All Springer/NP/PCP Air Gun Discussion General > "Bob and Lloyds Workshop"
Inward opening valve for big bores?
Jim Holmgren:
One problem that I have found when designing a new valve is to get enough throat area around the poppet especially with big poppets in big bores like .50/.73 cal.
For example lets make a .58cal with full bore porting, the transfer-port would be .58 dia or 170mm˛/0.263in˛ and the throat would need to be about 0.60 in in dia for the loss in valve-stem area and then the poppet would need to be even bigger to have a seal area so maybe a poppet dia of 0.65in?
Then the inlet have to be close to .90in in dia to give full flow around the poppet and if you have a balance camber in front of the poppet it may take up more area then the poppet it self,
And of course in bigger caliber like a 12ga would need something like 1.10in to get full flow around to poppet.
I have found it to be a problem to get enough area around the poppet in some designs, example when bottles are used the are no need to have a big 1.5in+pressure tube to get enough cc instead one can have a smaller transfer tube from bottle to valve, also the bigger tube you will use the bigger walls do you need so the in dia grows much slower then the outer diameter and weight.
Now to the question, is a valve that opens inwards a practical solution instead of a poppet going outwards?
It would means that the inlett would not need to be any bigger than the throat diameter, however it would need to travel the whole way past the transfer-port, and making it reacting balancing like the corthan valve that blow the valve open (to the close it by a bleed port) is maybe more complex...
The hammer would also need to be reversed.
A simple sketch of it
Jim Holmgren:
Here is a valve that don't have a o-ring in front of the transfer-port that could cut it self on the transfer port.
And here is a design that blow open and the closes thought a bleed hole like the Corthan valve among others.
jackssmirkingrevenge:
Looking at it quickly, roughly speaking a poppet would get 50% flow after moving just 12.5% of the seat diameter. As drawn you would need to move the piston 4 times as much to get the same flow, because the % flow is directly proportional to the percentage of the port that is exposed. This therefore might have proportionally more flow than a balanced poppet in a similarly sized tube, however it will reach maximum flow later, so the overall performance might not be superior.
Jim Holmgren:
--- Quote from: jackssmirkingrevenge on February 15, 2019, 11:47:53 AM ---Looking at it quickly, roughly speaking a poppet would get 50% flow after moving just 12.5% of the seat diameter. As drawn you would need to move the piston 4 times as much to get the same flow, because the % flow is directly proportional to the percentage of the port that is exposed. This therefore might have proportionally more flow than a balanced poppet in a similarly sized tube, however it will reach maximum flow later, so the overall performance might not be superior.
--- End quote ---
Yeah it would need to be opened fast enough for it to be useful, the drag of o-rings and heavy weight of it may slow it down so much that it may not make full flow ???
Did a simple sketch of the hammer if someone did not get it.
rsterne:
As Jack mentioned, a sliding valve would need a lot of travel to flow the full area.... A poppet valve only need to open 25% of the throat diameter to have full flow.... I think getting the valve to open quickly and fully, and then close without wasting air, would be quite a challenge....
Most really large caliber PCP builders don't bother making full bore area ports for exactly the reasons you gave.... They can develop huge FPE without bothering, simply from the large base area of the projectile available for the pressure to act on.... For instance a .58 cal with bore area porting and a 24" barrel on 3000 psi might have a "lofty goal" of 750 FPE, but with 7/16" porting that drops to about 600 FPE.... With a similar 12 gauge, 1200 FPE might be possible, drop the port back to 1/2" and you are still over 800 FPE.... It's easier to just increase to 4500 psi than to worry about bore area porting, and you are back up to a potential of 1200 FPE.... Need more, use Helium.... ;D
To support 1/2" porting, with a balanced valve using a 5/32" valve stem, the throat needs to be about 9/16", and the poppet OD about 5/8".... That means the valve ID needs to be about 7/8" to achieve a larger flow area than the throat or ports.... You could fit that inside a 1.25" OD tube.... Yes, if you want full bore-area porting you will need to go bigger.... but think about how much onboard air you need for just one full power shot.... :o …. 1200 FPE needs about a 1200 cc plenum, minimum, and even then you would get a pressure drop of about 10% during the shot.... To really get full advantage of bore size porting on a 12 ga. you would need a reservoir of many litres, and passages between the bottle(s) and the valve equal to the bore area as well.... The threads on a 500 cc bottle are only 18mm, so you can only get about a 1/2" hole through the attachment at the most.... See the challenges?.... They don't just stop with the valve diameter and tube diameter, when you want to go whole hog on an air hog.... ;)
Bob
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