A turbofan engine, sometimes referred to as a fanjet or bypass engine, is a jet engine variant which produces thrust using a combination of jet core efflux and bypass air which has been accelerated by a ducted fan that is driven by the jet core. The ratio of the mass of air bypassing the engine core versus the mass of the air going through the core is referred to as a the bypass ratio. A turbofan engine that derives most of its thrust from the jet engine core efflux is referred to as low bypass engine whereas an engine that derives most of its thrust from the fan is referred to as a high bypass engine. In general, low bypass engines are most commonly found in military applications, and may be equipped with an afterburner, whereas high bypass turbofan engines are the prevalent design in today's commercial aviation jet engines.
The core of a turbofan engine is quite similar to that of a turbojet and most are of the two spool design. The core consists of high and low pressure, multistage compressors, a combustor section and high and low pressure, multistage turbine units. The turbine driving the low speed spool (N1) of a turbofan engine is more robust, often containing more stages, than that of a turbojet of similar size. This is necessary as the low pressure turbine also powers the fan. In a conventional turbofan engine, the fan and the low pressure compressor turn at the same speed whereas in a geared turbofan, they do not. The fan itself is contained within a duct that surrounds the engine core. Low bypass engines often have a multistage fan which generates a low volume but relatively high speed air stream whereas high bypass engines usually have a single stage fan which generates a high volume but relatively low speed air stream. The fan airflow, referred to as the cold air stream, is accelerated by the fan and passes through the engine remaining outside of the engine core. The cold air stream moves much slower than the hot stream gas flow passing through the engine core. The cold stream serves to help cool the engine core and is then mixed with the hot stream exhaust to decrease both the temperature and the speed of the jet core exhaust flow. This speed reduction serves to both reduce the noise generated by the engine and to increase the engine efficiency by more closely matching the speed of the exhaust flow to the design speed of the aircraft.