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How does a piston hybrid work??

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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How does a piston hybrid work??

Unread postAuthor: CooperJS » Thu Mar 28, 2013 11:18 pm

Hey guys im thinking of building a piston hybrid because i hate the idea of cutting out burst disks and what not but i just cant seem to get my head around on how they work?

Iv got a metal lathe and all them goodies like that so building it shouldnt be a problem once i find out how it works.

Im planing to shoot golf balls if not homemade ammunition.

thanks in the future.

Cooper
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Unread postAuthor: Brian the brain » Fri Mar 29, 2013 6:42 am

A piston hybrid functions just like a pneumatic pistonvalved cannon.

BUT:
In stead of dumping pressure behind the piston, pressure is raised in front of it.
The effect is the same.

So if you'd put an ignitor in the chamber of a pistonvalved cannon and inject a fuel/air mix rather than just air, it should work.

Some recommend ( I would) to put a pop-off valve in the pilot area.
It seems logical to offer an exit for the pilot volume once it gets compressed by the piston sliding back.
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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: CooperJS » Fri Mar 29, 2013 7:00 am

so when the Fuel air mix ignites it shoots the piston back letting all the pressure out, am i correct??

Edit: What would you use for a sealing face? would you still use rubber or some other material?
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Unread postAuthor: matti » Fri Mar 29, 2013 8:38 am

Yes correct.. and by adjusting pressure behind the piston you can change the pressure when the piston valve opens. You can use rubber for the sealing face, I normally use NBR (Nitrile rubber) it works fine (it's the same stuff that basic O-rings are made of).
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Unread postAuthor: Technician1002 » Fri Mar 29, 2013 10:53 am

If you are into the technical side of the design, the ratio of the piston OD (id of the cylinder it is in the pilot) surface area in sq in or sq cm, etc, to the valve seat opening surface area, you can calculate the chamber pressure rise required to cause the valve to open. As the valve ratio approaches 1:1 the higher the chamber pressure must be before the valve will open, possibly even creating a failure to open. A large piston diameter with a smaller diameter seat may open well before the combustion rise has progressed far.

Be sure you keep fuel out of the pilot so it does not compression ignite.
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Unread postAuthor: CooperJS » Sat Mar 30, 2013 12:30 am

hmmm im starting think that i might use a burst disc for my first hybrid since its really simple, but how do you know what type of burst disk you need to use with the mix? E.g. Coke can, tin foil and so on.
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Unread postAuthor: Crna Legija » Fri Apr 05, 2013 5:57 am

CooperJS wrote:hmmm im starting think that i might use a burst disc for my first hybrid since its really simple, but how do you know what type of burst disk you need to use with the mix? E.g. Coke can, tin foil and so on.


i'd put a single coke can layer then fill with air till it burst then try two to see if doubles, if so put the amount of layers required for the mix
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Unread postAuthor: CooperJS » Fri Apr 05, 2013 9:18 pm

Thanks Crna ill try that soon once I get some building materials
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