Both piston (reciprocating) engines and gas turbine engines are internal combustion engines. They have a similar cycle of operation that consists of induction, compression, combustion, expansion, and exhaust. In a piston engine, each of these events is a separate distinct occurrence in each cylinder. Also in a piston engine, an ignition event occurs during each cycle in each cylinder. Unlike reciprocating engines, in gas turbine engines these phases of power occur simultaneously and continuously instead of successively one cycle at a time. Additionally, ignition occurs during the starting cycle and is continuous thereafter. The basic gas turbine engine contains four sections: intake, compression, combustion, and exhaust. [Figure 15-1]

To start a gas turbine engine, the compressor section is normally rotated by an electric starter. As compressor revolutions per minute (rpm) increase, air flowing through the inlet is compressed to a high pressure, delivered to the combustion section, and ignited. In gas turbine engines, not all of the compressed air is used to support combustion. Some of the compressed air bypasses the burner section within the engine to provide internal cooling. The fuel/air mixture in the combustion chamber burns in a continuous combustion process and produces a very high temperature, typically around 4,000° Fahrenheit (F). When this hot air mixes with bypass air, the temperature of the mixed air mass drops to 1,600 – 2,400 °F. The mixture of hot air and gases expands and passes through the turbine blades forcing the turbine section to rotate. The turbine drives the compressor section by means of a direct shaft, a concentric shaft, or a combination of both. After powering the turbine section, the combustion gases and bypass air flow out of the engine through the exhaust. Once the hot gases from the burner section provide sufficient power to maintain engine operation through the turbine, the starter is de-energized, and the starting sequence ends. Combustion continues until the engine is shut down by cutting off the fuel supply.

Note: Because compression produces heat and pressure, some pneumatic aircraft systems tap into the source of hot compressed air from the engine compressor (bleed air) and use it for engine anti-ice, airfoil anti-ice, aircraft pressurization, and other ancillary systems after further conditioning its internal pressure and temperature.