SpudFiles

Ok, I am always thinking of upgrades and improvement for my pneumatic and I recently thought of a way to minimize piston exhaust (wasted air). Since I have a 1" valve (1/2" port), I have been thinking that a 3/4" sprinkler valve is probably (though fast opening) not the most air conservative pilot. So, i came up with this; basically a plug that screws into the exhaust with 2 or 3 very small constriction holes in it.

So, tell me what you think.

As long as the piston fits fairly well in the piston track, there should be no need to try to minimize piston exhaust.

I know, this would be for use with tight fitting projectiles

Trying to catch a piston's exhaust will slow it's actuation speed, and cost you muzzle energy.

As Velocity says, if your piston is a good fit with minimal blow by, or ideally a form of check valve, the wasted energy is absolutely minimal, and trying to catch this air will actually be detrimental.

The same applies to trying to direct the piston's exhaust back into the barrel, which limits the pressure to which the pilot volume can fall (ideally, you want it at atmospheric pressure as quickly as possible), again slowing actuation, and potentially causing piston bounce.

Invest your time in tight-fitting, but free moving O-ringed piston, and you'll get good results, rather than the negative ones this method will provide you with.

Ok, this was just an idea to get rid of the problem of more wasted air when your projectile fits very tight, is there another way of fixing this?

FishBoy wrote:Is there another way of fixing this?

Yes, having a piston that fits properly.
If your piston fits properly with the minimal blow-by (which should ideally be check-valved) needed for the equalization hole, the amount of gas used will be pretty constant regardless of projectile mass or fit.

I could do the maths to prove that trying to trap the gas is inferior to just preventing the gas in the chamber from reversing back into the pilot volume, but I'm too lazy at the moment.

Also, there's little point in fixing a piston valve up with a modded sprinkler to vent it if the next thing you're going to do is prevent it venting to atmosphere.

To refer back (again ) to HEAL, in addition to being fitted with a check valve in it's design (picture from when I was first building it, rubber washer in the middle allows air right to left, but not the reverse), the piston is designed to seal against a rubber bumper when it's fully back, providing a double ward against backflow from the chamber... but it's deliberately overpiloted with a 1/2" QEV to absolutely slash opening times, which vents straight to atmosphere. (Well, actually, it vents through a tee threaded into the QEV's exhaust port for no real reason other than to look fancy.)

Damn, I hadn't realised I had some of these pictures of HEAL's internal construction.
This one for example, shows two things - the fact that HEAL has an over bored valve seat (it may look a bit messy, but it was later cleaned up), and a corner of the combination of electrical tape and plaster holding the side of my finger on after I shredded it trying to make a springer.

Thanks a lot, I didn't quite understand that. My Piston has a very good fit, its just that I assumed that with a tight projectile more air would go out the pilot.

FishBoy wrote:Thanks a lot, I didn't quite understand that. My Piston has a very good fit, its just that I assumed that with a tight projectile more air would go out the pilot.

It's not likely to be a problem with a tight projectile, which can actually be a benefit. Less blow-by, more pressure/force on the projectile, it leaves the barrel in less time than a loose projectile, and therefore less leakage due to less time.

You will get fractionally more leakage into the pilot by having a projectile that remains the barrel longer - heavier, excessive friction or whatever - by virtue of the fact that there is longer for the air to leak through. But with a good piston, this change should be insignificant.

As above, if you want less wasted air optimise the efficiency of your exhaust system. Keep pilot volume to a minimum and even a schrader valve will serve as an adequate pilot