Computer drawing of a turbofan engine with the equation
 for thrust. Thrust equals the sum of the exit mass flow rate times exit velocity
 minus free stream mass flow rate times velocity for both streams.

To move an airplane through the air, thrust is generated by some kind of propulsion system. Most modern airliners use turbofan engines because of their high thrust and good fuel efficiency. On this page, we will discuss some of the fundamentals of turbofan engines.

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 orange on the figure) passes through the fan and continues on into the core compressor and then 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 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. 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

A turbofan gets some of its thrust from the core and some of its thrust from the fan. If we denote the exit of the core as station "e", the exit of the fan as station "f", and the free stream as station "0", we can use the basic thrust equation for each stream to obtain the total thrust:

F = (m dot * V)f - (m dot)f * V0 + (m dot * V)e - (m dot)c * V0

We can combine the terms multiplying V0 and use the definition of the bypass ratio (bpr) to obtain the final thrust equation:

F = (m dot * V)e + bpr * (m dot)c * Vf - (m dot * V)0

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.

You can explore the design and operation of an afterburning turbojet engine by using the interactive EngineSim Java applet. Set the Engine Type to "Turbofan" and you can vary any of the parameters which affect thrust and fuel flow.

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byTom Benson
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