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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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Dual Ignition problems

Unread postAuthor: ZRTMWA » Tue Dec 22, 2009 3:54 pm

I drilled four screws into my chamber 2 sets of two. The sets are about 6 inches apart and the screws are about 2mm apart inside the chamber. Then I connected the two sets with thick wire. I attached a bbq igniter two one set but can only get the spark to jump through one set. Anyone had this problem and know the answer? Thanks.

By the way does dual ignition really make a spud gun much more powerful? Thanks again.
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Unread postAuthor: Heimo » Tue Dec 22, 2009 4:11 pm

I would say a BBQ igniter is a little too weak to fire two spark gaps get a stun gun or a something similar this is just me guessing cause I am no expert by any means
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Unread postAuthor: Hubb » Tue Dec 22, 2009 4:17 pm

A diagram would help, but I would think that if you are getting just one spark, then you have them wired wrong. Search the forum and the wiki on wiring the spark gap and revealed to you, the answer will be, young jedi.
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Unread postAuthor: SpudBlaster15 » Tue Dec 22, 2009 5:01 pm

It sounds like you have the gaps wired in parallel. You need to wire them in series, otherwise the current will flow through only the circuit with the least resistance.

Heimo wrote:I would say a BBQ igniter is a little too weak to fire two spark gaps get a stun gun or a something similar this is just me guessing cause I am no expert by any means


A BBQ igniter will jump 10 gaps if you make them small enough (~1mm). 2 moderate sized ones is not an issue at all.
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Unread postAuthor: ZRTMWA » Tue Dec 22, 2009 6:23 pm

Oh yeah, I feel like an idiot, realizing a wired them wrong thanks for the quick responses. Here is a diagram of how I wired them:

http://s779.photobucket.com/albums/yy80 ... g&newest=1

I am still a little confused about how to wire in series though if someone could explain a little further it would be much appreciated.
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Unread postAuthor: SpudBlaster15 » Tue Dec 22, 2009 6:32 pm

Think of the circuit as a path the electricity needs to take in order to return to the igniter. You want to make this one continuous path, with no way for the current to make a loop without crossing both spark gaps.
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Unread postAuthor: ZRTMWA » Tue Dec 22, 2009 7:04 pm

Oh so I need to connect the two screws in the far set to each other, right? Thanks a lot you guys are the best.

Edit: Obviously the wrong answer because I just thought about it more and that would require no sparks to jump. I guess I'm electrically retarted or something.

Double Edit: Just thought some more and realized, would I connect one end of the igniter to one screw in set A and the other end of the igniter to one screw in set B? Then connect one wire from the other unused screw in set A to the other unused screw in set B? Pretty sure this is it!!


A couple of minor questions as well:

1. Any good links you can provide for an explanation of how dual ignition eventually leads to more power.

2. Is it really true that the volume of the chamber should be about 1.5 times the volume of the barrel? Thanks as usual.
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Last edited by ZRTMWA on Tue Dec 22, 2009 7:20 pm, edited 1 time in total.

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Unread postAuthor: kjjohn » Tue Dec 22, 2009 7:18 pm

More ignition points greatly increases the power of a cannon, because it adds more heat to the mixture. In some high-mix hybrids, the pressure in the chamber can increase by as much as 200psi by adding several more spark points. In fact, I prefer to use at least 8 spark gaps in my launchers.
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Unread postAuthor: ZRTMWA » Tue Dec 22, 2009 7:24 pm

Jesus christ!!! 8 gaps?!?!? I guess I might add some more. But in only combustion (completely non- pneumatic) spudders the pressure rarely exceeded 30 to 40 psi right?

Edit: Care to make me a diagram of how to wire six sets of screws in series, kjjohn? I can only figure out how to wire two sets of screws in series :(
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Unread postAuthor: niglch » Tue Dec 22, 2009 8:11 pm

8 gaps could probably be considered a little over the top for most cannons. However, I have gotten 2 gaps working pretty consistently in my old aerosol cannons with a regular piezo BBQ igniter. You just need to make the spark gaps small enough (they don't need to be very big anyway to do their job).

And yes, aerosol fueled guns usually do not reach very high pressures. You'll get a slightly higher peak pressure with more spark gaps since you are essentially just decreasing the fuel burn time by igniting it in multiple places (this is where the increased power comes from).

As far as the chamber to barrel volume ratio, 1.5:1 is typical probably because it provides good power without wasting too much fuel to doing nothing more than making noise after the potato leaves the barrel.
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Unread postAuthor: SpudBlaster15 » Tue Dec 22, 2009 8:13 pm

Multiple gaps can increase power by burning the air/fuel mixture more quickly, which creates higher chamber and average barrel pressures under some circumstances.

How much effect they actually have depends on many variables, such as chamber geometry, barrel length, type of fuel, etc. Most 'standard' sized guns won't see much improvement, but if you have a relatively long chamber or short barrel, there will be larger gains.

Unless your chamber has the volume of a salt mine, more than 2 gaps is overkill, and doesn't add much, if anything, to performance.
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Unread postAuthor: STHORNE » Tue Dec 22, 2009 8:24 pm

This is my quick diagram for a quad-spark system.

Shouldn't need labels but if you do, just ask =]
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Unread postAuthor: theBOOM » Tue Dec 22, 2009 8:30 pm

Well, I've always thought that more spark gaps didn't increase effiency of a cannon by burning the fuel more quickly, I thought that it increased it's performance by creating more flame fronts, which in turn would generate more pressure, thus giving better results... correct me if im wrong...

Jimmy101 did some research on this, I'll wait until he jumps in to clarify this ..
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Unread postAuthor: SpudBlaster15 » Tue Dec 22, 2009 8:34 pm

theBOOM wrote:Well, I've always thought that more spark gaps didn't increase effiency of a cannon by burning the fuel more quickly, I thought that it increased it's performance by creating more flame fronts, which in turn would generate more pressure, thus giving better results... correct me if im wrong...


Well, what happens when you create more flame fronts? The fuel burns more quickly of course. That's where the pressure increase comes from.
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Unread postAuthor: starman » Wed Dec 23, 2009 1:45 am

kjjohn wrote:More ignition points greatly increases the power of a cannon, because it adds more heat to the mixture.


The extra heat from extra spark gaps is so small as to be irrelevant. The effectiveness of multiple gaps is all about their location in the chamber.

Imagine you have one spark gap, located at one end of the chamber. You will get effectively one flame front that must travel the entire length of the chamber. Simply moving the single spark gap to the center of the chamber will allow 2 simultaneous flame fronts traveling in both directions toward both ends of chamber, burning all of the chamber fuel twice as fast as installing the gap at the end of the chamber. Now install 2 gaps, one at the 1/4 mark and one at the 3/4 mark in the chamber. You have 4 flame fronts, 2 headed toward both ends of the chamber and the other 2 meeting in the middle, all completing the burn about the same time and again roughly halving the burn time of the single gap in the middle.

This faster burn means applying higher pressures to the projectile sooner in the barrel. The slower burning chamber typically has the projectile leaving the barrel well before full pressure has built up in the chamber, wasting much of its energy as heat and noise.

Check out Trip Thunder. It was designed with 3 roughly equally spaced mid-chamber gaps. Ignition time is very fast...and coupled with a burst disk breach, you have high and peak chamber pressure applied to the projectile very early in its travel down the barrel.

It's all about gap location in the chamber guys. Multiple gaps that are close to each other relative to the size of the chamber or otherwise misplaced are useless. I would rather throw a well though out single central gap and call it a day. Faster burn chambers have a performance difference that is very humanly noticeable.

Heimo wrote:I would say a BBQ igniter is a little too weak to fire two spark gaps get a stun gun or a something similar this is just me guessing cause I am no expert by any means


A BBQ ignitor is easily capable of arcing 2 or more properly configured gaps. Keep the gaps as narrow as possible. 1/16" or so...
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