Pressure and Efficiency in PCPs
I was recently having a discussion about the practical limits of PCPs and what we can expect for efficiency with various pressures.... I did some "what ifs" using Lloyd's internal ballistics spreadsheet.... I was trying to duplicate .22 rimfire ballistics, using a 40 gr. bullet, and using where I am currently at with my .224 cal. Hayabusa with a 29" barrel as a starting point.... It currently shoots a 41.3 gr bullet at 1077 fps (106 FPE) at 3000 psi at 0.61 FPE/CI when maxed out.... I set up the spreadsheet to reflect that information, and then came up with the following predictions....
At 3000 psi:
Maximum velocity (dump shot): 1080 fps @ 0.48 FPE/CI
Valve closes at 50% of barrel length: 1050 fps @ 0.80 FPE/CI
Valve closes at 33%: 1000 fps @ 1.20 FPE/CI
At 3500 psi:
Max: 1130 fps @ 0.47 FPE/CI
50%: 1100 fps @ 0.87 FPE/CI
33%: 1060 fps @ 1.11 FPE/CI
25%: 1020 fps @ 1.32 FPE/CI
At 4000 psi:
Max: 1170 fps @ 0.42 FPE/CI
50%: 1150 fps @ 0.78 FPE/CI
33%: 1110 fps @ 1.05 FPE/CI
25%: 1065 fps @ 1.30 FPE/CI
20%: 1030 fps @ 1.46 FPE/CI
At 4500 psi:
Max: 1210 fps @ 0.41 FPE/CI
50%: 1190 fps @ 0.75 FPE/CI
33%: 1150 fps @ 1.02 FPE/CI
25%: 1110 fps @ 1.24 FPE/CI
20%: 1070 fps @ 1.41 FPE/CI
15%: 1020 fps @ 1.65 FPE/CI
There are a few things we can deduct from this information.... First of all, having the valve open after the pellet is half way down the bore only adds 20-30 fps to the velocity, but causes a HUGE increase in the air used to achieve that.... The difference between having the valve close at 33% of pellet travel compared to 50% is about 40-50 fps, but again there is a big savings in the amount of air used.... The 1.0 FPE/CI efficiency point comes when the valve closes somewhere between 33% and 42% of barrel length, compared to a dump shot which is under 0.5 FPE/CI.... The difference between having an efficiency of 1.0 FPE/CI and less than half that is about 60 fps (5-6% of the MV)....
As the pressure increases, the velocity increases as well.... At maximum (ie a dump shot), a 50% increase in pressure gains about 140 fps (12% MV).... At an efficiency of 1.0 FPE/CI, that same 50% increase in pressure gains about 130 fps (13% MV).... At the 33% valve closure point, a 50% increase in pressure gains about 150 fps (15%).... You can see that increasing the pressure results in a gain in the potential for the gun to not only produce more velocity, but also to be more efficient at any given velocity level....
Please realize that these results are simple extrapolations achieved using Lloyd's spreadsheet.... We have experienced a reduction in efficiency at constant pressure when trying to push the velocities to and beyond Mach 1.... ie it takes a LOT more air to get there.... However, we have been playing around with pressures below 3000 psi (at least I have, I can't speak for Lloyd).... I don't know if the results indicated above will hold true or not.... but I think the trend is clear.... If you are limiting out because of barrel length or pressure, then increasing the pressure (providing it can be done safely, of course) is one way to gain more velocity, energy, and efficiency.... Lengthening the barrel is the other way....
Bob
