Turbofans have been rapidly replacing turbojet engines on commercial air transports for years. Although limited to slower top speeds than turbojets due to their larger cross-section (among other factors), the turbofan is much more fuel-efficient. As kermitov notes in his writeup, the turbine is used to power a large fan at the front of the engine. The airflow through the engine is split into two parts; one flow goes through the compressor section and into the combustor in order to power the turbine. The larger flow of air, however, is shunted around the turbine, bypassing it. The more air that is pushed by the fan (and by the higher-speed air flowing from the rear of the turbine as it mixes with the bypassed air) the more efficient the engine. Essentially, the higher the difference of the exhaust gas temperature from ambient, the less efficient the engine, as energy has gone into creating heat rather than thrust. A high-bypass turbofan is just that; one which by design attempts to push for maximum efficiency at the cost of top-end performance. When looking straight into a modern airliner's engine nacelle, it is possible to see daylight through the turbofan's edges; you are looking through the bypass.

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