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Igntion placement in Combustion Chamber

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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Igntion placement in Combustion Chamber

Unread postAuthor: Rob748 » Sun Feb 22, 2009 9:40 pm

Is it better to have the ignition in the middle of the chamber or the very end of the chamber ?


I came across something that said the best ignition is in the center of the chamber, I figured you would want the ignition at the very back so the gas can expand the direction you want them to, other than explanding in both directions.

Im a newbie and I probably am wrong I'd just love for someone to explain which one is better.
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Unread postAuthor: trigun » Sun Feb 22, 2009 9:45 pm

Well there is no defiant answer on this subject. For the middle chamber theory it is that: There is two flame fronts going on at once, So the fuel is burned faster and the pressure is created faste

The rear theory is just as you stated I think..... I dont fully understand that principle
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Unread postAuthor: Rob748 » Sun Feb 22, 2009 9:47 pm

Yeah. that makes sense in the middle. but I bet its more dangerous due to the pressure exerted more around the chamber and less towards the barrel.. unless the pressure exerts qually in all directions like archimedes law, or someones law :D
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Unread postAuthor: ramses » Sun Feb 22, 2009 9:51 pm

yeah, as long as you didn't use cell core, you should be fine for a spray 'n prey
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Unread postAuthor: Rob748 » Sun Feb 22, 2009 9:52 pm

I like my at the end theory !!!! Lol :D :D
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Unread postAuthor: Radiation » Sun Feb 22, 2009 10:15 pm

The better answer is multiple gaps spaced evenly through out the chamber.
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Unread postAuthor: ramses » Sun Feb 22, 2009 10:16 pm

or a single gap the length of the chamber, created by a marx generator. but that's just complicating things.
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Unread postAuthor: starman » Sun Feb 22, 2009 11:13 pm

If you have a spark at the end of the chamber, it could take 20-30 milliseconds to burn through the fuel/air in the full length of the chamber. This means a slower build up of pressure and a low powered shot. The projectile has left the barrel before the pressure peaks. A burst disk can remedy some of this problem but not all.

Now imagine moving the single gap to the middle of the chamber, both longitudinally and radially. The burn will run to both ends of the chamber in half the time as before. Now install 2 gaps, each one 1/4th of distance from the both ends of the chamber. Now you have 4 flame fronts...2 running toward the ends of the chamber and 2 running toward each other and meeting in the middle. This theoretically cuts the burn time in half yet again.

Multiple gaps, if placed properly, will have the effect of a fast valve in a pneumatic gun. If you put 2 gaps in your chamber 2" apart and next to the chamber wall, you will not see an improvement.

I think this is why there has been confusion on the multiple gap concept. It's all about location location and location....pure and simple.
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Unread postAuthor: inonickname » Mon Feb 23, 2009 2:35 am

For a single spark gap, dead center in the chamber is best. Ideally, you should have multiple spark gaps spaced along the chamber. Remember that as you use more gaps you will need a more powerful ignition source, for example a stungun, flyback, camera flash dumped into a coil etc.. Multiple sparks down the chamber is often referred to as a " spark strip".

Information on spark strips
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