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Suitable valve for piston-hybrid?

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Suitable valve for piston-hybrid?

Unread postAuthor: Alex345 » Fri Mar 04, 2016 4:08 pm

Hey guys, i'm thinking of turning my pneumatic piston cannon into a piston-hybrid. Already got a shorter chamber, but i'd still like a second opinion on the valve i would like to use as it is a pre-made valve instead of a custom built one.

I'm thinking of increasing the pilot hole from 1/8 to 1/4, drill a hole into the piston stem and thread it, add a few bolts and discs and an O-ring. Then solder a piece of copper pipe to a 1/4 fitting into which the O-ring fit's snugly, screw it into place and when the valve is forced open the O-ring will pop past the pipe and release all the remaining pilot air.
valve01.jpg
Valve split in half
valve02.jpg
Closed
valve02.jpg (12.11 KiB) Viewed 466 times
valve03.jpg
Open
valve04.jpg
Top view of the pilot with cap removed
valve06.jpg
Piston
valve7.jpg
Piston 2
valve08.jpg
Where the sealing face of the piston is, thinking of adding a big rubber disc to dampen the piston during firing.
valve09.jpg
Piston stem seal
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Re: Suitable valve for piston-hybrid?

Unread postAuthor: matti » Sat Mar 05, 2016 4:49 am

From what I know about piston valved hybrids (and I have build some) you need to have air in pilot volume side and air+propane (or other mix) in your chamber. So you don't want to have an equalization hole or leak between chamber and pilot and what I believe most of the valves like you have use equalization hole or no seals in piston for equalizing pilot/chamber pressure.

For a piston hybrid you want to have 100% sealed piston, this way you can change the pilot pressure while keeping the chamber pressure (mix) the same, this means you can adjust the valve opening pressure.

I also don't think the piston will survive from even low mix pressures if there is no rubber stopper or end cushioning there.

But you probably can modify the valve for piston hybrid use, I see no problem here.. :D
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Re: Suitable valve for piston-hybrid?

Unread postAuthor: Alex345 » Sat Mar 05, 2016 8:48 am

Thanks for replying. The pilot and chamber are in no ways connected, as you can see, the chamber sealing face of the piston is larger then the sealing face on the pilot side. So the pilot pressure needs to be slightly higher than the chamber pressure in order to keep the valve shut.

Before i knew that, i tried to make a connection between pilot and chamber for easier filling. The correction of that mistake can be seen at picture 7 as a small set screw where the hole once was.

The way it would have normally opened was that the pilot side has 2 inlets; the one on top which i use pushes the piston down. And the one on the side (not visible) actually pushes the piston up. So this was some kind of pneumatically controlled venting valve or some sort.

On pictures 7 and 8 you can see the valve body has 1 rubber "disc" and 2 O-rings to keep it all airtight. On pic 7 i plan to add a rubber disc to dampen the blow of the piston shooting upwards.

The pilot chamber is over 1cm thick brass, would be nasty if fuel got into there and ignited... I'd like to take it to x5 mixes with a remote set up. Just not really sure at what pressure the valve needs to be kept shut tho.
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Re: Suitable valve for piston-hybrid?

Unread postAuthor: matti » Sat Mar 05, 2016 9:09 am

Ah I see, that set screw hole was little misleading..

On the sealing pressure thing.. You can use HGDT or GasEq to test what kind of pressures you get with different mix numbers and then calculate the pilot pressure with the area differences in your valve. Opening pressure should probably be little lower then theoretical max pressure that you can get with the mix.
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Re: Suitable valve for piston-hybrid?

Unread postAuthor: Alex345 » Sat Mar 12, 2016 7:54 am

I can't seem to install any of those programs... I've already started adapting the valve, I'll post an update soon.
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