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Hybrid piston pilot valve. Your input please

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: BigBang J » Fri Sep 17, 2010 9:53 pm

Awww come on!!! I hate dial up! My internet connection locked up and didn't send my last reply that I told it to submit 2 days ago. O well.

Frocksie: So your pilot has a certain amount of air pressure sealed behind the piston? How would that compare with a pop valve? I would think that it would almost be the same since the air pressure is not vented. How much psi do you have in your pilot and what mix are you running?

SpudBlaster15: i don't think I understand the pilot part of it & the vent valve. In one picture there is 3 brass thingys in the pilot chamber end cap,
and only 2 in another picture. also is there a hose that connects those hose barbs, the one that is in the propane meter area and the other in the end cap? Cause it isn't shown in the picture.
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Unread postAuthor: Lockednloaded » Fri Sep 17, 2010 11:35 pm

Image

Here's a simple hybrid valve that does not require a pilot
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Unread postAuthor: Technician1002 » Sat Sep 18, 2010 12:04 am

Without a KE energy absorbing device in the assembly, the possibly exists to launch the piston into the spring, then the spring return the same energy back to the piston as momentum. How hard do you expect the seal to be impacted?

If the piston is too light, the rebound may occur early into the power stroke of the barrel. If too heavy, it will provide high flow restriction to flow while it opens.

I wish someone had a good simulation program to simulate the operation of this valve under normal hybrid use. It is hard to guess how it will perform. Dialed in right it could be a real performer.
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Unread postAuthor: Lockednloaded » Sat Sep 18, 2010 12:13 am

I remember a thread where either JSR or POLAND_SPUD showed some mathematical equations for a full auto pop-off valve. IIRC it could probably help determine the efficiency of this valve
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Unread postAuthor: BigBang J » Mon Sep 20, 2010 7:42 pm

Technician1002: I expect the piston will impact the seal fairly hard because of the incoming 90psi behind the piston will shoot the piston forward quickly.

One thing I was looking for when drawing this design was a design where the valve would stay open after the shot thus alowing the spring piston to vent the chamber out the barrel, and then the valve would reset in the fueling process. I was thinking about just using a pop valve before, but then I would have to have manualy open a valve to vent the chamber.

So can you see any advantages in this design over other designs?
Or should I just forget the whole thing and go with a pop valve or something?
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Unread postAuthor: Lockednloaded » Mon Sep 20, 2010 7:52 pm

If you have the facilities to make o-ring grooves, I suggest a design similar to this
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Unread postAuthor: BigBang J » Tue Sep 21, 2010 10:41 am

yes I have made o-ring grooves before and plan on it for my valve, That is pretty much just a simpler version of my design.
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Unread postAuthor: Lockednloaded » Tue Sep 21, 2010 10:53 am

I'm just saying that the check valve is not necessary to the design. Both valves are pretty much just copies of earlier hybrid valves
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Unread postAuthor: BigBang J » Tue Sep 21, 2010 8:11 pm

So you think I should ditch the check valve? It is not neccesary?
I thought it was a good idea.
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Unread postAuthor: Technician1002 » Tue Sep 21, 2010 8:35 pm

I'm thinking for the dwell time of the pop off valve, the pop off valve may be too small. I'm toying with a flange and spring loaded rear breech plate that will perform like a relief valve (not a pop off) so the energy of the piston and compressed air behind the piston is vented rapidly, but at pressure to eat the KE of the piston, so the potential energy of the compressed air is safely vented so the piston has low energy returning it closed. When the piston closes after the shot the volume of air is very low so it's energy is very low.

This design is based on the air bags the stunt men use for falling off buildings. Large vents held closed pop open on impact so the bag rapidly deflates on impact so the actor does not get rebounded off the bag. The bag vents slow enough to break the fall safely.
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Unread postAuthor: Lockednloaded » Tue Sep 21, 2010 9:27 pm

I can use a 200psi pop-off. The area behind the piston is ~150 psi at pre-ignition. When the piston is forced back it compresses the air and much of the air is vented, but some still remains and acts as a bumper
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Unread postAuthor: Gun Freak » Wed Sep 22, 2010 4:00 pm

It is getting more and more like SB15's valve, with a tapered seal at the back, you can completely vent the air behind the piston for greater flow and you could just add a bumper like rubber.
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