The brayton cycle:

The working cycle of the gas turbine engine is similar to that of the four-stroke piston engine.  There is induction, compression, ignition and exhaust in both cases, although the process is continuous in a gas turbine.  Also, the combustion in a piston engine occurs at a constant volume, whilst in a gas turbine engine it occurs at a constant pressure.

The cycle, upon which the gas turbine engine functions, in its simplest form, is the Brayton cycle, which is represented by the pressure/volume diagram, shown in figure.

The air entering the engine is compressed.

• Heat is added to the air by burning fuel at a constant pressure, thereby considerably increasing the volume of the resulting gas.
• The gases resulting from combustion expand through the turbine, which converts some of the energy in the expanding gases into mechanical energy to drive the compressor.
• The remainder of the expanding gases are propelled through the turbine and jet pipe back to the atmosphere where they provide the propulsive jet.

There are three main stages in the engine working cycle during which the changes discussed occur:

• During compression. Work is done on the air. This increases the pressure and temperature and decreases the volume of air.
• During combustion. Fuel is added to the air and then burnt. This increases the temperature and volume of the gas, whilst the pressure remains almost constant (the latter being arranged by design in a gas turbine engine).
• During expansion. Energy is taken from the gas stream to drive the compressor via the turbine; this decreases the temperature and pressure, whilst the volume increases. The rapidly expanding gases are propelled through the turbine and jet pipe to give a final momentum that is much greater than the initial momentum; it is this change in momentum which produces the propulsive jet.