A cooperative research program has been completed evaluating the impact of fuel composition (volatility, aromatics and sulfur) on the combustion and emissions performance of a Caterpillar 3406B turbo-charged diesel engine, which is representative of diesel truck engines of the late 1980s. Tests included both steady-state and transient operation measuring regulated and unregulated emissions. The fuel set was blended using only commercially available refinery stocks typical of those which could be considered for use in distillate fuel. The compositions of the blends were selected so that direct measurements of the individual effects of 10% and 90% distillation temperatures, aromatic content, and sulfur content could be made independently.Engine combustion performance data indicated that all fuels operated satisfactorily; aromatic content was as high as 50% and cetane number as low as 39. Further, the cetane number did not predict the engine measured ignition delay in this program.In steady-state emission tests, the data indicate that changes in the volatility or aromatic content had no substantial effect on particulates, smoke emissions, or gaseous emissions except NOx. Increasing aromatic content increased NOx emissions. Sulfur content had the expected impact of increasing the weight of particulate emissions but had no evident effect on the smoke opacity.Transient testing did not change any trends except for the effect of aromatic content on HC, particulate, and smoke emissions; increasing aromatic content increased these emissions. An examination of the transient emissions engine test cycle indicates that this trend may in fact be due to engine overfueling as a result of the test cycle or possibly fuel-related factors such as fuel density or a combination of both. This is an area for further study.No significant changes in the concentration of unregulated emissions were noted that would increase their level to an extent that any specific compound would exceed current threshold limiting values (TLV) or levels of concern. Carbonyl (aldehyde) measurements confirmed earlier good correlations with HC emissions. Engine operating conditions, rather than fuel composition, had the major determining effect on carbonyl levels. Polynuclear aromatic hydrocarbon (PAH) emissions levels did not substantially increase with increasing aromatic content at levels below 35%. Fuel sulfur conversion to SO4 emissions, engine type, and operating mode are the dominant factors in establishing the rate of formation of particulate sulfate levels.