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Author Topic: Balanced valves in pcp's  (Read 979 times)

rsterne

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Re: Balanced valves in pcp's
« Reply #15 on: September 26, 2017, 04:11:26 PM »
Tim, now I'm even MORE confused.... The hammer #3 slides on the outside of the part with the lip, #2, that hits the valve stem.... The hammer spring pushes against the inside of the inner sleeve, #1.... which is threaded to the rear cap?.... or #2.... or what?.... What powers the hammer if it just floats freely?....  ??? ??? ??? ??? ???

I have NO idea how this is supposed to work.... I would suggest you build one and show us....

In this thread I have summarized how a conventional, sleeve, and two types of balanced valves work.... That is about as far from "too many secrets" as you can get.... I am not about to examine in detail the internals of the Cothran or ART/SS valves, as a lot of time and effort has gone into their development as commercial products.... They work on the PRINCIPLE of using the air pressure inside the valve to counteract the normal force holding the poppet against the seat, which is the source of the cracking force required.... That force, and the balancing force, are BOTH variable, depending on the pressure inside the valve.... Any mechanical system, working on the hammer, is going to be fixed, and at best could only be adjusted to work at a given air pressure, ie in a regulated PCP.... If you have somehow come up with something different, then I suggest you build it and prove to us how it works....

Bob
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Shorty

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Re: Balanced valves in pcp's
« Reply #16 on: September 26, 2017, 04:42:48 PM »
Sorry for the confusion. I think I am not getting across the concept.

For the other balanced valves. I really do not care how they work or are designed (it is interesting though). My hopes were just to fully understand why some work and "some" almost work. Or is there a better way than using air pressure to counter balance cracking force.

It's Ok. I will try to get something in hand for a picture and then maybe it I will be more understandable.
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rsterne

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Re: Balanced valves in pcp's
« Reply #17 on: October 19, 2017, 02:02:23 PM »
I did this drawing up for a thread in the GTA, but thought I should add it to this thread.... This is a different way to reduce the opening force, by using a smaller poppet inside the main poppet, which opens first to equalize the pressure across the main poppet.... The small poppet stem is inside the main stem, which is tubular, concentric, and slightly shorter, so that the hammer first opens the small black poppet, (which allows the pressure to equalize across the main poppet through the vent holes in it, making it easier to open), and then the main red one....

Valve closed.... The hammer strikes the small, internal black stem first and opens the small black inner poppet before the large red main one...



Valve fully open.... Once the hammer strikes the outer red tubular stem, both poppets move together, opening the large valve throat....



I know of at least two guys who suggested or tried this, one was Travis (who built several working prototypes), the other one may have been Tim, but I honestly can't remember.... It is NOT my idea, but I don't see any reason it should not work as intended.... The key would be sufficient mechanical strength of all the components, and finding the optimum difference in the stem lengths.... If the small center stem is longer than needed, the pellet might start to move before the main poppet opened, and if it is too short, the pressure difference on the main poppet may not drop enough to reduce the opening force on it as desired.... My gut feel is that the stems only need to be 0.010" or less different in length, but I have never made one to test....

Bob
« Last Edit: October 20, 2017, 12:39:33 PM by rsterne »
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Alan

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Re: Balanced valves in pcp's
« Reply #18 on: October 19, 2017, 03:23:22 PM »
I'm not a dynamic fluid engineer, but I'd bet the most important consideration is the ratio of the small valve's airflow vs. the larger valve's airflow. I further suspect it would be more reliant on the chamber pressure as well. Nonetheless it is an interesting valve design, but not really a balanced version per se.
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Alan

I have an Omega compressor. If you're a fellow Guild member, and you pass through Roswell, NM, I'll fill your portable tank as a courtesy.

rsterne

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Re: Balanced valves in pcp's
« Reply #19 on: October 19, 2017, 07:32:58 PM »
You are 100 correct, this is NOT a balanced valve per se.... it uses a smaller poppet opening first to balance the pressure on both sides of the main poppet.... hence making it easier to open.... I included it in this thread because it has a similar effect (lower cracking force) arrived at via a different method....

If you look at the dynamics of what happens when any poppet valve opens, the air molecules flow through the initial gap at the average molecular speed of about 1650 fps... because the molecules that would have randomly hit the closed poppet now escape through the opening.... The molecules are TINY relative to even a 0.001" gap, so they pressurize the volume between the valve seat and the base of the pellet very quickly.... If that distance is 1", the first molecules hit the base of the pellet in only 0.05 mSec (0.00005 sec.).... When you consider that the distance the air molecules have to travel before they can bounce off the back of the main poppet is only a fraction of an inch, I'm betting that the pressure will equalize across it (maybe not 100%, but say, 70%) before the small poppet has lifted 0.010" off the main poppet.... There is no real flow volume during the stage before the pellet starts to move, only an extremely rapid pressure rise in the wasted volume between the valve seat and the pellet base.... That is why keeping that volume as small as possible is important (short port passages).... because all that happens during that time is a loss of pressure in the system before the pellet starts to move....

Consider this.... Valves in PCPs lift about 20-50% of the caliber.... After they are open about 33% (1/3) of the caliber, the flow rate peaks because of the curtain effect (even less with restricted valve throats).... If we assume the pellet can't/doesn't move until the air has traversed 1" of length (from seat to pellet base), which takes 0.05 mSec.... and we further assume that the valve dwell in PCPs is between 1-2 mSec (it pretty much has to be to get the velocities we get).... then the period during which all that is happening is the pressure in the ports is rising is 1/20 - 1/40 of the dwell.... The curve of poppet lift vs dwell is pretty much a parabola.... which means that at 1/20th of the total dwell, the valve has opened about 1/5th of the total lift.... For a .22 cal, (1/5 x 1/3 x 0.22) = 0.015".... At 1/40th of the total dwell, the valve has opened about 1/10th of the total lift.... For a .22 cal, (1/10 x 1/3 x 0.22") = 0.007".... That is why I think that having the small inner poppet open about 0.010" before the main poppet opens is in the right range.... It might be twice that, or half that.... only building one and trying it would tell you.... The larger the diameter of the small inner poppet, at least in theory, the less it should have to open to equalize the pressure across the main poppet before the pellet starts to move, and flow rate becomes important.... Of course the larger the inner poppet, the harder to drive it open.... so it's all a compromise....

Bob
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Alan

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Re: Balanced valves in pcp's
« Reply #20 on: October 20, 2017, 06:42:43 AM »
Makes perfect sense.

I'm still taken back with the idea of using piezo valves. The biggest issue is powering them, but assuming that can be overcome, they have some very unique properties. They open and close in almost sub-micro seconds, and there is no practical pressure limit.

It is all conjecture of course, but the possibilities are intriguing to say the least.
  • Roswell, New Mexico
Alan

I have an Omega compressor. If you're a fellow Guild member, and you pass through Roswell, NM, I'll fill your portable tank as a courtesy.

Aerotulz

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Re: Balanced valves in pcp's
« Reply #21 on: November 01, 2017, 08:01:55 AM »
Hi Bob, you are talking about flow in a PCP valve at 33% of caliber and then the curtain
effect restricts further flow. What is the "curtain effect" plz?
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rsterne

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Re: Balanced valves in pcp's
« Reply #22 on: November 01, 2017, 08:35:15 AM »
The area available through a poppet valve for flow is goverened by two factors.... the throat area (area of the hole minus the area of the valve stem), and the area under the poppet, between it and the seat (called the curtain area).... The former is a constant, the latter varies with lift.... At some point, no matter how far you open the valve, the flow will not increase, all that happens is that the flow occurs for a longer period of time (the dwell)....

It turns out that the area between the poppet and the seat equals the throat area when the lift is 1/4 of the seat diameter.... It actually occurs before that, because of the stem area, but there is some restriction to flow because of the corners the air has to turn to get through the seat, so the commonly accepted number is lift = 1/4 seat diameter....

Throat area = diameter of seat^2 x PI / 4

Curtain area = lift x diameter of seat x PI

For them to be equal.... 

lift x diameter x PI = diameter x diameter x PI / 4

diameter and PI cancel out, giving....

lift = diameter of seat / 4

This means that if the valve is open more than 1/4 of its diameter, the flow rate does not increase.... However, if you continue to drive the poppet open more than that, it takes longer for the poppet to complete its round trip from seat to full open and back to closed again.... This means that the dwell is proportional to the lift, and can be approximated by a parabola.... The amount of air that can flow through the valve is proportional to the area under that parabola.... However, since the flow rate cannot increase after the valve is open 1/4 of it's diameter, the flow rate plateaus, or is "clipped" at that maximum rate.... Here is a drawing of that, where 100% flow rate represents when the lift is 1/4 the diameter....



Once the ports in a PCP equal the area of the bore, there are no further gains to be made in flow rate.... It helps to have the throat ID slightly larger than that, because of the obstruction from the valve stem and turns in flow past the poppet seat, I like to see the area of the throat about 10% larger than the port area (bore area).... I have measured the lift of quite a few valves, and I find that in unregulated PCPs, when they are operating under a bell-curve with a 4% ES, that the lift varies from about 1/4 (at the beginning) to 1/2 (at the end) the valve diameter.... By extension, regulated PCPs, when operating up on the velocity plateau (where additional hammer strike and lift adds no power) are opening about 1/2 the diameter.... and when operating on the "knee" of the power curve, where there is a good balance between power and efficiency, the valve is opening about 1/3 of the diameter.... That is where my 33% came from.... After that, the flow rate is capped (by the curtain effect and port size, AND adding further dwell drops the efficiency, because adding air when the pellet is already halfway down the barrel or more adds little power, but wastes air quickly....

I hope that description clears up my statement....

Bob
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Aerotulz

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Re: Balanced valves in pcp's
« Reply #23 on: November 01, 2017, 09:51:45 AM »
Bob, Thanks for that. At the end of your explanation you mentioned the pellet is halfway down the barrel. How would a maximum weight/bore slug respond to more dwell?
John
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rsterne

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Re: Balanced valves in pcp's
« Reply #24 on: November 01, 2017, 10:37:15 AM »
There isn't a huge difference in the ultimate efficiency regarding when the valve closes as a fraction of barrel length.... regardless of slug weight.... What happens is, that the heavier slug will carry more FPE because of it's weight.... For example, at the 950 fps, every bullet has 2 FPE per grain of weight....

Think of it this way.... If the valve is open after the bullet leaves the muzzle, all air exiting the valve after that point is 100% wasted, because it can no longer accelerate the bullet.... The limiting example would be a dump valve, which stays open until the reservoir is empty, like in a pumper.... So at the very maximum, the dwell should be no longer than what is required for the bullet to exit the muzzle.... The velocity at which it does that will depend on its Sectional Density (how hard it is to accelerate), the pressure (force available), and the barrel length (distance that force is applied through).... The maximum FPE, at the limit, is bore volume (cu.in) x pressure (psi) / 12 (in/ft).... If a PCP gets to 50% of that value, it's a good one....

Most of the acceleration of the bullet happens early in the barrel.... Once the valve closes at or before the bullet reaches the muzzle, the amount of air released by the valve is the volume of the barrel behind the bullet at that instant, (plus the wasted volumes of the transfer and exhaust porting systems).... Ignoring those, the maximum FPE is reached when the valve dumps the barrel volume of HPA.... If, however, the valve closes when the bullet is only halfway from breech to muzzle, it has only released (just over) half that volume of HPA.... The interesting thing is, that at that point, the bullet has already reached a large percentage of it's maximum possible velocity at the muzzle (with a dump shot).... Then, even after the valve has closed, the air trapped between the valve and the bullet continues to expand, and continues to add energy and velocity to the bullet.... It turns out that closing the valve when the bullet is only halfway to the muzzle only loses about 3-5% of the maximum possible velocity.... and yet only uses half the air to do that.... That nearly doubles the efficiency in FPE/CI....

As you close the valve earlier and earlier, with the bullet closer to the breech, you use less and less air.... but the velocity also continues to drop.... The efficiency continues to increase (although the FPE is going down too) until the valve is closing when the bullet is only 10-20% of the way down the barrel.... maybe even less with large ports and short dwells and high pressures.... This occurs regardless of bullet weight.... At some point, there is not enough air inside the barrel to continue acceleration efficiently as the bullet approaches the muzzle, and the efficiency drops again.... Effectively, the barrel is too long to take advantage of such a small amount of air.... Eventally, the bullet actually starts to slow as it approaches the muzzle, and efficiency tanks....

If you hold the dwell constant, but increase the bullet weight.... the FPE tends to increase, and so does the efficiency.... There are exceptions at the limits, but in general what is happening is that the heavier bullet is moving slower, so hasn't moved as far down the barrel when the valve (which has constant dwell) closes.... This means less air has been released by the valve (which should increase shot count).... and also that air has more barrel length available to expand and add energy to the bullet (increasing FPE).... In addition, the residual muzzle pressure will be lower, and the report reduced.... HOWEVER, if you add dwell, you will increase the velocity and FPE of that heavier bullet.... and once again you will get back to the point where the valve is closing at about 1/4 way down the barrel when you are shooting with a good balance between power and efficiency.... Generally, there is no point in closing the valve more than halfway down the barrel, you start wasting a lot of air for a tiny gain in FPE (and a large increase in noise)....

Bob
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