In gas turbines air is first compressed and then heated by combustion with fuel in a combustion chamber and the major part of this is used for propulsion of the aircraft.
A part of the energy contained in the hot air flow is used to drive the turbine, which in turn drives the compressor.
Turbojet engines use only energy from the expanding exhaust stream for propulsion, whereas turbofan and turboprop engines use energy from the turbine to drive a fan or propeller for propulsion (see also: aviation engines).
(EMEP/CORINAIR 2006, Emission inventory guidebook, Group 8: Other mobile sources and machinery)
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