Not trying to stir the pot, what OCTANE gas do you run in your saws?
- Thread starter Freakingstang
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Thanks for clarifying.
BTW There are no explosions going on a properly operating engine. Combustion does not equal explosion.
Freakingstang
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bwalker said:
It's Ben, right? That was a great, unbiased post that explained it in simple terms. There is more to it, but that is about as clear cut as one can get without options and in depth analysis.
Boy, this went on a little longer that I first suspected....
Steve
ShoerFast
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Sounds a little like the chicken or the egg predicament
Ben
If a hot spot somewere in the combustion chamber could cause a pre-ignition.
Why couldent a detonation cause a hot spot?
This is were your loosing me.
kevin
bwalker said:
Ben
If a hot spot somewere in the combustion chamber could cause a pre-ignition.
Why couldent a detonation cause a hot spot?
This is were your loosing me.
kevin
Freakingstang at some point in an off topic talk about your mustangs and I'll tell about some of mine. Don't do it here. I see the direct link from motorhead to chainsaw also. Don't take what everyone says at face value. Research on your own a bit the same as with the cars. We sometimes just dunk on one another not agreeing with ourselves 100%.
Fred
Fred
CNYCountry
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Detonation and deflagration/combustion are two different things. Detonation is an actual chemical decomposition of the compounds in question. Deflagration is extremely fast burning (oxidation) of the compounds in question. Detonation is much more fast and violent and disturbs the boundary layer of cooler gasses that lay along the surfaces of the piston, cylinder and combustion chamber. Once that boundary layer is blown away the metal parts are exposed to the true heat of the detonation or even normal deflagration/combustion (many thousands of degrees) it melts.
Interesting definitions-explanations for the detonationCNYCountry said:
Detonation is oxidation as well as much I know, only the combustion speed and temp is very high and the shockwave is generated which acts like the hammer onto the piston, degrading it and following parts, bearings, etc.
ShoerFast said:
By that, do you mean on the subsequent cycle or cycles?
Combustion does and combustion not equal explosion.
It did?
ShoerFast
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Bill
Yes, and taking the explanation a step further as CNYCounry states. It had always seemed to me that this is the very core of the differant octane ratings.
It seems to me that an engine at speed and temp, is desinged to use every speck of fuel, that also puts it just at the brink of detonation, with possable micro-detonating pockets of fuel / air. (some leaner then others)
If I may, This idea came to me at simanar by Bear Scopes, in a the spark fireing sequence, if you strech out one spark as is bridges a gap, bringing it full screen. You can see the varing ocolations of the deminishing spark.
Coil-impedance could show up as a varing ocolation, but that would be the same for each cylinder, This could also be demnstrated by leaning cylinders or tweeking the timing, as those cylinders would get extreme.
If propane was used to inrich the cylinders as the gas was leaned, the engine behaved much better.
The only explanation I could see is varing cylinder burn rates and pressures, changing the cylindars resistance to the spark.
That could very well cause hot spots in the cylinder and be the start of a pre-ignition condition.
Try this with any engine with a carb, (fuel injection shuts off with the switch) ,,, after a good load was on the engine, hold the throttle open , some and turn off the switch,,,,,,,, most engines will try to stay running from the hot spots in the cylinder.
Kevin
Yes, and taking the explanation a step further as CNYCounry states. It had always seemed to me that this is the very core of the differant octane ratings.
It seems to me that an engine at speed and temp, is desinged to use every speck of fuel, that also puts it just at the brink of detonation, with possable micro-detonating pockets of fuel / air. (some leaner then others)
If I may, This idea came to me at simanar by Bear Scopes, in a the spark fireing sequence, if you strech out one spark as is bridges a gap, bringing it full screen. You can see the varing ocolations of the deminishing spark.
Coil-impedance could show up as a varing ocolation, but that would be the same for each cylinder, This could also be demnstrated by leaning cylinders or tweeking the timing, as those cylinders would get extreme.
If propane was used to inrich the cylinders as the gas was leaned, the engine behaved much better.
The only explanation I could see is varing cylinder burn rates and pressures, changing the cylindars resistance to the spark.
That could very well cause hot spots in the cylinder and be the start of a pre-ignition condition.
Try this with any engine with a carb, (fuel injection shuts off with the switch) ,,, after a good load was on the engine, hold the throttle open , some and turn off the switch,,,,,,,, most engines will try to stay running from the hot spots in the cylinder.
Kevin
OK, Kevin, list for me the events that might cause the original detonation that would cause the hot spots leading to pre-ignition...if you would. That is, in a properly tuned and maintained engine.
By the way, I don't think that spark occilations would have much effect as they occur in an area of burned fuel and would not present a second wave front.
By the way, I don't think that spark occilations would have much effect as they occur in an area of burned fuel and would not present a second wave front.
Crofter
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My understanding was tha pre ignition was ignition of the charge by sources other than the intended spark. It usually indicates something operating hotter than normal. I think combustion should be used for what is happening in a normally running engine. Flame burns outward from ignition source at a slow rate and pressure rise is gentle. Explosion is the same as detonation . From memory combustion is around 75 feet a second and detonation approaches 5000 feet a second. Pressure 5 to 10 times higher than combustion. Usually detonation starts out as a part of the charge going bang (pinging) If something like preignition or crossfiring initiates extremely early ignition, then detonation can start to occur in nearly the whole charge and it can take your engine apart in less than 2 seconds.
I would say that what initiates ignition is separate from describing whether the fuel is burning or exploding
It may be only quibbling abot definition but I think I have to agree with Ben, dang! lol!
I would say that what initiates ignition is separate from describing whether the fuel is burning or exploding
It may be only quibbling abot definition but I think I have to agree with Ben, dang! lol!
ShoerFast
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Bill,,,,
Second comment first here,, In order for the spark line on a scope to disapate smoothly, the resastance accrosed the plug gap would need to be the same, but it dose not. And the harder it's working, the worst it gets.
It is clear as a bell that the gap resisatance changes, you stating that it would be in a primarely burnt section of the cylinder, the only thing that would chance the resastance of the plug gap would be varing pressure changes. It also seems that about the same time that the spark lines start to disapate eraticaly, is about the same time the EGT starts to hit a heavy J curve up.
Just from test that I have seen.
You say that, "I don't think that spark occilations would have much effect" is missing the point I was trying to make, they represent whats happing.
Were did you get that "second wave front" idea?
Could you define your first question better? As you say "in a properly tuned and maintained engine" If your looking for an example to fallow your guide line of proper i'm not intrested. But if your talking about real lynda, say an older saw with a few cylinder fins pluged over a little extra dirt on the rest of them, a carbon build up on the inside, an age hardend metering chamber diaphram, sticky inlet valve, a few extra floaties in the tank and a batch of questionable pre-mix on one of the hottest days you would ever get cought working with you mill, then maybe, I still couldent awnser your question.
Kevin
WRW said:
Second comment first here,, In order for the spark line on a scope to disapate smoothly, the resastance accrosed the plug gap would need to be the same, but it dose not. And the harder it's working, the worst it gets.
It is clear as a bell that the gap resisatance changes, you stating that it would be in a primarely burnt section of the cylinder, the only thing that would chance the resastance of the plug gap would be varing pressure changes. It also seems that about the same time that the spark lines start to disapate eraticaly, is about the same time the EGT starts to hit a heavy J curve up.
Just from test that I have seen.
You say that, "I don't think that spark occilations would have much effect" is missing the point I was trying to make, they represent whats happing.
Were did you get that "second wave front" idea?
Could you define your first question better? As you say "in a properly tuned and maintained engine" If your looking for an example to fallow your guide line of proper i'm not intrested. But if your talking about real lynda, say an older saw with a few cylinder fins pluged over a little extra dirt on the rest of them, a carbon build up on the inside, an age hardend metering chamber diaphram, sticky inlet valve, a few extra floaties in the tank and a batch of questionable pre-mix on one of the hottest days you would ever get cought working with you mill, then maybe, I still couldent awnser your question.
Kevin
ShoerFast said:
I'm beginning to think we're talking about two different things here.
Pre-ignition at a site other than the regular ignition site causes the fuel/air mixture to burn. This causes a pressure wave to spread from that site outward. When the spark plug fires, that will cause a second pressure wave. Where the waves meet, there can be a magnification of pressure sufficient to cause an explosion of the remainder of the mix. That is my understanding of pre-ignition caused detonation...and once again I think Crofter has explained things far better than I ever could.
Read this link for more info than you ever wanted to know.
http://www.motorcycle.com/mo/mcrob/rt-fuel2.html
http://www.motorcycle.com/mo/mcrob/rt-fuel2.html
