Turbofan Jet Engine

Overview

A turbofan is the jet engine that powers nearly all modern airliners, producing thrust mainly by accelerating a large mass of air through a ducted fan. The high-bypass arrangement routes most of that air around the engine core rather than through it, which yields high thrust at the moderate speeds of subsonic cruise while burning far less fuel and running much more quietly than a pure turbojet. Two concentric spools let the fan and core each turn at their own optimum speed.

Air passes through a series of modules along the engine axis. The Fan Module draws in air and splits it between bypass and core streams. Core air is squeezed first by the Booster (LP Compressor) and then the HP Compressor, burned in the Combustor, and expanded through the HP Turbine and LP Turbine. The Shaft & Bearing System system carries the two spools, the FADEC Controller governs the engine, and the Nacelle streamlines the installation and houses the thrust reverser.

How it works

The engine follows the Brayton cycle: compress, burn, expand. The HP turbine drives the HP compressor on the inner spool, while the LP turbine drives both the booster and the fan on the outer spool. Combustion adds energy to the flow, and the turbines extract just enough of it to keep the compressors and fan turning; what remains accelerates rearward as thrust.

Most of the thrust comes from the bypass stream, the fan acting as a many-bladed propeller inside a duct. To survive the gas leaving the combustor at around 1,600 °C, the first turbine blades are cast as single crystals, internally cooled with compressor bleed air, and coated with thermal-barrier ceramics. The FADEC meters fuel and schedules the variable stator vanes to keep the engine efficient and stall-free across its operating range, and on landing the nacelle's thrust reverser redirects bypass air forward to help slow the aircraft.