I recently had this point clarified that when we place the mode selector to CRANK on the A320 we can dry crank the engine up to 5 mins. But the moment we place the mode selector to IGN/START then we are limited to 2 mins (I'm mainly referring to the 2 mins with regard to a manual engine start) Could someone please explain why there are these two different limitations?
Per the Airbus maintenance training manual, the crank limit whether it's dry (CRANK) or wet (IGN/START + MAN START) is 2 minutes.
An ECAM warning will notify of exceeding the limit: ENG 1 (2) START FAULT (condition: starter time exceeded).
This video shows the cranking of an A320 jet engine as a test after changing the IDG filter.
In CRANK mode, the air turbine starter (the thing that converts APU or bleed air into engine rotation) is working pretty hard, but nothing else in the engine is really doing very much. Speeds are slow, no fuel is being burnt, not much going on. After about 5 minutes, the starter will start to get hot, so you have to shut it down to let it cool off.
In START position, now there are lots of other things going on. Specifically, you are now introducing and burning fuel. That means a lot of things are going to get hot, besides just the starter. For example, you now have a bunch of hot gases going through the turbine. On a normal start, you will get to idle within 2 minutes or so. But if you don't get to idle within 2 minutes, that means something is wrong. There could be a bunch of different causes for this, but you don't want to continue to sit there and burn fuel while you figure this out. If you do that, you could potentially overheat some other components (like the combustor or turbines). Depending on the cause, you might not overheat anything, but you don't want to risk it.
In summary, the 5 minute limit really just affects the starter. The 2 minute limit is for a bunch of other things (besides the starter).