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Problem with Spark Plug

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Problem with Spark Plug

Unread postAuthor: MrCrowley » Fri Aug 31, 2012 10:22 pm

Had some problems with my piston hybrid the last few days. I finally installed a spark plug to replace the home-made electrodes but I was never able to get ignition using the spark plug. I tried on four separate occasions, twice at a 10x mix and twice at an 8x mix.

This seems to rule out electrode gap as a problem as I should have no problem getting ignition at 8x with the gap spacing. There are no leaks in the system and mixing shouldn't be an issue either as I can get ignition with the home-made electrodes (which I put back in) no problem.

I also filled the chamber to 150PSI with air and removed the top wire to the spark plug and held it a small distance from the top terminal. I was getting arcing, which should also imply that the spark gap is adequate.

I think the problem might be due to either the spark plug not protruding as far in to the chamber as the home made electrodes (as well as having a fitting surround it) or it's possibly short circuiting in the spark plug somewhere. Here are some photos:

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Unread postAuthor: MrCrowley » Sun Sep 09, 2012 5:17 pm

I posted this in the short question topic a week or so ago but didn't get a reply. Thought I'd just make a new topic. This post is to bump the topic since it retained the original posting date.
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Unread postAuthor: mattyzip77 » Sun Sep 09, 2012 8:49 pm

I have absolutley no clue what so ever about hybrids however I have had my share of faulty and fouled out plugs. You may be getting spark but the plug may still be no good. Try a new spark plug, only a few dollars and a extra if it dont work. Good luck....
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Unread postAuthor: Zeus » Sun Sep 09, 2012 9:53 pm

What Matty said, I reckon the ceramic is cracked, which would cause a;; your troubles.
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Unread postAuthor: POLAND_SPUD » Sun Sep 09, 2012 11:15 pm

you have checked it doesn't spark on the ouside, right?

Took me a while to figure that out
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Unread postAuthor: jackssmirkingrevenge » Mon Sep 10, 2012 12:23 am

Silly question, does it spark if you try it out removed from the launcher?
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Unread postAuthor: MrCrowley » Mon Sep 10, 2012 1:04 am

jackssmirkingrevenge wrote:Silly question, does it spark if you try it out removed from the launcher?
Yeah, it sparks fine. Also, as I said above, when the chamber is compressed to 150psi, by wiring one stungun wire to the spark plug body (or metal chamber) and holding the other wire ~1cm from the top spark plug electrode (the insulated one), a spark jumping the 1cm gap should be indicative of the spark also jumping the proper spark gap inside the chamber... shouldn't it?

POLAND_SPUD wrote:you have checked it doesn't spark on the ouside, right?
Not 100% sure, but pretty sure I would have caught it by now if it was.
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Unread postAuthor: jackssmirkingrevenge » Mon Sep 10, 2012 1:10 am

MrCrowley wrote:Also, as I said above, when the chamber is compressed to 150psi, by wiring one stungun wire to the spark plug body (or metal chamber) and holding the other wire ~1cm from the top spark plug electrode (the insulated one), a spark jumping the 1cm gap should be indicative of the spark also jumping the proper spark gap inside the chamber... shouldn't it?


yup...

Have you tried enlarging the spark gap a little?

In my experience with piezos, tiny spark gaps work great for high mixes but lower mixes then won't ignite...
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Unread postAuthor: MrCrowley » Mon Sep 10, 2012 1:13 am

Yeah that's true, will have to give it a go. Though, can't really test it until summer holidays as I'm having fewer and fewer chances to fire this thing. The amount of times I've driven to that navy base location only to turn around because of the groundkeepers/people playing sport/navy personnel running, etc. I usually only stay for one shot as well.
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Unread postAuthor: jackssmirkingrevenge » Mon Sep 10, 2012 1:18 am

I don't remember what you're using for ignition but I guess it's a got a bit more juice than a piezo, even so 8-10x is not exactly at the high end of mixes so that could be the issue.
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Unread postAuthor: jimmy101 » Mon Sep 10, 2012 3:26 pm

Yeah, it sparks fine. Also, as I said above, when the chamber is compressed to 150psi, by wiring one stungun wire to the spark plug body (or metal chamber) and holding the other wire ~1cm from the top spark plug electrode (the insulated one), a spark jumping the 1cm gap should be indicative of the spark also jumping the proper spark gap inside the chamber... shouldn't it?.
If you have a cracked insulator you'll still get a spark jumping when you do that.

I would get a new spark plug. The one you have looks pretty worn. Spark plugs will fail at high pressure even if they work OK at atmospheric.

You might also boost the gap a bit. 0.035" (IIRC) is the standard gap for a plug in a 10X full auto. (i.e. a car) using a 30KV or so HV source.
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Unread postAuthor: wyz2285 » Mon Sep 10, 2012 3:30 pm

I had a simular problem today with my hybrid, and the problem really shocked me, I will let the diagram to explain.
The result, enlarging the spark gap, the cannon worked wonderfully.
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Unread postAuthor: jsefcik » Tue Sep 11, 2012 2:44 pm

maybe your gap is to big?? that was a problem ive came across helping my neighbor
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Unread postAuthor: MrCrowley » Tue Sep 11, 2012 2:56 pm

I'm pretty certain it's not that. Mostly because the gap is probably smaller than it needs to be already and also because I couldn't get ignition at a lower mix (8x).
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Unread postAuthor: jimmy101 » Tue Sep 11, 2012 8:58 pm

Do you have continuity through the plug when the system is pressurized? (You shouldn't)
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