The Jet-Engine

Jet airliner’s engines slow evolution towards fuel efficiency

1. Early jet engines
Above formula shows thrust depending on the mass of air passing through the engine and on the (excess) speed at which this air leaves the engine’s nozzle. Early jet airliners used engines which ejected all air with a very high velocity thought their nozzles. These engines were most effective at speeds close to the speed of sound, e.g. Mach (M) 0,85.

2. Second generation jet engines focus on mass of air
Since the first engines were quite fuel thirsty, second generation jet engines put more focus on the mass of air passing through. More and more air was by-passing the engine’s combustion chamber and ejected from the nozzles at high velocities without use of any additional fuel. The turbo-fan engine was born. Still, this engines were effective at high speeds, e.g. 900 km/h.

3. Third generation of jet engines https://en.m.wikipedia.org/wiki/Jet_airliner
New technologies allowed even more air to be by-passed. The high-bypass turbo-fan engine‘s efficiency made the wide body airliners possible.

4. Fourth and later generations of airliners
A jet engine’s fuel efficiency increases, when a large amount of air is accelerated by a relatively small amount. Because of this, aircraft engines got bigger and bigger fans. Thanks to recent technological developments, only a small amount of air goes through the fuel consuming combustion chamber.

https://en.m.wikipedia.org/wiki/Turbofan#High-bypass_turbofan

5. Future aircraft engines

Fuel efficient engines’ fans do not rotate as fast as older ones did. For their greatest fuel efficiency, these engines require slower aircraft speeds, e.g. 750 km/h or M 0.75. However, airlines need a decisive economic stimulus to accept the higher costs for crew, maintenance and usability slower aircrafts have. Politics have to provide the incentives and the framework!