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In the last years, there has been an increasing concern on the emission of ultrafine particles in the atmosphere. A detailed study of formation and oxidation of these particles in the environment of the diesel cylinder presents many experimental difficulties due to the high temperatures, pressures and extremely reactive intermediate species. In this paper, in order to follow the different phases of diesel combustion process, high temporal and spatial resolution optical techniques were applied in the optically accessible chamber of diesel engine, at 2000 rpm and A/F=80:1 and 60:1. Simultaneous extinction, scattering and flame chemiluminescence measurements from UV to visible were carried out, in order to study the diesel combustion process from the droplet ignition to the formation of soot, through the growth of its precursors.

Ignition delay in diesel engine combustion comprehends both a chemical and a physical amount, the first depending on fuel composition and charge temperature and pressure, the last resulting of time needed for the fuel to atomize, vaporize and mix with air. Control of this parameter, which is mandatory to weight the relative amount of premixed to diffusive stage of the hydrocarbon combustion, is here considered. Experimental measurements of flame intensity spectra obtained by in situ measurements on an optically accessible test device show the presence of peaks corresponding to radicals as OH and CH appearing at the pressure start of combustion. Since OH radicals result from chain branching reactions, a numerical simulation is performed based on a reduced kinetic scheme which allows to measure the branching agent concentration, and whose approximate nature is adequate to the proportion chemical aspects contribute to the overall delay.