GTA
All Springer/NP/PCP Air Gun Discussion General => Air Gun Gate => Topic started by: Smaug2 on December 16, 2016, 01:28:38 PM
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I noticed on my old Stoeger X20 that it ALWAYS dieseled with its favorite pellets: Crosman Premier Light. (which it shot at about 920 fps) By dieseling, I don't mean detonation and going supersonic, just smoke coming out of the barrel after every shot. I had shot many hundreds of rounds through this, and never added any lube in the compression chamber, barrel, or breech area.
When I went to a heavier pellet, the dieseling stopped.
Now, that gun has moved on, and I've got a Benjamin Summit NP2, .22. I've got maybe 100 rounds through it, but with the CPHP and CPUM pellets, it smokes out the barrel after every shot. Only the first shot detonated.
I suspect that when I go to a heavier pellet, the dieseling will stop, just as it did on the Stoeger.
I have no idea why this is. It seem to me that it should diesel MORE with a heavier pellet, since there will be more compression behind the pellet before it moves out.
I haven't lubricated the pellets either.
...or is it not dieseling, and it's something else that's causing the smoke?
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I love these sort of engineering problems. I suppose we are a little loose with our use of the term dieseling. As I think we know, the diesel engine cycle relies on compression alone (no spark) to ignite and combust a fuel-air mixture. I suppose without detonation or combustion in the compression chamber, we don't really have dieseling. Then again, there is such thing as slow combustion, which may produce smoke, but not the small explosions associated with detonation and "dieseling" per se.
I think we generally agree that heavy pellets are more likely to increase the compression rate (the key to detonation or combustion) than lighter ones.
I think friction may be more likely to cause smoking without detonation, but that's just a guess. And I think some kind of fuel is necessary to produce a smoke, so a lube or even a polymer (piston seal) must be being slightly consumed when there's smoking.
I think there is also the possibility of generating some steam during the firing cycle, but steam doesn't look like smoke even when you can see it. Maybe if it is steam mixed with some lube? Or maybe the instantaneous heat is boiling off some lube, generating smoke? But that doesn't explain the difference with heavy and light pellets.
Just some food for thought. People with more experience (empirical always trumps theoretical in mechanical things, in my experience) may have some good ideas/explanations.
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I notice an orange smoke in the barrel of my Walther terrus in .177 after shooting JSB RS or AA Falcon 7.33 grain, with no noticeable difference in report. They fit loose in the breech as do the regular exacts, to the point where they feel like they will fall out. Other pellets of similar weights with a better fit in the breech leave no orange smoke in the barrel. Maybe the piston isn't slowing down enough and trace fuels are thrown into the bore, rather than being burned efficiently in the chamber.
Vs
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hmmm
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Its heat ....
A HEAVY pellet sits put longer and the falling piston finds a greater cushion of air decelerating its motion and delivers a longer lower pressure pulse with compression generated heat fairly low.
A LIGHT pellet will break free sooner and allow piston to fall at a higher rate creating a much shorter higher amplitude pressure spike that sets off a combustion process we know as Dieseling.
It is the rate in which the compressing/ed air heats up. Faster the rise in pressure greater is the heat generated.
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Why is there ANY smoke with ANY pellet? Something is getting into the combustion chamber and burning.
It is possible you've got a bit of leak by when you cock your gun. Any lubricant on the spring will be thrown onto the cylinder walls when the gun is fired. If your seal is less than perfect instead of scraping all this lube it can pass a tiny amount of it. This tiny amount of lube is then in front of the seal when the gun fires. The resulting hot air ignites it.
Your seal may be defective.
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Actually, NO parachute seal can wipe off all the lube on the back stroke.
If you want a gun to NOT "diesel" / combust ANTYHING at all, the only answer is to NOT lube the internals with ANYTHING that can combust. And that cannot be done with a parachute seal and normal airgun lubes.
All commercial airguns derive between 10% and 30% of their power from the added pressure and temperature generated by the combustion gases. When people "cross hatch" their compression cylinders what they are doing is creating tiny spaces where lube can remain on the back stroke to be used as propellant on the compression stroke.
To really know what your gun is doing you would have to strip it down, clean and degrease completely ALL the internals, assemble with very little Ultimox and then test.
Now, a limited, controlled, low degree of combustion is not bad in itself. It just requires more maintenance over the seasons. But if for the amount of pellets you shoot through the gun per year you're happy with the shot cycle and maintenance regime, then do not worry. Maintenance is/should be part of the fun.
Now, Match guns are different because they are shot 10's of 1,000's of times per year. And you want more consistency than a normal/semi-internal-combustion engine can provide.
So, unless you want to run your gun like a Match gun, just enjoy the smell.
;-)
Héctor Medina
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Its heat ....
A HEAVY pellet sits put longer and the falling piston finds a greater cushion of air decelerating its motion and delivers a longer lower pressure pulse with compression generated heat fairly low.
A LIGHT pellet will break free sooner and allow piston to fall at a higher rate creating a much shorter higher amplitude pressure spike that sets off a combustion process we know as Dieseling.
It is the rate in which the compressing/ed air heats up. Faster the rise in pressure greater is the heat generated.
Thanks Scott. This "feels" like the right answer to me, but it could be a bit of some of the others too.
Why is there ANY smoke with ANY pellet? Something is getting into the combustion chamber and burning.
I wish it WASN'T a combustion chamber! :D
It is possible you've got a bit of leak by when you cock your gun. Any lubricant on the spring will be thrown onto the cylinder walls when the gun is fired. If your seal is less than perfect instead of scraping all this lube it can pass a tiny amount of it. This tiny amount of lube is then in front of the seal when the gun fires. The resulting hot air ignites it.
Maybe burns it, but not ignites it. Kind of like if a frying pan is hot, the oil will smoke, but not burn. So it's probably some oil in the compression chamber, combined with what Scott/Motorhead said.
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Its heat ....
A HEAVY pellet sits put longer and the falling piston finds a greater cushion of air decelerating its motion and delivers a longer lower pressure pulse with compression generated heat fairly low.
A LIGHT pellet will break free sooner and allow piston to fall at a higher rate creating a much shorter higher amplitude pressure spike that sets off a combustion process we know as Dieseling.
It is the rate in which the compressing/ed air heats up. Faster the rise in pressure greater is the heat generated.
I don't doubt this empirical explanation, but I have heard alternative explanations that a heavy pellet is more likely to cause dieseling/detonation because its higher inertia causes a higher compression rate in the compression chamber than a lighter pellet.