Zero-stage

Jet engines are often uprated by adding a zero-stage to the front of a compressor. 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:

$w_2 = (w_2 \sqrt{T_3}/P_3) * (P_3/P_2) * (\sqrt{T_2/T_3}) * (P_2/\sqrt{T_2}) \,$

where:

core mass flow = $w_2 \,$

core size = $(w_2 \sqrt{T_3}/P_3) \,$

core total head pressure ratio = $(P_3/P_2) \,$

inverse of core total head temperature ratio = $T_2/T_3 \,$ i.e. f( $P_3/P_2 \,$ )

core entry total pressure = $P_2 \,$

core entry total temperature = $T_2 \,$

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.