Why are turbofan engines more fuel-efficient and quieter than turbojet engines?

Turbofan engines are more fuel-efficient and quieter than turbojet engines primarily due to their ability to move a larger volume of air at a slower speed.1

In a turbofan, a large fan at the front of the engine forces a significant portion of incoming air to bypass the engine's core (the combustion chamber). This "bypass stream" mixes with the high-velocity exhaust from the core, effectively increasing the total mass of accelerated air. By moving a greater mass of air at a lower velocity compared to the high-speed, concentrated jet of a turbojet, the engine achieves higher propulsive efficiency.231

The bypass ratio—the ratio of air bypassing the core to air passing through it—is a key factor in this performance; generally, higher bypass ratios result in better fuel economy. Furthermore, because the bypass air acts as a buffer and reduces the velocity of the exhaust stream, the engine operates with significantly less noise than a turbojet, which relies entirely on a high-speed jet of combustion gases.241