What is the purpose of the bypass air in the engine?

The purpose of bypass air in a jet engine primarily relates to its use in bypass turbojet and turbofan engines, where the bypass air enhances overall engine performance. Bypass air is the portion of the air that enters the engine but does not pass through the combustion chamber. Instead, it flows around the engine core. This airflow is a critical component in generating thrust.

In turbofan engines, which are commonly used in commercial aviation, the bypass air generates a significant amount of thrust by accelerating a large mass of air at relatively low speed compared to the air that goes through the engine core, which is used for combustion. The combination of fast-moving core exhaust and the larger volume of slower-moving bypass air results in a more efficient thrust generation, thus improving the performance and fuel efficiency of the aircraft.

This understanding highlights why the provision of additional thrust is essential when considering the function of bypass air in the context of overall engine design and performance.