Why are all APUs turbines?
 

As far as I know, efficient turbine engines have never been developed for road vehicles. Around 45 years ago there was an experimental Rover turbine-powered car, but it never got beyond being an experiment experiment.

If turbine engines are so inefficient, especially at ground level, why are piston engines, whether spark or compression ignition, not employed as aircraft auxiliary power units?

I would have thought that paraffin was near enough Diesel to power a compression ignition engine, so fuel is probably not much of a problem.

So is the problem weight, installation complexity, engineering inertia, or something else?

Turbines are very efficient at delivering high power from a lightweight unit. They are especially efficient when they can be designed to deliver a relatively constant output. They are also relatively maintenance-free.

A diesel would be too heavy and maintenance-intensive, and would not work well airborne.

Power to weight ratio, fuel compatibility and of course bleed air output that is native to their operation.

Several of the aircraft I used to fly employed piston APU's; typically ranger V-2 motors with a direct drive generator.

Respect your elders
 

Did the aircraft designation start with a B and end with a number? :ooh:



and were there large doors fitted to the underside? :)

I believe that the heat generated by a piston engine would require extra drag for cooling. Also the simple fact of the usage of jet fuel simplifies a turbine/centrifugal compressor type APU simplifies turbine APU usage, supplying an alternate fuel source would be impracticable.

Not to mention the regulated air source for a piston engine that would require a supercharger for apu's that ate certified to operate at altitude. The other factor is creating a piston powered apu that could maintain the 400 hz required for the generator and reliable power to do so.

Both airbus and boeing do work on a fuel cell APU system as far as i know. Especially boeing is very interested in that approach with their newfangled idea of the "more electric" aircraft as there is no need for bleed air supply from the APU anymore.

No need for bleed air from the APU?

- how would you start the engines?
- how would one dry crank an engine in case of tail-pipe fire?
- how would one keep cool in all of those greek airports which doesnt even have ground power :P

I think the apu will live for a long long time... =)

There was a gent, in the '70s who was running around in a B25 with a Briggs and Stratton 4hp 2 stroke engine to supply electical power.

power weight ratio would be critical. You would be lugging a great big piston engine about the sky for the entire life of the airframe.

ability to start and run at high altitude would also be critical. A piston engine's power drops off rather fast at high altitudes. And probably could not run at all above 250 without supercharging, let alone providing any power offtake.

another killer is the separate fuel system required. Maybe piston APUs on piston aircraft made sense, but not on a jet aircraft. The US Navy did fly jets off carriers using avgas for a while, but I don't know with what results. Certainly not commercially sensible.

Thanks for starting the thread. I think we need to ask ourselves fundamental questions like this to get some clear thinking on the subject. Like, why don't we tow aircraft to the takeoff point instead of sitting around with engines running.?

Nope. PB4Y, P2V, DC-3 (an addition), C82, C97, etc.

For the APU, no. Just a small exhuast. For mission purposes of dropping things, yes.

An lot of aircraft don't have APU's to begin with, and don't need bleed to start, using electric starter-generators. Or separate starters and generators. So far as a tailpipe fire...if no bleed is required to start, lack of it from an APU has no bearing on continuing to crank during the start process.

Cooling drag is irrelevant for ground power, and many aircraft do not or cannot use their APU in flight. We could use our piston APU's in flight, with no additional drag penalties.

Applicable for those aircraft which use 400 Hz...which not all do, and for those which do, stable inverter output can be regulated as needed.

While some operators do use commercial generation, more than likely he was using the standard four stroke APU, which was generally a two cylinder carbureted piston motor with a direct drive generator.

Reliability.
Turbines are much more reliable than their reciprocating counterparts, especially at the lower operating temps found in most APUs.

Power to weight has t obe another biggie, as well as providing its own compresor for the airstart case.

All true in conventional aircraft, but no longer true in Boeings future scheme of things starting with the 787. "More electrical" airplanes, as Boeing calls it, do not need bleed air for all that. The engines are started electrical with their starter generators (787 two 250kVA per engine), not with bleed air anymore, which eradicates the first two problems. And the packs are driven electricly as well which helps keeping your head cool.

Granted, in existing planes we will have turbine APUs for a long time come, even in the 787 we will have it for quite some time (driving two 225kVA starter/generators), but in other future projects or even as a replacement packs fuel cells are quite possible.

As Boeing is doing with the 787, all that can be done with electricity...

"...driven at LeMans by Graham Hill and newcomer Jackie Stewart, was 10th overall at 99mph (13.5mpg) in spite of engine damage."

That's 1965 - doesn't sound that inefficient.

Bit of a red herring, most diesel engines will run quite happily on Jet A1.

ButThat is the big problem with a diesel piston.

Even on the ground Truck mounted Air starters for jet engines have changed from Diesel engines to turbines. The equivalent turbine is much smaller and compact than the diesel.

We had a piston APU in our DC-3, it provided electrical power for lighting, avionics and the air conditioners. We had window style air conditioners mounted in the foward and aft cabin bulkheads.

To be honest, I don't believe these air conditioners would be approved for aircraft use in this day and age. But thirty years ago you get away with a lot of things that are strictly forbidden today.

The majority of small jets do not have APUs, some have been retro fitted to have APUs installed and some newer versions of the older jets now have APUs as options. However, for the most part they are only for comfort during ground operations and are not required for normal aircraft operations.

Like everyone has written here, it's mainly have to do with the fuel compatibility and power to weight ratio and ease of maintenance. A 744 APU provides around 1500++ horsepower if I can remember and the APU is still smaller than a heavy duty truck's engine which provides half of that horsepower.

I didn't think modern diesels would run on Jet A1, as it doesn't meet the lubricity requirements of the higher pressure pumps?

I think older 'clockwork' diesels would be ok though.

When I stick it in my '02 BMW I don't go above a 25% A1/ 75% diesel mix and it runs fine! :)