Complicated thinking: Why match cylinder porting and barrel length?

[highny]

Originally Posted by ILLusion

Are you referring to your imaginary scenario or real world?

Because I was hoping you would explain it in really simple English...


What does "bore up unported cylinder" mean?... There's different types of unported cylinders?!?!... Also, what does it mean when people say a cylinder is "oversized"?

Is there somewhere were I could read about them?...

Thanks for answering ILLusion, that really help me understand the fundamental of airsoft's gearbox.

[ILLusion]

Yes, there are different types of cylinders. Standard cylinders and bore up cylinders. Bore up cylinders have thinner walls, which allow for greater air volume (and thus, a larger cylinder head stem, larger air nozzle, and a larger piston head to make up for the larger surface area that needs to be covered.)

[Jackarutu]

Having just joined this forum I am total captivated by these very technical discussions. I find them very helpful in rounding out my own knowledge and experience.

Even though MadMax started this thread back in 2005 I was drawn to his explanation of bb size compared to piston head size.

The point of making the piston area so much larger than the barrel area is to acheive a good pneumatic coupling between the bb and the piston. Because the bb is a small diameter compared to the piston, you have a force division, a kind of pneumatic leverage if you will. It's related to the ratio of the square of the cross sections involved.

I’m sure we all remember as a child receiving a popgun (perhaps better named a cork gun) for Christmas where the Piston Head was approximately equal in diameter to the size of the cork. Well, we all know how far the cork ranged thus I believe offers a good empirical proof of MadMax’s theory.

[Zephyranthes]

So I did some math calculations and I found some interesting results. I wanted to see if bore-up cylinders were really necessary for long barrels like thePSG-1 (650mm). The volume of a barrel using the equation for the volume of a cylinder V=pi*((d/2)^2)*l, where d is 6.05mm and l is 650mm. I found the volume to be 18685 cubic mm. I then took measurements of an AEG cylinder (check me if my measurements are wrong), and I found the diameter to be 24mm and the length to be 72mm. Using the same equation above, I found a cylinder's volume to be 32572 cubic mm. That means a type-0 no hole cylinder has 175% the volume of the longest barrel on the market. This makes me question why bore-ups are necessary, considering their purpose is to increase the cylinder's volume to fit the barrel. However, I understand the entire volume of the cylinder is not used because the piston head and cylinder head take up some room inside the cylinder. I also understand that aeg's are not 100% efficient. This still doesn't justify to me the need for a bore-up cylinder when it's volume is 75% more than the barrel. Cylinder volume should not be an issue even with these considerations.

[Cushak]

Keep in mind, among other things, that you're compressing that air quite a bit to get the BB going fast, the pressure in the barrel right before the BB leaves the barrel is quite above atmospheric pressure I imagine.



Disclaimer, this statement is an educated guess.

[ILLusion]

Originally Posted by Zephyranthes

So I did some math calculations and I found some interesting results. I wanted to see if bore-up cylinders were really necessary for long barrels like thePSG-1 (650mm). The volume of a barrel using the equation for the volume of a cylinder V=pi*((d/2)^2)*l, where d is 6.05mm and l is 650mm. I found the volume to be 18685 cubic mm. I then took measurements of an AEG cylinder (check me if my measurements are wrong), and I found the diameter to be 24mm and the length to be 72mm. Using the same equation above, I found a cylinder's volume to be 32572 cubic mm. That means a type-0 no hole cylinder has 175% the volume of the longest barrel on the market. This makes me question why bore-ups are necessary, considering their purpose is to increase the cylinder's volume to fit the barrel. However, I understand the entire volume of the cylinder is not used because the piston head and cylinder head take up some room inside the cylinder. I also understand that aeg's are not 100% efficient. This still doesn't justify to me the need for a bore-up cylinder when it's volume is 75% more than the barrel. Cylinder volume should not be an issue even with these considerations.

Many people have done your same calculation and have come to the same conclusion, however:

1) You are not factoring in the "dead" volume of air between the piston head and the tip of the nozzle (when extended) where it touches the BB.

2) You are also assuming a perfect system with zero air loss due to air seal inefficiencies. A lot of air is lost due to such inefficiency. Losses occur around the piston head, the cylinder head, the connection between the cylinder head and air seal nozzle, between the nozzle and BB, and finally, air escaping around the BB as it travels through the barrel.

Your assumptions are only theoretical, but practice shows that in real world scenarios, bore up cylinders ARE required for barrel lengths that long.

[m102404]

Originally Posted by Zephyranthes

So I did some math calculations and I found some interesting results. I wanted to see if bore-up cylinders were really necessary for long barrels like thePSG-1 (650mm). The volume of a barrel using the equation for the volume of a cylinder V=pi*((d/2)^2)*l, where d is 6.05mm and l is 650mm. I found the volume to be 18685 cubic mm. I then took measurements of an AEG cylinder (check me if my measurements are wrong), and I found the diameter to be 24mm and the length to be 72mm. Using the same equation above, I found a cylinder's volume to be 32572 cubic mm. That means a type-0 no hole cylinder has 175% the volume of the longest barrel on the market. This makes me question why bore-ups are necessary, considering their purpose is to increase the cylinder's volume to fit the barrel. However, I understand the entire volume of the cylinder is not used because the piston head and cylinder head take up some room inside the cylinder. I also understand that aeg's are not 100% efficient. This still doesn't justify to me the need for a bore-up cylinder when it's volume is 75% more than the barrel. Cylinder volume should not be an issue even with these considerations.

Go for it then...and when you end up with a 200fps rifle, try a bore up kit.

[Blackthorne]

Also doesn't take into account the variables created by full auto fire.

[Wetmelon]

Though I feel a little bad about bumping the topic again, I am curious about an aspect of cylinder and nozzle design, and am hoping someone can set me straight.

I understand the ported/unported, volume matching, and loss characteristics of the cylinders discussed in this topic, but I was wondering why nozzles are not pressure sensitive. That is to say, when the piston comes forward, it ejects a bit of air through the port (if there is a port) to help acceleration and create a more aggressive pressure curve. If this is the reasoning, then why are nozzles not designed with a pressure-sensitive valve designed to snap open at the right time, dumping pressurized air faster than the piston is capable? Would that not create an even more aggressive curve (as pressure waves travel at the speed of sound) than letting the low-pressure air push the BB down the barrel for the first couple of inches? Or would you have pressure loss around the BB greater than the effect of a slower acceleration as it tries to overcome inertia?

Thanks for your time,
Melon

[coach]

The mechanics and engineering aspect of getting something reliable would cost an arm and a leg to buy. Plus, having an extra moving part, inside the nozzle, would wreak havoc in your gun in the event it breaks.

Also, where would you put said valve? The nozzle already sits pretty deep on the cylinder head. Anything inside would be incredibly small and impossible to fix. How would it 'snap' closed? Spring loaded or a total redesign of something that works. What if it jams closed? You'd end up with a back pressure that could possible aide in stripping your piston and gears. How good can a valve be to work fast enough for full auto?

In the end, the amount of air lost is insignificant. IMO, you will lose more air leaking down the feed tube then you would gain.

[Wetmelon]

Good point(s). I guess there's a point at which reliability greatly outweighs the need for a slight performance increase. Besides, people have no trouble hitting field FPS caps anyway, so why add something when it would be useless?

[coach]

I'm not saying it's useless but at this point it's not practical. the costs of trying to perfect something reliable would exceed the potential gains. current cost for a reliable nozzle is < $10. I can't imagine how much someone would charge for a valved nozzle. I can see what you're thinking but just it's not feasible.

[THEMAKKER13]

I recently sent my MP5 into a local store for repair. I put a Madbull 6.03 363mm tbb in it, and so I asked about changing the cylinder to an M4 cylinder while they were in my mechbox, and the guys there said that they find the best results with Type 0 cylinders in all guns. So thats what I ended up putting in my gun. Ill let you guys know what the results are when I get my gun back in a couple weeks.