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Same concept but altering the geometry
It doesn't. The valve opens on the chamber rise in pressure. A pop off can be added to the tube and valve act like a dashpot, but it won't fully vent. Due to mass and motion, the valve will pop open and the opening valve will increase the valve dwell time well beyond what a massless piston would provide. Due to the pressure involved, the projectile should be well on it's way downrange by the time the piston rebounds and closes due to the pressure in the pilot. The pop off can be internal to the chamber so it does not pop off while filling, but would pop off when fired and the chamber pressure drops while the pilot pressure rises due to compression. This would aid the dashpot in reducing stored energy in the pilot when fired. It would also prevent an accidental discharge due to overfilling the chamber and having a pop off vent the pilot.
Interesting. You don't think the valve would spring back closed before the pop off has a chance to allow any venting? I was thinking of having the pilot isolated from the chamber for two reasons. One is to allow opening pressure adjustment independent of chamber pre ignition pressure, this can fine tune performance. The other, is when the pilot is compressed by the piston, the pop off will be outside the chamber at atmosphere to allow a quick burst of air pop through. Maybe this will reduce the violent rebound of the piston as well as increase dwell time as you suggested.
The pop off can be an adjustable one, so it can be set to be equal to the pilot pressure so that it opens at the slightest increase. Assuming it doesn't dump too quick, it can also be used as an easy signal to let me know the pilot is up to pressure. Once the pilot os up to pressure, the valve would pop.
Being an isolated design, I could fill the pilot first and this would eliminate the risk of an accidental discharge of pre ignition gasses.
Once I actually try out the pop off valve, I might very well decide that its useless and remove it. Who knows. I could then replace it with a drain cock or push button release valve for pilot drainage, that way I can maybe vent the chamber without the piston blocking the way.
Instead of a pop off valve a spring loaded check valve could be used. The advantage would be a low pop pressure of only a few PSI and a large valve seat. The piston moving into the pilot compresses the gas. Venting that will reduce the rebound. Venting to the chamber will give a short flow path, a large opening, and enough restriction to arrest the piston movement in a dashpot configuration. The spring would be used to hold it closed while the pilot pressure is used to close and seat the piston.
How about this variation, the piston is spring loaded and a tight fit but with seals only at the barrel. During filling, the mix can flow to the pilot area so eventually it all equalises. On firing, the pilot area dumps into the firing chamber.
Not sure it would work, just one of those at-work-and-busy-but-not-concentrating brain farts
download this it is great way better then the window7 paint.
'' To alcohol... The cause of, and solution to, all of life's problems.”
Add me on ps3: wannafuk, 8/11/11 cant wait
Then with the heat of compression of the fuel air behind the piston, the secondary explosion shatters the piston and ejects shrapnel out the barrel..
We decided to purge the fuel from behind the piston for a reason.
In regrds to paint, at home I prefer The Gimp.
Where's your sense of a adventure
Actually I have gimp, just downloaded it a week or so ago. I figured it was overkill for something so simple.
Disclaimer: Below I complain about paint. Nothing in this post is useful.
I never used the w7 paint program before so I though "Oh, no problem! I can just bang out a quick diagram in 15 minutes on paint!" My problems first arose when I tried using one of the built-in shape tools for the weld beads. Want to draw a triangle? Great!, but it can only be oriented in a specific direction. Want to try drawing with the line tool instead? Ok, but it seems to think every time I click nearby the line should jump to the cursor instead of anchoring permanently. Also the "end point" is always a pixel or two off from where your cursor is. That and there's a blatant bug in the secondary color picker. And a couple other things I forgot about now because I was too pissed at the time.
I am now using an LCD screen, but I think I'm going to switch back to CRT... I miss being able to slam my fist into the screen when I have technical difficulties.
Oh, and moonbogg, the main idea behind my diagram was to keep machining cost down. Even without a bumper I did not have trouble with a 6 oz piston slamming into 2" steel fittings at 4x, but I stress cracked one when I went up to 6x.
Now that sounds pretty good. It would eliminate an isolated pilot, but maybe thats something I should live with.
EDIT: My concern with ditching an isolated pilot is that i'll be stuck with whatever opening ratio I initially choose. If friction or other unforseen things hinder valve performance, I won't have a way to adjust it from what I can tell.
I laughed really hard at that..... errrhem! back on topic.
I found that an adjustable pop off vented to slowly to bother with would not even pilot a 3/4''QEV.
If i was you, i would pilot the piston by building a simple spring loaded hammer valve that vents to atmosphere. hit by the piston.
The spring strength could be significantly more that what is needed to hold it shut under pilot pressure, as the piston would be moving with such force, it would have no problem opening it.
"stock xp" ms paint diagram on the way!
EDIT again: More suited to the shape of your launcher.
due to the design of the piston the force acting on it prior to ignition isn't really that great... why not use a spring and air cushion the piston ?
just to make it clear - I am suggesting putting a spring behind the piston and using air cushioning as a bumper
Children are the future
unless we stop them now
The piston style with an o ring in the barrel will launch the o ring every shot as it is pulled out the barrel. Been there, done that.
The black is from trying graphite. The fix is a longer piston nose and QDV style ports so the piston opens ports instead of pulling out of the barrel. This early piston worked, but my ports were too small so performance was poor. This was not good enough to post in the showcase. This was built before my first QDV which fixed the port size problem.
@Moonbogg, This is the piston I couldn't find in chat last night.
If your first build sux, don't feel bad. Mine sucked too. I learned and got better.
I beg to differ. The pneumatic Trident I made is clear proof it isn't always blown out. In fact, once I upgraded my piston I did at least 20 shots, no damage to or loss of the o-ring.
I've had good luck with pneumatic cannon that use a "in the barrel" o-ring.
But this is for a hybrid, so it might be wise to consider a lost o-ring a possibility.
(First post from the new desktop!) its a windows 7pc.... paint for windows7 DOES blow!
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