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Does size matter? -lol

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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Does size matter? -lol

Unread postAuthor: broken_system » Wed Jan 06, 2010 6:21 am

Question: Does the size, or rather the dimensions of the combustion chamber matter for a good combustion? I.e. Does it have to be a big round combustion chamber or can it be longer and skinnier?

Thanks!
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Unread postAuthor: jackssmirkingrevenge » Wed Jan 06, 2010 6:33 am

There's no doubt that a short fat chamber provides a better burn than a long skinny one of the same volume, though I don't think anyone has ever quantified the actual advantage.
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Unread postAuthor: broken_system » Wed Jan 06, 2010 6:36 am

Say the fuel/air mixture could be pulled into the chamber, evenly distributing it, would this still combust the same as a big chamber?
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Unread postAuthor: jackssmirkingrevenge » Wed Jan 06, 2010 7:37 am

Have a look at the high speed videos in this thread,it's to do with the propagation of the flame front.

Technically, a short and fat chamber is better, but I cannot tell you if it will give you 20% better power, 50% better power etc. because as far as I know no one has done any testing to find out what a difference it actually makes.
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Unread postAuthor: Brian the brain » Wed Jan 06, 2010 1:15 pm

Wouldn't ..one side going off prior to the other- result in the unburnt side being compressed before combustion??

wich would mean a higher peak perhaps, late in the process??

I'm not sure..no combustion guy..
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Unread postAuthor: JDP12 » Wed Jan 06, 2010 1:29 pm

I would say short and fat. That's how I've always built mine
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Unread postAuthor: broken_system » Wed Jan 06, 2010 1:39 pm

Mainly the reason I wanted to go for longer and skinnier was to have a plunger-type-thingy that would vent the waste and bring in fresh air/fuel (I'm trying to make a semi-auto). So in my case for convinience a smaller longer chamber would be better. I could set up something similar for the larger rounder variety however I wanted to go for a more compact design.
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Unread postAuthor: jimmy101 » Wed Jan 06, 2010 3:06 pm

Broken_

If you are actually going for a semi then I wouldn't worry about long skinny versus short fat chamber. The semi auto should be what is driving your design. If it is easier to do long and skinny then that is fine, the difference in performance isn't going to be all that great anyway.
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