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Not Sure if it's been done...

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: SPG » Sun Mar 15, 2009 3:17 pm

Fnord, what kind of volume did you have in your piston hybrid behind the piston?

I was thinking about air-springs yesterday (and after all that's basically what you've got) and then of course Boyle's Law struck me as being useful in air-springs.

Boyle's law gives us:

p1 x v1 = p2 x v2

So by altering the size of the chamber behind the piston you can change the rate of the air-spring.

We'll call the distance the piston moves the "swept volume". It's the ratio of this swept volume to overall voume which is important. If the swept volume is a high proportion of overall volume the air-spring will have a very strong rising rate, as air pressure behind the piston will increase quickly with volume decrease.

However if it's a very small proportion then the air-spring gets closer to having a constant rate, the air pressure wont change much.

I reckon this second type is what you need for a hybrid, that way you'll get a large movement in the piston, and so maximum airflow into the barrel.

I wonder if this makes any sense?
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Unread postAuthor: Brian the brain » Sun Mar 15, 2009 3:49 pm

It makes perfect sense.
I believe I may have invented the idea of a piston hybrid ( or not) back in spudtech days.
It felt like a logical idea, although I've never even tried a hybrid myself.

My suggestion at that time was to use either a pop-off, or a large pilot space, just like you said.

I don;t mean to claim being the creator of piston hybrids, as I was just thinking out loud without really understanding hybrids at the time..
I had never thought of using it for multiple shots though..
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Gun Freak wrote:
Oh my friggin god stop being so awesome, that thing is pure kick ass. Most innovative and creative pneumatic that the files have ever come by!

Can't ask for a better compliment!!
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Unread postAuthor: SPG » Sun Mar 15, 2009 3:55 pm

Brian, I think the way to phrase it is:

"I believe I might have lead the brain-storming" :wink:

I only cited Fnord's cos he's built one and was commenting in this thread, so thought he might have experience. Did you build one too? My occasional disappearances mean I don't quite know who did what, just that it's been done.
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Unread postAuthor: Brian the brain » Sun Mar 15, 2009 4:01 pm

No never built one.
And you are right, I invented brainstorming...

Oh no...I mean...well.. :roll: :D

I believe I posted a diagram of the general idea..

Care to share your thoughts on the dieseling option?

To get auto ignition you're generally talking compression ratios on the order of 20:1. I doubt you'll see that in the dead space on a pneumatic.



My brother managed to do it in a CO2 pistol, using a drop of pure ( medical) ether.
So compressed air should be easier..
I think 45 bar should be enough...
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Gun Freak wrote:
Oh my friggin god stop being so awesome, that thing is pure kick ass. Most innovative and creative pneumatic that the files have ever come by!

Can't ask for a better compliment!!
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Unread postAuthor: Fnord » Mon Mar 16, 2009 10:57 am

SPG wrote:Fnord, what kind of volume...
...boyle's law... ...other stuff.



Yeah, I deliberately put a lot of piston travel in my hybrids because of this. I think about the same as the porting diameter should work well.


However there is something else I need to point out.
If you use a high port diam to piston diam(for high opening pressure), your piston needs a good seal. In my newest version I used a rubber piston that fit tightly, but it did not have o-rings.

It did not work correctly like this. I had about a dozen 4x tests where the piston did not actuate. Instead it bulged out of the seat in a somewhat scary fashion(This was a 2" thick chunk of rubber, forming a 1/4" high bump across only a 1" space :shock: ) until the gasses cooled off.
The pressure was leaking around the piston too fast, and not applying enough force to the small area around the port to open it.


Then I cut a floating oring groove and it worked perfectly.
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