Why do turbofan engines use less fuel and make less noise than simple turbojets?

Turbofan engines are more fuel-efficient and quieter than simple turbojet engines primarily because of their ability to move larger volumes of air at lower speeds.1

In a turbofan, a large fan located at the front of the engine accelerates a significant amount of air that bypasses the engine's core (the combustion chamber). This "bypass stream" mixes with the high-speed exhaust from the core, increasing the total mass of accelerated air. By moving a greater mass of air at a lower velocity rather than a smaller mass at an extremely high velocity, the engine achieves higher propulsive efficiency.21

This design directly impacts performance in two ways:
* Fuel Efficiency: The higher propulsive efficiency allows the engine to generate the necessary thrust while consuming less fuel, particularly during subsonic flight phases like takeoff, climb, and landing.13
* Noise Reduction: Noise in jet engines is largely generated by the high-velocity exhaust stream. By increasing the mass of the air and reducing its exit velocity, the turbofan significantly lowers the noise levels compared to the high-speed, high-energy exhaust characteristic of a simple turbojet.1