The basic turbojet engine has five stages, four of which are designed to add energy to the airflow and one designed to release that energy to increase the available thrust. The diffuser at the front of the engine and the nozzle at the rear simply change the velocity of the airflow. The turbine in the rear drives the compressor in the front, and the combustion chamber in the middle adds heat energy to the flow. Some engines, like those found in fighter aircraft have an additional stage called an afterburner, which adds an additional “kick” of energy as the flow is leaving the nozzle.
The diffuser is sometimes called the inlet. Air enters the engine and travels a short distance to the compressor face. In that distance, the cross-sectional area of the opening is increasing. The law of conservation of mass states that the velocity of the flow must decrease in this situation and Bernoulli’s Principal states that the pressure in the flow must increase. The increase in pressure and decrease in velocity adds energy to the flow and prepares the flow for the compressor.
The air pressure is further increased when the airflow enters the compressor. There are two basic kinds of compressors used in jet engines. The most common is the axial compressor, which consists of a series of coaxial sets of blades. The blades typically get smaller and closer together as they approach the combustion chamber. The other, less common type of compressor is the centrifugal compressor, which uses a single spiraled blade set.
Fuel is added to the highly compressed air and ignited in the combustion chamber. This adds more energy to the flow. The effect is similar to what happens in an internal combustion engine, where the pistons compress the air which is then mixed with fuel and ignited.
The resulting controlled explosion creates the energy to run the engine.
The flow leaves the combustion chamber and is driven through sets of turbine blades. A shaft connected to the turbine is what drives the compressor in the front of the engine. In order to get the engine started, a separate motor or engine is necessary to spin the shaft until the engine can sustain itself.
The final stage is the nozzle, which is the exact opposite of a diffuser. The cross-sectional area decreases, which causes the velocity of the flow to increase. This acceleration creates a force which, according to Newton, requires a reactive force. That reactive force is the thrust which move the aircraft the engine is attached to. Some nozzles have adjustable apertures, which allow the pilot to change the thrust adjusting the cross-sectional area at the rear of the nozzle rather than by using the throttles. Still other nozzles, like those on the AV-8B Harrier or the F-22 Raptor, can change direction, which changes the direction of the thrust. This vectored thrust makes the aircraft more maneuverable and, in the case of the Harrier, can allow it to take off vertically or on an extremely short runway.
Many fighter jets have an additional stage between the turbine and the nozzle. Additional fuel is injected into the flow and ignited. The afterburner adds an additional “kick” of energy to the flow right before it leaves the nozzle. This greatly increases thrust, but is highly inefficient. The amount of fuel needed to operate an afterburner limit its use to very short periods of time.