A program to explore the effects of natural and additive-derived cetane on various aspects of diesel performance and combustion has been carried out. Procedures have been developed to measure diesel engine fuel consumption and power to a high degree of precision. These methods have been used to measure fuel consumption and power in three heavy-duty direct-injection diesel engines. The fuel matrix consisted of three commercial fuels of cetane number (CN) of 40-42, the same fuels raised to CN 48-50 with a cetane improver additive, and three commercial fuels of base CN 47-50. The engines came from three different U.S. manufacturers and were of three different model years and emissions configurations. Both fuel economy and power were found to be significantly higher for the cetane-improved fuels than for the naturally high cetane fuels. These performance advantages derive mainly from the higher volumetric heat content inherent to the cetane-improved fuels.A preliminary study on a fourth engine, which was outfitted with in-cylinder pressure transducers, was performed. The pressure data were utilized to derive heat-release profiles in order to investigate the effects of natural and additive-derived cetane on combustion. There was no apparent difference in heat-release behavior between a naturally high cetane fuel and a fuel raised to the same cetane number with cetane improver.