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Three spark gaps

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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Three spark gaps

Unread postAuthor: Sticky_Tape » Fri Jun 27, 2008 11:03 am

Hello I am making a combustion spray and pray olny a small one and I am putting in three spark gaps and I am hoping that the piezo will jump all of the gaps. The gaps are about 8mm across I am wondering about the normal kw a bbq piezo puts out and if it can jump 3 8mm gaps or olny one.
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Unread postAuthor: starman » Fri Jun 27, 2008 11:13 am

You can easily jump 3 gaps but you'll want to shorten each gap down to 1/8" of an inch or so. Place them relatively evenly down the center of the chamber for optimum performance.

Are you sure you want to go to all that trouble on a small spray n pray?
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Unread postAuthor: Sticky_Tape » Fri Jun 27, 2008 11:19 am

Why not? Btw I was wondering if the piezo could jump the gaps I have or will I have to test it?
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Unread postAuthor: bigbob12345 » Fri Jun 27, 2008 11:23 am

It will not jump them if it is an ordinary piezo.
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Unread postAuthor: Sticky_Tape » Fri Jun 27, 2008 11:25 am

Is that your final answer? well I will have to buy a piezo and try it if it doesn't work I will fill four of the holes.
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Unread postAuthor: starman » Fri Jun 27, 2008 11:30 am

Sticky_Tape wrote:Why not? Btw I was wondering if the piezo could jump the gaps I have or will I have to test it?


Because the effort would be much better placed on a metered propane gun. It can be a lot easier than you think....

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3 - 8mm gaps will be very challenging for a piezo. Like I stated before you can pull it off if you shorten your gaps.
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Last edited by starman on Fri Jun 27, 2008 11:46 am, edited 1 time in total.
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Unread postAuthor: Sticky_Tape » Fri Jun 27, 2008 11:38 am

I don't have any places to buy pressure rated pipe for a metered propane gun so I think that I will stick with the regular sap. I could make one though.
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Unread postAuthor: starman » Fri Jun 27, 2008 11:44 am

Sticky_Tape wrote:I don't have any places to buy pressure rated pipe for a metered propane gun so I think that I will stick with the regular sap. I could make one though.


You should use pressure rated pipe on spray and prays as well. Sydney, Nova Scotia isn't so far out that you can't find a PVC supplier....
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Unread postAuthor: Sticky_Tape » Fri Jun 27, 2008 1:37 pm

I looked around no prssure rated pipe that I could find.
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Unread postAuthor: FLONE » Fri Jun 27, 2008 3:19 pm

In my spray and pray with a BBQ igniter powering 3 sparks I get consistent results with .050" to .060" each. Metric I assume would be 1.2 mm to 1.5 mm each. Good luck!
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Unread postAuthor: psycix » Sat Jun 28, 2008 5:31 am

3 8mm gaps are the same as one 24mm gap. If the ignitor can jump that, it will work.
Try to make the gaps like 2mm, then it will certainly work.

Oh ant btw, I dont know how long that chamber is going to be, but your gaps should be evenly spaced at 1/4th, 1/2 and 3/4th of the chamber length for optimal performance.

Placing all 3 spark gaps near to each other in the middle wont improve performance alot, and then even two gaps spaced at 1/3 and 2/3 would already be better.
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Unread postAuthor: jimmy101 » Sat Jun 28, 2008 10:08 am

psycix wrote:Oh ant btw, I dont know how long that chamber is going to be, but your gaps should be evenly spaced at 1/4th, 1/2 and 3/4th of the chamber length for optimal performance.

Placing all 3 spark gaps near to each other in the middle wont improve performance alot, and then even two gaps spaced at 1/3 and 2/3 would already be better.

(1/6)L, (1/3+1/6)L, (2/3+1/6)L, simplifying gives (1/6)L, (1/2)L, (5/6)L, might be a bit better for three spark gaps. L= chamber length and the distances are all measured from on end of the chamber. For a 12"L chamber and three gaps you would put the gaps at 2", 6", 10".

In general, the first gap is at (chamber L)/(#sparks)/2 from the end of the chamber. Subsequent sparks are at (chamber L)/(#sparks) from the preceeding spark.

This gives the optimal spacing for a closed chamber. Nobody really knows what the optimal spacing is in a gun. The difference in performance between my gap spacing and psycix's is probably unmeasurable.

For a generic piezo BBQ igniter I would try to keep the total spark gap to 1/4" (6mm) or less.
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Unread postAuthor: psycix » Sun Jun 29, 2008 3:23 am

You're right jimmy, 1/6th, 1/2 6/5th is what I meant :P
My brain is getting old already and I'm only 19! :shock:

That way you will create 3 DOUBLE flamefronts, for a total of 6 flamefronts, each burning off exactly 1/6th of the chamber.
Now an inch forth or back wont make much difference but the point however, is that the gaps should be spaced out over the chamber, and not stick together at one point, since that will be near useless.

I would try to keep the total spark gap to 1/4" (6mm) or less.

Yep, 2mm per gap it would be.
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