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Need help with piston valve design

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: wyz2285 » Sun Oct 23, 2011 6:27 am

So the piston+pilot was in a balance, when the chamber pressure increases, the piston move back, open the pilot and then "boom" the piston fully opened?
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Unread postAuthor: Technician1002 » Sun Oct 23, 2011 11:29 am

Due to the increased piston face exposed to the chamber pressure as the piston opens, the boom, it's open tends to happen anyway. What venting the pilot does is preventing the piston from slamming back closed as the chamber pressure drops. Remember the chamber pressure vents out the barrel and the pressure drops. The pilot if unvented, remains at pressure and will slam it back closed.
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Unread postAuthor: wyz2285 » Sun Oct 23, 2011 11:42 am

That's what i thought, but if i use a mix >10 the pilot pressure will rise quite a lot if without a vent, that's not good for the piston (i can easily go up to 700 psi with a pneumatic, so a <10 mix just doesn't worse all the work)
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CpTn_lAw wrote::D "yay, me wanna make big multishot pnoob with 1000 psi foot pump compressor using diamond as main material. Do you think wet bread make good sealant? " :D
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Unread postAuthor: Fnord » Sun Oct 23, 2011 11:48 am

Btw Crowley, I had a pop-off on mine once but removed it. I didn't have a bumper so I figured it would be better to let the air absorb the shock. The launcher wasn't very consistent anyway so piston bounce wasn't the major concern.
On the images in my old 2.0 thread you only see a tortured 160psi pressure gauge on the pilot port.
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Unread postAuthor: Technician1002 » Sun Oct 23, 2011 12:05 pm

The pressure rise in the pilot is entirely dependant on Boyles Gas laws. In a hybrid, to reduce the pressure rise, add pilot volume. Do limit the piston movement distance to prevent excessive displacement. A small tube to an external chamber can act as a dashpot for the piston reducing impact energy and damage to the piston.
http://en.wikipedia.org/wiki/Dashpot
A dashpot is a mechanical device, a damper which resists motion via viscous friction. The resulting force is proportional to the velocity, but acts in the opposite direction, slowing the motion and absorbing energy.

I am working on a design for a future QDV to utilize a dashpot as part of the bumper.
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Unread postAuthor: Lockednloaded » Sun Oct 23, 2011 4:35 pm

Technician1002 wrote:The pressure rise in the pilot is entirely dependant on Boyles Gas laws. In a hybrid, to reduce the pressure rise, add pilot volume. Do limit the piston movement distance to prevent excessive displacement. A small tube to an external chamber can act as a dashpot for the piston reducing impact energy and damage to the piston.
http://en.wikipedia.org/wiki/Dashpot
A dashpot is a mechanical device, a damper which resists motion via viscous friction. The resulting force is proportional to the velocity, but acts in the opposite direction, slowing the motion and absorbing energy.

I am working on a design for a future QDV to utilize a dashpot as part of the bumper.


Sorry if I've misinterpreted, but you're saying that in a hybrid valve without a pilot, more "pilot" volume would be better for the piston?
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Unread postAuthor: Technician1002 » Sun Oct 23, 2011 7:05 pm

Lockednloaded wrote:Sorry if I've misinterpreted, but you're saying that in a hybrid valve without a pilot, more "pilot" volume would be better for the piston?


A small pilot will increase pressure as the piston moves into the pilot space, Not counting compression heating, 1/2 the space, the pressure doubles. Add a small tube to a secondary pilot volume and this compressed air can be forced out the one pilot volume through the tube providing piston braking so the impact is softer. If the piston almost completely moves into the first pilot volume and then tries to rebound on pilot pressure, the expansion of the remaining gas will limit the speed it is slammed back closed increasing the valve dwell time (it will close as the air volume returns to the area behind the piston. This can reduce the speed the piston hits the back of the pilot area and then again by limiting the speed the piston closes and hits the valve seat.


For a simple visual on how this works, take a bicycle pump and slam the plunger down as fast as you can as if to smash a walnut. The small hose will prevent the piston from slamming down hard on the bottom of the pump, even though the end of the hose is not under pressure. Now imagine the same pump without a check valve and a basket ball on the hose. Slam the handle down and let go. It does not crash hard on the down stroke and then from the pressure rises gently back up due to the flow restriction of the hose. Use the same air restriction to dampen your piston movement.
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Unread postAuthor: spudlicker2x » Sun Oct 23, 2011 7:35 pm

ok so i have come to the realization that i know how to make a barrel sealing piston valve. but the kind i know how to make only pertain to air cannons. i guess i am still not understanding how it would release pressure into the barrel in a hybrid launcher
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Unread postAuthor: MrCrowley » Sun Oct 23, 2011 7:45 pm

How does a piston in a pneumatic release pressure in to the barrel? Pressure differential. The chamber pressure acts on the face of the piston and is able to move the piston back when the area behind the piston (pilot) is quickly exhausted, resulting in a larger force pushing the piston back than pushing it forward.

A piston hybrid works more-or-less the same way. The mix ignites in the chamber, and if built correctly, the resulting combustion pressure greatly exceeds the pressure behind the piston so even though the combustion has less area to act as a force on the front of the piston (like in a pneumatic), the much higher pressure (therefore, more force) is able to overcome the force of the pressure in the pilot forcing the piston forward (unlike a pneumatic) even though the pilot pressure has more area to act upon.

The difference between a pneumatic piston and hybrid piston is that the hybrid piston forces the piston back without exhausting the pilot pressure, whereas in a pneumatic you dump the pilot pressure so that the piston can move back The reason the two designs operate differently is because you can't store the pressure generated from combustion for long, due to heat loss (I believe), unlike in a pneumatic where you can store the compressed gas for as long as you feel like.

If you further break down the problem outlined the in the paragraph above (combustion pressure not being able to be stored as in a pneumatic cannon), you will deduce that for the piston to be able to work it must be able to pilot at the same time the mix ignites in the chamber. Spend some more time figuring out how to achieve that and you'll come to a similar design of my and SB15's piston hybrids. That is, the pilot is exhausted when the piston starts to move back (near peak combustion pressure), this movement unseats the pilot valve and exhausts the pilot pressure allowing the piston to open fully.
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Last edited by MrCrowley on Sun Oct 23, 2011 10:04 pm, edited 2 times in total.
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Unread postAuthor: Moonbogg » Sun Oct 23, 2011 9:48 pm

Excellent explanations here. Do you still not get it? A picture might help. I'm sure theres one around here somewhere.

@tech: that tube and secondary pilot volume sounds great. I think the size of the tube might have to be fine tuned or experimented with to see where the flow sweet spot is at. Otherwise, if its too small, I can see the piston moving back and slamming shut before the tube even has a chance to allow enough air through it to help. If that could be pulled off, that would be the way to do it for sure.
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Unread postAuthor: spudlicker2x » Mon Oct 24, 2011 9:08 am

thank you mrcrowley. I really appreciate it. It finally totally makes since. so basically one side of piston has air pressure to hold valve closed. once the combustion happens the force pressure is greater forcing piston back to allow the air to escape through barrel. thanks for the details.
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