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A critical evaluation of the current ASTM method of rating diesel fuels, and of the available non-engine techniques for the estimation of cetane nunbers of diesel fuels is presented. The relationship between ignition quality and fuel composition is reviewed and it is shown that each member of an homologous series of hydrocarbons does not have the same ignition characteristics as the other members of the series. It is emphasized that the belief that paraffins have relatively high cetane ratings as compared to aromatics and cycloparaffins is not always correct. The basic flaw in the cetane index correlations, which use the easily measurable physical properties of the fuels as independent parameters, is explained. A fuel data base has been used to compare the different correlations.