Why are all APUs turbines?
Thread Starter
Joined: Feb 2008
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From: Stockport
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?
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?
Joined: May 2000
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From: Seattle
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.
A diesel would be too heavy and maintenance-intensive, and would not work well airborne.
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From: USA
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.
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.
Joined: Mar 2001
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From: I wouldn't know.
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.
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From: EU
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... =)
- 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... =)
Joined: Feb 2005
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From: India
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.?
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.?
Joined: Oct 2005
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From: USA
Did the aircraft designation start with a B and end with a number?
and were there large doors fitted to the underside?
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 you start the engines?
- how would one dry crank an engine in case of tail-pipe fire?
I believe that the heat generated by a piston engine would require extra drag for cooling.
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.
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.
Joined: Jul 2007
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From: Germany
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.
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.
Joined: Mar 2001
Posts: 4,563
Likes: 35
From: I wouldn't know.
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... =)
- 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... =)
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.
Joined: May 2000
Posts: 3,204
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From: Seattle
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... =)
- 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... =)
Joined: Apr 2005
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From: Stockholm Sweden
another killer is the separate fuel system required.
But
power weight ratio would be critical.
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.
Aviator Extraordinaire
Joined: May 2000
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From: Oklahoma City, Oklahoma USA
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.
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.
Joined: Jan 2009
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From: Sin City
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.
Joined: Apr 2008
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From: London, UK
Bit of a red herring, most diesel engines will run quite happily on Jet A1.
I think older 'clockwork' diesels would be ok though.