According to new research from Sweden, two aircraft engine concepts, a
geared turbofan and an open rotor, can enable a significant reduction to
aircraft fuel consumption. With an open rotor, the potential reduction
is 15%. An open rotor engine generates most of the thrust from two
counter-rotating propellers instead of a ducted fan. This enables a
larger engine diameter and a higher propulsive efficiency.
Read More - http://www.rdmag.com/news/2014/05/new-engine-design-could-reduce-aircraft-fuel-consumption-15