The Combustion Myth

Back in the 1950s the father and son team of G. V. Cardew and G. M. Cardew published the results of an interesting study in which that proposed that a large part of the power of airguns came from the combustion of lubricants in the compression chamber of spring-air guns. Thier experiment was simple, and convincing: They fired a .22 cal Weihrauch HW 35 that had been purged of air and filled with nitrogen. Muzzle velocities obtained were significantly lower than when the gun was fired in air.

I’ve always been somewhat distrustful of the Cardews’ experimental methodology, which involved placing a gun in a plastic bag, pumping out air, and bleeding in nitrogen, and then opening the bag and firing the gun, but I’m willing to believe it worked, and that they obtained the results they published. Certainly these guns were burning part of their lubricant; anyone who’s owned a Daisy BB gun is familair with the wisp of smoke and the smell of burning oil that follows a shot from a well-oiled gun.
The airguns of the Cardews’ era had leather piston seals which had to be fairly well saturated with lubricant in order to provide a good seal. Some of this oil would invariably be sprayed into the barrel in the form of a very fine aerosol on firing, and that aerosol would in turn be ignited by the hot air coming through the transfer port, and that would add to the propulsive power of the gun.

But there would also be lubricant burning in the compression chamber- and that should interfere with propulsion, by setting up a shock wave that would drive the piston back before maximum pressure was built up in the barrel. I have, on a number of occasions, encountered just such a situation in guns in which some combustible material- usually a pellet lube or cleaning solvent- has gotten into the chamber. The result is usually a loud report, a blackened (and sometimes ruined) piston seal, often a damaged mainspring, and a drop in muzzle velocity.
And there’s another issue. But in the 1960s, Ladd Fanta started experimenting with using silicone oils- which do not burn- in place of the combustible hydrocarbon oils traditionally used in air guns. Airgun dealer Robert Law also began promoting the use of these new lubricants. And looking over some of the catalogs that Law produced back then, no where does he mention any loss of velocity from using silicone based lubricants. So I remain- let’s say- skeptical, but open to being convinced.

Whether or not the Cardews’ test showed what was claimed, the results still have no applicability for today’s guns. Modern airguns- which the possible exception of some of the very cheapest Chinese guns still sitting in a warehouse somewhere- don’t use leather seals. Any lube beyond a fraction of a gram is quickly shot out. Modern synthetic lubes are made of silicone- a tightly bonded silicon-oxygen molecule that simply will not burn. And despite using only a miniscule amount of lubrication, and being fired regularly for years or decades without any additional lubricant, today’s spring airguns commonly achieve muzzle energies and velocities unheard of in the Cardews’ time.

5 thoughts on “The Combustion Myth”

  1. A test for combustion on sliding breech airguns: Take a breath and blow air into the breech while you close the lever. Chronograph the velocity and compare to velocity with fresh air. I have a rifle that consistently does 20-30fps lower when introducing “used air”. The “used air” is partially depleted of oxygen and is CO2 rich. So it impedes the combustion somewhat.

  2. The relative amounts in O2 in inhaled and exhaled air are 21% and %17, and I’m not sure that’s enough to explain the difference you’re getting- and 20fps is a very small amount in, say, a, 800fos gun. Simple shot to shot variance could easily exceed that.. How many trial did you do this over, and with what gun and lubricants? Unless you’re using a combustible lube there wouldn’t be any differences.

    The bigger difference in inhaled and exhaled air would be the amount of humidity- exhaled air is fully saturated at the temperature of the air. So in dry conditions, the difference would be 0% versus 100%. Even without any combustion that could have a significant effect, with much of the latent heat going to vaporize the condensed moisture.

  3. I prefer to have a gun that does not rely on combustion. That is why I did the impromptu test. I presented it as a quick way for someone to confirm combustion in a gun.

    The gun is a TF58/QB58. The piston had moly lube on the synthetic seal with 3-in-1 oil behind the seal on the piston body. The seal appeared to be picking up a little oil with each backward stroke of the piston. There was a small wisp of smoke from the breech when I cocked the gun after each shot. The velocity was fairly consistent at around 650fps. I suspected combustion but was not sure how much it was actually affecting velocity. The next 12 shots I alternated between “fresh air” and “used air”. The “fresh air” shots were at least 20fps faster in every case with no overlap. I disassembled the gun and cleaned out all traces of lube. I re-lubed with 60% moly. The average velocity dropped by 40fps and there was no more smoke.

    I believe that you are correct about the humidity as it helps impede combustion. The same way that water injection is used to stop detonation in internal combustion engines. As you stated, normal exhaled air still contains a high concentration of oxygen. I held my breath for a while prior to exhaling. To make the test more affective, you need to hold your breath before exhaling over the chamber. The longer you hold your breath, the better as this gives your lungs time to exchange the O2 for CO2. I guess you could breath normally in and out of a bag for a minute. That should remove about half of the oxygen. The test was not very scientific but it did indicate to me that combustion was taking place and affecting the velocity.

  4. I would agree that the smoke is absolute confirmation that combustion is taking place! The fact that it’s a Chinese gun is also significant, as these are typically lubed with hydrocarbons. I’m guessing your cleanup and re-lube probably significantly improved the smoothness of the gun.

  5. You have more than one variable being changed in your experiment. The air you exhale is slightly depleted in O2, but it contains much more water, and that water absorbs a significant amount of the heat generated when the air is compressed on firing.

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