What key feature distinguishes a turbofan engine from a turbojet engine?

A turbofan engine is primarily distinguished from a turbojet engine by the presence of a fan located in front of the compressor. This fan plays a crucial role in generating additional thrust. In turbofan engines, air is drawn into the engine by this large fan, and a portion of it bypasses the core of the engine, which enhances efficiency and reduces noise compared to turbojets. The bypassed air, combined with the high-speed exhaust from the core, produces thrust more efficiently, making turbofans especially suitable for commercial aviation where fuel efficiency and noise reduction are significant considerations.

In contrast, turbojet engines lack this front-mounted fan; they rely solely on the core engine for propulsion. Therefore, the distinguishing feature of a turbofan, the large fan, directly affects the engine's performance characteristics and applications in aviation.