**2. Definition of concept power unit**

### **2.1 Purpose of aircraft power unit**

The purpose of airplane power units is to generate thrust, supply energy to some onboard systems, and solve specific tasks. Power units are a set of engines and subsystems providing their operation on all possible modes of operation (Mach flight number M, altitude H, temperature T, etc.) for the aircraft.

### **2.2 Power units' subsystem content**

As was said above, a power unit contains not only engines, which are the main object of it. However, depending on an aircraft type, some subsystems and auxiliary components can be present in it. Brief contents for more common cases are presented in **Figure 1**.

So, about the purposes of the power unit structural members and subsystems.

Engine is needed for thrust or power generation and supply some of auxiliary subsystem such as electrical, hydraulic, pneumatic, etc. Brief classification of the engine by type of thrust creation is presented in **Figure 2**.

*Review of the Aviation Power Units Development DOI: http://dx.doi.org/10.5772/intechopen.112741*

### **Figure 1.**

*Brief content of a typical power unit of an aircraft [7].*

### **Figure 2.**

*Aviation engine classification by type of thrust creation [7].*

A propeller creates thrust as it rotates by throwing air back with some additional velocity (on piston, turboprop, and prop-fan engines). It has a propeller spinner. An example of an aircraft propeller is shown in **Figure 3**.

The fuel system is a complex of interacting subsystems designed to supply the engine with fuel for all operating conditions permitted for that aircraft and may also perform a number of additional functions (e.g., oil cooling, maintaining a specified center of gravity position, etc.). A brief diagram of a typical fuel system is shown in **Figure 4**.

The lubrication system is a set of units designed to lubricate the engine, dissipate heat from the engine units, and remove solid particles formed between friction surfaces under all aircraft operating conditions (**Figure 5**).

The engine mount (**Figure 6**) is designed to attach an engine with its installed auxiliaries and other equipment to airframe attachments. This means that it must support all possible loads for all allowable operating conditions for that airplane.

The air ingestion system is designed to intake and deliver the required amount of air to the air consumer in all operating modes. Therefore, it is necessary to ensure the transformation of the kinetic energy of the flow into the potential energy of the pressure with minimum losses (**Figure 7**).

*Propulsion Systems – Recent Advances, New Perspectives and Applications*

### **Figure 3.**

*Common an aircraft propeller structure [8]: 1—Spinner; 2—Multiposition blade; and 3—Engine fitting.*

### **Figure 4.**

*Typical fuel system for modern transport category aircraft [9]: 1—Engines; 2—Auxiliary power unit; 3—Valves; 4—Pumps; 5—Pipelines; and 6—Fuel tanks.*

### **Figure 5.**

*Typical lubrication system for an engine [10]: 1—Pressure oil channel; 2—Scavenge oil channel; 3—Vent channel; 4—Filters; 5—Pumps; 6—Compressor; and 7—Turbine.*

*Review of the Aviation Power Units Development DOI: http://dx.doi.org/10.5772/intechopen.112741*

### **Figure 6.**

*Typical engine mounts for a turbojet bypass (a) and a turboprop engines (b) [7]: 1—Front attachment; and 2—Rear attachment.*

### **Figure 7.**

*Aft fuselage engine air ingestion system [11]: 1—Air-intake; 2—Duct channel; 3—Engine; and 4—APU.*

### **Figure 8.**

*Exhaust system for a typical turbojet bypass or turbofan engine with separate thrust reverse [7]: 1—Hot channel nozzle; 2—Hot channel thrust reverse; 3—Cold channel; and 4—Cold channel thrust reverse.*

The exhaust units (**Figure 8**) of the engines are designed to convert the thermal and potential energy of the gases into kinetic energy of the outgoing flow. For modern airplanes of the heavy transport category, it is also necessary to provide thrust reversal for deceleration.

### **Figure 9.**

*Cooling and de-icing schemes for the typical turbojet bypass or turbofan engines [12]: 1—De-icing channels and components; 2—Cooling for the combustion chamber channel; and 3—Internal and turbine discs and blades cooling channels.*

### **Figure 10.**

*Brief scheme for a typical engine control system [13]: 1—Engine; 2—Sensors; 3—Sensors location; 4— Hydromechanical components; 5—Electronic and electrical components; and 6—Full authority digital engine control.*

The cooling system for engines and their subsystems is needed for the cooling of the whole engine directly or its some parts or subsystems (**Figure 9**).

Engine control (**Figure 10**) includes ground and in-flight engine start, false start, cold start, engine normal and emergency engine shutdown, engine operating mode control, and engine thrust revers control.

The starting system is used to transfer the aircraft engine from the non-operating condition to the low-gas steady state, which is characterized by the lowest turbine speed at which it can operate continuously for a long period of time.

The fire protection system (**Figure 11**) is designed to ensure flight safety by detecting and preventing the occurrence of fire in the power unit subsystems.

The de-icing system (**Figures 9** and **12**) is a set of units designed to: prevent ice build-up on engine structural elements or subsystems, remove ice build-up to provide all-weather conditions, and improve flight safety in icing conditions.

The nacelle or cowling (**Figure 13**) is designed to reduce drag, organize airflow for engine cooling, and reduce engine noise.

The Auxiliary Power Unit (APU) is designed to start the main engines under certain conditions and to supply power to systems not related to the generation of the main thrusts, such as electrical, hydraulic, pneumatic, etc. The APU is usually located in the aft section of the fuselage (**Figure 14**).

*Review of the Aviation Power Units Development DOI: http://dx.doi.org/10.5772/intechopen.112741*

### **Figure 11.**

*Typical fire protection system for an engine [14]: 1—Engine; 2—Control unit; 3—Alarm signal; 4—Cockpit; 5—Antifire substance balloons; 6—Discharge nozzles; and 7—Engine fire loops.*

### **Figure 12.**

*Typical de-icing system for an engine [15]: 1—Air intake of the engine; 2—Inlet guide vane; 3—Compressor; 4—Air intake of the hit-exchanger of the lubrication subsystem; and 5—Components of the de-icing system (valves, gages, etc.)*

### **Figure 13.**

*Structure of a nacelle of a turbojet bypass or turbofan engine [7]. (a) Assembled; (b) disassembled: 1—Wing; 2—Pylon; 3—Air intake; 4—Lips of the air intake; 5—Inner surface of the air intake, 6—Duct channel; 7—External skin of the air intake; 8—Fan cowl; 9—Reverser cowl; 10—Nozzle assembly; and 11—Engine.*

As shown above, the content of the power unit subsystem has a very wide range. Really, it is variable with which depends on an aircraft purpose and flight performance.
