Turbojet engine. Diagram of a type of aircraft jet engine known as a turbojet engine. The bottom half of the main diagram has the external casing partly removed to show the engine. The top half shows how the engine works. Air enters the engine from left, is compressed through low (blue) and high (red) pressure compressors, and passed into a combustion chamber (yellow). Fuel is added and the mixture is ignited. The hot gases produced expand rapidly out of the rear of the engine and thrust the aircraft forward. Some energy from the exhaust gases turns turbines (centre right) to power the compressors. Turbofan engines have often replaced turbojets, as turbofans are quieter and more fuel efficient.