Review of the Aviation Power Units Development

Abstract

The development of the aviation industry depends on many factors. One of the most important factors is the development of power units. A power plant is a source of energy for an airplane. Without power units, flight is possible, but the transportation efficiency of such flights will be low. So, an in-depth analysis of the power unit structure is an actual task every time. The power unit is composed of several components. Obviously, the main structural element is the engine. However, the overall efficiency of the propulsion unit does not depend only on the engine. Subsystems and subcomponents of power units should have rational performance that can provide high overall efficiency. So, a detailed analysis of each of these structural members is also an actual task. To perform such an analysis, it is necessary to know the characteristics of these components and ways to increase their efficiency. In this chapter, all these features are briefly explained. In addition, some trends for the future development of civil aircraft power units are discussed.

1. Introduction

The history of aviation is inextricably linked to the history of engine development. Throughout its existence, progress in aviation has been based primarily on progress in the development of aircraft engines. In turn, the ever-increasing demands for improving the efficiency of air transportation have been a powerful driver for the development of aircraft engines.

There have been many attempts to create a heavier-than-air, powered aircraft. One of the first aviation pioneers to create such an aircraft was the Russian researcher A.F. Mozhaisky [1]. Between 1881 and 1885, he created an airplane powered by a steam engine. The energy efficiency of the engine was not high, and this, along with other structural specifics, influenced the failure of this project. About 20 years later, American aviators, the Wright brothers, created the first successful airplane [1]. But this aircraft was already equipped with a piston engine. The piston engine provided sufficient energy efficiency for airplane flights at an early stage of aviation development.

In the 30s of the twentieth century, engine engineers developed engines with self-igniting fuel, also known as diesel engines. Thanks to it, an airplane achieved a speed record of 756 km per hour [2]. But by that time, internal combustion engines had exhausted their energy efficiency reserves.

Problems with the power-to-weight ratio of aircraft required new approaches from engine designers. Since the beginning of the twentieth century, researchers had been developing the theory of gas turbine engines. Scientists from Russia, Great Britain, and Germany were active in this field and were awarded several patents on the principle of operation and design of such engines. The first aircraft to successfully fly with a turbojet engine was the He-178, developed in Germany in 1939. However, the first generation of turbojet engines provided high power but had a very high fuel consumption. This did not allow the use of these engines for commercial aviation. With the development of turboprop and bypass turbojet engines, it became possible to achieve sufficient energy efficiency.

The first aircraft with a turboprop engine was the British Trent-Meteor, which made its first flight in 1945 [3]. The first bypass turbojet engine was the Rolls-Royce Conway, developed in the late 40s of the twentieth century. It was installed on the first commercial airliners such as Boeing 707-420, Douglas DC-8-40, etc. [4]. From that moment on, gas turbine engines became the main engines for civil aviation.

Currently, commercial aviation has several developers of high-performance engines. However, as in the past, existing engines have reached their limits in the new operating conditions. Without the development of new approaches to propulsion, the new conditions proposed by ICAO in the field of environmental protection cannot be satisfied.

ICAO has stated that the amount of harmful emissions must be reduced by a factor of 2 by 2050 [5], and ICAO is also tightening the requirements for noise [6]. Stringent emission reduction requirements can be achieved in several ways. But the main direction that can ensure the reduction of emissions is, of course, the development of new engines with new operating principles. Such engines are: electric, hydrogen and, as an intermediate stage, hybrid. But a separate engine cannot provide the required own efficiency without related subsystems.

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.

Figure 1.

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

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.

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.

Figure 3.

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

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.

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.

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).

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.

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.

Figure 6.

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

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).

Figure 7.

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

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 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 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).

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.

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.

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 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.

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.

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.

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.)

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

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.

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).

Figure 14.

APU location onboard of an aircraft [16]: 1—Main engines; and 2—APU.

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.

3. Analysis of the future trends for aviation power units improvement

Modern power units have a high level of performance, and for ordinary people, the question is why they need to be improved. There are several reasons that can answer this question. First of all, the aviation industry is the leader in engineering because it requires the highest possible performance, which can provide more higher efficiency of this transportation industry. The second reason is the environmental impact. In recent years, society demands a stronger approach to environmental safety. And the aviation industry has to make its own contribution. So, these two reasons push aviation engine engineers to improve the power units.

A number of ways are used to increase the range and altitude, speed, cargo capacity, also to improve the take-off and landing performance of aircraft and reduce the environmental impact.

3.1 Thrust and power improvement

Required thrust P, measured in [N] or [kg], and power N, measured in [W] or [h.p.] of the power unit.

Maximum airspeed is determined by the following equations:

• for (bypass) turbojets or turbofan engines

$$V_{\max }=\sqrt{\frac{2\cdot P}{C_{\mathit{Da}}\cdot S\cdot {\rho }_H}},$$E1

where C_Da is an aerodynamic drag coefficient; S is a wing area; ρ_H is the atmospheric density at a given altitude H;

• for piston and turboprop engines

$$V_{\max }=\sqrt{\frac{2\cdot N\cdot {\eta }_p}{C_{\mathit{Da}}\cdot S\cdot {\rho }_H}},$$E2

where η_p is propeller efficiency factor.

As an example of the efficiency of modern engines, it is possible to explain the maximum performance of different types of engines:

• The GE90-115B has a maximum thrust of 513,950 N (it showed a world record thrust of 569,000 N at the time of testing) for turbojet bypass or turbofan engines;

• The NK-12 has a maximum power of 11,000 kW (15,000 shp) for turboprop engines;

• The VD-4 K has a maximum power of 3200 kW (4300 shp) for piston engines;

• Cargo capacity, cruising altitude, runway length, ceiling and zoom altitude, and maneuverability are primarily determined by the available thrust of the aircraft’s engines.

3.2 Specific weight of an engine

The specific weight of an engine indicates the weight efficiency of an engine, or in other words, how much engine weight is required to produce a given level of thrust or power. The equation for specific weight is

• for turbojet bypass or turbofan engines.

$${\mathrm{\gamma }}_{\mathrm{en}}={\mathrm{m}}_{\mathrm{en}}\mathrm{g}/\mathrm{P},$$E3

where m_en is the engine weight; g is gravity acceleration;

• for piston and turboprop engines, [daN/kw] or [kg/h.p.]

$${\mathrm{\gamma }}_{\mathrm{en}}={\mathrm{m}}_{\mathrm{en}}\mathrm{g}/\mathrm{N}.$$E4

So, the minimum specific weight of the power unit is required.

Modern time, the lowest specific weight for different types of engines are

• for piston engines is γ_en = 0.67–1.3, [daN/kw];

• for turboprop engines is γ_en = 0.27–0.33, [daN/kw];

• for turbojets is γ_en = 0.2–0.25;

• for turbojets with afterburner is γ_en = 0.15–0.2;

• for bypass turbojets or turbofans is γ_en = 0.165–0.22;

• for bypass turbojets or turbofans with afterburner is γ_en = 0.1–0.15.

For an example, Prof. Mozhaisky airplane steam engines had γ_en = 10.7, Brothers Writes airplane with piston engine had γ_en = 8.4 and now, the most powerful turbojet in the world GE90-115B has γ_en = 0.167.

3.3 Specific weight of a power unit and fuel

The weight of a power unit is determined not only by the engines but also by all the subcomponents and subsystems, including the fuel. And here we can see the collision. On one side as smaller weight then better, but on the other side as more higher weight of fuel than more longer range and duration of a flight. The required weight of fuel is determined not only by the required range and duration of a flight but also by fuel consumption, which will be discussed below.

The minimum specific weight of an engine and fuel is determined by the eqs. [7].

$${\overline{m}}_{p.u}=\frac{m_{p.u}}{m_{t.o}}=n_{\mathit{en}}{k}_{p.u}\frac{m_{\mathit{en}}}{m_{t.o}}=n_{\mathit{en}}{k}_{p.u}\left(\frac{m_{\mathit{en}}g}{P_0}\right)\left(\frac{P_0}{m_{t.o}g}\right)=n_{\mathit{en}}{k}_{p.u}{\gamma }_{\mathit{en}}{t}_0,$$E5

$${\overline{m}}_f=\frac{m_f}{m_{t.o}},$$E6

where m_p.u is the weight of a power unit; m_t.o is the take-off weight of an aircraft; n_en is the number of engines; k_p.u is an empirical factor for the additional weight of a power unit of subcomponents and subsystems for an engine, engines (approximately k_p.u = 1.2–2.2); t₀ is the specific weight to thrust ratio of an aircraft, t₀ = P₀/m_t.og; m_f is weight of the fuel; ${\overline{m}}_f$ is the specific weight of the fuel.

Some values of ${\overline{m}}_{p.u}$ and ${\overline{m}}_f$ based on statistical data for different types of aircraft are presented in Table 1.

Type of aircraft	${\overline{m}}_{p.u}$	${\overline{m}}_f$
Subsonic passenger and transport	0.08–0.14	0.18–0.40
Maneuverable	0.18–0.22	0.25–0.30

Table 1.

Examples of ${\overline{m}}_{p.u}$ and ${\overline{m}}_f$ based on statistical data.

3.4 Specific fuel consumption

The specific fuel consumption indicates the efficiency of the engine because it shows how much fuel the engine needs under given flight conditions to produce 1 N of thrust (or 1 W of power) in 1 hour.

Minimal specific fuel consumption can be achieved in several ways: good aerodynamics of an airframe, rational choice of flight parameters (altitude, speed, etc.), rational operation of subsystems of a power unit, and, of course, it depends on the engine performance. Table 2 shows some examples of the fuel consumption for the most powerful engines.

Engine	Cp [kg/N⋅hour]	Cp [kg/kw⋅hour]
GE90-115B	0.033	—
NK-12	—	0.224
VD-4K	—	0.251
D-36	0.0662	—

Table 2.

Examples of the fuel consumption for the most powerful engines.

Specific cruise fuel consumption is the primary determinant of an aircraft’s economics and has a significant impact on the amount of emission emitted into the atmosphere.

3.5 Aerodynamic drag

The engine has very bad shape from the point of view of aerodynamics. This problem is usually solved by using nacelles. But subsystems of an engine have several external components such as air intakes of a ventilation system, access doors for maintenance operations, etc. All these structural features must be optimized by the next approaches:

• Reducing the aerodynamic drag generated by the engine and minimizing shock pressure losses by optimizing the design of engine air intakes, radiators, cooling systems;

• Propeller shape and control optimization;

• Nozzle shape and control optimization (the latter is for supersonic and thrust vectoring aircraft);

• Optimization of thrust rever device performance.

3.6 Environmental impact

The environmental impact of aviation has two parts: (1) emissions; (2) noise. Both are shown in the ICAO environmental impact forecast (Figure 15).

Figure 15.

Forecast environmental requirements: (a) Emission CO₂ [5]: 1—Emission growing without any actions; 2—Emission reducing forecast; 3—Baseline for CO₂ level for 2020; 4—Technology approaches; 5—Operational approaches; 6—Infrastructures approaches; and 7—New fuels types (b) Noise level [6]: 1—ICAO Chapter 2 (1973); 2—ICAO Chapter 3 (1977); 3—ICAO Chapter 4 (2001); 4—ICAO Chapter 14 (2013); 5—Noise reducing forecast; 6—Different international programs.

3.6.1 Emissions

First of all, the approaches to reduce emissions will be explained.

As it has been said above, fuel consumption has an important place in the reduction of environmental impact. It is especially actual for the engines with traditional oil-based fuel. So, how it is possible to improve the environmental performance of the traditional for the modern time engines. There are several approaches:

• Increase the bypass ratio. This ratio for the modern aircraft can reach 12.5 (for example, Pratt & Whitney PW1000G [17]). But, for the improvement of the environmental impact, it is reasonable to increase it up to 20 (for example, NK-93 experimental engine had 16.6 [18]);

• Increase the pressure ratio of the engine. This factor for modern aircraft has level about 40 (for example, GE90-115 has 42 [19]). Future trend for this ratio is level about 60 (for example, GE9X experimental engine has 60 [20]);

• Increase the temperature in ahead of the turbine. The gas temperature indicates the thermal load on the turbine. Now the operating temperature before the turbine is about 1400°C [2]. But there is an example with the temperature about 1560°С (PD14 [21]);

• Reduction of aerodynamic drag by improving the geometric parameters of the airframe. In this way, the reduction in fuel consumption can reach 7% [21, 22, 23, 24].

The first three approaches can provide fuel consumption (emission level correspondingly) decreasing about 15%.

But, as it seems, that all these approaches cannot provide decreasing in emission of 50% as it is required by ICAO. And nowadays there are new solutions, which can provide the required level of emission for the aviation industry. They are new types of fuel or engines.

First of all, about new types of fuel. In 1988, the USSR started test flights of the TU-155 (Figure 16), it was the first aircraft of the transport category with hydrogen fuel [25]. It had an experimental NK-88 engine.

Figure 16.

Tu-155 is the first transport category aircraft with hydrogen fuel power unit [25]: 1—Hydrogen fuel tank; 2—Engines; and 3—Passenger compartment.

Liquid hydrogen, with its high specific calorific value, which is three times higher than that of traditional oil-based fuels, and exceptional environmental purity, has shown great promise as a fuel for various engines.

Another possibility is the application of electric or hybrid power units. This process started with electric drones at the beginning of the twenty-first century. Nowadays, there are many projects for the general aviation aircraft (for example, Figure 17).

Figure 17.

NASA X-57 Maxwell modification IV is testing electrical aircraft [26].

But until now, the application of electric power units for the transport category aircraft is still under development and research.

For the light aircraft, the main power source has been lithium-ion batteries. The use of batteries as the main source of energy limited the capabilities of aircraft—range, flight duration, cargo capacity. Therefore, aviation engineers began to consider alternative options for obtaining energy. Some of them are as follows:

• Solar panels that convert radiation energy into electricity;

• Fuel cells, which convert the chemical energy of the fuel into electrical energy without combustion; hydrogen is most commonly used as the fuel.

A hybrid power unit converts energy twice: first into mechanical energy with the help of traditional engines, then into electrical energy with the help of generators. A hybrid power unit consists of an electric part (electric motor, generator, battery) and a traditional internal combustion engine using chemical fuel. And if today it is oil-based fuel, in the future it will be hydrogen, which opens great prospects for the development of aircraft based on the “all electric aircraft” approach.

An all-electric aircraft produces no emissions. However, it is not yet considered completely environmentally friendly because the production of batteries pollutes the environment and their structure and chemical composition make them difficult to dispose of.

3.6.2 Noise

Noise is also a dangerous factor that affects the environment. There are two sources of noise: airframe and engine. Improving the geometric parameters of an airframe can reduce the noise for cruise flight [27]. At the time of landing, the airframe has a landing configuration for its high-lift devices. Engine noise depends on engine operating modes (that also depends on flight mode). Most higher noise is presented for the maximum thrust mode, which is present for the take-off and maximum flight speed, and also for the thrust reverse mode, so in time of the landing (Figure 18).

Figure 18.

Typical airport noise map [28].

The maximum flight speed mode is appropriate for the high flight altitude, and the noise in this case is dangerous only for the persons onboard: crew members and passengers. The noise can be reduced by the correct choice of fuselage skin panels and engine nacelle panels. These two structural solutions are usually sufficient (see Figure 19).

Figure 19.

Typical noise absorption methods [7, 29] (from fan, compressor, combustion) for bypass turbojet or turbofan engines: (a) structural solutions maps for an engine; (b) typical soundproof honeycomb panel; 1—Acoustical covering of power unit elements (thick lines); 2—Optimum clearances; 3—Optimal number and configuration of blades; 4—Optimal nozzle location to reduce exhaust velocity; 5—Fan without inlet guide vanes; 6—Backing skin; 7—Honeycomb with soundproofing features; and 8—Perforate lining.

However, for the near-ground flight modes (takeoff, climb, approach, landing, thrust reverse), such approaches are not sufficient. The main sources of engine noise are: propeller (for turboprop and piston engines), fan, combustion, turbine and nozzle (for turbojet engines). Noise from internal structures can also be absorbed by nacelle panels [7, 29]. However, propeller and jet noise are difficult to reduce.

The blades of a propeller can be optimized by shape, twist angle, control device, etc. (Figure 3). The specific shape was developed for the exhaust nozzles to reduce the noise level (see Figure 20).

Figure 20.

Typical noise absorption methods [30] (from the jet and thrust reverse) for bypass turbojet or turbofan engines: 1 and 2—Special shapes for nozzles of hot and cold channels; 3—Thrust reverse only for the cold channel; and 4—Cold channel.

Thus, all of the above structural approaches can provide noise reduction for all modern high-efficiency engine types.

4. Summary and conclusions

Future development of power units requires an in-depth analysis of existing design, manufacturing, and logistics solutions. Their optimization can provide increasing in power unit efficiency. However, new ideas can provide more best performance, but they require a lot of time for research, testing, and analysis of results. New approaches (e.g. hydrogen power units) create significant impact on the traditional (for aviation engineering) design process.

Explained above detailed separation of the power unit structure by subsystems and subcomponents can provide more best understanding areas where it is necessary to apply extra efforts for the power unit efficiency increase, and can help to understand dependencies between different structural members of a power unit.