An Engine is a thermal device that converts heat energy into mechanical energy. Mechanical energy is principally derived in the form of torque on the output shaft of the engine and is utilized for necessary driving works.
Energy input to an engine is ‘heat’. Heat so used by the engine is derived from different sources, such as: (i) Solid fuel, (ii) Liquid fuel, (iii) Gaseous fuel, (iv) Nuclear fuel. Energy input system of the engine ensures efficient release of heat from the fuel. For the case a chemical fuel (solid/liquid/gaseous), combustion is the process that is to be carried out to liberate heat from fuel through an exothermic reaction. For nuclear fuel, a nuclear reaction is to be carried out in a nuclear reactor so that energy is liberated from atoms through nuclear chain reaction in which ‘nuclear binding’ energy is released as a result of re-arrangement of atomic particles.
Depending upon where the combustion process is carried out, (that is, outside the engine or inside the engine), the engine is classified as External Combustion Engine (ECE) and Internal Combustion Engine (ICE). For an ECE, combustion is carried out externally in a furnace and heat is utilized to produce working fluid, such as, generation of steam from water in a boiler by heating, heat being produced by burning coal. Here, steam is the working fluid that is taken to drive a steam engine. ECEs have applications in the field of industrial electric power generation. In the ICEs, combustion of fuel is carried out in a space or chamber inside the engine itself. This space/chamber is called the combustion chamber. Metered and atomized fuel is burnt in compressed air taken in combustion chamber and the hot gas so produced, called the flue gas with heat energy is the ‘working fluid’ that is directly used to stroke a piston or rotate a turbine wheel. ICEs are compact, all sections being integrated into single unit and hence, they have got wide-spread application in industries, locomotives and aircraft. They are in the form of piston engines and gas turbine engines.
A gas turbine engine is an ICE that uses turbines to convert heat energy of a gas into torque; the gas is the combustion-product produced by burning fuel in compressed air in its combustion chamber inside the engine. A turbine is a rotary device with arrangement of series of blades around the periphery of a wheel mounted on a shaft so that energy of the working fluid, when impinged over blades, will rotate the wheel. In short, turbine is an energy transfer mechanism, transferring energy from working fluid to its shaft in the form of rotation or torque.
In aircraft application, an engine is a propulsive device that provides propulsive force (thrust) to propel the aircraft forward overcoming atmospheric drag. Thus, as long as aircraft propulsion is concerned, the objective of the aircraft gas turbine engine is not directly the work output at its shaft but is the propulsive force. This is fundamental difference between the primary objective furnished by the gas turbineengines in industrial applications and in the aircraft applications. However, the basic aerodynamic and thermodynamic considerations are almost the same.

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