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My first combustion: input needed

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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My first combustion: input needed

Unread postAuthor: souljahz_unite » Mon Apr 23, 2007 7:27 pm

Hey,

I am making my first combustion cannon and through reading posts in the forums and reading the wiki along with tutorials and sites, i still found some questions un-answered.

First of all, I am using abs pipe, but as i read, it should be fine for the average combustion cannon, which is exactly what im going to build. The combustion chamber will be a 3" diameter and 18" long. The barrel is 1.5" abs pipe and 36" long. My initial plans involved having a barrel only 28 inches long but changed it to make a C:B ratio of 2:1.

The first question: By having a combustion chamber half the size of my barrel, will i lose power or lose ignition altogether due to the size of the chamber? A video tutorial i just watched called for a 10" - 12" chamber which seems small. The video did state however, that pressure would be lost if too big a chamber was used. Is this true and will i lose ignition because of the sizes?

My second question is dealing with the ignition. I plan on using a piezo bbq lighter but wanted to know if i would have a weaker spark if i made the wires longer. Sort of like having remote ignition except it is wired. Just a thought to try the cannon for the first time and not have it blow up right beside me (if it does).

Sorry for the long post but i just want to make sure im not doing anything wrong before i make it. My parents arent fond of the idea but id rather have it right rather than them telling me they told me so.

Thanks for any help.
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Unread postAuthor: Kenny_McCormic » Wed Apr 25, 2007 4:23 pm

1 you wont lose any power by having to large of a combustion chamber it will just be louder and less efficient, and making too big of a combustion chamber wont make it lose pressure

2 ye it will get weaker but 3 feet of wire wont bother it to much(just put a pallet between it and you if you are worried but i wouldnt abs doesnt tend to frag when it fails)
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Unread postAuthor: paaiyan » Wed Apr 25, 2007 4:29 pm

Well power shouldn't be a problem. As for the spark, does anyone know the actual amount of power a piezo puts out? Anyhow, my ignitor uses 8 AA batteries that heat up a heat up a smashed christmas bulb causing it to oxidize rapidly with a very bright flash and intense heat. I have about 30 feet of wire going both ways that carry the current fine.
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Unread postAuthor: homeflake210 » Wed Apr 25, 2007 5:30 pm

I think you would get a bit more power going with a 4'+ barrel.
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Unread postAuthor: spudthug » Wed Apr 25, 2007 5:35 pm

4'+ naw use 12 inches of 4 inch down to a 2 inch coupler threaded and then u can have interchangeable barrels 2 inch for big ass potatoes and caulk tubes and 1.5 for standard potatoes..why do u want remote ignition? this is only a regualr combustion its not like u cant hold it because of intense power...and a peizo with a foot minus wire shoudlnt decrease it any..a piezo puts out like a 3/8 gap i think sgort said on his site
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Unread postAuthor: souljahz_unite » Wed Apr 25, 2007 8:14 pm

i would go for a 4" CC but for now i just wanted a tester that would produce suffiecient results for me to want to continute making spud guns. Its sort of a project i wanted to show off to my friends and having the knowledge to produce better looking guns too.

Sound isnt too much of a problem for me. Where i live a little bang or anything wont do much...unless its huge i can drop it and run so sound wont matter.

The *sortof* remote dentonation was just more of a random idea so that it would be nice for a little video i might make. Having it look likes it fires itself might look cool so i dont know... just a thought.

Im really interested in the whole pnematic idea with copper pipe but have NO clue whatsoever the use of a piston or sprinkler valve or whatever is used does. I was just hoping that startign with a basic combustion would help be get on with learning more about the better stuff.

I bought my materials today :

-3" ABS - a 3 ft long pipe i'm going to cut to 3' pieces.
-1 1/2" ABS - 6 ft long pipe ill cut in half for the barrel
-3" - 1 1/2" reducer
-Female threaded adapter 3"
-Male threaded End cap 3"
-The piezo ignitor- which i still gota figure out how to have two wires coming from
-ABS cement

Is anything missing from the list or am i good to go?

Thanks again and sorry for the long post.
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Unread postAuthor: Juicey » Wed Apr 25, 2007 10:55 pm

Looks like you have all the mats you need for it. And just to clarify in your first comment. There really is no such thing as a chamber that is too big, the problem comes into play that a barrel is too short to harness the full power of the combustion. To have maximum performance, you want the potato to exit the end of the barrel just as the combustion comes to an end. Kenny pretty much summed it up though by just saying its less efficient :wink:
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Unread postAuthor: souljahz_unite » Thu Apr 26, 2007 5:52 am

Alright :) . Well i decided to stick with the 1 1/2 ft barrel, just cut it to length like 5 minutes ago. I think im going to go go ahead with teh contruction and then see where it goes.

GOT THE PIEZO WORKING :D im proud =) now onto the ocnstruction. Ill have pix up i guess when it done. Thanks for the help :D.
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