The turbojet engine was the earliest type of a turbo-propulsion machine. It had a simple design as it was made up of three basic components; a multi-stage compressor, a combustor, and a single or multi-stage turbine. However, in addition, an air intake and an exhaust system are needed to process the airflow, so that it is able to function efficiently and produce thrust.
In a turbojet engine, the air first goes into the intake section, which must supply a smooth and steadily stream of air to the compressor. The function of the compressor is to raise the pressure of the air, since it is a fast rotating air “pump”. This energy transfer makes also the temperature and density rise aside from the pressure.
As it comes out of the compressor, the pressurized hot air enters the combustion chamber where fuel is injected and burned. This adds even more energy to the airflow. At this stage, the energy transfer is carried out by a chemical reaction. The combustion process makes the temperature increase sharply, but the pressure remains constant. It is here where the hot airflow is converted into a useful gas that produces mechanical work efficiently.
The gas turbine is the complementary part of the compressor as it is the first station where energy is absorbed from the gas. They rigidly linked by a hollow shaft. The function of the turbine is to turn gas energy into mechanical work to drive the compressor and some accessories necessary for engine operation.
When the gas discharges from the turbine, the hot gas energy content is not depleted. The major part of the heat and pressure energy of the gas, which remains available, is converted by the exhaust nozzle into kinetic energy. High exhaust velocity is a prerequisite for the thrust generation.
Adding more heat downstream of the turbine, increases exhaust velocity by afterburning. However, it is a fuel exhausting method. The General Electric J79, which powered the F4 Phantom II and the F-104 Starfighter, was representative of this type of turbojet engine.
Components of the General Electric J79 turbojet engine: 1) compressor front frame; 2) Bevel gear; 3) transfer gearbox; 4) accessory gearbox; 5) compressor casing; 6) rotor; 7) compressor rear frame; 8) combustion casing; 9) combustion assembly; 10) turbine casing; 11) turbine rotor; 12) turbine rear frame; 13) rear cone; 14) reheat fuel manifold assembly; 15) flame holder; 16) afterburner; 17) exhaust nozzle.
Below, a diagram of a turbojet engine
