Experimental and modeling study of particulate formation in high-pressure diesel-like conditions

The diesel combustion process has been characterized by combined measurements of flame intensity and extinction in the UV-visible range in an optically accessible divided-chamber diesel engine at different air/fuel ratios and constant engine speed, using n-heptane and commercial diesel oil as fuels. The analysis of the diesel combustion performed by multiwavelength extinction measurements has shown that in the high-pressure conditions typical of diesel combustion, two classes of compounds are formed: soot particles and organic aerosol. The latter structures are present just after fuel ignition, and their concentration suddenly increases as the combustion proceeds, reaching a maximum value well before the formation of soot particles, which occurs after an induction period of about 1 ms. A detailed kinetic mechanism previously developed to predict the total concentration of particulate carbon in rich flames of aliphatic hydrocarbons has been used to model the formation of particulates in diesel combustion. The kinetic scheme, coupled to a simplified model of diesel combustion, has been able to correctly predict the total concentration and the initial fast formation rate of aromatics.