Because no one has answered yet, I will give it a shot….however I know little beyond ATPL theory on the subject, and that was a while back!!
Engine speed is controlled by amount of fuel injected into the engine, which in turn is controlled by pressure at the nozzles/injectors. In the past this was controlled by an FCU (fuel control unit) which was a mechanical device which included a fuel pump, speed control governor which sensed engine speed, a variable metering orifice (VMO) controlled by the thrust lever position and a few other devices e.g. pressure drop control orifice, pressure drop control governor, pressure drop control valve etc.etc. This would mechanically control pressure of fuel to the nozzles and thus amount of fuel burnt and therefore engine RPM. Modern engines use FADEC, full authority digital engine control or EEC (electronic engine control) similar to the ECU in a modern car. This uses essentially a computer system which has many data inputs on top of engine speed and thrust lever position which controls fuel pressure, and can offer advantages over mechanical control such as overboost protection etc.
Air flow is not controlled per say, however I believe in jet engines at low RPM the compressor produces too much air for the combustion section, therefore some is bled off through bleed valves. (nb compressed air is also bled off the engines in all/most stages of flight to provide compressed air for pneumatics e.g. aircond/pressurisation, de-icing etc...ignoring the new 787 eh!!) Being pedantic, there is no throttle system on a jet much like there is no throttle on a diesel engine.
Hope this helps a little, I will await corrections from people far more knowledgeable than me!
Cheers.
Last edited by tom775257; 11th Jul 2005 at 14:21.