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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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Piston Valve Question

Unread postAuthor: SPG » Tue Nov 13, 2007 3:16 pm

I've been reading about people's piston valves, great to see them working. One question I've not worked out though.

How are you venting the chamber of combustion gases after firing?

Are you in fact even bothering? Or are you adding a known quantity of air/fuel mix?

Hmm would that work?
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Unread postAuthor: Fnord » Tue Nov 13, 2007 3:23 pm

I guess this is kinda directed at me...

If you'll notice, I have 2 schraders on my hybrid, one on the valve, one on the chamber. when I need to vent, I just pump a bit of air through the chamber airline and allow it to vent through the valve.

Also, I can fire without venting, but the power and sound output drop noticeably
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Unread postAuthor: SPG » Tue Nov 13, 2007 3:34 pm

Right so you pump enough air in to force the valve open and vent the chamber that way. Or am I missing something and the valve stays open till it's reset ? Perhaps that's it?
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Unread postAuthor: Fnord » Tue Nov 13, 2007 3:57 pm

The valve re-seals itself after shooting(at a very low pressure), but if pumping in air doesn't cause it to open I can pilot it with the schrader in the back.
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Unread postAuthor: SPG » Wed Nov 14, 2007 4:00 am

I can see I'm being dumb here, sorry, hope you're feeling patient.

So the firing order (from rest) would be

    1. load ammo.
    2. charge gun with fuel and air to firing pressure through rear schraeder, sealing piston valve.
    3. fire gun.
    4. pump in air through front schraeder to vent chamber through barrel (valve stays open after firing).
    5. repeat.


Hmm, I'm only asking questions because for years I've been pursuing the Holy Grail of the auto-loading, repeating hybrid, and the chamber venting is one of the two remaining problems I've got to resolve.

Who knows, perhaps you don't need to vent it at all? If you add the correct fuel/air mix into a chamber full of combustion gases would it actually through out the fuel/oxygen in air ratio?
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Unread postAuthor: Fnord » Wed Nov 14, 2007 10:24 am

Yes, that's about right. I add fuel from the front, then air from the back, but that's just because of the way my meter is attached.

You can fire without venting and probably only get a slight power decrease if you figure the fuel mixture right. I was running too rich with the shots that I didn't vent before.
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Unread postAuthor: SPG » Wed Nov 14, 2007 12:46 pm

Excellent, I'm looking at injecting a pre-mixed, pre-pressurised mix into the chamber anyway, so if I don't have to vent, it makes life so much easier.

*off to the drawing board*
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