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Firearms thermodynamic efficiency

Discussion in 'General Gun Discussions' started by PercyShelley, Mar 19, 2012.

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  1. PercyShelley

    PercyShelley Member

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    Some years ago I ran across this article:

    http://www.z-hat.com/Efficiency of the 300 Hawk.htm

    On the thermodynamic efficiency of firearms; that is, how much of the energy of the burning gunpowder gets converted into motion of the bullet.

    Does anyone know if the figures in this article are correct? They certainly pass the smell test; about 30% efficiency is close to what most internal combustion engines achieve, and a firearm is not so different from a piston engine.
     
  2. The Lone Haranguer

    The Lone Haranguer Member

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    It doesn't seem unreasonable. Perhaps a little on the high side, particularly with "overbore," very high velocity rifle cartridges that produce a huge amount of muzzle blast and flash, or some handgun cartridges from short barrels. Once the bullet has left the barrel, all that noise isn't doing anything to add additional "push" or speed to it.
     
  3. Owen

    Owen Moderator Emeritus

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    looks good to me.
     
  4. Loosedhorse

    Loosedhorse member

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    What would an 100% efficient rifle be like? Silent, cold barrel, cold bullet, 0 chamber pressure on bullet exit, no recoil.

    Not likely.

    This is a bit of a marketing gimmic. You can make any cartridge seem more efficient if you choose a powder charge to achieve the preferred chamber pressure quickly and minimize the pressure as the bullet exits. (Heck, you can also goose it a bit using moly.)

    To the extent that you mimic the "magic chamber pressure curve" that is illustrated, you will achieve similar "efficiency." I see no reason to believe that curve is uniquely available to one cartridge only.

    But yes; we all know that you pay a lot for a little extra speed. That is the magnum downside.
     
  5. Driftertank

    Driftertank Member

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    Read an article a while back where a guy charted "cartridge efficiency" by measuring velocity, energy, calculating "L factor," then comparing these numbers to grains of powder in a typical load. The most efficient cartridge he tested, IIRC, was the .35 Whelen, Ackley Improved.

    It was a good read, while his calculations were not all-inclusive, he pointed out that higher cartridge efficiency pays dividends in things like barrel life and cost of components when reloading.

    It also sort of vindicated my love of the .308, as it was the most efficient of the .30 caliber rounds. Before reading the article, i just knew it was a popular, flexible round that offered good performance for a lot of uses with less recoil than the magnums.
     
  6. brickeyee

    brickeyee Member

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    Energy efficiency is not the goal of a firearm, so who really cares?
     
  7. Owen

    Owen Moderator Emeritus

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    Thermodynamic efficiency is something that internal ballistics people pay lots of attention too.

    <mod hat> If you don't care about a topic, you can feel free to not post in a thread </mod hat>
     
  8. eye5600

    eye5600 Member

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    The gun designer cares. Better efficiency means more velocity with a given round.
     
  9. Old krow

    Old krow Member

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    Looks close to me, even then it may be a little on the high side in some cases. In this case I believe that it was a 24" barrel used. If the exact same bullet (round) that was shot out these guns was shot out of a 20" barrel, or a 22" barrel, it would be even less efficient.

    Reloaders/handloaders use at least some of this, whether they know it or not. The Ideal Gas Law (PV = nRT) still applies, it's why we don't seat bullets too deep or change powder charges when we change bullet weights. Some even go so far as to compare the amount of powder used to gain a given velocity to determine the relative "value" of a given powder. Speer Gold Dot "short barrel" ammo is a good example of trying to find an efficient sweet spot. Barrel life is another reason.

    Interesting read, thanks for posting that.
     
  10. brickeyee

    brickeyee Member

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    No, they care about meeting a performance goal.

    X ft/s at the muzzle within allowable pressure.

    The thermodynamic efficiency is not even on the list.

    If you even calculated the efficiency of an Otto cycle motor, you would understand that SOME things have efficiency concerns.

    Firearms are not one of them.
    Actually, since the gasses in the barrel are still undergoing chemical reactions it does NOT apply.

    To the extent the gases are doing work and increasing their volume as the bullet is driven down the barrel, Charles's law can be applied, but as the gases continue to react even it becomes less than perfect.

    Improvements in thermodynamic efficiency are not going to help a firearm very much.
    It is far more profitable to improve the powder and its burn curve through chemistry, and things like granule shape (not exactly chemistry, but related to how the powder burns).
     
  11. Shadow 7D

    Shadow 7D Member

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    Kinda, but according to Newton, the best you can hope to achieve is 50% (equal and opposite)
    So losing 20% to heat and friction isn't bad.

    It gets complicated, the gas is produced by a pressure dependent reaction (for speed) that is also influenced by the rapid increase in volume as the bullet moves down the barrel.
     
  12. Old krow

    Old krow Member

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    This will easily get off topic fast, so, I'll just say this, read some on Julian Hatcher, specifically "Hatcher's Notebook." They do indeed care about thermal efficiency and it is measured. The Army researched it quite a bit. And, Charles's law is a special case of the Ideal Gas Law. It does apply.

    On the OP. Hatcher measured the thermal efficiency of the Browning Machine Rifle and found a thermal efficiency of 29%, so I'd say that the article is on target. Other small arms ranged from 17% - 37%.

    Heat to cartridge case 4%
    Kinetic energy to bullet 29%
    Kinetic energy to gases 19%
    Heat to barrel 22%
    Heat to gases 19%
    Heat to bullet friction 7%

    That was the Browning Machine Rifle though and will vary.
     
  13. Shear_stress

    Shear_stress Member

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    I guess that's why they call guns "heaters."

    Still, they'd work even better if they didn't squander a 1/3 of their energy trying to fling a small chunk of lead down a narrow tube. Just like those stupid ceiling heaters that waste 10% of their energy making light for some reason.
     
  14. Loosedhorse

    Loosedhorse member

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    Not for energy. If the rifle is unimpeded, it will achieve a momentum equal to the bullet's momentum (and in the opposite direction), but the energy of the bullet will be much greater (in fact, by the same ratio that the rifle mass is greater than the bullet mass).

    And of course, if you impede the rifle (let's say bolt it rigidly into bedrock), then you effectively increase the mass of the rifle tremendously, so essentially all of the kinetic energy goes to the bullet.
     
  15. PercyShelley

    PercyShelley Member

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    Thanks all.

    If I had to guess, pistol caliber carbines are the most thermodynamically efficient firearms, since they have an enormously long barrel for the amount of powder they're burning. Magnum rifles, especially with shorter barrels, are probably the least efficient.
     
  16. Shadow 7D

    Shadow 7D Member

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    no, actually the barrel length doesn't add that much to it after a certain portion
    pistol powders burn fast and are designed for tossing lead out of 3" barrels.

    Rifle powders are for longer barrels
    a .22 will SLOW down after 22? inches
     
  17. PercyShelley

    PercyShelley Member

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    How fortunate are we in the twenty first century to have experimental data concerning just this matter, and available for free on the world wide intertubes!

    http://www.ballisticsbytheinch.com/


    Looks like even very high expansion ratio, low pressure cartridges like .45 ACP are still accelerating most loads past the 10" mark, albeit pretty marginally.

    The pattern appears to be that the light bullet weights see the biggest gains in the long barrels. It would be interesting to know the powder charges that they're sitting on.
     
    Last edited: Mar 21, 2012
  18. denton

    denton Member

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    PV=nRT does not apply. This equation only works for ideal gasses, which are monatomic. Propellant gasses do not meet this requirement.

    Efficiency is always (effectively employed energy)/(total energy).

    Energy thankfully does not split evenly between the bullet and the rifle. Otherwise, none of us would fire more than one shot per lifetime. The accepted formulation is that the mass of the rifle + cartridge system remains at rest unless acted on by an outside force, if the system is freely recoiling, which a shoulder fired rifle is not. So for a rifle free to slide on a frictionless surface, the center of mass would not move.

    Much of the lost energy disappears when the bullet uncorks the barrel. All the propellant gas that is still in the barrel is vented into the air, and the energy stored in it is lost to propulsion.
     
  19. Shadow 7D

    Shadow 7D Member

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    Wrong
    the way energy works in ballistics is by pressure
    see a PUNCH has more Energy (joules) than a bullet does, yet a bullet may go right through you due to exerting it's force over a SMALLER surface, but a punch can knock you down by exerting a MUCH STRONGER force over a much larger area.

    (and we have proven time and time again a bullet lacks the ability to knock you down short of causing you to collapse due to mechanical structural or neurological damage)

    Equal force is exerted, newtons law is correct, but your mass added to the gun is effected by it orders less than a bullet measured in GRAINS, now there are 7000 grains to the pound, want to do the math, and find out how many times MORE massive you, and your gun are than a bullet?
     
  20. denton

    denton Member

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    Shadow 7D, you're debating physics with a physicist. Energy does not split evenly between the bullet and the rifle... not even close.

    Here's a recoil calculator for you: http://www.handloads.com/calc/recoil.asp

    Enter a 180 grain bullet, and 2800 FPS as the muzzle velocity. Choose 57 grains of powder, and a 9 pound rifle and scope. Hit CALCULATE.

    The 18.87 foot lbs of energy shown in the results is the kinetic energy of the rifle if it is allowed to recoil freely (sliding on a frictionless surface). Yet the KE of the 180 grain bullet is in the neighborhood of 3,000 foot lbs. That's nowhere near an even split. It just doesn't work that way.

    Conservation of Momentum is a wonderful thing. That is the accepted method for analyzing firearm recoil.
     
  21. Loosedhorse

    Loosedhorse member

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    :D

    As I made exactly the same point about energy in post #14--perhaps Shadow has me on his "Ignore" list?--I appreciate your post, denton.
     
  22. Shadow 7D

    Shadow 7D Member

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    Yield, I'm more of a chemistry type of person, if I understand the principle I can usually see it in my head.
    And Loosed I took you off my list, I was just pretty annoyed that I was trying to agree with you for a page, and just didn't seem to get it, but bygones are bygones.
    As for the other guy, I'd have to go look at my list, he's still on it.
     
  23. Blackstone

    Blackstone Member

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    Very interesting read, will pass this on to some friends of mine
     
  24. Loosedhorse

    Loosedhorse member

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    Sincerely appreciated; no sarcasm. And no hard feelings here at all.
     
  25. Chemistry Guy

    Chemistry Guy Member

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    I don't know exactly how the 70%+ of the energy that doesn't go towards driving the bullet is dissipated, but my gut tells me that the vast majority of it is heat. Acoustic energy and PV work done by the expanding gas on the atmosphere are probably peanuts compared to the heat released both inside and outside the barrel. I would imagine that this type of research is also very important in the design of heat shielding, suppressors, barrels, and the like.
     
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