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Pre loaded fuel/air mix chamber help?

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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Pre loaded fuel/air mix chamber help?

Unread postAuthor: JDP12 » Mon Feb 20, 2012 11:41 pm

Hey all... so recently I've been contemplating ideas for simplifying the hybrid fueling procedure. This would be for a small hybrid, with a firing chamber of 1" by 4" or so.

Basically I want to avoid valves/syringes on the actual gun. So my idea was for a removable preloaded chamber that has the fuel air mix already inside it, and all you have to do is fill the firing chamber from this secondary chamber. For about half a dozen shots or so optimistically. However, I have a few questions.

1) How would you calculate the fuel air mixture to put in this secondary chamber? Obviously you have to account for all the dead space.
2) How would you mix it if you wanted this secondary chamber to store the mix for a few shots? Would you just calculate how much fuel/air you would need in the firing chamber (plus any dead space) for x amount of shots, and then just keep injecting it into the firing chamber for each shot? The fuel/air should achieve a homogeneous mix, or as close as possible obviously.

Thoughts would be appreciated as I'm not sure here.
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Unread postAuthor: D_Hall » Mon Feb 20, 2012 11:49 pm

Thoughts: You're sitting on top of a pipe bomb. Bad idea.
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Unread postAuthor: jackssmirkingrevenge » Tue Feb 21, 2012 12:22 am

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Unread postAuthor: JDP12 » Tue Feb 21, 2012 12:27 am

lol ok. well I get the idea. The idea being, its a bad idea. lol.
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Unread postAuthor: Moonbogg » Tue Feb 21, 2012 1:08 am

It has been discussed before, and the dangers are obvious. If you did it in a non-hand held cannon, and had a large burst disc at the end of the pre-filled chamber, then it might be OK to be used at a distance. The cannon would always be "loaded" and the pre fill chamber could go off without warning and for no apparent reason (static electricity etc). Not worth the risk if you ask me, but I agree it is very tempting to make hybrids easier and more convenient to fuel.
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Unread postAuthor: JDP12 » Tue Feb 21, 2012 1:19 am

excellent points, should have thought of those. Idea is abandoned.
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Unread postAuthor: Zeus » Tue Feb 21, 2012 3:58 am

I don't understand this fear, if it's precompressed to 8X, even if it ignites, what would happen? It's not a firearm, pressure doesn't increase with confinement, so what's the problem?

Anyway, it's a steel container, where the heck does static come from? Use all metal fittings, there'll be no problems.

The fueling would be a sod though, unless you purge the chamber with an inert gas, then you'll have to compensate for the extra oxygen.

So it'd be hard, and the effort put in wouldn't be worthwhile, but it's not unsafe.
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Unread postAuthor: Technician1002 » Tue Feb 21, 2012 6:16 am

Instead of pre-mix, a volumetric pair for air and fuel can be used. Use pressure below the liquid gas transition phase (Don't use Propane at 300 PSI) and dispense both fuel and air in proportion into the chamber. The net result is the same in charging the chamber, but the compnents are stored seperately for safety in the supply tanks.
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Unread postAuthor: POLAND_SPUD » Tue Feb 21, 2012 6:24 am

@technician
you do realise that it's even more difficult? that in order to inject set amounts of air and butane into the chamebr you either need manometric or volumetric fueling ?
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Unread postAuthor: Technician1002 » Tue Feb 21, 2012 11:02 am

I said Volumetric. Make a air tank of X size. Make a meter pipe of about 4% of the volume. Charge both to the same pressure. Dump them together into the chamber.
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Unread postAuthor: CpTn_lAw » Tue Feb 21, 2012 11:31 am

If you have acces to some regulators, you can set them so for each 'X' of air that is injected, a certain amount of gas is injected aswell. Preset the gas injection regulator prior to use, according to your chamber volume.
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Unread postAuthor: Technician1002 » Tue Feb 21, 2012 1:06 pm

Without a regulator and just a pair of gauges, you could fill the fuel meter pipe to supply pressure (butane, propane, etc.) and then pump the air meter up to the same pressure.

On the thread of dispense rate, a pair of shield gas regulators with flow meters would work well. I have some flow meters and plan something like this in the future.
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Unread postAuthor: spudamine » Tue Feb 21, 2012 3:34 pm

It's not just been discussed before, it's been done, see the first link in my sig.
Instead of using a burst disk in my pre-loaded chamber, it was the burst disk. In the unlikely event of it going off accidently it would of probably scared the crap out of me but that would be about all as I kept my distance from the thing. Anyway it worked a treat, I filled the 6 litre chamber with a 5x mix and got quite a few 2x shots off with by regging it down, using the mix to purge the chamber between shots worked a treat.
Admittedly this would be a trickier thing to pull off with a handheld and might be impractical if you're aiming at high mixes.
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Unread postAuthor: Fnord » Tue Feb 21, 2012 7:10 pm

Uhh, you use a premix chamber that can take the full pressure from an ignition. A typical painball tank would be fine for 15x with a 2:1 safety factor, a HPA tank would be good for 30-40x. Those red disposable oxygen tanks for torches are rated for ~490 psi if you want large, low-pressure chambers.

As to fueling, it would probably be most practical to just empty the tank before refilling it. Take 4% off the meter to account for the one atmosphere you have left.

Or you could just use cartridges ^^;)
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