High efficiency dynamic valve plenum

[CableStop]

So this was inspired pretty heavily by a design jacksmirkingrevenge posted in another thread and I wanted to get opinions on whether or not this would work.  The big difference here is the inverted regulator type mechanism at the front which should enlarge the plenum size as pressure drops in theory.  Then it will snap forward when the poppet opens to shrink the plenum and keep pressure up at the exhaust port.

[CableStop]

Sorry meant to post this in the research gate not machining.

[sb327]

Interesting. I think to have the effect you want, you need to lose the small oring. Otherwise the pressure on the backside of large oring would not equalize. If it’s not equalized, it will forever overpower the small one.

I like outside the box thinking. 👍

Dave

[CableStop]

I think it needs the front o-ring to dynamically scale the plenum volume with the pressure.  It relies on the difference in surface area between the two pistons and a constant spring rate, similar to a regulator.  If the air can leak into the chamber between the o-rings it will equalize pressure on each side of the piston and allow the spring to decompress until it's at rest, it would still slide in toward the poppet when you fired to keep plenum pressure equal during the shot but you'd always have a fixed plenum size.

You do need a vent in the wall between the o-rings to vent out to atmosphere.

[rsterne]

There is something I'm not getting.... The passage through the piston is smaller than the exhaust port, so the pressure inside the valve (at the seat) will decrease faster during the shot than without the piston in place.... Not only that, it will work like a VERY small plenum, which is OK for low power applications, but exactly the wrong way to go for higher power.... In order for the main reservoir (whatever is feeding the valve) to act as a plenum, the passage through the piston should be larger than the exhaust port, in which case I see no advantage....

As you said, the piston is like a backwards regulator, so if the area between the O-rings is vented to atmosphere it will move towards the poppet any time the force differential across it is stronger than the spring.... It requires a VERY strong spring to overcome even relatively little pressure.... For example, if the small end is 1/2" and the large end is 3/4", with 1000 psi inside the valve/reservoir (poppet closed), there is a force of 245 lbs. pushing the piston towards the poppet (opposed by the spring).... As soon as the poppet opens, that force increases, and the piston must move towards the poppet.... If there is something I am not understanding, please explain....

Bob

[sb327]

Bob is correct as I understand.

If the center is vented to atm, the piston will never move to the right, unless acted upon by a REALLY powerful spring. The trouble with that is they offer little movement.

I also do not understand fully what exactly is trying to be achieved. Not knocking the idea, just trying to understand it better.

Dave

Dave

[CableStop]

Long story short I think it has the potential to be super efficient, as well as speeding up discharge and increasing pressure, air can't expand into the plenum until after the poppet closes. This means the ratio of plenum to total volume will be nearly zero during the shot cycle. Basically you just have the air expanding in the barrel and not back into the plenum wasting energy.

Nope I think you got it all, this was mostly just a thought experiment.  Only reason for designing it is for it to be out there I guess. For now I just want to see what happens.

My big questions are about barrel length, speed of plenum discharge, and tuning.  The first two are related, I want to know how quickly the plenum empties compared to the rate the pellet leaves the barrel.  There is also going to be a tuning window with this design based on spring rate, barrel length, and starting pressure. 

As pressure drops and the spring rate stays the same the piston will compress slower. This is going to be compounded by the increased length of travel the piston experiences at lower pressures. Eventually it may not be able to keep up with the pressure drop or be able to completely empty the plenum before the round leaves the end of the barrel.  The latter of those two issues could be solved with a longer barrel though. 

Then there's a hard cut off when the air pressure in the reservoir drops below the spring pressure and it can no longer compress it even when the plenum is empty. Sort of like falling off reg.

I've been thinking about the factors involved and how to mathematically figure this stuff out.

I think similar designs could have a place in certain niche areas, like 10m competition's where consistency is more important than power.

However if you reverse the piston assembly and make the spring push it towards the poppet it would enlarge the plenum. It could potentially change the forces involved but I'm unsure about that.

I'd need to sketch it out for my own sake to see if it'd actually change any force on the spring but my gut says no.

[rsterne]

If it is your intention to have minimum plenum volume, then are you not actually designing a dump valve, where all the plenum dumps with each shot (and then refills at a different volume dependant on reservoir pressure).... As sketched, the plenum volume is so small (a fraction of the barrel volume) that you would be limited to very low power applications (eg. a 10M target rifle).... They are about 7 FPE....

Bob

[CableStop]

Basically I want to decrease plenum volume over the course of the shot. What I was getting at is that air pressure gets divided by the volume it expands into so displacing the plemum volume temporarily will mean air can only expand inside the barrel.

I'm working on building a basic version that's simply a vented piston held back by a spring. It does the same thing as the piston in this design except for the self adjustment. Once air equalizes on each side it extends back to the same position.  It'll just get driven towards the valve upon firing and help keep pressure up. I'm not entirely sure if it's a dump valve or not, hadn't thought about it like that.

[rsterne]

Does it help keep the pressure up?.... Maybe, if you start with a strong enough spring to hold the piston away from the poppet....

Is there enough volume to create decent power?.... That depends on how large you make the plenum to start with, as a percentage of barrel volume....

An example, a .22 cal with a 24" barrel has a barrel volume of 15 cc.... If you have a 3 cc fixed plenum with 3000 psi expanding into that, the pressure when the pellet reaches the muzzle is only (3 x 3000) / (15 + 3) = 500 psi, and the average pressure during the shot is (3000 + 500) / 2 = 1750 psi....

If the plenum collapses to zero volume, then you have a pressure at the muzzle of (3 x 3000) / 15 = 600 psi, and the average pressure during the shot is (3000 + 600) / 2 = 1800 psi....

A 3 cc plenum would measure 1/2" diameter x 0.93" long.... and to reach zero volume would have to collapse completely.... for an FPE gain of about 3%.... You can use the same method for other plenum volumes, pressures, collapse ratios and barrel volumes....

Bob

[CableStop]

My drawing wasn't meant to really show scale. I would say a more accurate plenum size for a .22 would be 15cc which is 0.5 x 2.94 inches.

Using those numbers it would be around a 9% gain at 3000 psi.  You're correct it isn't huge but it'll scale with size.

Ultimately will it be efficient enough to make up for the space it takes up? I highly doubt the dynamic design I posted will achieve that efficiency. I'm building a simpler design that's just a version of this one using a single diameter piston.  This design should take up about the same volume as a regulator does now so it could potentially work.

What I want to test is the actual pressure and duration of the airflow with and without a collapsing plenum. If it keeps the pressure higher it should be readable with a gauge. If it does, I'm wondering what increase in efficiency and power there will be. 

[sb327]

I want to throw something in here. In my self cocking valve I have a single diameter piston. I have described it as having a ‘collapsing’ plenum. IT DOES NOT

My apologies for that misrepresentation. I was trying to explain why it is different than a dump valve.

My shuttle is more like a SAIL on a boat. It moves with the wind. It is very effective at gathering all the wind moving during the shot cycle. Not 100% though. It will equalize after the shot to reposition.

My plenum is ACTUALLY chamber A+B.

Of course I have introduced losses in there but my gain is a self cocking valve. But for conversation let’s leave that out.

Even in this simple drawing, we can see there will be losses due to friction and acceleration of the piston. (Maybe small, but there) Without those, we would have the same effect as if it were not there. (Up to the point it stops moving)

I only capture this movement to close my valve and I accept the losses.

So again, mine is not a collapsing plenum.

Dave

[rsterne]

plenum size for a .22 would be 15cc which is 0.5 x 2.94 inches.

I don't know where you are going to get a spring that will supply about 250 lbs. of force at a length of 2.94", while having nearly that travel, in a size that will fit inside a valve.... Just sayin'....

Bob

[CableStop]

Dave - No worries it was actually inspired by jacks design on page 4 or your valve thread. I understand your valve (except the floating sheer pin you described) and didn't realize we started using the word "collapse" because of it.  Gotta start reading my own threads better.

I had actually just finished reading about valve modding in multipump pneumatic guns which is all about raising pressure/lowering volume. The two ideas just kind of clicked and I came up with this concept.  The design is unique compared to either MPP valves or jacks idea though.

Everything Bob said so far has been correct, and he's right that this design is probably impractical for most applications. However I do think it's interesting to experiment with and still believe the theory is sound. A long barrel .177 with a large plenum would probably see the most improvement using this.

I'll post a picture of the new prototype once I get it finished up tonight. It gets rid of the dynamic adjustment and spring and improves the collapse function. It also takes up less reservoir volume.

[CableStop]

The main issue I'm running into with the variable version I posted initially is that as pressure in the reservoir falls the spring will have more bias against the piston. Not only will the piston have further to travel, it will have less energy where as the spring loses none as air pressure drops.

[Rob M]

Ron who built a funtioning 19 fpe SSP built a follower piston , but not laid out like the diagram above.. His had a pressure chamber, and that was seperate from the firing cchamber completely.. When the charge from the pump was fired, the precharged chamber had a moving wall that would chase the air out and down the barrel , maintaining some peak pressure.. But , the storage chamber was sealed off from the firing chamber by a dynamic seal

[JPSAXNC]

The weight of the piston would have to be less than the weight of the projectile, and I don't think it could generate more energy than is already present.

[Rob M]

The weight of the piston would have to be less than the weight of the projectile, and I don't think it could generate more energy than is already present.

James , if you think back to the webley paradigm , it was arranged in a similar fashion.. The follower doesnt create new energy , it simply chases the air out of the valve ( with equal pressure on the far side )  keeping the total volume somewhat constant for a split second.. Its worked in 3 different examples i can think of.. Heres an example where its currently being used in a similar fashion with positive results.
https://www.gatewaytoairguns.org/GTA/index.php?topic=175248.msg156061603#msg156061603

[JPSAXNC]

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?

[CableStop]

Hey sorry it's taking a while to make progress on this.  My dremel died (it had a good life) and it's making things more difficult. I finally have things set up for cutting the internal o-ring groove though so it's close.

I only have the capability to drill a hole smooth enough for the piston to seal up to 5/8 which will limit the plenum volume for now. Hopefully the simpler design works. If it does I have a 1322 pcp conversation I want to use it in.