Eng n2 - apu - airbus320
 

Hi guys just wondering why
sometimes during engine start .. i notice while monitoring parameters N2 rotates very slowly, and sometimes very fast why is that!!

and the same for APU sometimes when I press the start PB the APU N rotates and starts quickly and sometimes very slow.. why is that ??

Also depends on pneumatic pressure.

Well ..it also depends how "fast" you are observing those instruments...

Most of the time it is related to bleed pressure

Also depends on EGT. If you take a/c after a short time turnaround with eng EGT above lets said 100º C or more, Engine EEC will slow the start process by reducing fuel flow to avoid a high egt start.
The same is true for APU

On the RR 777 and the RR A330 the FADEC won't put the fuel in until the EGT drops below 100c during Auto start. It motors the Engine at Max Motoring until the temp drops.

As to the Engine N2 either accelerating fast or slow, that would depend on bleed pressure in the duct at the time wouldn't it? Have a look at the Duct pressure when it happens. :ok:

As to the APU N2 varying with speed, that would be down to the APU Battery condition?