kind of abnormal combustion, it's not the only kind.
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What exactly is the difference between post-ignition and detonation? I don't exactly have a firm grasp of post-ignition, but it seems to me that it would be really similar to detonation. Can you enlighten on that?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by JimBlake &raquo;</TD></TR><TR><TD CLASS="quote">Hot spots in CC can produce surface-ignition.
When it happens before the spark it's called pre-ignition.
After the spark (but before the flame-front gets there) it's called post-ignition.</TD></TR></TABLE>I always thought detonation is supersonic combustion. But if maybe that term is used differently for automotive engines??

So (imho) detonation is when the flame-front travels supersonic. If conditions are right for this to happen, it can be ignited by the spark or by some other hot surface.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by JimBlake &raquo;</TD></TR><TR><TD CLASS="quote">What seemed wrong to me, is the top row is also labeled BDC, but you see the flame kernel coming from the spark plug. That doesn't happen at 20 before bottom dead center, it happens at 20 before TOP dead center.

Yeah, I think so, mostly. But detonation is only one kind of abnormal combustion, it's not the only kind.</TD></TR></TABLE>

i think we are misunderstanding each other; in the picture, yes they mean before top dead center. thats detonation

what i was saying is totally different and NOT regarding that picture. I ment that pre- ignition occurs most likely close to BOTTOM dead center; where the mixture is not compressed and a lot easier to ignite.
hopefully this clears things up

 

OK, that makes sense. But I always figured pre-ignition happens somewhere during the compression stroke, since there's a lot of heating that goes on along with the compression. At the beginning of the compression stroke, the mixture is pretty cool. I wonder where we can find out about that... got any sources?

This is turning into a pretty interesting thread.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by JimBlake &raquo;</TD></TR><TR><TD CLASS="quote">OK, that makes sense. But I always figured pre-ignition happens somewhere during the compression stroke, since there's a lot of heating that goes on along with the compression. At the beginning of the compression stroke, the mixture is pretty cool. I wonder where we can find out about that... got any sources?

This is turning into a pretty interesting thread.
</TD></TR></TABLE>

I found a very interesting read about detonation and pre-ignition. The link is below:
http://www.streetrodstuff.com/...6.php

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Engine Basics: Detonation and Pre-Ignition &raquo;</TD></TR><TR><TD CLASS="quote">
Keep in mind the following sequence when analyzing pre-ignition. The charge enters the combustion chamber as the piston reaches BDC for intake; the piston next reverses direction and starts to compress the charge. Since the spark voltage requirements to light the charge increase in proportion with the amount of charge compression; almost anything can ignite the proper fuel/air mixture at BDC!! BDC or before is the easiest time to light that mixture. It becomes progressively more difficult as the pressure starts to build.

A glowing spot somewhere in the chamber is the most likely point for pre-ignition to occur. It is very conceivable that if you have something glowing, like a spark plug tip or a carbon ember, it could ignite the charge while the piston is very early in the compression stoke. The result is understandable; for the entire compression stroke, or a great portion of it, the engine is trying to compress a hot mass of expanded gas. That obviously puts tremendous load on the engine and adds tremendous heat into its parts. Substantial damage occurs very quickly. You can't hear it because there is no rapid pressure rise. This all occurs well before the spark plug fires.

Remember, the spark plug ignites the mixture and a sharp pressure spike occurs after that, when the detonation occurs. That's what you hear. With pre-ignition, the ignition of the charge happens far ahead of the spark plug firing, in my example, very, very far ahead of it when the compression stroke just starts. There is no very rapid pressure spike like with detonation. Instead, it is a tremendous amount of pressure which is present for a very long dwell time, i.e., the entire compression stroke. That's what puts such large loads on the parts. There is no sharp pressure spike to resonate the block and the head to cause any noise. So you never hear it, the engine just blows up! That's why pre-ignition is so insidious. It is hardly detectable before it occurs. When it occurs you only know about it after the fact. It causes a catastrophic failure very quickly because the heat and pressures are so intense.
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This was an interesting read. I now see why pre-ignition is so bad. Ouch!!!!

 

Gonna give this a bump up for some good info

 

yep, this turned out to be pretty good.

Lets now talk about engine geometry effects on timing.
For example lets take an engine with same displacement + compression; lets make rod ratio a variable.
long rod or short rod would need more timing? If there is any difference...
thoughts?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by mmuller &raquo;</TD></TR><TR><TD CLASS="quote">For example lets take an engine with same displacement + compression; lets make rod ratio a variable.
long rod or short rod would need more timing? If there is any difference...
thoughts?</TD></TR></TABLE>So it's the same bore & stroke, but different rod ratio...

I imagine that's a 2nd-order effect, since it's only subtle differences in the piston acceleration. Just a guess, tho. Short rod puts the largest piston velocity at a higher point in the compression stroke.

 

I did a bit of reading about piston and head designs and their affects on timing earlier. It seems to me that a flat piston with a pent roof head seems to be the best design for combustion. One of the worst designs was the hemispherical head used by chrysler in there Hemi powered Barricuda's. Coupled with a nice domed piston, the hemi appeared to be quite a good motor. But it was severly prone to detonation due to the already advanced timing (I think it was close to 38 degrees advancement). Whenever tuners advanced the timing more and more, they were constantly detonating their motors and breaking parts too quickly. This makes me wonder why though, the hemi is still revered as one of the most potent v-8's in existance.

 

One last bump up for a good thread. Good job guys

 

I don't know how I over looked this thread.

Props to all you guys excellent info.

 

Ok to my understanding it seems to me that short rod/ vs/ long rod will not have much of an effect in timing; as Jim sayd
but a long rod engine will have more dwell time at tdc, thus giving more cylinder pressure after tdc. After tdc the long rod piston speed will be gained slower than a short rod engine in proportion to crank angle.
So im thinking a long rod will need a little less timing than a short rod.
But then the short rod will have slower accelerating pistons speeds away from BDC; but piston speed will get faster the higher up the stroke compared to a long rod.
So im thinking it will need a little more timing...

Yeah or nea?

 

back from the dead bump. mmuller brought up something good, any thoughts?

 

bump?

 

I think you're right about long-rod needing (or tolerating) less spark advance. If a short-rod design can tolerate more spark advance, the different mechanical advantage from piston-to-crank might not take full advantage of it. I'm not really sure which influences are more important.

My hunch is that this stuff is getting subtle even compared to differences in combustion chamber shape. It's interesting stuff but since I'm not an engine designer, I feel like I'm doing a lot of guessing & BS.

 

It seems to me that the shorter rod will have quicker accel/decel times than a longer rod would. I believe it to be because of dwell time. Once the piston stops, the rod still has X amount of degrees left for the crank to turn before the rod reaches TDC or BDC. Wouldn't it be because of the length that it would require more time to slow down/speed up since it has to move farther, or would it be the opposite?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by dubman410 &raquo;</TD></TR><TR><TD CLASS="quote">... Once the piston stops, the rod still has X amount of degrees left for the crank to turn before the rod reaches TDC or BDC...</TD></TR></TABLE>No this is just geometry of the crank & rod. Technically the piston never truly stops except at the exact points of TDC & BDC. The difference comes from the rod going at it's angle. The piston never does perfect harmonic motion unless the rod is infinitely long.

 

I understand that the piston doesn't actually stop until TDC and BDC, I was referring to the appearance of a still piston. It doesn't actually stop until TDC and BDC, but somewhere after 15 degrees (is that right?) the piston is moving extremely slowly (relative, of course), that the motion isnt considered when calculating timing curves.