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QEV Hybrid Questions

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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QEV Hybrid Questions

Unread postAuthor: PVC Arsenal 17 » Sat Aug 29, 2009 2:28 pm

I've been building spudguns long enough, it's about time I finally try out hybrids.

For my first, I'd like to build something very small and compact. Instead of using burst discs, I would like to make a piston hybrid. For the sake of making the build easier and potentially safer, I plan to use a QEV instead of attempting to make a piston myself.

I've chosen an 1/8" SMC quick exhaust. It seems pretty thick all around. It's a diaphragm type which worries me. The diaphragm is reinforced by a metal ring though. Stupid me forgot to measure the thickness of the rubber. I'd guess it's around 1/16" thick.

For you guys more experienced in hybrids, what mix would feel comfortable taking this valve up to assuming the chamber and all other parts are capable of the same?

Thanks!

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Unread postAuthor: POLAND_SPUD » Sat Aug 29, 2009 3:22 pm

I can't answer your questions... but it seems you'll have to fill the gun with water to measure it's exact internal volume... getting the right mix on such a small gun is going to be tricky
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Unread postAuthor: ALIHISGREAT » Sat Aug 29, 2009 3:36 pm

I would go for a piston QEV because its more rigid and therefore would work better.
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Unread postAuthor: ramses » Sat Aug 29, 2009 3:46 pm

I would recommend a larger valve. HGDT overestimates my piston hybrid significantly, which is probably because a piston valve is less ideal than a burst disk. Compensating with a larger porting may offset this.

I would also recommend that you build a barrel sealing piston valve. If a QEV requires a tiny drop in pressure (often disconnecting a schrader is enough) to pilot, A small increase may be enough to open the valve, too. That would mean that the valve would open at a far less than optimal pressure, reducing performance.
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Unread postAuthor: psycix » Sun Aug 30, 2009 1:35 pm

You can always replace the diaphragm by something sturdier. Or just try out how much the original one can handle.

Keep crankin the mix up until it fails. It will fail way way way before the QEV housing.
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