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The APU (auxiliary power unit) does not need to supply bleed air when the main engines are not operating. Aerodynamics are improved due to the lack of bleed air vent holes on the wings. By driving cabin air supply compressors at the minimum required speed, no energy wasting modulating valves are required.
The compressed air can be supplied from an on-board auxiliary power unit (APU), a portable gas generator used by ground crew or by cross feeding bleed air from a running engine in the case of multi-engined aircraft. [19] The Turbomeca Palouste gas generator was used to start the Spey engines of the Blackburn Buccaneer.
Low pressure (40–70 psi or 280–480 kPa), high volume air from the compressor section of the APU is bled off through a system of pipes to the engines where it is directed into the starting system. This bleed air is directed into a mechanism to start the engine turning and begin pulling in air. The starter is usually an air turbine type ...
Air bled from the engine fan is blown across the pre-cooler, located in the engine strut, and absorbs excess heat from the service bleed air. A fan air modulating valve (FAMV) varies the cooling airflow to control the final air temperature of the service bleed air. Notably, the Boeing 787 does not use bleed air to pressurize the cabin.
Compressed air is sent to the ATS turbine from the aircraft auxiliary power unit ( bleed air from the gas generator or from a free-turbine load compressor, eg PW901 APU), from an already-running engine (bleed air) on a multi-engined aircraft or, for early jet aircraft, from an air compressor mounted on ground support equipment.
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 intake diverter, essentially functioning as a pioneering example of an auxiliary power unit for starting a jet engine.
The most common source of compressed air for pressurization is bleed air from the compressor stage of a gas turbine engine; from a low or intermediate stage or an additional high stage, the exact stage depending on engine type. By the time the cold outside air has reached the bleed air valves, it has been heated to around 200 °C (392 °F). The ...
Deutsch: Environmental Kontrol System ECS der Boeing 737-300, bei laufenden Treibwerken (1) Zapfluft vom linken Triebwerk (engine bleed air - stark komprimiert, deshalb sehr heiß, hinter dem Kompressor abgezapft); (2) Kühlluft aus dem vorderen Triebwerksbereich (fan air - fast nicht komprimiert, deshalb fast nicht erwärmt; (3) Luft von der APU (APU-air - eine der Hauptaufgaben der APU ...
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