"What sort of pressures are reached within the combustion chambers of a typical modern jet airliner (say the RB 211 on the 75 or CFM 56 on the A320, etc). AND, what are the differences between max power and idle. "
G'day "W",
I have a diagram showing the pressure, temperature and velocity changes at various points in an RB211 for what appears to be a fixed rpm, but unfortunately it doesn't specify what rpm it is. However, at this particular rpm, the combustion chamber pressure is shown at somewhere between 410 and 420 psia (1psi = 6.895kPa)
"can any of you lucky 75, 76 captains etc enlighten me as to their exact function, do they work like I think they do and under what circumstances the would be used."
Ignitors are simply high intensity spark plugs which ignite the fuel in the combustion chamber. These are normally controlled by aircraft automatics. During start, when you put the fuel levers to RUN, the ignitors fire up i.e. until the engine reaches a certain rpm
(de-energizing the start switch), at which point the ignitors are shut off automatically (The flame in the combustion chamber is kept alight with the continuous supply of fuel and air just as a gas stove keeps running after you have fired it up with a match). However, large chunks of melting ice and heavy rain can put a dampner on this continuous combustion process. Switching on the ignitors manually by pushing the CONtinous IGNition switch on the overhead panel simply _helps_ keep the flame alight. Some airlines select CON IGN during critical phases such as takeoff and landing as an additional safety precaution (even though airplane automatics will usually keep the ignitors zapping when the flaps (e.g 747) or slats (e.g 767) are deployed or if the Nacelle Anti-Ice is activated. More sophisticated aircraft may bypass these cues completely and simply keep an eye on engine acceleration: A rapid deceleration indicates that the engine is flaming out and the EEC's (Engine Controllers) will activate the ignitors automatically.
Anyway, hope this helps.
Rgds.
Q.