Zero-stage

Jet engines are often uprated by adding a zero-stage to the front of a compressor.^[1]

At a given core size, adding a stage to the front of the compressor not only increases the cycle overall pressure ratio, but increases the core mass flow.

This is demonstrated by the following relationship:

where:

core mass flow =

core size =

core total head pressure ratio =

inverse of core total head temperature ratio = i.e. ()

core entry total pressure =

core entry total temperature =

So basically, increasing increases .

On the other hand, adding a stage to the rear of the compressor increases overall pressure ratio, decreases core size, but has no effect on core flow. This option also needs a Turbine with a significantly smaller flow capacity to drive the compressor.

References