# Auxiliary Power Unit (APU): Functional Analysis

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1. Functional analysis Auxiliary Power Unit (APU) This section will give the necessary information about the APU. The focal point will be on the APS 5000, which is the APU for the Boeing 787 Dreamliner. This section is divided in two paragraph; APU in general (1.1.) and the APS 5000 (1.2.). 1.1. APU in general An aircraft APU is a small gas turbine specifically designed to meet the on-board demands of the aircraft when the Aircraft is On Ground (AOG). The APU drives both an electrical generator and an air compressor. With the produced energy the APU is able to provide:  Electric power from the electricity generators.  Pneumatic pressure for starting the engines and air conditioning.  Hydraulic pressure for oil pumps. This can be done on the ground as well as in-flight. Generally the APU is installed inside the tail cone area of the aircraft. The operation of an APU is almost the same as with a normal gas turbine and is also based on the Brayton-Joule cycle. First the compressions of the induced air by means of a compressor than the air is mixed with fuel and heated by the combustion of fuel. After this the exhaust gases drives the turbine and leaf the exhaust. The air comes…show more content…
Between the air inlet door and the APU, an air inlet plenum is installed. The ambient air is then compressed by the centrifugal compressor impeller, which increases the air velocity. When the air flow meets the diffuser, the air velocity will be convert into pressure. The pressurize air then reach te combustor, where the air is mixed with fuel and ignited by the ignition system. The combustion is a constant process that heats and expands the released gases before they reach the two-stage axial turbine. The gases clash on the turbine blades, which causes a rotation of the common rotor shaft. After the gases leave the turbines they flow into the APU exhaust and are released in the open