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?
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.
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.
- 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
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.
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.
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.