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interior fuel pipe

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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interior fuel pipe

Unread postAuthor: bluerussetboy » Tue Nov 06, 2007 12:09 pm

i'm currently building a combustion tater rifle. my chamber is out of 2" sch40. i'm trying to keep the weight as low as possible, so i don't want to add a fan or a chamba chain.

i was wondering if anyone has tried this configuration to disperse/mix propane/mapp gas? air and fuel will be mixed before injected into chamber.


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did it work? what were the issues if it didn't.
did the fuel disperse sufficiently?
should i use multiple spark gaps?
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Unread postAuthor: hyldgaard » Tue Nov 06, 2007 12:20 pm

Im not exactly sure i understand where youre going here, but if you are just trying to mix the fuel without the fan for keeping the weight down, I think you'd be better off with a fan..
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Unread postAuthor: bluerussetboy » Tue Nov 06, 2007 12:33 pm

as in the above post

air and fuel will be mixed before injected into chamber
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Unread postAuthor: benstern » Tue Nov 06, 2007 1:01 pm

Just to let you know, Nobody uses "chamba chains."
It was a brainfart by Joel that never caught on because fans are faster and easier. Oh and people don't want to look retarded shaking a cannon around.

The configuration you showed would still deliver more fuel to the rear. Also, all the fuel would not exit the pipe, there will still be residual gas inside it. (especially if the device has holes facing up)
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Unread postAuthor: bluerussetboy » Tue Nov 06, 2007 1:26 pm

i have both 1/8" and 3/16" brakeline to use. i can't imagine much fuel would be left in the pipe.

i know there is at least one cannon here in the showcase that uses a chamba chain.
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Unread postAuthor: BigGrib » Tue Nov 06, 2007 2:54 pm

I think you would be better of with a external pipe and a chamber fan it's just that much easier open a valve flip a switch push a button kaboom
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Yea, that's definitely going to get you at least a tazer.

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Unread postAuthor: bluerussetboy » Tue Nov 06, 2007 3:09 pm

funny thing, i know i don't want an external pipe or a chamber fan on my launcher. an external meter and a fan control box will both deter from the sleek looks and lower weight i'm shooting for. not to mention the 2" chamber is not all that easy/fun to put a fan in.
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Unread postAuthor: sandman » Tue Nov 06, 2007 3:16 pm

they may take from the sleek looks, but the performance improvement will make people forget about the looks
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Unread postAuthor: bluerussetboy » Tue Nov 06, 2007 3:39 pm

a properly mixed combination of fuel and air is going to give you improved performance no matter how it was mixed. the final product is still a properly mixed combination of fuel and air.

if i want performance i'll just drag out my advanced combustion that has the fan, external fuel meter and stungun ignition and shoot golfballs a million-zillion light years away.
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Unread postAuthor: jimmy101 » Tue Nov 06, 2007 4:40 pm

bluerussetboy wrote:i have both 1/8" and 3/16" brakeline to use. i can't imagine much fuel would be left in the pipe.


So, sit down and figure out how much fuel will be left in the pipe.

If your chamber is 2"D and the "injector pipe" is the same length as the chamber, and given that you want 4% fuel, the radius of the pipe would be about SQRT(0.04)*1" = 0.2", or an ID of about 0.4" for it to contain 100% of the fuel.

So even using 1/8" tubing you are going to be leaving a heck of a lot of the fuel in the "injector pipe".

You could just overfuel by that amount but it won't be very precise.

Instead of using a pipe with gross holes drilled in it, you might try a bubbler from a fish aquarium. Check the local pet store to see if they have long scintered (sp?) glass tubes or metal tubes. You might be able to find one long enough to get OK mixing in the chamber. If the bubbler has enough back pressure you might even be able to get away with just counting 1-potato 2-potato ... to measure your fuel.

Personally, I would add a chamber fan. Get a cheapo DC motor from somewhere and make a fan blade. The blade doesn't have to be anything fancy, use a hunk of plastic soda bottle, cut the fan blades then use gentle heat to bend them into position. Use a glob of epoxy to hold the fan on the motor shaft. Put the motor and battery inside the chamber so only the switch shows on the outside. Probably end up weighing next to nothing, especially if you use a 1.5V motor and a single AA battery.

If you want it really tiny go to a surplus electronics place or "borrow" an old cell phone from someone, and rip the vibrator out of it. That's a really tiny DC motor. Electronics Goldmine has several that are ~$1 each.
Image
EDIT: In the photo, the "10cent Dime" label is to differentiate that 10cent Dime from the less common 8cent and 12 cent dimes. :?

The usual caveats apply to using a brushed DC motor in a combustion gun.
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Unread postAuthor: Pete Zaria » Tue Nov 06, 2007 4:50 pm

bluerussetboy wrote:as in the above post, air and fuel will be mixed before injected into chamber


Sorry to burst your bubble, but unless you're building a hybrid, this isn't going to work.

In a normal metered propane cannon, there's a static volume of air inside the chamber, to which you add an exact amount of propane to create a stoichiometric fuel/air mixture.

If you intend to mix the air and fuel outside of the chamber, you either need to be injecting this fuel/air mixture into a vacuum, or you need to calculate in the amount of air inside the chamber, which defeats the purpose.

If you inject both air AND fuel into a chamber already containing air, your mixture will be far too lean, again, unless you're building a hybrid.

Additionally, you'd need to be able to seal off your fuel mixing/metering system from the chamber, to prevent combustion inside the mixing system, which could be catastrophic.

Also, any length of PVC or copper pipe and fittings, as you've drawn, would weigh significantly more than a small motor and battery, as several other people have pointed out. Perhaps you could somehow make your inside-chamber-gas-distribution-pipe also act as a spark strip.

My only other thought would be to use a venturi, much like the one on the tip of a propane torch, inside the chamber where the fuel comes in - this would effectively mix the fuel, and shouldn't weigh much.

Keep in mind a chamber fan has other advantages besides just mixing the fuel - it creates a hotter, faster combustion, and also aids in venting the chamber between shots.

Good luck with your build.

Peace,
Pete Zaria.
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Unread postAuthor: ShowNoMercy » Tue Nov 06, 2007 4:57 pm

How about getting a fuel injector off an old car and using that to inject your fuel?
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Unread postAuthor: bluerussetboy » Tue Nov 06, 2007 11:02 pm

thanks for the help jimmy101 and pete. your info gives me a few other options.
the cellphone vibrator option is very viable. i have a broken cellphone just sitting here doing nothing already.

i actually was going to use the dispersal pipe as part of the spark strip.
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Unread postAuthor: jimmy101 » Wed Nov 07, 2007 1:33 pm

Pete Zaria wrote:
bluerussetboy wrote:as in the above post, air and fuel will be mixed before injected into chamber


Sorry to burst your bubble, but unless you're building a hybrid, this isn't going to work.


Actually, this does work and has been used in cannons before. The trick to using "premix" is that you need a large excess of it.

Before loading the ammo you just flush the largre excess of the premix through the gun. This displaces the combustion products from the last shot and replaces it with fuel+air for the next shot. In this setup, you just need a single gas inlet point near the breech plug (the breech plug is not removed between shots).

Not terribly efficient but it does work. Some folks have just used the standard bernzomatic torch head to create the premix.

Of course, you are creating a big cloud of propane+air outside the gun that is probably within the combustion limits. :shock:
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Unread postAuthor: bluerussetboy » Wed Nov 07, 2007 2:15 pm

so i would need to flush the chamber with enough premix. so now my question would be what is enough premix? would it be 1.5X,2X. anything larger than 2X in what i was going to use as a meter will put my meter pressure too high. i will have to build a larger meter.


i have used the torch heads before, but results were less than favorable. living at 9,275 ft. above sealevel might of had something to do with it.
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