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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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TayE
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    The APU also provides bleed air and/or hydraulic pressure. – mins Dec 03 '17 at 00:13
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    And why the main engines are turbines instead of piston engines? The answer is same. – Agent_L Dec 03 '17 at 09:58
  • the title says "Why isn't the APU a standard diesel generator?" but the text says "So why not use a cheaper piston engine as opposed to a jet turbine?". which one is it? as there is a big difference between these two questions. – JOW Dec 03 '17 at 15:51
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    @JOW: The question is perfectly understandable to me, and has been answered. – mins Dec 03 '17 at 20:49
  • @JOW Because "piston engine" includes diesels, while "jet" or "turbine" excludes both. – Agent_L Dec 04 '17 at 14:07
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    Try to restart a diesel generator at 30,000ft. There isn't enough oxygen there. – abelenky Jun 13 '18 at 13:29
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    @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). – Vikki Jun 13 '18 at 15:06

4 Answers4

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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
Koyovis
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    The APU is also a pneumatic source for running the packs and starting the engines, which favors the turbine as well. – casey Dec 02 '17 at 17:18
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    ..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;) – Martin James Dec 03 '17 at 06:08
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    @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. – Koyovis Dec 03 '17 at 06:59
  • Would freezing temperature be another reason to avoid diesel? – Hugh Dec 03 '17 at 07:42
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    @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 - http://papers.sae.org/2004-01-1858/ etc – quetzalcoatl Dec 03 '17 at 08:46
  • @Hugh Diesel gels at about -10°C, Jet A freezes at around -40°C, Jet B around -60°C. Feed jet fuel to a diesel engine and there shall be no gelling... – Koyovis Dec 03 '17 at 14:56
  • Even if you compare a diesel generator with no enclosure, you get 0.05 kW/lb, compared to 0.25 kW/lb for the turbine. – fooot Dec 03 '17 at 20:02
  • Jet-A will blow a diesel engine if it cannot tolerate the additional stress of higher octane combustion. This can be fixed with bulking additives mixed into the fuel line.

    You have several thousand pound foot of forced induction already from flight itself, so you can design a system that will intake far more air and output far more power in-flight, but any system not built for it will go bang pretty fast. Off the shelf APU's are designed for use at rest, and pointing their intake out of a plane window is probably none-too clever.

     – user1901982 Dec 04 '17 at 00:00
  • @user1901982 simple, low cost modifications to a diesel engine would make it perfectly capable of running on jet-A. It still wouldn;t be a good APU. – Chris H Dec 04 '17 at 09:26
  • Late comment: All modern military tank and most other AFV diesels are designed to run smoothly on Jet-A, specifically for fuel commonality reasons. Some military diesels can even run on gasoline. Fuel is not a problem at all for diesels. But they're still heavier than turbines. – Therac Jun 19 '19 at 18:14
  • @Therac not for turbines either. – Koyovis Jun 19 '19 at 23:22
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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.

Romeo_4808N
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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.

Peter Kämpf
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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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tj1000
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