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Gas Turbine Engine
A Jet Engine is a reaction engine - that is, an engine which provides propulsion or thrust by expelling a reaction mass - and works in accordance with Newton's third law of motion: "For every action (force) there is an equal and opposite reaction (force)".
Most jet engines used in aviation are air breathing, axial flow, gas turbine engines. A gas turbine is a rotary engine that extracts energy from a flow of combustion gases. Ambient air is drawn into the engine intake where an axial or a centrifugal compressor (or both) increases both the pressure and temperature of the air before feeding it into the combustion chamber. In the combustion chamber, fuel is added to the hot, compressed air and ignited. Once ignition has occurred, it is self sustaining as the constant flow of air and fuel provide for continuous combustion. A high energy exhaust stream (reaction mass), produced by burning fuel/air mixture, leaves the combustion chamber passing through one or more turbines which serve to drive the compressor(s). The remaining exhaust gas is ejected through a nozzle providing thrust (force) to propel the aircraft forward.
A turbojet engine is most efficient when the speed of the aircraft that it propels approximates the speed of the exhaust gas. In many cases, aircraft are designed for speeds much slower than that of typical jet exhaust so the engine turbines are also used to drive other components such as a fan, propeller or other machinery. In this way, turboprop, turbofan and turboshaft engines are optimised for the speed and the type of the aircraft that they power.
Engines under development for very high speed applications eliminate the need for a powered compressor. In a "ram" engine such as a ramjet or a scramjet, air entering the engine is compressed due to the intake and compressor section geometry and the high forward speed of the aircraft. As a consequence, these types of engine do not require a compressor or the turbine to drive it but the engine cannot operate while the aircraft is stationary.