Fuel and Soot Oxidation in Diesel-like Conditions

Diesel combustion has been studied under simplified experimental conditions in a nearly quiescent, high-temperature (900 K), high-pressure (4 MPa) environment by means of two-dimensional (2D) laser light scattering techniques and chemical analysis of gaseous and condensible material sampled by a fastacting valve. Two model fuels, constituted of a simple alkane hydrocarbon (tetradecane (TD)) and an aromatic/aliphatic mixture (a-methylnaphthalene/tetradecane), have been used in order to study the effect of the fuel specificity on the combustion process. Temporal profiles of the scattering intensity and of the evolution of the oxidation progress evaluated byCO and CO2 determinations, have shown the same behaviour independently on the fuel type and on the sampling location. For both fuels, the combustion proceeds through the formation of CO and subsequent oxidation to CO2, which is anticipated with respect to the appearance of a scattering signal due to soot formation and is almost completed in correspondence of the beginning of soot oxidation. The early phase of the latter process takes place in correspondence of the CO depletion when OH radicals become available. The importance of this result consists in the recognition that mechanisms of soot oxidation by an OH attack can realistically occur in diesel engine combustion and that this is the only pathway through which soot oxidation takes place in characteristics timescales comparable to those of its formation.