6.3.2: Turbofans

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Most modern commercial aircraft use turbofan engines because of their high thrust and good fuel efficiency at high subsonic regimes. A turbofan engine is similar to a basic jet engine. The only difference is that the core engine is surrounded by a fan in the front and an additional fan turbine at the rear. The fan and fan turbine are connected by an additional shaft. This type of arrangement is called a two-spool engine (one spool for the fan, one spool for the core). Some turbofans might have additional spools for even higher efficiency.

The working principles are very similar to basic jet engines: the incoming air is pulled in by the engine inlet. Some of it passes through the fan and continues on throughout compressor, combustor, turbine, and nozzle, identical to the process in a basic turbojet. The fan causes additional air to flow around (bypass) the engine. This produces greater thrust and reduces specific fuel consumption. Therefore, a turbofan gets some of its thrust from the core jet engine and some from the fan. The ratio between the air mass that flows around the engine and the air mass that goes through the core is called the bypass ratio.

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Figure 6.14: Turbofan.

There are two types of turbofans: high bypass and low bypass, as illustrated in Figure 6.14. High bypass turbofans have large fans in front of the engine and are driven by a fan turbine located behind the primary turbine that drives the main compressor. Low bypass turbofans permit a smaller area and thus are more suitable for supersonic regime. A turbofan is very fuel efficient. Indeed, high bypass turbofans are nearly as fuel efficient as turboprops at low speeds. Moreover, because the fan is embedded in the inlet, it operates more efficiently at high subsonic speeds than a propeller. That is why turbofans are found on high-subsonic transportation (typical commercial aircraft) and propellers are used on low-speed transports (regional aircraft).