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Enhancing a Piston Valve

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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Enhancing a Piston Valve

Unread postAuthor: Hubb » Wed Oct 28, 2009 11:05 am

I was thinking the other night about piston valves. One of the things that come into my head was, "How can I take away the mass factor from a piston to make it open faster?" Then a thought occurred to me about having just the sealing face of a piston with support, and running a guide rod through it. Of course, this would probably limit the launcher to a chamber seal type, but are chamber sealing launchers really so much less powerful than barrel sealing ones to begin with?

(That thought then sprang up how I would envision a chamber sealing coaxial launcher...)

I would imagine that by taking the "mass" from a piston and just having a sealing face would increase efficiency as well as power. Efficiency gains would be from the fact that the pilot volume could be much smaller, which would also lead to a smaller, lighter valve design. Power gains would come from the fact that the piston will open faster.

All of that brings up the question, as well as the basis for this topic: How can a conventional piston valve be made to go beyond the limits we have come to expect from them?
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Unread postAuthor: POLAND_SPUD » Wed Oct 28, 2009 11:15 am

maybe just build a piston like the ones used in QEVs?
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Unread postAuthor: jackssmirkingrevenge » Wed Oct 28, 2009 11:23 am

When I thought about this I had come up with this idea:

the piston is in two parts, the purple section starts to accelerate when the pilot is opened, so that instead of starting to open gradually, the red portion sealing the barrel is yanked open suddenly when the purple part hits it. The diagram is not to scale, the purple section can afford to be a little larger in diameter.


In effect, it's a pneumatic hammer valve with full flow and no dead space:

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Unread postAuthor: john bunsenburner » Wed Oct 28, 2009 11:26 am

Hmm, one can also have a barrel sealing version of this. just a sealign face n a guide rod with a stopper at the end. I will make a diagram later, out of time now.
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Unread postAuthor: Gippeto » Wed Oct 28, 2009 11:31 am

POLAND_SPUD wrote:maybe just build a piston like the ones used in QEVs?


That's exactly what I've been tossing around. :)

I bought some qev's on evilbay last week. (10 in 3/8" for 24.95 shipped :D ) The "piston" travel is roughly twice the exhaust port diameter. If the air pressure can keep the piston from wobbling at that travel, perhaps a re-think is in order as to piston design.

I crunched some numbers using measurements taken from the body, and then hydrotested the assembled qev at 1000psi for 2 hours. The piston won't take that sort of abuse for too long. A new and improved piston will have to be made for reliable high pressure usage (1000psi+). :)

I think I've the makings of an idea. :idea:


Edited for "Brain Fart"...The piston travel is HALF of the port diameter...D/2. :oops:
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Last edited by Gippeto on Wed Oct 28, 2009 12:03 pm, edited 1 time in total.
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Unread postAuthor: Technician1002 » Wed Oct 28, 2009 11:36 am

I tackled that very problem in designing my valve. In summary here is what we wanted to do.

1 High flow when open. The amount of plumbing to and from the valve is to be cut to the bare minimum.

2 Very fast. Ideally in the t shirt launcher the valve should fully open before the shirt in the barrel moved 2 inches.

3 High reliability.

How we did it.

1 Coaxial design with large diameter chamber and valve ports with an area greater than the cross sectional area of the outlet. Flow is subject to only 1 bend in direction.

2 Piston was designed to be as light as or lighter than the projectile by using low mass HDPE. The same pressure that pushes the projectile also pushes open the valve. The pilot area was designed to operate at no pressure so it doesn't slow the piston. The initial opening is done by an impact to initiate motion at much higher initial speed than an air operated traditional piston valve. It cracks open faster and then is slammed open by the chamber pressure.

Edit, I noticed a common thread of using an impact.

3 A robust solid piston was used with a proper bumper. It is mechanically initiated which eliminated the problems of stuck pistons and high blow by that prevents firing.

From measurements the valve appears to fully open in about 1 ms.
We considered a 2 disk spool, but found the forces on the center of the bobbin would break things. The forces in our design would cause materials to fail in that design. The force evenly distributed prevents breakage.
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Unread postAuthor: jeepkahn » Wed Oct 28, 2009 12:37 pm

in my coaxiels I use a "cup" style piston... sealing face inside the cup, the cup can be lightweight(it's just for stability)... Here's a 2"piston that weighs less than an oz...
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Unread postAuthor: Ragnarok » Wed Oct 28, 2009 1:31 pm

I've detailed my thoughts on piston valve opening times already.

Any decent piston valve already opens fast enough (a couple of milliseconds) that even being able to halve the opening time will only yield a few more feet per second.

For those that like data, figures calculated for 3vo with various valve speeds can be found at the link above. In its case, the improvement of a valve that opens instantly is only 1% more energy than a 4ms valve - and in reality, its valve will open in around about 1 ms. Any further effort to further lower that can't even offer me another whole joule on the muzzle energy.

If you want to find ways to improve piston valve performance, then don't focus on valve opening time, it's good enough already. Focus on bringing up valve flow.
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Unread postAuthor: psycix » Wed Oct 28, 2009 1:59 pm

On high performance launchers, its the sound barrier which limits the performance. Using lighter gases or hot gases (think springer-type airgun) are the only profitable solution if you want to go faster then 300 m/s.
At 250 m/s and up the SOS of the gas already limits the effective pressure significantly.

A good opening time is important, but flow is even more. After all, opening time does nothing more then to quickly allow full flow. As the projectile is still standing still, there is not much flow restriction going on yet, thus the flow of the valve doesn't matter very much in the beginning moment. This means that opening a valve really fast doesn't grant much more performance then an adequately opening one.
Only on short barreled light projectile launchers the valve needs to open fast.

The future of piston valves? Whenever I have the time and money, I will build one this way:
-Machined
-Larger porting then barrel
-Flow! Rounded edges n stuff.
After that, it's just about making the pressure high enough.
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Unread postAuthor: Hubb » Wed Oct 28, 2009 2:05 pm

So, this means that to increase a conventional piston valve beyond it's current limits, the flow of the actual gases makes more of an effect than opening times.

Maybe the thoughts should lean towards trying to improve the flow? Obviously rounded edges would increase the flow as the gas will be able to "slide" rather than "turn". But how can this be enhanced?
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Unread postAuthor: Technician1002 » Wed Oct 28, 2009 3:38 pm

Hubb wrote:So, this means that to increase a conventional piston valve beyond it's current limits, the flow of the actual gases makes more of an effect than opening times.

Maybe the thoughts should lean towards trying to improve the flow? Obviously rounded edges would increase the flow as the gas will be able to "slide" rather than "turn". But how can this be enhanced?


1 avoid sharp bends.
2 avoid sharp edges
3 large cross sectional area in places with bends and edges to reduce the velocity for the same volume delivered.

# 3 on the list is one of the reasons I'm building a 2.5 inch valve to launch golf balls. The valve area is oversize. The transition to the barrel will be a smooth transition down to the GB barrel. Flow speed in the valve will be relatively low while the volume of flow will be very high. Goal is high flow at high pressure into the barrel with little pressure drop in the valve.

In the contest we designed to maximize both flow and speed. For raw distance we were beat by a 3 inch traditional piston valve. Flow matters. A slower 3 inch valve with more bends and edges will out flow a 2 inch valve with rounded edges and few bends. We still won best overall. :D
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Unread postAuthor: Ragnarok » Wed Oct 28, 2009 4:33 pm

Hubb wrote:So, this means that to increase a conventional piston valve beyond it's current limits, the flow of the actual gases makes more of an effect than opening times.

Definitely. Valves are already fast enough, which makes promotion of any valve as "faster than a piston valve" questionable.

To take Tech's QDV as an example, while it has other advantages which are worth promoting*, as far as opening time is concerned, the improvement of the pilot-less QDV over a (well built) piston valve is completely negligible.
*Indeed, for certain of these reasons, I changed 3vo's plans to include a QDV as a trigger valve. (The main valve will still be a straight piston valve, for certain other reasons.)

Anyway, to move back, the area where piston valves need to improve is without a doubt, flow.
Not that I'm suggesting that people shouldn't still endeavour to keep piston weight as low as is practical, but I don't advise taking any special measures on the matter.
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Unread postAuthor: jeepkahn » Wed Oct 28, 2009 5:06 pm

And lets not forget, that keeping the chamber volume as close to the breech as possible... A 4"diameter 5" long chamber into a 1" barrel will perform better than a 2"diameter by 25"long into a 1" barrel... even though they are both 58.9"^3.... ggdt shows identical performance, but If i'm not mistaken ggdt doesn't account for chamber pressure loss due to friction and flow rate within the chamber(d-hall can say for sure), it only looks at total chmber volume, not chamber shape...
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Unread postAuthor: psycix » Wed Oct 28, 2009 5:38 pm

The flow in the chamber is neglectable. As long as the chamber is larger in diameter then the valve and barrel diameter, you shouldn't worry about chamber length restricting flow.

With an over under, you should focus on the elbow (apart from the valve). It needs to be smooth and large enough in diameter.
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Unread postAuthor: jackssmirkingrevenge » Wed Oct 28, 2009 10:05 pm

I would agree that it's better to find ways of increasing your operating pressure than trying to squeeze every last drop of performance from your valve.
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