Turbofan

A jet engine in which a turbine-driven fan sends air through the engine core and around it. Thrust comes from both the bypass stream and the hot core exhaust.

Bypass ratio: The ratio of air mass flowing around an engine core to air mass flowing through it. It distinguishes high-bypass turbofans, which produce much of their thrust from fan-driven airflow.
High-bypass turbofan: A turbofan whose bypass airflow greatly exceeds the airflow through its core. Its efficient thrust at subsonic speeds makes it common on airliners.
Turbojet: A gas-turbine jet engine that sends nearly all incoming air through its core. It lacks the large bypass stream that gives most turbofans efficient subsonic thrust.
Propulsive efficiency: The fraction of an engine's energy transfer that produces useful thrust power. A large bypass stream can improve it by accelerating more air by a smaller amount.
Fan (aircraft engine): A large ducted rotor that accelerates air at the front of a turbofan. Its blades drive the bypass stream and supply airflow to the core.
Low-bypass turbofan: A turbofan with a relatively small bypass stream compared with its core flow. Its compact, high-specific-thrust design suits many combat aircraft.
Turboprop: A gas-t turbine engine that drives an external propeller to produce most of its thrust. It moves more air at lower speed than a turbofan, favoring slower aircraft.
Specific thrust: Thrust produced per unit of airflow through an engine. Turbofan designs trade specific thrust against fuel economy and engine size.
Brayton cycle: A thermodynamic cycle of compression, heat addition, expansion, and exhaust used by gas turbines. The core follows this cycle to power the fan and generate hot exhaust.
Pratt & Whitney PW1000G: A family of geared turbofan engines developed by Pratt & Whitney. Its gearbox lets the fan and low-pressure turbine rotate at different speeds.

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