Springer pressure at end of stroke

[MartyMcFly]

Does anyone have reliable information about the amount of pressure that is generated by a typical springer at the end of the compression stroke? I've seen estimates that are as high as 18,000 psi to as low as 1,200 psi (Tom Gaylord).

Better yet, does anyone know how to calculate it based on swept volume and spring force? I've been thinking about a two stage compression chamber design and I want figure out if it is is worth making into a prototype. The goal is to maximize the pressure at the end of the stroke, while keeping the spring energy the same.

Thanks,
Marty

 

[outdoorman]

  According to the Cardew book on spring guns, with a tube diameter of 1 inch and piston stroke of 2.5 inches, the pressure would rise to 1366psi

[subscriber]

Marty,

This is a tricky calculation because it is not a simple closed system:  If the pellet were "glued" in place you would generate the highest pressure.  That said, I doubt a "magnum" springer would ever get to 18 kPSI, even then. 

Because the pellet breaks free at a given force, the peak pressure can't be less than that equivalent pressure.  For instance, if a .177 pellet breaks free at 12 lb, the minimum pressure is 500 PSI.  Pellet inertia will also contribute to peak pressure, but only once acceleration starts.  As in, while and after the pellet has broken loose.

If you could capture pellets fired from springers gently, and compare the extent of skirt flaring with those shot from PCPs, that may help.  This borrowed image shows the skirt bulged from air pressure, compared to the un-fired pellet:





Indirect springer peak pressure "measurement":
Charge an unregulated PCP to 500 PSI. Adjust the hammer such that it dumps more air than normal (to be sure the pellet sees a full 500 PSI).  Then compare the pellet skirt flare at 500 PSI to that of a springer, firing the same pellet.  Then the PCP can be filled to 1000, 2000 and 3000 PSI, and the same examination performed again.  Find the PCP pellet with skirt flaring that most resembles the springer pellet; and Bob's your uncle...

I suspect that typical springers have peak pressures in the 1000 to 4000+ PSI range (depending on power and pellet weight), when not dieseling in an overt manner.  The above image source article contains pertinent information that would seem to bolster my guess:  https://www.ctcustomairguns.com/hectors-airgun-blog/at-the-moment-of-firing-and-fit-of-pellet-to-the-rifling


Do you want to use the spring to charge a chamber that is then dumped. Would this design have a piston that "sticks" or is captured at the end of the stroke to stop it from rebounding?  Or, you could have a reed valve that traps the air so it can't follow a rebounding piston.

The pressure does not just depend on the spring force, swept and residual volumes.  It depends mostly on the piston speed near the end of the stroke.  This is an acceleration/velocity/compression/deceleration equation, and not at all trivial...

[MartyMcFly]

subscriber, thank you for the well informed reply and the link to Hector's article. I figured that the calculations would be tricky due to the non-constant resistance offered by the pellet, which as you pointed out is acting like a plug before it's over-come by air pressure. I was hoping there are approximations I could make. I take it that you have some experience in this field, can you point me to a technical source that describes the math needed to solve it? It probably requires some differential equations or linear algebra, no?

The Design:

The piston I have in mind would be machined as one piece, with a large base acting as the low pressure piston. In the center of this piston base is a protruding smaller solid cylinder, which acts as the 2nd stage high pressure piston (This is the same piston design that is present in some two stage compressors). At the end of the compression tube the chamber narrows to accept the second stage piston, like a single male-to-female plug. Put simply, its like a finger going through an elongated donut at the end of the compression stroke, the finger being the second stage piston and the inside of the donut the 2nd stage compression chamber ahead of the transfer port.

Clearly, an issue with this design is that air is still present in the larger compression chamber as the head of the 2nd stage piston enters the smaller chamber, trapping air behind it. If the air isn't bled it will create back pressure and prevent the 2nd stage from traveling very far into the next chamber. I don't want to waste this left over air volume by bleeding it. I think I've solved this issue by drilling holes into the outer circle or front face of the "donut". These holes (maybe 4 of them) would be at a 45 degree angle channeling the air into the forward space of the smaller compression chamber, right before the transfer port. These "relief" channels would have a simple backstop valve to prevent back flow as the 2nd stage piston travels forward and builds up more pressure behind the pellet. A variation of this design can have the relief channels drilled at varying angles, that way each one empties into a different section of the smaller chamber and each is sealed by the body of the piston as it moves past them sequentially, eliminating the need for backstop valves.

I am not an industrial engineer, so there might be some fundamental conservation of energy effect that I am missing. When I came up with this idea my thinking was that concentrating the same spring force over a smaller area (the 2nd piston head) would result in a faster build-up of pressure. I thought this would result in the air being converted into plasma quicker and therefore increase the overall amount of pressure available to act on the pellet. Alternatively, if there is no change in absolute pressure then perhaps a quicker build-up allows the pellet to start traveling down the barrel earlier, reducing the time for hand movement or recoil to have an impact on the pellet's trajectory.

The goal of all this fanciful thinking is to increase efficiency over a traditional design, while not introducing more moving parts or increasing weight. Maybe the 2nd stage pressure build-up slows down the piston so much that it negates any improvement? I don't know, but I needed to get it out of my head. I'd upload some diagrams, but I don't have enough posts to do it yet. Hopefully you can visualize it. 

-Marty

PS: I thought that doing a sticking piston would require some kind of capture mechanism, so I'd like to avoid it. But if you have a simple solution I am all ears.

[outdoorman]

  Came across some recent and very interesting articles on spring guns including this one. The analysis was done on a Beeman RS2 using various JSB pellets. In paragraph 6.2, Firing Cycle, it says that peak pressure and temperature are achieved after the pellet has proceeded down the barrel and that the peak pressure was over 4,200psi

https://www.researchgate.net/publication/274638905_Internal_Ballistics_of_Spring_Piston_Airguns

[MartyMcFly]

Thanks, nice find!

  Came across some recent and very interesting articles on spring guns including this one. The analysis was done on a Beeman RS2 using various JSB pellets. In paragraph 6.2, Firing Cycle, it says that peak pressure and temperature are achieved after the pellet has proceeded down the barrel and that the peak pressure was over 4,200psi

[subscriber]

Hi Marty,

The first step in finding a solution is in correctly defining the problem.  From the sounds of it, you have identified the challenging aspect of a concentric piston on piston design, as it relates to your goal. 

Your goal may be described as using coaxial two stage compression; where the low pressure stage uses a large area piston to maximize swept volume.  This; while the second stage uses a small diameter piston to boost air pressure; and to reduce the area that high pressure air acts on, thereby minimizing the force that results in piston bounce.  Does that sound accurate?

I suggest you look at how coaxial two stage air compressors work.  What is the air flow path and what valves are involved.  My gut instinct is that such compressors do not have the residual large piston air volume problem you describe, because the sequence of compression is actually broken into two steps; like with a multi stage hand pump.

Here Jorg Sprave takes apart a two stage coaxial piston compressor and explains how it works: 

As you might know, there is a reed valve that controls air from the large piston compression space and delivers it to feed the small piston space.  As such, the two compression events happen one full crank revolution apart.  This is not what you want.  You want the two compression events to happen as one continuous event, yet without valves. 


As I am typing this, I can think of only one way to achieve this.  More precisely, one idea popped into my head and no other ideas have joined it :

Rather than use too many words, I created this simplified sketch: 



It has the smaller coaxial piston slide in the larger one, that is propelled under its own inertia (probably needs to be longer or made from tungsten to have sufficient weight).

Obviously there are sliding seals required between the two pistons; and between the smaller piston and its cylinder bore.  Also, between the large piston and the main cylinder.

I have not shown the main spring; that would energize both the larger and smaller piston together.  Nor have I shown a soft return spring that hold the smaller piston all the way back inside the larger one, until the sudden stop at the front of the cylinder under firing.

I think with this arrangement the residual volume problem of the larger piston and apparent need for flow paths or valving goes away.  The pistons themselves become the valves that act at just the right stages of operation.

It won't matter if the larger piston bounces back a little, because the smaller one would have already trapped and sealed in the high pressure air volume.

The challenge is going to be the ratio of piston areas and masses.  Specifically to prevent the larger one from crashing into the end of the cylinder.  The smaller piston will need to be "heavy" to work, but will at least have more of a high pressure air cushion in front of it.

You will also need reasonable alignment of the small piston to its bore, with generous lead-ins to prevent crashing against the edge. 

The question will be if the small piston carries the cylinder seal near its nose; or if the small cylinder bore holds the seal near its entrance?  Obviously, the small piston also requires a seal with the large piston bore; placed so as to minimize wasted air volume.


What am I missing....

[subscriber]

  Came across some recent and very interesting articles on spring guns including this one.
https://www.researchgate.net/publication/274638905_Internal_Ballistics_of_Spring_Piston_Airguns

This is an impressive body of work, with a lot of info that is also easily digested.

[MartyMcFly]

I was about to tell Eric that he found the holy grail! A lot of great information and the equations in that paper can be used to build a simulator in software. I hope the gentlemen who wrote it visits these forums, because it will take me a year to review all the concepts he discusses.

Subscriber, you captured my thoughts perfectly. My initial idea was a design very similar to yours, either having the interior piston “fall out” of the main piston via inertia or having a secondary spring fire it out. My first concern was that it would require more moving parts and increase production complexity. The other concern was that it can create two piston slaps, which can’t be good for accuracy.

I’ll watch the video you suggested to get a better idea of what is going on in a compressor, but my ultimate goal would be to have the piston body act as a valve and avoid reed valves if possible.

It might be wishful thinking, but given that only 28% of the springs energy is imparted to the pellet there has to be some way to get more efficiency out of airguns without making them as complex as a Swiss watch.

Thank you both for the intellectual stimulation.
-Marty

[MartyMcFly]

Subscriber, I’ll sketch the second stage chamber (hopefully tomorrow) showing how the relief channels work and how they are sealed as the 2nd piston moves forward.

-Marty

[Back_Roads]

The piston I have in mind would be machined as one piece, with a large base acting as the low pressure piston. In the center of this piston base is a protruding smaller solid cylinder, which acts as the 2nd stage high pressure piston (This is the same piston design that is present in some two stage compressors). At the end of the compression tube the chamber narrows to accept the second stage piston, like a single male-to-female plug. Put simply, its like a finger going through an elongated donut at the end of the compression stroke, the finger being the second stage piston and the inside of the donut the 2nd stage compression chamber ahead of the transfer port.

Clearly, an issue with this design is that air is still present in the larger compression chamber as the head of the 2nd stage piston enters the smaller chamber, trapping air behind it. If the air isn't bled it will create back pressure and prevent the 2nd stage from traveling very far into the next chamber. I don't want to waste this left over air volume by bleeding it. I think I've solved this issue by drilling holes into the outer circle or front face of the "donut". These holes (maybe 4 of them) would be at a 45 degree angle channeling the air into the forward space of the smaller compression chamber, right before the transfer port. These "relief" channels would have a simple backstop valve to prevent back flow as the 2nd stage piston travels forward and builds up more pressure behind the pellet. A variation of this design can have the relief channels drilled at varying angles, that way each one empties into a different section of the smaller chamber and each is sealed by the body of the piston as it moves past them sequentially, eliminating the need for backstop valves.

I am not an industrial engineer, so there might be some fundamental conservation of energy effect that I am missing. When I came up with this idea my thinking was that concentrating the same spring force over a smaller area (the 2nd piston head) would result in a faster build-up of pressure. I thought this would result in the air being converted into plasma quicker and therefore increase the overall amount of pressure available to act on the pellet. Alternatively, if there is no change in absolute pressure then perhaps a quicker build-up allows the pellet to start traveling down the barrel earlier, reducing the time for hand movement or recoil to have an impact on the pellet's trajectory.

The goal of all this fanciful thinking is to increase efficiency over a traditional design, while not introducing more moving parts or increasing weight. Maybe the 2nd stage pressure build-up slows down the piston so much that it negates any improvement? I don't know, but I needed to get it out of my head. I'd upload some diagrams, but I don't have enough posts to do it yet. Hopefully you can visualize it. 

-Marty

PS: I thought that doing a sticking piston would require some kind of capture mechanism, so I'd like to avoid it. But if you have a simple solution I am all ears.

Sounds like the design would need to be cocked 2 - 3 times to multiply the pressures ?

[subscriber]

Subscriber, I’ll sketch the second stage chamber (hopefully tomorrow) showing how the relief channels work and how they are sealed as the 2nd piston moves forward.

That would help me understand the concept.  Right now it seems like a blend between hard to make, and magic.

[Motorhead]

With enough weigh / mass you may get the secondary piston to compress the air required to launch said pellet ... But "Newton" and his proven 3rd law of physics  ( equal & opposite reaction ) would have the CYCLIC motion of the Machine / mechanics so reactive to the actually process as it happens.   While in application it very well could work, tho having the air gun stable in motion enough to get any kind of accuracy from ...  

Just sharing thoughts

[Back_Roads]

 Probably why PCP was re-invented

[subscriber]

While we are conjuring up springer variants; how about this one:

You cock the airgun by moving the piston back by means of barrel leverage, and capturing it with the spring compressed - just like any break-barrel springer.

Before you want to shoot; you trip the "compression sear" to precharge a dump chamber.  If you don't like that; trip it immediately after cocking and allow the temperature to equilibrate to ambient.

When you actually want to launch the pellet, you have a small spring drive a small hammer to open a dump valve; just like a single stroke pneumatic.  As vibration free as any PCP.

What might the advantage be over a SSP?  the cocking stoke is more user friendly.  You can capture the air at a higher pressure, without ridiculously peaky effort.  The higher the pressure, the more energy you can get out of the air and into the pellet.

You could even make such a setup "multi-pump" if you want to, but with usable performance with only one spring dump.

I don't think you want to start conventional diabolo pellets at over 4000 PSI, due to the distortion to their skirts...

[subscriber]

One of the main ways that "silly ideas" add value is in spurring other thoughts that had not occurred to anyone before:  "This won't work; but if you turn it upside down and cut that off, you may really have something"...

[outdoorman]

  I also came across some other very interesting articles on air guns. Tons of other articles on this site. Very interesting to note that peak compression is over 4,000psi and temperature over 2,000F in your typical spring gun.

https://www.researchgate.net/publication/277953420_Spring_Buzz_and_Failure_in_Spring_Piston_Airguns

https://www.researchgate.net/publication/272168018_Dieseling_in_Spring_Piston_Airguns_-_A_Conceptual_Analysis

https://www.researchgate.net/publication/326191398_Internal_Ballistics_of_PCP_Airguns

https://www.researchgate.net/publication/312199345_Energy-dependent_expansion_of_177_caliber_hollow-point_air_gun_projectiles

[anti-squirrel]

Some fascinating information- much appreciated for the reading!  Yet there's a lot of other considerations.  a "typical springer" such as my CZ-634 will have a wildly different max pressure than my run-of-the-mill gas-piston Hatsan 95QE or even the B-3 Chinese underlever.   The CZ has less than 10 FPE, the 95QE hits about 22 FPE, and the B-3 was about 14 FPE.  All three of these are "typical" airguns in every respect.  MY point is such generic statements are worthless without providing more insight into the caliber, chamber volume, preload on the spring (or gas-piston) and stroke as well as the expected power level; all must all be taken into consideration even when considering generic airgun pressures at the end of the stroke- and that's before involving the pellet and pellet's movement past the leade into the barrel proper.

While the mathematics and dynamics of this is fascinating, in the end, if you want a manageable high-powered airgun, springers are NOT viable.  By high-power I do not mean something putting 20 to 30 FPE, I'm talking 30+ FPE, 40+, 50 and well past that.  Springers pushing more power than 20-ish FPE tend to be more hold sensitive as power levels increase in my experiences.  Though I also desire a Hatsan 135 in .30 since lobbing "mortar"-like pellets onto squirrel heads is very appealing to me.

[outdoorman]

Some fascinating information- much appreciated for the reading!  Yet there's a lot of other considerations.  a "typical springer"

  Point taken. The gun used in the study was a Beeman RS2 and pellet was JSB 4.5mm 10.34gr. Muzzle velocity of 812fps for 15fpe

[subscriber]

if you want a manageable high-powered airgun, springers are NOT viable.  By high-power I do not mean something putting 20 to 30 FPE, I'm talking 30+ FPE, 40+, 50 and well past that.

You could make a 40 ft.lb springer, but you are not going to like cocking it, aiming it offhand, shooting it or toting it. 

To reduce cocking effort to the point that makes 40 ft.lb possible, the barrel or cocking lever would have to be super long.  The rest of the hardware super heavy.  This would increase the moment of the gun to the point that no one would want to shoot it offhand.  And no one would carry it hunting.

The fun limit for me is the RWS 350; and those are just over 20 ft.lb guns.  I much prefer the handling qualities of my HW50S and R9.  The R9 is harder to cock than the 350, because the R9 has such a short barrel.  I have a Webley Patriot, but it seems like a monster, for so little gain.

Anyway, I think the OP is trying to increase efficiency of spring air guns.  I doubt his goal is to double their power.  What if he succeeds at making a light compact springer that produces 12 ft.lb, but has only a 20 lb peak cocking force?  That would be something.