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.