Editing Auxiliary power unit (section)

===Sections===
A typical gas-turbine APU for commercial transport aircraft comprises three main sections:

====Power section====
The power section is the gas-generator portion of the engine and produces all the shaft power for the APU.<ref name="aertecsolutions.com">{{Cite web|url=http://www.aertecsolutions.com/2015/05/11/the-apu-and-its-benefits/?lang=en|title=The APU and its benefits {{!}} AERTEC Solutions|website=www.aertecsolutions.com|date=10 May 2015 |language=en-US|access-date=2018-06-20|archive-date=2018-06-20|archive-url=https://web.archive.org/web/20180620124922/http://www.aertecsolutions.com/2015/05/11/the-apu-and-its-benefits/?lang=en}}</ref> In this section of the engine, air and fuel are mixed, compressed and ignited to create hot and expanding gases. This gas is highly energetic and is used to spin the turbine, which in turn powers other sections of the engine, such as auxiliary gearboxes, pumps, electrical generators, and in the case of a turbo fan engine, the main fan.<ref>{{Cite web |title=Turbojet Engines |url=https://www.grc.nasa.gov/www/K-12/airplane/aturbj.html |access-date=2022-03-20 |website=www.grc.nasa.gov}}</ref>

====Load compressor section====
The load compressor is generally a shaft-mounted compressor that provides pneumatic power for the aircraft, though some APUs extract [[bleed air]] from the power section compressor. There are two actuated devices to help control the flow of air: the inlet guide vanes that regulate airflow to the load compressor and the surge control valve that maintains stable or surge-free operation of the turbo machine.<ref name="aertecsolutions.com"/>

====Gearbox section====
The [[gearbox]] transfers power from the main shaft of the engine to an oil-cooled generator for electrical power. Within the gearbox, power is also transferred to engine accessories such as the fuel control unit, the lubrication module, and cooling fan. There is also a starter motor connected through the gear train to perform the starting function of the APU. Some APU designs use a combination starter/generator for APU starting and electrical power generation to reduce complexity.

On the [[Boeing 787]], an aircraft which has greater reliance on its electrical systems, the APU delivers only electricity to the aircraft. The absence of a pneumatic system simplifies the design, but high demand for electricity requires heavier generators.<ref name=nobleed1>{{cite web|last=Sinnet|first=Mike|title=Saving Fuel and enhancing operational efficiencies|url=http://www.boeing.com/commercial/aeromagazine/articles/qtr_4_07/AERO_Q407_article2.pdf|publisher=Boeing|access-date=January 17, 2013|year=2007}}</ref><ref name=Design_News_20070604>{{cite news |url=http://www.designnews.com/document.asp?doc_id=222308 |title=Boeing's 'More Electric' 787 Dreamliner Spurs Engine Evolution: On the 787, Boeing eliminated bleed air and relied heavily on electric starter generators |publisher=[[Design News]] |date=June 4, 2007 |editor=Ogando, Joseph |access-date=September 9, 2011 |archive-date=April 6, 2012 |archive-url=https://web.archive.org/web/20120406062451/http://www.designnews.com/document.asp?doc_id=222308 }}</ref>

Onboard solid oxide fuel cell ([[SOFC]]) APUs are being researched.<ref>{{cite journal |last=Spenser |first=Jay |date=July 2004 |title=Fuel cells in the air |journal= Boeing Frontiers |volume=3 |issue=3 |url=http://www.boeing.com/news/frontiers/archive/2004/july/ts_sf7a.html}}</ref>