How does a turbofan engine differ from a turbojet engine?

The primary difference between a turbojet and a turbofan engine lies in how they handle incoming air and generate thrust.12

• Turbojet: This engine passes all incoming air through the combustion chamber. It functions by compressing air, injecting fuel, and igniting the mixture; the resulting expanding gases pass through a turbine to power the compressor before being ejected at high speed through a nozzle to create thrust.21
• Turbofan: This engine features a large fan at the front that accelerates a significant volume of air. While some air enters the engine core to be processed like a turbojet, a portion of the air—the bypass stream—is directed around the combustion chamber.21

By moving a larger mass of air at a slower speed, the turbofan increases propulsive efficiency, which reduces both fuel consumption and noise compared to the simple turbojet. While turbojets were common in early jet aircraft, turbofans have largely replaced them in many applications due to these economic and performance advantages.1234