High efficiency dynamic valve plenum

[Rob M]

Hi Rob, Yes, the piston chases the air out. But, to be able to maintain a constant pressure in the plenum, the piston would have to be moving faster than the escaping air. No?

the piston has to move the same speed as the forward plenum air.. The reason it possibly can is sectional density././ The shuttle ( piston ) is lower sectional density than then pellet, so in theory , it actually can move faster than the pellet but for a very limited distance..This is how I understand it, hope all is well James.

[sb327]

Hi Rob, Yes, the piston chases the air out. But, to be able to maintain a constant pressure in the plenum, the piston would have to be moving faster than the escaping air. No?

The escaping air is moving through a small throat. Speed there is fast. Speed of piston is slower due to being a larger diameter.

If plenum and throat we’re equal, yes , the piston would have to travel as fast as the escaping air to keep the pressure up.

Dave

[JPSAXNC]

I had made some displacer pistons that I used in a multi pump valve. I did get an increase in velocity, but I think it was only because the volume of the piston reduced the valve volume, so the pressure was higher for a given number of pumps than without the piston. I made three pistons, the assemblies were the same except for the piston diameters, .525 .550 .571, the stroke length was .377 The velocity was the same with all three pistons. I couldn't think of a way to determine if the pistons were having any positive effect.

[CableStop]

I had made some displacer pistons that I used in a multi pump valve. I did get an increase in velocity, but I think it was only because the volume of the piston reduced the valve volume, so the pressure was higher for a given number of pumps than without the piston. I made three pistons, the assemblies were the same except for the piston diameters, .525 .550 .571, the stroke length was .377 The velocity was the same with all three pistons. I couldn't think of a way to determine if the pistons were having any positive effect.

The easiest way would be to use the same size reducers in dynamic and static configurations to compare.  If you could find a way to install a pressure sensor in the valve body that'd work too and give a more accurate view of what's going on.

[JPSAXNC]

Hi Tom, I thought about using static reducers, but my piston assemblies were made from five parts and a spring, and I couldn't figure out a way to determine what the volume of the piston assemblies was so I could make a reducer of the same volume.  Thinking about it now though I realize that as I bored the valve out for the larger pistons, I was increasing the valve volume so the pressure for a given number of pumps was decreasing, but I was still making the same velocity. The three piston assemblies had about the same volume.

[JPSAXNC]

The pistons I made fit inside a Benjamin 392 valve, the pistons were mushroomed shaped, with a thin head and stem, the end of the stem was threaded for a nut that retained a very light spring that retracted the piston after the shot, the nut was a thin cross shape. The whole assembly fit in the front half of the valve, There was room for the check valve and a short check valve spring. I used an external valve return spring. The piston was the only moving part. The first piston I made was brass with a brass cross shaped nut, even though the piston travel was only .377 after firing the brass piston stem had bent due to the inertia of the brass nut, I made new Delrin nuts and that fixed the problem. So I made the pistons out of Delrin with a DYI Teflon o-ring, to have the lowest possible inertia, the Delrin piston weighted 1.5 grams.

[CableStop]

Very interesting, how did you do the external spring?

[JPSAXNC]

Hi Tom, for the external valve return spring, I just machined a groove the diameter of the spring on the back of the valve, (not very deep) and made a groove in the end of the valve stem for an e-clip and washer with a step in the washer to hold the id. of the spring. I found some pistons and parts of the valve, I'll have make some parts that are lost but if I get it all together I'll take some pics.

[CableStop]

Thanks that makes sense now. The pumper valves have a lot more valve stem sticking out the back than the pcp valves for some reason. Interesting way of doing it.

[JPSAXNC]

I found enough parts to put a 392 valve together with a piston inside. The valve dumps half the volume when fired. The power is poor, but on the plus side the piston seems to prevent any valve/ hammer bounce, and the piston seems to make an efficient regulator.

[Hack21]

I didn't sort through all of the links and discussion here, so I apologise if my crude sketch below isn't a new proposal.  The thought would be to use differential area on the piston as a spring to bias the piston to the right to a greater degree than what the soft poppet spring would. This on-deck area would be recharged through a small passage through the center of the piston. The pressure reflected across the piston during the time when the poppet was open would be unfortunately reduced due to the difference in areas, but it would replace the wire spring from the original concept with an air spring of sorts. This would add two dynamic seals with the potential to leak to atmosphere and the seal friction and mass of the piston might be a problem. Otherwise, I see merit in the original concept of a fixed volume dumped at a relatively high pressure rather than one that decays as the volume is expanded.

[CableStop]

I have a theory but it's just speculation based on other people's testing. It seems like there needs to be a strong buffer at the end of the piston travel to slow it down before it impacts the valve.

This is based off the fact that Dave's front exhaust valve works and Jack's design (basically what you drew) shattered the poppet. Both use the same principle but Dave's has a piston on the valve stem that counteracts the closing force.

[rsterne]

O-rings make a great buffer, because they act like a progressive spring, increasing resistance as the compression increases.... If the O-ring OD fits snug inside the valve ID (outboard of the poppet and well clear of it) it should not dislodge.... Think bstaley hammer buffer and you will get the idea....

Bob

[JPSAXNC]

Hi All, I machined a pocket in the face of the piston the same dimensions as the poppet, so the piston can close the poppet with out damaging it. That's also why I made the piston as light as possible, so it wouldn't damage anything on impact, kind of like a feather in a hurricane.

[JPSAXNC]

Well. Seeing that the .520 piston seemed to work ok, I decided to bore out the back half of the valve to take the larger .571 piston. On firing the piston was bouncing noticeably, I couldn't figure out why. I thought that when the piston closed the poppet that there must have been a vacuum, and when the retracting spring pulled the piston back, that the vacuum must have sucked the poppet back open allowing the piston and poppet to cycle a number of times. So I set the piston up in my mill and milled a small channel down the side of the pocket that the poppet fit into, and across the bottom of the pocket to the vent hole. So that air could flow past the poppet when it was in the piston face and relive any vacuum. When I fired the gun there was a high pitched buzz that drained all the air from the valve. So I realized that it was air slipping past the poppet that was causing the buzz and bounce. So I punched a small circle of rubber out of a thin sheet, slightly smaller than the poppet diameter and super glued it to the front of the poppet to act as a seal to block off the vent hole when the piston closed on the poppet. That cured the bounce.  hth

[JPSAXNC]

I realize now that the piston had to keep the poppet closed just long enough for the hammer bounce cycle to stop, and then retract. The bore and stroke for the piston is .571 x .385 giving a swept volume of .9 cc's, on 8 pumps the velocity with .22 cal. 14.3 grain pellets was .452fps. adding 4 pumps to the remaining air gave 430 fps.

[rsterne]

Sounds like maybe an SSG to stop the hammer bounce might help?....

Bob

[JPSAXNC]

Hi Bob, The displacer piston in the pumper valve was an early idea at making an air conserving pumper. The piston was supposed to control air metering, add something to the power of the shot, and act as the hammer debouncer, by holding the poppet closed until the hammer bounce cycle ended. The bleed hole in the piston is .015, there's a compression spring that retracts the piston as the remaining air is bleeding through as the plenum is refilling. If the timing is right everything works as it should. On this setup the spring I'm using to retract the piston is a little to strong, and doesn't let the hammer bounce subside completely before retracting the piston. The speed that the piston retracts at is a combination of the spring force and the bleed rate of the vent hole. This was Steve Woodward idea.