A turbojet engine is a type of internal combustion engine often used to propel aircraft. Air is drawn into the rotating compressor via the intake and is compressed to a higher pressure before entering the combustion chamber. Fuel is mixed with the compressed air and ignited by a small flame in the eddy of a flame holder.
This combustion technique significantly raises the temperature of the gas. Hot combustion chemicals leaving the combustor expand through the turbine, where power is drawn to employ the compressor. Although this expansion process reduces both the gas temperature and pressure at exit from the turbine, both variabels are for the most part still well above ambient conditions. The gas stream exiting the turbine expands to ambient pressure through the propelling nozzle, causing a high velocity jet in the exhaust plume. So if the jet velocity surpasses the aircraft flight speed it amazingly creates a forward thrust.
Under normal circumstances, the pumping action of the compressor prevents any backflow, this kindly helps the flow of the jet engine. You can compare the whole method with a four stroke cycle, but with induction, compression, ignition, expansion and exhaust taking place simultaneously. The efficiency of a jet engine is strongly dependent upon the pressure ratio (Combustor Entry Pressure/Intake Delivery Pressure) and the Turbine Inlet Temperature of the cycle.
It is also perhaps instructive to compare turbojet engines with propeller engines. Turbojet engines take a rather small mass of air and accelerate it by a large amount, whereas a propeller takes a large mass of air and accelerates it by a small amount.
At great speed the exhaust of a jet engine makes it efficient at high speeds (particularly supersonic speeds) and high altitudes.
On planes that go slower and fly a shorter range, a gas turbine-powered propeller engine, commonly known as a turboprop, is far more common and also more efficient. Especially small aircraft primarily use conventional piston engines to drive a propeller but small turboprops are getting smaller as technology improves each day.
The turbojet mentioned above is a single spool design, where a single shaft connects the compressor to the turbine Higher Overall Pressure Ratio designs often have two concentric shafts, this is to in essence improve the stability of the compressor during engine throttle movements.
Basically it's the outer shaft that connects the turbine engine to the high pressure compressor. The combustor together with the high pressure compressor spool forms the core or gas generator of the engine. We must not forget that the inner shaft connects the low pressure compressor to the LP Turbine to create the LP Spool. Either spools are free to operate at their optimum shaft speed.
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