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Advanced combustion problems

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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Advanced combustion problems

Unread postAuthor: hi » Tue Apr 19, 2011 10:51 pm

I have been spudding for years. certainly long enough to know that this is a very novice question, but here we go:

I made an advanced combustion with a propane meter. I am having trouble finding the "sweet spot". I find that anything between 15 and 23 PSI in the metering system will ignite. Right around 20 PSI works best, but it only launches the potato about 150-200 feet... very lame.

chamber is approx. 14'' of 3'' ABS. Barrel is approx. 4 1/2 feet of 1 1/2'' pvc.

It has a chamber fan that blows towards the barrel. The ignition is a BBQ ignitor that is in the forward part of the chamber. The gas is shot in via a copper line that is near the end cap.

I have tried every thing I can think of. I have have gone up and down in 1 PSI increments, ran the chamber fan for short periods of time and longer periods of time. I let the gas just flow in freely without being measured. Nothing seems to work. The best shot i got all day went MAYBE 200 feet.

The hose running from the tank to the meter is about a foot. It goes to a tee and pressure gauge, then to 3/8'' nipple that is 2'' long, then to a ball valve, then into the gun. the chamber fan is a small computer fan running of a 9v battery.

Again, I know this is a very noobish post, but for the life of me I cannot get thins thing to go bang the way that is should. suggestions are welcome.
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"physics, gravity, and law enforcement are the only things that prevent me from operating at my full potential" - not sure, but i like the quote

you know you are not an engineer if you have to remind yourself "left loosy righty tighty"
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Unread postAuthor: mark.f » Tue Apr 19, 2011 11:24 pm

Have you plugged you values into HGDT? I'm getting an estimate of 386 feet/sec. with the dimensions of your gun, propane, and a ~65 gram 2" long potato slug. Plugging that into GGDT's range calculator gives a maximum distance of about 435 feet with a CD of 1.3.

However, a potato is nowhere near a predictable projectile, and will probably travel much less than that.
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Unread postAuthor: hi » Tue Apr 19, 2011 11:53 pm

I have not plugged it into HGDT; however, i have made enough cannons to know that it should go much farther and be much louder.

The one and only thing i can come up with is the fan is pushing the gas to the forward part of the chamber and it isn't mixing well. Or maybe i need two ignition points.

I didn't actually measure the cannon, the dimensions I posted are rough estimates.
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"physics, gravity, and law enforcement are the only things that prevent me from operating at my full potential" - not sure, but i like the quote

you know you are not an engineer if you have to remind yourself "left loosy righty tighty"
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Unread postAuthor: Technician1002 » Wed Apr 20, 2011 1:46 am

It is essential that the chamber and barrel are completly vented of all spent gas before the next shot. Remove the cap with the fan running and let it vent for at least 30 seconds. Incomplete venting between shots will cut down on the power.

With a spud in place to plug the barrel, the chamber can be filled with water and then the volume of water measured to get an accurate value for HGDT. Do the same for the volume of the gas meter. With those you can get a good value for the gauge pressure to use. Even then, you will need to test small changes from that as the space between the chamber and meter can hold some of your metered gas charge and cause lean shots. This distance needs to be as small as possible.
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Unread postAuthor: wyz2285 » Wed Apr 20, 2011 7:39 am

Maybe you bought a bad propane, it has happened to me once :x
And as in you cannon the gas get in the chamber from the end cap but the ignition is in the front... :roll: try multiple spark points :)
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Unread postAuthor: Moonbogg » Wed Apr 20, 2011 10:59 pm

I think maybe the ignition point should be in the middle of the chamber or the back. Just a hunch, but maybe you aren't getting a full combustion of the gasses becuase of the spark placement.
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