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double chamber combustion. need advice

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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double chamber combustion. need advice

Unread postAuthor: alex bennett » Thu Sep 14, 2006 4:25 pm

Okay so I'm at homedepot and I find myself on the PVC isle, and just by chance my eye catches this PVC peice, a 90 degree elbow, 3'' on one side, 4'' on the other. So I thought why get 4 of these, and make a double chamber combustion. It is 3''x20''ish on the bottom with a latern sparker screw on plug, then on each end a 3'' to 4'' 90 degree elbow, with 4''x12'' on each of those, then another pair of elbows to a 2 3''in, 1 2'' out with a 2''x5' barrel. what i need advice on is what fuel to use, and how to deliver it best. Currently i use denatured alcohol and the latern sparker, it works i just have to use the leaf blower to clear the chamber before i refire.
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Unread postAuthor: schmanman » Thu Sep 14, 2006 4:30 pm

good idea, I've never heard of anyone using a leaf blower to vent the chamber. are the pieces pressure rated?
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Unread postAuthor: cannon freak » Thu Sep 14, 2006 4:32 pm

I would suggest propane, I have a very similar two chamber gun except much larger and propane seems to work just fine and is easy to use and obtain.
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Unread postAuthor: jrrdw » Thu Sep 14, 2006 4:46 pm

I want to see a picture of your duel chamber set up, can you post some?
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Unread postAuthor: alex bennett » Fri Sep 15, 2006 9:29 pm

All the peices are SCH 60, I cant find my camera right now...so ill post pics tomorrow
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Unread postAuthor: frankrede » Fri Sep 15, 2006 11:00 pm

sch 60 im iffy on that but you should be fine.

My version would have 2 igniters and burst disks for multiple shots.
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Unread postAuthor: Blackspire » Fri Sep 15, 2006 11:19 pm

How do you plan on getting both chambers to ignite at the same time?
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Unread postAuthor: nicholai » Sat Sep 16, 2006 12:34 am

frankrede wrote:sch 60 im iffy on that but you should be fine.

My version would have 2 igniters and burst disks for multiple shots.


why is that? its thicker/stronger than sch 40.....
sch 40 has a thinner wall thickness than sch 60, sch 60 (rare, ive never really seen it in a store i dont think) has a thinner wall thickness than sch 80, etc etc
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Unread postAuthor: Blackspire » Sat Sep 16, 2006 1:53 pm

Has anyone done this before. I am at the stage right now where I am about to leave on a mission to find all the parts. The theory works, but will the 90 degree elbows and the T stand up to the pressure? Will all the gases explode out of the barrel "efficiently"? If anyone knows that this will not work, let me know asap. If no one has a friggin clue..I guess we'll find out.
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Unread postAuthor: Pete Zaria » Sat Sep 16, 2006 2:10 pm

If the parts are pressure-rated they'll definitely hold the pressure. You want NSF-PW Sch 40 part if possible. Keep the distance between the chambers and the barrel as short as possible, and use the biggest 90's and T you can afford (to increase airflow and reduce constriction), 2" minimum.

Also, I'd put one spark gap in the middle (or towards the back) of each chamber, wired to the stun gun.

Are you using propane injection? This project probably won't work without it. Getting a perfect fuel/air mix in both chambers every time without propane injection is very unlikely. How I'd solve this problem would be to mount the fuel meter so that it injects fuel into the T. Then, put a chamber fan in both chambers, at the barrel side of the chamber, facing backwards towards the cleanout. This way, the fans will draw the fuel into the chamber and mix it properly with the air. You should get a good mixture every time.

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Unread postAuthor: cannon freak » Sat Sep 16, 2006 2:21 pm

Pete Zaria
I'd put one spark gap in the middle (or towards the back) of each chamber

Pete Zaria
How I'd solve this problem would be to mount the fuel meter so that it injects fuel into the T. Then, put a chamber fan in both chambers, at the barrel side of the chamber, facing backwards towards the cleanout


I think you may have a problem with having the spark gap so far away from the fans. The flame front will propagate further away and will have some force once it reaches the fan possibly damaging it.
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Unread postAuthor: Pete Zaria » Sat Sep 16, 2006 2:26 pm

I've never seen the combustion alone damage a fan. The pressure is equal on both sides of the fan, so it doesn't take much shock. Heat from repeated firing can damage them, and if the barrel is too horribly long they can be damaged by the vacuum effect of a way-too-long barrel (check out http://www.burntlatke.com/15cb-testday.html for info).

I figure having the spark gaps towards the back is better, because that way the flame will propagate forwards, towards the barrel and projectile, increasing in force as it goes. I have no evidence to back that up; just my suspicion.

To be safe, if you're using a stun gun, just do two gaps in each chamber (one near the fan in the front, one in the back). They don't need to be very big, only 1/8th" or so will work.

By the way, you can wire both fans to two 9v batteries with an off/on switch and they'll work fine. It's overvolting them a bit, but since you wont be running them very much, it should be fine.

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Unread postAuthor: cannon freak » Sat Sep 16, 2006 2:46 pm

Most people use a smaller chamber and have the fan near the spark gap, however having it further would mean that the flame front hits the fan, the pressure won't be equal on both sides at least not when the flame fron hits it. Anyway thats just what I think, I could be wrong.
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Unread postAuthor: Blackspire » Sun Sep 17, 2006 9:40 am

Well I priced it out yesterday, the ABS alone would be around $80 canadian. Plus if I did a bbq lighter set-up it would come out somewhere around $130. Eventually I might build it, but I want to take my time and make sure it works. Thanks for all the input guys, keep it coming as I am sure other people are interested. My buddy made a drawing of an idea that he had, so once I get it I will post it on here.
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Unread postAuthor: SpudStuff » Sun Sep 17, 2006 10:36 am

This Guy Did It
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