Problem (???) with diesels is that the fuel has to evaporate and mix with the hot air and get into the combustible range before it can ignite. It's approaching this point always from the point of a stream of liquid diesel into an environment of really hot air.
So the point of initial combustion (and subsequently all fuel injected thereafter) is always at the rich end of the combustible range...yellow.
Multi stage injection can get some of the fuel evaporated and on the lean side of combustion before the ignition event, and before it's hot enough to ignite.
Nice vids.
I like the gasser, where you can clearly see why valves can stay cleaner on port injection.