COMBUSTION (in IC engine)

Is there a diffusive combustion phase in internal combustion engine or only the kinetic combustion phase, since the air/fuel premix injected in the cylinder chamber are already...well, premixed?

I'm asking this, because a lecturer of mine is constantly repeating the same story on how some fuel additives shorten the diffusive combustion phase in fuel combustion process in IC engines. At the same time this guy says that the diffusive combustion phase takes usually few seconds... Yeah, seconds... In an IC engine.

Is he right and am I to dumb to comprehend this? Or he doesn't know what he's talking about?