> The principle of Carnot efficiency dictates that the larger and hotter you can make any heat engine, the more efficient it becomes. While this is true, for a

> The principle of Carnot efficiency dictates that the larger and hotter you can make any heat engine, the more efficient it becomes. While this is true, for air-breathing propulsion engines there's another factor: It is more efficient to accelerate a lot of air a bit, than a bit of air a lot. That's why the jet engine was replaced by the turbo fan with increasingly higher bypass ratio: they have a jet engine in the middle, but the fan also pushes through a lot of additional air that passes around the turbine itself entirely. In particular, the propulsive efficiency under some simplifying conditions is 2/(1+v_ex/v_in): the higher the exhaust velocity the worse the efficiency. https://en.wikipedia.org/wiki/Turbofan https://en.wikipedia.org/wiki/Turbofan https://en.wikipedia.org/wiki/Propulsive_efficiency#Jet_engines https://en.wikipedia.org/wiki/Propulsive_efficiency#Jet_engi...