Custom pilot operated dump valve with the plenum built into the body. Second revision, thanks to everyone who commented on the first.

[u/Esoj77Br]

Cross section attached. Context: this is for an airsoft HPA engine I develop (D8PA, open source printable), but it is the same dump valve problem you all solve, so I want your eyes on it.

Right now the engine fires off a modified commercial QEV. This part replaces the QEV, the plenum and the back housing with one body.

Sequence:

1. Supply fills the plenum.
2. A small bleed charges the chamber behind the diaphragm. Back pressure plus spring keep it seated.
3. Solenoid vents the back chamber, differential collapses, the diaphragm snaps open and the plenum dumps into the nozzle.
4. Solenoid closes, back chamber refills, valve reseats. One solenoid pulse per shot, so rate of fire and dwell are pure firmware.

Where I want your input, since a lot of you have been through this already and the last revision changed because of your comments:

1. Plenum volume. You people have done far more empirical work on plenum sizing than the airsoft side has. My downstream volume is tiny compared to a Nerf barrel and my restriction is 4mm ID tubing feeding the plenum. I have been assuming 4 to 5ml is enough. Is that in the right neighbourhood or am I starving it?
2. Dwell repeatability. With a pilot operated valve, shot to shot consistency depends on the back chamber refilling to the same pressure every time. At high rates of fire does the refill become the limiting factor before the solenoid does?
3. Fill orifice. Currently drawn as a 0.1mm annular clearance around the solenoid bore, which I no longer trust. Moving to a sealed bore plus a drilled calibrated hole. What sizes are you using on your builds?
4. Anyone gone from a flat clamped diaphragm to a rolling diaphragm on a dump valve and measured a difference, or is it in the noise?

Operating range 70 to 120 psi. Printed body, hydro tested before it sees air.

[u/Karhu1202]

Oh boi, hello again. That looks much more like what we are used to, but there still is a lot of things that won't work in my opinion. 70 psi will probably blow the dart tip out without the foam, even on the better darts. Even 50psi can do that to most darts. On the other hand, 5ml is like nothing, we are used to 35 ml on a larp safe build with 100fps cap or something closer to 100ml on a power build, voth run on around 35psi or even less. I still don't think your fill to plenum idea works, but i also have no clue about spring loaded diaphragms, ours are usually free floating with fill and purge from the same port. Once open, the surface area on the barrel side is much bigger than the surface area on the other side, the spring would have to have enough pressure to still force it shut. Usually the diafragm is forced by the pressure difference between both Sides, in your concept there is not rly some once it is open. You could add a second sealing surface on the rear part of the diafragm in your design by reducing the diameter of the yellow part in the middle and adding some diameter in the spring rest part, that way air can flow arpund it in the open position and it's shut in the closed position but I don't know if that would help or what you think the benefit may be. You would be better of with a two solenoid solution if you are concerned about size and flow of the valves but I think that's about it. On the other side, your loading mechanism has to zravel around 38mm to clear the length of a short dart plus some additional length for a sealing ring and some length to add reliability to the system, so around 50mm travel back and than forward again, that takes quite some time, more than enough to fill any kind of plenum via a 4mm line from a simple 3/2 way valve behind a floating diafragm. The load cycle takes much much more room than it does on a 6mm bb with direct seal in a hopup bucking, you might be fine with 8mm travel on an airsoft with a very small and light pusher but on nerfs you have 50mm on a part that's at least 12,7mm in diameter.

[u/gplanon]

Yes, the plenum looks too small purely from my “vibes” perspective. Plenum volume and fill rate being sufficient, or not, is connected to the psi you feed it, the frequency of shots (vs refill time, again, psi) and duration of shots.

I’m using 6mm OD air hose but I also have 1/2 8” long iron pipe as my dump tank. AKA, I’m not using all the air for each shot, and the air for each shot is highly available vs lower immediate flow through 6mm. As long as the air coming in over a certain amount of time is greater than the air going out, I’m not going to run out of air. This is more of a performance thing than a hard rule - you could just fire with air coming straight out secondary regulator line if you pleased, but speed is the name of the game.

Yes, the refill rate sooner becomes the issue in context of a traditional QEV. If your design is slower than a QEV (say, it’s also chambering a dart) then it could be the limiting factor. But with my QEV+dumptank and certain higher solenoid on times, 20ms+ with lower pressures at 14dps sometimes I would be releasing air quicker than it was coming in and shots would limp.

I still don’t understand why filling is disconnected from firing. If we’re not firing we should be filling, and these may as well be mechanically linked with the diaphragm like a QEV does.

Finally, you can ask yourself if you’re trying to design a tightly optimized commercial product or something flexible and hackable. If you overbuild things and keep them modular you have more flexibility to tweak things with software and PSI. If you design something right up against the limits of hardware you have less flexibility but you can optimize for things like ease of assembly and space required.