Both NOx and particulate emissions are affected by combustion temperatures, although in opposite directions. Combustion temperature, in turn, is related to injection timing. Thus, in general, NOx emissions increase and particulate emissions decrease when injection timing is advanced.For a realistic comparison of diesel fuel quality effects on emissions, therefore, injection timing is important. In this study, fuels of different ignition qualities, but with the same aromatics content, were tested such that a phase of combustion of all fuels was made to occur at the same point in the engine cycle. Under this method, injection timing was optimized for each test fuel such that Peak Rate of Pressure Rise was made to occur at 4.5 degrees after top dead center (ATDC).Testing fuels of different ignition qualities and the same aromatic content under these conditions showed that NOx emissions of high cetane number fuels were significantly lower than emissions of a low cetane number fuel. The effect of cetane number on particulate emissions was more complex, and was somewhat dependent on load conditions.