Floating Piston Valves?

[rsterne]

I've been giving some thought to the idea of a valve for a PCP based on the idea of a floating piston.... What I mean is a valve where the firing chamber is separated from a reservoir by a piston which has constant pressure on one side, while the other side (the firing chamber) is filled from the onboard air supply through a regulator.... The firing chamber then dumps all of its air during the shot cycle, with the piston travelling to maintain a nearly constant pressure in the firing chamber during the shot.... The concept is similar to that used in the SSP detailed by rbg1 in this thread.... http://www.gatewaytoairguns.org/GTA/index.php?topic=72744.0

Dump valve are notoriously inefficient at high power.... They work well when their volume is tiny compared to the barrel volume, but as you make them larger to increase the power, they blow increased amounts of air out the muzzle.... You can get a feel for that relationship by reading through this thread.... http://www.gatewaytoairguns.org/GTA/index.php?topic=71683.0 .... The problem is that the pressure in the valve drops in direct proportion to where the pellet is along the barrel.... If the valve is the same volume as the barrel, when the pellet exits it's pressure is half what it started at, and 50% of the air gets blown out the muzzle after Elvis has left the building, doing no useful work (just making noise).... Our typical "knock-open" valve used in PCPs avoid much of that by closing the valve while the pellet is still inside the barrel, and as a rough rule of thumb, if the valve closes when the pellet is a third of the way down the barrel, the PCP will deliver about 1 FPE/CI efficiency....

The concept of the floating piston results in a shot cycle that emulates a PCP with early valve closing, while using a dump valve.... The resulting shot cycle is almost identical to a PCP using a reservoir of the same volume as the firing chamber plus the "contained" reservoir combined, at the pressure at the start of the firing cycle.... For example, if we have a reservoir at 1800 psi that is 50 cc, with a floating piston, and we add air pushing against the piston until the pressure both sides of the piston is 2000 psi, the piston will have moved 1/10th of the length of the reservoir, leaving 45 cc on the reservoir side and 5 cc on the firing side.... If we then dump the air in the firing chamber into the barrel, the piston will move as the firing chamber empties, so that the pressure in the firing chamber, which starts at 2000 psi, is still 1800 psi when the entire 5 cc has moved into the barrel.... At that point, the piston hits the end of the reservoir, and the air in the firing chamber starts to expand in exactly the same manner as when a conventional PCP valve closes.... The results are (at least in theory) exactly the same as if we had a valve of 50 cc at 2000 psi and released 5 cc of air and then closed the valve.... If the barrel volume is 15 cc, the pellet would be 1/3rd of the way down the barrel at that instant....

Now for a short list of the pros and cons of this concept....

PRO:
A dump valve is used, which by designing something like a spool valve can be opened by minimal hammer strike (or even directly by the trigger)....
By using a dump valve, and a regulator to fill the firing chamber, exactly the same quantity of air (all of it) at the same pressure is released for each shot....
By changing the output pressure of the regulator the effective timing of the valve can be altered, more pressure, longer dwell....
By changing the precharge pressure of the floating piston reservoir relative to the regulator pressure the volume released and the timing can be altered....

CON:
The floating piston will have a LOT of energy when it hits the end of the reservoir, which could easily damage things....
The momentum of the piston, and its collision, could cause a double recoil similar (but much less in magnitude) to a springer....
Adjusting the effective dwell when you change pellet weight would have to be done by altering the pressures as above (more difficult than a preload change)....

I'm sure there are more advantages and disadvantages than this short list.... but the most important one I can see is the violent stop the piston comes to at the end of the firing stroke.... Anybody have any comments about the practicality of dealing with that?.... If not, this whole idea could be a moot point....

Bob

[plinker99]

Hi Bob,
    After reading this post a number of times, I still don't get it.  May be a picture would helpd
   Any way how about this idea.  For a PCP with a dump valve, put another valve between the air supply an the dump valve that opens only to charge the valve then closes during the shot.  If using a regulator, each charge to the valve should be the same.   That should give low shot divations.

[UK KIWI]

Hi Bob,

This is an interesting concept but as your other reply indicated, a difficult one to get your head around (if it is not your own idea) , I do not get regularly involved with PCP's, so you probably have much greater experience on this one, but from an engineers standpoint, My understanding of the concept you are trying to develop, is that of a variable cylinder to give a change in volume of the dumped air charge, in order to overcome the issue of deviation due to falling air-tank mean pressure - am i right??

The issue which many will have found is that whilst the concept of larger volume of lower pressure (roughly) equals smaller volume of higher pressure is that of variable "Dwell" on release of the air-charge, pushing the pellet, (of the same mass), thus the higher pressure smaller charge will tend to  reach peak pellet velocity in a slightly different position, due to the more rapid acceleration of the pellet, hence you will still be left wondering why?, it should be seen , i think, that in order for this concept, (that is a good one, by the way), to work, that there needs to be a variable sized port delivering the charge to the pellet, linked to the pressure/volume change in order that it is delivered to the pellet at a constant velocity, hence consistent pellet acceleration, muzzle velocity etc.

whilst considering this a few years ago, it was the main reason, (along with the required equipment and cost of PCP), that drove my decision to stick with spring guns, the concept I was considering at the time, (which would really throw a spanner in the works of your categorization of airgun types), was to develop a repeating Airgun, with a Pre-charged chamber, which would auto cock a spring and piston firing chamber! - this would achieve the same goal, just maybe the re-load time would increase marginally as the PCP chamber pressure reduced, but each shot would still deliver a standard FPE output -  now how would you rate that? -

when we stop development it all becomes pointless!

Mark

[rsterne]

This concept is for a regulated PCP, the idea being to have exactly the same volume and pressure of charge released for every shot by the dump valve.... Therefore, no variable port diameter or timing would be needed, as the pressure change in the tank has already been looked after by the regulator, which comes before any of what I am suggesting....

Sorry, but I don't follow your idea for the repeating springer, a bit off topic in this thread anyway....

Bob

[David.Soliman]

This may sound ridiculously stupid, but here goes....
Why not have a spring on the valve to help open it instead of air? The spring mechanism could have a sear to keep it locked during filling/any other strange situation.

[Sfttailrdr46]

  Here goes and I hope I do not embarrass myself but  why not insert a dense neoprene bushing at the end of the chamber to act as a shock absorber?

[rsterne]

No idea of the benefit and design of the spring to assist the valve opening.... we are talking a dump valve here....

The neoprene (or tougher) bumper idea would have to be employed in some way, I can't imagine it surviving steel to steel for long.... The problem is that with a 45-50 cc chamber at 1800-2000 psi, it might be, say, 1" diameter with a 3.9" length and a 0.39" stroke.... That makes the force on the piston averaging about 1500 lbs.... which would produce a lot of energy and momentum in even a 1-2 oz. piston accelerating over that 0.39" distance in about a millisecond, and a large compressive force when static....

Bob

[Rescue912]

Sounds kinda cool. If I get it right, the onboard air reservoir moves your firing chamber to a point where it has the same volume every shot and pressure is regulated already. Sounds a little like an intensifier and I rebuild those all the time. Very neat idea...

[rsterne]

Not really.... the floating piston changes the way the pressure in a dump valve declines over the shot.... Instead of the pressure in the firing chamber dropping in a linear fashion as soon as the valve opens, the piston moves to keep the pressure above the preload chamber pressure until the firing chamber is empty.... It makes a dump valve act like a conventional knock-open valve.... It is the regulator that determines the pressure and volume of each shot, as per normal....

Bob

[PakProtector]

Hey Bob...would like to confirm some numbers before I decide on how bad the 'CON' looks. Call the time under which this piston would be moving the lock time. Lock time defined by trigger pull to pellet departing the barrel. It has repeatedly been noticed that for a PCP with adequate chamber volume, it is rather pointless to keep the valve open much past barrel midpoint re. pellet travel. So roughly 3/4 lock time is all the pressure balancer has to operate in. this piston acceleration is dependent on pressure differential...at the beginning of the shot, the piston is stationary...it will only start accelerating once the shot chamber pressure drops.

So how far will it have to drop in order to get anything useful done? Lemme see V/dv=(integral)a*dx....or V^2/barrel length=a IFF we assume a is constant...with a we get barrel transit time.

IDK, I am thinking it will be lots of mass per benefit.
cheers,
Douglas

[rsterne]

I figure that the piston has to complete it's travel within 1.5 mSec at the longest.... Pretty rapid acceleration and then coming to a crashing halt.... a rough life for the floating piston, for sure.... It's the only way I can see to get any decent efficiency with a dump valve at higher power, however, so worth the analysis....

Bob

[Cal]

Snubber?

A bleed passage added as a detail that would pass the valve charge pressure to the "impact" side.

timing and volume TBD.