Complicated thinking: Why match cylinder porting and barrel length?

[MadMax]

I got a PM with a pretty good question that I've pondered for some time and come to some explaination for myself, but never shared. I may be right, I may be wrong. I don't have enough emperical observation to back me up, but what the heck. I did a lot of writing to answer his question thoroughly to the best of my understanding so I might as well share here and generate some interesting technical neuron work.

Simplified Questions: Why does TM use ported cylinders apparently matched to barrel lenght? If "barrel suck" is an issue, why not always use unported cylinders? Why do most stock AEGs perform similarly (FPS) even though their barrels are different length?

Setup:
When a mechbox cycles, the motor does the same amount of mechanical work irregardless of the presence of a bb or the cylinder arrangement used assuming that the mainspring force profile remains constant i.e. a full compression cycle of the spring always contains the same amount of potential energy when it's fully compressed.

However, the amount of energy transferred to the bb can be significantly affected by numerous factors. I surmise the following:

A cylinder matched to the barrel:

The piston travels forward pushing air ahead of it. If the piston head is behind the window cut into the cylinder, no air compression is acheived and the air blows out. Since no compression is acheived and the hole is faily large, little work is done in this first phase since the air blows out with little resistance.

Because no significant work is done pushing vented air, most of the potential energy in the spring must be converted to kinetic energy in moving parts up to that point. This means that the piston must be accellerating rapidly and front of the mainspring must be similarly accelerating. Since these two parts are the only two moving parts at this point, their kinetic energy should account for most of the change in potential energy change in the mainspring (conservation of energy).

When the window is finally passed by the piston head, air is trapped and compression begins. Because the piston and spring have absorbed nearly all of the potential energy change in the spring up to this point(actually the centre of gravity of the spring is probably moving about 1/2 the speed of the piston at all times), they're moving pretty quick which means that cylinder pressure should be rising pretty fast. With increased cylinder pressure, an increased backpressure force slows down the piston and spring i.e. kinetic energy is transferring from the piston/spring to compressing air and accelerating the bb (a further fraction transferred to the pellet).

A measured quantity of air is compressed rapidly and accelerates the bb down the barrel.

Unported cylinder and a short barrel:

As soon as the piston begins to move forward, air pressure starts to rise and starts the bb on it's trip down the barrel. Because the cross sectional area of the cylinder is so much larger than the barrel ID, the piston would be moving significantly slower than the bb. As the piston moves forward, air blows the bb down the barrel.

At some point, the bb leaves the barrel and there is no restriction slowing the piston. If the barrel is very short, the piston is still a fair ways from the cylinder so the remaining spring potential energy is converted to kinetic energy in accellerating the piston and front of the spring.

There is no bb present so the remaining potential energy is spent further accelerating the piston/spring which is eventually dissapated in the mechbox when the high energy piston/spring slams into the mechbox.

Conclusimifications:

I am guessing that a matched cylinder/barrel is more efficient than an over long cylinder (or short barrel) because the bb is given the opportunity to absorb kinetic energy from the ENTIRE piston travel. In the case of a matched cylinder, the energy from the initial quick acceleration (window blowby) of the piston is recovered when it rapidly compresses the air in front of the window and socks out the bb.

This energy recovery could be a good explaination why all stock TM AEGs seem to perform the same despite their significant barrel length differences. Notionally a longer barrel should give more time for acceleration. However a ported cylinder means that a shorter cylindered AEG would have a faster initial acceleration so it may perform similarly to a long barreled AEG.

Instead of: long barrel, more fps because of longer acceleration time. The actual situation may be: matched cylinder gives rise to: same stock spring = same energy despite different barrel diameter because of higher or lower acceleration magnitude.


I hope this at least muddies up the waters in an entertaining way.


--Carl

[Brakoo]

Thanks a lot for the explanation Carl. It does make perfect sense when you think of it.

[Daes]

So there could be significant damage to the mechbox with a unported cylinder and a short barrel? Would there also be a loss of FPS (shorter barrel not allowing the BB time to reach full exit velocity)? Does this mean carbine kits (with shorter inner barrels) for armalites could be potentially damaging mechboxes unless the cylinder is changed?

Thanks,
-Daes-

[MadMax]

Originally Posted by Daes

So there could be significant damage to the mechbox with a unported cylinder and a short barrel? Would there also be a loss of FPS (shorter barrel not allowing the BB time to reach full exit velocity)? Does this mean carbine kits (with shorter inner barrels) for armalites could be potentially damaging mechboxes unless the cylinder is changed?

Thanks,
-Daes-

That could be one problem with M16 cylinders pushing M4 barrels. I can't say without making measurements somehow. As it happens I'm working on a piston head of my own design. A prospective new product for AI. I'm slogging through impact measurement trials at the moment. One day when the damn thing is finished and proven, I could use the impact test apparatus to see if cylinder matching greatly affects mechbox impact.

In the meantime, do not dryfire your mechbox. I can show that dryfire imposes a peak impact force about 60% higher than when firing a 0.2g bb. The situation is even worse when you dryfire test your mechbox without a barrel. Even the barrel can produce a measureable reduction in piston impact.

[Daes]

Originally Posted by MadMax

That could be one problem with M16 cylinders pushing M4 barrels. I can't say without making measurements somehow. As it happens I'm working on a piston head of my own design. A prospective new product for AI. I'm slogging through impact measurement trials at the moment. One day when the damn thing is finished and proven, I could use the impact test apparatus to see if cylinder matching greatly affects mechbox impact.

In the meantime, do not dryfire your mechbox. I can show that dryfire imposes a peak impact force about 60% higher than when firing a 0.2g bb. The situation is even worse when you dryfire test your mechbox without a barrel. Even the barrel can produce a measureable reduction in piston impact.

You know, from what you are saying, if you want to make a quick project, create a barrel plug that will allow dry firing. Alot of people dry fire to see if a battery is working, if you have your plug in, like you should when you store or transport, a plug with an inner resistance tube could be cheap to make, and with the data presented, save the wear and tear on your gun...

Oh yeah, another test that I would like to see, related to this, is the impact difference of having an one-piece and a TM three-piece hopup. Does the quick takedown (armalite) TM hopup leak enough that it is causing more impact when shooting, and especially when dry firing...

=Daes=

[TheYellowDart]

TM barrel plugs are already designed to allow for dry firing. That's why they have small holes in them. They create enough pressure to simulate a bb in the barrel, but still allow air to escape so the pressure doesn't build up too high.

[MadMax]

Originally Posted by Daes

You know, from what you are saying, if you want to make a quick project, create a barrel plug that will allow dry firing. Alot of people dry fire to see if a battery is working, if you have your plug in, like you should when you store or transport, a plug with an inner resistance tube could be cheap to make, and with the data presented, save the wear and tear on your gun...

Oh yeah, another test that I would like to see, related to this, is the impact difference of having an one-piece and a TM three-piece hopup. Does the quick takedown (armalite) TM hopup leak enough that it is causing more impact when shooting, and especially when dry firing...

=Daes=

I'm not sure how effective the TM barrel plugs are effective in reducing mechbox impact. There isn't much of a good seal between the inner and outer barrel so there might not be much of a cushioning if the air can just rush between inner and outer barrels.

I have actually worked out a bit which can be pressed onto a mechbox nozzle so I can simulate a shot bb when testing a mechbox outside of an AEG (worst case mechbox impact). I also considered a barrel plug for field testing assembled AEGs.

Both items are deferred projects until the piston head testing is completed. I'm timesharing between this airsoft project and a hunk of lab equipment I'm working on so I can't really start any more design projects before none of them get anywhere.

If I didn't need to eat or pay rent, I'd stop retailling airsoft or adaptors so I could put nearly all of my time to getting my design projects done. Unfortunately it kind of defeats the purpose of inventing new stuff if you never take it to market.

[Maelstrom]

the description makes perfect sense in the way i thought of it as well. It's like turbocharged cars and their volumetric effeciency calculations to create high power with properly matched turbo's. After all 10PSI in and almost 10PSI out is more efficient and less power is lost due to excess exhaust heat in the chamber that reduces combustion temps.

[Primus]

Let's get you thinking a little more Carl With all that said what do you suspect happens (or not) when you upgrade the bits in the mechbox (stronger spring, spring guide, etc). Will these affect/distort the ratio of the cylinder port/barrel lenght?

What I'm getting at is if I upgrade basically everything in the mechbox should I be looking at a cylinder with different porting or is this not affected at all?

[CDN_Stalker]

Great stuff Carl, clears the air a bit (once you get by all the conplicated explanationisms.)

Here's a bit of my experience which proves Carl's theory: I have a TM MP5A5/SD "Frankengun" (Primus knows what it looks like....... M4 slither stock and SD front end) that has the basic upgrade of PDI 150% spring and metal bushings. Also have the Hurricane reinforced mechbox (but isn't a performance part, just figured I'd mention). It would shoot pretty damn consistantly (within +/- 2fps for test shots) at 352fps using .2g BBs without hop up. When I was about to install a bearing spring guide, I had Apoc's M733 sitting open on my workbench, and it had a full cylinder in it, compared to my ported MP5 cylinder. I swapped it over to see what would happen. Since my gun was shooting about 350fps, I figured the bearing spring guide should give me around 15fps higher. Once I had the gun back together, I tested it. All shots were sitting around 330fps!?!?

So a 229mm barrel with 150% spring and ported cylinder shot 350fps, but a 229mm barrel with 150% spring, full cylinder and new bearing spring guide shot 330fps. I put the ported cylinder back in, chronied my MP5 again and was shooting about 368fps, and with a bit of hop up, 364fps. So I was right about the spring guide adding 15fps, and was neat to see what the full cylinder did in my gun. Now that I've read Carl's awesome explanation, I can fully see why.

[Digital_Assasin]

What about a long barrel on a short barrel cylinder?

I ran it to the question in the process of upgrading from my TM G36C to the G36K (see sig). Here is I what I came up with after a lot of thinking and visualizing. (Unfortunately I didn't have any chrony tests.):

Since the compression cycle is starting later because of the port, the bb won't be receiving the proper volume of air compared to the barrel perhaps a slight drop in FPS. Also the piston will start moving back before the bb has left the barrel, so you will get some suck back on it.

I may be completely off. What do you guys think?

[Blastyman]

I been using a m4 cylinder with my .50 which has a psg-1 tightbore length barrel. Seems to work alright. Chronos about 415-420.

At first I put tape and stuff over the ports but the last time I had it apart I didn't bother.

Anyhow I have a full cylinder I plan on putting in it sometime I take the gearbox apart for maintenence but I not going to do it till then.

anyhow maybe I am getting the dreaded suckback but if I am I certainly don't see it. Don't want the gun shooting much hotter anyhow. hehe

[MadMax]

Originally Posted by Primus

Let's get you thinking a little more Carl With all that said what do you suspect happens (or not) when you upgrade the bits in the mechbox (stronger spring, spring guide, etc). Will these affect/distort the ratio of the cylinder port/barrel lenght?

What I'm getting at is if I upgrade basically everything in the mechbox should I be looking at a cylinder with different porting or is this not affected at all?

I would guess that a stiffer spring would change ideal matched cylinder.

My reasoning is that an upgrade which results in a higer final bb velocity with the same length barrel would have to apply a higher acceleration rate to the bb. This would require a higher pressure. Actually more like a pressure profile as the pressure probably varies throughout the bb travel.

A higher pressure profile would mean that they cylinder air would compress to a smaller volume and you'd have a higher leak rate around the pellet, nozzle, and nozzle-hop up junction. So if you previously had a perfectly matched cylinder and then you upgraded the spring, you'd have a short cylinder.

However, it's hard to say if you'd actually get barrel suck. Barrel suck is a pretty simple phenomenon if you think of it.

If the:

(cylinder volume - leakage losses) < barrel length

then you don't push enough air to get the bb out the barrel and the bb at some point starts to decellerate in the barrel as it pulls the air below 1atm.

However if the cyl' volume is significantly higher than the barrel volume and the piston head hits the cylinder before the bb leaves it doesn't necessarily mean you get barrel suck. The air pressure behind the bb is still higher than 1atm until the pellet runs down the barrel for awhile. If the pellet leaves just as the volume behind the pellet matches the cylinder volume (plus leakages) then you actually have ideal efficiency.

The actual piston and bb trip times are probably so fast that the idea that the piston would be pulled back by the mechbox before the pellet leaves probably doesn't happen.

Consider an AEG ROF of 900spm ~15shots per sec (0.067sec/shot)

If said AEG shoots at 330fps with a 340mm barrel (1.13')

assume average speed of 165fps (0 at beginning, 330 at end). This implies constant acceleration which is probably not the case, but probably not a bad approximation.

barrel trip time: 1.13ft/165ft/s = 0.00685sec

0.00685sec <<< 0.067/2 (0.0335sec)

I divide the piston cycle time in half because by the time the piston hits the cyl, half the motion cycle has passed. In one more half cycle, they piston will be withdrawn. However this is generous as the piston moves forwards a lot faster than it moves back which means I should allow more time for the sector to come around.

[mcguyver]

easiest way to measure "barrel suck" would be to measure velocity on single shot and full auto. there would be zero barrel suck on semi as the piston will stop at the end of it's compression stroke and the velocity difference between semi and full would have to be due to barrel suck, all other things being equal. you also brought up an interesting notion on dry firing. so what happens to guns where guys use low-caps and there is always dry firing at the end of each mag (10-15% depending on mag capacity). do these guys put undue stress on their guns when you couple this with the dy firing that occures during testing and weapons clearing. i seriously doubt it. and does the presence of the bb in the chamber, considering losses around the nozzle/cylinder head junction, nozzle/bb junction. bb/barrel gap and around the hop bucking and chamber apply any measurable resistance to the piston in it's stroke? the mass and velocity difference between the piston in motion vs. a bb at rest are so phenominally different not to mention the discharging air at higher pressure in the barrel "feeding back" energy to a larger, lower pressure area like the cylinder, the laws of physics just don't allow for that hypothesis. the only resistance to be offered by this system would be if the hop-up made a very good seal and the bb required significant force to push its way thru the hop bucking and travel down the barrel. so in essence the back-pressure from bb to piston could only come from the hop system and no place else. so depending how good you hop-up is will determine pressures.

[midgetspy]

Interesting thread Max... but irregardless isn't a word, dammit! ;-P

Nic