These cycles will be discussed as they apply to a gas turbine engine. In the intake cycle air enters at ambient pressure and in a constant volume and leaves at an increased pressure and decreased volume. In the compressor section air is received from the intake at increased pressure slightly above ambient and slightly decreased volume. Air enters the compressor where it is compressed, leaving with a large increase in pressure and decrease in volume. This is caused by the mechanical action of the compressor.
Pressure remains relatively constant, but a marked increase in volume takes place.
The expanding gases move rearward through the turbine assembly and are converted from velocity energy to mechanical energy by the turbine. The total energy in a flowing gas is made up of static and dynamic temperatures and pressures. A nozzle or a diffuser does not change the total energy level but rather changes one form of energy to another. By varying the area of a pipe, velocity can be changed into pressure and pressure into velocity. A turbine engine is just such a pipe, with areas where air pressure and velocity are constantly being changed to achieve desired results.