What is the difference between a turbofan and a turbojet?

What is the difference between a turbofan and a turbojet?

• A. A turbofan has more thrust at high altitude
• B. A turbofan has a bypass duct that increases efficiency at lower speeds
• C. A turbojet uses a bypass duct
• D. A turbofan produces less overall thrust

Answer: (B)

The key idea is that a turbofan adds a large bypass stream of air around the engine core. This bypass air comes through a duct and exits the back, providing thrust without burning extra fuel or reaching extremely high jet speeds. Because a lot of air is moved at relatively modest speeds, the overall propulsion efficiency improves, especially at subsonic, lower-speed flight. The core still produces high-speed exhaust, but the bypass flow contributes a big portion of thrust more efficiently. In a turbojet, nearly all intake air goes through the core, so thrust comes mainly from the high-velocity exhaust of the combusted fuel. There isn’t a significant bypass path, so efficiency at lower speeds is lower and fuel burn is higher for the same thrust. That’s why the defining difference is the bypass duct in a turbofan, which boosts efficiency at lower speeds.

The key idea is that a turbofan adds a large bypass stream of air around the engine core. This bypass air comes through a duct and exits the back, providing thrust without burning extra fuel or reaching extremely high jet speeds. Because a lot of air is moved at relatively modest speeds, the overall propulsion efficiency improves, especially at subsonic, lower-speed flight. The core still produces high-speed exhaust, but the bypass flow contributes a big portion of thrust more efficiently.

In a turbojet, nearly all intake air goes through the core, so thrust comes mainly from the high-velocity exhaust of the combusted fuel. There isn’t a significant bypass path, so efficiency at lower speeds is lower and fuel burn is higher for the same thrust.

That’s why the defining difference is the bypass duct in a turbofan, which boosts efficiency at lower speeds.