Gas turbine Engine Notations

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 will be noted as P2.4