Volume of Air Used

[rsterne]

Have you ever given any thought to how much air is used to propel the pellet down the bore of an airgun?.... Here is some food for thought....

Weihrauch HW95: 45 cc
Diana 34: 49.3 cc
Crosman Quest: 49.1 cc (these guns are about 15 FPE in .22 cal) = ~3 cc / FPE produced
Diana 350: 70.8 cc (about 24 FPE in .22 cal) = ~3cc / FPE produced


Crosman 1322/2289 .22 cal (18cc per stroke, F-T-P fitted):
10 pumps: 180 cc (about 9 FPE with 14" barrel) = ~20cc / FPE produced
20 pumps: 360 cc (about 14 FPE with 14" barrel) = ~26cc / FPE produced

Uber-Pumper .22 cal (2200 based pump, 24 cc per stroke, with F-T-P):
10 pumps: 240 cc (about 14 FPE with 24" barrel) = ~17 cc / FPE produced
20 pumps: 480 cc (about 21 FPE with 24" barrel) = ~23 cc / FPE produced
30 pumps: 720 cc (about 26 FPE with 24" barrel) = ~28 cc / FPE produced

Crosman Disco .22 cal: ~285 cc (averaged over 25 shots @ 22 FPE with 24" barrel) = ~13 cc / FPE produced

So why the big differences?.... It has to do with the way the air is compressed for one thing.... and with the way the pressure is released....

In a springer, the air is compressed in an ADIABATIC manner.... which means that the temperature increases dramatically during the shot cycle.... I have seen figures of about 1400 psi and over 800*F at the peak of the cycle.... In addition, the temperature is so high that there is reportedly some combustion taking place of the thin film of oil inside the chamber which adds as much as 60% to the power of the shot.... This would account for the high efficiency of springers compared to other airguns....

In a multi-pump pneumatic, the air is compressed in a near adiabatic manner during each stroke, but the heat soaks out into the gun so by the end of pumping the air temperature is only a bit above ambient.... I have seen a Delrin piston melted (~347*F) from enthusiastic pumping.... and yet the gun is only slightly warm to the touch.... If you wait until the gun cools to ambient temperature before firing, you will loose a bit of pressure and velocity compared to firing the gun right away, especially at very high pump numbers.... In addition, there may be pump losses which account for the reduced efficiencies of the MPP design.... A stock Crosman 1377/2289 begins to retain air at about 1500 psi.... with different springs that can be pushed to about 2500 psi or beyond....

In a PCP such as the Disco.... the air is compressed outside the gun in the case of a SCUBA tank being filled.... or in a near ISOTHERMAL manner if using a hand pump.... When the gun is filled, the temperature rises (quite a bit if filled from SCUBA) but it is allowed to cool to ambient temperature before shooting, just like in an MPP.... The air simply expands out of the reservoir to propel the pellet, actually lowering the temperature inside the reservoir slightly if the shots are close together.... Since all pumping / compression losses occur long before the shot cycle, the efficiency is higher than with an MPP design....

The extreme conditions inside a springer don't need a long barrel to produce excellent results.... High powered pneumatics, however, benefit from longer barrels.... I hope the above helps explain why that is the case....

Bob

[glassman98]

Thanks for the information. Now I think I know what they mean by warmup in a springer. gman

[QVTom]

Bob,

Interesting data.  Curious to know method used  to calculate the volume of the Disco?

-T

[Mark 611]

Great stuff Bob!

[rsterne]

The reservoir on the Disco is 135 cc.... For a 25 shot string the pressure started at 2000 psi and dropped to 1230 psi.... That is a 770 psi drop which is 770/14.5 = 53 Bar.... That gives a total of 135 x 53 = 7155 cc of air used which is 7155 / 25 = 286 cc per shot on average.... In actual fact, it's not linear at all.... I just used the average for comparison purposes....

The first shots, at high pressure, are much more efficient.... the pressure only dropping about 100 psi for the first 5 shots.... That works out to 100/14.5 = 6.9 Bar x 135cc = 932 cc / 5 = 186 cc per shot.... 186 / 22 = 8.5 cc / FPE.... The last shots in the string are the least efficient.... and the pressure drops about 200 psi for the last 5 shots.... That gives 200/14.5 = 13.8 Bar x 135 cc = 1864 cc / 5 = 373 cc per shot.... 373 / 22 = 17 cc / FPE.... The Disco is only about half as efficient at 1300 psi as it is at 1900 psi.... That is why it is a lot quieter at the beginning of the shot string.... less air being wasted.... The 13 cc / FPE quoted above is the average over the 25 shot string....

Bob

[QVTom]

Thanks for the explanation.  Good stuff!

Wandering of topic a bit.  I don't know much about how the metering valves on PCPs operate.  Looking at the data, it seems as the the pressure drops the valve closure delays. Is this the intended function of the valve design to try to compensate for pressure drop and maintain E or just a byproduct of less pressure to return to the closed position?

-T

 

[rsterne]

It is both a result and a design feature.... In a non-Regulated PCP, the highest velocity occurs when the valve opens fully (or nearly so) against the pressure trying to close it, which is a combination of spring pressure (minor) and air pressure (major).... At higher pressures, the air is acting to prevent the valve from opening as much (and/or close it sooner).... reducing the amount of air released and limiting the velocity.... At lower pressures, the valve is still opening the maximum amount but there is less air escaping.... This produces the classic "bell curve" of velocities that we strive for....

It all comes down to a balancing act between air pressure and hammer hit.... More pressure requires more hammer hit to fully open the valve.... If you have a heavier hammer, more hammer travel, or a stronger spring (or any combination of those).... you can use a higher fill pressure.... The result will (usually) be more energy delivered to the pellet.... but fewer shots available....

The transfer port can be used to restrict the airflow and flatten the bell curve, adding more shots by supressing the peak velocity.... Conversely, opening up the porting of the valve, transfer port, and barrel port.... will usually result in more peak power but a more highly curved (and hence shorter) shot string.... Again, it's a balancing act....

Bob