[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