A cycle is a process that begins with certain conditions, progresses through a series of events, and returns to the original conditions.

As an introduction to gas turbine operation, consider first the reciprocating engine, which operates on the Otto cycle. (fig 6-9 view A). The Otto cycle consists of four basic events that occur at different times but in the same place,

Figure 6-9.-A comparison of reciprocating and gas turbine engine cycles.

inside a cylinder of the engine. The events are (1) intake, where a mixture of air and fuel is drawn into the cylinder; (2) compression, where the mixture is squeezed into a much smaller volume; (3) power (or combustion), where the mixture is burned; and (4) exhaust, where the burned fuel/air mixture is forced from the cylinder. Now consider the gas turbine engine.

The gas turbine engine operates on the Brayton cycle (fig 6-9 view B). The Brayton cycle consists of the same four events as the Otto cycle. However, all four events occur at the same time, but in different locations within the gas turbine engine.

Figure 6-10.-Centrifugal compressor.

During the Brayton cycle, air enters the inlet (1) at atmospheric pressure and constant volume. As the air passes through the compressor (2), it increases in pressure and decreases in volume. In the combustor (3), the air mixes with fuel and burns. During combustion, pressure remains constant, but the increased temperature causes a sharp increase in volume. The gases at constant pressure and increased volume enter the turbine (4) and expand through it. As the gases pass through the turbine rotor, the rotor turns kinetic and thermal energy into mechanical energy to do work. The gases are released through the exhaust (5), with a large drop in volume and at constant pressure. The cycle is now completed.