The exhaust emissions from heavy-duty diesel engines (HDDEs) contribute to the degradation of ambient air quality; therefore, environmental agencies have created stringent emissions standards. Since the implementation of these standards, overall engine and fuel technology improvements have created a significant reduction in emissions. This study was completed in order to evaluate oxides of nitrogen (NOx) emissions from fuels with and without cetane-improving additives in recent and early production electronically controlled HDDEs. Five engines spanning the model years from 1991 to 2004 were tested using the Federal Test Procedure (FTP) dynamometer cycle with both petroleum-based diesel and B20 as the neat fuel. It was found that the additives had the most impact on reducing emissions at low engine powers, but the engine power range with an NOx benefit varied between engines. The cetane improvers were found only to reduce NOx below a cylinder gas density of 35kg/m3 at top dead centre. The lower compression ratio of the 1992 DDC S60 engines reduced the cylinder gas density and provided a larger optimal operating range for the cetane improvers. The cetane improvers reduced NOx at low engine powers and cylinder gas density for the B20 fuel but were less effective than for the neat petroleum fuels.