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Venting?

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Venting?

Unread postAuthor: Tshylea » Sat Dec 09, 2006 12:46 pm

How necessary is it to vent a hybrid? By that I don't mean do you need new air, but do you need to get rid of whatever is left after it is fired? Say you were using a no-burstdisk piston, like a supah, and you closed it immediately after firing. Could you just inject more propane and more air into the chamber, or would you have to get rid of whatever exhaust might be left inside? If you had the correct amount of propane and the correct amount of air, would it matter if you didn't let the exhaust out?
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Unread postAuthor: benstern » Sat Dec 09, 2006 2:41 pm

Yes, you will need to vent all residual exhaust gases.
This also restores the O2 concentration to atmospheric levels which is the main point of venting.

Keeping exhaust gases in would screw up the fuel/air ratio and lower performance significantly.
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Unread postAuthor: Tshylea » Sat Dec 09, 2006 4:28 pm

benstern wrote:Yes, you will need to vent all residual exhaust gases.
This also restores the O2 concentration to atmospheric levels which is the main point of venting.

Keeping exhaust gases in would screw up the fuel/air ratio and lower performance significantly.


But what if you metered your air, just like your propane? Put in, say, 100cc's of air and 4cc's of propane, why would having other stuff in there make a difference?
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Unread postAuthor: Pete Zaria » Sat Dec 09, 2006 4:32 pm

Because the left-over gases from the previous combustion would upset your perfect fuel/air ratio.

Lets say you've just fired a shot out of your hybrid.
When you inject your mix for the next shot (lets say a 2x mix), that pre-calculated mix takes into consideration that there's fresh air in the chamber; but there's not. When fueling, hybrid mixes assume that there's 14.7psi (atmospheric pressure) of fresh air in the chamber. If this fresh air is actually exhaust from the last shot, the mix will be off, and it either won't ignite or it'll be weak. You could calculate your mixture to compensate for the exhaust gases in the chamber, I suppose, but that'd be kind of pointless.

Just put a ball valve and quick connect on the chamber somewhere to blow out the chamber with your air source between shots.

Hope that explains it.

Peace,
Pete Zaria.
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Unread postAuthor: runningawaypyro » Wed Dec 20, 2006 3:45 pm

so wait are you saying that in hybrids you cant have a computer fan in the chamber like in normal combustions? how would you safely connect a pvc ball valve up to a sch80 chamber? do they make sch80 pvc ball vavles?



another queston i have is does the barrel of the hybrid also have to also be sch80 just like the the chamber? you can just use sch40 because the pressure in the pipe isnt as great because the pressure is decreaseing as the projectile is exiting right? but how do you connect the sch40 pipe to the sch80 union because im sure they have different diamerters.


another queston i have is about the ignition source. i have a stun gun set up on my combustion which i love to death. But with i hybrid incase something goes wrong i would like to have at least a 20ft wire to fire a little more remotly away from the cannon. but if i were to just lengthen the wire from the stungun to the chamber the spark would decrease right? im thinkin if there is some sort of circuit i could put together that would use a few batteries at the far end of the wire to activate a switch somehow that would flip the switch on the stun gun itself. sort of like a momentart switch?

i know its alot of questons but i would REALLY aprecciate your guys help!!


thanks alot
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Unread postAuthor: Insomniac » Wed Dec 20, 2006 4:18 pm

Well, with both regular combustions and hybrids, you take off the threaded cap and the fan blows out all the used gasses out the barrel.
It would be a good idea to use a sch 80 barrel. This is a hybrid we are talking about, you REALLY don't want to mess up.
Well I'm not sure of the circutry in a stun gun, but if you have any knowlege in electronics heres what I suggest doing. Open the case of the stungun and cut the wires going to the switch that makes it spark. Remove the switch and attach some really long wires to the wires you just cut, connecting the switch and the stungun back together. I'm not sure this will work though, as I have never seen the internals of a stungun.

Also, I STRONGLY ADVISE that you don't build a hybrid. THEY WILL KILL YOU if you screw up. Really, If you haven't built a lot of combustions and pnumatics yet, DO NOT build a hybrid. You need to understand all the concepts involved, have plenty of experience working with pvc, and LOTS of experience with spudguns in general.
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Unread postAuthor: runningawaypyro » Wed Dec 20, 2006 8:13 pm

Thanks for the advice!! im not so sure that i can take apart my stungun. it doesn't have any bolts holding it together. Wouldn't a 10 ft of sch80 2" pipe be really expensive? Im thinking the large the combustion chamber and the longer hte barrel the more time the projetile has in the barrel alowing the velocity to increase makeing further distances. right? but the larger the chamber would be more dangerous right? because the chamber's volume is increasing but so is the fuel. so is there a size that would just be ridiculously dangerous to surpase that i should know?


i did some serious research due to a low budget and managed to get a great 100,000volt stungun off ebay for $17 including shipping. :lol: I do alot of high explosives work and i have built two combustion cannons and a large pneumatic. I mihht seem a little unexperinced but i plan on takeing small careful steps.
Your help is much apreciated!!
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Unread postAuthor: squeaks » Wed Dec 20, 2006 8:17 pm

okay people. For venting a hybrid it really isn't that hard. You're using an air compressor, right? Then why don't you just give it a few second jet of compressed air to vent it? Hello! It's just that simple!
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Unread postAuthor: Insomniac » Wed Dec 20, 2006 8:22 pm

@runnignawaypyro
Well if you have the experience then go for it! Instead of building a big one, why not make a smaller metal one? oh and for remote ignition you can use speaker wire to run a cable from the stungun electrodes to the gun. Also, If you deal with explosives, why not put a very small 'ignitor' charge in the gun?. Not an explosive, just somthing to make a flame?
There is not so much of a size limitation on hybrids( although bigger ones are definatly more dangerous) The real limit is the mix. Most hybrids run at a maximum 3x mix (3 times atmospheric pressure), and some push it to 4x. Anything higher than 4x runs a huge risk of causing DDT ( you may be familiar with the term if you deal in explosives, but Ill explain it anyway) Which is deflagration to detonation transition. This happens when the flame front moves faster than the speed of sound, causing a shockwave that will blow most materials to bits, or at the least cause extreme stress on the chamber.
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Unread postAuthor: runningawaypyro » Wed Dec 20, 2006 9:42 pm

hmmm... so im a little lost on how conecting speaker wire to the sg would work. do you mean in line with the switch? yes well i will try that and as for puting in a fan that last comment made me realize how obvious that problem could be solved. thanks. oh and about useing ematches to ignite the chamber fuel would be a total waiste for a spud gun. its not like pyro hobiests just run through igniters like water. they do take work and money to make instead of pushing a button uesing a ready available battery when i already have a stun gun and a 15,000v neon sign transformer. but still im not sure if i can aford to buy the chamber AND the barrel in sch80... but i see you right. il just have to save up. i think iv found what i want for chritmas.

"so what would you like for christmas this year?"

> "oh ya know just some sch80pvc pipe and fittings..."
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Unread postAuthor: runningawaypyro » Wed Dec 20, 2006 9:48 pm

oh and about useing metal. i cant weld or fabricate like that. :cry: has anyone made a hybrid with resoanble priceing that didnt envolve a bunch of expensive welding equipment?
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Unread postAuthor: pyrogeek » Wed Dec 20, 2006 9:59 pm

Running wires from the electrodes of the stungun will probably reduce the amount of current that can flow and create the spark because of the resistance of the wire. But, I have done it, and it still works just fine on a regular combustion. In the future I plan to wire up a stungun with a relay on the switch, so I can have the stun gun on the spud gun, and just have a long wire running off that relay to trip the stun gun. Just a wire and a lantern battery would do, but I want it to look cool so I want to have an LED to show that theirs power, a couple switches and have it all in a nice box. This would also double as an ignition unit for "model rockets".

The cases on stun guns usually have to be pryed apart since they are glued. I just used a little water tight tackle box thing for the ignition on my big combustion. It holds everything easily, and it's easier to get to batteries that way. They aren't that expensive, just a few bucks.
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Unread postAuthor: Insomniac » Wed Dec 20, 2006 10:24 pm

Runningawaypyro: In my first post I was talking about running the wires from the switch, so the sg was still attached to the launcher. In my second post I ment that you could just hold the sg and have the high voltage electrodes wired to the gun via long speaker wire. Pyrogeek is right though, running the long wires will make the spark shorter. If you can't afford both barrel and chamber, just lessen the dimensions of the gun until you can afford it. Even 'little' hybrids pack a hell of a punch, and your better off with a well built, smaller hybrid than a badly built, larger one. Have you thought about the metering system you will use??
I saw a hybrid the other day, I think it was called the mauler hybrid. The guy who built it said the recoil made his shoulder go purple! (It was mounted on a rifle style stock)
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Unread postAuthor: runningawaypyro » Wed Dec 20, 2006 10:44 pm

hhmmm yes i agree. However, in p=my opinion from what iv heard from about hybrids i WOULD NOT should one. thats REALLY inteligent. its ok with a combustion because your only looking at max 50psi and a pneumatic is very controled but a chamber full of EXTREMLY rapidly expanding gas inches away from my face... no thanks.

i will build a stand for sure.
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Unread postAuthor: Insomniac » Wed Dec 20, 2006 10:48 pm

Well my point wasn't to encourage shoulder fireng (except at a 1.5 mix or somthing low like that) I was just saying that hybrids are so ridiculously powerful that even a small one would shoot incredibly fast.
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