Spring Piston Designs - Delayed Pressure Release??

[Duane38]

Just musing....

Has there ever been a design in a springer powerplant to delay the exhaust pressure? 

I posit that, in conventional design, the pellet begins to move well before the piston ends it's travel. 

Is it possible, or even beneficial, to design a system where the swept heated charge of air is compressed into a sub-valve that is subsequently knocked open upon piston bottoming out?  This would produce a higher pressure charge to be exhausted all at once over the conventional design. 

Understandably, in the conventional design, the pellet begins to move when the equilibrium in force is no more.  A pressure of X is developed at point of release while the piston is still moving and pressure is somewhat maintained until piston stops moving. 

There will need to be some careful design parameters for a knock-open valve at the end of stroke. 

[mackeral5]

Just musing....

Has there ever been a design in a springer powerplant to delay the exhaust pressure? 

I posit that, in conventional design, the pellet begins to move well before the piston ends it's travel. 

Is it possible, or even beneficial, to design a system where the swept heated charge of air is compressed into a sub-valve that is subsequently knocked open upon piston bottoming out?  This would produce a higher pressure charge to be exhausted all at once over the conventional design. 

Understandably, in the conventional design, the pellet begins to move when the equilibrium in force is no more.  A pressure of X is developed at point of release while the piston is still moving and pressure is somewhat maintained until piston stops moving. 

There will need to be some careful design parameters for a knock-open valve at the end of stroke.

Makes sense to me--Seating a pellet deeper into the rifling will yield slower velocities in many piston guns, as the pellet begins to move at a much lower pressure.... At what point in the pressure spike and at what time in the piston stroke is ideal, I bet the piston gun gurus have this figured out and do their best to control it with breech/rifling entry design and pellet selection.

[Ronno6]

I am way out of my depth here, but......
I wonder if the ram would continue to the end of stroke with the same force in the face of mounting back pressure.

Kinda sounds like you are describing a hybrid break barrel/SSP air rifle.......

[sb327]

I don’t think it would make the whole stroke in conventional form. It would stop wherever the pressure and spring/piston diameter causes the forces to equalize. Maybe a touch more due to momentum of piston.

If it had maybe something that tripped the valve open along the piston’s travel, it might be used to optimize the operation. Maybe with a small holding plenum with x amount of volume. Have the sear actuated by pressure maybe.

Interesting to think about.

Dave

[KWK]

The volume of the air in that system would reduce efficiency

[mikeyb]

The beauty of springers is the simplicity of construction even though the actual energy "dance" from expanding spring to pellet muzzle energy can be quite complex.

fyi... Most pellets don't begin to move until the piston reaches 85% of its travel distance.

No need to make springers more complicated with fast acting valves that would be nearly impossible to build even as a one-off prototype. Forget about mass production or availability at any reasonable cost.

A proper fit pellet popping the lead already IS the ideal pressure release valve.

Don't let this post stop you from digging deeper. The only way you will truly learn the answer to your question is to study the math and physics involved.

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

[Duane38]

The beauty of springers is the simplicity of construction even though the actual energy "dance" from expanding spring to pellet muzzle energy can be quite complex.

fyi... Most pellets don't begin to move until the piston reaches 85% of its travel distance.

No need to make springers more complicated with fast acting valves that would be nearly impossible to build even as a one-off prototype. Forget about mass production or availability at any reasonable cost.

A proper fit pellet popping the lead already IS the ideal pressure release valve.

Don't let this post stop you from digging deeper. The only way you will truly learn the answer to your question is to study the math and physics involved.

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

I've got the time and tooling and a few extra springers lying around...  May be something to explore/experiment with.  That 85 percent hypothesis is the whole reason, adiabatic process aside (but not ignored).  If a piston is drawn 7", pellet begins to move at 5.95" (1.05" before stop).  At room temperature,  with a 1.25 bore compression chamber, that works out to be around 83 psi the pellet begins to move.  HOWEVER, the air is heated and expands.  We can say springers are running around 1,300 psi.  Is that to say the heat process has a multiplier of 15-16 times?  No.  This logic wouldn't be correct.

Say, the pellet has moved 3 inches in the bore when the piston has bottomed out; that would work out to be roughly 1,400psi in a .177 bore.  Right in the ball park of what is actually happening. 

The approach I'm thinking of is to minimize piston slam and bounce and subsequent suction on the bore.  Dunno.  Probably not worth venturing.  Though, curiosity has got me. 

[rsterne]

It might work, if you had a really tiny valve, so that the pressure would be higher than without it.... Achieving that with sufficient port size would be a challenge!....

Bob

[JPSAXNC]

In any type of air gun it would be hard to fill the valve and dump it from the same end.

[JPSAXNC]

I thought about making a double walled piston, the hallow would be filled half way with mercury. The mercury would act like a slide hammer, when the gun was fired the mercury would go to the back of the piston, when the piston bottomed out the mercury would slam forward preventing the piston from bouncing back. Maybe?

[Snatchcan]

My first thought was a plenum with a "double" poppet through it.  The air would compress through the first poppet and into the plenum. The first  poppet would essentially be free floating and sealed to the poppet stem with o'rings and held in place with a spring to act as a one way valve. Then the piston, at the very end of its travel would slam the valve stem opening the second poppet, that's fixed to the stem and release the compressed air to the transfer port. The second poppet would need a sufficiently stiff spring to hold back the air charge until the piston hits the valve stem.

With the first spring and poppet you may also be able to make the plenum collapse when hit with the piston,  squeaking out the last little bit of air.

It seems doable, and after reading your post on your mega pumper I would expect you have the machining skills to make it work.

[subscriber]

As stated, a proper pellet with its malleable, but larger than groove diameter skirt, already acts as the pressure build-up / instant opening valve, with perfect timing.

As James suggested, reducing piston bounce would improve efficiency and power.  Such piston de-bouncers have come and gone.  Probably because the added cost is not justified.  There may be some marketing and regulatory pushback against mercury being used to create a dead-blow hammer effect, but perhaps steel shot would be a much cheaper option, that could be made to work almost as well.

As for a pressurized plenum, the transfer port in a springer acts as high pressure air plenum.  Yes, it is tiny, but springer power is not very impressive.   Many a tinkerer has opened up a springer transfer port to "reduce flow restriction", only to loose significant pellet velocity, due to the much lower peak air pressure developed.  Due to the lower temperature reached in compression, and the larger surface area of steel in the larger port removing more heat from the already colder air, the average air pressure is also reduced.  Average air pressure down the barrel is what predicts the pellet energy.

If you want more performance out of a given piston and spring, shorten the transfer port.  Not an easy thing to do in an existing airgun.  There is a reason why most springers on the market have transfer port diameter somewhere between 3 and 4 mm.  Some are longer and some are shorter, but the range of designs that work well enough seems narrow.

If you want to see the effect of transfer port geometry on a springer, that Internal Ballistics of Spring Piston Airguns document linked above is a good primer on the topic.

[lefteyeshot]

Is this to keep recoil and vibration from effecting the pellet as it travels down the barrel?

What came to my minds eye was two triggers. One to release the spring and piston and compress the air and a second trigger to release the compressed air. Kind of like double set triggers or maybe the Mendoza dual triggers that overlap each other. The first is a safety the second releases the sear.

[subscriber]

Tim,

That could work, if you don't mind the massive loss in pellet FPE due to the air cooling down after compression.  So, you just make the piston and spring larger to compress more of it, as a form of single stroke pneumatic; which is how it shoots.

If you fire your two trigger system with 0.1 second delay, or a 1 second delay, or a 10 second delay; the pellet velocity will vary wildly.  So, you are better off with a conventional springer, or a pneumatic.

If you want to get rid of springer jolt, disturbing the gun before the pellet is out of the barrel, then engineer (another) opposed piston (or piston with counterweight) springer.  I think there is merit in revisiting this, but most airgun manufacturers offer either a conventional springer, or a PCP.  The lower prices of PCPs seems to make reinventing a better springer a marketing fool's errand. 

Better to sell a cheaper, lighter, easier to shoot airgun, that is also more powerful.  Of course, the advertising usually does not mention either the cost of a compressor; or the effort of a hand pump.  That is where a springer shines.  Just follow through like you are shooting flint-lock and accept that springers become large, heavy and hard to cock when you go over about 20 FPE. 

Between 10 and 20 FPE conventional springers are perfectly serviceable, for those who are willing to put in a little effort practicing with them.  For those folks that shoot everything off a bench rest, springers may not perform as well, as for those who can shoot well enough offhand.

[lefteyeshot]

I don't worry over the jolt. I got the artillery hold down pat. I'm a springer guy. I don't even care for gas ram, even if I have a few, forget PCP. I'm pretty happy with springers as is, cheap, expensive or in between. I have a few Mendozas and I just acquired a LG 55 with double set triggers (DST). I guess that's why I thought of that.

[subscriber]

Tim,

I am not worried about springer jolt to my person.  Only to it moving the airgun off the target before the pellet is out of the muzzle.

Anyone who notices springer recoil and describes it as more than a vibration is shouldering the airgun too hard.  Thus you are right, the artillery hold eliminates the sensation of recoil.  It also makes the point of impact more consistent, relative to the point of aim.

[lefteyeshot]

I exaggerate, but I say my artillery hold is so loose if I coughed I'd drop the gun. A way of describing it. After all the recoilin', vibratin' and teeth rattlin' the pellet comes out at the same time and if you let it do as it wants the end of the barrel will be in the same place when the pellet comes out and the POI will be the same or close. Zero accordingly.