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The concentration of pollutants in the combustion space of an indirect injection swirl diesel engine have been studied on a time basis. A fast acting water-cooled gas sampling valve was used for the in-cylinder sampling. The measured concentrations have been used to visualise a simplified reaction zone structure of diesel combustion. The results show that carbon monoxide appears first in the reaction zone followed by nitric oxide and soot. The high soot concentration exists only for a short period of time (3 ms) and is rapidly reduced to a low level whereas carbon monoxide is gradually oxidised to carbon dioxide over the expansion cycle. The reaction zone is found to be 10-20mm wide and is widest near the chamber wall.