*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

### **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*

environmental impact, it is reasonable to increase it up to 20 (for example, NK-93 experimental engine had 16.6 [18]);


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.

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**).

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—

### **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.*

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

**Figure 17.** *NASA X-57 Maxwell modification IV is testing electrical aircraft [26].*

range, flight duration, cargo capacity. Therefore, aviation engineers began to consider alternative options for obtaining energy. Some of them are as follows:


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 oilbased 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**).

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 18.** *Typical airport noise map [28].*

### **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.

### **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.*

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

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**).

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