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Author Topic: What Effect Does Dwell Have  (Read 59 times)

rsterne

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What Effect Does Dwell Have
« on: November 03, 2017, 02:45:52 PM »
There seems to be a fair amount of confusion about the effect of valve dwell in PCPs.... I thought I would take the time to plot some interesting charts using Lloyd's Internal Ballistics Spreadsheet for what happens inside a "typical" PCP.... I chose a .25 cal with a 24" barrel operating at 2000 psi (regulated), using a 50 cc plenum, with a fairly high efficiency of 75% (which can be reached with large ports).... I used a typical "wasted" transfer port volume, and pellet bore drag of 1 lb. and breakaway drag of 2 lbs.... I included the air mass in the calculations, and used Isothermal expansion while the valve is open and Adiabatic expansion after it closed.... You don't have to know all that stuff, just realize that I tried to choose typical conditions, to produce GENERIC charts.... The absolute values will not be valid for any particular PCP, but the RELATIVE things that happen should be.... I used two pellets, the 25.4 gr. JSB Kings and the 34.1 gr. King Heavy MkIIs.... Here are the charts....



In the first chart, above, the blue lines represent the velocity (fps, on the left axis), and the red lines the energy (FPE, on the right axis).... The solid lines are for the 25.4 gr. pellets, and the dotted lines are for the 34.1 gr. pellets.... and all are plotted against the valve dwell, in milliseconds (mSec).... Note that both pellets have equal FPE (~ 62.3 FPE) at a dwell of 1.8 mSec....




In the second chart, above, the black lines represent the pellet position (left axis), from breech (0%) to muzzle (100%), when the valve closes.... and the green lines represent the efficiency (FPE/CI, on the right axis).... Again, the solid lines are for the 25.4 gr. pellets, and the dotted lines are for the 34.1 gr. pellets.... and again, all are plotted against the valve dwell, in milliseconds (mSec).... Note that the 25.4 gr. pellet exits the muzzle in a bit under 3.0 mSec, and the 34.1 gr. pellet in about 3.3 mSec.... therefore any additional dwell simply wastes air, as it cannot add more velocity and energy to the pellet....



The table, above, compares the two pellet weights at 1 mSec (green), 2 mSec (blue), and 3 mSec (red).... You can see what happens to the pellet position at valve close, muzzle velocity and energy, and the efficiency, at those three dwells.... At 1.8 mSec, when the FPE is equal (62.3 FPE) for both pellets.... the 25.4 gr. has travelled about 42% of the way to the muzzle, but the 34.1 gr. only about 33%.... Even though the valve dwell is identical, since the lighter pellet has travelled further down the barrel, more HPA has been released by the valve (to fill the barrel behind the pellet).... This means that more air was used to produce the same energy.... so the efficiency with the lighter pellet was less also.... While the 34.1 gr pellet achieved 62.3 FPE at 1.21 FPE/CI.... the 25.4 gr., at the same energy, only managed 1.00 FPE/CI.... In fact, the 34.1 gr. pellet is more efficient than the 25.4 gr. at all valve dwells....



The last table, above, shows what happens if you close the valve when the pellet is halfway along the barrel.... The dwell is adjusted so that both pellets are at the 50% point when the valve closes....  The % fps and % FPE numbers show the loss compared to the maximum possible, and the % air numbers show the amount of air used compared to running flat out.... Note that closing the valve at halfway saves over 40% of the air, while losing less than 5% of the velocity and 10% of the energy.... That is why I have been saying it's not worth tuning for the last few fps, it simply costs way to much air to get them....

I hope that this post gives you some insight into what is happening inside the barrel.... I will leave this thread unlocked, providing you stay on topic with your questions....

Bob
« Last Edit: November 03, 2017, 04:13:38 PM by rsterne »


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