Pulse Bubble Distance

[rsterne]

Lloyd has a cell in his spreadsheet called the Pulse Bubble Distance.... It is an output, and represents how far down the barrel the pellet has moved when the valve closes.... This is directly related to the efficiency of the shot.... If the pellet has moved much more than halfway down the barrel when the valve closes, the air behind it has little room to expand.... Here is a graph that you may find interesting....



The lines in blue are based on my .25 cal Disco Pumper Carbine, which is actually a PCP with an onboard pump.... The valve has a small reservoir attached, and the total volume is 27 cc.... With this small volume, I can still get three shots of 31 FPE within a 3% ES, plus a backup shot.... If I set the gun up for maximum, it shoots about 845 fps with JSB Kings (40 FPE).... Set up that way, the valve is closing when the pellet has gone about 9.5" down the 20" long barrel, and the efficiency is 1.04 FPE/CI.... The valve starts out with 1500 psi in it, and after that shot, the pressure is down to about 1160.... If I fitted a stronger hammer spring, in an attempt to make the valve dump more air, could I get more velocity?.... It turns out the answer is not much.... Even if I kept the valve open until the pellet left the barrel, it would only gain about another 30 fps.... Doing so, would increase the amount of air used (and the pumps required to refill to 1500 psi) by about 60%.... That is a 3% gain in velocity for a 60% increase in air usage.... Hardly a worthwhile tradeoff, especially if you have to pump to refill....

So how does this relate to a more normal PCP?.... Well, a Disco has exactly 5 times the air reservoir volume, 135 cc.... Keeping all the numbers the same in the spreadsheet except for the tank size, I ran the same data, and those curve are in red.... I didn't increase the barrel length, I left it at 20".... Otherwise, they would represent a .25 cal Disco with the same valve, and running at 1500 psi.... Note that the velocity is greater because the larger reservoir (we are assuming no restrictions on the inlet side of the valve) keeps the AVERAGE pressure higher at the valve seat during the shot.... The efficiency curve is very similar, shifted slightly lower because of the increased power.... However, the key elements are still the same.... If the valve is open for half the barrel length, the gun would shoot within 30 fps of maximum.... but it would be 75% more efficient doing it....

The point I am trying to make is this.... The last few fps you can wring out of a PCP by increasing the valve dwell comes at an EXTREME price in the amount of air required to do that.... Backing the velocity down about 5% will use about half the air....

Bob

[airpuffhunter]

very interesting

[Backyardsniper1]

So for something like a P'rod adjusting the stroke and such would be beneficial vs.heavier hammer spring and bored out transfer port?

[rsterne]

Adjusting the stroke changes the dwell, just like changing the hammer spring.... In practical terms, then are nearly interchangeable.... Generally, increasing the flow though the ports allows a PCP to develop more power at a lower pressure.... Therefore if you keep the pressure constant, for a given power level with bigger ports, you can decrease dwell, which increases efficiency....

The problem is, that as you open up the ports, you shorten the shot string because you make the curve "peakier".... That is why you can't just say that "A" is better than "B", because it is the combination of "A and "B" that shapes the shot string....  The point of my post was to warn of the wastefulness of trying for the last few fps by increasing the hammer strike (travel, weight, or spring force).... It just wastes air....

Bob

[lloyd-ss]

............................  The point of my post was to warn of the wastefulness of trying for the last few fps by increasing the hammer strike (travel, weight, or spring force).... It just wastes air....

Bob

Bob,
No offense intended, but that might have been said by a drivers ed teacher.. or a nun.     Just joking!
But I think what you described in that last sentence is a lot of the purpose-built big bore hunting rifles.  Two or three good shots with a weak follow-up.  Really powerful, and loud, and the devil may care about efficiency.

But if you want a long session in the woods or at the bench and not have to refill, then what you are saying has serious merit.
The half the barrel length rule of thumb is pretty reasonable but it will vary with bullet dia to barrel length ratios and sectional density, and more.
I agree that those last few fps are very expensive and that backing off a little can make a LARGE difference in fuel economy! I always try and find the max vel of anything I build and then work down from there, unless I have altered something such that it cannot be tuned back down.

I sometimes think of an analogy with 1/8 mile and 1/4 mile drag cars.  A car that does 125 in the 1/8 might get just 160 in the 1/4.

Lloyd-ss

[Rescue35]

...I think what you described in that last sentence is a lot of the purpose-built big bore hunting rifles.  Two or three good shots with a weak follow-up.  Really powerful, and loud, and the devil may care about efficiency...

Lloyd-ss

Very true Lloyd. I think as a general rule that Bobs analogy applies to .22 and .177 PCPs. Even some .25 pellet rifles. Big bores are a beast of there own.

[rsterne]

I'm not so sure that it can't be applied equally well to big bores.... Here is the shot string from my DAQ .308 Exile as received.... 118.8 gr. Lee RN bullets....

Fill pressure 3000 psi
836
789
746
End pressure 1800 psi (pressure drop 1200 psi)
Average velocity = 790 fps, total FPE = 494 fps, Efficiency = 0.39 FPE/CI

I "set" the hammer spring by compressing it fully, letting it rebound, and then closed the coils on both ends.... That reduced the free length of the spring from 5.44" to 4.50".... There is no preload adjustment, so the uncocked force went from 11.9 lbs. down to 2.7 and the cocked force dropped from 27.9 lbs. down to 20.2.... Here is the resulting shot string....

Fill pressure 3000 psi
833
832
824
811
793
End pressure 2050 psi (pressure drop 950 psi)
Average velocity = 818 fps, total FPE = 883, Efficiency = 0.87 FPE/CI

The efficiency more than doubled, and I went from 3 shots with an ES of 90 fps to 5 shots with an ES of just 40 fps.... After 5 shots the velocity was still higher than the 2nd shot in stock form.... No changes other than the hammer spring....

Bob