Effect of Multiple Injection Strategies on Combustion and Emission Characteristics in a Diesel Engine

This paper presents an experimental and numerical investigation on the spray and combustion characteristics in a common rail diesel engine with two different multiple injection strategies involving a fixed injection interval (double injection strategy) and a fixed second injection timing (split injection strategy). In this study, the double injection strategy had a fixed 10 interval with before top dead center (BTDC) 40, 30, and 20 of first injection timing. The split injection strategy consisted of a two-stage injection, in which the second injection timing was fixed at top dead center (TDC), followed by a first injection that varied in timing from BTDC 40-10. A high-pressure chamber with a maximum pressure of up to 4 MPa was used to analyze the spray characteristics of the various injection strategies at high ambient pressures. Spray images were obtained using a spray visualization system to analyze the spray behavior with multiple injection conditions. The KIVA-3V code was coupled with a reduced n-heptane mechanism to analyze the combustion and emission characteristics. The numerical code was applied to calculate and analyze the effect of multiple injection strategies on the combustion and emission characteristics after the validation with the same experimental results [one single injection case and one multiple (split) injection case]. The spray behaviors exhibited different patterns according to the injection strategies, such as single injection and multiple injections, and injection timing. For double injection, the spray developed faster than that for split injection. The start of combustion was affected by the multiple injection timing, and for double injection, the heat release rate increased as the second injection timing was retarded. Furthermore, the combustion pressure and heat release rate that were calculated were consistent with the results obtained from the experiments with various injection strategies.