How are turbojet engines different from turbofan engines?

The primary difference between a turbojet and a turbofan engine lies in how they handle incoming air to generate thrust.

• Turbojet Engines: In a turbojet, all of the air ingested by the engine passes through the core, where it is compressed, mixed with fuel, and ignited in the combustion chamber. The resulting high-velocity exhaust gases are ejected through a nozzle to produce forward thrust. Because all air passes through the core, turbojets are considered "zero-bypass" engines.123
• Turbofan Engines: A turbofan is a modification of the turbojet that features a large fan at the front of the engine. This fan accelerates a large volume of air, only a portion of which enters the engine core; the remainder—the "bypass stream"—is directed around the core and exits through a separate nozzle or mixes with the core exhaust.213

By moving a larger mass of air at a slower speed, turbofans achieve higher propulsive efficiency, lower fuel consumption, and reduced noise compared to the simple turbojet. Consequently, turbofans have largely supplanted turbojets in most modern aviation applications.2134