On the question of why older diesel engines did not need EGR, they probably did but no-one gave a damn way back then.
EGR is just a way of trying to reduce NOX emissions to keep up with modern requirements. My visualisation of this is that having been burnt it is an inert gas and is used as a moderator to damp down the combustion process and keep the combustion temperature below the threshold where nasty NOX compounds are created. Is this caused by reducing the AFR or oxygen surplus, depends on your viewpoint really.
What I have certainly noticed is that many modern diesels have a particularly acrid or acidic smell to their exhaust whereas older diesels don't seem to do this. Is this a result of higher injection pressures and smaller droplet sizes perhaps? I might have been inclined to say that combustion pressures are increased due to higher boost pressures but this is not the case at idle when I have noticed the smell.
Many, if not most modern turbo-diesels seem to have a great deal of oil in the inlet tract downstream of the turbo, this oil must get involved in the combustion process and increase the amount of particulates in the exhaust gases, which of course get recirculated. obviously turbocharger oil seals are not as effective as they could be.
Following this thread with interest.
Shaun.