Turbofan Engine

A turbofan engine is the most modern variation of the basic gas turbine engine. As with other gas turbines, there is a core engine, whose parts and operation are discussed on a separate page. In the turbofan engine, the core engine is surrounded by a fan in the front and an additional turbine at the rear. The fan and fan turbine are composed of many blades, like the core compressor and core turbine, and are connected by an additional shaft. All of this additional turbomachinery is colored green on the schematic. As with the core compressor and turbine, some of the fan blades turn with the shaft and some blades remain stationary. The fan shaft passes through the core shaft for mechanical reasons. This type of arrangement is called a two spool engine; one "spool" for the fan, one "spool" for the core. Some advanced engines have additional spools for sections of the compressor which provides for even higher compressor efficiency.

How does a turbofan engine work? The incoming air is captured by the engine inlet. Some of the incoming air, colored blue on the figure, passes through the fan and continues on into the core compressor and then into the burner, where it is mixed with fuel and combustion occurs. The hot exhaust passes through the core and fan turbines and then out the nozzle, as in a basic turbojet. This airflow is called the core airflow and is denoted by (m dot)c. The rest of the incoming air, colored light blue on the figure, passes through the fan and bypasses, or goes around the engine, just like the air through a propeller. The air that goes through the fan has a velocity that is slightly increased from free stream. This airflow is called the fan flow, or bypass flow, and is denoted by (m dot)f. The ratio of (m dot)f to (m dot)c is called the bypass ratio - bpr.

bpr = (m dot)f / (m dot)c

The total mass flow rate through the inlet is the sum of the core and fan flows

(m dot)0 = (m dot)f + (m dot)c

Because the fuel flow rate for the core is changed only a small amount by the addition of the fan, a turbofan generates more thrust for nearly the same amount of fuel used by the core. This means that a turbofan is very fuel efficient. In fact, high bypass ratio turbofans are nearly as fuel efficient as turboprops. Because the fan is enclosed by the inlet and is composed of many blades, it can operate efficiently at higher speeds than a simple propeller. That is why turbofans are found on high speed transports and propellers are used on low speed transports. Low bypass ratio turbofans are still more fuel efficient than basic turbojets. Many modern fighter planes actually use low bypass ratio turbofans equipped with afterburners. They can then cruise efficiently but still have high thrust when dogfighting. Even though the fighter plane can fly much faster than the speed of sound, the air going into the engine must travel less than the speed of sound for high efficiency. Therefore, the airplane inlet slows the air down from supersonic speeds.


As we can see the working principle of the jet engine is pretty simple so now lets move onto the constructional aspects of it and discuss about the parts. Go to the next page to learn more