Good work so far Timothy , keep at it , don't lose interest.
Force large volume into small area = high pressure but there are so many variables like TP size and TP distance from valve head. Internal valve chamber volume plus TP volume and how it affects the initial pressure. So example - if I have a initial valve pressure area of 763 cubic mm but a combined TP and internal valve area of 1069 cubic mm it doesn't matter how much initial pressure I have if it moves into even a slightly larger area - it will be reduced. I have to keep all areas the same. And if I reduce the internal valve area and TP to less than the pressure chamber then I restrict it.
So to get a close 2800 psi I need to push 294524 cubic mm's of air into a chamber of 1545 cubic mm's and then release that pressure into the barrel via and internal valve route and TP of as close as possible to the initial volume of the pressure chamber without any restrictions. I am now on my 16th valve LOL
I am now getting around 600 FPS with 15.4 gr pellets and I know I am going to have to make another valve if I want higher. My aim is 850 FPS and the problem is reduced pressure because of the valve internal volume affecting initial pressure.
After I think my 16th valve I am finally happy. I am not a machinist and almost failed matrick so this was one serious learning curve LOL.
This valve is as close as I can get to keeping all my areas the same and there is a marked improvement. At least with this one I can adjust the internal volume for more power as I progress without having to make a new one every test
My only problem now is the hammer is not opening the valve long enough to release all the air. Am still retaining about a third in the valve after firing. Going to make a heavier hammer next week.
Been at this now for 16 hrs a day since Wednesday - wife has been very patient LOL
Well she shoots. Nice and quiet too. There is still so much small details to finish off I am wondering if she will ever be done. I am not getting the velocities I was hoping for but until my buddy pulls through with his chrono I won't know for sure. I suspect about 500 fps or less. Getting 20 mm penetration into pine at 3 mtrs but nothing at 20 mtrs - not very scientific I know but It's all I have for now and I am not about to put 3 K down on a chrono. I suspect my valve area is too small - the science says large volume into small volume = high pressure but if the amount of air is inadequate no amt of pressure will make a difference I think. Thank fully I can still tweak this valve a bit on both sides so will get to that once I know for sure what FPS I am currently getting.
Really happy with trigger set up - nice and light. Need to add a return spring to bring it back but for now she shoots
I tested today on a friends chrono and I am currently getting 430 FPS with one pump. My aim is 700 plus FPS with 15.5 gr pellets. I was also just blessed with a chrono that was given to me and should be here next week. So things will start getting much more scientific.
So here is the math:
My pump chamber is 289615 cubic mm or 289.615 cc and this is fixed/static and is volume at state 1
Using Boyle's law I worked out that I need a volume at state 2 of 1934 cubic mm (or 1.93 cc) to get the required 2200 psi (150 bar)
My current valve has a internal volume of 500 cubic mm (0.5cc) and this is giving me a constant of 430 FPS with 15.5 gr pellets and I couldn't understand why because common sense says smaller area = higher pressure and that's good - right? Well actually not. According to Boyle's law my current set up was giving me up to 8000 PSI. This didn't sound right so I went and re-did the maths and no matter how many times I put in the figures I was getting 8000 PSI in my valve. Now I know that what is on paper is not always reality and if any one is wondering if it is impossible I have a 5mm wall honed pipe around the valve and the valve has a 8.5mm wall around the chamber. The formula says 8000 psi so why am I not blowing myself up or shooting at the speed of light? Bear in mind that all the math and sizes were based on my original prototype in .177.
With a few hundred hours of surfing my friend google and getting more confused I found two comments on Airgun nation:
"The +/- 630 cc of air per shot for the Vulcan example is the volume of air that would expand from the
pressurized air reservoir to the atmosphere with each shot.
If you want to do the calculation for your own PCP, the equations can be simplified:
cc of air used per shot @ atmospheric conditions= (Bar used per shot)*(actual air reservoir volume cc)
**bar used per shot=(initial air reservoir fill pressure-ending pressure)/# shots fired
to convert volume from atmospheric conditions to air reservoir pressure of 150 bar:
cc of air used per shot @ 150 bar=(cc of air per shot @ atmospheric)/150
"I just checked my 22 Evanix and used 50 bar in 22 shots or 2.27 bar/shot. The Evanix has a 290 cc air cylinder so the cc used per shot is 2.27*290=658 cc per shot @ atmospheric pressure
Converting 658 cc to 150 bar, 4.4 cc per shot. The Evanix shoots 18.1 grain at 980 fps. Pretty sure these numbers are in the ball park for our PCP’s
using the numbers from your Wildcat I calculate 525 cc per shot atmospheric or 3.5 cc per shot at 150 bar."
So I double checked his math using Boyle's law:
P1 = 1 Atmosphere
V1 = 658 cc
P2 = 150 bar
and the output was 4.4 cc per shot as he stated.
This made me realize that the volume of my valve chamber was a quarter of the size it should be. Doesn't matter how high the pressure is if the volumes of the ports and the barrel are far higher - what should be a crack becomes a fizzle fart as the pressure is drastically reduced entering a larger volumetric area. My ports are 4.8mm just for interest sake.
So it all clicked. I checked the math again. If the above formula is right for the Evanix then I would need:
P1 = 1 Atmosphere
V1 = 289 cc (my pump volume)
P2= 150 bar
= 1.95 cc of air required in my valve chamber
And this all agrees with my original calculation a year ago when I started of a valve chamber of 1934 cubic mm but I forgot that and ran on my previous calculations for a .177.
I didn't take into account barrel volumes and that the volume of my .22 would be as much as four times that of the .177 so would need more air behind the pellet.
I also stumbled on some info about the Benjamin 392 that has a average air reservoir of 5cc and a pump volume of 50cc so at 10 pumps (10*50/5) = 100 bar so again I got the result of 8000 plus psi for mine - (1*289/0.5)=578 bar. I am still not sure if this is right but it does make sense. So if I enlarge my valve area to 2cc it goes like this (1*289/2)= 144.5 bar which is very close to my end goal. I will also be modding the valve to be able to multi pump and then run some chrono tests to see if my velocities match my mathematical assumptions LOL
Yes - it took a year for it to make sense LOL but I would appreciate it if someone/anyone could point out any errors in my calculations - saves me a whole lot of money.
The irony is I didn't want to spend any money on a PCP - I am way past that point now