The fan section of a turbofan engine is driven by the low pressure (LP) spool. This means that it is connected to two parts of the engine:
- The LP compressor, which must compress the air before it enters the high pressure (HP) compressor
- The LP turbine, which must receive the air after it exits the HP turbine
The problem is that the HP section is efficient at high RPM. But the fan will reach aerodynamic limits as the fan tips pass supersonic speed. Fan blades are also harder to contain in the event of a failure as they spin faster. So to accommodate the fan, the LP section has to spin at a slower speed, which makes it harder to match with the high RPM HP section.
There is also a limitation on air flow. The fan must be able to move a maximum amount of air, which is takeoff power at sea level. But it must also be able to function efficiently at cruise level, where the flow is faster but much less dense.
Engine companies use different strategies to deal with these limitations.
GE mainly makes the LP sections larger in diameter. This allows the blades to have a high velocity at lower RPM. This this requires the air flow to change direction more rapidly.
Rolls Royce has created a 3 spool design, which allows each section to spin at a more optimal RPM. This adds complexity to the design.
Pratt and Whitney is going with the geared fan approach. Similar to turboprops, there is a gearbox to allow the fan to spin at a lower speed than the engine spool that is driving it.
This allows the turbine spool to operate at a more optimal RPM while still allowing the fan to spin more slowly. A slower fan can be larger, allowing for a higher bypass ratio.
A high bypass ratio allows a turbofan engine to drive more air (produce more thrust) for less air going through the core (burn less fuel). Modern large aircraft have engines like the GE90 on the 777 (9:1 bypass), with the latest engines on the 787 going even higher with the GEnx (9.6:1 bypass), and Trent 1000 (10.8:1). Smaller aircraft like the 737 and A320 currently use engines like the CFM56 (5.5:1), with the new LEAP going higher (9:1 or 11:1). The PW1000G goes even higher (up to 12:1).