Dwell vs. Caliber

[rsterne]

I just had one of those "AHA!" moments while writing another post.... It's probably one of those things that lots of guys know, or at least allow for (I did), when tuning a PCP, but until moments ago, I hadn't really organized my actions into words, so here goes....

When you change the caliber, you need to make a corresponding change in the dwell (via hammer strike).... This is necessary to compensate for the larger (or smaller) bore volume.... It's a simple concept, but one I had somehow glossed over.... Here are my thoughts on the matter.... Let's take the three common calibers, and a barrel length of 24".... The bore volumes are:

.177 - 9.7 cc
.22 - 14.5 cc
.25 - 19.3 cc

For the moment, let's assume we start with a .22 cal PCP that is tuned for a beautiful, classic bell-curve starting at 2500 psi and down to 1500, all shots with a 4% ES, and peaking at 2000 psi.... What does that mean?.... Well, it means that the combination of valve dwell and pressure is balanced to produce velocities with that 4% range over that range of pressures.... At 2500 psi, the pressure is higher, but the valve is not opening for very long, so the volume of (higher pressure) air is smaller.... As the pressure drops, the valve dwell increases, and the valve releases more air but at lower pressure.... Eventually, the volume peaks, so does the velocity, and then as the pressure drops, the velocity declines with it....

Now let's assume we take off the .22 barrel, and substitute a .177 barrel.... It only has 2/3rds the volume, and yet our valve and hammer are balanced for the larger bore volume of the .22 cal.... Now that small amount of 2500 psi air is more than needed to make the velocity stay 4% below the peak it will be at 2000 psi.... and MUCH more than is required to balance the velocity at 1500 psi.... In fact, it may be enough volume so that the first shot (at 2500 psi) is the fastest in the string.... In order to rebalance the shot string over the same range of pressures, we need to REDUCE the hammer strike....

The reverse occurs if we install a .25 cal barrel.... Now the bore volume is 1/3rd larger than the .22 cal barrel, and the (relatively) small amount of 2500 psi air isn't enough to push the pellet to within 4% of what it will be at 2000 psi.... In fact, with the larger caliber, we would expect the peak of the velocity curve to move to a lower pressure, possibly down into (for example) the 1700-1800 psi range.... To rebalance the shot string over the original range of 2500 down to 1500 psi, we will need to INCREASE the hammer strike....

The same thing will happen in a regulated gun.... Most of you realize that I am a proponent of balancing the hammer strike against the regulator setpoint so that the gun is operating on the "knee" of the velocity curve.... which is basically the same thing as always operating at the top of the sweet spot (velocity peak) in an unregulated gun.... If you don't change the hammer strike and go to a smaller caliber, you will be moving up onto the "plateau" of the velocity curve, and therefore wasting air.... so reducing the hammer strike won't lose velocity, but will increase shot count.... When going to a larger caliber, you will be operating on the downslope of the curve, and increasing the hammer strike will move you closer to the "knee" and hence increase the velocity.... both of the above without changing the regulator setpoint....

I hope I've made the above argument clearly and understandably.... Sometimes things are obvious, and adjusted for, without knowing (or articulating) WHY they occur.... For me, until this morning.... that was the case....

Bob

[sperho]

I agree with your logic and would add that the pellet mass also contributes to this trend.  My thinking is that pellets with increasing mass would exascerbate the peak shift in the same direction as barrel volume.  I'm pretty sure a decent computer programmer could put together a simulation program with mass, CoF, contact area, barrel volume, and pressure pulse profile that would be useful in PCP gun design.  Maybe someone already has.  I know that if I worked for a PCP company (or had the tools to make my own guns), I certainly would be working on it.  Through simulation, one can not only gain a deeper understanding of what is going on, but could also use the learnings to influence/improve the mechanical design of the pneumatics.  (Just like any field that utilizes simulation, including mine:  chemistry; we simulate reactions, reactors, etc. to gain deeper insight into a process so that it can be improved).

[rsterne]

Lloyd already has developed a program on internal PCP ballistics, and has generously shared it with me.... and yes, you are correct, pellet mass has a similar (although not as drastic) effect.... Increasing the mass shifts the peak of the velocity curve to a lower pressure (usually just 100-200 psi).... However, that has already been discussed in detail.... The above comparison would be valid even for pellets of identical Sectional Density (mass to bore area ratio) however, ie comparing apples to apples.... Sometimes while looking at the details, we lose sight of the larger picture....

Bob

[sperho]

From the looks of the patent authored by one of our very own, he may have used such a program or at least thought about this phenomenon quite a bit.  I was thinking valve control by solenoid as well and, well, so did he .

http://www.google.com/patents/US8322329

[sperho]

I was "post ninja'd" by you Bob and you confirmed my suspicions that he developed a program to look at this...

[rsterne]

Here is a sample output from Lloyd's spreadsheet.... pellet position in the barrel on the horizontal azis....



Generic to some degree, but indicative of why unregulated PCPs are self-balancing....

Bob

[GecoGuy]

rsterne,

This makes perfect sense and yes it is not something you think of off the top of your head I am glad you came up with it/ shared your knowledge! I am in the very same predicament with my mrod and can't figure it out. I recently acquired a refurbished .177 cal mrod and swapped the barrel out for a .22. I took velocity readings with the .177 b4 swapping barrels and velocity readings with the .22. Both straight out of the box from the factory settings.

When I read this post I said AHAH! I know what to do to fix my curve, but then pulled the data up and forgot that the .177 and .22 curves are similar. From my results it looks like this gun was a victim of your first scenario where it was set for a smaller caliber than the .177 and bigger barrels were put on. But we all know there is nothing smaller then a .177! So this is were I get confused....?

Here are my curves I am talking about. Both exhibit the first scenario you described were the peak velocity is the 1st shot. How should I go about tuning this for the .22?

Mrod - .177


Mrod - .22

[sperho]

Thanks Bob (and Lloyd).  How much do the curves change when you leave the valve open for the same duration, all at the longest time?  Another graph would be great, if possible.

[sperho]

How should I go about tuning this for the .22?

If you back off on the dwell time at the higher pressures, you'll get a velocity lowering and less gas "wasted" to achieve a bell-shaped curve.  I did this by decreasing the hammer preload in my gun.  This allows for shorter dwell times due to higher back pressure on the valve at higher fill pressures and longer dwell times as the gun loses pressure.  I had to ask, too, as I hadn't thought about PCPs much until recently...    Now, the real experts can chime in.

[Gertrude]

subscribed

[rsterne]

GecoGuy.... sperho is correct, you have too much hammer strike.... back off the preload until the first shot in your string is about 4% below the peak velocity....

sperho.... Those charts show what is actually happening inside a Disco as it self regulates over it's normal pressure range.... The dwell changes inversely with pressure as shown.... I can do other charts, but they would be rather meaningless.... I will, however, do up a chart showing three different pressures with a constant dwell.... How you would accomplish that with a "knock open" valve would be problematic, of course.... With an electronic valve, not so much....

Bob

[rsterne]

Here is the graph done at constant dwell (the same as the 1200 psi graph above)....



You will note that as the pressure increases, the pellet accelerates faster, and is consequently further down the barrel when the valve closes.... The velocity is higher, but the efficiency is also much less.... The stock Disco returns about 1.2 FPE/CI over the 2000-1200 psi range, and it does that by delivering about 1.4 FPE/CI at 2000 psi (because the valve closes so early)....  Conversely, in this case, the efficiency at 2000 psi is only about 0.6 FPE/CI, but the power is much higher, at ~35 FPE instead of ~21 FPE....

I hope that answers your question....

Bob

[sperho]

Thanks Bob.  A few pictorial questions.  edit:  Nevermind the shoulder queston. this is barrel distance based, not time based.  The shoulders all appear at the same time, but difference barrel distances because the speed is faster at higher pressures and the abrupt shoulder appears because the valve closed so the barrel doesn't see the pressure of the feed cylinder anymore.  Nevermind on the fps and fpe clarifications, too.  I calculated it and the labels I put on are correct...

[sperho]

Is the valve opening modeled as a square wave pulse or something more parabolic in shape?  Also, where is the pressure being "measured" exactly?  At the breech?

[rsterne]

You are correct on all points.... The shoulder on the pressure curve occurs when the valve closes.... after that the air in the barrel is simply expanding, and hence the pressure dropping.... Since the dwell time is a constant, the pellet distance increses with pressure.... The efficiency is a function of the pellet position when the valve closes, to a very large extent.... as is the residual pressure at the muzzle, and hence the noise generated (and energy lost).... The lower curves are fps (right scale, also for pressure), the upper curves are FPE (left scale)....

AFAIK, the valve dwell is considered "on or off" ie a square wave, for the purposes of the calculations.... You would have to ask Lloyd to be sure....  Since the actual flow doesn't increase past a lift of 1/4 the diameter with any poppet valve, this is likely pretty accurate, particularly at lower pressures (eg. higher lift, longer dwell)....

Bob