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Dual Ignition 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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Unread postAuthor: theBOOM » Wed Dec 23, 2009 12:03 pm

Starman for burst disc combustions or hybrids isn't the number of sparks irrelavant? I say this beacause basically the burst disc determines when all the pressure will leave so having more sparks wouldn't really make a difference. :idea: :idea:
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Unread postAuthor: SpudBlaster15 » Wed Dec 23, 2009 3:40 pm

Multiple ignition points will not have as much of an effect on a burst disk launcher, but they still can help.

If you have a very large chamber with a single spark gap, combustion will take a long time to complete, and you probably lose a significant amount of heat energy to the chamber walls before sufficient pressure is generated to rupture the disk.

However, if you have something like a standard sized 4" x 12" chamber and a 40PSIG burst disk, I doubt that multiple ignition points would noticeably increase performance.
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Unread postAuthor: ZRTMWA » Wed Dec 23, 2009 4:45 pm

I am still having trouble. I decided to stick with only two spark gaps after reading the comments. This is how they are now wired:

http://s779.photobucket.com/albums/yy80 ... udgun2.jpg

However with this setup, I get no sparks. I believe it is because the gaps are too big. Or did I do something wrong with the wiring? Thanks.
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Unread postAuthor: SpudBlaster15 » Wed Dec 23, 2009 5:06 pm

The wiring is good; it sounds like the gaps are too large. I usually set them at about 1/8" (~3mm) each.
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Unread postAuthor: ZRTMWA » Wed Dec 23, 2009 11:24 pm

I just put them both really close but I still can't get anything! WTF?
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Unread postAuthor: theBOOM » Thu Dec 24, 2009 2:06 am

Make sure the electrodes have nothing on them, make sure you wired them properly... check out the wiring in www.advancedspuds.com it shows you how to wire as many as 4 spark gaps I think. Also make sure that noting metallic is close to the nails or to the igniter electrodes.

Lastly, what igniter are you using are you using the Red piezo igniters or the Electrical BBQ Igniters which use batteries.

If none of these work post again and I'll think of some other reasons why it does not work properly. :wink:
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Unread postAuthor: starman » Thu Dec 24, 2009 2:34 am

theBOOM wrote:Starman for burst disc combustions or hybrids isn't the number of sparks irrelavant? I say this beacause basically the burst disc determines when all the pressure will leave so having more sparks wouldn't really make a difference. :idea: :idea:


Yeah a burst disk makes up for lots of sins in regard to your spark gap setup. However, you still want as much fuel burning at the same time as possible, rather than a slower "run-up". A faster burn means a higher instantaneous pressure peak. You can actually play with your burst disk setting to take best advantage of this peak. It means not necessarily setting the break point as high as you might think...high enough to break the disk...then allow the highest pressure spike to drive the projectile directly.

In hybrids, you really don't have much choice because of the difficulty of arcing your gaps.

HGDT allows you to play with gaps and disk break points, chamber/barrel sizes, etc. Give it a try if you haven't already.
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Unread postAuthor: theBOOM » Thu Dec 24, 2009 2:56 am

Hm an offtopic question .. but I would't really want o make a whole new thread for a simple question... has there ever been a hybrid with more than 1 spark?? other than Vera :?:

I have actually pluged in numbers in HGDT apperantly having more sparks even in a hybrid makes alot of difference I just find it hard to make more than 1 spark with all the pressure in the chamber, but hey I'm electrically retarded...
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Unread postAuthor: Moonbogg » Thu Dec 24, 2009 3:31 am

My Venom hybrid has 2 gaps. I can't say what the difference would be with one because i haven't tried it. 2 seems to do really well and HGDT gives large improvement predictions with 2. Also, 2 is good to have for standard combustion use when not using it as a hybrid. So if you plan on using it as a 1x launcher as well, definitely use 2 gaps.
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Unread postAuthor: McCoytheGreater » Thu Dec 24, 2009 3:45 am

^
| |
| |
| |__
| | |__
| |/^| |__
| / | /| | |
|/_|/_|_|_|
| /
|_______ /

...What he said.
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Unread postAuthor: theBOOM » Thu Dec 24, 2009 4:23 am

Moonbogg the problem is with a stungun you can't get really anything more than a 3x or 4x mix... so you would have to go something bigger that isnt portable...
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Unread postAuthor: ZRTMWA » Thu Dec 24, 2009 9:49 pm

I know this is like the 10th time I've said thanks but I love the help I got on this one. I finally got dual sparks jumping!

In case anyone was wondering here are the designs for the combustion spudder I built (The Green Hornet, except with a 9" chamber):

http://www.members.tripod.com/potatogun ... nplans.htm

I made "The Green Hornet" (it's about half-way down the page), with a 1.5" by 3'6" barrel and a 1.5" by 1' barrel. I know it's really mini compared to what most of you guys build but I still love it, who knows maybe one day I'll build another spud gun! Merry Christmas!!!
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Last edited by ZRTMWA on Fri Dec 25, 2009 12:45 am, edited 1 time in total.

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Unread postAuthor: SpudBlaster15 » Thu Dec 24, 2009 10:54 pm

theBOOM wrote:Moonbogg the problem is with a stungun you can't get really anything more than a 3x or 4x mix... so you would have to go something bigger that isnt portable...


I've used a standard BBQ igniter at 10x mixes. A stungun should be capable of even more; you simply need to make the gaps small.
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Unread postAuthor: ZRTMWA » Sun Dec 27, 2009 9:56 pm

OK, so I tried it out a few days ago with a potato down the barrel and it wouldn't ignite. And as usual I haven't an idea why. I tried using WD40 and propane for accelerants but neither lit! Anyone have any idea what the problem is??? I haven't and I don't plan on opening up the chamber while the ciombustibles are inside, so I'm assuming the problem is that the sparks aren't gapping. Maybe the accelerants just add a tiny bit more resistance? Thanks.
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Unread postAuthor: Insomniac » Mon Dec 28, 2009 12:14 am

The propellant can indeed add a little resistance, but unless you are using high-mix hybrids (which have both more propellant and more air) this sholdn't be a problem.

Your issue is most likely that you don't have the right amount of propellant. For propane, you only need about 4% fuel and 96% air for the most powerful combustion. I bet you have flooded your chamber with too much fuel, and now there isn't enough oxygen for it to burn.

Try this. Empty out the gun, get rid of both the potato and let any fuel dissipate (take off the end cap). Once the chamber has been flushed of all fuel, give it a quick one-second spray, close the chamber up and see if it ignites. If not, open it up again, vent out all the fuel, and try 2 seconds. Repeat until you find the amount of fuel that lets it ignite, and then try it with a potato. Also remember to vent all the burnt fuel out of the chamber after each shot.
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