JA600L
Minister of Fire
I think it's more there's a tremendous difference in air quality when somebody runs a 5-15 year old engine vs. one made either in the last 5 years or more than 15 years ago
The latest regulations put a higher priority on nitrogen oxide suppression than on complete combustion. It alarmed me at first how many new-looking trucks I saw blowing visible smoke (but not enough that I think they were deliberately modded to "roll coal") until I looked into what the latest regulations were and what it takes to achieve them. In some instances, EPA regulations can represent significant progress. In some instances, they can make 2 steps forward and 3 steps back.
The technology I work with is SCR. Using diesel exhaust fluid correctly means no EGR or particulate filter. That in itself is a step in the right direction. Tier 3 or tier 4 interim emphasized EGR usage heavily. Tier 4 final done right with SCR means a clean burning engine with longer service intervals because less contaminates in the oil and combustion chamber. Some manufacturers still struggle to grasp the benefits of SCR.