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Piston Valve Hybrid

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 Hybrid

Unread postAuthor: Jolly Roger » Sun Aug 26, 2012 2:00 am

Hey all,

It's been a while since my last post, but I'm still bent on building my piston valve hybrid so I'm looking for some advice or tips on how I can improve the design.

The picture is pretty basic but it is to scale and should show the basics of how it works.

Firstly the arrows indicate non-return valves or check valves. The idea works basically like a co-axial piston valve pneumatic, except I wanted the two chambers completely separate so as to not let the fuel seep into the rear chamber and igniting, which means there would be little or no pressure drop in the rear chamber and the gun simply wouldn't work. So I introduced check valves to have equal pressure in each chamber, but with the rear just air/oxygen, and the primary the fuel mix.

The ignition points have't been drawn but they are in the primary chamber.

To ready the gun, the air line is opened but with the primary chamber air line valve turned off. This increases the pressure in the rear chamber only so the primary chamber can be pressurised with fuel without the rear opening. Next, the fuel line is opened and the primary chamber is pressurised to a specified pressure before the primary chambers air line is also opened. All lines are then closed.

As the piston valve has a greater surface area than the primary chamber's barrel seal and other side of the piston, it will hold closed until the primary chamber is ignited and the total surface pressure on those seals in the primary chamber go above that of the rear chamber. I've done some maths on the surface area of these seals to make sure it opens at the right pressure to maximise performance.

The black in the rear chamber indicates a rubber stopper to absorb the impact of the piston on ignition.

After ignition, the air lines are opened and closed to vent the chamber and the piston rod is then manually pushed back to form a seal.

I am wondering though if there is anyway I can make it more autonomous or simplify anything. Any advice is welcome.

Cheers,

Jacob
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Unread postAuthor: CS » Mon Aug 27, 2012 1:34 am

I don't see how the "rear chamber"/pilot is exhausted. As it is the valve will open for a short time, and then close again because there is pressure in the rear chamber.

I am wondering though if there is anyway I can make it more autonomous or simplify anything. Any advice is welcome.


Sabotage! You could improve the speed of loading a burst disk and get performance and simplicity.
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Unread postAuthor: Jolly Roger » Mon Aug 27, 2012 3:33 am

Yeah I should have drawn that a bit better. Where the yellow rod comes out the back, along there will be pretty free for any air to exhaust. I can mill some vanes in it if I find it too constricting.
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Re: Piston Valve Hybrid

Unread postAuthor: jackssmirkingrevenge » Mon Aug 27, 2012 3:40 am

Jolly Roger wrote:I am wondering though if there is anyway I can make it more autonomous or simplify anything. Any advice is welcome.


Image

:D :D :D
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Unread postAuthor: Jolly Roger » Tue Aug 28, 2012 2:49 am

:shock: Can they shoot spuds?
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Unread postAuthor: jackssmirkingrevenge » Tue Aug 28, 2012 5:13 am

Seriosuly though, cartridges are a good way to avoid on-board mixing difficulties if a high rate of fire is what you are looking for.
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Unread postAuthor: Jolly Roger » Tue Aug 28, 2012 5:35 am

Yeah I thought about that as well. Cartridges just don't seem to be as fun to design for some reason. Plus if I'm looking at very high pre-ignition pressures, they will be some seriously bulking cartridges.

I'm having second thoughts about the design now anyway. Not too keen on having the piston in there at high mixes. Will try to look into some other alternatives to burst disks
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Unread postAuthor: jackssmirkingrevenge » Tue Aug 28, 2012 6:17 am

Jolly Roger wrote:Yeah I thought about that as well. Cartridges just don't seem to be as fun to design for some reason. Plus if I'm looking at very high pre-ignition pressures, they will be some seriously bulking cartridges.


Very true, they're not easy to get round and the fact that you have to make lots of them is even more off-putting.

I'm having second thoughts about the design now anyway. Not too keen on having the piston in there at high mixes. Will try to look into some other alternatives to burst disks.


An o-ring detent could be an alternative if you're firing hard consistent projectiles, but long is the way, and hard, that out of Hell leads up to Light.
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Unread postAuthor: CS » Tue Aug 28, 2012 6:43 pm

Heat will create problems in fast cycling hybrid. An idea I'm working on is a 6 shot launcher, sort of like this, that uses permenant chambers instead of cartridges. You can fuel all 6 chambers as if it was one large volume, instead of indivually. The down time of fueling, and loading burst disks/ammo will allow some heat to dissipate.

There is dynamic and static seals, seals that move and seals that don't. Rubber is the medium to seal, and it responds to heat. So I see the goal of not using dynamic seals. Burst disks are the only valve that allows for a static seal.

edit: Omnipotence fail! The o-ring detent would be another static sealing valve...
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Unread postAuthor: POLAND_SPUD » Tue Aug 28, 2012 11:26 pm

what about metal to metal seals ?

as far as heat dissipation is concerned you can always use propane for cooling
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Unread postAuthor: jackssmirkingrevenge » Wed Aug 29, 2012 2:09 am

I wouldn't worry too much about heat, it's not like it will cause other rounds to "cook off" - though if you're mixing in a hot chamber, it will screw with your pressure gauges because of the different densities...
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Unread postAuthor: Jolly Roger » Wed Aug 29, 2012 3:10 am

jackssmirkingrevenge wrote:An o-ring detent could be an alternative if you're firing hard consistent projectiles, but long is the way, and hard, that out of Hell leads up to Light.


How does that design work? I'm keen to get rid of the piston without having to look at burst disks.

I've emailed a couple of suppliers of rupture disk valves, and pressure release valves to see if they stock or know of any dump valves that can exhaust all the pressure in the line once they go over a certain amount. Will update once they get back to me.

CS wrote:Heat will create problems in fast cycling hybrid. An idea I'm working on is a 6 shot launcher, sort of like this, that uses permenant chambers instead of cartridges. You can fuel all 6 chambers as if it was one large volume, instead of indivually. The down time of fueling, and loading burst disks/ammo will allow some heat to dissipate.

There is dynamic and static seals, seals that move and seals that don't. Rubber is the medium to seal, and it responds to heat. So I see the goal of not using dynamic seals. Burst disks are the only valve that allows for a static seal.

edit: Omnipotence fail! The o-ring detent would be another static sealing valve...


Something like that would be good, it's just a matter of engineering it into a combustion/hybrid.
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Unread postAuthor: jackssmirkingrevenge » Wed Aug 29, 2012 3:37 am

Jolly Roger wrote:How does that design work? I'm keen to get rid of the piston without having to look at burst disks.


The idea is that you have an o-ring at the breech of the cartridge which seals the projectile, with enough friction to keep it from escaping under pre-ignition pressure. When you ignite the mix, the force of the combusting gas is now stronger than the friction and the projectile fires.

That's the theory anyway ;) and sometimes, it works!

[youtube]http://www.youtube.com/watch?v=w8MZ5aS2-JA[/youtube]

Hard projectiles are a must though.
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Unread postAuthor: Jolly Roger » Wed Aug 29, 2012 3:47 am

Yeah I gotcha. Fairly specialised design. Had much luck with it?

I've been toying with the idea of a pressure release valve to act as my burst disk as you'd call it.

Basically something like this mounted inline before the barrel, only with some sort of latch to catch the shaft of the seal when the spring is compressed fully.

http://1.bp.blogspot.com/_YOjepBEvjc8/RtFJB1RGcKI/AAAAAAAAAfk/KxnSkQyz5ww/s320/valve101.gif

This way you can set it's opening pressure as well by using the screw jack to compress or loosen the spring.

The only thing I'm concerned about is the flow is diverted. If only I could come up with a way to have a clear run from the chamber to the barrel[/url]
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