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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 Answers 2

up vote 14 down vote accepted

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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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. –  kayson Sep 2 at 19:01

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.

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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? –  Skip Miller Sep 2 at 14:40
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@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. –  Manu H Sep 2 at 15:01
    
@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. –  RedGrittyBrick Sep 2 at 15:14
    
even on the CFM56 commercial presentation I cannot find consumption information –  Manu H Sep 2 at 15:35

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