Abstract
This paper describes an experimental and numerical investigation on the spray and combustion characteristics of diesel fuel under advanced and multiple injection conditions in a common rail diesel engine. The advanced split injection indicates the double injection, at intervals of 10 degrees injection duration at BTDC 40, 30 and 20 degrees. Multiple injection strategies were composed of a two-stage injection in which the second injection timing was fixed at TDC followed by the first injection timing varied from BTDC 40 to 10 degrees. In order to analyze the spray characteristics of various injection strategies at the high ambient pressure, the high pressure chamber which can be pressurized up to 4MPa was used. The spray images and injection rate were measured by using a spray visualization and an injection rate measuring systems, respectively. In addition, the combustion characteristics were analyzed from the KIVA-3V code coupled with a reduced n-heptane mechanism, a KH-RT breakup model, a two-step phenomenological soot model, and a GRI NOx mechanism. Calculated results were compared to experimental results obtained from a single cylinder diesel engine with a common rail injection system for various advanced and multiple injection conditions using the validated KIVA code. It was revealed that the spray behaviors show different patterns according to the injected fuel mass and injection timing. In the case of multiple injections, spray development of the double injection was faster than that of the pilot injection. Regarding the combustion characteristics, the start of combustion was affected by the double injection timing and rate of heat release increased as the second injection timing was retarded for two-stage injection. Furthermore, the calculated combustion pressure and heat release rate agreed well with the experimental results at various injection strategies.
Keywords: Spray, Combustion, Multiple injection, Common rail diesel engine