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Troulbeshooting

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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Troulbeshooting

Unread postAuthor: JET47 » Tue Jun 03, 2008 2:46 pm

Ok. I finished building my gun, posted it in the showcase section here, and promised all you guys damage pics. only problem is, i cant get it to fire :x

The sparker is working. i can see the spark when i look down the barrel (chamber has definately been cleared! lol) fuel is being mixed.

I think the problem comes from the amount of propane im putting in. I tried a few calculators, and they said to use something crazy like 120psi in my meter. that doesnt make sense to me, because my chamber is very small, but i tried it anyway, and first; no dice. then i went with 60 psi cause thats what it seems alot end up being, still no dice. so i pulled out the old hand calculator and pencil and did it by hand, figured out how many cubic inches were 4% of 52.224, my chamber volume, (2.089) and figured that in a meter volume of .25 cu in i would need about 8, 8.5 psi. drastically different from the other results, but i tried that too, pretty optimistically, but stillll, no dice.


Ok, before posting this, i figured i'd try a little more math. and im leaving that above paragraph there even though i know i messed up in the end. thats 8 and change bar, which, when converted to psi, turns out to be a little over 120 lol. anyways, it still isnt working at 120. any advice?


EDIT: redid calculations on my meter volume, came up with .723 cu in instead, which makes my pressure even lower. i guess i was using too much propane then. testing continuing.
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Unread postAuthor: Fnord » Tue Jun 03, 2008 5:07 pm

The calcs are right, actually.

You're not multiplying the ~8.3 you got by 14.7, which will give you 120 psi or so. The number 8.3 is "BAR" pressure, or how many meter volumes of gas you have in your meter.

1 bar = 14.7 psi.

Edit... You need to measure your meter with water instead of using pi*r^2 * L ; it's generally more accurate than the math given the odd meter geometry (ball valves, gauge).
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Unread postAuthor: jrrdw » Tue Jun 03, 2008 7:06 pm

Another thing to think about is that arcing under pressure is harder to do. Try using 1 spark gap.
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Unread postAuthor: starman » Tue Jun 03, 2008 7:52 pm

You've got a fairly smallish chamber there so your fuel metering will have to be a close tolerance to ideal. As suggested earlier, perform a liquid volume test on the meter as well as the chamber on up to the ball valve.

Keep working with it. Your meter looks proportioned about right for your chamber to run a reasonable psi in it. It will be much greater than 8 psi however. After each failed test, be sure you air out the chamber well.
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Unread postAuthor: JET47 » Wed Jun 04, 2008 12:29 am

thx for the advice. running with the new meter volume i got 44 odd psi, still didnt work though.

and its already at 1 gap, had to abandon the four because they very rarely worked, so i just bent all the screws except for one pair until they touched.

i cant measure my chamber volume with water because theres a fan in there.
and when i tried to fill the meter and pour it out i got so little water i figured it wasnt getting in. like barely a few drops. and the smallest scale i had was one for weighing envelopes, didnt make it move at all. and no measuring cup anywhere near small enough. id get a micropipette from the lab at school but i graduated on sunday =D
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Unread postAuthor: starman » Wed Jun 04, 2008 1:24 am

JET47 wrote:...didnt make it move at all. and no measuring cup anywhere near small enough. id get a micropipette from the lab at school but i graduated on sunday =D


See if you can find a syringe...or scour a kitchen store for small measuring devices. Also it might be worth pulling the fan out for the measurement. Assuming an 80 mm fan, you can subtract the fan volume of about 50 cc or so.
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Unread postAuthor: jrrdw » Wed Jun 04, 2008 6:48 am

If your eletrodes are touching each other they wont work. You will need at least a 2 mm gap. The smaller the gap, the hotter the spark!

Make sure you only have 2 points near each other just to eliminate possible errors.
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Unread postAuthor: JET47 » Wed Jun 04, 2008 9:02 am

heh ya i had four pairs of screws for gaps, but they sparked very inconsistantly so i bent three of the pairs together, and the single one left sparks every time.
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Unread postAuthor: jrrdw » Wed Jun 04, 2008 8:28 pm

So does it work now? I mean, does the cannon shoot now?
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