Abstract A two dimensional visualization of premixed combustion process was performed in a common rail (CR) Euro 5 optically accessible compression ignition engine. The effect of fuel properties on injection and combustion phenomena was evaluated by using commercial diesel fuel, as reference, rapeseed methyl ester (RME), and a paraffinic fuel, such as n-Heptane. The choice of monocomponent fuels, free of aromatic, allowed the analysis of the contribution to soot formation with respect to European diesel fuel. Evaluation of the soot volumetric fraction was made for all the fuels. The first soot occurs at the same location for the analyzed fuels but at different times. n-Heptane was the fuel which gave the lowest soot production in accordance with its better physical and chemical characteristics. The total amount of soot formed during the combustion of RME resulted to be intermediate between n-Heptane and diesel fuel.