For your amusment

What is dieseling? Compressing gases till it ignites. A diesel will run on any kind of oil. Years ago Dad was on oxygen. I had to fill his small tank for him to carry when he went somewhere. The man who brought the oxygen out showed me how to fill the tank. He told me to watch the temperature and if the tank started to get warm to slow the filling up. Isn’t this similar to the air pushing a pellet with oil in the barrel. As the air is trying to push the pellet out it is compressing. What is compression rate of a diesel engine. 18-1????Looks like it could happen to me.

Raptor,
Gippeto answered most of your question already. The fact is that liquids don’t burn, vapors do. Dieseling in either a PCP or springer or diesel engine has nothing to do with compressing liquids (especially since liquids are essentially incompressible). While friction creates heat, the friction of a pellet in a bore is not an ignition source.

Raygun was probably right to use lighter fluid in his experiment as lighter fluid vaporizes easily. His mistake was in assuming that his gun with light diablo pellets produces anywhere near the breech pressure of a gun shooting tight fitting slugs. He thinks the tight fitting slug moves instantaneously with the trigger pull (or nearly so) and thus volume is rising, pressure is falling and temperature must be falling throughout the gun’s airway. He will never convince himself otherwise as he depends on his available gun for proof. Of course, his near bore riding pellet head and tiny, thin pellet skirt will likely never resist movement enough to build up pressure and get to auto-ignition temperature of even lighter fluid.

Diesel engines use high pressure fuel injectors to inject finely atomized fuel into the cylinder at just the right time to maximize power and efficiency. If you look at the technical data for the newest diesels you might note that injector pressures are higher than the old engine and the injector has more but smaller holes. The reason is that higher pressure through smaller holes atomizes the fuel into smaller but many more tiny droplets. These many more droplets have a greater total surface area and thus more area for the fuel to vaporize faster and more completely. That means more of the fuel can burn before the power stroke is complete and thus you can use less fuel because you are more efficient turning liquid into a combustible vapor…

Aircraft (most larger aircraft that is) use high pressure hydraulic systems to move control surfaces. For decades the standard military aircraft used hydraulic fluid at 3000psi. The V-22 and some other new platforms use a 6000psi system. That fluid is flammable and gets pretty hot being run through the pressure pumps yet it does not explode. Now, the hydraulic system is sealed on the pressure side and there is no air with oxygen present to support combustion. Put a pin hole in a 3000psi hydraulic line though and you have the makings of a huge fireworks show if there is a jet exhaust or other ignition source near! Fluid going through a pinhole leak at 3000psi blows out in a mist and every tiny droplet has vapor at the fluid/air boundary…

My gun most likely dieseled because I got oil down the transfer port and the firing cycle sent air through that port at great speed. That atomized some of the solvents in the 3 in 1 oil and when that air hit the back of the slug, it stopped. The pellet didn’t just slip down the bore because it takes force to engrave the slug into the rifeling and get it started. That force comes from rising pressure. If the slug fits tightly enough, the pressure in the tank and the pressure behind the stuck pellet will equalize at damn near the 3200psi in the tank (the volume of the transfer port and air passage is tiny compared to the volume of the tank). So, go from 14.7psi to 3000psi in the duration of a firing cycle and think you might build up some heat? Remember, all the air between the tank exhaust valve and the back of the slug was at 14.7psi just an instant ago. Even if the slug starts moving when the breech pressure hits 500psi you may still have a higher top pressure as the air is flowing from the tank (ever hear of “ram air?” Even if the pressure peak is 500 psi, what was that compression ratio? Make the peak pressure 300psi and your at diesel ratios. So why don’t PCPs diesel very often? Well, remember that part about liquids don’t burn, vapors burn? A tube with walls moist with oil wont really produce a lot of atomized oil and the combustible vapor at the wet surface is minimal, it might actually burn some but there is so little, you’ll probably never be able to notice. With guns that don’t produce higher breech pressures it’s not a concern.

OK, so my gun has a transfer port aiding atomization of misplaced oil and produces high breech pressures supporting high power levels so dieseling can be explained. How did this happen to Pabs? He’s a liar and just thought it would be cool to say it happened to him too. 😆

OK, Pabs is not a braggart or liar so maybe we ought to give this some thought. His Condor does not have a transfer port in the traditional sense, it has inline air flow. It does have plenty of dead air space though (air between the tank and pellet at 14.7 psi). OK, there’s the compressible and thus heatable gas, being air, it can support combustion. How do we get that air hot? Well, we open the valve and let 3000psi move into the “dead space” taking it from 14.7 to some much higher pressure in a very short time (so short that the heat produced cannot be absorbed by the metal tubes fast enough to avoid auto-ignition temperature of any flammable vapor present). How do we keep the dead air in a confined enough volume to produce this compression, tight slug (hard to move). Now we have an oxidizer and heat…we’re going to need something to burn! Oil! OK, I said we need vapor and it can’t just be a moist bit of barrel. Well, what if you shoot a lot of slugs and lube them all? Is it possible that during normal firing cycles that lube doesn’t all get blown out the bore? Can some of that lube accumulate to where a particular shot cycle might pick up enough to atomize enough to support a noticable “diesel” event? Remember, Pabs didn’t advertise a frequent or even easily repeatable event.

Diesels are designed to have all the required elements for controlled combution come together 1000s of times a minute. If you understand the combustion cycle and what furthers it and hinders it, you will see how it can happen rarely to PCPs and also know when you’re getting BS’d about certain dangers…

Jerry,

I am sure you know what you are talking about, but I don’t think you and I are are the same page. Let me explain my logic. Lighter fluid (the liquid) is not going to ignite either by having its molecules compressed by a pellet forcing it between it (the pellet) and the barrel. In a springer or a NPSS the friction between the pellet and the barrel may, however create enough heat to ignite the vapors of lighter fluid which are sure to build up because lighter fluid has a low evaporation temperature. With these guns, the temperature of the air forcing the pellet down the barrel will have no cooling effect. In a PCP it will and therefore the heat generated between the pellet and the barrel will not likely reach the flash point of the lighter fluid vapor. Oils are much denser and therefore are much more sensitive to the heat generated when its molecules are compressed. If there is an oil that heats to its combustion point under low enough pressure in the barrel it will ignite. It will not matter what type gun it is if the compression pressures are met.

Raptor,

I agree that raygun is on another page. However, your concept of what is burning and how it is ignited is a bit off base as well.

Jeez Pabalas we can’t let you out for nothing! 😆

Yeah, I don’t think it’s an easy thing to replicate. When I saw it, I was using a slug that is not especially tight and I’ve lubed those slugs for years. I think it was because I used a patch too wet with oil and some got down the transfer port. That would be in a hi flow area that would atomize some of the oil and make it easier to ignite with momentary high temperatures. I think it’s one of those unusual things that certain conditions that are not normal allow. Of course, some guy who read Boyle’s law can tell me all about it… 😆

PB, I don’t know about anyone else’s PCP dieseling experience but I doubt mine would have registered a lot on the chrony. I didn’t get a big, loud bang and a noticeable change in POI. I just had a couple of “off shots” that were followed by smoke from the breach. At the time, I just thought it was curious and pointed it out to Gary as a mildly interesting aside… It really wasn’t a mystery or news flash to us. I’m sure that a gun without a full, heavy breech will sound a little different but I don’t think dieseling a PCP on purpose is going to pay off with huge numbers or usefullness….