Fuel Composition Effects on Air-Fuel Mixing and Self-Ignition in a Divided Chamber Diesel System by Optical Diagnostics

The influence of fuel composition on mixture formation and first stage of combustion, occurring in a small high swirl combustion chamber of an IDI Diesel engine, was analyzed from measurements of spectral extinction and flame emissivity. Measurements were carried out in an optically accessible combustion chamber in which an air swirling flow is forced from the main chamber through a tangential passage. A conventional injection system was used to inject Tetradecane, N-heptane and Diesel fuel. The distribution of liquid and vapor and the interaction of the jet with air swirl were detected by UV-visible extinction measurements. The autoignition phase was characterized by UV-visible chemiluminescence measurements. For all fuels examined, it was observed that initially the liquid fuel penetrates almost linearly with time until reaching a maximum characteristic length, slightly dependent on the fuel. Beyond this characteristic length, the fuel is entirely in the vapor phase, whose distribution revealed to be affected by the different fuel properties. The sites and timing of initial self ignition were measured by the first detectable chemiluminescence and the ignition delays were evaluated too. The spatial and temporal distribution of autoignition was observed to be dependent on chemical and physical characteristics of fuels. On the other side, a good correlation between the measured ignition delay and the cetane number was observed.