Turbine Throttle
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Turbine Throttle
Hey,
Just a quick question regarding how the throttle controls a jet turbine. I've searched around a fair bit but had no luck so far.
I understand that it must be controlled by one (or both) of the two variables - air and fuel. My guess is fuel, as air would be rather hard to handle. So how is the fuel injection rate controlled? In piston engines it is often pulsed (EFI at least anyway), but i doubt this would work quite so well for continuous ignition. I thought about varying pressure to the fuel delivery nozzle also, but (going off a garden hose example) wouldn't this vary how the fuel is delivered (ie shape of the spray cone)?
Any help much appreciated!
Cheers,
El`C
Just a quick question regarding how the throttle controls a jet turbine. I've searched around a fair bit but had no luck so far.
I understand that it must be controlled by one (or both) of the two variables - air and fuel. My guess is fuel, as air would be rather hard to handle. So how is the fuel injection rate controlled? In piston engines it is often pulsed (EFI at least anyway), but i doubt this would work quite so well for continuous ignition. I thought about varying pressure to the fuel delivery nozzle also, but (going off a garden hose example) wouldn't this vary how the fuel is delivered (ie shape of the spray cone)?
Any help much appreciated!
Cheers,
El`C
Join Date: Jan 2001
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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.
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.
Join Date: Feb 2005
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tom's basically right as far as he goes...
Whether FADEC or hydromechanical control, there are a few things the engine requires:
1) At any given instant, there's a maximum fuel flow limit the engine can stand. This is determined by an acceleration flow schedule. It's determined by pressure (usually compressor discharge pressure), temperature (usually at the HP compressor inlet), and RPM. If this limit is exceeded, a stall or surge or too-rich blowout of the burner is likely.
2) Also at any given instant, there's a minimum limit, to prevent too-lean blowout (flameout) of the combustor.
3) Fuel flow to hold constant RPM lies somewhere in between 1) & 2). A governor (linked to the throttle/thrust lever) trims fuel flow until RPM stabilizes at the desired speed. This is one point on the "steady-state line".
When the throttle advances, the governor senses an underspeed, calls for more petrol, and fuel flow rises until it reaches 1). As RPM reaches the new demand point, the governor backs off fuel to re-stabilize RPM. This is a new point on the "steady-state line".
During a decel, fuel flow can only drop to the min. limit in 2).
"Smart" controls like FADEC can impose other limits as well - like turbine temperature or pressure ratio.
Airflow control is another whole different matter - generally it's not directly linked to the throttle, though.
Whether FADEC or hydromechanical control, there are a few things the engine requires:
1) At any given instant, there's a maximum fuel flow limit the engine can stand. This is determined by an acceleration flow schedule. It's determined by pressure (usually compressor discharge pressure), temperature (usually at the HP compressor inlet), and RPM. If this limit is exceeded, a stall or surge or too-rich blowout of the burner is likely.
2) Also at any given instant, there's a minimum limit, to prevent too-lean blowout (flameout) of the combustor.
3) Fuel flow to hold constant RPM lies somewhere in between 1) & 2). A governor (linked to the throttle/thrust lever) trims fuel flow until RPM stabilizes at the desired speed. This is one point on the "steady-state line".
When the throttle advances, the governor senses an underspeed, calls for more petrol, and fuel flow rises until it reaches 1). As RPM reaches the new demand point, the governor backs off fuel to re-stabilize RPM. This is a new point on the "steady-state line".
During a decel, fuel flow can only drop to the min. limit in 2).
"Smart" controls like FADEC can impose other limits as well - like turbine temperature or pressure ratio.
Airflow control is another whole different matter - generally it's not directly linked to the throttle, though.
