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Fuel 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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Fuel question.

Unread postAuthor: hotknife ralphie » Tue Sep 04, 2007 6:36 am

Just a random question ( I do not intend to build a hybrid as I have a major lack of knowledge in that field).
Also, I am sorry if this had been answered, I looked through but did not see it.

I came across this:
"you idea is correct, you add more propane than you would in a combustion and to make up for the lack of oxygen, you add air."

And was wondering, to make up for the lack of oxygen you add air? What would happen if you just added pure oxygen? My guess would be its a bad idea, I am very curious however.
Once again I'm not making one, just gathering knowledge.(Also sorry if this is in the wrong section)
Thanks in advance.
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Unread postAuthor: frankrede » Tue Sep 04, 2007 6:52 am

Its been done several times.
But I wouldn't do it in any form of plastic pipe.
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Unread postAuthor: paaiyan » Tue Sep 04, 2007 7:56 am

If you added pure oxygen, then you would want to inject the stoichiometric amount of propane along with it. You can do that, but it's much more powerful and requires a strong combustion chamber.
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Unread postAuthor: SpudBlaster15 » Tue Sep 04, 2007 11:26 am

It has been done at least once before.

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Re: Fuel question.

Unread postAuthor: Dumbascii » Wed Sep 05, 2007 9:34 am

hotknife ralphie wrote:What would happen if you just added pure oxygen? My guess would be its a bad idea . . .

Good guess. There are two major issues with oxygen, but it is certainly possible to design around them:
1) Air is roughly 20% oxygen, so 100% oxygen needs 5 times the normal fuel charge to reach a stoichiometric (perfect combustion) ratio. This is way too much power for PVC. Adding that much fuel will also slighly pressurize the combustion chamber, either pushing your slug down the barrel or oozing the fuel/oxy mix past the slug - neither situation is good - so you'd need to operate similar to a hybrid.
2) If there is leftover oxygen after combustion (which is quite likely), I've experienced the following materials burning out of control after being ignited by the combustion process: PVC, paint, nylon, silicone, oil, spud starch. You wouldn't expect silicone to burn!

There is also some potential (though I have not experienced it) for DDT, Deflagration (burning) to Detonation (exploding), where the normal combustion starts a shockwave that spontaneously decomposes all of the fuel at once. The shockwave can shatter almost any material.
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