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Gas turbine engines are used to move an aircraft through the air, hereby a propulsion system is used to generate thrust. A gas turbine is actually an internal combustion engine, which uses air as a working fluid. In the following sub-paragraphs four different variations of gas turbine engines will be discussed. Firstly the turboprop engine will be discussed, whereby a propeller is attached to the jet engine (1.2.1). After the turboprop comes the turbofan engine, this engine is known of its fuel efficiency with high thrust (1.2.2). Then the simplest type of a gas turbine engine will be discussed, the turbojet engine (1.2.3). In a turbojet engine an afterburner can be employed, which increases the thrust dramatically (1.2.4). At last a conclusion will be drawn with regard to the engine which is most suitable for the Fokker 70 (1.2.5).
1.2.1 The turboprop engine
A turboprop engine generates thrust with a propeller, whereby the engine is attached to the propeller (Appendix X). In a turboprop engine the air enters the engine at the inlet. Then the air passes through the compressor, where an increase in temperature and pressure occurs. The high-pressure air than enters the combustion chamber. In this chamber fuel is added which then ignites. The air then goes through the turbines. The turboprop engine has two turbines, one rotating turbine wheel that drives the compressor by the hot gases which flows through the engine after entering the combustion chamber. Then the gases pass through a power turbine which is coupled to the shaft, this power turbine drives the propeller that is connected to the gearbox. So actually most of the energy is used to run the compressor and propeller, and the remaining energy provides a small amount of thrust. Also because the power output of the turbine is proportional to the squire of the speed of the propeller blades, high rotational speeds are limited because of the compressibility effects at the blade tips. This compressibility losses are avoided by the gearbox, which slows the rotational speed to values that keep the propeller tips on a save speed.
1.2.2 The turbofan engine
The turbofan engine is the most modern engine that can be found on an aircraft (Appendix X). This engine consist of five main parts: the fan, the low- and high-pressure compressor, the combustion chamber, the high- and low-pressure turbine, and the nozzle. At the inlet of the turbofan engine the air passes the fan, some of this air travels through the core and most of it travels around the core. The air that passes the core, also known as bypass air, is accelerated by the fan and exits the engine creating thrust. The air that goes through the core, which also is accelerated by the fan, enters the low-pressure compressor (LPC). Here the volume of the air decreases, because of the cross section are of the LPC. Hereby the temperature of the air increases and the air pressurizes. Then the air enters the high-pressure compressor (HPC). Here the volume decreases even more, creating a higher temperature and pressure state. This air then enters the combustion chamber where combustion occurs. In the combustion chamber fuel is added and mixed with the air, this air then ignites producing a rapid increase in heat energy. This mixture of air then enters the high-pressure turbine (HPT), which spins the HPC connected by the high-pressure shaft to the HPT. The cross sectional area of the HPT increases, creating an increase in volume and a decrease in temperature and pressure. Thus the air exiting the HPT is less pressurized and cooler than the air entering the HPT, which created a decrease in the overall energy. Then the air enters the low-pressure turbine (LPT), which spins the fan and the LPC by the low-pressure shaft. The LPT has actually exactly the same function as the HPT. But the air exiting the LPT is significantly cooler than the air exiting the HPT, but it is still hot. The hot air then enters the nozzle, providing thrust by propelling the flow out of core of the turbofan engine.
1.2.3 The turbojet engine
The turbojet engine is the simplest type of an aircraft gas turbine engine (Appendix x). This engine consist of four main parts: the compressor, combustion chamber, turbine , and the nozzle. At the inlet of the turbojet engine air is drawn into the engine by the compressor. In the compressor the velocity of the air will be decreased and compressed. The compressed air will be forced into the combustion chamber with a high pressure. In the combustion chamber fuel is added into the high-pressure air and ignited, which produces a rapid increase in heat energy. This gases will be send into the turbine at a high speed, where part of the energy is used to power the turbine. The turbine will now rotate a high speed which in turn drive the compressor to which is attached by a shaft. Thereby the compressor is continually drawing in more air. After the expanding gases leave the turbine, they will be blown out through the nozzle. The gasses have lost some its energy through the turbine, but it has enough energy left over to accelerate through the nozzle providing a fast moving propelling jet stream.
1.2.4 The afterburning turbojet engine
The afterburning turbojet is used by most modern fighter aircraft for supersonic flights. This engine looks exactly like a turbojet engine, but then with an additional combustion chamber after the turbine. In this chamber fuel is added into the hot exhaust and producing more thrust which allow supersonic flight. But this chamber is also very inefficient, because it uses to much fuel. When turning the afterburner of, the engine can then be used like a turbojet engine.
The new gas turbine engine that has to be mounted on the Fokker 70, needs to fulfill the standards of today. These standards consists of a high efficiency, therefore a low fuel consumption, powerful enough to provide the needed thrust of M = 0.73, high reliability. The gas turbine engines that cannot meet these standards are the turboprop engine, the turbojet engine, and the afterburning turbojet.
The turboprop engine is fuel efficient, but is not able to provide the needed thrust of M = 0.73. Providing such speeds, means that the propeller blades have to spin really fast. Which is an undesirable, because of limited compressibility effects at the blade tips.
The turbojet engine can provide the needed thrust. But this engine is very noisy during operation and at low pressures. Also the fuel consumption of this engine two to three times greater than the turbofan engine.
The afterburning turbojet is only used to fly at supersonic speeds and the added afterburner is also not fuel efficient. Because the Fokker 70 is not allowed to fly at supersonic speeds, this engine will not be discussed.
Thus the only engine remaining is the turbofan engine. The turbofan engine has a low fuel consumption, is powerful enough to provide the needed thrust, and has a high reliability. So in the upcoming paragraphs and chapters the turbofan engine will be elaborated in more detail.