Engine Cycle / Brayton Cycle
The engine cycle is the sequence in which air and fuel is mixed, compressed and combusted in order to produce thrust. These steps basically consist of intake, compression, combustion and expansion. In a pressure / specific volume graph, these steps are expressed as the state of the gas at all four points.

The cycle described above is also known as the " Brayton Cycle". In the previous century, George Brayton, described the continous combustion cycle scientifically. The Brayton Cycle is given to the continous thermodynamic cycle of the gas turbine engine is also widely known as the "continous pressure cycle". This is because the pressure within an engine remains fairly constant during combustion as volume increases as well as velocity.

The Brayton cycle consists of four continous events that can be shown on a pressure-volume graph (figure 1). As we will see later on, the process is closed-in by two isobars. Referring to the graph, point A to B indicates air entering the inlet at a lower than ambient pressure due to the divergent shape of the duct. The next segment, from B to C, indicates air pressure returning to the ambient while volume decreasing.

Segment C-D shows a further decrease in volume while pressure builds up rapidly due to compression. From D to E the graph shows a slight pressure decrease due to heat added in the combustion chamber while volume increases again. This pressure decrease is approximately 3% and is controlled by the carefully sized and designed exhaust nozzle opening. This small pressure drop also ensures the correct direction of gas flow through the engine towards the combustor. Segment E to F displays another pressure decrease which results from the increasing velocity which is accelerated by the turbine. Eventually, segment F to G shows a volume increase which results in rapid acceleration of the gas flow. After this, the pressure will return to ambient and the process is completed.

Engine Cycle / Brayton cycle
Figure 1 - Pressure/Volume graph

Figure 2 shows roughly the same plots as displayed in figure 1 except that now the temperature during the various stages of the process can be observed. The internal engine temperature graph shows a slight temperature drop at points A, B and C, as air is sucked into the duct. Segment C-D shows a temperature increase again as air is compressed in the engines' compressor. Point D-D represent another temperature rise which corresponds to a slight pressure rise. Segment D-E shows a temperature peak as compressed air is combusted after which it decreases again as the burned mixture enters the rear section of the combustor section. From point F on, the temperature continues to drop as the volume of the air increases untill it exits the engine at higher than ambient temperature.

Engine Cycle / Brayton cycle
Figure 2 - Pressure/Volume graph

The velocities we are dealing with in the discussion above are all below Mach 1. We'll discuss the operation of gas turbines operating under supersonic conditions in the future.