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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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Unread postAuthor: ramses » Thu Nov 12, 2009 6:01 pm

I disagree about the leak and the static. Whatever mix is leaving the reservoir through the leak is most likely moving near mach1. Propane flame fronts won't move at anything near that speed unless you engineer something to do just that. I would be more worried about internal static, leaving a valve open during firing, etc.

Simply use a container rated for the peak pressure of the mix you will fill it with, and/or use a burst disk to relieve the pressure spike. A 20 oz CO2 tank for paintball would work very well. Don't point the burst disk at anything important, because it will be destroyed.
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Unread postAuthor: SPG » Sat Nov 28, 2009 6:23 am

I'm not certain why people are talking about enormous forces on pistons, etc.
If you look at the diagram below of a simple coax piston hybrid:

1. Fill with fuel/air behind the piston b.
2. The fuel/air mix bleeds through the equalisation holes in the piston c.
3. The pressure equalises both sides of the piston a. and b.
4. The fuel/air is ignited in the chamber a., the pressure rises pushing.
5. The piston c. back causing.
6. The pressure in b. to rise.
7. The combustion gases in the chamber a. leave via the barrel.
8. The piston c. is pushed forward by the pressurised gas in b.

The maximum pressures in this system will always be the same in both a. and b.

Piston c. shouldn't hit the rear of the gun as the pressures behind it in b. will increase exponentially till they're equal to the pressure of the combusting gases.

As for a pre-mixed fuel tank, it is no more dangerous than a hybrid of the same size. Simply fitting the tank under the barrel, with a burst disc at the muzzle end will mean that you have the same safety as any other hybrid as long as you carry out the simplest of safety measures and never point the muzzle at anyone.

PS. In the diagram you can see a simple check valve if you're worried about flame front entry behind the piston. It could be something like a disc of silicon rubber (go get a silicon baking tray) fastened to the piston only in the centre, so the fuel/air is free to pass, but the pressure at combustion in the chamber a. pushes it against the face of the piston.
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Unread postAuthor: ramses » Sat Nov 28, 2009 3:41 pm

there will still be a large amount of force on the piston. Before the valve opens, the piston will be subjected to a very high pressure in the radial direction. Then the valve will open and as the pressure builds in the pilot, the piston will be subjected to some slightly lower pressure in the axial direction (end to end), while still being subjected to that lower pressure radially. once the pressure subsides, the piston will be accelerated back closed by the air in the pilot area, and impact the sealing face of the barrel. if you get ignition in the pilot area, you are turning your piston into a projectile fired from a whatever mix hybrid at a small metal edge.

To answer the question about material, I used aluminum on mine, but if/when I upgrade to a 2" valve, I will try UHMW.
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