How Gas Turbine Engines Work

Large jetliners use what are known asturbofanengines, which are nothing more than gas turbines combined with a large fan at the front of the engine. Here's the basic (highly simplified) layout of a turbofan engine:

You can see that the core of a turbofan is a normal gas turbine engine like the one described in the previous section. The difference is that the final turbine stage drives a shaft that makes its way back to the front of the engine to power thefan(shown in red in this picture). Thismultiple concentric shaft approach顺便说一下,在燃气轮机极为常见。In many larger turbofans, in fact, there may be two completely separate compression stages driven by separate turbines, along with the fan turbine as shown above. All three shafts ride within one another concentrically.

The purpose of the fan is to dramatically increase the amount of air moving through the engine, and therefore increase the engine'sthrust. When you look into the engine of a commercial jet at theairport, what you see is this fan at the front of the engine. It is huge -- on the order of 10 feet (3 m) in diameter on big jets, so it can move a lot of air. The air that the fan moves is called "bypass air" (shown in purple above) because it bypasses the turbine portion of the engine and moves straight through to the back of the nacelle at high speed to provide thrust.

Aturbopropengine is similar to a turbofan, but instead of a fan there is a conventionalpropellerat the front of the engine. The output shaft connects to agearboxto reduce the speed, and the output of thegearboxturns the propeller.