What is the difference between a turbofan and turbo jet engine?

Answer 1

A turbojet is one of the oldest kinds of jet engine designs. The air flow enters the jet engine at one end and is compressed while it travels through rows of rotating blades (called compressors; sometimes referred to as stages). It is then ignited in the combustion chamber and as the gas expands and speeds up, it causes the turbine blades to turn, which provides the torque to the compressor stages. The force of the exhaust gases exiting is what provides the forward motion of the engine and is referred to as thrust.

A turbofan jet engine is the most widely used jet engine on modern aircraft today. It uses a fan placed directly in front of a series of compressors to compress the air. The operation is similar to the turbojet engine, except not all of the air moved by the fan goes into the engine's combustion chamber. A portion of the air flows over the engine itself, called bypass air. Bypass air also produces thrust in conjunction with the exhaust gas pressure.

Turbofan engines come in two varieties: high bypass and low bypass.

With high bypass turbofans, the majority of the total engine thrust (as much as 80%) is produced by the bypass air. With low bypass turbofans, the majority of the thrust is produced by the exhaust gases. Large commercial airliners (such as the Boeing 747 Jumbo Jet), large military cargo jets (such as the Lockheed-Martin C-5 Galaxy) and some small military tactical fighters jets (such as the Fairchild-Republic A-10 Thunderbolt II) use high bypass turbofans because they offer better subsonic efficiency at low to medium altitudes, are quieter, and still offer strong thrust at the cost of supersonic capabilities.

Low bypass turbofans are popular on small, high-performance tactical fighter aircraft (such as the Lockheed-Martin F-22A Raptor) and sacrifice fuel efficiency for improvements in the following areas: high altitude performance, supersonic performance, supersonic efficiency, and afterburning fuel economy. In afterburning engines, fuel is sprayed into the exhaust stream and produces the famous flame trail that jet engines are known for. Activating the afterburner can increase thrust output by as much as 50%.

Turbofans as a whole offer better efficiency than turbojets at high sub and transonic speeds, and at low supersonic speeds with little sacrifice in thrust performance.