Second opinion regarding article about pre-ignition

[wondersparrow]

In the obvious quest for more power I like to do a lot of reading about basic engine theories and such. I also know that I shouldn't believe everything I read on the internet. A thread here at HybridZ pointed me to an article on detonation and pre-ignition here -> http://www.streetrodstuff.com/Articles/Engine/Detonation/Page_6.php Great article, good read; though I do have a question....

 

The article states:

 

"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."

 

This seems odd to me. I would have thought that in thinking of the whole concept of a diesel engine that this would be the opposite. Is this a case of "the intake charge has not had enough time to cool the hot spots" type of thing? I would expect pre-ignition to happen much later than BDC, in fact probably much close to the actual spark event. Instinct tells me that you would have to be running quite lean to get this type of pre-ignition.

 

What are the thoughts from the gurus around here?

[cygnusx1]

Based on the knowledge that a spark plug will "blow out" under extremely high boost, I think it might be correct that at BDC the mixture would be easier to ignite but I have a hunch that the energy from combustion would be very low at that point. It might ignite easily but burn slowly undoer near zero compression..just a hunch. I could easily be wrong. Any pyro experts here?

 

I think that in your quote above, they could also be referring to just the difficulty of jumping an arc across the plugs electrodes at TDC as compared to BDC.

[Daeron]

you intake valve is open when the piston is at BDC. When detonation occurs with the piston at this position, you get a backfire out of the intake. KABOOM!

[wondersparrow]

I think in terms of what the article is saying is that worst case scenario pre-ignition is when the fuel ignites just after the intake valve closes. It goes on to discuss how at that point the compression cycle has to compress all the hot expanded mixture. This then causes extremely high pressure and heat to build up usually resulting in a hole in the piston within a few cycles.

 

I just found it surprising that it stated that almost anything can ignite the charge at BDC. I could understand it if Dave is right that that what they are talking about is the difficulty of creating a spark near TDC. Instinct still leads me to believe that the charge is more volatile when compressed, not when at near atmospheric pressures.

[Tony D]

Go back and read that article again, if the thing ignites at BDC, it's preignition, a wholly different thing than detonation...

 

As for a Diesel, you are confusing spark ignition with heat of compression ignition.

 

A diesel has NO fuel in the compression chamber before the point of Ignition. Ignition happens when you take 17:1 compressed air at a heat of around 600F+, and spray in your fuel. Which ignites from the heat present, and burns from there.

 

Spark ignited engines, on the other hand are compressing a fuel/air combustible mixture. And the more dispersed the fuel/air is, the easier it is ignited and the faster it will burn.

 

If you think it will burn better compressed, think about it like this:

When Sherman went through the South, the first thing he did was find the local flour mill. He sent his men to get sacks of flour, and take them to the rooftops of the buildings along the main street, where they would cut them open and flay the flour out into the air to waft down to earth. They would then chuck one stick of dynamite into the middle of the street amid the flour-fog, and BOOM! The entire main street was efficiently dispacthed.

 

Now, stick a single stick of dynamite in the middle of 50 sacks of flour and set it off and see what it does. (Not Much...)

 

That's a rough equivalent of the piston at BDC with a combustible mixture present, and the piston at 37 degrees before TDC with a compressed, combustible mixture present.

 

It's the pressure trace of Preignition that will get you. You can see when it starts burning, and as it burns it expands---sure, it's burning slooooowly, that's why it's usually silent. This expansion happens while the space for the expansion is getting mechanically smaller. It's why they call it the silent killer. Think of it as hypertension for your car. Detonation is when the expanding gasses are rapidly burning, faster than the piston is going down (expanding) and that pressure spikes. While it's a pressure spike, the volume available for expansion is expanding...far less potentially damaging than preignition when the space is mechanically decreasing.

[wondersparrow]

Thanks Tony, makes sense now. The flour analogy made it click. The rest of the article made sense and was quite helpful.

[Daeron]

I am just now reading this article, and its actually rather good. I've read at least a half a dozen different articles/groups of articles like this, and all of them are useful because each is a review and re-statement of essentially the same stuff.....

 

--edit

VERY good article... interesting illustration here:

Consider the Northstar engine. If you do a full throttle 0-60 blast, the engine will likely run up to 6000 RPM at a 11.5:1 or 12:1 air fuel ratio. But under sustained load, at about 20 seconds, that air fuel ratio is richened up by the PCM to about 10:1. That is done to keep the spark plugs cool, as well as the piston crowns cool. That richness is necessary if you are running under continuous WOT load. A slight penalty in horsepower and fuel economy is the result. To get the maximum acceleration out of the engine, you can actually lean it out, but under full load, it has to go back to rich. Higher specific output engines are much more sensitive to pre-ignition damage because they are turning more RPM, they are generating a lot more heat and they are burning more fuel. Plugs have a tendency to get hot at that high specific output and reaction time to damage is minimal.

--end edit

 

...but whats blew MY mind (and inspired me to comment here) was the fact that I started saying to myself that the guy chose a poor font. It looked, at first, as if the "r" and the "n" kept running together and looking like an "m." Hence, instead of

 

b u r n r a t e

 

it appeared he was writing

 

b u m r a t e

 

which is two totally different things.

 

So, just for S's and G's, I went to highlight one instance of the term "burn" to investigate.

 

somehow, I picked one that was a typo and actually was (and I cut-and-paste) "bum rate"

 

I'm going to sleep now. The world has treated me badly today, and that was the final straw