4.4 Engine Materials & Structures

4.4.1 Overview

Different types of materials used in engines include plastics, metals, ceramics and composites. Each of these has its own advantages and disadvantages. Plastics cannot withstand high temperatures but are lighter than many other materials, less breakable and rust-free. Metals used in engines usually refer to alloys and superalloys that have high strength and can withstand high temperatures and stresses. The three main groups of superalloys are nickel-based, cobalt-based and iron-based. Ceramics can withstand high temperatures but are quite fragile. They are usually combined with other material to form composites. Composites are able to withstand high temperatures and are light in weight. The three main types include polymer matrix composite (PMC), metallic matrix composite (MMC) and ceramic matrix composite (CMC).




4.4.2 Turbofan

Different parts of the engine have different requirements for its material. We shall look into a turbofan engine and explore the usual requirements in the materials of the fan, compressor, combustor, turbine, mixer and nozzle. Figure 4.4.2.1 below outlines the 6 main parts of a turbofan engine and the requirements and materials used for these 6 parts are elaborated in table 4.4.2.1.

As seen from table 4.4.2.1 above, different materials are used in different parts of the engine due to different requirements and temperature working range. Materials are being constantly improved upon to facilitate the best functioning of engine. Most of the time, materials of fans, combustors and turbine are being looked into for better efficiencies. For instance, General Electric has introduced the use of composite fan blade of the typical titanium ones. The advantage of using composite is that the engine will become lighter and the engine can have a higher bypass ratio. These allow the engine to have lower fuel consumption and hence are more efficient.

Below is a diagram taken from Rolls Royce that highlights the improvements in efficiency of engines in relation to the bypass ratio.

Combustors in jet engines require high temperatures for optimum combustion process. The higher the temperature, the better the fuel burns to completion and hence the higher the fuel efficiency and the lower the emissions. Usually, combustion temperatures exceed the melting point of most materials. As such, air must be cooled by a cooling system before entering the turbine. Alternatively, materials that can withstand this high heat can be used together with some insulating coatings.

Under the ultra efficient engine technology, ceramic matrix composite, polymer matrix composites and advanced disk alloys will be looked into for use in combustor liners and turbine vanes in order to withstand high temperatures so that complete combustion of fuel can take place. As for the static engine structures, lightweight materials will be developed to reduce the overall weight and hence reduce fuel consumption.

In addition, ultra efficient engines will also make use of active combustor control to achieve a lower combustor pattern factor and hence reduce the turbine cooling requirements. An extra benefit generated by this method is that thermal fatigue and failure can be reduced.