Application of Spontaneous Raman Scattering for Studying the Diesel Mixture Formation Process Under Near-Wall Conditions

A central point for the further development of direct injection engines is the optimization of the mixture formation process, because all subsequent processes as ignition, combustion and pollutant formation are mainly influenced by the local air/fuel-ratio inside the cylinder. Especially for passenger car engines the interaction between the spray and the combustion chamber walls is an important issue for mixture formation. For that reason this interaction was object of the investigation described.

The investigations were carried out in a heatable high pressure high temperature chamber under typical diesel engines conditions of 450°C temperature and 50 bar pressure. A passenger car common rail system was used as injection system equipped with a 6 hole nozzle with common rail specific seat geometry, mini-sac hole geometry and double needle guide. In order to allow a detailed evaluation of the quality of the mixture formation, a measurement technique based on spontaneous Raman scattering was applied which enables the quantitative measurement of the local air/fuel-ratio along a line of a few millimeters. By the careful adaptation of the optical set-up it was possible to separate the weak Raman signals from the present background influences and this allows a distinct determination of the density of air and the fuel vapor phase. The measurements were performed at a distance of 2 mm from the wall directly above the impact location of the spray beam as well as inside the wall jet regime. For each observation time 180 individual injection cycles were used for statistic evaluation. The results obtained show the improvement of the mixture formation inside the wall jet in consequence of the increased air entrainment. Additionally to the investigation of the temporal development of mixture formation the influence of the injection pressure to the mixture formation has been studied.