In 1997 the US EPA formed a Heavy-Duty Engine Working Group (HDEWG) in the Mobile Sources Technical Advisory Subcommittee to address the questions related to fuel property effects on heavy-duty diesel engine emissions. The Working Group consisted of members from EPA and the oil refining and engine manufacturing industries. The goal of the Working Group was to help define the role of the fuel in meeting the future emissions standards in advanced technology engines (beyond 2004 regulated emissions levels). To meet this objective a three-phase program was developed. Phase I was designed to demonstrate that a prototype engine, located at Southwest Research Institute, represented similar emissions characteristics to that of certain manufacturers prototype engines. Phase II was designed to document the effects of selected fuel properties using a statistically designed fuel matrix in which cetane number, density, and aromatic content and type were the independent variables. Phase III is planned to be a transient validation of the results of Phase II. Phase II results are presented in this and two other papers.The test engine and the experimental methods are described in detail. The test results are summarized in tabular and graphical form. The results are discussed based on scatter plot analyses of the data. Conclusions are presented regarding the effects of the fuel properties on emissions and fuel consumption.