Fact or fiction?

[aimless]

@ turnup:

"3) Fact from personal experience. In my youth. cautiously experimenting with deliberate dieseling using a Webly pistol. I found that, while it clearly increases MV, it greatly reduces accuracy. I stopped my experiments when the gun re-cocked. OK it does not take much to re-cock an old wobbly Webly pistol spring, but it surely did happen."




Maybe, if the weight faktor pellet-piston is matching. But how can a Weihrauch Barakuda then work safely?????????

[duomatic410]

Gun can recock on fireing? YES. Have had it happen on Webley mk3 in 177, and have had cocking rods broken.
This was caused by dieseling(on purpose for power) using aero fuel😐.
(65 years ago).

Springs finish up totally kna..red, a new spring will look like one that's been in years when it it's been in weeks.

Heavy tight pellets will put the pressure up when dieseling, and can cause it to diesle when it perhaps it wouldn't useing a light pellet.

And if you have a mk3 or other air rifle that doesn't seem to reach power as it should, there is a chance that your cylinder as bulged due to the above.

[trick cyclist]

The physics can only answer so much - if we take a 'snapshot in time', where the pellet is moving up the barrel, and the piston is still reaching (or has reached) the very end of its travel, then using the simple formula of Pressure (P) = Force (F) / Area (A), we can see that by having to make the assumption that P is the same everywhere in the barrel/breech (as Turnup said), the force on the pellet will be clearly many magnitudes greater than the force on the piston. However, P=F/A only really works for static systems, and as Jim Tyler and others who really study this will say, the springer firing cycle is so complicated, it is very hard to truly calculate what is happening at any one time in such a dynamic environment, soooo, in the case of a piston that is approaching the end of its travel, with the air temperature now at several hundreds of degrees, a detonation at the piston face could conceivably produce a shockwave that flings it back up the compression tube. I'm in, though I've never seen it!

[Turnup]

@ turnup:

"3) Fact from personal experience. In my youth. cautiously experimenting with deliberate dieseling using a Webly pistol. I found that, while it clearly increases MV, it greatly reduces accuracy. I stopped my experiments when the gun re-cocked. OK it does not take much to re-cock an old wobbly Webly pistol spring, but it surely did happen."




Maybe, if the weight faktor pellet-piston is matching. But how can a Weihrauch Barakuda then work safely?????????

OK, quick google reveals that the Barakuda is designed as a compression-ignition firearm. It works safely by being designed to be safe?????

[aimless]

OK, I know - , I have one; but ...... IF the pressure spreads out without determinded direction , it would NOT stop the forward moving piston and spring (with large kinetic energy) AND let him move backwards. And certainly not, because the pellet is already moving towards the muzzle......

I am quite sure, that there is NO planned diffussion/direction of the explosion by the BARAKUDA-rifle. Car-engines do have one....

So IMHO the piston (as described before) can not move backwards, if the mass-factor overrules the area-factor.

(My mass- and area-factors were taken from a vintage HW50 and without spring)

[Turnup]

OK, I know - , I have one; but ...... IF the pressure spreads out without determinded direction , it would NOT stop the forward moving piston and spring (with large kinetic energy) AND let him move backwards. And certainly not, because the pellet is already moving towards the muzzle......

I am quite sure, that there is NO planned diffussion/direction of the explosion by the BARAKUDA-rifle. Car-engines do have one....

So IMHO the piston (as described before) can not move backwards, if the mass-factor overrules the area-factor.

(My mass- and area-factors were taken from a vintage HW50 and without spring)

It is not correct to take the example of a firearm designed to use compression ignition, and generalise that to apply to a firearm not designed to be used in this way. I expect that the Barakuda has a heavy piston specifically to control piston bounce.

Note that tuners on here report that in normal operation, without dieseling some air guns exhibit piston bounce, that is where the piston is arrested and rebounds by air pressure alone. This is an effect which tuners try to eliminate. Now add dieseling into that and you can see that it is perfectly possible to move the piston backwards by a considerable amount. Note also that two posters on this thread report re-cocking has been found when deliberately inducing dieseling in guns not designed to do it.

Any dieseling event will be transitory (very fast by comparison to the moving parts of the gun). Once the very fast combustion has occurred, it will release combustion products (mostly gaseous)within the compression cylinder, and they will be much hotter, thus the pressure in there will be increased. It is this pressure which can push the piston backwards.

Within a diesel engine, controlling the propagation of combustion has nothing to do with directing a pressure wave towards the piston, the designers are trying to ensure that all parts of the fuel/air mixture are evenly ignited to promote complete burning. Ignition pressure waves will be of such short duration as to provide no meaningful force to the piston. It is the liberation of combustion gasses and heat which increases the pressure and moves the piston.

[aimless]

" Note that tuners on here report that in normal operation, without dieseling some air guns exhibit piston bounce, that is where the piston is arrested and rebounds by air pressure alone. "



so, if you close the muzzle completely, this would/should occur (every time)???
You invented a perpetuum-mobile

And that would be more than magic and worth a nobel-price

[shabee]

i knew this thread would get complicated
eric

[aimless]

Eric, NO: Interesting

And for me a challenge and a pleasure to improve my english

[Leonardj]

Thank you to all that have responded to the thread.
Now that everyone has shared their experience, it is my turn to weigh in on the three points in question based on my own experience.

1) Fiction. It is quite common knowledge that a freshly lube-tuned springer that is dieseling violently, to the point of achieving detonation, can be settled by shooting a few heavy pellets through it. Shooting the heavy pellets result in a much milder dieseling, which will slowly burn off any excess lubes in the compression chamber. After several heavy pellets, the mild dieseling ceases, and at that point, shooting the lighter pellets will no longer cause dieseling nor detonation. This practice that has been around for decades seems to directly contradict the theory presented in the first part of the noted statement.

2) Fiction. It is highly unlikley that an experienced airgunsmith would make a statement like that. I would be more inclined to expect some inexperienced wannabe to make such a claim in hopes of gaining some "cred". Yes, some abuses can appear readily apparent from the condition of the spring, but unless the suspected abuse can be verified by the owner of the gun, you are just guessing. For example, I had a Diana 34 in for repair, which had it's mainspring shattered into 6 pieces, with numerous fractures in the coils as well. I suspected dry firing as the cause. In speaking with the owner, I discovered that he had been using plastic, 3 grain pellets in the gun exclusively. IMO, that is about as close to dry firing the gun as you can get. The owner was advised on the best pellet weight to use in his freshly repaired gun going forward.

3) Fiction. I have had some guns in for repair that had some major damage due to detonation, and not one of the owners had experienced having the gun re-cock itself. Bulged compression chambers, front plug (complete with barrel still attached) blown completely out the end of the compression chamber, broken stocks, etc. Just to elaborate, dieseling is when there is a puff of smoke, of varying degrees, emitted upon firing, but if there is sufficient "fuel" and heat generated by the dieseling, then detonation is achieved - producing a very loud bang. It is the energy produced by that contained explosion in the compression chamber that is what causes the damage to the airgun.

[shabee]

The reason this thread has got so confusing is that many answers have deviated far from the original questions, none more so than yours.
You originally referred to a gun at the critical point of dieseling now you talk of one dieseling violently to the point of detonation in your answer
Obviously no one believes guns have re cocked due to detonation even though there are people on here who have personally witnessed or created the effect.
It takes all kinds..
eric

[DT Fletcher]

Can't really say much about the spring piston guns, since I've not worked on them much, but, I can say something about CO2 guns that might add to the discussion. I've worked on certain CO2 rifles that had a restrictor plate installed; presumably to reduce the output energy. However, what happened was that the CO2 flow would encounter this restriction and reflect backwards and reopen the exhausts valve so that the rifle would "chatter" with multiple openings of the exhaust valve.

The lesson here is that the forces involved in an airgun are not just simple physics. It's not just the size of the pellet vs the piston. Instead, there are forces vectoring/reflecting off any surface in the way of the moving air/gases. Especially if the forward end of the piston housing is straight, the forces produced by the combustion could hit it and directly reflect/vector back against the piston.

So, 1) the weight of the pellet and or how much resistance it produces to forward air/gases would probably make a difference.

2) I'm pretty sure that an experience airgun smith would be able to determine if a gun has been subject to repeated dieseling; there's going to be residues from the combustion.

3) can't see why not. Lot's of variables would be involved; amount of dieseling, shape of the "combustion" chamber, resistance of the pellet, strength of the spring....

[pennineway.fswo]

I have seen Webley Mk 3,s with a bulge towards the front of the cylinder due to violent dieseling/detonation due to over lubrication and/or too strong a spring

I have no doubt that such an event could send the piston back up the cylinder to damage piston rod and or trigger sears.
Shooting the Mk3 with the loading tap left open may well contribute to a violent detonation if the spring and piston are overlubricated

[Leonardj]

The reason this thread has got so confusing is that many answers have deviated far from the original questions, none more so than yours.
You originally referred to a gun at the critical point of dieseling now you talk of one dieseling violently to the point of detonation in your answer
Obviously no one believes guns have re cocked due to detonation even though there are people on here who have personally witnessed or created the effect.
It takes all kinds..
eric

Sorry if you find my answer confusing, or deviating from the topic, but IMO, my answer simply points out a contradiction of the original statement.

"Using a heavier pellet will cause the pressure (and temperature) to spike higher and earlier in the shot cycle.... In a gun that is marginal on dieseling, this increase in pressure and temperature can cause dieseling to occur...."

A lot of folks are not aware of the difference between dieseling and detonation - they just lump any such reaction of a spring gun as "dieseling". Typically, one will often hear comments such as, "it dieseled so bad it sounded like a 12 gauge shotgun going off!". Obviously, what is being described as "dieseling" is in fact, "detonation". How would anyone be able to determine that a gun was "marginal on dieseling"? That is what leads me to believe that perhaps the author of that statement is not familiar with the difference between dieseling and detonation.

Therefore, in my answer, I used what I know to be the correct terminolgy to describe the event:

It is quite common knowledge that a freshly lube-tuned springer that is dieseling violently, to the point of achieving detonation, can be settled by shooting a few heavy pellets through it. Shooting the heavy pellets result in a much milder dieseling, which will slowly burn off any excess lubes in the compression chamber. After several heavy pellets, the mild dieseling ceases, and at that point, shooting the lighter pellets will no longer cause dieseling nor detonation. This practice that has been around for decades seems to directly contradict the theory presented in the first part of the noted statement.

Thus, in the simplest possible terms, the statement in question claims that use of a heavy pellet induces dieseling, whereas, my answer describes the use of a heavy pellet to quell dieseling.

[shabee]

In that case,
In your final answer:

""It is quite common knowledge that a freshly lube-tuned springer that is dieseling violently, to the point of achieving detonation, can be settled by shooting a few heavy pellets through it. Shooting the heavy pellets result in a much milder dieseling, which will slowly burn off any excess lubes in the compression chamber. After several heavy pellets, the mild dieseling ceases, and at that point, shooting the lighter pellets will no longer cause dieseling nor detonation. This practice that has been around for decades seems to directly contradict the theory presented in the first part of the noted statement.""

You have still failed to explain to the unknowing the process by which the dieseling "can be settled" and by which "much milder dieseling" is eventually achieved and that is: by using the heavier or tighter pellets that the chamber pressure is further increased, resulting in a higher temperature which further aides combustion and burns off the excess lube quicker to a point whereby changing to a normal or lighter pellet will no longer induce dieseling. Using heavier or preferably tighter pellets will for a time increase the dieseling effect and get rid of the excess quicker, not as implied simply tame the dieseling.
The only thing that you have said regarding this point which i can agree with is that "how could anyone know that a gun is marginal on dieseling" and i agree its impossible but is it not the theory we are talking about?
Anyway all this takes us back to the original question point 1. FACT entirely feasible in theory.

For those struggling with grasping the heavy or tighter pellet part of this discussion: Possibly the easiest bit to grasp here is if we use a tight pellet in the bore ie. one whereby it is more difficult to push down the barrel, hopefully you can see that this will increase the chamber pressure above what a looser fitting pellet will, by causing a blockage or better seal?
The heavier pellet is a bit more difficult to understand but basically heavier things (in this case a pellet) have more inertia or "reluctance to get moving" it takes more energy to get a heavy pellet moving than it does i light one, in an airgun pellet for this scenario this will have little effect but heavier pellets will have thicker skirts resulting in a better resistance to deformation in the barrel so creating a better blockage if this is easier to understand. Another way of thinking about it is this: the heavier or tighter the pellet is in the barrel breech, the higher will be the chamber pressure and thus temperature at the point where the pellet starts to move.
Hope this helps
Eric