DI Injector Duty Cycle

Well if we know that the cycle time(where 4 strokes happen) is 20ms and the duty cycle of the injector is 50%, we have 10ms of pulse width.

But the cycle time is relevant only because we are injecting out of the cylinder where we can inject fuel while the engine is in the compression, combustion or exhaust stroke, so it doesn’t matter, because the intake valve is closed, with a direct injector, the time that we have to inject is actually 4 times less because we can only inject while the engine is in the intake stroke, am I right?

So we would only have aprox. 5ms to inject?

Yes, that's why the fuel pressure is so high on DI engines. So you can deliver the mass of fuel required in the short time available.

Even less than that, as you do not want to be injecting when the inlet valves are open, nor when the piston near the top of it's stroke. You are also limited by the ignition timing as you need to have the injection pulse finished before firing the coil.

It's very interesting as modern DI engines manage to inject fuel even a few times during one cycle...

Not as common with GDI, but diesel will do multiple pulses (Pilot 1, Pilot 2 and Main 1 are typical, with the Pilots smoothly raising the pressure and temp in the combustion chamber, removing noise and vibration compared to a single main pulse) at much higher rail pressures compared to gasoline. Some diesels will also do post injections for emissions (firing off the DPF).