How Everything Works
How Everything Works How Everything Works

Question 558

How do the different jet engines work on aircraft—the turbojet, the turbofan, and the turboprop? — KB, Charlottesville, VA
All three engines start with a turbojet engine. In a turbojet engine, a stream of air is first compressed by a rotary compressor. The air is then mixed with fuel and the mixture is burned. Finally, the hot burned gases are allowed to expand through a rotating turbine and they flow out of the back of the engine at very high speed.

To understand how all of this works, let's follow the flow of energy through the turbojet engine. Assuming the plane is moving forward, the air is moving fast when it encounters the engine's inlet duct. This inlet duct slows the air down substantially and the change in its speed causes the air's pressure to rise—an effect observed by Bernoulli. The air's energy doesn't change, but its kinetic energy (energy of motion) is partially converted to pressure potential energy. The now pressurized air is further pressurized by its passage through the rotary compressor at the front of the turbojet. The compression process adds energy to the air by doing mechanical work on that air. Now fuel is added to the high-pressure air and the mixture is burned. This combustion adds an enormous amount of energy to the air. The exhaust gases immediately expand and their speeds increase substantially as they pour out of the combustion chamber. These gases flow through a rotating turbine on their way out of the back of the engine. Even though the gases do work on the turbine, they still have lots of energy and flow out of the jet engine at a much greater speed than the air had when it arrived. Much of the fuel's chemical potential energy has become kinetic energy in these exhaust gases. The turbine provides the mechanical work that operates the rotary compressor, or the fan of a turbofan or the propeller of a turboprop. Overall, the exhaust gases leave the turbojet engine traveling faster than the air did when it arrived. Since the gases carry backward momentum with them as they leave the engine, they have evidently pushed the engine forward to give the engine and the plane forward momentum.

That's all there is to a turbojet engine. A turbofan engine uses the mechanical work from an enlarged turbine to operate a large fan that's in front of the turbojet engine itself. This fan takes air that has slowed down on entry into the jet's inlet duct and adds energy to this air. The air then speeds up as it flows out the jet's outlet duct and the air leaves the engine traveling faster than when it arrived. Once again, the engine experiences a forward thrust force as it pushes this air backward.

A turboprop engine uses mechanical work from an enlarged turbine to operate a propeller. The propeller pushes air flowing past the engine backward and the air pushes the engine and airplane forward. Because there is no duct around the propeller blades, the air passes the blades at full speed (a turbofan engine uses its duct to slow the air down before pushing on the air with its fan blades).


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