Editing Auxiliary power unit (section)

==Transport aircraft==
===History===
[[File:Jumo 004.jpg|thumb|250px|The intake diverter of the Jumo 004, with pullcord starter handle for Riedel APU and its sparkplug access ports]]
[[File:Riedelanlasser.jpg|thumb|right|250px|The [[Norbert Riedel|Riedel]] 2-stroke engine used as the pioneering example of an APU, to turn over the central shaft of both World War II-era German [[BMW 003]] and [[Junkers Jumo 004]] jet engines (pullcord starter variant shown).]]
[[File:BMW 003 Riedelanlasser.jpg|thumb|right|The Riedel APU installed on a preserved BMW 003 jet engine (electric starter variant shown).]]

During [[World War I]], the British [[Coastal class blimp]]s, one of several types of airship operated by the [[Royal Navy]], carried a {{convert|1.75|hp|kW}} [[ABC Motors|ABC]] auxiliary engine. These powered a generator for the craft's [[Transmitter|radio transmitter]] and, in an emergency, could power an auxiliary air blower.{{refn|group=Note|A continuous supply of pressurized air was needed to keep the airship's [[Ballonet]]s inflated, and so maintain the structure of the gasbag. In normal flight, this was collected from the propeller slipstream by an air scoop.}}<ref>{{cite book | title=The British Airship at War, 1914–1918 | publisher=Terence Dalton | author=Abbott, Patrick | year=1989 | pages=57 | isbn=0861380738}}</ref> One of the first military fixed-wing aircraft to use an APU was the British, World War 1, [[Supermarine Nighthawk]], an anti-Zeppelin [[night fighter]].<ref name="Andrews p21">Andrews and Morgan 1987, p. 21.</ref>

During [[World War II]], a number of large American military aircraft were fitted with APUs. These were typically known as ''putt–putts'', even in official training documents. The putt-putt on the [[B-29 Superfortress]] bomber was fitted in the unpressurised section at the rear of the aircraft. Various models of four-stroke, [[Flat-twin engine|Flat-twin]] or [[V-twin engine|V-twin]] engines were used. The {{convert|7|hp|kW}} engine drove a ''P2'', DC generator, rated 28.5 Volts and 200 Amps (several of the same ''P2'' generators, driven by [[Wright R-3350|the main engines]], were the B-29's DC power source in flight). The putt-putt provided power for starting the main engines and was used after take-off to a height of {{convert|10000|ft}}. The putt-putt was restarted when the [[Boeing B-29 Superfortress|B-29]] was descending to land.<ref>{{cite book | title=Boeing B-29 Superfortress: the ultimate look: from drawing board to VJ-Day | publisher=Schiffer | author=Wolf, William | year=2005 | pages=205 | isbn=0764322575}}</ref>

Some models of the [[B-24 Liberator]] had a putt–putt fitted at the front of the aircraft, inside the nose-wheel compartment.<ref>{{cite book | title=Under the Southern Cross: The B-24 Liberator in the South Pacific | publisher=Turner Publishing Company | author=Livingstone, Bob | year=1998 | pages=162 | isbn=1563114321}}</ref> Some models of the [[Douglas C-47 Skytrain]] transport aircraft carried a putt-putt under the cockpit floor.<ref>{{cite book | title=Flying the Hump: In Original World War II Color | publisher=Zenith Imprint | author=Ethell, Jeffrey | year=2004 | pages=84 | isbn=0760319154 | author2=Downie, Don}}</ref>

====As mechanical "startup" APUs for jet engines====
The first German [[jet engine]]s built during the [[Second World War]] used a mechanical APU starting system designed by the German engineer [[Norbert Riedel]]. It consisted of a {{convert|10|hp|kW}} [[Two-stroke engine|two-stroke]] [[flat engine]], which for the [[Junkers Jumo 004]] design was hidden in the engine nose cone, essentially functioning as a pioneering example of an auxiliary power unit for starting a jet engine. A hole in the extreme nose of the cone contained a manual pull-handle which started the piston engine, which in turn rotated the compressor. Two spark plug access ports existed in the Jumo 004's nose cone to service the Riedel unit's cylinders in situ, for maintenance purposes. Two small "premix" tanks for the Riedel's petrol/[[Two-stroke oil|oil]] fuel were fitted in the annular intake. The engine was considered an extreme short stroke (bore / stroke: 70&nbsp;mm / 35&nbsp;mm = 2:1) design so it could fit within the in the nose cone of jet engines like the Jumo 004. For reduction it had an integrated [[planetary gear]]. It was produced by [[Victoria (motorcycle)|Victoria]] in [[Nuremberg]] and served as a mechanical APU-style starter for all three German jet engine designs to have made it to at least the prototype stage before May 1945&nbsp;– the [[Junkers Jumo 004]], the [[BMW 003]] (which uniquely appears to use an electric starter for the Riedel APU),<ref>{{cite web |url=http://legendsintheirowntime.com/LiTOT/Content/1946/Av_4603_DA_BMW003.html |title=Design Analysis of BMW 003 Turbojet - "Starting the Engine" |last1=Schulte |first1=Rudolph C. |date=1946 |website=legendsintheirowntime.com |publisher=United States Army Air Force - Turbojet and Gas Turbine Developments, HQ, AAF |access-date=September 3, 2016 |quote=Starting procedure is as follows: Starting engine is primed by closing electric primer switch, then ignition of turbojet and ignition '''and electric starting motor''' of [[Norbert Riedel|Riedel engine]] are turned on (this engine can also be started manually by pulling a cable). After the Riedel unit has reached a speed of about 300 rpm, it automatically engages the compressor shaft of the turbojet. At about 800 rpm of the starting engine, starting fuel pump is turned on, and at 1,200 rpm the main (J-2) fuel is turned on. The starter engine is kept engaged until the turbojet attains 2,000 rpm, at which the starter engine and starting fuel are turned off, the turbojet rapidly accelerating to rated speed of 9,500 rpm on the J-2 fuel |archive-date=September 29, 2018 |archive-url=https://web.archive.org/web/20180929074301/http://legendsintheirowntime.com/LiTOT/Content/1946/Av_4603_DA_BMW003.html |url-status=dead }}</ref> and the prototypes (19 built) of the more advanced [[Heinkel HeS 011]] engine, which mounted it just above the intake passage in the Heinkel-crafted sheetmetal of the engine nacelle nose.<ref>Gunston 1997, p. 141.</ref>

The [[Boeing 727]] in 1963 was the first jetliner to feature a [[gas turbine]] APU, allowing it to operate at smaller airports, independent from ground facilities. The APU can be identified on many modern airliners by an exhaust pipe at the aircraft's tail.<ref>{{cite news|last1=Vanhoenacker|first1=Mark|title=What Is That Hole in the Tail of an Airplane?|url=http://www.slate.com/blogs/the_eye/2015/02/05/what_s_that_thing_unveils_the_mystery_of_that_hole_on_the_tail_of_the_airplane.html|access-date=20 October 2016|work=[[Slate (magazine)|Slate]]|date=5 February 2015}}</ref>

===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>

===Manufacturers===

The market of Auxiliary power units is dominated by [[Honeywell]], followed by [[Pratt & Whitney]], [[Motor Sich|Motorsich]] and other manufacturers such as [[PBS Velká Bíteš]], [[Safran|Safran Power Units]], [[Aerosila]] and [[Klimov]]. Local manufacturers include [[Beit Shemesh#Economy|Bet Shemesh Engines]] and [[Hanwha Aerospace]]. The 2018 market share varied according to the application platforms:<ref>{{Cite web |date=September 27, 2018 |title=Case M.8858 – Boeing/Safran/JV (Auxiliary power units), Commission decision pursuant to Article 6(1)(b) of Council, Regulation No 139/2004 and Article 57 of the Agreement on the European Economic Area |url=https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:32018M8858&qid=1660219951114&from=EN |access-date=August 11, 2022 |website=[[EUR-Lex]] |publisher=European Commission |page=14}}</ref>

* Large commercial aircraft: Honeywell 70–80%, Pratt & Whitney 20–30%, others 0–5%
* Regional aircraft: Pratt & Whitney 50–60%, Honeywell 40–50%, others 0–5%
* Business jets: Honeywell 90–100%, others 0–5%
* Helicopters: Pratt & Whitney 40–50%, Motorsich 40–50%, Honeywell 5–10%, Safran Power Units 5–10%, others 0–5%

On June 4, 2018, [[Boeing]] and [[Safran]] announced their 50–50 partnership to design, build and service APUs after regulatory and [[antitrust]] clearance in the second half of 2018.<ref name="4jun2018PR">{{cite press release |title= Boeing, Safran Agree to Design, Build and Service Auxiliary Power Units |date= June 4, 2018 |url=https://www.safran-group.com/media/boeing-safran-agree-design-build-and-service-auxiliary-power-units-20180604 |archive-url=https://web.archive.org/web/20180617115653/https://www.safran-group.com/media/boeing-safran-agree-design-build-and-service-auxiliary-power-units-20180604 |archive-date=2018-06-17 |website=Safran}}</ref>
Boeing produced several hundred [[Boeing T50|T50]]/[[Boeing T60|T60]] small [[turboshaft]]s and their derivatives in the early 1960s.<!--<ref name=Flight5jun2018>--> Safran produces [[helicopter]]s and [[business jet]]s APUs but stopped the large APUs since [[Labinal]] exited the [[Auxiliary Power International Corporation|APIC]] joint venture with [[Sundstrand Corporation|Sundstrand]] in 1996.<ref name="Flight5jun2018" />

This could threaten the dominance of [[Honeywell]] and [[United Technologies]].<ref>{{cite news |url= https://www.flightglobal.com/news/articles/boeing-and-safran-partner-to-disrupt-apu-market-449184/ |title= Boeing and Safran partner to disrupt APU market |date= June 4, 2018 |author= Stephen Trimble |work= Flightglobal}}</ref> Honeywell has a 65% share of the [[Mainline (air travel)|mainliner]] APU market and is the sole supplier for the [[Airbus A350]], the [[Boeing 777]] and all [[single-aisle]]s: the [[Boeing 737 MAX]], [[Airbus A220]] (formerly Bombardier CSeries), [[Comac C919]], [[Irkut MC-21]] and [[Airbus A320neo]] since Airbus eliminated the P&WC [[Hamilton Sundstrand#Aircraft Systems|APS3200]] option.<!--<ref name=Flight5jun2018>--> [[Pratt & Whitney Canada|P&WC]] claims the remaining 35% with the [[Airbus A380]], [[Boeing 787]] and [[Boeing 747-8]].<ref name=Flight5jun2018>{{cite news |url= https://www.flightglobal.com/news/articles/analysis-how-will-boeing-safran-venture-shake-up-ap-449234/ |title= How will Boeing-Safran venture shake up APUs? |date= June 5, 2018 |author= Stephen Trimble |work= Flightglobal}}</ref>

It should take at least a decade for the Boeing/Safran JV to reach $100 million in service revenue.<!--<ref name=AvWeek27jun2018>--> The 2017 market for production was worth $800 million (88% civil and 12% military), while the [[maintenance, repair and overhaul|MRO]] market was worth $2.4 billion, spread equally between civil and military.<ref name=AvWeek27jun2018>{{cite news |url= http://aviationweek.com/commercial-aviation/opinion-why-boeing-diving-apu-production |title= Opinion: Why Is Boeing Diving Into APU Production? |date= June 27, 2018 |author= Kevin Michaels |work= Aviation Week & Space Technology}}</ref>