12 replies to this topic

[Curt44319]

A question that's been dogging me lately.
In my country, the "power" of an airgun is legally not limited, so this can be applied several ways.
A standard CO2 powerlet contains 12 grams CO2. Fine.
Some guns get a few shots, and some get many shots.
Is it not possible to know the weight of the pellet, and the muzzle velocity, thereby giving
some number of foot pounds, and by extension the number of shots at what power per shot
is theoretically possible from a given amount of CO2 ?
Ergo, how many foot pounds are contained per gram of CO2 ?
This can be extended to PCP also, but for now my personal interest is in CO2.

Oh, howdy. First time poster. yadda yadda, etc. etc.

[rikthornton]

Your question is an interesting one, but (in my opinion) is not one with as simple an answer as X quantity of CO2 contains Y amount of energy - there are other factors to consider, barrel length for one...

Short tubes are inefficient, as the pellet has left the barrel before the gas has fully expanded - conversely, long tubes can be a drain on energy as the gas will have fully expanded and the pellet is then being slowed by friction of the rifling.

A friend of mine with considerable experience on all things airgun also holds that the .22 can be more efficient than .177 as the larger caliber provides more volume for a given length than the smaller. This sort of thing can promote longstanding arguments though, my apologies if one ensues.

Edited by rikthornton, 28 February 2012 - 04:15 PM.

[Curt44319]

Methinks it is a simple question, and the answer also simple, HOWEVER.......
The equation, and all of the variables needed to solve the equation, not so simple, and also why I pose the question.
It may be that one caliber uses gas more efficiently than another. It's very true that there is an optimum barrel length, but I know not if that optimum length changes with velocity, just as barrel length changes velocity. ( it may )
Any and every barrel will have friction with the pellet, and so consume energy, dissipate power, in that friction.
Still, no barrel will have no friction, and so dissipate no power, but also provide no acceleration at all, hence no velocity, and so no muzzle energy. In such case, power still exists, dissipated as dispersal of the propellant gas, and noise only.
Still, with some optimized barrel length, at some optimized velocity, and some optimized pellet weight, there will be a maximum total foot pounds able to be developed from a powerlet, or some other defined amount of CO2.
I can see, taken to extreme, a needle with a great deal of friction area, yet virtually no mass, and no base area to push against, should be incredibly inefficient. Still, a 3 kilometer wide base area, providing a great deal of surface to push against, just seems wrong. There are diminishing returns, to be sure.
We may be lead toward the largest slowest caliber possible. I hope not.
( I aspire to a .38 caliber CO2 pistol, but that may be modified by this discussion )
Since I pose an academic question, there should be no arguments, but possibly some spirited data mining. ;)

Still, if the book with my Crosman 454 ( BB ) is believed, ( 325+ FPS and "more than" 60 shots ) suggests "more than" around 75 total foot pounds in a powerlet.
If a post elsewhere on this forum is to be believed, ( 25 shots at 8 foot pounds each ) then there are some 200 potential foot pounds in a powerlet.
From this we can conclude that the 454 makes a very inefficient use of the available energy, but kinda sorta sets a floor value.
I'd assume there is at least a potential of 75 foot pounds, or slightly more than the legal limit in GB for about 6 shots, at a minimum, and something less than the legal limit for 7 shots, to an empty powerlet at the same total overall efficiency.
Great ! Now, what's the maximum anyone has ever gotten, and we can probably extrapolate a reasonable usable number, as well as get a good feel for where whatever we have falls within the range.
My suspicion, and my motivation in posing the question, is that there is a good deal more to exploit in CO2 than many of us ( OK, me ) realize.
Your ponderings are appreciated.

[rich]

With CO2 power, the potential work done comes from evaporating the liquid CO2 into the gas phase. If the latent heat of vaporisation is 574 kj/kg then a 12g cartridge "contains" 574 * 12 joules, which comes to say 7,000.

If you get 70 shots from a cartridge at something under 7 joules per shot (to stay legal with a pistol) the useful work is nearer 500 joules, about 7 % efficiency.

Does that help?

[Curt44319]

It helps tremendously. Thanks.
It also stuns !
It seems incredulous that so much potential energy is somehow lost in conversion inefficiencies.
I understand it takes a certain amount of energy to simply push the gas through the powerlet cap.
I understand it takes a certain amount to get through the valve, and ports, into the breech.
I understand some is wasted in muzzle blast. ( optimum barrel length minimizes this one )
I Understand some is lost in barrel friction.
Discovered just this morning, a good deal is wasted in my 454 blowing the magazine
BB's back against their spring, with considerable force as it hit my finger. ;)
( the 454 is not a potential mod piece )
If I did my conversion right, there's nearly 5000 foot pounds potentially in a powerlet.
That's 416 shots at the rifle limit, 833 shots at the pistol limit, at 100% efficiency, which is
obviously not attainable, but even with a despicable 25% efficiency, ( 104 shots rifle and
208 pistol ) it seems there is yet much to be gained in careful research and engineering.

Still reeling from the realizations......

[Curt44319]

It helps tremendously. Thanks.
It also stuns !

//snip//

If I did my conversion right, there's nearly 5000 foot pounds potentially in a powerlet.
That's 416 shots at the rifle limit, 833 shots at the pistol limit, at 100% efficiency, which is
obviously not attainable, but even with a despicable 25% efficiency, ( 104 shots rifle and
208 pistol ) it seems there is yet much to be gained in careful research and engineering.

A little more research.....
My friend ( a PHD chemist ) suggests that this potential energy available in CO2 really isn't,
until quite a bit of heat is added. At least enough to vaporize all of the CO2, and get it into
the "ideal gas" behavior region, at *considerably* more PSI than is practical.

So, using my 454 again, it would appear that something like 84 potential foot pounds
worth of CO2 *is* expended per shot, and that if that quantity were heated enough, the
full 84 foot pounds *would* be expended.

The loss in pushing the gas through the valve, ports, potential loss to cooling as the
gas expands, and such, would be higher, but although the loss would be higher,
there would be considerably more TO lose, while still leaving more available for
the pellet than is available in total at room temperature.

His guess is that we're getting something in the 90+ percentages of what
is likely available at human survivable temperatures and heat flows in the
highest efficiency CO2 guns available off the shelf.

*I* certainly learned a few things.

[magicniner]

if that quantity were heated enough

You'd be adding a huge amount of energy to the system that isn't in your initial calculations rendering them totally invalid!

[Adam77K]

Very interesting question.

If it helps, here is some anecdotal data from my experience with real world CO2. I get between 60 and 80 shots from a powerlet in an Umarex type auto replica, at around 2-2.5 ft.lbs, with pellets from a short .177 barrel, with losses around the magazine. These being basically a revolver design. I get about 40-50 shots from my Umarex s&W 586, 6" barrel at a very slightly higher power. With my Umarex Winchester rifle which takes 2 powerlets, when I used one empty and one full powerlet thereby charging the system with only 12g of C02, I got at least 40 shots at 6 ft.lbs. The gas path on these rifles is very long, which clearly must hit efficiency somewhat, but the long barrel clearly helps a lot here.

[Jon]

Not ignoring the powerlet price even when buying in bulk.
Most pistols 25 to 40 shots pushing to extremes. May well get 70 or 80 shots but at least 40 useless being way underpowered.

For me CO2 is a dead loss, power dependant upon how warm. Ok for 10M where temp is stable.
Rather than reg them, constant temp better. How much would that add wrapping heat coils round the container with some form of regulated heat output.

[Shootist]

Not ignoring the powerlet price even when buying in bulk.
Most pistols 25 to 40 shots pushing to extremes. May well get 70 or 80 shots but at least 40 useless being way underpowered.

For me CO2 is a dead loss, power dependant upon how warm. Ok for 10M where temp is stable.
Rather than reg them, constant temp better. How much would that add wrapping heat coils round the container with some form of regulated heat output.

That last bit sounds like a good idea, and with rechargeable batteries in the stock maybe not too difficult. Fudge a solid state thermostat in the right place?

[gedfinn]

A question that's been dogging me lately.
In my country, the "power" of an airgun is legally not limited, so this can be applied several ways.
A standard CO2 powerlet contains 12 grams CO2. Fine.
Some guns get a few shots, and some get many shots.
Is it not possible to know the weight of the pellet, and the muzzle velocity, thereby giving
some number of foot pounds, and by extension the number of shots at what power per shot
is theoretically possible from a given amount of CO2 ?
Ergo, how many foot pounds are contained per gram of CO2 ?
This can be extended to PCP also, but for now my personal interest is in CO2.

Oh, howdy. First time poster. yadda yadda, etc. etc.

What do you think of this !
http://www.network54...from 12 gr. CO2

[magicniner]

At least the information is internally consistant and may not lead to a loss of forum access ;-)

[yana]

CO2 isnt very suitable for high powers really. Its at its best at matchlevels, 7.5J/6Ftp imo.
More power and co2 looses its inherent capability to self-adjust to pressure changes etc (keeping the co2 constant by boiling off).
hence make it less consistent over less shots,and give less shots overall.
So for 12Ftp power, of more, take a springer or pcp.

And yés, I ám a co2 fan, especially 12gr bulbs, but for 7.5J use only.