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An experiment was performed using an optically accessible direct injection (DI) diesel engine to investigate the effects of exhaust gas recirculation (EGR) on diesel combustion. EGR was simulated using nitrogen and carbon dioxide as intake air diluents. Timing was adjusted to maintain constant start of combustion for all cases. Both diluents were found to be effective in reducing emissions of oxides of nitrogen. Soot emission was not changed by the addition of nitrogen; however, carbon dioxide substantially reduced soot emission while simultaneously reducing NOx emissions. NOx is reduced by intake air dilution is a change in flame temperature.