High bypass improves propulsive efficiency
It is generally more efficient to accelerate a large mass of air moderately than a small mass extremely fast. Larger fans help turbofans exploit that principle.
Fan size is limited by the aircraft around it
Engines have to fit beneath or around the wing with enough ground clearance. Nacelle drag, landing-gear length and structural loads prevent unlimited growth.
Lighter materials make large fans practical
Composite fan blades and cases can reduce weight while maintaining strength. Without material advances, increasing diameter would impose unacceptable mass penalties.
Hotter cores improve thermal efficiency
At the same time, engine cores use advanced materials and cooling to operate at higher pressures and temperatures. Fan and core improvements work together.
Gearboxes offer another path to optimization
A geared fan allows different engine sections to rotate at more favorable speeds. That can support large efficient fans without forcing the turbine into an inefficient operating point.
Future gains become progressively harder
Every improvement interacts with noise, durability, cost and maintenance. Engine development increasingly depends on small gains across aerodynamics, materials, controls and manufacturing rather than one dramatic breakthrough.
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