Changing fuel quality, increasingly stringent exhaust emission standards, demands for higher efficiency, and the trend towards higher specific output, all contribute to the need for a better understanding of the ignition process in diesel engines. In addition to the impact on the combustion process and the resulting performance and emissions, the ignition process controls the startability of the engine, which, in turn, governs the required compressions ratio and several of the other engine design parameters. The importance of the ignition process is reflected in the fact that the only combustion property that is specified for diesel fuel is the ignition delay time as indicated by the cetane number.The objective of the work described in this paper was to determine the relationship between the ignition process as it occurs in an actual engine, to ignition in a constant volume combustion bomb. The ultimate goal is to develop a new procedure for rating the ignition and combustion quality of fuels for diesel engines. The short-term goal, and an interim step in the development effort, is to establish the bomb approach for determining cetane number.The ignition delay time versus temperature relationships for several blends of the primary reference fuels and for several distillate fuels were determined in a constant volume combustion bomb as well as in a specially designed, variable-compression ratio diesel engine. The engine and bomb results have been compared and found to be in excellent agreement. The effects of air-fuel ratio, air pressure, air temperature, and diluent composition have also been examined in the bomb. The ignition delay time is primarily a function of the temperature prevailing during the ignition delay time. The cetane numbers of the distillate fuels were determined from the combustion bomb data and compared to those measured using the standard ASTM D 613. Based upon these comparisons, it appears that the bomb technique does provide a good measure of the cetane number.