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| PVC Arsenal 17 |
 Posted: 07/17/2007 17:51 PM Post subject: Mini Piston Hybrid |
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WITTY PHRASE
Joined: 27 Mar 2006
Posts: 1232
748.69 Spud Bux
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First off, I thought of this while I was falling asleep. So there's a chance I'm missing some things and this whole idea could in fact be entirely stupid. If so, feel free to ignore it.
Basically, what I wanted to accomplish was a small, burst-disc-less hybrid. I kind of modified the concept of a coaxial piston gun. The idea is the same, except there is no manually operated pneumatic trigger, instead, there is only the piezo ignitor and a pop-off valve.
You fill the gun the same way you would any coaxial piston gun, but in this case you're filling it with premixed propane/air. You fill it to pressure slightly LOWER than the pop-off valve is set for. When the piezo is clicked, the propane/air mix ignites and pushes the piston back in the same fashion a typical pneumatic would when the pilot is vented. Only here, the pilot is compressed until the pressure inside overcomes the pop-off, allowing faster and easier movement of this piston backwards. O-rings on the piston will prevent the pilot from igniting as well. All the hot gasses then escape out the barrel. Hopefully...
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| spanerman |
| Posted: 07/17/2007 17:55 PM Post subject: |
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Lieutenant Colonel
Joined: 17 Sep 2006
Posts: 447
296.08 Spud Bux
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| looks like it would work, the piston would need a cheack valve, and a nice big bumper. but it would work...
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| noname |
| Posted: 07/17/2007 18:04 PM Post subject: |
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Mr Vortex
Joined: 10 Apr 2006
Posts: 2509
2698.56 Spud Bux
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| Great minds must think alike. I had this exact same idea, down to the pop off valve and O-ring'ed piston a while ago, but never built due ot lack of funds and that the massive pressure of a hybrid could explode the gun because of the self-modified bushing.
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| homedepotpro |
| Posted: 07/17/2007 18:06 PM Post subject: |
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Lieutenant Colonel
Joined: 26 Nov 2006
Posts: 437
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the only thing i see wrong is IF the pressure somehow reaches behind the piston, and the pressure equalizes, then you have a bomb. also the temperature might eat at the O-rings
btw: great diagram, very detailed
Last edited by homedepotpro on 07/17/2007 18:07 PM; edited 1 time in total |
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| anatov |
| Posted: 07/17/2007 18:07 PM Post subject: |
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Private
Joined: 16 Nov 2006
Posts: 20
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| Hmmmm...I just had an idea. Take the propane injection system and put that further up, say in the front part of the gun, threaded into the coupling/bushing and pipe. Then replace the pilot valve with a schrader With that there, you pressurize the piston to be like your burst disk. The area behind the piston would be set at a much higher pressure than the hybrid's fuel is injected as. That way, when you press the ignition, the combustion would only push the piston out of the way when the pressure exceeded that of the air pressure behind the piston. This would allow for two things that are new to me as far as hybrids go, 1. You'd get a controllable pressure to release at, 2. You wouldn't need a burst disk.
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| anatov |
| Posted: 07/17/2007 18:08 PM Post subject: |
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Private
Joined: 16 Nov 2006
Posts: 20
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| Actually, homedepotpro, if that happened then the pop-off valve would kick in.
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| Flying_Salt |
| Posted: 07/17/2007 18:58 PM Post subject: |
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Post whored my way up
Joined: 09 Oct 2006
Posts: 785
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Yeah homedepotpro, it's only a very short pressure spike, and the pop of valve can be pulled off.
I've seen this concept floating around for a little pvc, it should work.
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| homedepotpro |
| Posted: 07/17/2007 19:02 PM Post subject: |
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Lieutenant Colonel
Joined: 26 Nov 2006
Posts: 437
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| i don't think the pop off valve would vent the air fast enough, if the air behind the piston ignited
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| Flying_Salt |
| Posted: 07/17/2007 19:07 PM Post subject: |
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Post whored my way up
Joined: 09 Oct 2006
Posts: 785
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| Well then it would just pop off.
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| _Fnord |
| Posted: 07/17/2007 21:14 PM Post subject: |
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reenigne
Joined: 13 Feb 2007
Posts: 1198
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Let me just confirm this- Your design WILL work, as it's working for me right now.
The only difference is I fill the propane from the front, and then compressed air from the back (and I don't use a coax setup, because of the better flame front propagation an open chamber gives)
I don't even use a pop-off valve, and it still works just fine. Make sure you use metal for the chamber, I wouldn't trust pvc with this design.
The only problem I can see is rough metal could wear out the o-rings and allow the mixture behind to ignite. However, sch 40 steel can contain a 4x mix safely. I also know that because it happened to me before(projectile sealed itself inside the barrel).
Just smooth it out and grease er' up 
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| PVC Arsenal 17 |
| Posted: 07/17/2007 22:41 PM Post subject: |
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WITTY PHRASE
Joined: 27 Mar 2006
Posts: 1232
748.69 Spud Bux
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Now I'm really considering making this... hmmm.
But I have an r/c helicopter to buy parts for, and a $200 plane to get. Eventually you'll see it for real.
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| PVC Arsenal 17 |
| Posted: 07/19/2007 14:54 PM Post subject: |
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WITTY PHRASE
Joined: 27 Mar 2006
Posts: 1232
748.69 Spud Bux
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Maybe I'll work on this sooner than I thought.
In that case, how much travel do you think a piston should have if the chamber is 6" of 1" brass pipe? The piston will most likey be inside it.
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| lukemc |
| Posted: 07/19/2007 15:13 PM Post subject: |
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Colonel
Joined: 25 Feb 2007
Posts: 561
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| What the heck? i had a very similer idea but with a burst disk in back instead of a blow off
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| Pete Zaria |
| Posted: 07/19/2007 16:03 PM Post subject: |
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Resident Computer Guru
Joined: 31 Mar 2006
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I think this concept would work, however, I would try hard not to pressurize the area behind the piston with an air/fuel mixture. I know the pop-off valve should take care of it if the pilot volume ignited, but I wouldn't trust a pop-off to vent the combusting, expanding pilot volume fast enough to prevent pipe failure.
A half-inch union with a burst disk behind the piston might be better.
I'd use two or more O-rings on the piston (to hopefully prevent any fuel from getting back there) and inject the fuel from the front of the chamber. Use anatov's idea of putting a schrader (or quickconnect) behind the piston so that you can pressurize the pilot area with air (it needs to be only slightly higher than the pre-combustion mix pressure...). You could also employ a light spring to keep the piston closed while fueling. Keep the pop-off behind the piston, too, just in case.
I would NOT build this out of any type of plastic pipe (I don't care if you find 800-psi rated sch160 nsf-pw PVC, I still wouldn't recommend it).
Just my input.
Peace,
Pete Zaria.
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| PVC Arsenal 17 |
| Posted: 07/19/2007 19:44 PM Post subject: |
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WITTY PHRASE
Joined: 27 Mar 2006
Posts: 1232
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I want to stick with the pop-off behind the piston since the idea was to make a small, disc-less hybrid.
I'm planning on building this out of brass, so I should be safe as far as that goes.
If I want to thread a schrader into brass pipe (for propane filling), should I solder on layers of copper for a better hold?
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