Engines and pressures and loads more...
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Engines and pressures and loads more...
Ok, got another question for you in the know...
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.
Oh oh oh, almost forgot, I know on most airliners (spec 757 cos that’s what I remember) one has the option to activate the ignitors(?), so that they effectively ignite at pre-set intervals for extreme conditions (jolly chilly days and high water intake I guess), 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.
Once again, thanks for any replies, you’re a great bunch.
w
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.
Oh oh oh, almost forgot, I know on most airliners (spec 757 cos that’s what I remember) one has the option to activate the ignitors(?), so that they effectively ignite at pre-set intervals for extreme conditions (jolly chilly days and high water intake I guess), 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.
Once again, thanks for any replies, you’re a great bunch.
w
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"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.
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.
Present day compressors have pressure ratios over 25:1. With the addition of a fan pressure ratios of more than 25:1 are achived.
Compressor pressure ratio is available from manufacturer data.
For instance CFM56-5A3 pressure ratio=28; for an ambient inlet pressure of 14.7 psia the final pressure is 147x28=411 psia.
The pressure ratio will increase and decrease with engine speed. As inlet temperature increases, the compression ratio will tend to decrease due to the combined effects of air density decrease and the temperature effect on the angle of attack on the blades.
Regards
Compressor pressure ratio is available from manufacturer data.
For instance CFM56-5A3 pressure ratio=28; for an ambient inlet pressure of 14.7 psia the final pressure is 147x28=411 psia.
The pressure ratio will increase and decrease with engine speed. As inlet temperature increases, the compression ratio will tend to decrease due to the combined effects of air density decrease and the temperature effect on the angle of attack on the blades.
Regards
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The 767 with the C6-80C2 Engines which have FADEC control has a combustion chamber pressure limiting function internally within the Electronic Engine Control (EEC) Unit which limits the Combustor to 430 psi relative to under-cowl pressure.
This is achieved by a sensing line from the combustor area providing a pressure signal to a transducer in the EEC.
The EEC will then down-trim fuel scheduling to prevent overpressure conditions occuring.
With "AUTO" selected on the Eng Start Panel, this will enable the Ingitors to operate automatically when either the leading edge slats are not fully retracted or if Eng Thermal Anti Ice (TAI) is selected on.
This is achieved by a sensing line from the combustor area providing a pressure signal to a transducer in the EEC.
The EEC will then down-trim fuel scheduling to prevent overpressure conditions occuring.
With "AUTO" selected on the Eng Start Panel, this will enable the Ingitors to operate automatically when either the leading edge slats are not fully retracted or if Eng Thermal Anti Ice (TAI) is selected on.
