ALL Closed loop systems require some form of power input in order to function.
A self contained closd loop is IMPOSSIBLE.
Perptetual motion does not exist !!

Edited June 25, 20204 yr by Doug72

Not to revive a very old thread ... but a closed system where all the air is never exchanged with the outside world is not realistic. Those pneumatic switches and other connections have leaks, and as such, will run out after a 'long' while. Now, if such leaks are extremely small, and provided the compressor's motor is powered externally by a DC motor, then yes, a closed-loop system is possible. In fact, this is how Air COnditioners work. The fluid in the loop is some Freon (or something similar), the gas is constantly cooled/heated (depending on where it is in the AC's loop), the leaks are extremely minute, and the system runs continuously WITH the electrical motor running the compressor's motor.

Hey there DrJB,

Not at all, I appreciate the further input. As I think I mentioned earlier in the thread, I probably misnamed the title of the thread that has made people misinterpret my question.

Perhaps a better name for what I have in mind is a moderately "self-sustaining" system with use of different subsystems. Those subsystems would include 1) tanks of compressed air, 2) two pneumatically-powered triple expansion engines, 3) four to six pneumatic electric generators (each using a PF XL motor), and 4) two Lego-built compressors. If we were to combine any excess air leftover from the pneumatic engine/generators AND  surrounding air as well, would such a system be able to keep up with itself, more specifically regarding electric output and input?

Looking up a bit of statistics, it looks like the Power Functions Large motor uses about 80 mA of power on no-load (see here), and the PF XL motors use 120 mA. In videos it seems that they output only 1/2 to maybe 2/3rds that amount of power if used as generators. Does anyone have any proper stats on what kind of power these motors can provide?

Thanks again for everyone's replies thus far, and I apologize that I am such a dork mechanically and terminologically speaking.