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I am interested in understanding how electric power is generated in Jet engines. If a pilot continuously keeps altering the throttle to suit the flight requirements, how does the jet engine maintain constant torque to the generators, given the load on the generators is relatively stable? I thought of three potential solutions

  1. Install a set of gears along the main shaft to handle the torque differential.
  2. Feed output of few combustion chambers to a separate small turbine connected to a generator. This arrangement would allow controller to independently control fuel to these combustion chambers ensuring there output is optimal for feeding to generators.
  3. Use bleed air from engines and drive some wind turbine. I feel this option very unlikely.

Please share some schematics as well. My intention is to just learn and I am an amateur, so feel to correct my assumptions

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    $\begingroup$ Given, pilot continuously keeps altering the throttle to suit the flight requirements, how does jet engine maintain constant torque to the generators given the load on the generators is relatively stable. I don't see why you feel this is a problem. Cars have the same "problem" (and throttle use is way more variable than in a plane) and it is solved by a simple voltage regulator $\endgroup$ Aug 12, 2022 at 13:29
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    $\begingroup$ Are you asking about the engine specifically? Or a jet aircraft as a whole? Because you ask about the engine but then talk about pilot inputs. On some aircraft they have a little dedicated turbine (or even a piston engine) just for the power generation, not propulsion: the auxilloary power unit (APU). $\endgroup$
    – DKNguyen
    Aug 12, 2022 at 18:49
  • $\begingroup$ But the APU is turned off once the main engines are running typically. Simply to save fuel. $\endgroup$
    – U. Windl
    Aug 13, 2022 at 0:31

4 Answers 4

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All jets and turbofans run a gearbox driven off the outer spool shaft. The gearbox has drive pads for all of the powered accessories like hydraulic pumps, fuel pumps, and the generator, and an input drive pad for the starter. The engine runs at whatever speed it wants to run at as set by the pilot, and the demands from the services driven off the gearbox just add to the load it has to deal with. In the end, if the electrical load goes up, it just means the fuel controller has to dump more fuel in the burner to maintain the speed it's being told to maintain.

The generators on most modern jets are normally alternating current 3 phase 110 volt, at a constant frequency of 400 Hz. They will use a variable input gear drive that holds the generator at an RPM that gives a steady 400 Hz frequency output, regardless of the engine input speed, which is required by all the electronics. These are called Integrated Drive Generators.

You may also see variable frequency AC generators that just run at whatever speed the engine runs at with AC frequency changing with speed. You can't use variable frequency AC for avionics, so those types of generators may only power things like heaters and other frequency insensitive services, and the fixed frequency AC will be provided by static inverters that run off DC power, provided by a separate DC generator.

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  • $\begingroup$ Thanks John. Also are there any engines with dedicated combustion Chambers feeding to a dedicated gas turbine and generator. I am just trying to understand the reason for going with gearbox. $\endgroup$ Aug 12, 2022 at 4:41
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    $\begingroup$ The idea of "dedicated combustion chamber" is more or less what an APU is; their fuel control can adjust the fuel to keep the RPM steady, thus avoiding the need for constant-speed drive and gearings. As for separate combustion chambers within an engine, no, that isn't used AFAIK. $\endgroup$
    – Ralph J
    Aug 12, 2022 at 4:53
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    $\begingroup$ The gearbox is because you need to power a number of services. It's not a gear reduction per se like a car, just a gear train that distributes the input power to a sequence of output gears at the mounting pads for the different components. Just google "jet engine accessory gearbox". $\endgroup$
    – John K
    Aug 12, 2022 at 4:58
  • $\begingroup$ Is one of the approaches you mentioned (using a gearbox to produce a preset frequency vs. just generating at the engine's frequency and then generating fixed frequency using electronics) newer than the other? I imagine the latter costs more, but saves some weight, so might be preferred in newer designs? Or does it just depend on the engine (or aircraft) manufacturer? $\endgroup$
    – rob74
    Aug 12, 2022 at 21:17
  • $\begingroup$ @rob74 once the power electronics for ACFW had matured commercially, it wasn't only lighter than a gearbox, but much cheaper too - replace all that high grade rotating stuff with stuff that doesn't move. $\endgroup$
    – Neil_UK
    Aug 14, 2022 at 6:07
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There are in fact at least three (I say that because I know of 3, and there could be more) different methods used. So to say “all aircraft” is not quite right. Lets start with what I think is the single most important equation, Faraday’s Law (simplified): $$ \mathcal{E}=-N\frac{\mathrm{d}{BAcos{\omega t}}}{\mathrm{d}t}\ $$ This tells us there are a number of ways to produce a given output EMF ($ \mathcal{E} $). The two that are relevant for this discussion are B and $ \omega $. Note $ \omega $ is the angular frequency, which can be scaled to RPM.

GA aircraft, including biz jets, will likely be using a DC generator or a DC alternator. In a DC system, given these are small light aircraft, we actually control B in Faraday’s Law. That is, if the RPM increases, then we reduce the magnetic field strength inside the generator. This results in a balanced output. The generator control unit (GCU) is responsible for doing this. Effectively, it is continuously measuring the output of the generator (or DC alternator) and as the output voltage falls, the current provided to the field windings (the coils or wire that produce the magnetic field, which is an electromagnet) is increased. Similarly, if the output goes high, the current to the field windings is decreased. The circuit is shown below [1].

Regulation of generator voltage by field rheostat

For large transport category aircraft, there are two options. These are the legacy systems (as answered by others) and the modern more electric aircraft solution, used in the B787 and the A350 (and I am sure on modern variants of older airframes). As previously mentioned, for legacy large transport category aircraft, the part of Faraday’s law that is controlled is the RPM. This cannot be done inside the AC alternator, so it needs to be done before it. As mentioned, the constant speed drive unit (CDU) is combined with the AC alternator to give an integrated drive generator (IDG). The CDU is a hydrostatic transmission system. In this, the input rotation pumps a hydraulic fluid, which depending on the input RPM will increase and decrease in pressure. The GCU now actuates a wobble plate in the transmission systems which ensures the output hydraulic fluid pressure is constant. This constant pressure fluid is then given to the output “turbine” or hydraulic drive, which rotates the output at a constant rate. This gives a constant RPM for the alternator. A figure is show below, although not super helpful [1].

A hydraulic constant speed drive for an AC alternator

For modern large transport aircraft, none of the above happens. These are called ACFW systems, that is AC (alternative current) frequency wild systems. Here, we let the starter generator (which still does have a GCU, so there is an operating band that needs to be maintained) rotation change with the RPM change of the engine. This generates wildly different AC frequencies at different times (engine RPMs). This is then immediately converted into DC. For this to be utilised in systems that require AC current, the typically 3 phase 115V/120V 400Hz signal, then static inverters are used to convert the DC signal into the require AC signal. In these, we effectively switch on and off a DC signal, give it to a transformer (and other conditioning circuitry) to make the required AC signal. The same kind of power system is utilised in the F-22 and F-35, for example.

References:

[1] FAA. (2018). Aviation Maintenance Technician Handbook - Airframe Volume 1. Federal Aviation Administration. PDF (194 MB)

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  • $\begingroup$ "3 phase 115V/120V 400Hz": Do you mean "115V/200V"? Variable frequency current from ACFW is also used as-is (e.g. for heaters: ovens, deicing), but ACFW aircraft are not common (A380, B787). More, including this picture. $\endgroup$
    – mins
    Sep 4, 2022 at 16:32
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It wasn't exactly a wind turbine, but the older model C-130's had an "Air Turbine Motor" which did exactly what the OP's 3rd point suggests, extracting the energy from bleed air to power a generator.

I'm familiar with the E model, which was produced from 1962-1972, and the 1973 H-model (also called the Super-E, and distinct from the 1974 -H1 model). The H1 (1974), H2 (1980's), H3 (1990s) and J model (current) don't use an ATM, and have an APU that can be started and run in flight. The E model birds had a "Gas Turbine Compressor" or GTC, which was like an APU but which couldn't be run in flight. It didn't have a generator, but produced bleed air for the ATM to use. The ATM could also use bleed air from the engines, so it was a viable backup generator in flight, even though the GTC wasn't available then.

This arrangement isn't common, but Lockheed evidently liked it well enough to use it for a decade or more; I can't speak to what the Herks older than the E model used in this regard, though I wouldn't be surprised if the A and B model birds also had GTC's and ATM's too.

John's answer describes how the engine-driven generators on the Herk (and nearly everything else) worked, which is by far the most common solution. But yes, bleed air certainly can be used to power a generator.

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Many aircraft engines has an axillary gear drive shaft to a hydraulic constant speed drive or CSD. The CSD input can be a fairly wide range of RPM. the output is constant. It turns a generator at a constant speed resulting in a constant frequency of electrical output. The AC on most aircraft is 400 HZ.

It is common to have more that one generator on multiengine aircraft. The generators outputs are bussed together. a frequency / load controller will make small adjustment to the CSD to balance the load across the generators.

Older aircraft often have pneumatically driven generators that used bleed air from the engines.

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