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Mini hybrid => help?

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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Mini hybrid => help?

Unread postAuthor: CpTn_lAw » Thu Mar 20, 2008 10:44 am

Hey everybody! This afternoon i have some time, so i decided to try an make a really mini hybrid. Ok, first, it's all steel, 1/2" chamber, i inject butane via a syringe, and the sparker is a piezo sparker from an old BBQ.
Ok, the volume of my hybrid is approximately 40 ml, (40 cubic centimeters),
so i calculated that i need 1.5 cc (ml) of butane per X. I do so, opening the butane bottle a little, placing the syringe directly in, and gently pulling the piston until i reach any multiple of 1.5 (just to make it easy).
I inject this by plunging the syringe into the chamber through the union, then i take it out, and quickly put the burst disc, sealing everything. I then add air, until i reach 1 bar (15 psi) clode the safety valve, clic clic....nothing....

It works on normal combustion at 1x, although it's not very consistent in the number of working shots. the spark gap is roughly 3-4 mm.
here is a picture. Any help is greatly appreciated!

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Unread postAuthor: Novacastrian » Thu Mar 20, 2008 11:14 am

Looks like i have a rival- my mini hybrid is 47ml. Try reducing your spark gap Captain, i know that bbq piezos are good up to 8x mixes, to obtain this i run a gap of around 1mm. A gap of 3-4mm should work at a 2x though and leads me to believe that you may have fueling problems, either the wrong amount of fuel or inadequate mixing after injection.

Much luck with getting it to work consistently, i have found that mini hybrids are heaps of fun when you get the bugs worked out. :)
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Unread postAuthor: CpTn_lAw » Thu Mar 20, 2008 11:22 am

lol, i ain't tryin to rivalize don't worry!!! i pumped it to see how much pressure could 8 layers of aluminium foil could take in the 1/2" union...my pump didn't go high enough to break it...(280 psi). 4 layers burst at 150 psi with a very very loud noise, even for a mini (i was like "wtf!!").
I'll try reducing the gap in a few moments, and make the fueling as rigorously as i can. I'll keep you posted.
BTW thanks for the hints :D
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Unread postAuthor: SpudFarm » Thu Mar 20, 2008 11:37 am

may be that when you use a syrlinge like that you get butane in but the air goes out.. try put the disk in and make a seal so you can fill through the ball valve :)
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Unread postAuthor: CpTn_lAw » Thu Mar 20, 2008 12:18 pm

yeah i know air goes out, but in that case, i can simply add a little more air, and still get a 4 % mix. The gap is now 1mm....still no ignition. burst disc is so loud when i get to fire with air...that is like a flashpowder fire cracker.
Still...it freaks me out that it doesn't work...any onther help?
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Unread postAuthor: POLAND_SPUD » Thu Mar 20, 2008 1:56 pm

wow it' great.. I hope you find way to make it work.. I am also thinking about building 1/2" hybrid

I have no idea why it doesn't work... are you 100% sure that you have measured the chamber volume correctly?? try filling it with water or something - it is very difficult to predict the volume of fittings
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Unread postAuthor: CpTn_lAw » Thu Mar 20, 2008 4:30 pm

Man...you just solved my hypothetical problem xD i'm gonna do like you said and i'll see if i miscalculated something. in a mini, the smallest change can affect everything...This said, in pneumatic mode, it's freaking powerfull.
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Unread postAuthor: DYI » Thu Mar 20, 2008 5:13 pm

1.5 cc (ml) of butane per X


Well there's your problem...
Read "Hybrid Fueling 101". Even if you do a liquid volume measurement of the chamber, your calculations won't give you perfect fueling.
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Unread postAuthor: POLAND_SPUD » Thu Mar 20, 2008 6:34 pm

yeah DYI is right but one way or another you have to measure it correctly... you know with a burst disc placed in the union etc.
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Unread postAuthor: Novacastrian » Thu Mar 20, 2008 8:59 pm

I thought that the correct percentage of butane was 3.2%, had any luck Captain?
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Unread postAuthor: CpTn_lAw » Sat Mar 22, 2008 6:49 am

well thank you all for your tips, i'm gonna try everything you suggested right after lunch ^^ . i'll keep you posted with the advancements.
btw, i've done some 300 + psi launches with air only, and CO2. Well, it's loud, powerful and as it's small, you can feel the recoil !



EDIT :
Ok now correct me if i'm wrong :
perfect mix with butane and air is 3.2% butane for the rest of air. now, i have a chamber volume of roughly 37 ml. I must then inject 1.1 ml of butane but the volume of air in the chamber is now 35.9 ml. So i have my 3.2% of butane and the rest of air. if i leave it a few seconds in the hybrid and ignite, it will 98% of chances ignite without problems right?
But as a matter of fact it only works when doing a normal combustion.
any idea?
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