What Happens Inside a Disco

[rsterne]

Lloyd_ss recently sent me a copy of his latest spreadsheet that we use to examine what is happening inside PCPs.... The variables include the caliber, pellet weight, air pressure, valve dwell, reservoir volume, transfer port volume, barrel length, pellet drag (sliding), pellet breakaway force, and an efficiency factor (typically from 50-70%).... Most of the numbers are easy, the "art" in making the spreadsheet output useful information comes from balancing the valve dwell and the efficiency factor to make the outputs match observed results (ie FPE and air usage).... For any given set of results, there is only one value of dwell and efficiency that provides a match.... There are many outputs, but the coolest thing on the latest version is a graph which allows you to compare three sets of data visually.... I input the data from a stock Disco shooting 15.9 gr. JSB Exacts for three pressures, 2000 psi (fill pressure), 1700 psi (velocity peak), and 1200 psi (refill pressure).... Those pressures are the range that produced velocities within a 4% window, starting at 774 fps, peaking at 809 fps, and falling back to 779 fps.... The average was 797 fps (22.4 FPE) and the efficiency over the 25 shots was 1.28 FPE/CI.... However, at the beginning of the string, each shot only uses about 20 psi, while at the end it is using over 40 psi....



There is a LOT of information on that graph.... First, look at the pressure curves, and note where the valve closes (where the plateau ends), and what the residual pressure is at the muzzle (end).... At 2000 psi the pellet has only travelled about 2" when the valve closes, and the muzzle pressure is about 200 psi (and the gun is relatively quiet).... At 1700 psi, the valve is open until the pellet has moved almost 4", and the residual pressure is roughly 300 psi (gun getting louder).... At the end of the string at 1200 psi, the valve is open until the pellet has travelled 7.5", and the gun is much louder with the muzzle pressure being nearly 400 psi.... While it's not on the graph, the efficiency for these three cases is about 1.75 FPE/CI at 2000 psi, 1.27 FPE/CI at 1700, and only 0.9 FPE/CI at 1200 psi.... A short pulse of high pressure is much more efficient than a long, steady push....

Now look at the velocity curves.... They are different shapes, but they all end up at nearly the same place, within a 4% spread averaging 800 fps.... The FPE curves tell the story about how the pellet gains energy from the air charge.... At 2000 psi, most of the energy is applied to the pellet in the first third of it's travel (8"), at which point it ends up with the same energy (15 FPE) as the 1700 psi case.... At 1200 psi, the pellet only reaches the same energy as the 2000 psi case at the very end of the barrel, getting a long, slow shove all the way to the muzzle.... I think it's a real eye-opener what is actually happening inside a PCP.... I'd like to personally thank Lloyd Sikes for the countless hours he has spent developing and continually improving this spreadsheet.... and in particular for sharing it with me.... I find it invaluable....

Bob

[lloyd-ss]

Bob,
Thanks for the thanks, so to speak.     
Your explanations are spot on and Iappreciate your taking the time to word everything so succinctly. Another thing that can be  observed on this chart is what will happen if you chop the barrel.  At first glance, it looks like the barrel can be easily cut back several inches with almost no loss in velocity.  But the velocity scale is a little compressed, and if you look at the fpe curves instead (heading up to the top right)  it will show more clearly what your real loss will be.  You can also see how the muzzle report (due to the muzzle psi) will go up as the barrel is shortened.
As you said, a lot of info there.
Lloyd

[Tom SC]

  A question gents if I may, does this mean the pellet stays in the barrel longer at lower pressure shots than shots taken at higher pressures and at similar muzzle velocities.  Will this affect accuracy by the pellet being subject to barrel vibrations for longer or shorter periods of time . Am I making any sense?

[lloyd-ss]

You are correct that he low psi shot will take longer to leave the barrel than one that accellerates faster to the same vel. But i don't know if one is more likely to be more accurate. Low psi seems easier to hold steady. 10m rifles are really slow.
Lloyd-ss

[rsterne]

The lock times are 3.49 ms @ 2000 psi, 3.57 ms @ 1700 psi, and 4.01 ms @ 1200 psi.... so yes, the pellet stays in the barrel about 1/2 millisecond longer at 1200 psi than it does at 2000 psi.... If the barrel harmonics are such that makes a difference, then it is possible the accuracy could be affected.... but which way (better or worse) is anyone's guess....

Good point, Lloyd, about being able to see what happens if you chop the barrel.... Thanks for adding that comment.... Note that the velocity and FPE loss is greater at lower pressure.... Geez I love your spreadsheet !!!

Bob