From SKYbrary Wiki
A turbojet engine is a jet engine which produces all of its thrust by ejecting a high energy gas stream from the engine exhaust nozzle. In contrast to a turbofan or bypass engine, 100% of the air entering the intake of a turbojet engine goes through the engine core.
The component parts of a turbojet engine are the inlet, the gas turbine engine, consisting of a compressor, a combustion chamber and a turbine, and the exhaust nozzle. Air is drawn into the engine through the inlet and compressed and heated by the compressor. Fuel is then added in the combustion chamber and ignited. The burning fuel adds energy to the exhaust stream by heating and expanding the air. Sufficient energy to drive the compressor is extracted from the exhaust stream by the turbine. The remainder of the exhaust energy is used to produce thrust, a process which is enhanced by the geometry of the exhaust nozzle. As the exhaust gas passes through the nozzle, it is accelerated to high speed as it expands thus providing propulsion. The thrust produced by the engine can be selectively increased by incorporating an afterburner or re-heat into the engine design.
Turbojet aircraft work on the principle of accelerating a relatively small mass of air to a high speed. As optimum efficiency is achieved when the speed of the accelerated air approximates that of the aircraft, turbojet engines do not reach peak efficiency until speeds approaching mach 2. For sub-mach speeds, a turbofan engine is significantly more efficient and also much quieter due to the greater mass and lower speed of the exhaust stream leaving the engine.