Turbofan engines
What is a turbofan engine
Most modern Boeings, Airbuses and executive jets use Torbofans as their source of power. These are the most modern form of turbojet engine.
You can recognise them as you get on board by the fans that sit inside the engine wall, quietly purring away while the passenger boards the aircraft.
Many passenger refer to these engines as Jet Engines, and while that is partially true, where they differ from the true jet engine is that they don't use an afterburner for thrust - they use propellers built into the engine.
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 aircraft while external propellers are used on low speed aircraft.
These engines are built with cruising in mind rather than rapid acceleration or decelleration as in a true jet - such as concorde - which was one of the few aircraft passenger aircraft to use true jet engines with afterburners.
An additional benefit, particularly for commercial aircraft, is that the use of propellers in this way considerably reduces the noise level that the engine produces so that at cruising altitude, the noise level becomes little more than a background hum.
