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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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Ignition Problem

Unread postAuthor: etlprts » Wed Jul 28, 2010 12:47 pm

Hi,

I've got an issue with my combustion cannon where the ignition will spark for the first couple of goes, but then nothing....
If I then short the ignition pins with my finger, it'll spark again. As I'm still trying to figure the best fuel mixture (it's a hot dog cannon built with a grease trap, so the combustion chamber is fairly small) it's not too easy as I've got to remove the cap on the grease trap everytime to make sure it's sparking.
Does anyone else have this issue, and how do I get around it? Do I need to earth one of the pins?

Thanks
Paul
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Unread postAuthor: SpudFarm » Wed Jul 28, 2010 12:49 pm

What kind of ignition system are you using?

I am using a camera capacitor and a ignition coil and that have worked great for every gun I have ever built.
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Unread postAuthor: etlprts » Wed Jul 28, 2010 12:52 pm

I'm using a BBQ ignitor, that's arcing via 2 wood screws (I live in the middle of nowhere so I'm being all low-tech!)
When it fires it's great, but it seems a bit too hit and miss to me!
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Unread postAuthor: jor2daje » Wed Jul 28, 2010 12:56 pm

What fuel are you using, you could be gumming up your wood screws so they wont spark, and then cleaning them slightly when you put your finger on them.
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Unread postAuthor: etlprts » Wed Jul 28, 2010 1:02 pm

Hi,

I'm using Lynx (Axe in the rest of the world) deodorant, I'm pretty sure it's not down to dirty contacts, as even with no propelant I'll get 2 or 3 arcs and then nothing.

Cheers
P
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Unread postAuthor: ramses » Wed Jul 28, 2010 1:56 pm

it sounds like it's building up a field between the contacts that interferes with the operation of the piezo. When you short them, the voltage becomes effectively zero, and the piezo can function again.

Do you feel any kind of shock when you short it with your finger?
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Unread postAuthor: jimmy101 » Wed Jul 28, 2010 4:59 pm

Spark gap distance?

You should be using sharp screws (fine thread drywall screws are excellent) and the gap should be 1/8" or less for a typical piezo BBQ sparker.
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Unread postAuthor: etlprts » Wed Jul 28, 2010 5:18 pm

Thanks for all the pointers, there's no shock when I short the points, and I'm using wood screws with around a 60mm gap, it was sparking happily yesterday (and flinging hotdogs happily too, when it wasn't blowing apart!)
I'm wondering if it's something as annoying as humidity, it's felt a bit too clammy for my liking in these parts today!
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Unread postAuthor: jimmy101 » Wed Jul 28, 2010 6:57 pm

etlprts wrote:Thanks for all the pointers, there's no shock when I short the points, and I'm using wood screws with around a 60mm gap, it was sparking happily yesterday (and flinging hotdogs happily too, when it wasn't blowing apart!)
I'm wondering if it's something as annoying as humidity, it's felt a bit too clammy for my liking in these parts today!

60mm (2.4 inches)? Doesn't sound likely, do you mean 6mm (0.25")? I would cut it down to about 3mm (1/8") or a bit less.
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Unread postAuthor: etlprts » Thu Jul 29, 2010 1:25 am

Sorry, you're right, I' meant 6 mm! I'll give it another go with 3mm and see where I get to.
Cheers
P
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Unread postAuthor: etlprts » Mon Aug 02, 2010 5:49 pm

Thought I'd let you now, it's now working a treat!
I've shortened the gap slightly, it's now around 4mm, but I still need to short the contacts after it's fired.
The biggest difference came from airing the combustion chamber each time. I'm using a sink grease trap as part of the combustion chamber, and removing the bulb from this and airing the chamber whilst loading another hotdog made the thing fire pretty much first time everytime - 15/16 so far.

Thanks for all the pointers, need to find something else to fling now 8)
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