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Spark plug ignition problem

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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Spark plug ignition problem

Unread postAuthor: thunderskunk » Sun Jun 08, 2014 10:16 pm

Ok fellas, I've got an odd one for you.

So I've been working on this howitzer. It's supposed to use shells instead of being muzzle loaded, so the system is fairly complicated which... complicates things.

Here's a load-out of my system:
-Spark plug ignition, which is an actual spark plug with the ground electrode ripped up so the gap is much larger.
-One barbecue button as an electronic initiator
-hinged "door" which completes the circuit upon closing

The pictures are gonna explain this a thousand times better than I am now. My problem is getting a consistent ignition. I've gotten it to work with light loads and small charges of hairspray, but I got nothing when I went to do a full blown test. I know I can get an arc using the spark plug and through my circuit, and I've tested the connections while the shell is in the tube.

SO since I've got a bunch of experienced spudders out there, I need some suspicions and suggestions. I have a few theories:
A) The door system I am using causes too much resistance, and the current at the spark gap is not hot enough to ignite large doses of gas
B) By spraying gas in the front of my shell, I am not getting the gas all the way to the back of the combustion chamber, thus the spark never gets near enough to ignite the gas
C) By spraying gas in the front of my shell, my spark plug is getting wet, shorting out... or rather unshortening, my spark gap.

My dilemma is I do not want to abandon my shell-fed system, nor my door. Yes, I could turn this thing into a regular spud gun, but I've already got one of those. This **** makes you feel like a redleg, haha.

Any help is appreciated

Matt P

PS Here's the link to the rest of the build. It's just a bunch of pictures, and my feeble attempt at rifling the barrel.

https://www.facebook.com/media/set/?set ... ploaded=10
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Attachments
SUNP0107.JPG
This is the door and the back of the shell
SUNP0113.JPG
That's the best picture I have with the shell out of the chamber
SUNP0127.JPG
Here's the whole dang thing compared to a regular potato gun, with a shell next to it.
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Re: Spark plug ignition problem

Unread postAuthor: matti » Mon Jun 09, 2014 4:29 am

Getting the right mix of air and fuel is whats normally causing problems when manually spraying fuel in the chamber. You can't put more fuel in and hope for more power..

I suggest fuel meter of some sort, maybe fuel it with syringe using propabe or butane gas.
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Re: Spark plug ignition problem

Unread postAuthor: mobile chernobyl » Mon Jun 09, 2014 11:31 am

This a very cool build first off!

Cartridges are great if your trying to store a pre-metered fuel/air and you seal it off.
-Do you have a method of sealing off the cartridge? since you state you are using hair spray, I assume it's just a mixture of static air and injected hair spray, so there is no pre-ignition pressure over 1 ATM to worry about - you could get away with simply sealing the tip of the cartridge with saran wrap and a rubber band. I highly recommend using propane instead of hair spray as creating a mixture that will stay suspended is much easier.

I'm not a very big advocate of cartridges when it comes to using gaseous combustible mixtures though - simply injecting the gases into a static combustion chamber can make life a lot simpler and reduce your "load-out" significantly by not needing excessive materials.
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Re: Spark plug ignition problem

Unread postAuthor: Anatine Duo » Tue Jun 10, 2014 4:02 am

really neat
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Re: Spark plug ignition problem

Unread postAuthor: Juggernaut12121 » Tue Sep 30, 2014 6:51 pm

I can't see the rest of the pictures for some reason, pretty cool though!
I like the concept of being able to have shells, it makes it more realistic and I'm guessing the ROF would be increased since you could just replace the chamber instead of having to vent and fuel every time
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