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Diffuser Duct
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 A.  The diffusers main purpose is to change high velocity air pressure to low or zero velocity air pressure. The APU has two diffusers. The power compressor diffuser delivers air to the combustion chamber. The load compressor diffuser delivers compressed, low velocity air to the APU bleed duct for airplane pneumatic systems.
 B.  Airflow to the load compressor impeller is modulated by variable inlet guide vanes (VIGV's) before passing through the twenty-three pipe load compressor diffuser assembly.
Combustor
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 A.  Air flows to the combustor from the power compressor diffuser. Finely atomized fuel is added to the the air entering the combustor and the air-fuel mixture is ignited by the ignition system. Ignition is required only during the starting cycle, as the heat of combustion sustains the burning as long as the fuel supply is not interrupted. Only part of the air is used to sustain combustion, the remainder of the air is used for cooling the combustion chamber liner.
Turbine Section
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 A.  The turbine section consists of two counter rotating axial-flow turbines, a turbine housing, and an exhaust port. The high pressure turbine (N2) drives the power compressor and turns clockwise (rear view). The low pressure turbine (N1) drives the load compressor and turns counterclockwise (rear view).
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL

 6.  Engine Bearings
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 A.  The APU rotating components are supported by five main bearings. The gas generator rotor assembly is supported by a ball and a roller bearing designated as No. 2 and 3 bearings respectively. Thrust is absorbed by the ball bearing which is of regular deep groove design with a split inner race. The No. 2 bearing is installed in the intake casing ahead of the power section impeller and adjacent to the AGB tower shaft drive gear. The No. 3 roller bearing, which is also of conventional design is located in the gas generator casing between the power section impeller and high turbine disk assembly.
 B.  Two roller bearings of conventional design support the power turbine shaft and are designated No. 1 and 4. The No. 1 bearing, mounted on the front end of the power turbine shaft and forward of the AGB drive bevel gear, is housed in the load compressor intake casing and shares a common compartment with the No. 2 bearing.
 C.  The No. 4 bearing, housed in a cavity at the center of the exhaust case, is installed on the power turbine shaft downstream of the turbine. The load compressor rotor assembly is supported by a ball bearing, identified as No. 0 bearing. Axial thrust from the power turbine/load compressor rotor assembly is absorbed by the No. 0 bearing. The No. 0 bearing is situated at the front of the load compressor impeller.
 7.  Accessory Gearbox
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 A.  The accessory gearbox (AGB) is mounted beneath the air inlet case and is driven by the high compressor stubshaft through a tower shaft and bevel gears. The accessory gearbox provides drives for the starter, fuel pump and an oil pressure pump and scavenge pump assembly. A cover on the front of the gearbox provides access to a square-end drive for manual rotation of the high rotor. Also mounted beneath the air inlet case is the fuel metering unit (FMU) which regulates fuel flow. The FMU is mounted on the fuel pump via a fuel metering adapter.
 8.  Load Gearbox
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 A.  The load gearbox drives two 90 KVA air cooled generators and a cooling air fan. The load gearbox consists of an arrangement of simple spur gears supported on roller bearings. A quill shaft transmits torque from the load compressor rotor assembly to the gearbox input pinion. An idler gear distributes torque from the input pinion to two generator drive gears which incorporate oil lubricated output splines. A second idler gear, also meshing with the input pinion, powers the cooling air fan drive gear situated in the upper portion of the gearbox. The oil tank is located in the load gearbox between the front and rear housings.
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