Chinese air rifles and Gas spring conversions

[MarkFive]

Here is a rough thought I had on the gas ram conversion subject. If you have an advanced degree in physics please speak up.

The more powerful ram will force the piston to move faster, relative to a "stock" or lesser ram, but only marginally because they both are moving the same volume of air from the compression chamber.

So the best method of increasing performance would not be only a stronger ram, but also more air volume. I believe the NP XL guns are an example of this, but I haven't measured their compression chamber to be sure, only looked at the schematics.

I plan to look for ways to increase volume while converting my gun. I have no idea if it is even possible. Custom parts would obviously be involved, so don't try this at home.

Thoughts?

[Scotchmo]

MarkFive -
I don't have an advanced degree in physics but I'll speak up anyway. You are correct. The primary limiting factor in piston gun power is the swept volume. The head space also plays a role. Those two factors determine the compression ratio. Knowing those properties makes it possible to determine the maximum possible amount of energy that can be transferred. Adding more spring energy beyond that makes no difference in power output. It just makes the piston slam harder.

Increasing the swept volume returns a proportional increase in power. Increasing the spring energy gives diminishing returns and maxes out at some point.

It is sometimes possible to increase the swept volume by a small amount. A large change requires a redesign of the whole mechanism. But if spring energy is already near the maximum, a small increase in chamber volume can yield a substantial power increase.

[Paul68]

"The primary limiting factor in piston gun power is the swept volume."

I keep seeing this said, but something seems like it doesn't add up to me.

In the end, you are transferring energy to the pellet. The spring stores the energy you provide by compressing it, the air is a "vehicle" for transfering that energy from the spring to the pellet, and the pellet then transfers the energy to the barrel, whatever it hits, and to a lesser degree, the atmosphere and losses that get all scientifiky like and stuff.

The problem I have with the idea that chamber volume limits energy is that it doesn't seem that volume alone dictates how efficiently energy is transferred, how much can be stored, or what limits the total transfer of energy. We naturally view air as gaseous and without much energy or resistance, but in a closed system (chamber) applying energy to air causes the air to absorb energy and store it until it is released or transferred. This is how an air compressor works to power tools and the like, which is the same principal for an AG. Putting more energy into the "air", stores more energy until it is released.

For example, if we pump up our air compressor to 250 psi, we have stored a whole bunch of energy, and we can then run an impact wrench for some time as the rate of energy release is controlled and spread out over a long period of time.

Now, if we take that same compressor and simply open a hole in it without any regulation or control over the rate of energy release, we get a catastrophic and extreme release of energy over a short period of time. **Kaboom.** The restricted release of energy over a long period of time results in a little force of long duration, the full release over a short period results in a ginormous force of short duration. The restricted release would simply loosen lug nuts, the full release would blow the wheel off. We are simply altering how the energy is applied.

Also, lest you think that by adding air to the compressor in order to adde energy we are in effect increasing the total "volume" of air, which is also true, consider that this is not the only way to add energy to get the same results. We could also apply heat to air in a closed chamber instead and still store a whole lotta energy.


So, it would seem, that applying more energy to a specific amount of air would result in a cooresponding increase in energy stored in that  air. Put a little energy into a volume of air, get a little out; put a lot of energy into that same volume of air, get a lot out. Of course there are limits and losses due to friction, heat and the like that reduce possible returns, but I think this gets the idea across.

It seems like the main limiting factor below the total limits of how much energy a volume of air can store, would be how efficiently the energy can be transferred to the pellet rather than how much volume there is. Since I doubt that an airgun is capable of containing energies high enough to represent the total storage limits of a volume of air, perhaps something else is causing the poor returns on energy represented by larger springs?

(I think I hurt my brain with this one)

[rsterne]

You can't compare how a springer works to the energy stored in an air tank.... In a springer, the compression of the air occurs in a fraction of a second and incorporates a large degree of heating.... enough to cause some combustion of the film of lube on the chamber walls according to the Cardew brothers in their work "The Airgun from Trigger to Target".... They proved this by filling and firing a springer in a Nitrogen atmosphere and it lost a lot of power.... when used in air, the power returned....

There is an old saying in automobile engines that "nothing beats cubic inches".... While it's true that you can produce incredible amounts of horsepower from a tiny engine by supercharging it.... if you do the same modifications to a larger engine you get even more power.... If you push any engine far enough you will blow it up.... If you push a springer far enough, you will simpy crash the piston into the end of the chamber and start destroying parts.... Add volume, and you have a larger amount of air to cushion and harness the energy of a larger spring....

Up to a certain level, increasing the spring power will add performance.... Once that level is reached, trying to exceed it only makes the gun harsh shooting and eventually will destroy it....

Bob

[Scotchmo]

Paul68,
We can take a simplified view of it so that it is more clear:

You have a 6 inch long cylinder with a piston inside it. There is 1 inch of head space at the bottom of the stroke. That is a lot but it is just for demonstration. You manually push the piston until it comes to within 2 inches of the bottom, resulting in a 6 inch column of air squeezed into a 2 inch space. It took a certain amount of energy to do that. If you input some additional energy, you can further compress that column of air down to 1 inch. At that point, no more energy can be accepted. The piston is bottomed out and the head space contains all of the compressed air.

If you use a spring to launch that piston down the tube, all of the energy in the spring wants to go somewhere. Once the spring energy is high enough to bottom out the piston, you have reached the limit. Any excess energy just ends up as energy transferred into smashing the seal and jarring the rifle.

[blaqheart]

I got a buddy with a rws 48,he put a webley patriot spring in it,he got more power,or so he believes since its never been put trough a crony.The gun is no longer a smooth  shooter and it spends a lot of time in the shop.

[william71743]

All of this is very interesting and all the forces of physics certainly apply . That said, it's PSI, pounds per square inch right, and the base of a pellet isn't very big . I'm sorry I'm brain dead already,  but is it something like .0246 for a .177 cal.  Wouldn't it take a big change to make just a little difference, someone with a degree in physics again ?

[Paul68]

Paul68,
We can take a simplified view of it so that it is more clear:

You have a 6 inch long cylinder with a piston inside it. There is 1 inch of head space at the bottom of the stroke. That is a lot but it is just for demonstration. You manually push the piston until it comes to within 2 inches of the bottom, resulting in a 6 inch column of air squeezed into a 2 inch space. It took a certain amount of energy to do that. If you input some additional energy, you can further compress that column of air down to 1 inch. At that point, no more energy can be accepted. The piston is bottomed out and the head space contains all of the compressed air.

If you use a spring to launch that piston down the tube, all of the energy in the spring wants to go somewhere. Once the spring energy is high enough to bottom out the piston, you have reached the limit. Any excess energy just ends up as energy transferred into smashing the seal and jarring the rifle.

Thanks. I hadn't taken into account the need for extra volume to compensate for compression itself. This would explain the ability of PCP's to produce more power, because they aren't limited by a single compression of a small air volume. In other words, we are limited in springers by the PSI that can be produced with a certain volume of air.

[Scotchmo]

Thanks. I hadn't taken into account the need for extra volume to compensate for compression itself. This would explain the ability of PCP's to produce more power, because they aren't limited by a single compression of a small air volume. In other words, we are limited in springers by the PSI that can be produced with a certain volume of air.

True, and there is a practical limit to the maximum pressure. So once we reach that practical limit, the only way to transfer more energy is to increase the swept volume.

[rsterne]

Could I request that the Admins sticky this thread?.... I for one, don't want to lose it....

Bob

[Mike 4888blues]

  yes  I agree this should be saved great discussion 

[peterdulux]

Just what I've been looking for too. I just bought a Walther Talon in .25, what a hammer. Took out the cheap scope stop in less than 30 shots.
Three questions for now; Will these pistons fit the Talon and a Diana 350?
How about a little piston for a Diana 24?
Would like a little more punch from my XL1100. It shoots predators very well and would like to get them up to mid 800's. Is that possible?

I'll keep reading, and thanks so much. Complete information is always so refreshing.

[Kailua]

Scotchmo
I know this is an old thread.  But if you can get back to this.  I read this post and might have missed something.  When looking at the N-forcer pdf the nitrogen springs are pressure adjustable.  Have you tried to adjust the pressure.  Also do you think air can pressurize the cylinder.

[Scotchmo]

Kailua,
I have not tried to adjust the pressure. I have not needed to yet. I thought about the use of air. It might be OK for experimenting. Even though air is mostly nitrogen, it also contains some oxygen which promotes corrosion and oxidation. So use it at you your own risk. I would stick with pure nitrogen for long term use.

[Kailua]

The reason why I had asked was the numbers needed for the pressure adjustments were in the PCP range and wondered if a PCP pump or tank could work instead of renting a nitrogen tank for a such a small amount.  Good point on moisture.  May need a good air filter.  Not sure if there is a filter to hold that kind of pressure.  Thanks for the reply.

[SMOKEY]

another reason to use only nitrogen is that it dosent expand with heat !

[Scotchmo]

another reason to use only nitrogen is that it dosent expand with heat !

Yes it does, just like air.

PV=nRT

[codytwoeyes]

You wrote (I tested a red label gas spring in my TF99) have you tried a yellow in a tf 99.
do you think it is OK just to Strong for the 99? can you tell the size me of the ram you ordered.
sorry there is just so much I dont know. is there perhaps a really strong spring that would be better?
I  asking all this because  I now have a 99. nice gun but my XL has just about ruined me.I really like power! I think the 99 can be up power some however I do want to learn to do this the right way.
So any help you would or can give would greatly appreciated.-- I will pass it on   

[Scotchmo]

You wrote (I tested a red label gas spring in my TF99) have you tried a yellow in a tf 99.
do you think it is OK just to Strong for the 99? can you tell the size me of the ram you ordered.
sorry there is just so much I dont know. is there perhaps a really strong spring that would be better?
I  asking all this because  I now have a 99. nice gun but my XL has just about ruined me.I really like power! I think the 99 can be up power some however I do want to learn to do this the right way.
So any help you would or can give would greatly appreciated.-- I will pass it on   

What does your TF99 do in stock form?