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HC emissions and ignition delay were investigated in a research single-cylinder DI diesel engine. Correlations were made between the measurements and different air-fuel mixing parameters calculated from a gas-jet spray model and expressions from the literature. The change in ignition delay was caused by varying engine inlet conditions of pressure and temperature and by adding a special cetane improver to No. 2 diesel fuel. In order to be able to interpret the experimental results a zero-heat-transfer heat release model was used in evaluation of the engine pressure data. It was found that the too-lean-mixed fuel could explain a maximum of 20% of the HC emission; the remaining amount is caused by other sources.