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The Working Principle of Auxiliary Power Units

Auxiliary power units (APUs) are specialized devices installed in vehicles to provide energy for various functions. Found in large aircraft, naval ships, and large land vehicles, APUs allow them to operate autonomously without the need for GSE such as ground power units, external air-conditioning units, or high-pressure air start carts. That being said, this blog will cover APUs, how they work, and their importance.

Towards the back of many aircraft tails, you can locate the APU inlet. The inlet is equipped with a small APU which is categorized as a type of turbine engine. In aircraft, APUs do not provide thrust. Instead, they supply power to start the engine through the use of the aircraft’s battery. Furthermore, APUs provide electrical power to the aircraft, and bleed air for engine starting and air conditioning. One example of an aircraft with this type of APU was the Boeing 727 which was the first jetliner to contain a gas turbine APU in 1963.

In the Space Shuttle, APUs provided hydraulic pressure. In fact, this spacecraft had three APUs installed in it, all of which were powered by hydrazine fuel. At this point, APUs were exclusively used during ascent, re-entry, and landing operations. Within armored vehicles, some tanks were equipped with APUs, providing electrical power without high fuel consumption. For instance, the American M4 Sherman, the most used tank in WWII, was fitted with a small APU to charge the tank’s batteries. In the refrigerators found in trains, an APU and a fuel tank help maintain low temperatures while in transit.

APUs for commercial transport aircraft contain three principal sections. The first is the power section which is the gas-generator portion of the gas turbine APU engine, producing shaft power for the APU. Next, a shaft-mounted compressor supplies the aircraft with pneumatic power. For some aircraft, this section extracts bleed air from the power section to start up the main engine. The inlet guide vanes within this section regulate airflow to the load compressor. Meanwhile, the surge control valve maintains the stable operation of the turbo machine. The last section consists of the gearbox which transfers power from the main shaft to an oil-cooled generator for electric power. 

Within aircraft, APUs produce 115 AC voltage at 400 Hz frequency. More than that, they produce 28 V DC voltage to run the electrical system. Additionally, they supply power via a single or 3-phase system. Prior to take off, the APU engine is switched on, providing high speed bleed air to the main engine. Usually equipped with a smaller APU, this unit size draws low power and is easy to install. There are also a few other functions it performs. To begin, it provides cabin air and electric power before the engine gets started, allowing the aircraft to save battery power. Moreover, APUs provide an emergency source of electric power in the case that the engine fails. Lastly, they have the ability to start the engine mid-flight if there is an emergency.

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