How a Jet Airplane Engine Works
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Diffuser
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The first component of a jet engine is a diffuser. This is the space between the mouth of the engine and the first set of blades on the compressor. The cross-sectional area increases as the space gets close to the compressor, so the velocity of the flow is decreased and the pressure is increased. This aids the compressor and begins the process of adding energy to the flow.
Compressor
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The airflow in the jet engine moves from the diffuser into the compressor. The first jet engine compressors were centrifugal compressors, and they are still used on some small jet airplanes. Axial compressors are used on most modern jets as they are much more efficient. Regardless of the type, the purpose of the compressor is to add energy to the flow by increasing pressure. This step is similar to the piston in an internal combustion engine adding pressure before the fuel and spark are added.
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Combustion
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Heat energy is added to the flow in the combustion chamber. Fuel is added to the flow and then ignited. The flow is then passed through to the turbines.
Turbines
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The high energy flow is passed through a set of turbine blades attached to a shaft that drives the compressor. The shaft passes axially through the engine; since the flow through the turbines has more energy, the engine will run itself as long as the combustion chamber is operational. Until the turbines are spinning, the compressor can't work. Jet engines actually need a separate starter engine because of this design characteristic.
Nozzle
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The final stage of a jet engine is the nozzle. A nozzle is essentially the opposite of a diffuser. The cross-sectional area decreases, so the flow velocity increases. The force created when the flow leaves the nozzle is called thrust. This is what propels the airplane forward. The higher the velocity, the more thrust that is created. Some jets, especially military fighter jets, have adjustable nozzles. The exit diameter can be increased or decreased as needed to optimize thrust.
Vectored Thrust
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A few fighter aircraft use "vectored thrust." The nozzles on the jet engines can rotate so the thrust is no longer acting axially through the engine. This helps in maneuverability because the thrust can be used to help change directions along with the airplane's control surfaces. One jet, the AV-8B Harrier, uses vectored thrust to take off on short runways or even vertically. This capability has given the Harrier the nickname "Jump Jet."
Afterburner
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Most military fighter jets also have an afterburner. This is like a "turbo-boost" for a jet engine. Raw fuel is dumped into the airflow after it leaves the turbines, but before it enters the nozzle. The fuel is ignited, giving the airflow an acceleration boost which drastically increases the velocity going into the nozzle. The result is increased thrust. This process is very inefficient, though, and can only be done for short periods of time. Otherwise, the pilot would quickly run out of fuel.
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