Is this becomming the "Geek Gate" ?

[Big Bore Bart]

   Yes "delta P" is boiler engineer shorthand for differential pressure.   The point I am trying to make is in response to this quote from Sean about his valves,

"So at the top of the curve there is virtually no force being applied by pressure differential. (Again I have physical working valves that you could expereince this on. To open them requires a 6 foot cheater bar but to adjust the flow rate in the middle of the flow curve you can do it with a finger) Anyhow, at the top of the curve the predominant factors then become the reciprocating masses and in our case the forces acting on them ( return springs, or my finger)"

   And as far as high speed flow causing his valve to open, it is the first few milliseconds when there is a large delta P across the valve.

[rsterne]

And as far as high speed flow causing his valve to open, it is the first few milliseconds when there is a large delta P across the valve.

With typical total dwell in our PCPs running around 2 milliseconds, the length of time there is a large pressure differential is on the order of 0.1 mS.... During that time, the valve is only open less than about 0.015".... but I understand what you are saying that the flow would have a "wedging action" as it starts to flow through the tapered gap....

Bob

[PakProtector]

You are absolutely correct, if the chamber is too short, a bullet can be very hard to load, and it IS possible to break a bolt handle.... I know, I've done it.... However, the problem is mostly that the bullet is oversize.... If it's enough oversize, hard enough, and wedged up against the leade tightly, even that 150 lbs. of force from 3000 psi isn't enough to fire it through the bore.... I've had that problem too, with .22 cal cast bullets, as you know....

You have to ask, though, if that can be considered a "normal" condition or not?.... IMO, it's a problem, and not one you can solve without matching the bullet OD to the groove ID properly.... Simply running the bullets that wouldn't even fire at 0.225-0.226" through a 0.224" sizing die, and they chambered fine and launched at 900 fps....

Bob

hmmm...make the chamber of the same geometry, just variable in length and the bullet will either seat with minimal bolt pressure or it won't. With a long chamber there will still be a significant push required to engrave the rifling on the boolit...just like trying to seat a long boolit in a short chamber...either the bolt( likely with a hammer of some sort ), or the air pressure will be making this push. Then we get into the soft seat, and 'running start' condition v. driving the boolit in with a hammer on the bolt. All this assumes preoerly groove-sized boolits...break that comandment and there will be issues.

Now I do intend to break this; boolit lapping will require some pressure at the groove diameter I think...in any case I plan to cut slowly with very fine abrasive( JB Bore Brite comes to mind ). Apply the rouge-coloured goo to the boolit, and fire it...
cheers,
Douglas

[lloyd-ss]

Very lively discussion!

Bob- I am starting to see what you are saying about varying the closing force apparently not having a lot of impact on the efficiency.  I worked up a little mathematical model (very basic) and am getting results like yours.  But obviously, various elusive  "things" improve the efficiency.

Sean- Your valves are getting very good efficiency, so you must be doing something right.  You have already mentioned a few characteristics that make your valves different.

 Big Bore Bart- your comment about the aerospike reactive forces on the bottom of the valve face in Sean's gun seems to make good sense. I think there is a lot to be gained by working with the flow characteristics of the valves. The standard practice in PCPs of 3000 psi air making a U-turn thru a bunch of sharp corners offers plenty of opportunity for improvement.

CarsonRatSniper- Your consolidated list of 5 things to include in any valve design seem to be a solid starting point, almost a list of "must do".

Douglas- do you have some fire lapping in the works, or have you already done it. A powder burner buddy of mine does it a lot and swears by it.

Lloyd-ss

[SeanMP]

Bob

Yes I'm cetainly seeing that the M/2*2V*F model is looking like it's heading in the right direction.Just for kicks I'd like to see the curve of M/2*2V*2F, and M/3*2V*2F. In my mind I'm wondering if the closing forces are being accurately represented. I'm feeling that it's almost essential to totally understand how that force is at a peak with the valve closed, possibly diminishing I think to a plateau (maybe even to negligable)as the valve accelerates forward, then building with the incoming pressure wave, then building exponentially to close. I believe that the timing of those events has as much of an impact as the event itself. For instance. Let's imagine that the mechanical acceleration of the valve is ahead of the pressure wave in the timeline. Then we are dealing with the valve being at full open before the closing forces even start to build. Or perhaps it's the exact inverse.

Lloyd and Bart
The idea of an aerospike is pretty darn interesting. But looked at in cross section the "nozzle" is divergent in my valve. It "should" comply with Bernoulli and generate a slight negative pressure... Maybe? In fact I machined in a 5deg divergent with purpose.
Lloyd . Yes I agree that the flow characteristics of a conventional valve are counterproductive. It's pretty clear if you compare a simple line sketch of the two valves and pencil in flow vectors at the minimum open point.
Further on that same thought yes, I think I made too many major changes at once and now I don't know if it's a chicken or an egg

Fire lappers....(shaking my head) It will take a few hundred shots to get to the point you could achieve in 15 minutes of lapping.

[rsterne]

Sean, the "closing forces" are there from the instant the valve cracks.... All they are is a force that is slowing the poppet/hammer "assembly" (they travel as a group because the force is on the poppet).... The reason that quadruping the stem area hardly affects the opening is that the velocity (relative to the force) is very high.... Once the poppet/hammer stop and reverse, it then starts to show up more, so the lift/dwell curve gets shortened more at the back end.... When I graphed the Lift/Dwell with and without the pressure differential, I adjusted the hammer weight and velocity to superimpose the curves (ie make them have the same flow volume).... If you want the raw data for the M/2*2V*2F and M/3*2V*2F I can do them in the raw state, that will show you exactly what is happening with no change of hammer velocity....

I think the best way to think of the closing forces is starting at the closed force (pressure x seat area), reducing to the "open" force (pressure x stem area) over a very short time (10-15% of the dwell), then plateauing where the stem force predominates, then increasing again over the last 10-15% of dwell,  as the valve seats it is again the (reduced) closed force ((reservoir pressure - barrel pressure) x seat area), and then finally returning to the original closed force when the pellet exits.... Again, that is the six stages from above, and the appropriate spring force must be added at each stage, plus an additional (5% of so) amount for the "drag", mostly applicable during the middle "open" stage when the flow is the highest....

I've been thinking about the roughly 1/3rd increase in lift when I changed from the (admittedly pressure damaged) Disco poppet to the PEEK poppet in my Disco Double.... That is proof that the RESIDUAL energy was about a third greater.... This may easily mean that with the original poppet,  1/3 - 1/2 of the energy of the hammer went into cracking the Disco poppet because it was so "springy" and the seat was hammered so deep.... In that case (pardon the lack of algebra skills):

Initial hammer energy = X
Energy used to crack Disco poppet = X/2
Residual energy (Disco) = X/2
Lift (Disco) = Y (created by energy X/2)
Lift (PEEK) = 4/3 x Y (observed)
Residual energy (PEEK) = 4/3 x X/2 = 4X/6 = 2X/3
Energy used to crack PEEK poppet = X - 2X/3 = X/3
Therefore PEEK used (X/3) / (X/2) = 2/3rd the energy to crack the valve compare to the Disco poppet.... (probably too high, given the condition of the Disco poppet)....

If the Disco poppet was only using 1/3 of the initial hammer energy to crack....

Initial energy = X
To crack disco = X/3
Residual (Disco) = 2X/3
Residual (PEEK) = 4/3 x 2X/3 = 8X/9
Energy to crack PEEK = X - 8X/9 = X/9
Therefore PEEK used (X/9) / (X/3) = 1/3rd the energy to crack the valve compared to the Disco poppet.... (likely a good fit, IMO)....

If the Disco poppet was only using 1/4 the initial energy to crack....

Initial energy = X
To crack Disco = X/4
Residual Disco = 3X/4
Residual PEEK = 4/3 x 3X/4 = X .... This is impossible, as it would mean no energy was used to crack the PEEK valve....

As you can see, MORE THAN 25% of the initial hammer energy went into cracking the Disco valve in it's damaged state.... If we assume the PEEK was three times better, to get a third more lift than with the Disco valve, then 1/3 of the initial hammer energy was going into cracking it.... while only 1/9 of the energy went into cracking the PEEK valve.... We had a previous calculation for a Disco valve operating at 2000 psi that indicated about 21% of the hammer energy went into cracking the valve (roughly 1/5).... It seems to me that for poppets operating within their limits (with one resilient seat) the "cracking" energy is likely between 10% - 25% of the hammer energy at the beginning (high pressure) end of the string.... The REST of that energy is then available to actuate the valve.... I know this is a rather circuitous way of coming to that conclusion, but it's the best my limited math skills will allow, based on our limited data....

Bob

[rsterne]

Sean, here is the graph you asked for....



I don't think those with increased closing force 2F are useful solutions, simply not enough airflow at such light hammer masses....

Bob

[CarsonRatSniper]

What about ditching the traditional poppet altogether and making a flow through valve stem?
I have some ideas - could that be a viable solution?
The stem could have a pin in the rear that rides in slots in the valve body to keep it from rotating out of alignment.

[rsterne]

Would your design be self-regulating, or only work in conjunction with a regulator?....

Bob

[Motorhead]

As you can see, MORE THAN 25% of the initial hammer energy went into cracking the Disco valve in it's damaged state.... If we assume the PEEK was three times better, to get a third more lift than with the Disco valve, then 1/3 of the initial hammer energy was going into cracking it.... while only 1/9 of the energy went into cracking the PEEK valve.... We had a previous calculation for a Disco valve operating at 2000 psi that indicated about 21% of the hammer energy went into cracking the valve (roughly 1/5).... It seems to me that for poppets operating within their limits (with one resilient seat) the "cracking" energy is likely between 10% - 25% of the hammer energy at the beginning (high pressure) end of the string.... The REST of that energy is then available to actuate the valve.... I know this is a rather circuitous way of coming to that conclusion, but it's the best my limited math skills will allow, based on our limited data....

Bob

Would not these losses of potential energy be even MORE detrimental as hammer weigh becomes less ... or visa versa becomes less a factor as hammer weight increases ?

A light hammer like a light pellet gives up it energy very quickly once meeting resistance. !!!
So in my world of lower power regulated guns that Love light hammers, these opening forces being minimal seem very worth having.

[CarsonRatSniper]

Its a self regulating design. I'm at the gym - I'll explain in detail once I'm home in a couple hours.

[rsterne]

Absolutely.... the more energy lost in cracking the valve, the more energy you need to compensate to still have the required lift and dwell to lauch the pellet at the FPE you want.... It's easy when you are working mostly with low to medium powered guns to be lulled into the thought that the lighter the hammer the better.... Once you start trying to launch bullets with high Sectional Densities and in larger calibers, you find that the price you pay is having to go to extremely long springs and hammer travel.... or some kind of a lever system to cock the very stiff spring.... It may not be worth the price of trying to get a very light hammer....

That's not to say they aren't a good idea.... but IMO the price you have to pay may not be worth it.... Certainly, again IMO, intentionally increasing the closing force seems counterproductive....

One of the things that concerns me is the assumption that is being made that the flow through the valve is always "clipped", or can be made so by increasing the hammer energy or eliminating poppet "stick" (for lack of a better term).... Having made quite a few measurements, I am of the opinion that at the beginning of a "normal" shot string, the lift is roughly the same as the curtain limiting height of the valve (ie about 1/4 the throat diameter).... so no clipping (or very little) is taking place.... Hitting the valve harder, to push it into the realm of clipping will just waste air, or increase the velocity to the point the first shot is the fastest.... We need a LOT more direct lift measurements on some of the newer valves before we can just assume we are operating in the clipped mode.... My new PEEK valve in the Disco Double certainly isn't at the high pressure end of the string....

Bob

[CarsonRatSniper]

Bear with me as I'm not on a desktop computer.

Here's my thoughts:

Instead of the traditional stem with poppet, the stem will be 'x' in diameter (5/16"?).
It will go from the rear of the valve where the striker hits it through to the back of the valve where the seat would be but there's no seat.

Instead the valve side of the stem is like a flat top piston with an o ring.
The stem itself is undercut behind the head so when it recesses into the valve body it has a seat to set against.
Behind the stem head/seat there are 2 or 3 angled ports for the air to flow into the stem.
Inside the stem is bored out to whatever your required throat diameter would be.
The exhaust port portion of the stem is an oblong hole proportionate to your total calculated lift.

The surface area of the stem the striker hits is much larger than the traditional 5/32" and pushing the entire width of the stem MAY result in easier cracking open (?).

Once cracked open the exhaust port lines up with the transfer port at the same time the head ports are exposed to pressure and the transfer of air.
Once the stem is filled with air the closing pressure would be against the face of the stem as well as the back of the stems 'throat', but the air would be shut off at 2 points upon closing - the ports at the head AND the exhaust port, effectively shutting off flow rather quickly.

You could also have the oblong transfer port so long that it is always exposed for air flow and the unused portion would seal into the valve body as it is moved forward.

I haven't figured where the valve return spring fits into this yet, but I figured I would throw this out there and see what you guys come up with.

[rsterne]

Sorry, but I can't visualize it without a diagram....

Bob