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hybrid fueling

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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Unread postAuthor: MrCrowley » Tue Jan 25, 2011 12:16 am

Well it all depends on the full scale and whether the accuracy is full scale or not (most gauges are rated to full scale). 2% accuracy at full scale on a gauge that only reads 15PSI as a maximum isn't that bad really and would work on a hybrid like mine. I'd say stay away from gauges that read over 30PSI and keep the accuracy to 2% or below.
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Unread postAuthor: Lockednloaded » Tue Jan 25, 2011 12:47 am

6746K17 is a regulator on mcmaster, but it has 0-25psi output pressure, and ±1% accuracy, so I need a gauge for it. BTW my chamber is about six inches of 1-1/4" galvanized pipe, so its very small.
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Unread postAuthor: jackssmirkingrevenge » Tue Jan 25, 2011 2:52 am

I've found volumetric fuelling using a simple syringe to be highly effective and reliable, without the need for gauges etc.

It's not suitable if you're aiming for mixes past 28x, want an on-board fuelling system as part of the launcher or have a particularly large chamber, but otherwise it's perfectly adequate.
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Unread postAuthor: DYI » Tue Jan 25, 2011 10:59 am

First off, I'd just like to say that I find it disturbing to see experienced members feeding the correct numbers to someone who clearly has no idea how to derive them for himself (and no interest in learning, by the looks of it). Regardless of the fact that modern hybrids are no longer the fragmentation-prone deathtraps they used to be, I still don't think they should be built by people who don't understand them.

Second, JSR is completely correct: for plinker type hybrids, syringe fueling is surely the best way to go. For a cannon though, requiring several liters of propane per shot, they're not the most practical option, and manometric metering becomes more attractive.

Now, as to gauges: McMaster sells a nice cheap range of +/-1% analog gauges (4053K211). They're accurate, well made and, at ~$40, practically free. They come in the required low pressure ranges (I used the 0-15psi model on my early hybrid experiments, which was suitable for up to 20X or so and worked well even at 2X) as well as higher pressure models useful for filling with your oxidiser and buffer gas of choice (or just air, if you're lazy and unambitious :lol: )
For those with more interesting projects, http://www.instrumart.com has a very wide selection, and ships outside the US.

Obviously, required accuracy depends on what you want to achieve - for proper experimentation, one wants to know as precisely as he can afford to just what is in the chamber. If, like most people, you're just trying to make something that goes boom when you pull the trigger, the only requirement is to make sure that the fuel concentration is somewhere in the ignitable range.

Finally, to clarify the usefulness of manometric metering: Yes, it does work at all mix levels. However, it may suffer in low pressure, low volume launchers due to insufficient mixing, and tends to be less cost effective than other available methods.
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Unread postAuthor: jakethebeast » Tue Jan 25, 2011 11:15 am

SUCCES! :D got the hybrid to work. Charged the chamber with about 2psi of propane, added 2BARs of air and BOOM she said :D . Tomorrow i will test differend kind of bursk disks, today i used 1 layewr of a side of an yoghurt can, when i used 2, it didnt bursk(mayby i used little bit too much propane on that one...) this gun is going to get 28mm OD copepr barrel that is about meter long, i will post this baby to the shohcase when i hav the ingnition system little more pretty :D
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Unread postAuthor: saefroch » Tue Jan 25, 2011 6:28 pm

Your mix is still off :roll: recommended that you do some calculations.
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Unread postAuthor: MrCrowley » Tue Jan 25, 2011 6:34 pm

@jakethebeast,

Are you not going to calculate the fuel ratio properly? Are you just going to keep on injecting roughly the right amount of fuel and air and hope it works?

I still don't think they should be built by people who don't understand them.

I'm with DYI on this one. Jake, figure out how the to calculate the correct fuel:air amounts or stay away from hybrids. While your rough mixes may work, your attitude is disconcerting.
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Unread postAuthor: saefroch » Tue Jan 25, 2011 8:55 pm

MrCrowley wrote:your attitude is disconcerting.
Seconded. Hybrids are not for those who are not willing to do some simple calculations.
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Unread postAuthor: omniscient » Tue Jan 25, 2011 9:55 pm

With 2BAR of compressed air and (about) 2psi of propane in the chamber, his mix was only 4.535% ...(Again: "about")

The only thing to really worry about, is that the peak combustion pressure of successfully igniting a mixture as he's described, is slightly over 300psi.

... Well, actually ; I do have to agree with the other replies, as guesstimating is not something you want to do, when messing with compressed mixtures of fuel & air.
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Unread postAuthor: jakethebeast » Wed Jan 26, 2011 12:42 am

crowley: no, im nmot going to leave it like this. Im just happy that i got it to work. today im going to measure the chamber and my fill area, to see if i can use my fill area as fuelmeter for more accurate measurements. And no worries, i know im dealing with high pressures.

EDIT: i made som calculations, looks like with 2psi of propane i need to add 32,9psi of air to get perfekt mix, i need to try that :D
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