Turbojets operate on the Brayton cycle. In this form of engine, cold air is inhales and is then compressed to a very high pressure, by passing it through an axial compressor, having many stages. The pressure of the air increases after every stage, and in modern models of this engine, it can reach up to 20:1.
When the pressure of the air is increased, it also results in significant increase in temperature. The hot air is then used in the combustion process, resulting in the production of exhaust gases, having temperatures in the range of 1200 to 1250 degree centigrade.
These hot gases are then directed to the turbine blades, which are mounted on a shaft, which is used to drive the compressor. The exhaust gases are then passed to a nozzle which increases its velocity.
The major issues related with turbojets are that they are noisy and quite inefficient at subsonic speeds. In fact, the efficiency of the turbojets is only optimum after Mach 2.
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Image Courtesy: airliners.net
In this version, the shaft work is used to drive a huge fan, which results in a better air intake. Now, the air sucked by this fan is divided into two parts. One part of the air goes through the normal turbojet procedure to drive the shaft, after passing through a series of compressive stages, and the other part is used directly for propulsion.
The ratio of the air allowed to go to the compressors to the air which is bypassed and helps directly in thrust protection is called bypass ratio. This ratio is set as per the specific requirement of the operation.
Read More: Difference Between Subsonic and Supersonic