Fact or fiction?

[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.

[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?

[aimless]

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

No, I never shot her with "fuel" and know no one who did it And original ampoules are rare and I have only seen a picture of them.

As I wrote in my threat:

http://www.airgunbbs.com/showthread....light=barakuda

it might work with "break-cleaner spray" (that´s what we call it in german)

[evert]

I'd guess "engine startup-spray" would work, as it is ether.

[shabee]

Yes ive used "Engine start" and i think the make was Bradex years ago, maybe sold under a different brand name now though?
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.