Port size and efficiency volume 2.
What does Port size have to do with 2 milliseconds? Is this a punchline to a joke? Absolutely not. 2 Milliseconds is roughly the amount of time you have to expel as much air into an airguns barrel prior to the pellet out pacing any further air being released. Its a race against the hare versus the turtle, or rather air molecules and a piece of lead, and against all odds, the turtle (lead) always wins...2 ms isn't a golden number I pulled from thin air or calculated, its generalized here simply because the average barrel length of an air gun is relatively 20"~, so with that, you can roughly state that for every 10" of barrel you have ~1 ms of potential effective dwell, and anything beyond is wasted air. In reality, the number is not static, and reduces as barrel length is greatly increased, but for 10"-20"-30" barrels, we can ballpark their potential effective dwells at 1ms-2ms-3ms to keep it simple. (its not quite this simple but I am choosing to simplify here for practical purposes)
So whats caliber/port size have to do with it? Why do bigger calibers/ports generate more power, with less or equal dwell than smaller calibers/ports? Is it as simple as there being more area for pressure to act upon as the projectile moves down the barrel? No. One cannot and should not dismiss a very important factor in air guns....Flowrate.
Having larger calibers or ports increases flow rate, which allows more air to enter the barrel within its ~2 millisecond window of opportunity, This in turn increases the effective barrel volume, which increases potential energy output. As shown in the graph below, flow rate and fpe output are directly tied together. I also used 23% thermal efficiency to calculate approximate maximum fpe per caliber at full bore porting and a 1925 psi starting pressure of the shot cycle. 23% thermal efficiency is by no means the upper bounds of limitation, but closer to the lower bounds, a typical pcp operates between ~20 and ~30% thermal efficiency. The biggest variable determining that is valve dwell / closure relative to pellet position in the barrel, the others being port size relative to bore size, pressure drop during shot cycle, barrel length and caliber. 23% best represents my current configuration when operating at 2ms~ dwell, where as a 33% reduction in dwell only reduces the power output by 10% but cuts down the air use by roughly 33% and increases thermal efficiency to the upper bounds of 30%.
This second graph will demonstrate what happens when you increase port size in a given caliber, this one being .25, with a 19.5" barrel and 1925~ psi.
To hit the 23% efficiency mark in the first graph (~89fpe for .25 cal) the second graphs rifle (mine) would require a longer barrel, and more plenum volume to keep the pressure up during the shot cycle. Both of which my spreadsheet predict loses for.
As you see, recoverable losses are calculated at 25 fpe, where my current peak output is 64. 64+25 = 89, which is the upper bounds of what fpe can be obtained in a .25 cal at 1925~ psi shown in the first graph.
This is why larger ports/calibers both make more power, and use more air...its not as simple as more area for the pressure to act upon, its both flow rate and area for pressure to act upon.
-Matt
What does Port size have to do with 2 milliseconds? Is this a punchline to a joke? Absolutely not. 2 Milliseconds is roughly the amount of time you have to expel as much air into an airguns barrel prior to the pellet out pacing any further air being released. Its a race against the hare versus the turtle, or rather air molecules and a piece of lead, and against all odds, the turtle (lead) always wins...2 ms isn't a golden number I pulled from thin air or calculated, its generalized here simply because the average barrel length of an air gun is relatively 20"~, so with that, you can roughly state that for every 10" of barrel you have ~1 ms of potential effective dwell, and anything beyond is wasted air. In reality, the number is not static, and reduces as barrel length is greatly increased, but for 10"-20"-30" barrels, we can ballpark their potential effective dwells at 1ms-2ms-3ms to keep it simple. (its not quite this simple but I am choosing to simplify here for practical purposes)
So whats caliber/port size have to do with it? Why do bigger calibers/ports generate more power, with less or equal dwell than smaller calibers/ports? Is it as simple as there being more area for pressure to act upon as the projectile moves down the barrel? No. One cannot and should not dismiss a very important factor in air guns....Flowrate.
Having larger calibers or ports increases flow rate, which allows more air to enter the barrel within its ~2 millisecond window of opportunity, This in turn increases the effective barrel volume, which increases potential energy output. As shown in the graph below, flow rate and fpe output are directly tied together. I also used 23% thermal efficiency to calculate approximate maximum fpe per caliber at full bore porting and a 1925 psi starting pressure of the shot cycle. 23% thermal efficiency is by no means the upper bounds of limitation, but closer to the lower bounds, a typical pcp operates between ~20 and ~30% thermal efficiency. The biggest variable determining that is valve dwell / closure relative to pellet position in the barrel, the others being port size relative to bore size, pressure drop during shot cycle, barrel length and caliber. 23% best represents my current configuration when operating at 2ms~ dwell, where as a 33% reduction in dwell only reduces the power output by 10% but cuts down the air use by roughly 33% and increases thermal efficiency to the upper bounds of 30%.
This second graph will demonstrate what happens when you increase port size in a given caliber, this one being .25, with a 19.5" barrel and 1925~ psi.
To hit the 23% efficiency mark in the first graph (~89fpe for .25 cal) the second graphs rifle (mine) would require a longer barrel, and more plenum volume to keep the pressure up during the shot cycle. Both of which my spreadsheet predict loses for.
As you see, recoverable losses are calculated at 25 fpe, where my current peak output is 64. 64+25 = 89, which is the upper bounds of what fpe can be obtained in a .25 cal at 1925~ psi shown in the first graph.
This is why larger ports/calibers both make more power, and use more air...its not as simple as more area for the pressure to act upon, its both flow rate and area for pressure to act upon.
-Matt