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Hybrid piston pilot valve. Your input please

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Hybrid piston pilot valve. Your input please

Unread postAuthor: BigBang J » Tue Sep 14, 2010 8:12 pm

O.K. I think I have got the idea now. Please let me know what you think of this design for a valve. The first design in the picture is the one that I am thinking of going with, I designed it first and I think is better than the bottom design. I started on the bottom design cause I thought that it was a good idea, but now I am thinking the top is better. I think it is pretty self explanatory but let me know if I left something out or you don't get something. So please give me your thoughts on this design.
O, I am going to be running a 7x mix, which I believe is about 90psi.
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Unread postAuthor: Gun Freak » Tue Sep 14, 2010 8:15 pm

I don't have input on the design, but I'm here to tell you that a 7x mix can generate about 700 psi :)
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Unread postAuthor: BigBang J » Tue Sep 14, 2010 8:25 pm

Gun Freak wrote:I don't have input on the design, but I'm here to tell you that a 7x mix can generate about 700 psi :)

Yes but I meant preignition pressures.

also one thing I am not sure about is the pop valve exhuasting of the pilot chamber. I mean will the pressure on the front of the piston force it back and start leaking out before the pop valve can open, or does it all happen so fast that it isn't noticeable? Also how do you calculate at what pressure the valve will open? I suppose you have to calculate the combusted gas pressure and surface area of the face of the piston that is exposed when sealed to the barrel?
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Unread postAuthor: Gun Freak » Tue Sep 14, 2010 8:28 pm

Have you seen SB15's piston hybrid? It is incredibly simple to understand, and after studying it for a minute you will understand how it works. I don't get how yours works, the design is sort of complicated.... Also I think the pre ingnition pressure of a 7x mix is 120 psi. 15 psi (atmospheric pressure) at 1x, 30 psi at 2x, 45, etc.
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Unread postAuthor: Lockednloaded » Tue Sep 14, 2010 8:45 pm

Have you seen fnord's early piston hybrid? Its extremely simple, but I'm not exactly sure how efficient in comparison to other valves
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Unread postAuthor: MrCrowley » Tue Sep 14, 2010 8:50 pm

Gun Freak wrote: Also I think the pre ingnition pressure of a 7x mix is 120 psi. 15 psi (atmospheric pressure) at 1x, 30 psi at 2x, 45, etc.

It's closer to 85-90PSI I believe (depending on where you live). At 1x, it is 14.7PSIA (PSI absolute) but really 0PSIG (PSI gauge). Then, at a 2x you start at 14.7 PSIG.

Not sure why we are still discussing this though.
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Unread postAuthor: BigBang J » Tue Sep 14, 2010 9:15 pm

come on guys, back on topic. I have studied SB15's piston hybrid, but I still don't think I quite get it. piston valves are new to me, before I just used pop valves. I am looking at fnord's early piston hybrid right now but he is using a pop valve design, which I was told and believe is not the best way to go.
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Last edited by BigBang J on Tue Sep 14, 2010 9:25 pm, edited 1 time in total.
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Unread postAuthor: Lockednloaded » Tue Sep 14, 2010 9:22 pm

did you look at the link i posted? it is essentially a giant pop-off valve
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Unread postAuthor: BigBang J » Tue Sep 14, 2010 9:32 pm

Yes I just finished looking at it. But was told that a pop valve was not the best way to go and over all a piston design seemed better and more efficient.

edit: The sequence is: The piston will start on the right side of the valve chamber, the incoming fuel mix will force the piston to the left side sealing to the barrel and opening the way to the combustion chamber through the check valve. The pressure will equalize between the pilot chamber and combustion chamber. So now there will be about 90psi behind the piston. when the mix is ignited the pressure will force the piston backwards which will increase the pressure and cause the pop valve to open and vent the pilot chamber. The piston then will be forced back and the gasses to go out through the barrel.
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Unread postAuthor: saefroch » Wed Sep 15, 2010 5:56 am

This looks to me like just an overcomplicated version of a normal piston valve. Really the only addition over a normal pneumatic piston valve is the check valve connecting the pilot volume to the chamber, which could even be incorporated into a piston design.

EDIT: Blurred a little on upload... but I think this is more the design we discussed... am I wrong?
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Unread postAuthor: ilovefire » Wed Sep 15, 2010 6:13 am

couldnt the mix behind the piston ignite when the piston is forced back even with the check valve it still might be able to build up enough pressure fast enough to ignite the mix?

edit: yes i think that that design would be safer because it doesn't have the fuel behind the piston as well
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Unread postAuthor: SpudBlaster15 » Wed Sep 15, 2010 11:34 am

BigBang J wrote:I have studied SB15's piston hybrid, but I still don't think I quite get it.


It's essentially identical to your proposed design (The first one in the diagram), however it uses a different type of pilot vent valve, and separate chamber and pilot fill ports.

Your check valve idea is interesting and will work, though I'd be concerned with the potential for having an ignitable air/fuel mixture left in the pilot volume after the chamber is filled.

To use a pop off as a vent valve, you'll want to minimize the dead volume in the pilot chamber. Pop offs tend to reseal at a relatively high pressure, so it's necessary to create a high compression ratio in order to remove as much air as possible from the pilot volume.
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Unread postAuthor: BigBang J » Wed Sep 15, 2010 12:36 pm

on the pilot area: I did not clarify something here, the propane will be introduced first, then the compressed air will come in behind it and force out the propane into the combustion chamber. So no ignitable mix will remain in the pilot area.

I don't think I quite understand the method of the pilot chamber in SB15's design. Can anyone enlighten me on that?

saefroch: EDIT: Blurred a little on upload... but I think this is more the design we discussed... am I wrong?
Ummm. I am not sure what you mean. I'll get back online lator if I can, I got to do some work now:)
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Unread postAuthor: frocksie » Wed Sep 15, 2010 2:58 pm

The second design in the first diagram is exactly what my piston hybrid looks like (<a href="http://www.spudfiles.com/forums/bolt-action-aluminum-hybrid-piston-valve-al-50-t21259.html">link here</a>). My launcher does not use a pop-off valve, for a few reasons. First, I'm not sure that a pop-off in a chamber sealing valve will really improve performance all that much. Second, the sealed pilot area acts is an air spring that slows the piston down, preventing it from impacting the back plug, and then resealing the piston. Finally, because the pilot does not need to be refilled from shot to shot, I can achieve a better rate of fire.
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Unread postAuthor: SpudBlaster15 » Wed Sep 15, 2010 3:08 pm

BigBang J wrote:I don't think I quite understand the method of the pilot chamber in SB15's design. Can anyone enlighten me on that?


Which part are you not understanding? The vent valve, the filling system, the piston design? None of it is really complex if you break it down into fundamentals.
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