Most of the air through a jetliner's engine never gets burned
In the engines on most passenger jets, the bulk of the air sucked in at the front never reaches the combustion chamber. It is pushed around the hot core by a big ducted fan. With a bypass ratio of 6, six times more air flows around the core than through it, and that cool air does much of the pushing.
The reason comes down to efficiency. A plain turbojet sends all its air through the burners and blasts it out at very high speed, which wastes a great deal of energy as a fast, hot wake. Moving a larger mass of air more slowly produces the same thrust for less fuel. A turbofan does exactly that, using an extra turbine in the core to spin a fan whose bypass stream typically supplies between 30 and 70 percent of total thrust.
Frank Whittle saw it early. His March 1936 British patent, aimed at flight speeds of 500 mph, laid out the principles of the turbofan before the name existed. The idea also untangles a problem that plagued turbojets: raising their pressure and temperature to improve thermal efficiency simply made the exhaust faster and wasted fuel. By shifting energy to the fan, designers can choose how much thrust comes from each stream independently of the core's cycle.
Helicopters show the same trade-off in a different form. A given weight can hang beneath a powerful engine turning a small rotor, or beneath a weaker engine turning a bigger rotor that moves air more gently and burns less fuel. At the extremes, channelling all the gas power into a nozzle suits high supersonic flight, while handing all of it to a huge, slow airstream suits hovering.
Between those poles sits the turbofan, which is the most efficient engine type from roughly 500 to 1,000 km/h, the cruising band of most airliners. Commercial jets therefore use high-bypass designs, while fighters favour low-bypass engines that can mix the streams and add an afterburner. Some military engines bypass only a sliver of air, just enough for cooling.
Source: Turbofan
