Fact or fiction?

[shabee]

Eric, NO: Interesting

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

Good day aimless....its certainly had my attention and you are correct some of the answers have been "interesting " to say the least,
Your english is good enough, certainly better that would be my German, i take my hat off to you coming on and joining in despite the language barrier.
have a good day
eric

[Leonardj]

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.

When mild dieseling occurs, some of the excess lubricant is slowly burned off, but the chamber temperature does not reach the point of ignition, and the resulting detonation. This mild dieseling slowly eliminates the excess lube so that when the mild dieseling settles after a few heavy pellets, there is insufficient "fuel" for detonation to occurr, when switching back to the lighter pellets.

Consider the theory that with the heavy pellet, the piston begins to slow sooner than with the lighter pellet, thus actually building the pressure more slowly against the added restriction. Slower compression, would result in lower chamber temperature, and thus less tendency to diesel.
Thus, the gun that is achieving detonation with a lighter pellet, the piston is moving faster, and further, compressing the air far more quickly - faster compression of air = higher temperatures = detonation.

[shabee]

You are now trying to split hairs and go the the Nth degree to disprove the theory 1)
What you say above will be true for a given weight of pellet (more likely a lead slug) or given restriction in the bore, but to slow the piston with back pressure enough to quell the dieseling will take more than a pellet marginally heavier than the norm.

your 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".

Still holds true for a pellet a few grains heavier, If we then talk about slowing the piston enough to slow the compression rate and hence lower the temperature to tame the dieseling then we are not talking about a slightly heavier pellet are we? because a slightly heavier pellet will not produce the effect you now decide to bring to the table. Also its not just about the inertia that a heavier pellets creates it is as much about slowing the outflow from the compression cylinder without unduly slowing the piston stroke, and a light but tight fitting pellet would have the same effect. There are so many measures of these events that can be created by using different pellet weights and upto a complete blockage that they cannot all be covered by theory 1. As someone else quite correctly wrote "its all a matter of extent". Piston weight, piston diameter, piston stroke, spring strength, caliber will all play a part in the events leading to the dieseling and what happens in one gun wont necessarily happen in the next, but for the purpose of this thread we are bound by your statement.
eric

[Turnup]

When mild dieseling occurs, some of the excess lubricant is slowly burned off, but the chamber temperature does not reach the point of ignition, and the resulting detonation. This mild dieseling slowly eliminates the excess lube so that when the mild dieseling settles after a few heavy pellets, there is insufficient "fuel" for detonation to occurr, when switching back to the lighter pellets.

Consider the theory that with the heavy pellet, the piston begins to slow sooner than with the lighter pellet, thus actually building the pressure more slowly against the added restriction. Slower compression, would result in lower chamber temperature, and thus less tendency to diesel.
Thus, the gun that is achieving detonation with a lighter pellet, the piston is moving faster, and further, compressing the air far more quickly - faster compression of air = higher temperatures = detonation.

If I understand you correctly, you use of the term "mild dieseling" means "evaporated by heat and pressure changes", without ignition?

And you use the term detonation to mean ignition of the fuel within the compression tube?

Is this generally accepted airgun terminology? What bothers me is that in a diesel engine the fuel/air mixture is ignited by the temperature rise due to compression. The fuel mixture deflagrates (burns) but does not detonate. Detonation is a different phenomenon and in engines is not desired. NB it is quite difficult to induce detonation with oil as a fuel, this is exactly why diesel engines run on oil rather than petrol, which detonates more easily.

I really do not see that enhanced evaporation/vaporisation of oil within an airgun, arising from the temperature/pressure changes, can in any way be called dieseling.

Really don't get this. With both a heavy and light pellet, the piston begins to slow before the pellet has moved.

Don't get this either, the lightest possible pellet (i.e. no pellet) leads to the highest pressures and temperatures?

[Leonardj]

You are now trying to split hairs and go the the Nth degree to disprove the theory 1) Seems to me like you are doing much the same to support the original theory.
What you say above will be true for a given weight of pellet (more likely a lead slug) or given restriction in the bore, but to slow the piston with back pressure enough to quell the dieseling will take more than a pellet marginally heavier than the norm. I have found that a gun that attains detonation with an 8 grain pellet, will not attain detonation when a few 10.5 grain pellets are fired through it. This is fact, not theory.

your original statement: Not MY 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".

Still holds true for a pellet a few grains heavier, If we then talk about slowing the piston enough to slow the compression rate and hence lower the temperature to tame the dieseling then we are not talking about a slightly heavier pellet are we? because a slightly heavier pellet will not produce the effect you now decide to bring to the table. Also its not just about the inertia that a heavier pellets creates it is as much about slowing the outflow from the compression cylinder without unduly slowing the piston stroke, and a light but tight fitting pellet would have the same effect. There are so many measures of these events that can be created by using different pellet weights and upto a complete blockage that they cannot all be covered by theory 1. As someone else quite correctly wrote "its all a matter of extent". Piston weight, piston diameter, piston stroke, spring strength, caliber will all play a part in the events leading to the dieseling and what happens in one gun wont necessarily happen in the next, but for the purpose of this thread we are bound by your statement. Once again, not MY statement.
eric

It appears that we shall have to agree to disagree, based on our own individual experiences.

[Leonardj]

If I understand you correctly, you use of the term "mild dieseling" means "evaporated by heat and pressure changes", without ignition?
And you use the term detonation to mean ignition of the fuel within the compression tube?

Is this generally accepted airgun terminology? What bothers me is that in a diesel engine the fuel/air mixture is ignited by the temperature rise due to compression. The fuel mixture deflagrates (burns) but does not detonate. Detonation is a different phenomenon and in engines is not desired. NB it is quite difficult to induce detonation with oil as a fuel, this is exactly why diesel engines run on oil rather than petrol, which detonates more easily.

I really do not see that enhanced evaporation/vaporisation of oil within an airgun, arising from the temperature/pressure changes, can in any way be called dieseling.
Really don't get this. With both a heavy and light pellet, the piston begins to slow before the pellet has moved.
Don't get this either, the lightest possible pellet (i.e. no pellet) leads to the highest pressures and temperatures?

Yes, Dieseling and detonation are generally accepted airgun terminology.

As for your comment regarding deflagration, the definition as taken from Wikipedia:

Deflagration (Lat: de + flagrare, "to burn down") is a term describing subsonic combustion propagating through heat transfer; hot burning material heats the next layer of cold material and ignites it. Most "fire" found in daily life, from flames to explosions, is deflagration.

Therefore, your explanation above with regard to deflagration not meaning the same as detonation, is really quite inaccurate. Ignition of any fuel produces detonation - an explosion due to the rapid burning of that fuel. Without detonation (ignition of the fuel), internal combustion engines would not operate.

[Turnup]

Yes, Dieseling and detonation are generally accepted airgun terminology.

As for your comment regarding deflagration, the definition as taken from Wikipedia:

Deflagration (Lat: de + flagrare, "to burn down") is a term describing subsonic combustion propagating through heat transfer; hot burning material heats the next layer of cold material and ignites it. Most "fire" found in daily life, from flames to explosions, is deflagration.

Therefore, your explanation above with regard to deflagration not meaning the same as detonation, is really quite inaccurate. Ignition of any fuel produces detonation - an explosion due to the rapid burning of that fuel. Without detonation (ignition of the fuel), internal combustion engines would not operate.

No - you cite Wikipedia, which states that deflagration is a subsonic reaction. Had you also checked Wikki on "Detonation" you would have found that it is a similar reaction but which proceeds supersonically. Very much not the same thing at all. Not all explosions are detonations. In respect of detonation, Wikki goes on to state

"In engines and firearms[edit]Unintentional detonation when deflagration is desired is a problem in some devices. In internal combustion engines it is called engine knocking or pinging or pinking, and it causes a loss of power and excessive heating of certain components. In firearms, it may cause catastrophic and potentially lethal failure."

Thus when I fire my centrefire rifle, there is an explosion but no detonation (thankfully).

Some materials can exhibit both reaction types depending upon the circumstances e.g. Dynamite will burn steadily and unspectacularly if set fire to, but will detonate if initiated by an appropriate shockwave. The chemical reactions and products (including the amount of energy released) are exactly the same, only the speed is different. Some materials will not detonate, but do explode, for example Black Powder (the stuff Muzzle Loaders use), which is "low explosive" and will self-confine (go "bang" rather than "wuff" when ignited in a loose pile) in quantities of around a pound, but has never been demonstrated to detonate.

You also state "Ignition of any fuel produces detonation" - when I ignite petrol in my cigarette lighter, there is no detonation, no explosion, only deflagration (burning) of a fuel/air mixture.

Dieseling is ignition of a fuel/air mixture by compression effects and in a diesel engine (and for the most part in an airgun) is an explosive deflagration reaction. Detonation inside an airgun is a much more destructive event, and probably quite difficult to achieve with oil as a fuel, but I am prepared to accept that it can happen.

[aimless]

I would like to add some ideas, that are probably not mentioned:

Deflagration (in firearms and engines) is desired .

Firearms and engines (even the BARAKUDA air rifle) are designed in the way, that the direction of the deflagration is predetermined (e.g. engines-from the spark-plug to the piston)

So dieseling -if occuring- can be e.g. ignited on the piston-sealing OR on the port; with quite different results as you can imagine.......but IMO will not lead to recocking. Even if the pressure-wave is directed towards the piston (as I wrote before).

[Turnup]

I would like to add some ideas, that are probably not mentioned:

Deflagration (in firearms and engines) is desired .

Firearms and engines (even the BARAKUDA air rifle) are designed in the way, that the direction of the deflagration is predetermined (e.g. engines-from the spark-plug to the piston)

So dieseling -if occuring- can be e.g. ignited on the piston-sealing OR on the port; with quite different results as you can imagine.......but IMO will not lead to recocking. Even if the pressure-wave is directed towards the piston (as I wrote before).

I can certainly accept that in a detonation, because it is supersonic, then shockwaves can be directed (and even focussed) and therefore cause localised effects. The shaped charge explosives use exactly this effect (Munroe effect IIRC).

I am not sure about the effects of direction in a deflagration reaction. By definition this is subsonic, so the surrounding gasses can flow and will equalise the pressure within the confinement at near sonic speeds, that is to say the pressure equalisation will be achieved faster than the combustion (I think). Thus the gasses produced by the burning, and heat, will expand uniformly away from the point of ignition, and will act equally in all directions. By comparison with the velocity of a piston, this is all over very quickly.

Your point about where the combustion actually starts is a good one and I can see that this could lead to different effects. With absolutely no data, I would speculate that combustion is more likely to start away from the cooling effect of the container walls, but there is also the problem that the fuel/air mixture is unlikely to be evenly concentrated and it may be that the necessary critical conditions exist only in one (random) area.

Whatever, I have experienced re-cocking in an air pistol when I was deliberately introducing fuel. It was at that point I decided to cease further experimentation as I realised that pressures could become unsafe. BTW the gun was not damaged in any detectable way by these few experiments and it still works just fine today, with it's original seals. I guess I was lucky to have worked up to it and stopped before it got too exciting.

[evert]

Consider the theory that with the heavy pellet, the piston begins to slow sooner than with the lighter pellet, thus actually building the pressure more slowly against the added restriction. Slower compression, would result in lower chamber temperature, and thus less tendency to diesel.
Thus, the gun that is achieving detonation with a lighter pellet, the piston is moving faster, and further, compressing the air far more quickly - faster compression of air = higher temperatures = detonation.

Sounds reasonable, and corresponds with my own experience.

In my head, this also corresponds with why we sometimes see lowered power output and harsh noisy firing behaviour when a spring gun is "oversprung".
My thoughts(based upon other peoples shared findings and my own experiences) are that the (overly)fast acceleration creates a too sudden buildup of pressure, that cannot be transferred to the pellet before heavy dieseling OR detonation OR piston bounce occurs- thereby reducing pellet velocity and causing a lot of recoil and noise.

[shabee]

Leonard, i am simply supporting an already proven theory, with my own experiences, i am not trying to impress my ideas on you, and one thing which you persistently fail to understand is : it is not all about pellet weight, it is about the restriction in the bore and how fast or slow you get it moving, this in turn determines the critical piston speed and ultimately the pressure. For the purposes of dieseling and detonation there is a sweet spot (both can be different) where the combination of pellet weight and tightness (bore restriction), piston speed and cylinder pressure culminate in the optimum condition to achieve the desired effect.
Just because you got the result you did with one type of 10.5 g pellet does not mean you will get the same result with another 10.5g pellet and it does not mean you will not get the same result with a lighter, tighter pellet.
Admittedly not an easy thing to understand unless you have studied physics, forces and moments. You will often hear shooters say they shoot "x" pellet in their gun and it shoots harshly and they then shoot "y" brand and it shoots sweeter, this is the cushioning effect of slowing the purging of the compression cylinder, both conditions will support dieseling when other conditions are optimum.
Maybe the original author and gunsmith is a little wiser than has been given credit for?
kind regards
Eric

[Leonardj]

No - you cite Wikipedia, which states that deflagration is a subsonic reaction. Had you also checked Wikki on "Detonation" you would have found that it is a similar reaction but which proceeds supersonically. Very much not the same thing at all. Not all explosions are detonations. In respect of detonation, Wikki goes on to state

"In engines and firearms[edit]Unintentional detonation when deflagration is desired is a problem in some devices. In internal combustion engines it is called engine knocking or pinging or pinking, and it causes a loss of power and excessive heating of certain components. In firearms, it may cause catastrophic and potentially lethal failure."

Thus when I fire my centrefire rifle, there is an explosion but no detonation (thankfully).

Some materials can exhibit both reaction types depending upon the circumstances e.g. Dynamite will burn steadily and unspectacularly if set fire to, but will detonate if initiated by an appropriate shockwave. The chemical reactions and products (including the amount of energy released) are exactly the same, only the speed is different. Some materials will not detonate, but do explode, for example Black Powder (the stuff Muzzle Loaders use), which is "low explosive" and will self-confine (go "bang" rather than "wuff" when ignited in a loose pile) in quantities of around a pound, but has never been demonstrated to detonate.

You also state "Ignition of any fuel produces detonation" - when I ignite petrol in my cigarette lighter, there is no detonation, no explosion, only deflagration (burning) of a fuel/air mixture. Sorry, my mistake on that one - I meant within a confined/closed space, but failed to specify that.

Dieseling is ignition of a fuel/air mixture by compression effects and in a diesel engine (and for the most part in an airgun) is an explosive deflagration reaction. Detonation inside an airgun is a much more destructive event, and probably quite difficult to achieve with oil as a fuel, but I am prepared to accept that it can happen.

Also from Wiki:
https://en.wikipedia.org/wiki/Deflag...ion_transition

Deflagration to detonation transition

Deflagration to detonation transition (DDT) refers to a phenomenon in ignitable mixtures of a flammable gas and air (or oxygen) when a sudden transition takes place from a deflagration type of combustion to a detonation type of combustion. The effects of a detonation are usually devastating.

A deflagration is characterized by a subsonic flame propagation velocity, typically far below 100 m/s, and relatively modest overpressures, say below 0.5 bar. The main mechanism of combustion propagation is of a flame front that moves forward through the gas mixture - in technical terms the reaction zone (chemical combustion) progresses through the medium by processes of diffusion of heat and mass. In its most benign form, a deflagration may simply be a flash fire. In contrast, a detonation is characterized by supersonic flame propagation velocities, perhaps up to 2000 m/s, and substantial overpressures, up to 20 bars. The main mechanism of combustion propagation is of a powerful pressure wave that compresses the unburnt gas ahead of the wave to a temperature above the autoignition temperature. In technical terms, the reaction zone (chemical combustion) is a self-driven shock wave where the reaction zone and the shock are coincident, and the chemical reaction is initiated by the compressive heating caused by the shock wave.

Under certain conditions, mainly in terms of geometrical conditions such as partial confinement and many obstacles in the flame path that cause turbulent flame eddy currents, a subsonic flame may accelerate to supersonic speed, transitioning from deflagration to detonation. The exact mechanism is not fully understood,[1] and while existing theories are able to explain and model both deflagrations and detonations, there is no theory at present which can predict the transition phenomenon.

A deflagration to detonation transition has been a feature of several major industrial accidents

1970 Propane vapour cloud explosion in Port Hudson
The Flixborough disaster
The 1989 Phillips Disaster in Pasadena, Texas
The damage observed in the Buncefield fire, see the 2005 Hertfordshire Oil Storage Terminal fire
The phenomenon is exploited in pulse detonation engines because a detonation produces a more efficient combustion of the reactants than a deflagration does, i.e. giving a higher yields. Such engines typically employ a Shchelkin spiral in the combustion chamber to facilitate the deflagration to detonation transition.[2][3]

The mechanism has found military use in the thermobaric weapon.

A deflagration to detonation transition (DDT) has also been proposed for thermonuclear reactions responsible for supernovae initiation;[4] see also Carbon detonation. Apart from the name,[dubious – discuss] this phenomenon is completely unrelated to the chemical combustion and flame acceleration phenomenon.

[Leonardj]

... it is not all about pellet weight, it is about the restriction in the bore and how fast or slow you get it moving, this in turn determines the critical piston speed and ultimately the pressure. For the purposes of dieseling and detonation there is a sweet spot (both can be different) where the combination of pellet weight and tightness (bore restriction), piston speed and cylinder pressure culminate in the optimum condition to achieve the desired effect.
Just because you got the result you did with one type of 10.5 g pellet does not mean you will get the same result with another 10.5g pellet and it does not mean you will not get the same result with a lighter, tighter pellet.

Admittedly not an easy thing to understand unless you have studied physics, forces and moments. You will often hear shooters say they shoot "x" pellet in their gun and it shoots harshly and they then shoot "y" brand and it shoots sweeter, this is the cushioning effect of slowing the purging of the compression cylinder, both conditions will support dieseling when other conditions are optimum.
Maybe the original author and gunsmith is a little wiser than has been given credit for?
kind regards
Eric

Based on your first paragraph, we are essentially in agreement. A conflict of terminology may be the problem - IE - "bore restriction" as opposed to "pellet weight".
Essentially, as you state, just far too many variables involved to be covered by a single, generalized statement.

I know of no way of quantifying "pellet tightness" in the bore in definable numeric terms, but have found that generally, the heavier pellets produce the desired result, thus I simply use the term "heavy pellets". Perhaps, as you state, the same result could be achieved with a lighter, tighter pellet, however, when a gun is detonating badly, my interest is to stop the detonation, and I reach for what has worked well for me in the past. Experimentation with different other pellets to find one that is lighter and tighter may not be feasible if the gun could be damaged during that experimentation process.

I have studied physics, forces and moments, albeit quite some time ago.
I was also involved with root cause, and failure mode analysis in an industrial environment.

[MarcDV]

Interesting propositions. My take on the claims:

1) Since the pressure is purely a function of the barrel bore and the distance traveled, the pressure (and therefore the temperature) must be the same regardless of pellet weight. There are several contributory issues not addressed here, such as pellet acceleration and final velocity, pellet fit, residual lubrication and friction, neglect these (very minor side effects) and this is a non-starter.
2) Invalid, see response above.
3) A guess at best. While it is highly likely that dieseling caused many a spring failure and also theoretically possible for the spring to re-cock, in 45 years of handling air guns, I have never seen this.

Good stuff to keep our grey matter churning!

Marc.

[shabee]

I would like to add some ideas, that are probably not mentioned:

Deflagration (in firearms and engines) is desired .

Firearms and engines (even the BARAKUDA air rifle) are designed in the way, that the direction of the deflagration is predetermined (e.g. engines-from the spark-plug to the piston)

So dieseling -if occuring- can be e.g. ignited on the piston-sealing OR on the port; with quite different results as you can imagine.......but IMO will not lead to recocking. Even if the pressure-wave is directed towards the piston (as I wrote before).

In a gun with a good seal the hotspot will indeed be around the port where there is most agitation of the compressed air molecules trying to escape, if the piston seal is leaking this can change things somewhat as you quite rightly point out.
eric

ps. do you still use your barakuda? and can you still obtain the glass ampoules?