Spray/wall interaction influences on the diesel engine mixture formation process investigated by spontaneous Raman scattering

A main topic for the further development of direct injection diesel engines is the optimization of mixture formation, as all subsequent processes like 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 spray and combustion chamber walls is an important issue for mixture formation. Therefore, this interaction was the subject of the investigations described. The investigations were carried out in a heatable high-pressure high-temperature chamber under typical diesel engine conditions of 450°C temperature and 50 bar pres'sure. A passenger-car common rail system was used as injection system which could be equipped with two different six-hole nozzles, both 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 mixture formation, a measurement technique based on spontaneous Raman scattering was applied, enabling quatitative measurements of the local air/fuel ratio along a line of a few milimeters. The careful adaptation of the optical setup made it possible to separate the weak Raman signals from background contributions, and this allowed a distinct determination of air and fuel density in the vapor phase. The measurements were carried out at a distance of 2 mm from the wall, directly above the impact location of the spray jet, as well as inside the wall jet region. The results obtained indicate the improvement of the mixture formation inside the wall jet as-consequence of the increased air entrainment. Additionally, for the investigation of the temporal development of mixture formation, the influences of injection pressure, nozzle orifice geometry, and wall surface temperature on mixture formation have been studied.