The role of radical species in diesel engine auto-ignition detection

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. By changing the engine operating conditions, ignition delay is shown to exhibit a trend characterized by a negative temperature coefficient. A good agreement is found between the location of a maximum in the computed rate of formation of the reaction cycle branching agent and that of a maximum of the measured emission intensity of early formed radicals.