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Ignition placement

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 placement

Unread postAuthor: Arcticpheonix » Thu Mar 13, 2008 10:24 pm

I tried searching for this, but I couldn't find anything. My question is, where in the chamber should the ignition source be? I'm thinking as far back as possible, so that as much gas as possible is used to propel the round. Am I right in this assumption?




A graphic in case my explanation sucked xD

$ = igniter
#'s = ignition sequence

:::::::4321$] As opposed to :::::::21$12] in which the right half of the gas is wasted.
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Unread postAuthor: bigbob12345 » Thu Mar 13, 2008 10:31 pm

just do duel ignition but if not I actually think you should put it in the middle of the chamber because it creates the pressure quiker as it will spread through the chamber quicker. Its not the flame accelerating to the projectile that propels it, it is the pressure generated and putting it itn the middle would create more pressure faster.
I knoow the above post is hard to understand but just read it over a couple times and you should get it.
and if im wrong will some more experienced members please correct me.
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Unread postAuthor: jon_89 » Thu Mar 13, 2008 10:34 pm

I put one in the back and one in the middle but that is for dual gaps. For one spark I would put it about one third of your chamber length from the end.
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Unread postAuthor: Arcticpheonix » Thu Mar 13, 2008 11:00 pm

bigbob12345 wrote:putting it in the middle would create more pressure faster.
me.


Hmm, that does seem to makes sense. Can anyone second this?


Jon_89 wrote: For one spark I would put it about one third of your chamber length from the end.


Which end, and why 1/3?[/quote]
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Unread postAuthor: keep_it_real » Thu Mar 13, 2008 11:38 pm

For one ignition source, put it in the middle. I remember reading this from someone else who experimented with it and found that the middle was best.

The theory (my theory): The flame can travel in both directions rather than starting from the back making for faster combustion. Once the entire chamber has been ignited, the gas can only move out the barrel.
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Unread postAuthor: jimmy101 » Fri Mar 14, 2008 12:22 pm

It is generally assumed that a central spark is the best. Like Keep_it_real said, this gives you two flame fronts, one moving forward and one moving backwards.

If the spark is at the back of the chamber one of the flame fronts burns out soon after ignition, cutting the remaining burn speed in half.

The same happens with a spark near the barrel, on front burns out well before all the fuel has been burned.

There have been suggestions that perhaps the spark should be near the barrel end of the chamber. Even though one flame front burns out early the remaining front is moving towards the breech. The gases at the breech end of the chamber are expanding (due to heating) and are being accelerated towards the flame front. This might be beneficial though nobody has ever proven it.

I don't believe anyone has ever actually measured the change in performance with the spark located in different places.

The peak pressure in the chamber is probably relatively insensitive to the spark location. It is the speed of combustion that is most likely to change as the spark location is changed.

You can avoid the entire question of spark location by putting in several spark gaps. A gap every chamber diameter would probably be optimal but in the typical sized chamber three gaps located at perhaps 0.2, 0.5 and 0.8 of the chambers length would be a good guess.
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Unread postAuthor: Jared Haehnel » Fri Mar 14, 2008 12:30 pm

I don't believe anyone has ever actually measured the change in performance with the spark located in different places.


Sound's like an interesting experiment if some one has some free time on their hands an a chronograph I would be very interested in the results 8)
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Unread postAuthor: BigGrib » Fri Mar 14, 2008 1:09 pm

I agree 100% with jimmy. If you did have just one gap, smack dab in the middle would be the best, but for two gaps, now that I think about it, you would want want to put your first gap 25% of the way in and the second 75% of the way in that way the flame propogation rate would reach the ends and the middle of the chamber at the same time. tell me if this doesn't make sense.
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Unread postAuthor: jimmy101 » Fri Mar 14, 2008 1:33 pm

BigGrib wrote:tell me if this doesn't make sense.

Makes perfect sense, of course, that doesn't mean it is true. :D
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Unread postAuthor: BigGrib » Fri Mar 14, 2008 3:27 pm

Well if I was smarter I'd do the math but ow my head hurts already just thinking about math. It would be cool if we had some sort of computer model showing how the flame spreads out from each ignition point.
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Unread postAuthor: Arcticpheonix » Wed Mar 19, 2008 10:54 pm

BigGrib wrote:Well if I was smarter I'd do the math but ow my head hurts already just thinking about math. It would be cool if we had some sort of computer model showing how the flame spreads out from each ignition point.


Agreed on both counts. As it is my math is shaky enough, I do all my calculations 3 times just to make sure I did them right. lol

jimmy101 wrote:A gap every chamber diameter would probably be optimal


The question then becomes, how much would that weaken the chamber? Do you think it would do so enough (with a well sealed chamber) to be a problem?
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Unread postAuthor: MaxuS the 2nd » Thu Mar 20, 2008 3:12 am

Wouldnt weaken the chamber anymore than usual. Just use a Spark Strip with X number of gaps. Say, if you had a 100mm chamber diameter and you were using 50cm of it. Then you would have 5 spark gaps evenly spaced.

Length/Diameter = Number of Spark Gaps needed.
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Unread postAuthor: mark.f » Thu Mar 20, 2008 6:31 am

The math has already been accomplished. Look in the archives for Jimmy's work in this area. With a gap in the middle of the chamber every chamber diameter, the flame front can expand in "spherical mode" instead of "laminar mode", in which spherical mode is faster.
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Unread postAuthor: jimmy101 » Fri Mar 21, 2008 2:31 pm

MaxuS the 2nd wrote:Wouldnt weaken the chamber anymore than usual. Just use a Spark Strip with X number of gaps. Say, if you had a 100mm chamber diameter and you were using 50cm of it. Then you would have 5 spark gaps evenly spaced.

Length/Diameter = Number of Spark Gaps needed.


I wouldn't use a spark strip. It seems to me that the physics of the flame propagation from a spark strip are going to suck.

You want the sparks as far away from the chamber wall as you can get them. A spark strip mounted on the wall (ala Latke) is going to burn slower and loose heat to the PVC faster than a spark out in the center of the chamber.

I believe this question falls into the "lots of theories, no real data" category.
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