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Do i need oxygen?

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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Do i need oxygen?

Unread postAuthor: wyz2285 » Tue Nov 02, 2010 4:18 pm

sorry if some body have already asked a same question :? But i have to solve this problem fast: i'm building a combustion rifle, the chamber size is only about 103 cm cubic, i use butane so i should inject about 3 cm cubic of it. i use a ignitor of a normal light, so if i want to make sure it works do i need inject also some oxygen too? if i do how much should i inject? plz help and thanks :)
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this is the prototype, the real gun will has the same ignition, the same gas tank and same size of chamber.
Last edited by wyz2285 on Fri Nov 05, 2010 4:50 pm, edited 1 time in total.
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Unread postAuthor: jackssmirkingrevenge » Tue Nov 02, 2010 4:21 pm

There's 20% oxygen in the air and that's enough ;) I would go with 4cm<sup>3</sup>, just make sure you vent it after every shot
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Unread postAuthor: wyz2285 » Tue Nov 02, 2010 4:23 pm

vent it well :? don't know if that's possible, i don't have a chamber fan
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Unread postAuthor: saefroch » Tue Nov 02, 2010 4:27 pm

You can set up a connector to vent it with a pump, or just use your breath. Chamber fans also do help mix...
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Unread postAuthor: jackssmirkingrevenge » Tue Nov 02, 2010 4:30 pm

By vent I don't mean "mix", rather blow a burst of compressed air after you shoot to remove the combusted gasses and replace it with fresh air for the next shot.
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Unread postAuthor: wyz2285 » Thu Nov 04, 2010 9:19 am

Any way i think i will Inject some oxygen
But i will also put a Valve one the chamber to vent it :)
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Unread postAuthor: SpudFarm » Thu Nov 04, 2010 3:26 pm

Boom goes the dynamite..
Oxygen and propane has a high probability for detonation if there isn't any buffer gas.
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Unread postAuthor: wyz2285 » Fri Nov 05, 2010 4:44 pm

new problem :? even though i vent the chamber it still doesn't ignite :x
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Unread postAuthor: reedfe » Fri Nov 05, 2010 4:54 pm

you prb aren't waiting long enough. gun freaks mini combustion sometimes takes 1-2 min before you can shoot it again. also check that your sparker or ignition is still sparking.

i chuckled at the title ;)
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Unread postAuthor: wyz2285 » Fri Nov 05, 2010 5:13 pm

i DO NOT want to wait 1 ou 2 min :shock: tomorrow i will see if i can found and small oxygen tanks, the sparker works well :wink:
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Unread postAuthor: jimmy101 » Fri Nov 05, 2010 9:00 pm

Don't use oxygen. You don't know what you are doing and you'll "put an eye out".

Just blow fresh air into the chamber between shots. Use your lungs, or a small fan, or a big floor fan, or a shop compressor, or just wave the stupid thing around in the air for 15 seconds.

If you use oxygen then you'll have to recalculate the amount of fuel. For pure oxygen + fuel you'll boost the energy in the chamber by about 500%. If the round jams in the barrel you will most likely exceed the burst pressure of most plastic chambers.
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Unread postAuthor: wyz2285 » Sat Nov 06, 2010 2:37 pm

Thanks that post helps :D don't worry the real gun I'm planning is made of metal, as the chamber is small i will need that 500% energy increase :D how much oxygen do i need for butane?
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Unread postAuthor: saefroch » Sat Nov 06, 2010 2:43 pm

Wait... if I'm reading that correctly, you don't have high-school chemistry and are building an oxy-propane combustion...

Fueling. I hope you can extrapolate how to fuel a combustion from that.
Please use the search function on the site. I know it takes a few more seconds than just asking in a thread but it makes you look knowledgeable. :wink:
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Unread postAuthor: wyz2285 » Sat Nov 06, 2010 3:35 pm

saefroch wrote:Wait... if I'm reading that correctly, you don't have high-school chemistry and are building an oxy-propane combustion...
well I'm 16 this year is my first year at high school and this project is only my second try on combustion
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Unread postAuthor: saefroch » Sat Nov 06, 2010 3:52 pm

Then it's probably not advisable to go all-out-crazy with oxygen, trying to make huge steps like that usually doesn't work out well. I'd get a propane meter system working, then try enriching the mix with oxygen.

The setup you have right now isn't much better than a traditional spray 'n' pray, and if anything fueling inaccuracy is increased by the small chamber volume. Syringe metering will probably work for such a small chamber volume, but above all be sure to vent the chamber.
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