Auxiliary Power Units (APU) Information


Auxiliary power units (APU) provide vehicles with energy for functions other than propulsion. They are employed in aircraft, ships, and some land vehicles to perform tasks such as starting main engines, heating motor blocks, and charging batteries. The devices supply energy in electric, pneumatic, or hydraulic form.


In aircraft, the elements assist in starting the primary engine. They generate electrical power in planes during pre-flight checks and energize cabin amenities while engines are off. Furthermore, the mechanism activates accessories in vehicles when it is idle. Long-haul trucking systems rely on the instruments for saving fuel and reducing air pollution caused by idling.




The products come in different types serving distinct purposes. They draw power from multiple sources, including:

 auxiliary power units specification guide

  • Battery
  • Hydraulic accumulator
  • Combustion engines using diesel, gas, or propane fuel 

Design characteristics vary depending on use and fuel source. Standard models include aircraft and military aircraft APUs.


Aircraft units


The tools comprise the following three sections:


  • Power: The section features a gas generator for producing the device's shaft power.
  • Load compressor: A compressor delivering pneumatic power for an aircraft is mounted on a shaft. Select versions extract bleed air via the power section.
  • Gearbox: The component transfers energy from a motor's main shaft to a generator cooled by oil to create an electrical current. Inside the gearbox, power transfers to areas, including fuel control units, lubrication modules, and cooling fans. A starter motor linked through the gear train executes the starting function. 

Military aircraft APUs for small jets deviate from the solutions found on commercial planes. In military aircraft, the following elements control the startup and supply electrical and hydraulic energy:


  • Jet fuel starter: Consists of diminutive turbo shaft engines bringing the jet to its self-sustaining RPM. The starter's output shafts are mechanically linked to an engine that runs when the device starts turning. This instrument does not transmit electricity when the aircraft is idle.
  • Emergency power unit: Performs without a gas compressor and uses a compound made up of hydrazine and water. The mixture burns after release and passes through an iridium catalyst. The formation of expanding gases prompts the turbine. The resulting energy drives both an electric generator and a hydraulic pump.
  • Fuel cells rely on diesel fuel and other energy sources to achieve an efficient reduction in emissions. The option is ideal for regulating exhaust emission or noise levels. These items are cost-effective for managing electrical equipment.
  • Electric alternatives are equipped with batteries. The battery pack gains charge from a vehicle's movement. This APU category offers advantages, including lower emissions, reduced maintenance expenses, and fuel savings.
  • Combustion engines are supported by substances such as gas, diesel and propane. A fraction of the substance is sufficient to ignite the main motors. This facilitates compact sizing, enabling convenient placement of the structures. 


 auxiliary power units specification guide

Auxiliary power units serve a diverse scope of applications, including:


  • Trolley buses
  • Military vehicles
  • Commercial conveyance
  • Luxury cars
  • Recreational vehicles (RVs) and campers
  • Spacecraft
  • Airports
  • Select stationary operations



The configuration of APUs depends on the working requirements. In aircraft, the technology helps in startup and distribution of electrical current when flight engines are inactive. Combusting the fuel involves pneumatic turbine rotation and fuel pumps combined with an electric spark. When starting additional motors, the active element replaces the APU.


ETOPS (Extended-range twin-engine operations) aircraft deploy the devices to ensure safety. The components assist in transmitting reserve electricity and compressed air in the event of a motor or generator failure.


For military vehicles, the product delivers supplementary power, permits silent operation, and lowers logistical and further ongoing expenditures. Such automobiles require instruments with capabilities, including:


  • Ability to handle extremely heavy dust conditions
  • Ability to maintain superior performance in situations involving extreme temperatures and at high altitude
  • Ability to provide stationary or moving operation
  • Full fuel and control system integration
  • Compact size 

The solutions are deployed in airport equipment responsible for executing tasks such as clearing runways. They support cooling, heating and hydraulic warming if required.


Furthermore, commercial trucks are fitted with the tools comprising diesel motors with independent cooling systems and other structures. The primary objective is to minimize wear on motors and eliminate idling losses.




These instruments offer an array of types and dimensions. Some elements are designed for individual applications while others are more flexible. Check manufacturer's specifications to ensure the device is best suited for the intended use. Factors such as power source, type of fuel, and fuel availability in the operating environment should be considered when evaluating a particular option.


Image credits:


Carrier | tweakle.cnc / CC BY 2.0



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