NASA Logo - Web Link to NASA.gov Vertical Line

+ Text Only Site
+ Non-Flash Version
+ Contact Glenn

Go
ABOUT NASA NEWS AND EVENTS MULTIMEDIA MISSIONS MyNASA WORK FOR NASA
Computer drawing of kids page link
This page is intended for college, high school, or middle school students. For younger students, a simpler explanation of the information on this page is available on the Kids Page.

NASA Meatball Image

Turbofan Engine

Glenn
Research
Center

Computer drawing of turbofan engine - still picture

Button to Make Engine Button to Display Low Speed Rotation Button to Display High Speed Rotation Button to Display Flow Button to Display Temperature Variation Button to Display Pressure Variation Button to Stop Action

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 to 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 even higher efficiency.

How does a turbofan engine work? The incoming air is captured by the engine inlet. Some of the incoming air 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. The rest of the incoming air 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. So a turbofan gets some of its thrust from the core and some of its thrust from the fan. The ratio of the air that goes around the engine to the air that goes through the core is called the bypass ratio.

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.

The mathematics describing the thrust of a turbofan engine is given on a separate slide.


Activities:
Button to Display Grade 9-12 Activity Button to Display Grade 9-12 Activity
Guided Tours
  • Button to Display Previous Page Jet Engines: Button to Display Next Page
  • Button to Display Previous Page Turbofans: Button to Display Next Page


Navigation ..

Button to Display Propulsion Index
Beginner's Guide Home Page

 

     First Gov Image


+ Inspector General Hotline
+ Equal Employment Opportunity Data Posted Pursuant to the No Fear Act
+ Budgets, Strategic Plans and Accountability Reports
+ Freedom of Information Act
+ The President's Management Agenda
+ NASA Privacy Statement, Disclaimer,
and Accessibility Certification

 

NASA Logo   
Editor: Nancy Hall
NASA Official: Nancy Hall
Last Updated: May 13 2021

+ Contact Glenn