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Help - fuel 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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Help - fuel problems

Unread postAuthor: codshead » Mon Jan 05, 2009 8:20 am

Help please
I have built a very basic cannon out of MDPE black sdr11 pipe work. to create the chamber though it had to be enclosed ( the back does not unscrew) so to get the fuel in (propane camping gas cooker ) I drilled a 1mm hole in the chamber where I can spray the gas. It worked the first time but has not worked again. I don't know what the problem is as the peazo is working correctly - always sparking across 2 screws out of 4. is it because of the lack of oxygen or should I use a different fuel ( I have also used lighter fluid gas but no different)
Any help would be appreciated
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Unread postAuthor: inonickname » Mon Jan 05, 2009 8:38 am

Dimensions would help. You really need an endcap, makes it easier to fuel etc. and it allows you to purge the spent fuel out. With this you can't do that, which means the inert gasses will just stay in there. To check this is the case, use a blowgun (atmospheric air, not pure O2!) and purge it through the small fuel hole. If it wont fire still, you should try different fuel ratios.

Good luck and welcome to spudfiles.

Also if that pipe isn't pressure rated (I don't know if it is or not) you'll cop an earfull about it from everyone :(.
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Unread postAuthor: starman » Mon Jan 05, 2009 10:00 am

A couple of things possible. First, you are over or under fueling. Propane has a fairly narrow firing band. And second, as nickname mentioned, you aren't airing out each firing very well. This is absolutely critical.

If you don't build a cleanout cap fueling system, you almost always have to install a fan and a vent in the rear to allow fresh air to vent in and spent gases to exit the breech or barrel. And, installing a fan in your closed chamber is going to be very difficult if not impossible. I would suggest you rebuild you chamber with at least a clean out cap on the rear.

Also, the 1 mm fueling hole is a little dubious.
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Unread postAuthor: codshead » Mon Jan 05, 2009 10:41 am

The pipe is MDPE which is used in pressurized water installations and i have tested this kind of pipe up to 12 bar and it has never moved which is a lot more than pvc would take but the fittings are not as easy to come by hence the stopped end rather than screw off end. the combustion end is 250mm dia by 600mm long and the barrel is 55mm x 1m long.

Thank you - i will try some thing to purge the spent gas out - or start again with PVC
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Unread postAuthor: jimmy101 » Mon Jan 05, 2009 5:31 pm

starman wrote:Also, the 1 mm fueling hole is a little dubious.

Get a syringe and inject the fuel through that 1mm hole. Much simpler and cheaper than a meter but just as accurate.

Use a compressor with a sports ball needle on it to flush the chamber between shots using the 1mm hole.

When firing just cover the hole with a piece of tape.

codshead wrote:
... as the peazo is working correctly - always sparking across 2 screws out of 4.

Do you mean you have four spark gaps and only two or sparking or four screws to make two spark gaps?

Mutliple gaps should be wired in series, sounds like you've either got them wired in parallel or you have a short somewhere.
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