Pressure buildup dump valve for springers?

[DIYengineer235]

As an engineer, i tend to look at things a little differently. When I was putting my Beeman Sportsman back together the other day, I got to wondering how much pressure is actually built-up by the spring before that pressure hits the pellet? My conclusion was, not very much. So i began to wonder if anybody had tried some sort of pressure buildup valve that would only allow the air to flow once it reached a certain pressure withing the plunger tube. Once that pressure was achieved, the valve would dump it into the barrel, all at once, instead of a slower, limited buildup.

You see, I realized springers have one major inhibitor for their efficiency. The pellet in the barrel starts to move as soon as enough pressure is built up to overcome friction. That means the spring cannot build up its max pressure before the pellet leaves the barrel. By adding in a buildup valve, you can overcome that inhibitor. Also, because the plunger is still moving, pressed by the spring, that pressure would remain higher as the pellet moves down the barrel than it otherwise would have.

There would be several other advantages as I see it.

1. You would get performance more like that of a PCP, because you would have a chamber of built-up pressure released instantly before the pellet even moved instead of a relatively slow buildup as the pellet moved down the barrel.

2. Reduced recoil. Because that built-up pocket of air would act as a cussion, slowing the plunger so it doesn't smack into the tube as hard. You'd get less recoil, reducing wear and tear on the rifle.

3. You can TUNE the pressure by adjusting the valve to the ideal pressure for a given pellet. You wouldn't have to change springs! Simply adjust a screw to change the release pressure, just like a regulator!

Keep in mind, this would all happen within milliseconds, so I doubt a delayed valve, when tuned propey, would affect POI, especially with a reduced recoil. I'm confident a dump valve could be designed to release that pressure completely and quickly enough to improve performance.

I realize that I may be forgetting some things so please feel free to ask questions or point out obstacles I may have missed.

Feel free to call me crazy too. My inner mad scientist is cackling as I type this.

[Roadworthy]

I have no idea how the pressure relief valve would be implemented within the airgun design - it may not be too difficult though once opened the valve should probably stay open for the duration of the shot.  I do not see how recoil would be reduced, though.  The recoil is caused as an "equal but opposite" reaction to the spring pushing the piston forward.  I don't think holding the air back as it compresses a wee bit would necessarily have a significant (if any) effect on recoil - interesting thoughts though.

[Jim-in-UK]

Thinking out loud...

I have measured .177" pellet start pressures from ~120psi (7.3 grain JSB, soft alloy and thin skirt), to >600psi (6.9gn RWS Hobby, hard alloy, thick skirt), and a friend did the same experiment with .22" pellets and found start pressures broadly 66% those of .177", in line with their CSA.

Given that the pellet gains most of its energy while it and the piston are travelling in the same direction (between pellet start and piston bounce), the low start pressure pellet (about 78% into the available stroke) is exposed to lower pressure air for longer, the high start pressure pellet (about 93% into the available stroke) to high pressure air for a shorter time. If the two pellets are of broadly equivalent mass, the muzzle energy will usually be roughly the same.

If you introduced, say, a 600psi pressure valve, it would have no effect on the high start pressure pellet, but would give the low start pressure pellet much higher force, but for less time, and whether that would translate into higher muzzle energy I honestly don't know. Sounds like a job for an experimental engineer.

[ballisticboy]

I know that while springer models are not good at predicting springer performance without calibration data, they are not usually too bad at assessing the effects of changes. Years ago when I was keen, I created a complex springer model which did seem able to make a reasonable estimate of the effects of changes to a calibrated model (based on a lot of your data, Jim). From what I remember, energy increased slightly when start pressure was reduced from some of the high values, everything else being kept constant. There did come a point where the trend reversed, but at a very low pressure. The reason seemed to be, as Jim says, that at lower start pressures the piston and pellet are going in the same direction for longer, though it would also depend on the barrel length how big an advantage there is.

There is also the problem in that the pressure seen by a pellet is a dynamic pressure, and most metals will withstand a much higher dynamic pressure than they will a static pressure. Hence, the rate of increase of pressure may also be a factor in when the pellet starts.

[Jim-in-UK]

I know that while springer models are not good at predicting springer performance without calibration data, they are not usually too bad at assessing the effects of changes. Years ago when I was keen, I created a complex springer model which did seem able to make a reasonable estimate of the effects of changes to a calibrated model (based on a lot of your data, Jim). From what I remember, energy increased slightly when start pressure was reduced from some of the high values, everything else being kept constant. There did come a point where the trend reversed, but at a very low pressure. The reason seemed to be, as Jim says, that at lower start pressures the piston and pellet are going in the same direction for longer, though it would also depend on the barrel length how big an advantage there is.

There is also the problem in that the pressure seen by a pellet is a dynamic pressure, and most metals will withstand a much higher dynamic pressure than they will a static pressure. Hence, the rate of increase of pressure may also be a factor in when the pellet starts.

I'm glad my data was of some use, Miles.

You're of course right that the trend reverses at abnormally low start pressures (such as from a barrel with an 'eased' breech); the air does not have enough internal kinetic energy to do much work accelerating the pellet, and by the time the air has gained sufficient energy, the pellet is too far up the barrel to benefit from it.

The figures I gave for pellet start pressure are dynamic, by the way, and are from my HW77. Other barrels will vary to some degree.

[subscriber]

Matt,

The reason pressure builds up ahead of the piston is because the pellet acts like a temporary plug.  So, the pellet sees the same pressure as the piston.  Firing a springer without a pellet has the air escape through the transfer port, such that the air cushion escapes at low pressure and the piston then crashes harshly into the cylinder endwall.

It is not friction that causes the pellet to act as a plug, but more like a car stopped with its front wheel against a steep but short speed bump.  The force required to start the car moving is increased momentarily to climb up the bump.  In a similar manner, the pellet skirt is larger in diameter than both the land and groove diameter of the barrel.  That interference and the process of reshaping the pellet skirt in the breech cone acts like climbing the speedbump - except the bump effectively gets flattened out as soon as it is overcome by the pellet.

Sure, there is friction during this pellet skirt sizing operation, but just like with the lesser friction all the way down the barrel, that friction is a net loss of energy.  The pellet being held and released by the breech cone is not a loss of energy.  Making that cone too shallow will actually reduce the pellet energy because it won't be able to hold onto the pellet.  Ditto for sizing down the pellet skirt "to reduce friction".  Unless the skirt was way oversize, sizing it down kill the pellet holding function in the breech cone, along with peak and average pressure - thus reducing pellet velocity.

Perhaps a better analogy is holding an jet aircraft on its brakes with the throttle wide open just before takeoff.   Then, only letting off the brakes after the turbine has spun up so that a large amount of fuel can be burnt by the now much more significant airflow through the engine.  Else, the aircraft starts moving at low initial thrust, using up runway space before the engine is on full boost.  This is a problem on a short runway, as the aircraft may not have reached takeoff speed before the end of the runway.

The weight (inertia) of the pellet also acts as part of the resistance against which the air pressure builds.  So it is not just the dynamic release force of the pellet skirt against the breech cone that matters.

The high starting air pressure on firing a springer tends to expand the pellet skirt even harder against the breech cone, so the actual peak pressure tends to be higher than if you push pellets into the breech with a probe using a force meter, then back calculating that to equivalent pressure.

Your two stage system, with piston movement followed by captured air pressure released to propel the pellet has a few problems:

If there is a long delay between releasing the piston and firing the pellet, the air that was heated very significantly by compression has time to cool before the pellet is launched.  Springers have 2 to 3 times the efficiency of PCPs, when you consider the work you put into cocking a springer, or pumping up a PCP, compared to the kinetic energy of the pellet.  That is because the pumping of a PCP happens so long before the shot that the thermal energy added to the air when raising its pressure has long gone.  Compared to a springer, where that thermal energy is part of what increases the pressure of a much smaller volume of air.

If there is a short, but perceptible delay in releasing the piston before firing the pellet, the airgun will be harder to shoot well than a regular springer.  The residual bounce from recoil will still be there and the delay will force an even longer follow through; much like a black powder flint-lock rifle: ka-------POW.  Or for your proposed system; POW--------ka.

In practice, springer pistons bounce before the pellet has left the muzzle.  That bounce represents energy that was not transferred to the pellet.  There have been some attempts to introduce a brake that captures the piston at the forward-most extent of its travel.  While these were sold as an improvement, they seem to have disappeared from the market.  Certainly, when it comes to the over 12 FPE market where customers want more power from their springer, rather than adding cost with extra parts that add a small percentage improvement, and might not last very long, the answer is to add swept volume and a larger spring, with a heavier piston.

Those in the know, prefer lower power springers that easier to shoot well, rather than miss a lot with faster pellets.

Could one make a PCP pump that has you cock a spring, then discharge a piston to pump up a tank with a shutoff valve to prevent back flow?   For a small volume, the instantaneous peak pressure one could reach might be high, but I think that repeating the cocking by hand for 100s of "shots" to fill a modest PCP would be very tiring.  The reason being that the more abrupt the compression, the more heat will be lost - unless the shot cycle follows immediately, as with a conventional springer design.

The air cushion in front of a conventional springer already prevents the piston from slamming directly into the endwall of the cylinder.  Too light a pellet or dry firing would remove or reduce that cushion.  When the right type of pellet is used, springers will shoot tens of thousands of shots without any problem.

Actual rearward recoil from launching a heavy piston via a strong spring would not go away.  The fundamental problem that makes springers hard to shoot well, would remain with your proposed valve system:   The gun jumps significantly before the projectile leave the barrel.  Introducing any further delay will make the problem worse.  Hence, springer shooters often shorten their barrels, so the pellet is out of the barrel sooner, because  this makes them easier to shoot well.

Don't let this put you off.  Nothing lost in dreaming up new ideas.  Some new ideas have come and gone.  A very good one for a springer is two opposing pistons that compress the air between them.  Properly configured, such a system is recoilless and nearly vibration free.  It is heavy and expensive, so the air rifle sporting this design are long gone. 

Want PCP shooting qualities?  An actual PCP is much lighter, shorter and cheaper than a springer configured to identify as a PCP.

[JMJ in NC]

I recommend you read “from trigger to target” by cardew & cardew.

https://www.abebooks.com/9780950510828/Airgun-Trigger-Target-G.V-Cardew-0950510823/plp

It explains everything about the dynamics of spring piston guns. The pressure developed before the pellet starts to move is on par with PCPs.

JMJinNC

[oldair]

Actually, Crosman used this idea in its push-barrel BB guns: V350, V3500, M-1 Carbine.  There's a "pop valve" in the transfer port that opens when pressure has reached some level.  It's generally accepted that this design was intended to increase velocity of these BB shooters, and it seems like the extra cost wouldn't have been put into them if it had no effect.  Obviously smooth-bore BB vs rifled pellet behaviors are different.

Don R.

[subscriber]

You can buy a used copy of from trigger to target for lots of money; or you can download a copy for free.   If one owes the Cardew estate a royalty, how would that be paid? 

$92.11: www.amazon.com/Airgun-Trigger-Target-Gerard-Michael/dp/0950510823/
$129.95: https://www.ebay.com/itm/185351239869?

http://www.mediafire.com/file/5mcemcyutrm/The_Air_Gun_From_Trigger_To_Muzzle_s.pdf/file

[subscriber]

Cardew measured the pressure peak of their .22 springer as 1250 PSI when shooting just over 400 FPS (hardly a magnum springer).  The static pressure they measured for various breech cone shapes, and the resultant pellet velocity is attached below from a screen capture.  As you can see, the higher the pellet's holding force (within reason), the higher the velocity.

Based on skirt ballooning of pellets shot into water from spingers, and comparing that to the skirt ballooning seen with PCPs fired at known pressures, peak pressures over 3000 PSI are common with 15 to 20 FPE springers.

[rsterne]

Read Cardew's books.... The pressure and temperature is a LOT higher than you might expect....

Bob

[DIYengineer235]

You can buy a used copy of from trigger to target for lots of money; or you can download a copy for free.   If one owes the Cardew estate a royalty, how would that be paid? 

$92.11: www.amazon.com/Airgun-Trigger-Target-Gerard-Michael/dp/0950510823/
$129.95: https://www.ebay.com/itm/185351239869?

http://www.mediafire.com/file/5mcemcyutrm/The_Air_Gun_From_Trigger_To_Muzzle_s.pdf/file

Thanks! I've been looking at that book, but it was just far too expensive.

[mikeyb]

I don't understand why springers are perceived as inefficient when they are usually the most efficient type of air guns. Considering all the energy storage/exchange transitions it is a surprisingly efficient process. Arm effort (energy), mechanical linkage to spring compression (potential energy), spring to piston mass-velocity (kinetic energy), compression of air (potential energy), air pressure to pellet velocity (kinetic energy). All of that approaching ~30% from arm to pellet. May seem low compared to some other systems, but it's actually quite good for this system.

Pellets transitioning from the lead-in to the rifle bore ARE a "pressure buildup dump valve" in it's simplest and IMO most elegant form. This is one of several reasons certain springers like specific pellets. When a pellets fits the lead-in & bore just right the peak pressure reached forms the pellet into the rifling we usually(but not always) see the most accurate & consistent results for that particular rifle.

Selection of a good-fit pellet can satisfy the perceived advantages 1) and 3) of a springer dump valve. I don't see less recoil 2) as that is determined by piston & rifle mass and spring force. Less vibrations "yes" when the whole system is optimized for best energy transfer.

Tuning the release pressure with an external adjustment 3) might have some benefit under certain conditions. I just don't see any practical (repeatable, reliable) way to implement such an adjustable Pressure-Relief-Valve into the powerplant of a springer. This valve would need to work in the SUB-millisecond time frame and fit into the space of a typical transfer port. It should be noted that the pressure inside the compression tube only gets high enough to start moving any pellet in the last 2mm-5mm of piston travel.

Sorry to all the fans of the Cardews' book "Trigger to Muzzle", but that work is dated. I haven't read the revised edition "Trigger to Target" yet, but I have read the older version as it is the only version I can find as a free download. It was the best REFERENCE work for many years and admittedly STILL HAS some very good information. However polymer & metallurgical material science, electronic/optical/acoustical measurements, and computational resources have all advanced significantly since the initial work. The Cardews' book is often quoted and assumed correct without consideration of these advances.


I suggest additional reading of these papers to update that older work...

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

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

Also this paper on coil spring behavior in air guns...

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

(or the GTA sticky here... https://www.gatewaytoairguns.org/GTA/index.php?topic=185967.msg156132856#msg156132856)

[subscriber]

Pellets transitioning from the lead-in to the rifle bore ARE a "pressure buildup dump valve" in it's simplest and IMO most elegant form.

In a nutshell!

[Doug Wall]

In “The Airgun from Trigger to Target”, pg 12, they show an ocilliscope trace that shows the pellet starting to move at a point very close to the peak pressure in the gun. At a different part in the book, they figure the max pressure between 1300 and 1400 psi. Your quest for a pressure relief valve might be an exercise in futility.

[Ribbonstone]

Generally springers “bounce back” from the air they compress, not from slamming into the end of the tube...don’t see a “pop off” system changing that.

Any delay, even a small one, robs the system of some of the heat...which is part of the springer-pressure. Delay it long enough, make the trigger the opening device, and you have a SSP?

As to coming up to PCP performance...just don’t see the air volume to do that.

I’d love to be proven wrong about the above...do like springers.

In a way, springers already do match PCP’s in power.  Consider the non FAC shooters in the U.K. limited to 12 foot pounds in any air gun.  Springers do seem to have a large following, but the PCP’s are still preferred for their match shooting (steel or paper).  Much the same could be said for U.S. shooting when the game is limited to 20 foot pounds. 

[lefteyeshot]

Seems to me the transfer port will regulate the presure to the pellet regardless of the presure behind the transfer port. That would depend on how much pressure the compression chamber will produce but that's it.
Varible transfer port?

[Jim-in-UK]

In “The Airgun from Trigger to Target”, pg 12, they show an ocilliscope trace that shows the pellet starting to move at a point very close to the peak pressure in the gun. At a different part in the book, they figure the max pressure between 1300 and 1400 psi. Your quest for a pressure relief valve might be an exercise in futility.

On pellet start pressure: I turned short 25mm OD cylinders from Acetal that were a snug fit inside the cylinders of my 25mm HW77 and TX200, and made sure there was plenty of spring energy to ensure the piston hit the end of the insert cylinder. By varying the length and bore of the insert cylinders, I was able to set peak cylinder pressure anywhere I wanted, and I tested a range of pellets at a range of pressures to see at what pressure each pellet had started to move after I shot the rifle. This is 'dynamic' air pressure testing, and represents the true start pressure of a pellet in a normal shot cycle.

I tested only in .177", and a retired chartered engineer friend tested in .22", the .22" pellet start pressure being circa 66% that of the .177". The pressures in .177" ranged from 120psi to a little over 600psi, so why did Cardew come up with pellets starting closer to the peak pressure?

For a start, he almost certainly tested with Eley Wasp pellets, which were the best by far available in the UK at the time, and which had a very high antimony content and were extremely hard, so it took a lot of force to get them to conform to the rifling. Second, Eley Wasp pellets were 5.6mm, and the test rifle (HW35) had the Continental 5.5mm bore.

Cardew's book is the most important work on the spring/piston air rifle, but it is based on the assumption that all rifles and pellets behave in the way his test rifle and pellets did, as in his pellet start pressures, and his assertion that all springers need to diesel to make 12 ft. lb.. They do not so, while I encourage people to read the book, I I suggest they do so with an open mind.

[subscriber]

Jim,

Thanks for sharing.  Did you measure your pressure data, or calculate it?  If you used a simple compression ratio calculation, then you values may be artificially low:

If you slam 50 CCs of air into a 5 CC space the compression ratio is 10:1.  If ambient air pressure was 14.7 PSI you might expect that compression should increase the pressure to 147 PSI.  However, due to the heat generated by sudden compression, the actual pressure increase is much greater than that.

Cardew measured their springer pressure with an instrument.  We can speculate about the calibration of their set-up, but instrumented is likely to be closer than calculated.  Unless you used a fancy formula for adiabatic compression, rather than slow compression, where the heat has had time to leave.

The higher and more abruptly the applied pressure, the more the pellet skirt balloons.  This increases engagement of the skirt in the breech cone, thus raising the peak pressure before pellet release even further.   This effect is diminished with a more gentle application of air pressure, so there is no risk of the pellet jamming itself in the breech.

It is not only the interference fit of the pellet in the breech that drives up dynamic pressure.  The inertia of the pellet starts to matter when abrupt acceleration is involved.  So, if you experimentally lower the acceleration, the peak pressure reached is likely to be reduced also, due to a smaller inertia component.

At 120 PSI a .177 pellet has 2.8 lb acting on it.  I am not sure I want to shoot pellets from my springers that have such low starting forces in their breech, because that seems close to dry firing.  Just loading the pellet head into the barrel until the skirt is flush with the breech often takes more than a few pounds on a break barrel.  The force required to swage the skirt down as it enters the bore proper, is usually more than the loading force.

Considering that the pressure profile for a springer is a spike, without any plateau, the higher the peak pressure, the greater the average pressure down the barrel.  For a 12" barrel length to achieve 12 FPE, the average pressure acting on a .177 pellet needs to be just over 500 PSI. 

An average air pressure of 500 PSI is impossible if the peak pressure is lower than 500 PSI.  Much more likely for the peak pressure to be four or five times the average.  If the barrel is longer than 12" (as they are on break barrels to provide cocking leverage), then the peak pressure appear even more spiky, compared to the pressure in the last third of the barrel length.

[Jim-in-UK]

I calculated the pressure for adiabatic compression; the compression ratio raised to the power of 1.4 and multiplied by start pressure. So 14.7 at 10:1 is 10^1.4 x 14.7, or 369psi.

I must apologise, but I have to go out now. I'll read your post fully and reply after I get back.