Most Efficient Piston Valve?

[Ragnarok]

The air piloted piston can never be as good as a sear released one, simple as that.

Define "good" - no, it won't open quite as fast, but the eventual difference on the muzzle velocity will be inconsequential compared to a well designed air piloted one.

Given it's a decent valve and I have the specs handy, I'll model my current project with a sear based valve over an air piloted one. In this case that means a ~20% increase in opening force, given the pilot trigger pressure is approximately 1/5th of the chamber pressure - and correspondingly, a ~9.6% increase in opening time. We shall ignore the mass of the rod leading to the sear for now.

Anyway, the gains are in centimetres per second, because like I say, after a valve gets faster than a couple of milliseconds opening time, further increases are pretty moot.

Now, if you consider the extra weight inherent in a sear rod, the opening time gain is barely worth considering.

It will also be harder to build and maintain - that sear is going to have to take a lot of force reliably. For that reason, I have to prefer your co-axial version to the "chamber sealer" version of sniper hero's - it has to take less static force, because less area is exposed when it is sealed.

Additionally, the sliding O-ring seals on a sear valve have to be damned near perfect, or the pressure will leak away pretty fast. A air piloted piston can get away without perfect seals, because it only has to hold pressure against a differential for milliseconds - and doesn't even have to be perfect at that.
A sear valve might have to hold pressure for minutes. Unless you have a lathe and some patience, you'll actually probably lose more air than you could save.

I also need not mention the potential for injury on exposed moving parts, so that will need to be considered - no bare sear rods in other words.

Given the microscopic gains, but vastly increased maintenance and construction efforts, I can't really say it's worth it.
So I at least will be sticking with air piloted - well, for my cannons. I'd have an interesting time trying to make my air rifle air piloted...

EDIT: I can't really vote in the poll, because none of the options are appropriate.

Where's the "Difference is moot, and this isn't an original idea*" option?

*But then again, what is an original idea these days?

[jackssmirkingrevenge]

Define "good" - no, it won't open quite as fast, but the eventual difference on the muzzle velocity will be inconsequential compared to a well designed air piloted one.

Most designs I see here have woefully oversized pilot chambers and usually inadequate pilot valves, so the ideal valve you describe above is a relatively rare phenomenon. So lets just say that over your average exhaust valve, performance increase will be significant

[Fnord]

Here is an idea. Make a sear valve cannon from one of these:



80% of the work is already done, you just need to make the chamber and piston. The stress on the gun frame should be acceptable to use below 125 psi or so.

[Ragnarok]

The ideal valve you describe above is a relatively rare phenomenon. So lets just say that over your average exhaust valve, performance increase will be significant.

Not really.

Like I said, faster than a couple of milliseconds on the opening time isn't necessary - faster than 4 milliseconds doesn't usually make much difference either.

To rip some velocities straight from GGDT, using the same GGDT model as before (more specifically, my kind of high pressure, high velocity and portable style), but using the generic valve and altering just the valve opening time:
20 ms: 186.5 m/s - 347.8 J
18 ms: 190.7 m/s - 363.7 J
16 ms: 195.8 m/s - 383.4 J
14 ms: 201.5 m/s - 406.0 J
12 ms: 208.2 m/s - 433.4 J
10 ms: 215.9 m/s - 466.1 J
8 ms: 224.0 m/s - 501.8 J
6 ms: 231.0 m/s - 533.6 J
5 ms: 233.6 m/s - 545.7 J
4 ms: 235.5 m/s - 554.6 J
3 ms: 236.3 m/s - 558.4 J
2 ms: 236.5 m/s - 559.3 J
1 ms: 236.7 m/s - 560.2 J
0 ms: 236.8 m/s - 560.7 J

As the numbers show, the gains of a faster valve are very much a law of diminishing returns. We'll talk in terms of energy here.
Each 2 ms gain in the 6 to 20 ms range earns you 5-7% more energy on the shot, so worth while. A jump from 6 ms down to 4 ms - around 4%. 4 ms down to 2 ms - less than a percent. The 2 ms between 0ms and 2 ms - about a quarter of a percent.

And I'm not talking about just "ideal" valves. A decent piston valve will hover at the 1 or 2 millisecond mark - an acceptable one, maybe 3 or 4 milliseconds. From that point, jumping to even an instantaneous valve earns - at most - around a percent on the muzzle energy in this case.
I can't call jumps of fractions of a percent "significant performance increases", and I rather doubt you honestly could either.

Like I said, gains on opening time when you're talking about something in the same speed range as a piston valve are pretty irrelevant.

[jackssmirkingrevenge]

A decent piston valve will hover at the 1 or 2 millisecond mark - an acceptable one, maybe 3 or 4 milliseconds.

Where did you get these numbers from? I used to think that my piston valves were very efficient - low friction, tight fit, miniscule pilot volume - but the difference between them and a burst disk - 0 ms - was extremely significant.

[Brian the brain]

I agree with Jack.
This sear method IS more efficient. Period.
And Rag, you should know GGDT is more of a guideline than an actual rule...

Supersonic velocities remember..

( notice how I shy away from any scientific explanation )

[jeepkahn]

A decent piston valve will hover at the 1 or 2 millisecond mark - an acceptable one, maybe 3 or 4 milliseconds.

Where did you get these numbers from? I used to think that my piston valves were very efficient - low friction, tight fit, miniscule pilot volume - but the difference between them and a burst disk - 0 ms - was extremely significant.

that's comparing apples to oranges, differant flow characteristics, probably differant configurations as well... Something I see overlooked a lot on this sight is the hidden variables such as inertias effect on gases, flash heating, and the like.... There are many dynamic variables that effect flows and forces beyond what many of us calculate for, I'm sure even d-hall will tell you that ggdt/hgdt don't can't won't calculate for because they are too complex to be bothered with for a hobbyist sake, but they are real nonetheless...

[jackssmirkingrevenge]

that's comparing apples to oranges, differant flow characteristics, probably differant configurations as well...

I'm referring to the launchers mentioned here, some differences in configuration but not enough to account for the dramatic differences in performance, the most significant factor must have been valve opening time.

[Ragnarok]

Where did you get these numbers from?

It's not a complex task to calculate them.

I used to think that my piston valves were very efficient - low friction, tight fit, miniscule pilot volume

... and very often, piloted by a schrader, which is not exactly the peak of pilot valves.

I should also point out that miniscule pilot volumes can actually HURT performance due to adiabatic heating, which relates to percentage changes in system volume.

If the required piston movement takes up most of the pilot volume, then the percentage change in the volume is large, the adiabatic heating creates a large pressure increase, slowing the valve opening, and potentially causing pneumatic bounce.
If it takes up a smaller percentage, adiabatic heating is less, and the piston may well open faster.

Where the pilot valve is small, this is particularly important. Where the valve flow area is insanely high like in my best known project - yes, there may be a noticeable flow of air out of the pilot volume while the valve is opening, and potentially enough to counter the adiabatic heating of the pilot volume being small. (The pilot volume however, is not at it's potential minimum, it has deliberately been left larger to combat this problem. I'm taking this further with my current project, which is enjoying application of maths to work out the ideal.)
Conversely, the pilot valve is a schrader, there will not be enough flow to combat the adiabatic heating.

The "smallest possible pilot & dead volume" theory is not the most powerful. Dead volume, in moderation will create small performance gains - and too little pilot volume will, as I have said, slow valve opening.
Indeed, the mantra needs to be moderation, not minimalism.

For those reasons, although your piston valves were undoubtedly well built, I cannot be quite so glowing in what I think of their design.
To reiterate, my guess is that the primary thing creating the gulf between your results is the high degree of adiabatic heating that will be inherently involved with your minimal pilot volumes and valve, slowing the opening. Your results also seem to be indicative of some piston bounce, as evidenced by the fact that increases in pressure can result in decreases in velocity.

@ BTB: I'm using GGDT because people know it well and are thus more likely to trust it than my own model (even though my own model is far more pedantic). Both exhibit the same results as far as opening time is concerned though.

I can explain why this tendency exist by using maths and theory if needs be*, I need not rely on GGDT results. In this case, it was just an easy way to put some numbers down rather than talking in wishy-washy "Yeah, if you get fast enough, it doesn't really matter" terms" - and as I've said what I have on valve opening time many a time and it still seems to be forgotten, I figured I needed to throw some numbers at it.

*However, I'm tired, said brain melting will need to wait for tomorrow.

EDIT: Bloody Nora, I didn't realise the length of this post until I submitted it. No wonder I'm tired.

[jeepkahn]

that's comparing apples to oranges, differant flow characteristics, probably differant configurations as well...

I'm referring to the launchers mentioned here, some differences in configuration but not enough to account for the dramatic differences in performance, the most significant factor must have been valve opening time.

actually, there can be a huge differance in performance... with a burst dic, you're dealing with a gas that is already charged with kinetic potential and the burst disc fails when that kinetic potential crosses the threshold of what the disc can withstand so you have instantaneous propagation of the low pressure area(the barrel) by the gas, with ANY type of valve that opens in the direction opposite of the internal pressure there is an inertial delay of the gases themselves before they propagate the low pressure area(the barrel AND the volume displaced by the moving valve) that the valve created upon opening...

[jackssmirkingrevenge]

Surely the simple fact that all the above complex factors would be eliminated with a sear triggered valve is reason enough to make one? Too many numbers for my liking, I need real world results ;p

A good piston buffer is recommended, I neglected to add one the first time I shot my version and the piston shot out and hit me in the chest hard enough to draw blood. If one uses a sealer gun as _Fnord wisely recommended, adding a spring buffer would be easy.

[Ragnarok]

Surely the simple fact that all the above complex factors would be eliminated with a sear triggered valve is reason enough to make one?

Perhaps. It would require less consideration of dimensions as the creation of such a valve would not be dependent on factors which had a more complex ideal than "as much or as little as possible", so their design will be less of a task.

However, that does not mean that air powered valves are obsolete. The construction of one is certainly easier.

On the note of real world results, I cannot imagine you will see any difference but that which can be made out by a reliable chronograph, and any gain will no doubt be lost amongst natural variance.

Overall, as the performance of each type is essentially identical, and as I am not challenged by the design of such things, my choice will be for air piloted piston valves for the foreseeable future - I have little reason to change.

[Hotwired]

On the plus side for an air piloted valve there isn't a sear at all.

Which means higher pressure and larger valves are far more feasible.

There really isn't a limit on how large you can make a piloted valve because the forces are fairly evenly spread across the whole system but sears focus the whole force from the pressurised piston face onto a very small area and it has to be a small area or it's not really going to work too well as a trigger.

The only sear triggered pneumatics I'm aware of are springers and gas springers. The last of which have to have very good seals to minimise gas loss.

*oh look, another page


Justabout anything will work at appropriate pressure or scale, it's not something you're likely to see being used on a 3" porter for example but smallbore cannons at bike pump pressure would not be a problem for it at all. With enough input you probably could make a 3" porter at decent pressure but the cost (and hazards come to think of it) over air piloting would be substantial.




Of course if you took another look at how this concept works, you could just as well stop calling it a piston, give it a barrel to be launched from and call it a projectile.

Compressed gas one side, no restraints or compressed gas required on the other side, held back by a sear... why not?

[jackssmirkingrevenge]

Of course if you took another look at how this concept works, you could just as well stop calling it a piston, give it a barrel to be launched from and call it a projectile.

And where have we seen that before?

sears focus the whole force from the pressurised piston face onto a very small area and it has to be a small area or it's not really going to work too well as a trigger.

This is a drawback I had mentioned in the original thread, air piloted valves are certainly easier to make in terms of standard fittings, no doubt about that.

[sv490665]

The air piloted piston can never be as good as a sear released one, simple as that.

Thank you, that's the answer I was looking for. And I agree, in theory, that there's no reason (except if you use too hard of a return spring) that my sear released valve wouldn't open faster than any air piloted piston. 8) sweet. As for the issue of putting alot of stress on the sear itself, I like the option of making this coaxial as was mentioned earlier. I should probably mention that I am mainly into building high-power/small caliber guns. So this valve would be somewhat small in my personal application. Also, since my latest project, I've decided I only want to build spring-fed magazines for all of my guns because of the awesome ergonomic value, so my valve designs and chamber/barrel orientation have to be acceptable for this type of setup- so I don't know about the idea of eliminating the piston altogether and having sear released ammo, but it is an interesting concept that, if refined further, may be a sensible alternative for me if you don't have to hand-craft all of your ammo cause that could be very time consuming and easily innaccurate if you don't have a lathe or perfect mold or whatever. And yes, the pic doesn't show it, but I was already planning on having the guide rod covered to eliminate injury. All I'm focusing on is the performance of the valve itself, not the other details because other details can be changed.