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Dieseling in Airguns: The Myth That Refuses to Die.

I caught this today at a web site that also sells airguns:

“While RWS spring-piston models do rely on a tiny diesel effect to produce full power, sever (sic) dieseling must be avoided.”

Elsewhere in this company’s web site they recommend that “One or two drops every 5,000 to 6,000 rounds, or each 8-12 months, should be plenty.”

Modern airguns are lubricated with very tiny amounts silicone-based oils that have a flashpoint of between 450 and 550F, and simply do not burn any significant amounts of oil- unless the owner squirts  excess down the barrel, as Beeman still recommends. Many guns use a silicone-molybdenum disulfide paste that will not vaporize under any circumstances.

Supposing the oil did burn. Given that they say a properly lubricated spring piston airgun can fire at least 5,000 shots before needing a tear down and relubrication, how much oil do you imagine could burn on each shot?  Those two drops of oil- about 100 mm^3, weigh  roughly 7.15×10^-5  grams, which, divided by 5000 shots comes to 1.43×10^-8, or  0.0000000143 grams per shot.

You could be lubricating the gun with gasoline, and you’re still not going to get very much energy from 0.0000000143 grams per shot! Note that Beeman, who used to recommend two drops of oil with every tin of pellets, now say that:

The piston seal in most modern air guns is made of a synthetic material that is self lubricating. It should only be lubricated during routine maintenance performed by an authorized service shop.

So there you have it. No oil, and no dieseling.

23 Comments

  1. Pieter van de Beek wrote:

    Dear Airgunner, that myth will stay alive forever. Why? Because the great Cardew himself has said so. Nobody ever replicated his measurements, but he was the holy Cardew, and therefore always right.
    There are two (at least) cases documented in which somebody un-lubricated (new word?) an airgun and came to the conclusion that there was no significant speed difference between a dry and lubricated airgun.

    first you have:

    http://www.network54.com/Forum/79537/message/1173260392/Bone-Dry+Spring+Gun+Part+III

    of which the conclusion was: no cigar, speed stays for all purposes a constant;

    and then you have:

    http://www.luchtbuks.net/index.php?showtopic=37676&hl=cardew&st=0

    which is in Dutch, but came to the same conclusion.

    And yet people keep saying that Cardew could not be wrong. They invent reasons such as “the fatty dirt from your hands lubricates the pellets, therefore you still have slow burn” and “there is always oil left in the microgrooves in the steel of the piston and the chamber, therefore..” sigh.

    Be prepared to see this myth pop up every now and then.

    Keep up the good work,

    Pieter

    Wednesday, September 9, 2009 at 11:49 am | Permalink
  2. Titou wrote:

    I don’t really see the connection between the “dieseling myth” and what is explained here…
    You (very well) demonstrate that what could be called “maintenance lubrication” is far from being sufficient to generate a dieseling effect, due to the poor amount of oil that could burn each time. However, you did not demonstrate that dieseling is impossible when remaining oil quantity is enough for maybe two or three shots !

    I totally agree when you say that airgun manufacturers claim of a dieseling effect ALWAYS contributing to full power is bullshit ! Same thing for the idea of a lubrication from the skin of the shooter !

    Still, dieseling is not a myth, it just requires some specific conditions: enough oil, flash point within airgun operating pressure limits (gazoline is perfect: flashpoint between 200 and 300 bars)

    Quite every airgunner has already seen some smoke (with a very characteristic smell of burnt oil) getting out of the barrel when shooting an airgun that has been stored for some time with usually more oil than necessary !

    But what is more important is the danger that dieseling represents due to the quick increase of pressure inside the barrel. The barrel itself is not bound to explode as it is usually over-sized regarding pressure limits. Seals and piston are far from being as strong…You will probably observe dammages to the seals and get an increased rebound of the spring+piston (there is always one, but quite reduced under normal conditions).

    Wednesday, September 23, 2009 at 7:16 am | Permalink
  3. mje wrote:

    Titou: My point is not that dieseling is impossible- it is that dieseling is not a source of any significant propulsive energy in modern airguns- which is what the website I quoted was claiming. Certainly, as you say, we have all seen some small amount of combustion in over-lubricated guns.

    Wednesday, September 23, 2009 at 10:26 am | Permalink
  4. Adam wrote:

    I’m assuming–if one was so enclined–that the cavity in the back of the pellet could be filled in with some sort of ‘high-cetane’ material, such as parrafin wax…

    Tuesday, December 29, 2009 at 3:26 am | Permalink
  5. Juan wrote:

    When in the doubt it´s better you abstain…(please excuse my english)
    I think air rifles are not enginered for such experiments of dieseling! But if someone would like to attempt, please do it in a cheap chinese one. Be sure will get not any increase of velocity or power, but destruction of seals and piston…

    Monday, February 1, 2010 at 2:03 pm | Permalink
  6. GunScientist wrote:

    Dieseling of airguns is a fact, and it does increase velocity. In my own experimenting with a spring piston airgun and a number of fuels, some diesel and some don’t. You are correct that oiling a gun with 2 drops of oil will not cause dieseling for 5000 shots, but it may for a few shots. Gasoline will not diesel, and it doesn’t work in diesel engines either. The correct fuel/air mixture is key. I lubricated a clean gun with new seal with non-dieseling lube, and of course a lubricated gun works better, but a gun lubricated with an oil that works as a fuel garners an increase in mean velocity. In addition, there can be ‘detonation’, a very violent burn of fuel that can damage the gun in one event. Cardew & Cardew knew what they were talking about. If you want to refute them, publish the results of a properly designed experiment.

    Tuesday, February 23, 2010 at 1:06 am | Permalink
  7. mje wrote:

    I don’t disagree with anything you say, but I do think you’re misreading my point. Dieseling has not been a factor since the advent of synthetic lubes. Even before then, a properly lubricated gun only exhibited very modest dieseling. After a few shots the dieseling would stop unless more oil was added. If you shot airguns in those days you might recall that accuracy and velocity were very uneven after lubrication until the excess burned off and the gun stopped smoking. I can’t really duplicate Cardew’s work without a leather-seal gun and there aren’t any made these days.

    Tuesday, February 23, 2010 at 8:42 am | Permalink
  8. GunScientist wrote:

    Fair enough MJE. I think we may all be more or less on the same page, and just not expressing it very well. As far as leather seal guns, some of the cheap Chinese guns (ie. Arrow brand) still have leather seals, and it is not a shame to do some destructive testing on them.

    Tuesday, February 23, 2010 at 12:03 pm | Permalink
  9. mje wrote:

    Good idea! I’ll see if I can’t find something to chronograph.

    Tuesday, February 23, 2010 at 12:05 pm | Permalink
  10. Vance wrote:

    I just came across this dieseling discussion, very interesting…

    I think mje’s analysis is an excellent overall approach. However, when I worked the numbers I got a bit different conclusion. Maybe you guys can point out if I’ve got an error here somewhere.

    1. Oil (or gasoline) has about 1.3E8 joules per gallon energy content. This works out to 3.4E4 joules per ml.

    2. A drop is around 0.05 ml. So energy content would be 1700 joules per drop

    3. If the drop’s energy were distributed uniformly over 2500 shots, that would be about 0.7 joules per shot.

    Granted this is not a huge number, but it’s not all THAT small. Also, if the distribution is nonuniform — more immediately after the lubrication, and less later — then the initial shots could get a substantial boost from this dieseling effect.

    p.s. — Cardew’s book has been reprinted recently (Nov 2010) and is available again. I just got my copy. As a retired engineer, I have to say I’m impressed.

    Friday, February 4, 2011 at 7:23 pm | Permalink
  11. mje wrote:

    Interesting addition to the debate, thanks. I might argue with how efficient the combustion of the oil would actually be, but that’s something we could test.

    Friday, February 4, 2011 at 7:31 pm | Permalink
  12. Jeffrey c wrote:

    I’ve inadvertantly put too much light air gun oil in my crossman .177 caliber spring break down type, and got a loud report with some smoke and a .75 in. pine board penetrated plus the bb bouncing around in the woods 200′ the board. It doesn’t happen every time, but when it does there’s no mistaking what took place.

    Sunday, March 13, 2011 at 10:30 pm | Permalink
  13. mje wrote:

    Yes, if you put too much lube in a gun it will diesel- as it will if you put any amount of hydrocarbon oil. The point of the article is that modern guns do not rely on the so-called dieseling effect to produce power.

    Monday, March 14, 2011 at 1:07 pm | Permalink
  14. fifo wrote:

    All I can say is that I add to every new can of crosman hollowpoints a smudge of synthetic grease and a couple of drops of synthetic oil for lubricating paint gun seals and I tumble the can carefully so as to not to damage the skirts of the pellets and then I shoot them with my crosman storm xt and the overall performance seems to get better results. (accuracy and velocity). I try to keep the bore dry though.There is some dieseling effect (smoke) but not that bad. Any comments?

    Friday, June 3, 2011 at 7:13 pm | Permalink
  15. mje wrote:

    You may be getting a small amount of dieseling- can you smell burning oil at the breech when you open the gun after shooting it?

    I used to lube my pellets for Field Target competition, after cleaning them with something like brake cleaner. I’d use a very small amount of a commercial English pellet lubricant- don’t recall the name. SOme of my fellow competitors used Lemon Pledge- just a very light spray.

    Whatever lube you use, just use the smallest amount that has an effect. To much, and you’ll get damaged springs and piston seals.

    Friday, June 3, 2011 at 8:07 pm | Permalink
  16. Paul Brown wrote:

    test a several non oiled pellets over a chrono then try it with lubricated ones dramatic power increase in Flbs & FPS readings in some cases more than double the energy, how can people say it has minimal effect?
    I must agree it seems better to keep the barrel dry

    Friday, January 13, 2012 at 2:21 pm | Permalink
  17. chris dieker wrote:

    I have a nitro venom dusk .177 and I have white smoke and bad smell every shot so far.(as in so much smoke you could be mislead the barrel is plugged on the other end not letting light in) I lubed after the 3rd shot and cleaning out the barrel til I got a white patch and continued to dry patch there after. The 3, 4, and 5th shot were louder and smoky. I haven’t lubed since then, and I get a loud crack every shot ,just assume it’s sound barrier penetration and muzzle pop.

    Sunday, August 17, 2014 at 12:20 pm | Permalink
  18. Jeff Rymer wrote:

    I am just a poor Mechanical Engineer, so my explanation may be ‘unworthy’.
    That said, a few points.
    First, when a gas is compressed in a relatively short period of time, ‘ignition’ can occur provided fuel is present.
    Second, the ‘combustion’ in the case of a ‘spring air gun’ would be during the ‘cocking phase’.
    Third, if such an event occurs, the increase in pressure would only be present for a very short period of time since the ‘event’ occurred during the ‘cocking’ phase and the ‘increase in pressure was due to ‘ignition of fuel’ (heat) and thus would cool too quickly to contribute to increased velocity when the trigger is pulled.

    Thursday, June 11, 2015 at 8:17 am | Permalink
  19. mje wrote:

    What you say is true for guns that compress air into a reservoir, but spring air guns compress air at the time of firing. Thus the pressure spike can contribute to the energy imparted to the pellet.

    Thursday, June 11, 2015 at 8:31 am | Permalink
  20. Steve Harrison wrote:

    If you are talking about unwanted natural dieseling with lubes I agree, will do damage eventually with lackluster results.
    As to force dieseling to achieve more kinetic energy and velocity it can be done safely if done right.
    In the 1960’s an experimental rifle, the HW EL54 Barakuda air rifle was designed to diesel by having an injector port where ether gas was injected before cocking the spring.
    Originally, the factory supplied leather seals would fail and the rifle was discontinued. However, with modern seals, the overly robust rifle using a lubricated ether gas (starting fluid) could achieve mythical results.
    Without the charge, the rifle shot a .22 lead pellet @ 850 fps. WITH the dieseling, up to 2000 fps was possible, but 1500 (with a heavy pellets) was the norm.
    The rifle is very hard to find and expect to pay a grand or more for one.
    I had one but alas sold it in the 90’s as I am a fool…..

    If you have heavy duty seals installed, and use heavier lead pellets, Take a can of starting fluid and after cocking, with the straw adapter, spray a 1/2 sec. spray in the piston chamber before closing the barrel and then rip one off…..you WILL be impressed. You now have a .22 firearm that is perfectly legal.
    Check out the Barakuda rifle for historical informational purposes.

    Friday, January 13, 2017 at 2:19 pm | Permalink
  21. mje wrote:

    Yours is a minority opinion when it comes to the Barakuda, I think. Seals weren’t the only problem. There were also broken springs. Are you familiar with the Daisy VR project?

    Friday, January 20, 2017 at 3:29 pm | Permalink
  22. Billy wrote:

    100 dollar beeman break barrel from Walmart both .22 and .17 I very seldom ever shoot dry I have put several hundred rounds by dieseling and the gun only seems to be improving, no problems at all.

    Friday, September 29, 2017 at 12:27 pm | Permalink
  23. mje wrote:

    It’s a good idea to chronograph and accuracy test to see if experiments like that help or hurt.

    Friday, September 29, 2017 at 5:58 pm | Permalink

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