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Why isn't the APU a standard diesel generator? Jet-A and diesel are interchangeable (with the addition of a lubricant). So why not use a cheaper piston engine as opposed to a jet turbine?

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V.S. enter image description here

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    $\begingroup$ The APU also provides bleed air and/or hydraulic pressure. $\endgroup$ – mins Dec 3 '17 at 0:13
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    $\begingroup$ And why the main engines are turbines instead of piston engines? The answer is same. $\endgroup$ – Agent_L Dec 3 '17 at 9:58
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    $\begingroup$ @JOW: The question is perfectly understandable to me, and has been answered. $\endgroup$ – mins Dec 3 '17 at 20:49
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    $\begingroup$ Try to restart a diesel generator at 30,000ft. There isn't enough oxygen there. $\endgroup$ – abelenky Jun 13 '18 at 13:29
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    $\begingroup$ @Agent_L: In fairness, one could build a turbine engine that ran on diesel, which would then be a turbine diesel engine. Not a turbine Diesel engine (as it wouldn't be using the Diesel cycle), but a turbine diesel engine (as it would be burning diesel). $\endgroup$ – Sean Jun 13 '18 at 15:06
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Weight.

Piston APUs for trucks are designed for frugal and quiet operation. This one produces 5.2 kW electrical power and weighs 375 lbs.

The APU for the A320 and B737 is a noisy screaming unit that produces 90 kW electrical power, 445 shaft kW. It weighs 375 lbs as well.

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The main difference is in the weight of the engine itself. The turbine engine of the A320/B737 APU produces 445 shaft kW and weighs 145 kg. This diesel engine also produces 450 shaft kW and weighs 1542 kg.

Power-to-weight ratios:

  • The turbine produces 3.1 kW/kg engine weight.
  • The diesel engine produces 0.3 kW/kg engine weight, it is a standard industrial machine and not optimised for weight.
  • The Rolls Royce Merlin was very much optimised for weight, it used aviation fuel (gasoline) and produced 1.58 kW/kg
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    $\begingroup$ The APU is also a pneumatic source for running the packs and starting the engines, which favors the turbine as well. $\endgroup$ – casey Dec 2 '17 at 17:18
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    $\begingroup$ ..and, if an off-the-shelf diesel engine was used, it would require diesel fuel, rather than aviation kerosene. Then you have an exra fuel tank and an extra refuelling operation. I'm sure that a diesel could be designed to run on the jet fuel, (after all, both diesels and gas turbines are compression-ignition), but off-the-shelf mass-production engines are cheap, a special 'APU diesel' would not be:( 'Jet-A and diesel are interchangeable' - not according to my source;) $\endgroup$ – Martin James Dec 3 '17 at 6:08
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    $\begingroup$ @MartinJames There was a bloke in Germany who drove his diesel car on cooking oil he collected from snack bars. The car wafted smells of fried chips when he drove, he just needed to filter out some of the crusty bits. And anecdotal evidence exists of airport employees who drive their diesel cars on jet A1 no problem. The old diesel engines were not very particular it seems, don't know about the high performance modern ones though. $\endgroup$ – Koyovis Dec 3 '17 at 6:59
  • $\begingroup$ Would freezing temperature be another reason to avoid diesel? $\endgroup$ – Hugh Dec 3 '17 at 7:42
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    $\begingroup$ @Koyovis: not just one guy. For example, in EMEA, a rapeseed oil is very popular in gastronomy and double-use it (and similar) as a fuel is really seriously researched - papers.sae.org/2004-01-1858 etc $\endgroup$ – quetzalcoatl Dec 3 '17 at 8:46
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Three words: They’re much heavier. Gas turbines have a very high power to weight ratio As opposed to reciprocating diesel engines. So for the needs of an airplane which requires large amounts of electrical and hydraulic power in order to operate, a gas turbine makes a lot of sense for use as an APU.

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    $\begingroup$ That's four words. $\endgroup$ – Transistor Jun 13 '18 at 17:47
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You need to look at the overall picture. Yes, a diesel is cheaper, but it will weigh more and be less reliable. Also, the operating time of an APU is rather a small fraction of total airframe operating time, so it will mostly be a dead weight.

To arrive at a number representative of overall cost, add to the acquisition cost all the maintenance and all the fuel needed, not only for operation, but also for lugging the APU around. If you need a handle on the weight-induced amount of fuel, use the Breguet equation. Here is an answer where it is used to calculate the extra fuel needed to have windows in the fuselage, and here it is used for a partially loaded A320.

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A primary reason, aside from weight considerations, is that a gas turbine APU has a built in air compressor - the compressor stage of the turbine. As such, it can supply air under pressure, or bleed air, just as the main engines can.

Bleed air is used for several purposes on an airliner. It is used not only to pressurize the cabin while in flight, but also serves as an air conditioner or heater, as the climate dictates. The APU can operate the climate control system on the airliner when the main engines aren't running.

Bleed air is also used to start the main engines. One running engine can start all of the others - the main engines have a small turbine that uses the pressurized bleed air to spin the main turbine up to starting speed. Or... the APU can also supply that bleed air. It is not uncommon to use bleed air from the APU to start the main engines, such as when an airliner has shut down its engines while waiting on the taxiway due to extended delays.

In some cases, APU bleed air may be the only option: BA flight 9 used APU bleed air to restart its engines, when all four had been disabled by volcanic ash.

So one reason gas turbine APUs are used on airliners is that they can supply the bleed air that is used for several purposes on the airliner, when the main engines are not running.

A diesel generator would have to power a separate air compressor to do the same, and that's more weight, and another thing to break or need service.

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