GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: shorty on September 26, 2018, 09:48:57 PM
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Can we discuss the losses further ? There's got to be something else besides these 6 items as most of them "to me" don't make sense to a 30% to 50% loss in potential energy
"1. Pressure drop in the reservoir or plenum (smaller volume reduces average pressure).
2. Energy used to accelerate the mass of air (or other gas) down the barrel.
3. Pressure drop due to the air cooling as it expands.
4. Sliding friction of the pellet in the bore.
5. Starting friction of the pellet in the bore.
6. Pressure drop due to wasted volume in the porting system between reservoir and pellet."
Are you sure we ain't missing something ?
#1 ) No plenum and reservoir ( way ) beyond barrel volume.
#2) Energy to accelerate the mass of air ( I have no idea what this means ) Dry firing maybe to measure air speed ?
#3) At approximately 30 psi, a 25.39 pellet (in 25 cal) can achieve 140 fps which is 1% of the a standard full charge of 3000 psi. What is the pressure drop at a given pressure/volume and does that pressure change vs temperature change at different room temp pressures.
#4) Dynamic COF ( never measured ) but, lubed pellets do not achieve a higher velocity ( I have never seen ).
#5) Static COF (never measured) but, most fluoropolymers such as PFPE greases/oils we use have an extremely low Static COF ( never seen any fps changes using them )
#6) Sure, makes sense only if not taking this volume into the equation for maximum theory power.
Not arguing or disputing. Somethings don't make sense ( common sense ) to me so I am asking for further explanations and possible examples with data to support it along with some possible experiments to prove or disprove.
This loss of power (since Bob shared how he figures it) bothers me to all heck. It's nice to see efficiency as Lloyd set up for us but the guns actual efficiency is getting really interesting to me and would like to see more on it.
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I thought I explained each item in the article.... but here goes (again)….
1. In a shot where the valve closes at the exact moment the pellet exits, the pressure drop can be calculated, using P1V1 = P2V2.... P1 is starting pressure, V1 is reservoir or plenum volume, V2 is V1 plus barrel volume, allowing you to calculate P2.... The average pressure is (P1+P2) / 2.... If the valve closes earlier, then you only add the "uncovered" barrel volume to get V2.... eg. if the valve closes when the pellet is halfway to the muzzle, then V2 = V1 + (barrel volume / 2)…. Remember that in the original formula, an infinite reservoir was assumed, so P2 = P1 (no pressure drop during the shot)…. In REAL PCPs, the pressure drops during the shot, and the smaller the volume, the greater the drop.... hence the need for plenums in regulated PCPs....
2. Air has mass, and HPA has a LOT of mass.... In maximum power shots, the air mass accelerated may be greater than the pellet mass.... This is explained in the article.... Let's say the air mass accelerated equals the pellet mass.... In that case, only half the maximum energy is available to accelerate the pellet.... Easy-Peasy....
3. If you don't understand the concept of Adiabatic Expansion, then Google it.... I'm not your Physics Prof.... When air expands, it cools, and the pressure drops more than if the temperature was constant.... Instead of using P1V1 = P2V2 (which is for Isothermal Expansion, eg. temperature constant), the equation is PV=nRT…. n is the number of moles of gas (basically the mass), R is the universal gas constant, and T is the temperature in Kelvin.... Only if all those are constant does P1V1=P2V2.... In Adiabatic expansion, the temperature drops, so the pressure drops even more than my answer in #1 above....
4. & 5. I don't have numbers for you for the sliding and starting friction (they vary with every pellet and barrel).... but surely you must admit that those EXIST, and that they rob energy from the pellet.... Let's say the force on the pellet from the air is 100 lbf…. and the sliding friction is 2 lbs.... Now you only have 98 lbf. left to accelerate the pellet.... Starting friction is greater than sliding friction, so at the very beginning of the shot, when the pellet just starts to move, there is less force than there would be without friction.... Again, the concept is Easy-Peasy....
6. Again, pretty simple concept.... If the plenum is 50 cc, and the volume between the valve seat and the base of the pellet is 1 cc.... then before the air reaches the pellet the pressure has dropped from the plenum pressure to (50-1)/50 of that.... Start with 2000 psi in the plenum, you can never have more than 1960 psi at the pellet....
As I stated in the article, the biggest loss is due to accelerating the air.... That is why Helium can provide more power, because its mass is only 4/29ths that of air.... Basically only 1/7th the loss due to accelerating the gas compared to air, so more of the potential energy ends up in the pellet.... In some cases (eg. a plenum that is way too small), that loss can be greater than the loss due to accelerating the gas, but it isn't really a "loss", because the energy was never there in the first place (smaller volume of HPA available to create the shot)…. However, compared to the "Maximum FPE" formula, which uses an infinite reservoir.... well then obviously it means you can never reach the maximum....
When I wrote that article, I had never heard of anyone achieving even 50% of the maximum FPE on air (70% on Helium can be done), which is why I call that my "lofty goal" formula.... Since I wrote it, Marco managed to hit about 60% on air.... My best is about 45% of the theoretical maximum when using a bullet travelling at about 950 fps (bullet weight = half the FPE)…. If you use a heavier bullet, travelling slower, the bullet is a greater percentage of the total (air plus bullet) mass.... and several people have exceeded 50% by doing that.... for single, maximum power shots, where the valve is open until the pellet leaves the muzzle.... This is a far cry from what is achieved in "normal" PCPs, which when tuned for efficiency only produce about 10-30% of the theoretical maximum.... For example, a .25 cal MRod has a theoretical maximum FPE of (0.25 x 0.25 x PI/4) x 3000 x 20/12 = 245 FPE.... My "lofty goal" would be half that, at 123 FPE…. A stock MRod does about 20% of that (49 FPE)….
HTHs....
Bob
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Thanks for taking the time AGAIN to explain further. Since your reply, I have been able to update my gun efficiency calculator properly as it was incorrect (it now looks even worse) and I can't believe how much loss there is. Thank you for your help.
I did come across a nice paper that nearly explains almost everything as you did but slightly different.One interesting thing to me is the pressure drop across the valve to predict exit velocity. Stating that the barrel pressure will never achieve the reservoir pressure.
That is an interesting statement to me. Along with how little air pressure is needed to move the projectile just like I stated above.
The diagram of the solenoid valve is also interesting (possible new inline PCP valve design). It helps visualize the opening and closing of the valve like a sine wave (which I have read here the discussions). Opening slow to a peak and then closing at the same rate.
When I now look at my gun efficiency calculator there is a clear observation of as the porting increases (throat and exit) that the gun efficiency goes up ( well duhh we know that) but, is the simple reason why is because the valve has shorten the sine wave since it does not have to open as far along with reducing the pressure drop across the valve ( what ever that is - flow and pressure across an orifice ).
Here's the paper:
http://www.iontrap.wabash.edu/publications/AJP_80_24_rohrbach_air_cannon.pdf (http://www.iontrap.wabash.edu/publications/AJP_80_24_rohrbach_air_cannon.pdf)
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The pressure at the bullet is indeed not full reservoir pressure, which is why Lloyd's spreadsheet (in addition to accounting for all 6 of the above losses) also has an "efficiency factor" which can be anything from 50-80%.... Smaller ports will see closer to the lower value, while bore-size porting closer to the upper one....
A plot of valve lift vs. dwell, while it looks a bit like a sine wave, is actually a parabola.... The lift is proportional to hammer energy and the dwell proportional to hammer momentum.... Since there is a flow limiting affect once the lift exceeds 1/4 the throat diameter (called the Curtain Effect*), if the valve opens more than that, the flow rate is "clipped", and the resulting flow rate starts looking more and more like a square wave....
*Curtain Area = Throat Diameter x PI x Lift.... while Throat Area = Throat Diameter squared x PI/4.... If we set Curtain Area = Throat Area we get the following....
D x PI x L = D x D x PI / 4.... D x PI x L = D x D x PI / 4.... L = D / 4
This means that the flow rate is limited by the throat area if the lift is greater than 1/4 of the throat diameter.... this is called the Curtain Effect or Curtain Limiting of a poppet valve....
(http://i378.photobucket.com/albums/oo221/rsterne/PCP%20Internal%20Ballistics/ValveFlow1_zps5a9bbe8d.jpg) (http://s378.photobucket.com/user/rsterne/media/PCP%20Internal%20Ballistics/ValveFlow1_zps5a9bbe8d.jpg.html)
Since there must be some choking of the flow occurring in the last part of the closing stages of the valve cycle, the descending part of that curve gets squared off even more, ie the flow drops very suddenly just before the valve closes completely.... This would be more abrupt at higher pellet (air) velocities (because the choking occurs sooner in the closing cycle)....
Lighter hammers, for the same "flow-through" of air (and in theory the same FPE) have more lift and less dwell, and therefore are more likely to have a "squarer" flow diagram.... This has been credited with the increased efficiency seen with lighter hammers PROVIDED you don't run in to a problem with the increased cocking force they require.... There is a contradiction, however, to the logic of square waves being better when we look at larger ports.... When the throat is larger, the valve can open further without the curtain area limiting (clipping) the flow.... However, when we use larger ports, we typically shorten the dwell (and hence the lift) that we use.... So, instead of using a longer "square" wave, we end up using a shorter parabolic flow diagram.... Regardless, larger ports and shorter dwell help to produce greater efficiency, regardless of why....
Bob
PS.... the paper you reference uses "shop air" pressure of up to 100 psig and produce velocities of under 350 fps.... The valve, as drawn, appears to be very restrictive.... I would suggest not a good fit compared to PCPs....
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So.....
It sounds like if the valve opens faster than normal and the poppet does not have to travel as far to be below the curtain, then a "gun efficiency" can be increased due to the pressure drop before the projectile begins to move. I think I said that and you are agreeing.
Is this possibly the "MAJOR" loss and not all those other things ?
I think/maybe ?
So it would appear to me, to get the best bang out of the buck would be to have an extremely fast opening valve rather than fast closing valve when looking for "gun efficiency".
Since I have read most of MJP's posts, he always uses some kind of balanced valve (piloted or traditional). And, I can only guess that your experiment involved a balanced valve as well to get you over the 50% mark.
Too me now, it sounds like the valve operation is way more important than the 6 items your addressing as a loss for a guns efficiency.
I say,
let's figure it out because I can't stand running a 40% efficiency at over 95 fpe at 3000psi using a 20" barrel with 43.2 grain projectiles.
I am gonna work on it anyway if your not interested but it sure would be great if you and other "smart guys/gals" with give some opinion so I don't feel like the only nerd.... ;D
Thanks for sharing Bob and taking the time to do what you do.
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Tim, the "major loss" is almost always the energy lost in accelerating the mass of air.... If the plenum is small, that can be just as important, or even more so…. As an example, let's use an MRod set up to shoot a 25.4 gr. pellet at 45 FPE on 3000 psi.... If you toggle off the air mass in Lloyds spreadsheet the FPE would increase to nearly 60 FPE.... In a higher powered application the loss is even greater.... Same gun, tuned to shoot a 34 gr. pellet at 70 FPE.... toggle off the air mass and the FPE would increase to 97 FPE.... Now add a 25 cc plenum into that same gun.... same 2000 psi pressure, 25.4 gr. pellet at 45 FPE.... Instead of 45 FPE, Lloyd's spreadsheet now predicts only 40 FPE....
Incidently, his spreadsheet, if you turn off ALL the losses above, predicts 244.4 FPE at 3000 psi with a 120 gr. bullet, just like my 245 FPE back of the napkin "maximum FPE" calculation.... If I turn on the losses, one at a time, here is the results.... I used a bullet weight of 1/2 the FPE in each case, so that we were always working at 950 fps....
25 cc plenum.... 189 FPE (loss of 55 FPE)
Include air mass.... 152 FPE (loss of 37FPE)
Cooling of air.... 139 FPE (loss of 13 FPE)
2 lbs. pellet drag and 5 lbs. starting friction.... 136 FPE (loss of 3 FPE)
1 cc of port volume.... 124 FPE (loss of 12 FPE)…. (port volume is particularly important with a small plenum)
What this means is that even if you could achieve 100% efficiency, instead of the 245 FPE theoretical maximum, the BEST you can expect is 124 FPE.... Gee, that looks pretty close to my "lofty goal" number of 123 FPE, doesn't it?.... How close have you come to 123 FPE with an Mrod at 3000 psi with a 20" barrel and a 62 gr. bullet?.... Now do you see why I call it my "lofty goal"?....
You may not like the math, and I know you have your own ideas.... All I can do is wish you luck in getting over 50% of the theoretical maximum power, and at the same time getting a usable shot count.... Incidently, at 3000 psi your 20" long, .25 cal barrel (volume of 16 cc) contains almost 60 gr. of air.... more than the weight of your pellet.... :o …. Does that give you some idea where a lot of the wasted energy is going?....
Bob
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Tim,
Think of it like a dragrace, you will win or loose on the first quarter of the track.
It is not how fast you can go but how quick you can get moving from stand still.
You need to get as much power as you can as fast as possible behind the projectile.
Lloyds pressure and release proved me that.
Marko
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Tim,
Think of it like a dragrace, you will win or loose on the first quarter of the track.
It is not how fast you can go but how quick you can get moving from stand still.
You need to get as much power as you can as fast as possible behind the projectile.
Lloyds pressure and release proved me that.
Marko
;D ;D Exactly ;D ;D
LAUNCH HARD !! ... shut it down and let momentum carry it down range.
While very hard on pellets because the skirts can get blown way out of shape and the distribution of weigh as well there shape upset. Bullets it does not mater.
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No question, most of the energy imparted happens in the first part of the bullet's travel down the bore.... That is why closing the valve when the bullet is halfway to the muzzle ends up getting about 97% of the velocity, and 94% of the FPE, from 50% of the air used (compared to closing the valve at the muzzle).... Close the valve at 25%, and you only use 1/4 the air, etc.etc…. but the gains in efficiency eventually drop off, and eventually the FPE/CI can drop off again....
These are two different kinds of efficiency though.... FPE/CI, which relates to shot count has very little to do with how much FPE you can produce in a maximum power shot, compared to the maximum possible for that combination of barrel volume and pressure....
Bob
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Great stuff and thank you all for entertaining my silliness.
I can only guess that Bob and Marco achieved over a 50 % gun efficiency ( The only 2 guys that seemed to have done this) by using some type of fully balanced valve system. (Please correct me if I am mistaken).
Open that valve up as fast as possible which means lighter hammer and fast as lightning spring to launch it (that's were Scott comes into play).
Apply these techniques to a standard valve and you get what I have (35% to 40%). Apply them together and gun efficiency goes over 50%.
I am not too much of a dumby but, it really makes me want to build or by a Cothran valve to begin verifying and proving out some stuff. The reason for the Cothran valve is that it appears as if it is extremely easy to crack open ( at pressures up to 3000psi) which means little or no loss in hammer energy while opening the valve.
I don't care about closing right now for shot string efficiency but, I now can't stop thinking about gun efficiency of lost potential power.
For some reason Bob, I can't put your equations for curtain effect into a spreadsheet so I can't run a trend line or chart to understand throat to stem area for maximum flow at a given lift. What I am looking for is to generate a trendline or chart that says if you run this stem size and this throat size, how much does the poppet have to open to be at the curtain or max lift to achieve max flow.
And,
If you don't open the valve to the curtain (efficiency-shot string wise) along with closing faster due to the shorter stroke of the valve, is there a balance of max potential power and shot string efficiency that we have not explored yet.
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I've never done over 50%, nor do I know of anyone except Marco that has when shooting bullets at 950+ fps.... Doug Noble has managed it with a Condor with heavy bullets (where the air mass is a smaller percentage of the total)…. All my powerful guns, so far, have used conventional valves.... The ONLY advantage at maximum power to a balanced valve is that it takes a lighter hammer strike to operate it.... which is why I eventually plan to retrofit my guns with it.... Cocking my Hayabusa .457 with a 1/2+ lb. hammer with 2" of travel and over 28 lbs. of cocking force ain't fun.... ::)
To get near the 50% "lofty goal", you are throwing "efficiency" out the window in the quest for maximum FPE.... It requires whacking the valve a ton, and holding it open until the bullet has left the muzzle.... This typically gives you less than 0.5 FPE/CI.... maybe a bit more with a long barrel.... As I said, if you can get over 50% of the maximum and at the same time get over 1.0 FPE/CI.... you will be in an area nobody has even come close to without using Helium....
You seem to have difficulty separating FPE/CI efficiency (which requires a short dwell, closing the valve when the pellet is 25% of the way down the barrel or less.... from a maximum power shot (valve open until pellet exits) measured as a percentage of the theoretical maximum.... They are not even remotely the same thing....
Bob
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This is starting to hurt my head. For some reason it is the only thing on my mind to understand and improve on. I believe that the 2 (fpe/ci) and maximum power (theory) go hand in hand.
No one said you have to run wide open if your gun is set up for theoretical maximum power.
I am starting to believe the "lofty goal" theory of 50%. Although, I do not think it is because of the 6 items listed as the loss. I have a feeling that something else restricts it that we are not calculating. I have no idea what it is but something is not making sense.
I made 2 tables based off of historical data and simple math for my .22 and .25 cal airguns. Each table can not achieve well over the 50% mark. Although, the smaller the caliber the less of the lofty goal can be achieved.
What's interesting when comparing other peoples valves in the same gun is that some people have higher or lower max "actual" FPE levels with the same porting. When I say porting, I mean valve exit to barrel and not throat.
Now this is interesting to me for a few reasons. The biggest reason why is because I am curious if the throat acts like a second plenum before the restrictive transfer port.
It's strange to me that my valve with a .25 throat and .22" porting to barrel has nearly the same power (within 5 fpe) of a war valve/Cothran valve/SS valve with .187"/.19" porting at the same pressure.
SO what gives ?
Full bore porting through out seems like the ticket, so why don't we always try for it.
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Tim, is that table actual results, or something you calculated?.... I ask because if you are achieving 120 FPE at 3000 psi in .25 cal with a 20" barrel, I haven't seen you post about it.... I would have though you would be shouting it from the rooftops....
I hope you find your elusive other factors that are preventing us (except Marco's high pressure, short barrel wonder) from reaching more than 50% of the theoretical maximum at 950+ fps.... When you figure out what they are, and quantify them, please share.... I also have no idea how you would approach even 1.0 FPE/CI at the same time you approach 50% of the theoretical maximum, let alone 2.0 FPE/CI at that power.... To me they are mutually exclusive.... However, I agree that bore-size porting is able to produce high FPE/CI (by using short dwell) as well as close to 50 of the theoretical maximum (at maximum dwell).... just not both at the same time.... I don't have anything else I feel I can contribute to this discussion....
Don't forget that in order to utilize bore-size porting you must retract the bolt face or probe so that there is no obstruction in the chamber.... and that the throat AREA (after you deduct the valve stem) must be bore area or preferably about 10% larger (to make up for the flow interference of the stem)…. To feed 0.22" porting, with a 1/8" valve stem, I would use more than a 1/4" throat (17/64"/0.266" is about right, that is what I use with 7/32" porting)…. Alternately, if you want to stay with a 1/4" throat, you could reduce the valve stem to 3/32" or about 0.090".... The same would apply to the bolt probe, to take best advantage of 0.22" porting in a .25 cal, your bolt probe should be only 3/32"....
Bob
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Deleted my original post.
Your right and thank you for kind answers.
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I hope this isnt off topic but I have ran many experiments with the SS valve compared to a conventional valve and I think I have some evidence(THINK) that shows the SS valve with its thimble does not produce a parabola. Ok comparing the two valves trying to make identical power at 3k Psi I have found that the SS valve lifts almost 1/3 higher than the stock valve with identical ports but uses only slightly more air 1.42 fpe/ci compared to 1.44 fpe/ci at the same power level and pressure. I believe the low holding force of the poppet allows the valve to come off fast and lift high and the Trapped 3k pressure in the poppet thimble drives it closed faster than a stock valve can. I believe getting anywhere near 50% efficiency or higher will require this type of valve action But i could be wrong. So the parabola ramp on a standard valve simply cannot lift and close fast enough to get the stored energy in the HPA to move the pellet up to speed fast enough but a high lift short dwell valve i think could be the answer to surpassing the 50% mark and with a few changes to the SS valve I think we could test this theory.
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The SS Valve is pretty dang efficient if you ask me. Thanks Travis!!!
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More than likely, the track of lift vs. drag is either a very HIGH parabola (which means clipping of the flow due to curtain effect is likely, squaring off the shape of the pulse)…. OR if as we suspect the valve is partially "blowing open", then the rise time to full open may be shorter and steeper....
I don't know how we could test this theory without some pretty fancy instrumentation, and actually plot the lift to dwell for several valve types.... The parabola is a theoretical shape anyways, based on the forces applied and the hammer velocity and mass.... It is really only a "discussion point" so that we can explain why changes to hammer mass or travel, or spring rate and preload, affect a valve like they do.... DomingoT showed in his calculations that the hammer may be hitting the valve stem multiple times just on the opening part of the cycle, so the whole idea of a parabola is probably inaccurate anyways.... What DOES apply is the idea that the bigger the area under the curve, the more air can flow, and that lift is proportional to hammer energy, while dwell to hammer momentum.... The SS valve curve is probably different, but still seems to follow that basic idea, IMO....
Realistically, on high powered PCPs like my 6mm and .257, I can't really see any significant difference in power or efficiency with the SS valve.... probably because I am running so close to wide open power.... What I really LIKE is how easy it is to open.... I can't wait to retrofit all the versions of my Hayabusa to get rid of the massive hammers and springs....
Bob
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My point is if you can get a valve to lift higher and close as fast as a standard valve you will always be ahead of the curve with power to efficiency so I think its worth perusing to see how high you can lift a valve with a short dwell to see how efficient one can get a valving system.
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Never stopped thinking about this and I like the way your thinking Travis.
It didn't dawn on me that the throat/valve stem may have a larger impact on MAX Gun (FPE).
To me (now), I would think that minimum full bore porting at the throat is critical and a valve that can open extremely fast (bigger the throat the better)
1) Poppet does not need to open as far
2) Throat now acts like a second HPA reservoir.
Getting the peak pressure faster and longer appears to be key.
I am curious to know if there is a pressure sensor fast enough that can be placed 1 inch after the barrel port. My gut tells me that the barrel pressure never reaches or will never reach HPA tank pressure due to it being an open dynamic system.
This is why I feel the MAX fpe theory is flawed with the listed restrictions. Now if we talk about max velocity, that's completely different but we are talking about max FPE.
Lately, I have been trying to understand how piloted and or QEV valves work. Pretty neat stuff but, I just do not have an understanding of it yet.
Using this thought process,
Now I think I know why it is so hard to get Max theory FPE out of a .17 cal vs using a .357 cal. I am pretty sure it's due to the volume of air can not flow at pressure through smaller orifices in an open system. Yes of course it can build to pressure if the projectile is being held back when the valve is open but that's not the case in our PCP's.
Thanks for continuing with the post as I think this is another holy grail to our PCP quest since the "shot count" efficiency stabilized around 2.0.
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Thanks Tim
on the .177 vs. .357 its mostly due to area as the .357 has much more area for the compressed air to push upon so this makes it vastly superior in using the available air column. I agree with your statement that all the losses are not accounted for in the formulas ive seen but these small percentages are likely irrelevant to the other listed losses. JMHO
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I am not seeing it the way you wrote.
I am seeing it as (with an open system-like a barrel) that the volume of air can not flow through as fast when comparing smaller to larger. Maybe the same thing.
Almost like how a small hole in a regulator works to refill the plenum. Huge droop and slow with small hole but less droop and maintained pressure with larger hole (like when coming off regulation).
There's something here that warrants investigation and all I can say is I am all in. I can't stand only having 35% to 40% of the guns potential power.
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Bob,
I am not doing this to be a thorn in your kidney. It didn't dawn on me to run "your" max theory fpe calc with a barrel length that can adjusted to the dwell of a valve as you posted.
The numbers still don't add up (worse) and now I am thinking that I am chasing my tail due to your calculation.
Believe me, I don't have the answers but I (and a lot of people) believe/trust in your calculations to advance our knowledge of PCP's. Maybe you can explain this table for us. I used your (max theory) calculation and did some fpe/fps back calculations.
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Your calculations for the maximum theoretical FPE (Col.1) are correct, and include the FPE given to the mass of the air, and all losses.... They of course won't work if the mass of the projectile (and included air) goes to zero (which is impossible), as the velocity (Col.2) would be infinite.... That is why the number in Col.1 is a THEORETICAL MAXIMUM.... Of course in the real world, the air has mass, which accounts for the largest loss in a conventional PCP.... To give some consistency to the results, you should hold the velocity constant (I use 950 fps, so that the pellet mass is half the FPE)…. This "evens out" the playing field so that the pellet weight is a relatively constant percentage of the total mass and energy.... This, and other losses, are taken into account in my "lofty goal" formula which is based on 50% of the theoretical maximum, at 950 fps.... For example, with your 10" barrel, half the max. would be 61 FPE, using a 30.5 gr. pellet at 950 fps.... If you use a lighter pellet, it will be harder to reach 61 FPE because the air mass becomes a larger percentage of the total mass (leaving less energy available for the pellet).... If you use a heavier pellet, it will be easier to reach 61 FPE because the air mass is less important....
The easiest way to think of this is hold the velocity constant at 950 fps, and vary the pellet weight to get the FPE.... Since the velocity is constant, then for a given barrel length, the time spent in the barrel is constant, but more pressure can accelerate a heavier pellet and produce more FPE.... As an extreme example, my 6mm, 28" barrel at 4200 psi reached 173 FPE with a 73.4 gr. bullet.... but when I used a 1.8 gr. airsoft BB, which reached 2092 fps, the FPE dropped to only 17.5 FPE (1/10th as much)…. because the mass of the air absorbed most of the energy of the shot.... The "lofty goal" for that gun is 227 FPE, but that would require a bullet weighing 114 gr.... and I still would not likely be able to reach that.... Most of my "high powered" PCPs reach only about 45% of the theoretical maximum with the appropriate bullet weight.... If I ever reach my 50% "lofty goal", I will be a VERY happy camper....
HTHs....
Bob
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My (.177 /24” barrel ) Condor_clone shots 13,43gr Monsters at 27 FPE buy using .22 transfer port from big air bottle reservoir at 1750 PSI .
My question is if the valve close before pellet is one half of barrel length.
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I need more information to give you a better answer.... I need the reservoir or plenum volume, and the efficiency (FPE/CI)….
Bob
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Bob,
0,45l bottle pressure fall from 200bar to 180bar after 10 shoots and 120 bar reservoir/plenum is about 10cm3 .
.........
Sorry ,
I probably made mistake. There was more shoots ( 20 ? ) . I need to check it again tommorow.
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Plenum is only 10 cc?.... About how long is the air path from the valve seat to the pellet base?.... and the approximate diameter of that?.... I am trying to get an idea of the "wasted volume" in the port system, that could have a big effect on performance with such a small plenum (big pressure drop)….
Bob
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Sorry,
I have measured the plenum today but from outside only and it seams that it is about 20cm3. If I calculate energy myself/EU whit ideal average 12Mpa and pellet career 0,3m the energy should be 3,14*2,25mm*2,25mm*12Mpa*0,3m= 57Jole/42FPE so the valve should to close a little bit sooner and/or air transfer way from valve up to pellet (3,14 *0,35cm*0,35cm*5cm = 2cm3) is dropping energy to 50J/37FPE.
..........
I have checked number of shoots again and calculate efficiency .
Air pressure at 0,45L bottle dropped from 2320PSI to 2030PSI by shooting 19*37FPE shots.
Internet calculator tell me efficiency 0,9 FPE/cuin .
Why the result is at cuin when I have putted cc ?
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Your 42 FPE calculation for the "lofty goal" is very unlikely to be reached with a small plenum volume and a large transfer port system.... For example, the highest pressure the pellet can see, with a 20 cc plenum and a 2 cc "wasted volume" is not 12 MPa, but only (20 / 22) x 12 = 10.9 MPa…. In addition, the "lofty goal" calculation is for a "dump shot".... ie the maximum you might see when the valve stays open until the pellet leaves the muzzle.... even then, you are unlikely to achieve it....
I will run your numbers later today and see if I can come up with an estimate of when the valve is closing.... but I can comment right away that an efficiency of 1.86 FPE/CI indicates the valve is closing VERY early in the shot cycle, when the pellet is less than 20-25% of the distance to the muzzle....
Bob
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I again made mistake se previous post.
soooory
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For your efficiency I get the following calculation....
0.45 L = 27.4 CI.... (2320 - 2030) = 290 psi = 20 bar.... Air used is (27.4 x 20) = 548 CI total
19 shots @ 27 FPE = 513 FPE total.... Therefore efficiency is (513 / 548) = 0.94 FPE/CI....
Lloyd's internal ballistics spreadsheet tells me that your valve is closing when the pellet is about 1/3 - 1/2 of the way from breech to muzzle.... If that is the case, you should be operating on the "knee" of the curve, and the plateau velocity would be just over 1000 fps.... Have you ever increased the hammer strike to find the plateau?....
Bob
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First calculate for pressure drop during shot. For you its (2320-2030)/19 * (450/20) = 343 psi. Divide that by half and subtract from initial shot pressure to obtain average. Which is 1578. Then calculate pressure drop from transfer volume. Using .75 cc I get 20/20.75 * 1578 and you get 1520 psi. Re run the lofty goal calculation and you get 37 fpe. Hope this helps.
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Mike, I'm trying to follow your calculation, so bear with me.... The 343 psi is the total pressure drop in the plenum during a shot, correct?.... It starts at 1750 psi and ends at (1750 - 343) = 1407 psi so the average is 1578 psi.... However, at the pellet, the pressure is lower because of the transfer port volume (I agree, including the chamber volume, 0.75 cc is a reasonable estimate, the OP used diameter, not radius).... That results in a starting pressure at the pellet of (20 / 20.75) x 1750 = 1687 psi.... The pressure at the end of the pellet travel must include the barrel volume exposed by the receding pellet.... That pressure is 1407 psi.... Therefore the pressure seen by the pellet is the average of those 2 pressures, which is 1547 psi.... Other than that, I like your reasoning....
You are correct that the average pressure during the shot is the correct one to use for calculating the "lofty goal" FPE.... Using 1547 psi, we get (0.177 x 0.177 x PI/4 x 24 x 1547) / 24 = 38.0 FPE.... However, that would be for "bore size" porting, and will not be achieved, even with a dump shot, with smaller ports.... The ports in this gun are 0.138" (3.5 mm) and you can get an approximation of the likely FPE by multiplying the lofty goal FPE by (0.138 / 0.177) = 29.6 FPE.... This is quite close to what skorec has achieved....
We can approximate the pellet position at valve close by using the pressure at the end of the shot, which is 1407 psi.... If the expansion is isothermal (which it isn't), the total volume when the valve closes would be (1750 / 1407) x 20 = 24.9 cc.... However, the plenum plus porting system is 20.75 cc, meaning that (24.9 - 20.75) = 4.15 cc of that is the barrel volume behind the pellet.... The volume of the barrel is (.177 x .177 x PI/4) x 24 x 16.4 = 9.7 cc.... Therefore the pellet must be at least (4.15 / 9.7) = 43% of the way from breech to muzzle.... The reason I say "at least" is that assumes equal pressure throughout the system when the valve closes, which is not the case, there is always some pressure gradient in the barrel or the air could not flow.... also the expansion is partly adiabatic, not isothermal.... and both those factors would move the pellet closer to the muzzle at valve close.... However, it provides an interesting, if approximate, confirmation for Lloyd's spreadsheet....
I guess this method could also provide an interesting approximation of the pressure drop in the plenum that would occur if the valve closed exactly at pellet exit.... If we add the barrel volume to the plenum and transfer port volume we get (9.7 + 0.75 + 20) = 30.45 cc.... At the instant the pellet exits, with a "dump shot" the plenum pressure would be (20 / 30.45) x 1750 = 1150 psi.... That (1750 - 1150) = 600 psi drop in pressure would translate to a pressure drop of (20 / 450) x 600 = 26.7 psi in the bottle.... This compares to the current pressure drop of 15.3 psi for the gun as tuned.... If the pressure drop in the bottle exceeds about 30 psi per shot, then we can assume the pellet has left the muzzle before the valve closed.... Wasted air and a loud report would result....
These numbers are only approximations, of course.... because the expansion is likely closer to adiabatic than isothermal, and there must be some pressure gradient in the barrel.... but it provides another way of looking at what is happening.... Thanks for making me look at this from a different perspective.... always nice to learn something new first thing in the morning.... 8)
Bob
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Ufff
I try to use now only metric system whit Mike magic value 1520PSI
If I have putted one shot average pressure 1520PSI=10,5Mpa to my calculation 3,14*2,25mm*2,25mm*10,5Mpa*0,3m=50J=37FPE answer is 0,3m .
or
If I have putted one shot average pressure 1547PSI=10,7Mpa to my calculation 3,14*2,25mm*2,25mm*10,7Mpa*0,29m=50J=37FPE answer is 0,29m .
My barrel has 0,635m.
Mike, Bob Is it correct ?
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I am confused.... In one place you said 27 FPE and in another 37 FPE.... For 19 shots at 27 FPE, the efficiency is 0.94 FPE/CI.... For 19 shots at 37 FPE it is much higher, 1.29 FPE/CI....
In addition, your calculation using the barrel length gives the MAXIMUM theoretical FPE, and the "lofty goal" is only half that.... This is why your calculations show a barrel length of only half what you have.... Does that make sense?....
Bob
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Ufff my next mistakes . Really too much. :(
YES 27 FPE is my ( Monster/13,4gr/950FPS lowest spring preload ) reality. Also internet calculator told me 0,93 FPE/cuin
http://www.calc.sikes.us/1/index.php (http://www.calc.sikes.us/1/index.php)
If I put one shot average pressure 1547PSI=10,7Mpa to my calculation 3,14*2,25*2,25*10,7*0,22=37J=27FPE answer is 2*0,22m=44cm . My barrel is only 63,5cm.
May be it is the reason why my China Condor valve is too noisy. Much louder then SS balance valve in my Puncher at the same pover.
Thanks for your time.
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I think you are looking at the problem the wrong way.... The "lofty goal" for your gun, with full bore area porting (ie 0.177" from valve to pellet), and maximum power (a dump shot) would be (0.177 x 0.177 x PI/4 x 1547 x 24) / 24 = 38.0 FPE.... However, your ports are only 0.138", so you can reduce that ESTIMATE to (0.138 / 0.177) x 38.0 = 29.6 FPE.... You are achieving 27 FPE, which is 91% of what you might reasonably expect.... IMO that is pretty good....
If you cranked the preload to get to the plateau, and managed to achieve that 29.6 FPE, you would likely be using about 26.7 psi or more per shot from the bottle, and the efficiency would be a dismal 0.58 FPE/CI or less.... That is why we don't tune guns to operate on the plateau.... and why the "lofty goal" is something you very likely will not achieve but you don't care anyways, because you would never tune a gun like that....
Bob
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Bob,
I use next Airforce Condor valve and the hole/port is 5,5mm.
Unfortunatelly I do not understand ( 0.177inch x 0.177inc x PI / 4 x 1547psi x 24) / 24 = 38.0 FPE
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That is my "lofty goal" formula in Imperial units.... It is developed from the basic formula for work (energy)…. Energy in (ft.lb), abbreviated as (FPE) comes from the force applied and the distance it acts through....
Energy (ft.lb) = Force (lbs) x Distance (ft)
Force (lbs) = Area (sq.in) x Pressure (psi) …. and Area (sq.in) = Bore (in) ^2 x PI / 4 = Bore (in) x Bore (in) x 3.14 / 4 …. This is the same as radius ^2 x PI
Distance (ft) = Barrel length (in) / 12
Therefore Maximum Possible FPE (infinite reservoir, constant pressure, no friction or other losses) for your gun can be found from....
Max FPE = (0.177 x 0.177 x 3.14 / 4) x 1547 x 24 / 12 = 76 FPE.... However, the are many large losses, the greatest of which is that part of that energy must be used to accelerate the mass of the air itself.... and air under pressure is surprisingly heave.... HPA at 3000 psi has a mass of 60 gr/CI.... The mass of the air can be as much or even more than the mass of the bullet in some cases....
Therefore, for my "lofty goal" calculation, I use HALF of the formula for the maximum, so the divisor becomes 24 instead of 12 (in/ft)…. For your gun the "lofty goal" would be....
Lofty Goal = (0.177 x 0.177 x PI/4) x 1547 psi x 24" / 24 = 38.0 FPE
You can do the same thing in Metric if you watch your units.... which you seem to have done by using the bore in (mm), the pressure in (MPa), and the barrel length in (M)…. with the result being the energy in Joules.... You can either use the caliber (squared) and multiply by PI/4 = 0.785.... or you can use the radius of the bore (squared) and multiply by PI (3.14)…. Both formulas give the correct bore area in (sq.mm)….
Bob
PS, the effective port can never be larger than the caliber.... However, if ALL the ports before the pellet are larger than 4.5 mm (0.177") then your lofty goal would be 38.0 FPE.... Currently your ports are smaller, so the goal goes down because of that....
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Thanks for explanation I didn`t know that:
1. 24”/12 = 2ft = full length of barrel
2. Mass of pressure is similar as mass of pellet
Yes I need to do serious inspection of next item.
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Is that a Condor clone valve stem?.... The airflow is hardly a "straight line" like people say, is it?.... *LOL*…. How big is the hole that the 3mm stem sits in.... That could govern the flow area.... I'm not that familiar with Condor valves....
Bob
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I have one Kral Arms Puncher ONE and some China Condor clones.
On Krall the ODs are 8mm,12mm, 6mm(ID probably 5mm ? ) and 3mm . Holes are 5,5mm and 2*4mm similar to these .22
https://www.gatewaytoairguns.org/GTA/index.php?topic=63857.40 (https://www.gatewaytoairguns.org/GTA/index.php?topic=63857.40)
China looks different.
The air flow seams to be semi strait line but the air transfer way up to pellet is probably two times longer then at standard PCPs.
I have bough it mainly because hammer striking is barrel centrically but I have found out that charging the pellets whit my big fingers is really not easy.
I am shooting whit it :
1. 13,4gr Monsters whit lowest spring preloading and reg120 bar at 955 FPS by using Weihrauch choked barrel
and
2. 21grPiledrivers whit highest spring preloading and reg180 bar at 945 FPS by using Poly LW NCH barrel.
I don`t understand now why I was wanting to calculate valve dwell directly from pellet Energy. Sorry again.