Airbus is about to pave the way for additional decades of service for some slightly aged aircraft types, which means most of their 2-engine models, the A319, A320, A321 and A330. The term "NEO engine" does not specify a certain engine manufacturer or engine type, but stands for a new class of engines, providing significantly better fuel efficiency, going hand in hand by noise reduction of at least 50%. Actually we are talking of 20% less fuel burn per seat, while Boeing's answer, the B737 MAX8, just saves 8%. In comparision to the actual models, the A320 carries 9 seats more, the A321 unbelievable 20 seats more!

CFM, Pratt & Whitney and Rolls Royce go different ways to achive that goal. The major problem of a high-bypass jet engine is, that for best efficiency, the fan should spin slower and the hi-pressure turbine should spin faster, as a conventional construction permits it. Reduction of rpm has always be achieved by "pneumatic" transmission, a high pressure turbine driving a low pressure turbine of larger diameter, with the fan mounted in front. The possible transmission ratio is limited, so none of the components runs at best rpm, and waist of energy is immense. Rolls Royce traditionally closes this gap a bit better by a third intermediary turbine, which in most cases resulted in a slight advantage in efficiency during flight, payed by higher fuel burn on the ground and higher costs for purchase and maintenance.

The first real breakthrough are the NEO engines. Pratt & Whitney's geared turbofan concept uses a surprisingly compact and light weight mechanical gearbox by the German manufacturer MTU. The gearbox enables the turbine as well as the fan to run at their optimal rpm. The single aisle bypass ratio jumps from ~5:1 to incredible 12:1. CFM's approach is rather convential compared to P&W. However, the bypass ratio of 1:11 is also surprisingly high, performance and fuel efficiency practically identical.

Though these engines look huge. There is less solid material inside of the nacelles, which are made of composite material,, providing less weight, less parasite drag and higher noise absorbtion. Carbon fiber also reduces weight of the big fan blades.

Here are two short videos, which excellently explain the principle and benefits: