What distinguishes Turbofan engines from Turbojet engines?

Turbofan engines are distinguished from turbojet engines primarily by their thrust generation mechanism. While both types of engines operate on similar thermodynamic principles, the way they produce thrust differs significantly.

In turbofan engines, a large fan at the front of the engine draws in a substantial amount of air, a portion of which is bypassed around the engine core. Most of the thrust produced by a turbofan comes from this bypassed air, which is accelerated by the fan. This allows turbofan engines to produce more efficient thrust, especially at subsonic speeds, making them ideal for commercial aviation.

In contrast, turbojet engines generate thrust primarily from the exhaust gases expelled from the combustion chamber. Although turbojets are capable of high speeds and altitudes, they are less efficient at lower speeds compared to turbofans.

Other options do touch upon certain characteristics of the two engine types, such as noise levels and operational altitudes, but these factors are not the primary distinguishing features. Instead, the fundamental differences in their thrust generation mechanism provide a clear, technical delineation between the two engine types.