by B.B. Pelletier

Sometimes, I have to be reminded that not everybody understands all the terms I use in my postings, and last week someone asked for an explanation of the term dieseling. This term is almost always used when talking about spring-piston airguns, though there is such a thing as a dieseling event with a precharged gun, as well. That’s something you NEVER want to witness. But, the most common use of the term dieseling applies to spring guns.

What it means
In 1894, Rudolph Diesel designed a working internal combustion engine that ignited the vaporized fuel by means of heat generated by compression, alone. It did not rely on a sparkplug, which meant that an entire engine subsystem was eliminated. The diesel engines of today are based on the work he started that long ago.

A spring-piston airgun generates a very small amount of highly compressed air to power a pellet or BB. When the air is compressed by the piston, the temperature rises to very high levels (Beeman has said 2000 degrees F) that can ignite tiny oil droplets, created when the lubricated piston seal scrapes the walls of the compression chamber. Thus, the diesel effect happens. Because the amount of “fuel” is very small, the force of the resulting ignition is usually also very small, but the presence of smoke in the barrel after a shot has been fired is one telltale indicator.

Read your Cardew
G.V. and G.M. Cardew wondered about the same things we do, so in the 1970s they conducted a series of experiments to determine what things were happening when airguns fire. Their book, The Airgun From Trigger To Muzzle, was a report on these experiments. In 1995, they updated their work and published The Airgun From Trigger To Target. The approach they took makes it obvious to me that they had read Dr. Frank Mann’s seminal work on firearm ballistics of nearly a century before, The Bullet’s Flight From Powder To Target. Like Mann, the Cardews used ingenious ways of testing various airgun principles, and I think that nowhere did they do better than when they described dieseling. You can buy this book here at Pyramyd Air. If you are curious about all technical aspects of airguns, this is one you really need.

Four phases of a spring-piston airgun
They categorized four phases of spring-piston power, based on what happens when the guns fire. Those phases are blowpipe, popgun, combustion and detonation. We are only concerned with the two most powerful phases; combustion (the Cardews call dieseling combustion in their second book) and detonation. Combustion is a true diesel event, but there isn’t enough fuel present to make a mighty bang. The reason they did not use the term dieseling for this phase is because the detonation phase is also a dieseling event. The difference between the two is the amount of fuel available.

A detonation is a very strong combustion. You will hear a bang and, in the most extreme instances, guns can recock themselves and even blow apart! Jess Galan wrote about this in Airgun Digest Vol. 2, when he wrote about “Oil-Can Louie.”

Obviously, there will be a lot of smoke with detonations, too; but with a detonation, you will sometimes see a small flame coming out of the muzzle of the gun! It helps to be in a very dark place to see this phenomenon. The Cardews warned of the danger of creating an intentional detonation, but they did it to learn as much as possible. They also proved beyond a doubt that all powerful airguns support combustion with every shot. That is the same as saying all powerful spring-piston airguns diesel with every shot.

Tomorrow, I will tell you how they did it, and also give you some hint as to how certain sly airgun manufacturers can cheat their velocities.

What about a PCP diesel?
When you charge a precharged gun from a scuba or carbon fiber tank, the rapid compression of air generates heat – a LOT of heat, as a matter of fact. So much heat that there have been a few accidents attributed to this heat and the presence of petroleum-based lubricants that do not belong in precharged reservoirs. These accidents are nearly always catastrophic, and at least one resulted in the death of the person filling the airgun. Another accident happened with a vintage big bore airgun, where lard is used inside the reservoir to trap airborne contaminants. These reservoirs were made at a time when scuba tanks and 3,000 pound pressures did not exist, so lard was not the danger we think it is today. But, if someone fills a vintage tank from a scuba tank, even though they are careful not to exceed the vintage pressure of 500 or 600 psi, the speed at which the air is introduced is enough to create sufficient heat to ignite the lard. When it did, the tank was blown off the connection, fortunately, injuring no one but resulting in this cautionary tale.