These can be divided into several categories.

Turbojet engines. The turbojet was the first type of jet engine developed. In this engine all the air passes through the core engine (i.e. the compressor, combustor and turbine). The engine may be single shaft as in the Avon engine, or twin shafted as in the Olympus 593 fitted to Concorde.

These engines are noisy and are not the most fuel efficient for normal use, however for high altitude high speed flight they are in a class of their own.

b. Low and Medium By-pass or turbofan engines. These engines will have two or three shafts. The Low Pressure (LP) shaft drives a larger diameter compressor. Some of the air produced by-passes the core engine (hence the name) and is used to provide thrust. The core airflow provides power for the compressors and thrust. These engine are quieter than turbojets and more fuel efficient. The Spey and

Tay engines fall into this category.

The by-pass ratio is determined by the ratio of the air in flowing through the by-pass to the air passing through the core of the engine. Low by-pass less than 2:1, medium by-pass 2:1 to 4:1, high by pass greater than 5:1.
c. High by-pass turbofan engines. These engines have very large fans driven by a relatively small core engine. Often the fan is geared to run at a lower speed than the LP turbine, which gives the turbine mechanical advantage and also allows it to run at higher speed where it is more efficient. The ALF 502, RB211 and the Trent engines are all high by-pass
High by-pass engines are very fuel efficient, powerful and quiet. These engines have a very large diameter which does give drag problems, and are not suitable for high speed flight as the blade tips will suffer compressibility problems as they approach the speed of sound.