They are, in particular the higher the in cylinder combustion temperature the higher the efficiency, and the higher the NOx production. This caused heavy diesel

They are, in particular the higher the in cylinder combustion temperature the higher the efficiency, and the higher the NOx production. This caused heavy diesels to get less efficient for a few years when NOx stringencies were tightened. Then they were tightened far enough to require catalytic reduction, and manufactures went back to high efficiency high NOx combustion and removed the NOx downstream.