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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 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 the bypass ratio. A turbofan engine which 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 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 containing a multistage compressor, combustor and multistage turbine section. The low speed spool (N1) of a turbofan engine also drives the fan which is contained within a duct that surrounds the engine core. The fan airflow, referred to as the cold air stream, is thus accelerated through the engine but outside of the engine core.