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Quick hybrid piston valve question

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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Quick hybrid piston valve question

Unread postAuthor: sharpshooter » Tue Nov 10, 2009 9:27 pm

Transferring from a private college to a state school has been a hell of a change. A) i have a hell of a lot more free time on my hands, and B) i now have access to tools and a full shop (as long as i can somehow relate parts to class). Sounds like an excellent time do design a new cannon right?

I have been catching up on what i have missed in the year or 2 since i built my last cannon, and have been intrigued by the idea of the hybrid piston valve.

From what i understand, it is just your standard barrel sealing valve, but instead of venting it, you just ignite the mix? Does the vent side of the valve need to be vented in some fashion? like using one of those pop off pressure valves? I would think there would be some pretty considerable pressure built up on the vent side of the piston from it getting compressed down to essentially nothing. Perhaps it simply seeps through to the chamber side?


thanks in advance guys!!
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Unread postAuthor: Moonbogg » Wed Nov 11, 2009 3:39 am

So few have ever been made I don't think there is a standard. Its pretty much whatever you come up with. Be a pioneer!
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Unread postAuthor: Technician1002 » Wed Nov 11, 2009 3:59 am

A normal piston valve is held closed by the pressure in the pilot area being equal to the chamber area, but the pilot area has more surface cross sectional area so the force is greater in the pilot area on the piston holding it closed against the chamber pressure.

In a hybrid, when the mix is ignited, the sudden rise in chamber pressure changes the balance so the force on the piston on the chamber side is greater than the force on the pilot side, so it opens. A low piston OD to valve seat ratio might not open. A piston with a little larger ratio would work fine. When the chamber suddenly increases in pressure 5 or more times, the valve pops open. To ensure a full opening, a relatively large pilot area is recommended so when the piston opens the pressure rise in the pilot is relatively low. This ensures a full piston stroke and the piston will remain open until a good portion of the chamber has vented. It should not close until the projectile has left the barrel.

Few people make them because the forces are very high. The piston is slammed open with high force and again as the chamber vents, the piston is slammed shut by the pilot pressure which is still there after the shot. It is not vented rapidly.

Use materials that can take the abuse if you decide to build one, and post damage pictures.

The piston would have a normal eq port so it balances pressure as the chamber is filled. If the flame front does make it through the eq port, the piston closing force will be quite high. Use a very tiny eq port to prevent this if possible.
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Unread postAuthor: Moonbogg » Wed Nov 11, 2009 12:29 pm

You can take a look at the Striker post. It uses a different kind of valve. Its a blow open stay open type with no pressure left over. Its held back mechanically instead of by pressure.
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Unread postAuthor: john bunsenburner » Wed Nov 11, 2009 12:39 pm

To prevent the flame penetration one can have O rings on the piston and it can act like an air spring, with a valve at the end to adjust pressure, and another vale on the chamber to fill the hybrid. Just putting it out there.

Also this would make a semi auto easier as filling is more rapid this way...:D

However a precise gauge is needed to fill the pilot spring to the correct pressure, though an apparatus that takes the mix pressure and equilizes the pressure of the pilot would be a doable little invention...
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Unread postAuthor: Technician1002 » Wed Nov 11, 2009 12:59 pm

john bunsenburner wrote:To prevent the flame penetration one can have O rings on the piston


For those who believe in the keep it simple stupid way of building things (KISS theory) is to simply fill from the pilot. Fuel first and then fill to pressure. This closes the valve, meters the fuel and then purges the pilot of all fuel. From there simply fire and don't worry about complicated fixes. :D
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Unread postAuthor: john bunsenburner » Wed Nov 11, 2009 2:15 pm

Well once calibrated the air spring meathod will give faster filling, will all for quick reloading, would eliminate the need for a manner of preventing the projectile from slipping back,and so on and so forth,overall i find it better.

Yes it is against the KISS principle. I don't support it however, as nature has proven the KISS principle as unfunctional, or else women would never have evolved.
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Unread postAuthor: sharpshooter » Thu Nov 12, 2009 1:15 am

Im intending to fill from a pre-mixed reservoir, so building the meter in is of no importance.

If im understanding the striker design correctly, the valve is held closed by some sort of snare? It appears to release at a given pressure and then returned by hand(or by the golf ball being slid back)?

I should think that if the vent side of the piston is a) actually vented and b) vented quickly enough, the piston will open quickly enough that there will be no worries of the flame front passing through an equalization hole or through the imperfect seal around the piston. This should prevent the piston from slamming back forward at any velocity greater than from the initial fill.

I was previously thinking of schedule 80 pvc, like my other hybrids, but this might be worth investing in some metal parts.
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Unread postAuthor: D_Hall » Thu Nov 12, 2009 1:36 am

sharpshooter wrote:Im intending to fill from a pre-mixed reservoir,

:shock:
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Unread postAuthor: SpudBlaster15 » Thu Nov 12, 2009 2:08 am

D_Hall wrote:
sharpshooter wrote:Im intending to fill from a pre-mixed reservoir,

:shock:


Yeah, I was thinking the same thing.
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Unread postAuthor: niglch » Thu Nov 12, 2009 2:19 am

Just out of curiosity, what kind of materials are strong enough to handle the abuse of being used as a piston in a hybrid? Are you pretty much limited to metals or can other stuff be used provided that the mix is reasonable (like ~4-5x).
I was thinking of using a small PVC cap filled with epoxy with a couple o-rings around the circumference. Perhaps two of these joined by a metal rod with one sealing the breech and another sealing the pilot (air-spring) volume. The whole thing would just spring back with a high enough pressure rise in the chamber. Maybe it's far-fetched though. I made a little schematic to sort of illustrate the idea. It's not very different from what has been discussed before, but I'm not sure what to use as materials because I don't have easy access to a machine shop. I'm limited to the stuff in the basement for now.
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Unread postAuthor: sharpshooter » Thu Nov 12, 2009 2:38 am

I didnt think a reservoir was that far fetched of an idea. Its far easier to get the mix right in something that is 200 ci than in something that is 10 ci. (I dont have a mammoth cannon in mind, if you didnt catch on :P ) My last portable hybrid had the propane tank and a shock pump built in.... not very pleasing to the eye, nor a fast reload time. There is no point in having a valve that is instantly ready, to have to sit and wait for the chamber to fuel.
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Unread postAuthor: Moonbogg » Thu Nov 12, 2009 4:10 pm

The design looks pretty good. I think UHMW would work with an aluminum connecting rod.
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Unread postAuthor: spudtyrrant » Thu Nov 12, 2009 4:43 pm

sharpshooter wrote:I didnt think a reservoir was that far fetched of an idea. Its far easier to get the mix right in something that is 200 ci than in something that is 10 ci. (I dont have a mammoth cannon in mind, if you didnt catch on :P ) My last portable hybrid had the propane tank and a shock pump built in.... not very pleasing to the eye, nor a fast reload time. There is no point in having a valve that is instantly ready, to have to sit and wait for the chamber to fuel.
they are not worried about how "easy" it is, they are worried about the safety risks involved, it is very dangerous having a "bomb" sitting around that can go off at any given moment from a very small leak and some static electricity, especially a stoichiometrically "correct" bomb :roll:

and as to niglch's design, the area behind the piston would have to be filled with a pressure higher than your pre-ignition pressure(and be completely air tight) or filling would just activate the piston
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Unread postAuthor: niglch » Thu Nov 12, 2009 5:50 pm

and as to niglch's design, the area behind the piston would have to be filled with a pressure higher than your pre-ignition pressure(and be completely air tight) or filling would just activate the piston


Right, I forgot to mention that detail. The pilot would be filled to a certain pressure so that the forces on either side of the piston become equivalent at the desired burst pressure. Any higher than that and the barrel seal breaks. The key is to have a relatively small surface area on the chamber side of the piston and a large surface area on the pilot side. That way, the pilot doesn't need to be filled to high pressure in order to get a good burst pressure, and the piston will not simply slam shut before the projectile leaves the barrel. With some luck, one couple probably tune the surface area ratios and pressure behind the piston to obtain maximum performace and efficiency at a close to ideal burst pressure.

That's how I think it will work anyway.
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