GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: Hack21 on October 26, 2020, 01:26:53 AM
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Bob / Lloyd / others,
Would a small plenum (dump shot) with an easily adjustable regulator pressure make sense? If so, what about a fixed regulator setting with an easily adjustable relief valve on the downstream side to knock the pressure down?
I have been thinking recently about the "variable size transfer port" type power adjusters. They must be awfuly convenient, but when the regulator setting and hammer spring are properly tuned for the maximum power adjuster position, I can't imagine that the reduced power adjustment is very efficient. Of course, a lot of the airguns with this feature would also have adjustable regulators and hammer spring preload, but I wouldn't think that these would be utilized to dial it down for a short plinking session with the intent to return it to maximum power when completed.
So that thought path leads me to thinking about alternative schemes to achieve an efficient maximum power tune with a power adjusting feature. The problem with an easily adjustable regulator setting would be that the hammer strike would be excessive as the regulator setting was reduced. This is because the valve requires less energy to open and the flow forces are less and so the valve stem will have greater lift and dwell resulting in wasted air since the "on-deck" volume is intentionally greater than what will be used for the shot. However, if the plenum size was say 20% of the barrel volume and the hammer strike was tuned to just barely dump it all, then could this yield a range of very efficient tunes by changing the pressure only? The porting could then be fixed and large.
I think the negatives to this approach would be that a greater regulator pressure would be required than when using a 1cc/fpe plenum and so the shot count would be reduced. Also, the shot to shot variance resulting from regulator inconsistency might be worse since the hammer strike wouldn't be tuned just below the knee.
It also occurs to me that a fixed regulator could have a variable relief valve downstream to reduce the "on-deck" pressure. A feature like this could potentially be 0-400 psi on one turn and compliment a 1200 psi set regulator for example. Since such a valve would typically require the flow path to turn at a right angle, this feature might easily be included in a drop block. The negatives to this are that there is now a second seat that may creep (leak) and that it would fall off of the regulator at the same pressure despite the in-valve pressure being reduced. Another negative is that the relief setting would need a shot cycle to take affect (once the valve is pressurized and flow has ceased, increasing the relief setting wouldn't lower the power of that particular shot, but it would for the subsequent one).
Another GTA post recently spoke to residual muzzle pressure and how this might affect accuracy by destabilizing the pellet. The logic seems sound. If so, would a higher pressure / small plenum shot have the additional benefit of increased accuracy?
Thoughts?
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Dump valves are notoriously inefficient, compared to "limited dwell" valves as used in conventional PCPs.... For the same volume of air (at STP) used, you have a longer dwell at lower pressure, and the valve is still open when the pellet exits the muzzle.... Any air released after that is 100% wasted, it cannot add to the pellet velocity.... The larger the valve volume, the greater the potential power, but the lower the efficiency.... Here is an example of how valve volume (as a percentage of barrel volume) is related to power and efficiency....
(https://hosting.photobucket.com/albums/oo221/rsterne/Millenium%20Pumper/.highres/DumpValves_zps8ff30e30.jpg) (https://app.photobucket.com/u/rsterne/a/2b6b9a8f-bf4c-41f4-8426-985eaf5ed6aa/p/a0fd3d9a-96c7-4f4f-bfc0-9e346f70e1db)
As you can see, a dump valve of 20% of the barrel volume yields about 0.75 FPE/CI, which is about half what you can get from a well tuned PCP at a similar power level.... The concept of closing the valve just as the pellet reaches the muzzle, while it can save air, is difficult to achieve, and the hammer strike required will change with pressure, and even pellet weight....
A lot of the inefficiency of dump valve is the higher residual muzzle pressure, so you would be going in the opposite direction you want for accuracy....
Bob
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Hi Hack, Wouldn't an adjustable transfer port accomplish your down stream power adjustment, even though it doesn't change the pressure going through your dump valve, it dose reduce the volume of air going into the barrel, and thus reduces the the final pressure in the barrel. Yes/No?
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James, I use that a lot, particularly with my retractable bolt designs.... I have a screw on the back of the receiver to limit how far the bolt face is withdrawn, and hence how much of the barrel port is available for flow.... Note the pin in the "J" slot.... The bolt has a flat face to load the pellet/slug ahead of the barrel port, and then the handle pulls back into the lower slot for firing....
(https://hosting.photobucket.com/albums/oo221/rsterne/30%20cal%20Disco%20Double/.highres/IMG_3800_zps811e14d1.jpg) (https://app.photobucket.com/u/rsterne/a/d8b60ed8-9651-478b-96e4-937970f0e59d/p/f9228c02-cd7d-4983-89db-8b70c83ed4ac)
It works great, and you don't have to retune the hammer.... However, the efficiency at reduced power is not as great as can be achieved by leaving the port large and reducing the valve dwell by reducing the hammer strike instead....
Bob