Table of Contents
1.0 Introduction
3
2.0 Basic Components of a Jet Engine
3
2.2 Compressor
3
2.3 Combustor
4
2.4 Turbine
4
2.5 Exhaust Nozzle
4
3.0 Classification of Jet Engines
4
3.1 Turbojet engines
5
3.2 Turboprop Engines
6
3.3 Turbofan Engines
6
3.4 Turboshaft Engines
7
4.0 Creation of Thrust in a Jet Engine
7
5.0 Conclusion
9
6.0 References
10
1.0 Introduction
According to Hunecke (1997), jet engines also known as gas turbine engines are the most widespread and most efficient method used for airplane propulsion currently. The use of jet engine began during the Second World War. Initially, challenges existed during the first designs of the jet engines. The transition to using of jet engines especially in the Navy brought changes such as the use of hydraulic flight controls, redundant hydraulic systems, air conditioning among others (Rubel, 2010). In the aviation industry, jet engines represent one of the most significant technological developments. Currently, the use of jet engines is being applied in all fields of aviation (Hunecke, 1997).
2.0 Basic Components of a Jet Engine
2.1 Air Intake
The air intake acts as a fluid flow duct, which directs the airflow to ensure that the engine functions correctly to generate thrust (Hunecke, 1997). The intake has to be designed to deliver the needed quantity of airflow to the engine and ensure that airflow entering the compressor is stable and uniform. All these conditions have to be met when the aircraft is on the ground and during flight. A good intake design is required to ensure that engine performance is close to figures obtained during standard testing.
2.2 Compressor
The function of the compressor is to increase the pressure of the airflow that comes from the air intake. Mechanical energy is supplied to the compressor via rotating blades that exert aerodynamic forces on the airflow (Hunecke, 1997). Important compressor performance parameters include compressor efficiency, compressor total pressure ratio and the airflow rate. These parameters influence the amount of energy required and the quality of energy conversion that is achieved (Hunecke, 1997). Compressor types used can be either axial-flow or centrifugal-flow compressors.
2.3 Combustor
The combustor provides a stream of hot gas that provides energy to the turbine and nozzle components of the engine. Heat is added by burning a mixture of compressed air and vaporized fuel (Hunecke, 1997). Minimal loss of pressure is required in the combustion chamber during combustion. Types of combustors used include can-type burners, annual-type burners and can-annular type burners.
2.4 Turbine
The turbine is used to drive the compressor by providing aerodynamic forces. A high turbine power is obtained by extracting all the energy present in the hot gas. On its own, a distinct turbine blade contributes approximately 250 hp (Hunecke, 1997).
2.5 Exhaust Nozzle
The function of the exhaust nozzle is to transfer gas potential energy into kinetic energy required to generate thrust (Hunecke, 1997).
3.0 Classification of Jet Engines
Jet engines are classified according to the tasks they perform. Some of the design characteristics that are used to differentiate the jet engine types include the principle of compression, number of spools, utilization of exhaust gas and the distribution of airflow within the engine (Hunecke, 1997). According to Hunecke (1997), there are four basic types of jet engines used in aircrafts. These include turbofan, turbojet, turboshaft, and turboprop. The turbofan and turbojet use reactive forces generated by the exhaust gas to provide propulsion forces. In the turboshaft, the entire utilizable hot gas energy is extracted and transferred into shaft power. This is made possible by the presence of an additional turbine. Helicopters use the turboshaft engine. For a turboprop engine, hot gas energy is utilized to drive an extra, but separate turbine, which supplies shaft power to drive the propeller.
3.1 Turbojet engines
The turbojet engine has the following components:
Multistage compressor
Combustor
Single or multistage turbine
Processing airflow requires an air intake and an exhaust system. This ensures that the turbine functions properly to produce thrust. Air entering the intake section is directed as a smooth stream of air to the compressor (Hunecke, 1997). The compressor raises the pressure of the air. During the compression, the energy transfer results to an increase in temperature and density. From the compressor, the compressed air enters the combustion chamber where there is fuel injection and burning. This causes an increase in energy in the airflow. The turbine, normally attached to the compressor, is used to convert the gas energy into mechanical energy that is utilized to drive the compressor.
The exhaust nozzle converts a major proportion of the heat and pressure energy of the gas to kinetic energy. Generation of thrust requires a high exhaust velocity. The exhaust velocity may be increased by using a thrust augmentation that adds heat downstream of the turbine. An example of a turbojet engine is the General Electric J79 engine used in the Phantom combat aircraft and the Starfighter.
3.2 Turboprop Engines
These engines have a gas generator that has the following components
The turbine component of the gas generator is designed in a manner that surplus energy is abstracted from the hot gas to drive the compressor and auxiliaries, whereas the surplus shaft power is used to drive the propeller. The turboprop engine differs from the turbojet since it has an extra turbine to drive the propeller, mechanical reduction gear, and a two-spool rotational machinery (Hunecke, 1997).
3.3 Turbofan Engines
This is one of the most common gas turbine engines being used for aircraft propulsion. The turbine component is designed similar to that of a turboprop engine. That is it abstracts more energy from the hot gas to drive the compressor plus other auxiliaries (Hunecke, 1997). The excess shaft power is used to drive a fan. This fan is a low-pressure compressor that has a larger diameter and is arranged upstream of the main compressor. A portion of the air entering the engine intake is processed in the fan component and bypasses the inner engine expanding through a separate nozzle to provide a cold thrust, which increases the propulsive efficiency (Hunecke, 1997). According to Hunecke (1997), the quantity of air that is bypassed in comparison with the air that passes through the inner engine is referred to as the bypass-ratio. The high bypass-ratio engine is mostly found in most commercial transport aircraft, whereas a low bypass-ratio is most common in supersonic combat aircraft. The advantage of a high bypass-ratio turbofan engine is that its high thrust level, mainly during take-off, results mainly from accelerating a large air mass bypassing the inner engine with the thrust from the inner engine being only about 15% of the total engine thrust (Hunecke, 1997).
3.4 Turboshaft Engines
This engine is similar to the turboprop apart from the function of the extra turbine. The turbine-driven shaft is joined to a transmission system that drives helicopter rotor blades instead of driving a propeller (Hunecke, 1997). Pressure of the incoming air is increased when it enters the compressor. Once the pressure is increased, the air is directed to the combustion chamber where the air is blended with vaporized fuel and burned. The hot gas expands through the two separate turbines, one drives the compressor and the other drives the helicopter rotor blades by means of a transmission gear. The gas is then released through an exhaust duct without generating any thrust. The turboshaft engine is mostly employed in small aircraft.
4.0 Creation of Thrust in a Jet Engine
The creation of thrust in a jet engine is based on Newton’s law of motion applied to a steady flow of air (Cumpsty, 2003). The momentum of air leaving the engine is supposed to be higher than the momentum of the air inflowing the engine. The outcome is a high kinetic energy for the jet. The high-energy input of the jet comes from burning of the fuel. According to Hunecke (1997), propulsion in a jet aircraft occurs based on the principle of reaction. This principle states that a gas jet exhausting at elevated velocity from a nozzle generates a force in the reverse direction that is referred to as thrust (Hunecke, 1997). The thrust force will depend on the airflow passing through the jet engine and the exhaust velocity. The jet engine increases the momentum of the airstream flowing through it.
Momentum change
Considering an engine on a pylon under a wing as shown below, the only force applied is through the pylon.
Assumptions made include the wing lift and drag being unaffected by the engine and the engine being unaffected by the wing. Additionally, it is assumed that there is uniform static pressure around the control surface (Cumpsty, 2003). A flow of fuel mf flows through the pylon, but with a low velocity, thus moment is insignificant. Consequently, a mass flow of air mair enters the engine. For a bypass engine, the velocity differences between the inner engine and the bypass streams have to mix out. Thrust will be calculated by taking into account the flux of momentum across the control surface around the engine. The air enters the control surface with a velocity V. Air entering the control surface passes along the engine with only a small portion, mair passing through the engine. According to Crumpsty (2003), the air that passes around the engine exits the control surface with the same velocity V as the flight speed; thus it does not contribute to thrust.
Therefore, the flux of the momentum entering the engine is given by the equation below
In addition, the flux of momentum leaving the engine is given by the equation below
The net thrust FN is given by the difference between the two fluxes
Increase in kinetic energy causes an increase in velocity between the flow of air entering the engine and that of the air leaving the engine (Cumpsty, 2003). The kinetic energy results from the effect of the work supplied by the engine to the air via combustion.
5.0 Conclusion
This report provides a explanation of the basic components of a jet engine and the workings of each component to produce thrust that is used for propulsion. Airflow is directed to the compressor via the air intake. The compressor increases the pressure of the airflow before releasing it to the combustor. The combustor increases the energy of the airflow through burning using fuel. The turbine provides energy to the compressor, which is used to increase pressure of the gas. The exhaust nozzle converts the hot gas energy to kinetic energy, which generates thrust.
6.0 References
Cumpsty, N. A. 2003. Jet propulsion: a simple guide to the aerodynamics and thermodynamic
Design and performance of jet engines. 2nd ed. Cambridge, U.K.: Cambridge University Press.
Hünecke, K. 1997. Jet engines: fundamentals of theory, design, and operation. Osceola, WI,
USA: Motorbooks International.
Rubel, R. 2010. THE U.S. NAVY’S TRANSITION TO JETS. Naval War College Review,
63(2), pp 49-59.
