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Oxygen-Propane Combustion

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Oxygen-Propane Combustion

Unread postAuthor: freyyr890 » Wed Dec 24, 2008 4:10 am

Hi,

My friend and I are upgrading his miniature spud gun to accept a Propane-Oxygen fuel mix. It has a small combustion chamber (about 75 cubic inches, or ~1100 cc). It's built from ABS. Our design is to use an oxy-propane cutting torch as a regulator to provide the fuel mix from tanks of propane and oxygen. The current igniter is a barbecue lighter, but I'm considering upgrading to a flyback transformer from an old TV set (he's got a couple of old 6V rechargeable lead-acid batteries lying around).

I was wondering if anyone had any experience with oxygen/gas mixtures and the safety (danger?) thereof. Also, if anybody has any input on the design I'd love to hear it.
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Unread postAuthor: jackssmirkingrevenge » Wed Dec 24, 2008 4:32 am

Oxygen, propane and ABS aren't really words that look pretty in the same sentence.
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Unread postAuthor: Cosmic Muffin » Wed Dec 24, 2008 5:52 am

oxygen is more explosive than propane, and if you are making a propane injected gun you use pressure rated parts. abs aint pressure rated so to add a more combustible gas to the mix would raise your chances of a blown chamber.
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Unread postAuthor: TurboSuper » Wed Dec 24, 2008 9:27 am

Cosmic Muffin wrote:oxygen is more explosive than propane, and if you are making a propane injected gun you use pressure rated parts. abs aint pressure rated so to add a more combustible gas to the mix would raise your chances of a blown chamber.


Ehh, oxygen isn't combustible; it's reactive.

Still, the point stands, using oxy-acetylene in any thing less than SCH 80 pressure-rated pipe is a horribad idea.
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Unread postAuthor: ramses » Wed Dec 24, 2008 12:06 pm

using oxy-acetylene in anything is a "horribal" idea, because the mix likes to detonate. Acetylene can't even be stored under pressure, they have to dissolve it in acetone to put it in the tanks.

besides Oxy- anything is really a hybrid.
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Unread postAuthor: freyyr890 » Wed Dec 24, 2008 2:34 pm

Sorry, I should have clarified. I wasn't considering a pressurized hybrid, but a propane cannon using oxygen at atmospheric pressure. All the oxygen is for is to enrich the fuel/oxygen mixture to reduce the fuel-to-air ratio and get a cleaner combustion.

Does anyone have any experience with this design, or is it doomed to failure?
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Unread postAuthor: Gippeto » Wed Dec 24, 2008 2:45 pm

To be honest, it sounds like you're playing with matches on the hay stack.

Since fuels only burn at certain ratios, why not just build a meter.

From the sounds of things, it'll be safer.
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Unread postAuthor: ramses » Wed Dec 24, 2008 4:33 pm

I forget the actual definition of hybrid, but in my book, an oxygen enriched combustion is a hybrid, because there is more than the stoichiometric amount of fuel in the chamber. It has been done, but it is almost certain to be frowned upon on this forum if it is made of plastic. Those who have toyed with oxygen include DYI, Larda, and others. Talk to DYI, because Larda isn't on often, and he might consider atmospheric combustion below him (if I were him, I would). They will also tell you that you shouldn't use oxygen in plastic. If you live in the US, small steel pipe isn't much more expensive, and you don't even have to solvent weld.

Keep in mind that if you do use oxygen, you will need an all oxygen rated meter, as well as a propane meter.

p.s.- pvc shrapnel doesn't show up on X-Ray.
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Unread postAuthor: starman » Wed Dec 24, 2008 6:15 pm

You've got plenty of O2 in the air around you for the cannon you have, safely buffered with lots of inert nitrogen. A little fuel meter system is all you need.

Don't worry about the bottled O2 on this cannon.
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Unread postAuthor: Fnord » Wed Dec 24, 2008 6:22 pm

I've seen oxy/propane used in abs before, and while it didn't explode violently, it did split enough to ruin the gun before long. I wouldn't recommend it.

I forget the actual definition of hybrid


Hybrid means a combination of two different things, such as gas-electric hybrid cars. Hybrid in our case means a combination of pneumatic and combustion guns. Adding oxygen doesn't make it a hybrid.

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Unread postAuthor: TurboSuper » Wed Dec 24, 2008 8:09 pm

Actually, a hybrid is more powerful than an oxy/propane mix 'cos it's adding the pneumatic energy of the (pseudo)inert gasses in the compressed air to the combustive force of the enriched oxygen/fuel mix.

Still a terrible idea to use in ABS though. Unless you fancy yourself a darwin award nominee.
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Last edited by TurboSuper on Wed Dec 24, 2008 9:34 pm, edited 1 time in total.
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Unread postAuthor: freyyr890 » Wed Dec 24, 2008 9:19 pm

Okay, thanks for the feedback.

I'll shelve the O2 for now until I get around to building a metal cannon, and just build a propane meter.

Thanks again.
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