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QEV + popoff --- Making it more efficient.

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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QEV + popoff --- Making it more efficient.

Unread postAuthor: psycix » Tue May 05, 2009 6:09 pm

QEV + popoff systems are nice due to their simplicity, but the big inefficiency in these designs is this:
You feed air to the pilot volume while pilotting.
It is ridiculous and has to be solved.

My solution:
Add a second piston, connected to the main popoff piston via a rod. This second piston has holes drilled through it so that air can pass right through it.
When the popoff pops back, the second piston shuts off the air supply (possibly O-ringed) while still allowing the air from the pilot volume to go through the popoff.


Well JSR, efficient enough nao? :D
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Unread postAuthor: jackssmirkingrevenge » Tue May 05, 2009 10:15 pm

That should work nicely, as long as there's enough air to ensure the pop-off actually pops and not fssssss's :roll:

Reminds me of the sort of lines we were going along here trying to achieve "pulsing" flow.
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Unread postAuthor: inonickname » Tue May 05, 2009 10:28 pm

Good design, but I suspect many people would go for the moar easy design over moar efficient.

I could make something similar to this as practice for when I get my mini lathe.
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Re: QEV + popoff --- Making it more efficient.

Unread postAuthor: Technician1002 » Tue May 05, 2009 11:47 pm

psycix wrote:QEV + popoff systems are nice due to their simplicity, but the big inefficiency in these designs is this:
You feed air to the pilot volume while pilotting.
It is ridiculous and has to be solved.

My solution:
Add a second piston, connected to the main popoff piston via a rod. This second piston has holes drilled through it so that air can pass right through it.
When the popoff pops back, the second piston shuts off the air supply (possibly O-ringed) while still allowing the air from the pilot volume to go through the popoff.


Well JSR, efficient enough nao? :D


I was looking at your design and have a couple thoughts.

1 Make a custom pop-off. Use a o ring full seal seat. Make the pop off piston with an o ring also. When closed the pressure builds until the piston moves. It keeps moving until it leaks, picking up the big piston, which completes the opening cycle.

2 Put the sealing piston on a sliding rod so the piston doesn't move until after the piston has hit the second step and is well on the way to popping open. This leaves the air supply on until the valve popped.

After the valve popped and the supply piston has shut off the source, the piston will remain closed until the pop-off closes completely. It would prevent sticking at the hiss position. The o rings will need to move with low friction to prevent a stuck valve.

Will this work? Great idea and drawing by the way. :)

Edit, added modified drawing to show proposed changes
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Unread postAuthor: Hailfire753 » Wed May 06, 2009 12:23 pm

As long as the surface area suddenly increases like a "snap-valve", the original design will work fine. Tech's idea might work better, but I don't think it is necessary.
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Unread postAuthor: psycix » Wed May 06, 2009 1:37 pm

JSR wrote:That should work nicely, as long as there's enough air to ensure the pop-off actually pops and not fssssss's
One word: TNAK! :D
JSR wrote:Reminds me of the sort of lines we were going along here trying to achieve "pulsing" flow.
I see. Though without changing the piston's effective surface (THE thing of a popoff), that kind of designs tend to go farting, as we learned from your experiments. But the idea of shutting off the air feed is about the same, yes.


Technician1002, I didnt go in detail about the main piston of the popoff / snapvalve because we all know that part. O-rings could indeed be a nice addition, and with the proper hole placement and piston length, allow an air spring to be used.
I would also use a stepped piston that seals via an o-ring instead of a sealing face, as a sealing face starts to leak when the popping pressure is approached.

I think that the sliding rod thing may be unnecessary. It may even cause even more unwanted air loss. I will keep it in mind for future ideas though.

I will probably NOT machine this valve as I have a better idea that is even more efficient... :D
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Unread postAuthor: covey12 » Wed May 06, 2009 2:00 pm

i dont get it :?: anyways, are these type of valves quicker than normal piston valves, i wonder how big you could make one
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Unread postAuthor: SubsonicSpud » Thu May 07, 2009 4:24 am

Looks like a great idea, will be good to see how much it affects the air usage. The biggest benefit would come when operating at high rate of fire due to the much greater flow of air available from the valve and able to flow out the popoff. If the trigger valve is only set to a low rate of fire then the would probably not be a great increase in efficiency.

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Unread postAuthor: psycix » Thu May 07, 2009 8:05 am

Yes, subsonicspud, it will indeed make more difference with high rates of fire.

covey12 wrote:i dont get it :?: anyways, are these type of valves quicker than normal piston valves, i wonder how big you could make one

No, the popoff is a pilotting mechanism to get a pulsing flow out of the piston valve, which allows a fully automatic cannon to be constructed.
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