been thinking about fuel injection, at which moment in the exhaust/induction cycle does the fuel start to be injected?

Is it done in 1 pulse or amny pulses?

And does the time when injection starts the same, the duration being how much fuel is injected or does the injection stop at the same moment but the start varies?

hope that makes snese.

Many electronic injection systems fire the injectors in pairs 1 with 4 and 2 with 3, some fire all 4 together.

Unless it is a sequential system controlled by a cam sensor then the fuel is injected in 1, 2 or 4 pulses per engine revolution on a 4 cylinder ,with no relationship between injection and engine position.
The fuel sits in the manifold until the valve opens.The injection starts at the same time but pulse length is varied depending on the load.
Have a read on the megasquirt site theres lots of info on the options and reasons for them.
Paul.