High temperature Combustion Promotes NOX. in Diesels. EGR Valves lower the temps but putting exhaust back through an engine reduces Efficiency and more CO2 and also causes Engine Problems (Crud filled Inlet Ports, and extra wear, Dirtier oil.). etc therefore even less efficiency. Non turbo'd diesels can Make More BLACK smoke ( Over fuelling) when you get to Higher Altitudes. They are pretty much Obsolete these days. Nev