Inline valve vs classic architecture

[madeInLV]

Greetings GTA,

Firstly, a shout out to Bob. Thank you for making me read between the lines:)

Here goes...The hope to create a perfect PCP rifle still lives in me. It just won't go away. A perfect system is one that produces the least resistance to gas flow thus resulting in the most energy transferred for a given calibre / pressure / barrel length.

In short, we are talking about an inline valve and how much a valve of this kind actually increases the efficiency of the PCP system.

My reasoning is as follows: in the classic design, air is forced to make 2 * 90 turns, which even on an intuitive level does not seem very good and most likely hides some potential to increase the efficiency of a PCP system.

I'll say right away that we are not talking about low powers here. I'm talking about slugs @ 100+ (J) in .22cal. In other words we're talking about subsonic 22LR performance levels on air. At low pressures / energy levels, the classic architecture gives you more than enough shots per fill. No need to fix the classic here, it's good enough.

To those about to reach their keyboards for comments like "the inline valve is BS, it's all about closing the valve on time/fast enough" .... calm down, drink tea, you're right ... this is clear and common knowledge by now, my question is a bit different.

Imagine two systems. In the first case, we have a classic design, in the other we have a system with an inline valve. IMPORTANT: barrel length, bullet weight, reservoir volume, regulator pressure as well as the plenum volume, are all identical. In both cases, we can adjust the valve, that is, the moment in time we wish to close it.

Question: If both systems are set at the same muzzle energy (let it be 100 J), how much will the shot count (before the regulator cuts out) differ between the two systems.

As I read the russian forum (guns.ru), GTA or the yellow forum (too bad it's gone btw), this topic basically ends all the same ... some argue that an inline valve simply has to be and obviously is better, but nobody can give you a numeric experimental value supporting their argument.

And how would you really measure the performance benefit of an inline design? Ideally it would be a setup free of any components that could potentially introduce uncertainty/error or additional unknowns into the experiment. So...the two platforms proposed above would probably give you a good chance at finding the truth... but would still leave enough room for arguing on interpretation of the results+getting an inline valve to reliably close is quite tricky:)

What if we could simplify the experiment?

Let's take a regular two stage dumb valve of about 20 (cm3) volume, fill'er up to say 200 (atm) and disconnect it from the HPA source. We shoot couple of 22LR slugs with it and measure the average speed. I emphasize that the pressure in the system must drop to 0 (atm). The system generates the maximum possible energy that we can then measure and record.

We then unscrew the barrel and place an adapter between the barrel and the valve. This adapter simply mimics the porting/ducting of a classic PCP architecture forcing the air to take 2*90 turns. In this case, the barrel will be looking backwards. We take several shots as before and record the average reading.

What will we get? We'll get different muzzle velocities and therefore different energy levels. How much different? No idea. My gut tells me it'll be around 15% which may or may not justify the extra effort.

In such a way, we will directly measure how much these 2 * 90 turns affect the energy transfer compared to an in-line system. There definitely are losses in a regular knock-open PCP valve compared to a smooth plunger of an inline valve and the reasoning favours the inline design here as well, however quantifying this loss/resistance would require a separate setup/test. Next time.

What do you folks think of the experiment? Would this experiment indisputably prove (or not) and quantify the benefits of an inline PCP valve? What am I missing? How would you design the experiment?

Does anybody have a lonely dumb valve laying around....

[rkr]

Inline valves tend to make about 10-15% more power than traditional valves when pressure, barrel length and bullet weight is about the same. That should translate to efficiency as well.

[madeInLV]

Rkr,

Thank you for sharing. Do you have a link to the source of this information?

[rkr]

Rkr,

Thank you for sharing. Do you have a link to the source of this information?

That's information I gathered when tuning my traditional valve .257. Airforce based guns made about 10-15% more power with same parameters.

[madeInLV]

Rkr,

Thank you for that. This really is getting more and more interesting. I'm working on a test rig design to test this concept. The experimental setup will be based on a dumb valve

[sb327]

This is quite interesting. My last two valves have been inline and reasonably close to the projectile base. I have no idea what it does for efficiency or power because my valves are really designed for high power from high pressure. I guess you could say I want high power with a standard length barrel. This takes a higher pressure to achieve and the high pressure is a bit much for conventional valves. Since my valves are not connected mechanically, I was able to locate them directly behind projectile.

Anxious to see what your testing shows.

Dave

[Mr.P]

I have a couple of guns with both configurations and have wondered about this.

Not sure if there’s anything more folks are going to add but I will follow this just in case.

Tangentially related topic that would interest me would be how to conceive of a spring stopping guide for an in-line forward cocking system like evanix rex or airforce, just to make the most of that in-line system.  So far all the spring guides I have seen have been for the other types. 

[Scotchmo]

It will also depend on the 100J projectile mass. Two 90 degree bends at <<sonic velocities won't restrict much. At sonic velocities, there should be a measurable difference. 90 degree sweeps at sonic velocities should perform better than sharp 90 degree bends.

[rsterne]

Just to complicate things, my "conventional" valves use an exhaust port angled 30 degrees, and a radiused bend at the poppet seat and below the transfer port.... The air can flow through the seat on an angle (reducing the turn), and there is little change in direction at the transfer port.... At the barrel, the transfer port blends from a circle to an oval, with the front radiused to again ease the airflow.... It is a long ways from two sharp 90 degree bends....

Axial valves, such as used in the Airforce, are NOT "straight through".... There are two small ports in the stem at 90 degrees to the airflow that the air must "slalom" through.... Of course that design can also be improved by angling those ports.... However, even in the best designs, the air does change direction to some degree....

It makes sense that an axial flow (aka inline) valve should have somewhat less resistance to the airflow, particularly when the flow rate is high, as Scott says.... My experience is that comparing the best conventional valve to the best inline valve, you might see 10% more pressure required for the same FPE for the conventional valve.... Even the best of them struggle to exceed 50% of the theoretical maximum FPE, unless you use heavy slugs at lower velocity....

Bob