I would like to know whether the APU of modern commercial airliners consumes more or less fuel than a jet engine. Of course, this will vary between aircraft types, so what comparisons in fuel usage are there on a modern jet such as a B737 or A320? What is the ratio between an engine hourly fuel consumption and the APU hourly consumption?
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2$\begingroup$ Nit-picky perhaps, but keep in mind that a typical APU IS a jet engine. It's just a smaller one. :-) $\endgroup$– LnafzigerCommented Apr 18, 2015 at 23:48
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$\begingroup$ @Lnafziger yet designed to produce only electrical and hydraulic energy, not kinetic energy as jet engines mounted underwing $\endgroup$– Manu HCommented Apr 20, 2015 at 9:22
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$\begingroup$ Absolutely! I would suggest rewording slightly so that it is more clear that you aren't saying that an APU is NOT a jet engine though. $\endgroup$– LnafzigerCommented Apr 20, 2015 at 21:18
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2 Answers
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To give a comparison with an engine at idle:
On the 737 NG, the APU can use 240 lb/hr when under load. It is rated at 90 kVA, same as the generator on each engine.
The engines can use about 850 lb/hr at idle, which from those figures is about 1/10 of fuel flow at full (takeoff) power. So the engines use about 4 times as much fuel (each) as the APU does to just provide power. Of course these values will vary based on ambient conditions and the load placed on the systems (like the bleed air system).
This is why if a plane has to stop away from the gate for a while, they will shut down the engines and run on the APU.
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3$\begingroup$ And remember that is idle engine power for an entire hour. In reality the engines will most likely be operating at some thrust and the fuel consumption is much larger. $\endgroup$– kevinCommented Sep 2, 2014 at 19:01
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$\begingroup$ With that fuel burn and output power. That translates to roughly 7% efficiency for the APU. $\endgroup$ Commented Jul 13, 2019 at 12:11
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$\begingroup$ @Bostwickenator: Not really, an APU at full-load also runs a compressor to provide pressurized air. $\endgroup$– user14897Commented Mar 11, 2021 at 18:54
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The APU consumes less fuel because it doesn't have to contribute thrust to the airframe.
Many APUs have the same generator capacity as a propulsion engine. But they don't produce thrust. They can run at an optimum RPM for the generator requirements.
A320
APU
Ground operations: 130 kg/hour 90 kVA
FL200: 65
FL300: 51
On a short flight each main engine might consume an average of 1260 kg/hour.
answered Sep 2, 2014 at 14:14
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$\begingroup$ A better comparison would be with the main engine idling and/or with sufficient RPM to produce 90 kVA. How much does the main engine consume with that as a load? $\endgroup$ Commented Sep 2, 2014 at 14:40
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1$\begingroup$ @SkipMiller That's true, depending on what you conisder. For normal on ground APU usage, you are right. To generate power in case of in-flight loss of engines (e.g. US Airways Flight 1549), one can always quibble. $\endgroup$– Manu HCommented Sep 2, 2014 at 15:01
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1$\begingroup$ @Skip: Interesting Q. I think it would be surprising if an engine primarily designed to produce thrust would generate electricity more efficiently than an engine specifically designed for that purpose. But the APU may be optimized instead to reduce weight I guess. $\endgroup$ Commented Sep 2, 2014 at 15:14
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$\begingroup$ even on the CFM56 commercial presentation I cannot find consumption information $\endgroup$– Manu HCommented Sep 2, 2014 at 15:35