Effect of Injection Pressure on Transient Behaviour of Wall-Interacting Jet Flame Base in an Automotive-Size Diesel Engine 2013-01-2536

Influence of the injection pressure on the temporal evolution of lifted jet flame base upon the bowl wall impingement has been studied in a small-bore optical diesel engine. Previous studies suggest that the jet-wall interaction causes re-entrainment of combustion products into the incoming jet, which shortens the lift-off length during the injection and thereby increasing downstream soot. After the end of injection, the flame base slowly moves downstream as the diminishing jet momentum results in reduced re-entrainment. How the injection pressure impacts this transient behaviour of the flame base is a main focus of the present study. Common-rail pressure was varied from 70 to 160 MPa at a fixed injection mass (10 mg per hole) and timing (7°CA bTDC). Ensemble-averaged hydroxyl (OH*) chemiluminescence images of 30 firing cycles at various times after the start of injection show that indeed the flame base moves towards the nozzle in accordance with increasing penetration of wall-reflected jet during the injection, supporting the re-entrainment theory. When the injection pressure is increased, the higher jet momentum allows higher penetration rate of the wall-reflected jet possibly enhancing the re-entrainment. However, the faster shortening of lift-off length at higher injection pressure conditions is not observed because the injection ends earlier and therefore the diesel flame develops mostly after the end of injection when the re-entrainment is unlikely. As a result, the shortening of the lift-off length before the end of injection shows no correspondence with injection pressure for tested injection conditions of this study. By contrast, the injection pressure affects the increasing trend of lift-off length after the end of injection. The 70 MPa case shows a very slow recession of the flame base towards the bowl wall due to low jet momentum. The 130 and 160 MPa injection cases also show a slow recession despite very high jet momentum because the original location of the flame base is observed near the wall and develops along the wall curvature. Therefore, the 100 MPa condition with high jet momentum as well as near-nozzle location of the original flame base shows the highest increase of lift-off length after the end of injection.