The auxiliary power unit (APU) is most often used to start the main engine. Often this equipment is used in aircraft. However, it can also be used on armored vehicles, ships, locomotives and cars.
The main characteristics of the APU
For such a power plant with air extraction behind the compressor, its consumption, the pressure of this air, and its temperature are the main parameters. However, it is worth noting here that such a characteristic as air pressure is not an energy indicator. In other words, it cannot be used as an estimate of the resource indicators of the auxiliary power plant of the Armed Forces. It will not work with it to evaluate the workflow either. For this reason, you have to resort to the use of such a conditional parameter as the equivalent air power. In addition, a parameter that is called specific fuel consumption is also important. For a power plant with air extraction behind the compressor, it means fuel consumption per hour per 1 kW of equivalent air power. In addition to these basic characteristics, there are also secondary:
- margin of stability of the compressor;
- coefficient of excess air in the combustion chamber;
- temperature and pressure of the working fluid;
- coefficient of performance (COP) of the compressor, turbine, etc.
A brief description of the APU for the car and locomotive
If we talk about locomotives, it is rare, but gas turbines are still used. On such a transport, the auxiliary power unit is mounted in order to start the main engine. In addition, with its help, maneuvers and the movement of a single locomotive are carried out.
If on a car with special equipment that requires electrical power and an idle engine, fairly widely known electrical units were used as APU. It is also worth noting that on a number of special machines it was also possible to start the main engine.
APU device for aircraft
As for the auxiliary power unit of the aircraft, it is a source of hot compressed air, as well as electrical energy with direct and alternating current, which can be used to power aircraft systems.
When the plane is on the ground, the APU can be fully used to ensure complete autonomy of transport. This autonomy is used in the preflight preparation process. Such a system can only be operated at airfields that are located at an altitude of no more than 3 km. It is also worth mentioning that an auxiliary power plant with 300 m or another model can be used simultaneously for intake of both compressed air and electricity. Compressed air enters the air conditioning system of the aircraft, and electricity to start the main engine. The APU is well suited for starting a gas turbine engine, its mounting system, air intake device, exhaust system, as well as for a system that provides engine start-up and provides the ability to control it.
APU compartment design
The system is equipped with a drainage system. At the lowest point is a device called a drainage collector. There is also a pipe, which is designed to withdraw fluid outward, by gravity. The gas turbine engine of the aircraft is also located in the APU compartment, which is located in the rear leaky part of the fuselage. The flight engineer’s remote control has an APU Launch panel. On this panel are all the controls and controls for the auxiliary power device.
APU TA-6A
This kind of auxiliary installation, such as TA-6A, is most often installed on board such aircraft as TU-154, IL-62M, IL-76, TU-144, IL-86M, TU-22M. It can also be installed on some ground transportation units. The main purpose is the supply of compressed air to start the main engines of the aircraft on the ground to supply compressed air to the air conditioning system.
It is important to note here that this APU can be used to power the on-board electrical system with both alternating and direct currents on the ground and, most importantly, can be used for the same purpose in flight when the main system fails. The installation itself is presented in the form of a single-shaft gas turbine engine with air extraction behind the compressor. This suggests that the main characteristics of the auxiliary power unit TA-6A are the flow rate, pressure and temperature of the taken air. This device consists of several basic elements. The first main unit includes a gearbox with a starter-generator. There is also an alternator, as well as several other mounted units. All of them are necessary in order to ensure the normal operation of the engine. As a compressor, a three-stage diagonal-axial element is used.
Indicators APU TA-6A
The device has the following main technical characteristics:
- The direction of rotation of the rotor from the nozzle side is right.
- The second important parameter is the rotor speed for the turbocharger. When debugging the engine at idle, the temperature range should be about 60 degrees Celsius. In percentage, the indicator should be 99 ± 0.5%. If we talk about rpm, then the indicator should be in the region of 23950 ± 48.
- As for the main mode of operation, the change in the rotor speed is allowed in the range from 97 to 101%.
- There is such a parameter as vibration overload of the engine. At the beginning of the service life, this ratio should be 4.5. At the end of the term, it can rise to a maximum of 6.0.
- There is such a parameter as the duration of a cold load cycle. The maximum value is limited to 32 seconds.
- During a cold load, the rotor speed must be between 19 and 23% of the maximum power.
Engine Operation TA-6A
During operation of the auxiliary power unit, atmospheric air will be sucked in by a compressor through a grid and a radial circular inlet. The compressor has three stages, after which air is compressed and fed into the casing of the gas collector. From here, the bulk of the selected substance enters the combustion chamber. The remaining part can be transferred to the cochlea of the exhaust pipe and through the exhaust pipe can be discharged back into the atmosphere, or it can be delivered to the consumer.
It is worth noting that the air that is supplied to the combustion chamber is divided into two streams - primary and secondary. As for the primary stream, it enters the combustion zone through the evaporation tubes, as well as the holes in the head of the flame tube. Through the same evaporator tubes, fuel is also supplied from the starting manifold.
The minor flow follows a certain number of holes. After passing through it, it enters the same compartment as the substance from the first stream. In this tank, these flows are mixed with gas, which makes it possible to achieve the required temperature conditions for the entire gas stream entering directly into the turbine. It is also worth noting that there are gaps in the walls of the chamber. A small amount of air passes through them inside and is used there to cool the walls of the chamber.
Helicopter auxiliary power unit
An auxiliary device for a helicopter is somewhat different from that mounted on board an aircraft. The main components for the device were a pair of engines, as well as a gearbox. If such a need arises, then the power of one engine will be enough to continue the flight. It is also worth noting that the right and left engines of the installation are interchangeable. However, this is provided that there is the possibility of a reversal of the exhaust pipe. The engine itself includes elements such as a compressor with rotary blades, a combustion chamber, a compressor turbine and a combined turbine that transfers power through a spring shaft to the VR-8 gearbox. Also here there is an exhaust device and a box of drive units.