The O-Ring buffer and Air Usage Efficiency

[bstaley]

LOL....yes many variables.

Not saying that it's absorbing and returning energy at the same time.  As the vavle is opening, the buffer absorbs some of the excess hammer energy that would otherwise go into increased lift and dwell.  Then once lift peaks, and the valve starts closing, some of that energy may then be returned to the hammer, increasing the closing speed.  Again, perhaps not a huge factor, but when combined with everything else results in significant net changes in dwell.

I think we basically agree.  Several things seem to be happening, all of which are tending to reduce dwell.

[FuzzyGrub]

As Bstaley can confirm, my thoughts on how this works, has been all over the map.     Anyway, my latest theory, given Bob's recent data, is that the o-ring compression range is what drives the best tune with the o-rings.  For it to work well requires a narrow valve throw range.  With a narrow valve throw range, the only way to get higher velocities, is by opening the transfer port and valve exhaust.   I can't say that I found any better efficiencies by increasing the transfer port, and detuning further.   ie, using 0.110" transfer port and tuning to 24 shots @ 16fpe, vs 0.125" transfer port and trying to that same level.  Actually, might not have tried that.      But, it does sound like something worth trying, just to see if it can extend shot count or flatten string further. 

[bstaley]

As Bstaley can confirm, my thoughts on how this works, has been all over the map.     Anyway, my latest theory, given Bob's recent data, is that the o-ring compression range is what drives the best tune with the o-rings.  For it to work well requires a narrow valve throw range.  With a narrow valve throw range, the only way to get higher velocities, is by opening the transfer port and valve exhaust.   I can't say that I found any better efficiencies by increasing the transfer port, and detuning further.   ie, using 0.110" transfer port and tuning to 24 shots @ 16fpe, vs 0.125" transfer port and trying to that same level.  Actually, might not have tried that.      But, it does sound like something worth trying, just to see if it can extend shot count or flatten string further. 

Yes, you should try opening up the TP port more, as well as the exhaust port on the valve.  Opening these both up I'm now able to get 38 shots at an average FPE of just over 15 FPE within 4% from my p-rod.

See teh 4th post down in this thread:
www.network54.com/Forum/581291/thread/1356374854/

[rsterne]

At some point, this trend to increasing efficiency with opening the ports and reducing the dwell HAS to stop.... It is the same situation as putting a large wasted volume between the valve and the pellet.... In Lloyd's spreadsheet we call that the "transfer port volume", but it's really a combination of all the volumes between the seat of the poppet and the base of the pellet.... and it's a negative in terms of efficiency.... What happens is that the pressure (and hence the force) available to accelerate the pellet drops as the air expands into that volume.... In a .22 cal Disco, you have a reservoir of 135 cc, and a TP volume of 0.34 cc, so the pressure drop is small, using those numbers about 0.25%, ie from 2000 psi down to 1995 psi.... However, in my .25 cal Pumper, with a valve of only 6.5 cc and a TP volume of 0.46 cc, and now the volume is 7%.... The pressure drops from 1600 psi down to 1494.... no longer insignificant.... Boring out the ports larger and larger, and then filling that volume with a smaller and smaller amount of HPA, obviously has to have it's limits.... Going too big on the ports just wastes volume, and hence pressure and efficiency....

Bob

[bstaley]

Yep, as with all components of a system, changing certain properties will provide benefits, until some point where other factors become more of a detriment.  This point is usually identified by the knee in  many of your graphs....the point of diminishing returns.

At some point the pressure drop due to the increased volume will start to overcome the decrease in pressure drop from the reduced restriction......now to just figure out where that is

The pumper's limited valve volume probably makes this point much lower than it would be in an unregulated PCP.

[rsterne]

You can't just generalize all unregulated PCPs, it depends on how much of the reservoir air is available to the shot cycle.... In the gun I'm using, with a Disco valve but a PRod gauge port, virtually all of the reservoir air is available to keep the pressure up in the valve during the shot.... The smallest restriction to flow on the inlet side is the 1/4" hole in the front of the valve, which is still about 3 times the area of the transfer port.... In a stock Disco, however, the hole in the gauge block is only 1/8", smaller than the barrel port, although between that and the valve seat there is "almost enough" air for stock power levels.... Start modding the Disco for twice the power output, and that is no longer the case, you need to allow more air INTO the valve by using a PRod gauge port.... I've never had one, but I understand that possibly Air Arms rifles have a "firing pot" of fixed volume that feeds the valve, and could possibly cause a similar starvation at very high power levels.... In regulated PCPs it's VERY common to not have enough air in the "plenum" to support high FPE levels, and it's something I always address in my regulated designs....

Incidently, it is possible that the O-ring buffer may have a use in regulated PCPs as well.... It just may be that using a buffer to limit the dwell and "shape" the pulse could provide increased efficiency.... if not directly, through control of hammer bounce.... It would also allow easy detuning, although with a regulated gun just backing off the preload is usually enough.... That does cause one weird thing though.... if you go too far, the velocity INCREASES below the setpoint as the valve tries to self-regulate over the lower pressures.... Using a rigid (or nearly rigid) buffer might prevent that....

Bob

[bstaley]

Yep, I understand that everything is dependent on everything else.  Every gun will be different.

I actually have tried tuning with this in my regulated .177 m-rod, and as it turned out there was a LOT of hammer bounce.  Could easily hear the burrrrping from the bounce......efficiency wasn't very good at all!  I think motorhead may have found the same thing when he first started tuning with his regulator.

Will have to try a rigid buffer....that might just help!

[rsterne]

Here is some food for thought....

In some ways, the buffer is working like a very stiff valve spring, in that it is limiting the lift and making the difference between the lift at high and low pressure smaller.... The major difference is that it is easily adjustable without tearing the valve out of the gun.... The more I think about it, the more I like that analogy.... The difference between a 70D and a 90D buffer is like the difference in using a stiffer spring (90D) but with less preload.... The major difference between the rubber buffer and a coil spring is that the O-rings are (I think) more progressive than most steel springs are.... and of course they can "absorb" some energy by converting it to heat instead of sending (nearly) all of it back to the hammer as rebound force....

Anyone?....

Bob

[PakProtector]

The spring rate on a stacked o-ring buffer changes remarkably quickly. The more o-rings, the slower the change to go along with a lower rate. I suspect that this spring is comparable to the closing force from the valve, perhaps a bit less...but a singnificant fraction at the bottom of the string, and therefore at its maximum compression. I am going to see just what this looks like; I have available #211 o-rings and a spring rate checker. Compression range oto of up to .1"...then get a curve fit function...better thanthen paper and pencil and geometry...LOL too late on Friday afternoon, will take it up on Monday...heh-heh-heh
cheers,
Douglas

[bstaley]

Here is some food for thought....

In some ways, the buffer is working like a very stiff valve spring, in that it is limiting the lift and making the difference between the lift at high and low pressure smaller.... The major difference is that it is easily adjustable without tearing the valve out of the gun.... The more I think about it, the more I like that analogy.... The difference between a 70D and a 90D buffer is like the difference in using a stiffer spring (90D) but with less preload.... The major difference between the rubber buffer and a coil spring is that the O-rings are (I think) more progressive than most steel springs are.... and of course they can "absorb" some energy by converting it to heat instead of sending (nearly) all of it back to the hammer as rebound force....

Anyone?....

Bob

Yup, with the advantage that it adds little or nothing to the force required to open th valve, but then can counter the excessive momentum that remains.

Just looked back at my original writeup on the mod, and that was the exact analogy I used.

[rsterne]

I'm currently thinking that the buffer works best when engaged even at the beginning of the shot string, or nearly so.... Of course it's hard to tell, as the initial "spring rate" is so low for the first few thou of movement, especially with a tall stack of 70Ds....

Bob