Most effecient cannon layout discussion...

[Technician1002]

I know blowing own horn, but these were the reason I progressed from piston to large homebuilt QEV, to Quick Dump Valves. I'll shut up now.

While sometimes you do drop in the pros of the QDV a little irrelevantly, I have to agree with you here. Apart from an inline combustion, the QDV is about as efficient as you get. The air doesn't have to change direction much if at all and is released quickly too. Apart from the mechanical actuation, there aren't really any disadvantages to the layout that I can think of.

Thanks. In the original planning stages as an engineering challenge, innovation counted in the scoring. The fact it wasn't a typical valve was worth points. The one splitting hair advantage we tried for was initial valve speed when it did open.



A piston sealer is basically at rest when it starts to open. This means a good part of the initial flow is through a mostly closed valve as it accelerates. The disadvantage (in a fair balanced comparison) the QDV has a disadvantage because the initial flow still has to pass part of the piston, so it is good design to put the o rings as close to the end of the piston as possible. In distance traveled, a barrel sealer is the winner. In initial opening speed, the QDV is the winner. The QDV has a poor flow path at initial opening as the end of the piston is in the way. It was a toss up which is better.

In the t shirt launcher the golfball rod assembly travels 2 inches before it hits the piston to start it moving.

A QDV on a sliding rod configured much like an auto body dent puller has both an impact driven speed start and a travel distance before the o ring unseats. In testing with our loads, this advantage was negligible. It was worth points to the judges. It may have been an advantage if we were launching Styrofoam balls instead of t shirts.

Under low pressure conditions (throttle ability) QEV's are unreliable as moving friction is high and initial opening force is low. A yank on the QDV ensures full stroke operation all the way to 0 PSI. We knew the judges were engineers and would eat this up. These factors were why we built it.

I'll shut up now.

Sorry..

[jeepkahn]

That's odd. I know for the same sizt the toolie has a larger chamber, but I would have thought that the barrel sealing part of the piston would obstruct flow at least a little...

I think the fact that the 4" to 2" bushing is funnel shaped it actually acts as a velocity stack, when the sealing face sides back it allows the air to not only move in a straight line into the barrel, but it also acts to increase the velocity of the air as it enters the barrel... if it was a smaller chamber to barrel diameter raitio, the sealing face would act as an obstruction, but with a 2" sealing face and a 4" chamber the air behind the sealing face doesn't have to flow around the face it merely has to take the place of the air around the outer part of the chamber as it exits...

This is all just theory mind you...

[Technician1002]

That's odd. I know for the same sizt the toolie has a larger chamber, but I would have thought that the barrel sealing part of the piston would obstruct flow at least a little...

I think the fact that the 4" to 2" bushing is funnel shaped it actually acts as a velocity stack, when the sealing face sides back it allows the air to not only move in a straight line into the barrel, but it also acts to increase the velocity of the air as it enters the barrel... if it was a smaller chamber to barrel diameter raitio, the sealing face would act as an obstruction, but with a 2" sealing face and a 4" chamber the air behind the sealing face doesn't have to flow around the face it merely has to take the place of the air around the outer part of the chamber as it exits...

This is all just theory mind you...

Wow.. I think it's time to build a piston with a nose cone into a barrel in a taper, a funnel piston into a funnel reducer. It would take care of the "dead space" volume and make the valve large. I wonder if the added mass is worth it?

Darn.. More projects to try to fill my full calendar


Edit, another crude mark-up. I was in a hurry so sue me..
Dead space, acceleration zone mod. Can either be a traditional QEV piston or QDV with slight modifications of the barrel seal locations.