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[[File:BMW 003 Riedelanlasser.jpg|thumb|right|The Riedel APU installed on a preserved BMW 003 jet engine (electric 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 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.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>Left blank intentionally</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>
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>Left blank intentionally</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>
Some models of the [[B-24 Liberator]] had a putt–putt fitted at the front of the aircraft, inside the nose-wheel compartment.<ref>Left blank intentionally</ref> Some models of the [[Douglas C-47 Skytrain]] transport aircraft carried a putt-putt under the cockpit floor.<ref>Left blank intentionally</ref>


====As mechanical "startup" APUs for jet engines====
====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 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>
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>Left blank intentionally</ref>


===Sections===
===Sections===
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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.
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>
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">Left blank intentionally</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>
Onboard solid oxide fuel cell ([[SOFC]]) APUs are being researched.<ref>Left blank intentionally</ref>


===Manufacturers===
===Manufacturers===
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* Helicopters: Pratt & Whitney 40–50%, Motorsich 40–50%, Honeywell 5–10%, Safran Power Units 5–10%, 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>
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">Left blank intentionally</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" />
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 (aeronautics)|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>
This could threaten the dominance of [[Honeywell]] and [[United Technologies]].<ref>Left blank intentionally</ref> Honeywell has a 65% share of the [[Mainline (aeronautics)|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">Left blank intentionally</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>
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">Left blank intentionally</ref>


==Spacecraft==
==Spacecraft==
The [[Space Shuttle]] APUs provided [[Hydraulic machinery|hydraulic]] pressure. The Space Shuttle had three [[redundancy (engineering)|redundant]] APUs, powered by [[hydrazine]] fuel. They were only powered up for ascent, [[re-entry]], and landing. During ascent, the APUs provided hydraulic power for [[gimbal]]ling of the Shuttle's three [[rocket engine|engines]] and control of their large valves, and for movement of the [[flight control surface|control surfaces]]. During landing, they moved the control surfaces, lowered the wheels, and powered the [[brake]]s and nose-wheel steering. Landing could be accomplished with only one APU working.<ref>{{cite web
The [[Space Shuttle]] APUs provided [[Hydraulic machinery|hydraulic]] pressure. The Space Shuttle had three [[Redundancy (engineering)|redundant]] APUs, powered by [[hydrazine]] fuel. They were only powered up for ascent, [[re-entry]], and landing. During ascent, the APUs provided hydraulic power for [[gimbal]]ling of the Shuttle's three [[Rocket engine|engines]] and control of their large valves, and for movement of the [[Flight control surface|control surfaces]]. During landing, they moved the control surfaces, lowered the wheels, and powered the [[brake]]s and nose-wheel steering. Landing could be accomplished with only one APU working.<ref>{{cite web
  | url = http://spaceflight.nasa.gov/shuttle/reference/shutref/orbiter/hyd
  | url = http://spaceflight.nasa.gov/shuttle/reference/shutref/orbiter/hyd
  | archive-url = https://web.archive.org/web/20010602114515/http://spaceflight.nasa.gov/shuttle/reference/shutref/orbiter/hyd/
  | archive-url = https://web.archive.org/web/20010602114515/http://spaceflight.nasa.gov/shuttle/reference/shutref/orbiter/hyd/
Line 63: Line 63:
  | publisher = NASA
  | publisher = NASA
  | access-date = 8 February 2016
  | access-date = 8 February 2016
}}</ref> In the early years of the Shuttle there were problems with APU reliability, with malfunctions on three of the first nine Shuttle missions.{{refn|group=Note|Early Shuttle APU malfunctions:
}}</ref> In the early years of the Shuttle there were problems with APU reliability, with malfunctions on three of the first nine Shuttle missions.Early Shuttle APU malfunctions:
*STS-2 (November 1981): During a launchpad hold, high oil pressures were discovered in two of the three APUs. The gear boxes needed to be flushed and filters replaced, forcing the launch to be rescheduled.<ref>{{cite web
*STS-2 (November 1981): During a launchpad hold, high oil pressures were discovered in two of the three APUs. The gear boxes needed to be flushed and filters replaced, forcing the launch to be rescheduled.<ref>{{cite web
  | url = http://www.nasa.gov/mission_pages/shuttle/shuttlemissions/archives/sts-2.html
  | url = http://www.nasa.gov/mission_pages/shuttle/shuttlemissions/archives/sts-2.html
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A refrigerated or frozen food semi trailer or train car may be equipped with an independent APU and fuel tank to maintain low temperatures while in transit, without the need for an external transport-supplied power source.{{Citation needed|date=July 2014}}<ref>{{Cite web|url=http://www.ooida.com/EducationTools/Info/weight-exemptions.asp|title=Vehicle weight exemptions for APUs}}</ref>
A refrigerated or frozen food semi trailer or train car may be equipped with an independent APU and fuel tank to maintain low temperatures while in transit, without the need for an external transport-supplied power source.{{Citation needed|date=July 2014}}<ref>{{Cite web|url=http://www.ooida.com/EducationTools/Info/weight-exemptions.asp|title=Vehicle weight exemptions for APUs}}</ref>


On some older diesel engined-equipment, a small gasoline engine (often called a "pony engine") was used instead of an electric motor to start the main engine. The exhaust path of the pony engine was typically arranged so as to warm the intake manifold of the diesel, to ease starting in colder weather. These were primarily used on large pieces of construction equipment.<ref>{{cite book|last1=Orlemann|first1=Eric|title=Caterpillar Chronicle: History of the Greatest Earthmovers|isbn=9781610605779|pages=35}}</ref><ref>{{cite web|title=Willard v. Caterpillar, Inc. (1995)|url=http://law.justia.com/cases/california/court-of-appeal/4th/40/892.html|website=Justia Law|access-date=13 December 2016}}</ref>
On some older diesel engined-equipment, a small gasoline engine (often called a "pony engine") was used instead of an electric motor to start the main engine. The exhaust path of the pony engine was typically arranged so as to warm the intake manifold of the diesel, to ease starting in colder weather. These were primarily used on large pieces of construction equipment.<ref>Left blank intentionally</ref><ref>{{cite web|title=Willard v. Caterpillar, Inc. (1995)|url=http://law.justia.com/cases/california/court-of-appeal/4th/40/892.html|website=Justia Law|access-date=13 December 2016}}</ref>


===Fuel cells===
===Fuel cells===
{{main|Fuel cell auxiliary power unit}}
In recent years, truck and fuel cell manufacturers have teamed up to create, test and demonstrate a fuel cell APU that eliminates nearly all emissions<ref>Left blank intentionally</ref> and uses diesel fuel more efficiently.<ref name="Weissler">Left blank intentionally</ref> In 2008, a DOE sponsored partnership between Delphi Electronics and Peterbilt demonstrated that a fuel cell could provide power to the electronics and air conditioning of a Peterbilt Model 386 under simulated "idling" conditions for ten hours.<ref>Left blank intentionally</ref> Delphi has said the 5&nbsp;kW system for Class 8 trucks will be released in 2012,{{update inline|date=August 2015}} at an $8000–9000 price tag that would be competitive with other "midrange" two-cylinder diesel APUs, should they be able to meet those deadlines and cost estimates.<ref name="Weissler" />
In recent years, truck and fuel cell manufacturers have teamed up to create, test and demonstrate a fuel cell APU that eliminates nearly all emissions<ref>{{cite news|first=Christie-Joy |last=Broderick |author2=Timothy Lipman |author3=Mohammad Farshchi |author4=Nicholas Lutsey |author5=Harry Dwyer |author6=Daniel Sperling |author7=William Gouse |author8=Bruce Harris |author9=Foy King |title=Evaluation of Fuel Cell auxiliary Power Units for Heavy-Duty Diesel Trucks |year=2002 |publisher=Elsevier Sciences Ltd. |url=http://www.uctc.net/papers/587.pdf |work=Transportation Research Part D |pages=303–315 |access-date=2011-09-27 |archive-url=https://web.archive.org/web/20120403131809/http://www.uctc.net/papers/587.pdf |archive-date=2012-04-03 }}</ref> and uses diesel fuel more efficiently.<ref name="Weissler">{{cite journal | title = Delphi truck fuel-cell APU to hit road in 2012 | journal = Vehicle Electrification | date = 2010-05-12 | first = Paul | last = Weissler| url = http://ev.sae.org/article/8222 | access-date = 2011-09-27 | quote = and Delphi says it will have a 5-kW APU on the market in 2012. }}</ref> In 2008, a DOE sponsored partnership between Delphi Electronics and Peterbilt demonstrated that a fuel cell could provide power to the electronics and air conditioning of a Peterbilt Model 386 under simulated "idling" conditions for ten hours.<ref>{{cite news | first = Mike | last = Jacobs | title = Solid Oxide Fuel Cell Successfully Powers Truck Cab and Sleeper in DOE-Sponsored Test | date = 2009-03-19 | publisher = National Energy Technology Laboratory | url = http://www.netl.doe.gov/publications/press/2009/09017-Fuel_Cell_Powers_Commercial_Trucks.html | work = NETL: News Release | access-date = 2011-09-27}}</ref> Delphi has said the 5&nbsp;kW system for Class 8 trucks will be released in 2012,{{update inline|date=August 2015}} at an $8000–9000 price tag that would be competitive with other "midrange" two-cylinder diesel APUs, should they be able to meet those deadlines and cost estimates.<ref name="Weissler" />


==See also==
==See also==
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* [[Uninterruptible power supply]]
* [[Uninterruptible power supply]]


==Notes==
{{reflist|group=Note}}
==References==
{{reflist}}


==External links==
==External links==
{{commons category|Auxiliary power units}}
* [https://web.archive.org/web/20010504090759/http://spaceflight.nasa.gov/shuttle/reference/shutref/orbiter/apu/ "Space Shuttle Orbiter APU"]
* [https://web.archive.org/web/20010504090759/http://spaceflight.nasa.gov/shuttle/reference/shutref/orbiter/apu/ "Space Shuttle Orbiter APU"]
* [http://www.freesound.org/samplesViewSingle.php?id=69203 "Sound of an APU from inside a Boeing 737 cabin"]
* [http://www.freesound.org/samplesViewSingle.php?id=69203 "Sound of an APU from inside a Boeing 737 cabin"]
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* [https://www.youtube.com/watch?v=8Azxzu1sqCU&t=75s YouTube video of restored Junkers Jumo 004 jet engine, being started with "integral" Riedel APU, from September 2019]
* [https://www.youtube.com/watch?v=8Azxzu1sqCU&t=75s YouTube video of restored Junkers Jumo 004 jet engine, being started with "integral" Riedel APU, from September 2019]


{{Aircraft components}}
{{Aircraft piston engine components}}
{{Aircraft gas turbine engine components}}


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Latest revision as of 21:37, 29 April 2025

A Honeywell GTCP36 APU mounted in the tail of a business jet
The APU exhaust in the tailcone of an Airbus A380

An auxiliary power unit (APU), is a device on a vehicle that provides energy for functions other than propulsion. They are commonly found on large aircraft and naval ships as well as some large land vehicles. Aircraft APUs generally produce 115 V AC voltage at 400 Hz (rather than 50/60 Hz in mains supply), to run the electrical systems of the aircraft; others can produce 28 V DC voltage.[1] APUs can provide power through single or three-phase systems. A jet fuel starter (JFS) is a similar device to an APU but directly linked to the main engine and started by an onboard compressed air bottle.[2]

Transport aircraft

History

The intake diverter of the Jumo 004, with pullcord starter handle for Riedel APU and its sparkplug access ports
The 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).
The Riedel APU installed on a preserved BMW 003 jet engine (electric starter variant shown).

During World War I, the British Coastal class blimps, one of several types of airship operated by the Royal Navy, carried a 1.75 horsepower (1.30 kW) ABC auxiliary engine. These powered a generator for the craft's radio transmitter and, in an emergency, could power an auxiliary air blower.A continuous supply of pressurized air was needed to keep the airship's Ballonets inflated, and so maintain the structure of the gasbag. In normal flight, this was collected from the propeller slipstream by an air scoop.[3] 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.[4]

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 or V-twin engines were used. The 7 horsepower (5.2 kW) engine drove a P2, DC generator, rated 28.5 Volts and 200 Amps (several of the same P2 generators, driven by 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 10,000 feet (3,000 m). The putt-putt was restarted when the B-29 was descending to land.[5]

Some models of the B-24 Liberator had a putt–putt fitted at the front of the aircraft, inside the nose-wheel compartment.[6] Some models of the Douglas C-47 Skytrain transport aircraft carried a putt-putt under the cockpit floor.[7]

As mechanical "startup" APUs for jet engines

The first German jet engines built during the Second World War used a mechanical APU starting system designed by the German engineer Norbert Riedel. It consisted of a 10 horsepower (7.5 kW) 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/oil fuel were fitted in the annular intake. The engine was considered an extreme short stroke (bore / stroke: 70 mm / 35 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 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 – the Junkers Jumo 004, the BMW 003 (which uniquely appears to use an electric starter for the Riedel APU),[8] 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.[9]

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.[10]

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.[11] 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.[12]

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.[11]

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.[13][14]

Onboard solid oxide fuel cell (SOFC) APUs are being researched.[15]

Manufacturers

The market of Auxiliary power units is dominated by Honeywell, followed by Pratt & Whitney, Motorsich and other manufacturers such as PBS Velká Bíteš, Safran Power Units, Aerosila and Klimov. Local manufacturers include Bet Shemesh Engines and Hanwha Aerospace. The 2018 market share varied according to the application platforms:[16]

  • 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.[17] Boeing produced several hundred T50/T60 small turboshafts and their derivatives in the early 1960s. Safran produces helicopters and business jets APUs but stopped the large APUs since Labinal exited the APIC joint venture with Sundstrand in 1996.[18]

This could threaten the dominance of Honeywell and United Technologies.[19] Honeywell has a 65% share of the mainliner APU market and is the sole supplier for the Airbus A350, the Boeing 777 and all single-aisles: the Boeing 737 MAX, Airbus A220 (formerly Bombardier CSeries), Comac C919, Irkut MC-21 and Airbus A320neo since Airbus eliminated the P&WC APS3200 option. P&WC claims the remaining 35% with the Airbus A380, Boeing 787 and Boeing 747-8.[18]

It should take at least a decade for the Boeing/Safran JV to reach $100 million in service revenue. The 2017 market for production was worth $800 million (88% civil and 12% military), while the MRO market was worth $2.4 billion, spread equally between civil and military.[20]

Spacecraft

The Space Shuttle APUs provided hydraulic pressure. The Space Shuttle had three redundant APUs, powered by hydrazine fuel. They were only powered up for ascent, re-entry, and landing. During ascent, the APUs provided hydraulic power for gimballing of the Shuttle's three engines and control of their large valves, and for movement of the control surfaces. During landing, they moved the control surfaces, lowered the wheels, and powered the brakes and nose-wheel steering. Landing could be accomplished with only one APU working.[21] In the early years of the Shuttle there were problems with APU reliability, with malfunctions on three of the first nine Shuttle missions.Early Shuttle APU malfunctions:

  • STS-2 (November 1981): During a launchpad hold, high oil pressures were discovered in two of the three APUs. The gear boxes needed to be flushed and filters replaced, forcing the launch to be rescheduled.[22]
  • STS-3 (March 1982): One APU overheated during ascent and had to be shut down, although it later functioned properly during re-entry and landing.[23][24]
  • STS-9 (November–December 1983): During landing, two of the three APUs caught fire.[25]}}

Armored vehicles

Template:Missing information APUs are fitted to some tanks to provide electrical power without the high fuel consumption and large infrared signature of the main engine. As early as World War II, the American M4 Sherman had a small, piston-engine powered APU for charging the tank's batteries, a feature the Soviet-produced T-34 tank did not have.[26]

Commercial vehicles

A refrigerated or frozen food semi trailer or train car may be equipped with an independent APU and fuel tank to maintain low temperatures while in transit, without the need for an external transport-supplied power source.[citation needed][27]

On some older diesel engined-equipment, a small gasoline engine (often called a "pony engine") was used instead of an electric motor to start the main engine. The exhaust path of the pony engine was typically arranged so as to warm the intake manifold of the diesel, to ease starting in colder weather. These were primarily used on large pieces of construction equipment.[28][29]

Fuel cells

In recent years, truck and fuel cell manufacturers have teamed up to create, test and demonstrate a fuel cell APU that eliminates nearly all emissions[30] and uses diesel fuel more efficiently.[31] In 2008, a DOE sponsored partnership between Delphi Electronics and Peterbilt demonstrated that a fuel cell could provide power to the electronics and air conditioning of a Peterbilt Model 386 under simulated "idling" conditions for ten hours.[32] Delphi has said the 5 kW system for Class 8 trucks will be released in 2012,[needs update] at an $8000–9000 price tag that would be competitive with other "midrange" two-cylinder diesel APUs, should they be able to meet those deadlines and cost estimates.[31]

See also


External links


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  1. ↑ 400 Hz Electrical Systems.  Aerospaceweb.org.  Retrieved from link
  2. ↑ A Jet Fuel Starter and Expendable Turbojet, ASME Digital Collection, by C Rodgers · 1986
  3. ↑ Left blank intentionally
  4. ↑ Andrews and Morgan 1987, p. 21.
  5. ↑ Left blank intentionally
  6. ↑ Left blank intentionally
  7. ↑ Left blank intentionally
  8. ↑ Design Analysis of BMW 003 Turbojet - "Starting the Engine".  (1946)  United States Army Air Force - Turbojet and Gas Turbine Developments, HQ, AAF.  Retrieved September 3, 2016 from legendsintheirowntime.com
  9. ↑ Gunston 1997, p. 141.
  10. ↑ Left blank intentionally
  11. ↑ Jump up to: 11.0 11.1 The APU and its benefits | AERTEC Solutions.  (10 May 2015)  Retrieved 2018-06-20 from www.aertecsolutions.com
  12. ↑ Turbojet Engines.  Retrieved 2022-03-20 from www.grc.nasa.gov
  13. ↑ Saving Fuel and enhancing operational efficiencies.  Mike Sinnet.  Boeing.  Retrieved January 17, 2013 from link
  14. ↑ Left blank intentionally
  15. ↑ Left blank intentionally
  16. ↑ 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.  (September 27, 2018)  European Commission.  Retrieved August 11, 2022 from EUR-Lex
  17. ↑ Left blank intentionally
  18. ↑ Jump up to: 18.0 18.1 Left blank intentionally
  19. ↑ Left blank intentionally
  20. ↑ Left blank intentionally
  21. ↑ Hydraulic System.  NASA.  Retrieved 8 February 2016 from spaceflight.nasa.gov
  22. ↑ Space Shuttle Mission Archives STS-2.  NASA.  Retrieved 18 February 2016 from www.nasa.gov
  23. ↑ Space Shuttle Mission Archives STS-3.  NASA.  Retrieved 18 February 2016 from www.nasa.gov
  24. ↑ Template:Cite interview
  25. ↑ Space Shuttle Mission Archives STS-9.  NASA.  Retrieved 18 February 2016 from www.nasa.gov
  26. ↑ IRemember.ru WW II Memoirs.  Dimitri Loza.  (September 21, 2010)  IRemember.  Retrieved June 13, 2017 from iremember.ru/en
  27. ↑ Vehicle weight exemptions for APUs.  Retrieved from link
  28. ↑ Left blank intentionally
  29. ↑ Willard v. Caterpillar, Inc. (1995).  Retrieved 13 December 2016 from Justia Law
  30. ↑ Left blank intentionally
  31. ↑ Jump up to: 31.0 31.1 Left blank intentionally
  32. ↑ Left blank intentionally
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