The modern day Gas turbine engines are usually cylindrical in shape and they have the necessary parts inside the engine in longitudinal way, the main components we will come across from front to rear end are usually as follows:
- Compressor
- Combustion system
- Turbine assembly
- Exhaust system
A shaft connects the turbine to the compressor which creates the engine to rotate by using the gas reaction and generate the axial flow. Fuel burners are positioned in the combustion system. Initial ignition is provided once the air flow is produced by rotation of the compressor, the pressure of the mass ensures the expanding gas in the combustion chamber travels in a rearward direction. Once ignition is achieved, the flame will be continuous, and the ignition device can be switched off. The hot gases crossing the turbine produce torque to drive the compressor, therefore the starter can also be switched off. This is usually known as self-sustaining speed of the engine.
In the past the all the sections in compressor was one single unit and turbine was one single unit. these two were attached using a spool or shaft. however, now days the engines can provide high thrust for the aircraft and the engine has made in different sections which allows rotation of the compressor stages if different speeds to get optium results. This kind of engines are named as multi spool engines which has 2- 3 shafts connecting different sections of turbines to matching compressor sections. usually they are termed as Low pressure Compressors / Low pressure Turbines and High pressure compressors/ High pressure turbines.
In order to simplify descriptions of the gas flow through an engine, the various stations through the engine are annotated. At each point where the condition of the gas is changed, the pressure and temperature will be noted as a specific number. The number will change depending on the number of spools there are in the engine so that, while all engine inlet pressures and temperatures will be ‘1’, the exhaust could be ‘4’, ‘6’ or ‘8’ for single, twin and triple spool engines respectively.
The notations for a single spool are shown above and, for comparison purposes, the notations for a low by-pass twin spool engine are shown below. the main difference between these are the way how the notations are made.
when we have multi- stage compressors or turbines the notation is given in such way that it shows the major state and then followed by the sub stage.
eg: low pressure compressor 3rd(P1 + 3 stages) stage would be noted as P1.3 same way High pressure compressor 4th stage.
source- http://www.aviationbirds.com/2014/05/aircraft-gas-turbine-engine-layout-and.html#.U39cNaO0L5U.facebook