هواوفضا.هواپیما.طراحی و آموزش

The Turbofan Engine

A high bypass turbofan engine.
A turbofan engine is basically a turbojet to which a fan has been added. Large fans can be placed at either the front or rear of the engine to create high bypass ratios for subsonic flight. In the case of a front fan, the fan is driven by a second turbine, located behind the primary turbine that drives the main compressor. The fan causes more air to flow around (bypass) the engine. This produces greater thrust and reduces specific fuel consumption.

A low bypass turbofan engine.
For supersonic flight, a low bypass fan is utilized, and an augmentor is added for additional thrust.

The Turbojet Engine

The turbojet is the basic engine of the jet age. Air is drawn into the engine through the front intake. The compressor squeezes the air to many times normal atmospheric pressure and forces it into the combustor. Here, fuel is sprayed into the compressed air, is ignited and burned continuously like a blowtorch. The burning gases expand rapidly rearward and pass through the turbine. The turbine extracts energy from the expanding gases to drive the compressor, which intakes more air. After leaving the turbine, the hot gases exit at the rear of the engine, giving the aircraft its forward push ... action, reaction !
For additional thrust or power, an afterburner or augmentor can be added. Additional fuel is introduced into the hot exhaust and burned with a resultant increase of up to 50 percent in engine thrust by way of even higher velocity and more push.

The Ultra High Bypass Engine

An ultra high bypass jet engine.

A logical approach to improving fuel consumption is even higher bypass technology. Mechanical arrangements can vary. During the 1980s, GE developed the Unducted Fan UDF® engine which eliminated the need for a gearbox to drive a large fan. The jet exhaust drives two counter-rotating turbines that are directly coupled to the fan blades. These large span fan blades, made of composite materials, have variable pitch to provide the proper blade angle of attack to meet varying aircraft speed and power requirements. Powerplants such as the UDF® engine are capable of reducing specific fuel consumption another 20-30 percent below current subsonic turbofans.