Cylindrical valve mechanism for regulated rifle?

[rsterne]

Solving the inefficiency of dump valves compared to timed valves which close before the pellet has travelled less than half the length of the barrel is the greatest problem.... With a valve of 50% of the barrel volume, if you dump it you use all the air in the valve.... If you close it when the pellet reaches the muzzle, you conserve 1/3rd of the air in the valve with NO loss of power.... If you close it when the pellet is only half way down the bore, you save 1/2 the air in the valve and lose only a few fps in velocity.... Close it even sooner, you save even more air.... but the velocity will start to drop off....

Bob

[Bill G]

Excellent summarization Bob.  Your graph really brought it home for me.  Above 50% barrel volume cant really be justified by power gains IMO.

Bill

[TimmyMac1]

Balanced chamber quick exhaust valve that uses a spring to upest the ballance.  pressur released fom backside of piston allows piston to move rearward thus removing poppet from flow path.  this allows contained pressure to evacuate bore obstruction violently.  work s like a dump valve and will require a shut off from reservoir during the cycle. 

Bingo. Certainly the most elegant of available solutions in my mind. Simple is always easier to make.
Airgun Designs Auto Mag Painball pistol worked on this concept. I saw a household drain cleaner that was a pump up that worked that way as well. The Shimel Air Pistol used that concept as well with manual charging to eliminate flow during the firing cycle. You could also control the flow during firing cycle with a small restrictor.

TimmyMac1

[Bill G]

Tim, I'm trying to get the shut off to be part of the cycle.  I think I have a concept that will work. The system requires pressure regulation for consistancy, but Ive realized that the system doesn't care if the piston is smaller.  It only requires a dump plenum at needed pressure, that is adiquate for the desired power output.  A smaller piston only increases efficiency and reduces applied forces.  This will make it a safe build by reducing component stress.  This concept when applied to Big Bore will allow hammer spring and hammer to be tiny in comparison to traditional valve works.

Bill   

[rsterne]

IMO, if you can't get the valve to close before the pellet is half way down the barrel (ie in about 1 mSec.) you are just wasting air.... or are doomed to operate with a very small valve to barrel ratio, and hence low power....

Bob

[PakProtector]

Napier Sabre, Bristol Centaurus and a few others ran sleeve valves. By the End of WWII the big poppet valve radials were giving them a run for their money and jets were about to deliver a knock-out. No way to get anything resembling a reasonable seal at the pressures you'd need for a PCP. While the Sabre was capable of significantly better breathing at high rpm, their 'junk box' would no way stand the effective rpm as defined by a PCP firing event...lol
cheers,
Douglas

[Bill G]

I may be posting in this thread incorrectly.  Just dawned on me that the concept initiated was for a ROTATING cylindrical valve and Im thinking in a balance piston type. 

I see your point Bob.  The theory is to find the combination of psi and volme to produce desired energy output.  For the most shot count with highest energy, I have found that my model matches what you had published about dump valves.  50% of barrel volume is the reasonable maximum. Yes we can get more fpe with highe ration of barrel volume, but the gains arereally small for the losses in efficiency.  I have modeled  a .45cal 230gr at 900fps using a dump chamber of 3.125ci at 2400psi. with a 22ci reservoir, this gives 5 shots at 906 -889 fps.  1.8% spread at 400+fpe for 1.59 efficiency.  FPS and shot count aren't exceptional but the efficiency seems to be a tad better than when I modle the traditional valve.  with a dump valve the valve is open the entire time, so my thought was to use as little as possible, at as low psi as possible, as long as it achieved the desired fpe range.  Then of course, we can change bullet mass, barrle legth ect and every thing else changes.

Bill

[rsterne]

Interesting model results, I ran those numbers in Lloyd's spreadsheet, using a barrel volume of 6.25 CI, which in .45 cal works out to 39.3".... and it requires 75% efficiency to get to 400 FPE when dumping 3.125 CI at 2400 psi.... That's high, but not impossible, although I can't imagine using a rifle with a 39.3" barrel.... and going shorter will require higher efficiency to reach 400 FPE, which is unlikely to be achieved.... Also, I don't see how you can get 5 shots of 3.125 CI with a setpoint of 2400 psi, as that works out to 5 x 3.125 = 15.6 CI x 166 bar = 2590 CI, and for a 22 CI reservoir to deliver that much air without dropping below 2400 psi (at which point it still contains 22 x 166 = 3650 CI), it would have to contain 3650 + 2590 = 6240 CI when filled, which means filling to about 300 bar, but perhaps that is what you are considering....

You are dumping 3.125 CI of air at 2400/14.5 = 166 bar x 3.125 CI = 517 CI of air per shot, producing 400 FPE, so the efficiency is 400/517 = 0.77 FPE/CI.... so you need to take a look at your efficiency number.... If you used a timed valve, closing it when the bullet left the muzzle, you get the same power and the efficiency jumps to 1.16 FPE/CI.... Closing it at 50% of the barrel travel, then you would still get ~ 385 FPE of energy and the efficiency would increase to 1.49 FPE/CI with no other changes.... You can see how inefficient a dump valve is compared to a timed one....

Bob

[Bill G]

I'm going to have to look into this.  My model was originally done for timed valves and I have made a copy that is SUPPOSED to take the  inputs and convert them to regulated out puts.  I think that I may be missing some element.  The idea is to have a known dump volume,  that is adequate to propel the desired bullet, at desired velocity and try to maintain that condition with a reg. Same as a timed valve.  But, as you know, a timed valve has to have a plenum that is sufficient to keep psi drop across the valve acceptable to produce the power desired.  As i see it, timed valve uses ever changing volume under ever changing psi, unless reg is employed.  Then you use same volume at the same psi until equilibrium is met between the reg and the reservoir.  under this condition, the dose of air from the plenum is modulated by poppet duration.  What I want to do is do away with the valve duration aspect of the shot.  I want to take 50% or less of the barrel volume and have it filled to the required psi for desired performance.  when the valve actuates, I want the reservoir feed to be interrupted until the dump volume(50% or less of barrel vol) is expended. once dump is expended, valve returns to home position allowing regulated flow to return and fill dump volume.  This should result in a drastic and sudden plummet of velocity once equilibrium is reached.  The now, lower psi, isn't adequate to produce the energy level desired and volume is static. Flow can't change to make up for the lack of psi, like a unregulated timed valve. 

The issue may be in converting the losses from the reservoir accurately.  Every shot at the known volume and psi of the dump chamber, degrades volume in the reservoir.  Perhaps this is where i'm failing.  Another possible point of contention is the break away friction of a lead bullet of that size, I have no real world measurement of this. I simply used 20lbs, If it is more, then obviously it will require either more psi or more volume and maybe both.  That will shrink thing quite a bit. 

Thanks for the input Bob

Bill     

[rsterne]

The first half of your post is a perfect description of what I think of as a dump valve.... filled after the shot cycle from a regulated chamber.... The air used is very simple, it is the volume of the chamber times the pressure, in the example you gave 3.125 x 166 = 517 CI.... I understand the desire to stay away from the duration aspect of the shot, but it is that which leads us to wasting a lot of the air.... With a valve having 50% of barrel volume, at the instant the pellet leaves the muzzle the air contained in the valve has expanded to fill the barrel (ie threefold), so your 2400 psi is at that moment 800.... The valve still contains 517 / 3 = 172 CI of air, which ends up exiting the barrel after the pellet has gone, contributing nothing to the shot.... If you closed the valve at that instant, capturing that 172 CI of air, instead of using the entire 517 CI for the shot, you would only use 345 CI.... without changing the muzzle velocity 1 fps.... However, your efficiency would go up from 400 / 517 = 0.77 FPE/CI to 400 / 345 = 1.16 FPE/CI.... THAT is the problem with dump valves.... Incidently, I was using a much lower breakaway and sliding force, increasing that to 20 lbs. would require even higher efficiencies to balance Lloyd's spreadsheet....

Bob

[Bill G]

So...... if I were to reduce the volume of the dump chamber, there would still be the issue of 1/3 that volume still being in the chamber when the bullet exits the bore. This results in lower fps.  I see.  To get the velocity I would have to have a 517 plenum so to speak and close it when 172 was still in the chamber. The question is then, how do i close the valve right before the bullet exits?  Well that doesn't get away from duration at all does it?   Thanks again Bob.

[rsterne]

If you made the dump valve smaller, relative to the bore, you waste less air, but have less power.... If it was 25% of the barrel volume, at the moment the bullet exits the air would have expanded 5 times and you would be wasting only 20%.... but the pressure drops faster during the shot, so you have less power (but you are only using half the air).... I don't see any way to get the kind of efficiency we expect/desire with a dump valve.... and once you close it at the moment the pellet exits, why not close it when the pellet has only gone 50% of the way down the barrel, lose only a few fps and get a big jump in efficiency....

Bob

[Bill G]

Agreed.  Originally when I started working on this valve design my intent was to make a valve for big bore guns that could be opened with the same force as is used to operate a .177 pcp valve.  My oversight lead me down a false path of efficiency and my attention was diverted.  I now see that i can probably get the desired low energy opening of the valve but at the price of efficiency.  After some serious mods to this dump model, I see that mid .7 to mid .8 range is likely the best that could be achieved with this valve design.  It is possible to get a tad closer to 1.0 if the range of motion and timing were right.  WAIT!!!! thats the original conundrum! .  It may be worth the effort, if opening the valve is reduced to a more reasonable cocking effort, as long as the efficiency can be closer to 1.0

Bill