What does dieseling mean? - Part 2
by B.B. Pelletier
WOW! This subject, which I felt was going to be too simple for many of you, has really raised a lot of good questions. I will try to address them today.
If the fuel is burned, why does the gun keep dieseling?
Thus asks Nathan, and it's a good question that has several possible answers. And, I will conjecture a little to explain them, which means I am not certain of what I am saying, either. First point: the amount of fuel that gets burned each time is very small. It doesn't take much to sustain it for years. However, if the fuel does get burned in the compression chamber, why does the gun keep dieseling?
One possibility (this is conjecture) is that all the fuel doesn't burn, that only a small fraction of what's there actually combusts. That would explain why you can keep on burning fuel shot after shot.
Conjecture No. 2: several writers, including the Cardews, have suggested that the lubricants in the spring tube behind the piston contribute fuel by slinging their lubrication onto the walls of the spring tube. When the piston is withdrawn in cocking, some of this lube is not scraped back by the piston seal and remains on the walls to go forward with the piston as the gun fires Remember, the piston seal works best when going forward. There is a continuing replenishment of lubricant ratcheting its way forward to the compression chamber through the action of cocking and firing. Combine this with the other explanation of all the fuel not burning each time, and you have a relatively self-sustaining fuel burning engine.
Is fuel even necessary?
For years, I was satisfied with this explanation until I read what the Cardews did to try to stop the burning. They rebuilt a powerplant after drying it completely. They then lubricated it with dry graphite powder. They cleaned all oil from the pellets they used. The gun sounded like a "bag of washers"; but, even after all of this, they could still smell the acrid smoke of combustion when the gun fired! Something was still burning, though there didn't seem to be anything left to burn. They left this observation unsolved.
The nitrogen experiment
The Cardews tested a .22 caliber Weihrauch HW35 that was getting 636 f.p.s. with a 14.4-grain pellet. They put the rifle in a large plastic bag and sucked out all the air. The left it that way for 30 minutes to get all the oxygen out of the piston seal, then they filled the bag with nitrogen, which doesn't support combustion. The muzzle was poking out of the bag, but they resealed it with a plug after each shot. They shot this gun in a pure nitrogen atmosphere for several shots and recorded a velocity average of 426 f.p.s. The gun had only 45 percent of its initial power (energy, not velocity) when it was not permitted to burn fuel. Then, they took the gun out of the bag and continued firing it and the velocity rose back to the initial figure. This is their proof that combustion happens with every shot. Not only that, but with some guns that have tight velocity spreads, it is also very well regulated!
Buy the book!
Don't think that I have told you everything that's in the Cardew book. Indeed, what I have told you comes from just 4 of the 235 pages.
Next question: Light or heavy pellet?
I have tried to follow the intense thread of discussion that Squirrel Killer started when he told us of his success with 16.1-grain Eun Jin pellets in his .177 Gamo CF-X. To that I responded that many shooters (not me) feel that heavy pellets will damage the mainspring of a spring gun. And, then, we were off to the races! To Squirrel I say - keep shooting the pellets that work.
For the rest of you, I do have some information about the effects of light and heavy pellets on detonation. Light pellets seem to make a gun detonate; heavy pellets don't. This has been my observation after years of shooting. Anytime I forget to load a pellet into a rifle (yes, it happens to me, too) and the one time I loaded a .177 pellet into a .22 Beeman R1, I got a detonation. Any pellet that is obviously too small for the bore is another almost certain candidate to detonate. Sometimes, I get detonations from pellets that seem to fit well but are extra light. This is one good reason why I do not like Gamo PBA ammo.
From my observations, I would have to say that backpressure seems to stop the detonation. When I shoot a very powerful airgun, I always start out with heavier pellets, though until I read Squirrel's story I never thought of using super heavy pellets in spring guns.
Here I must break off the last part of the report into part 3.