Effects of intake air conditions and micro-pilot (MP) injection timing on micro-pilot dual fuel (MPDF) combustion characteristics in a single cylinder optical engine

Abstract

The effects of intake air conditions and micro-pilot injection timing on the characteristics of micro-pilot dual fuel combustion were examined to enhance the combustion stability in marine engines. A minimum amount of diesel fuel was injected as the ignition source, and most of the fuel energy was obtained from methane gas. First, the intake air flow rate was varied. In addition, the methane gas flow rate and micro-pilot injection timing were changed depending on the intake air flow rate. Second, the micro-pilot injection timing was varied under intake air temperatures of 35 °C and 55 °C. The intake air and methane gas flow rates were maintained. Results indicated that increasing the intake air and methane gas flow rates promoted autoignition in the end-gas region. Nevertheless, the standard deviation of the peak cylinder pressure was within 3.1 bar. Under the intake air temperature of 35 °C, advancing and retarding the micro-pilot injection led to misfiring, which increased the standard deviation of the peak cylinder pressure to 10.2 bar and 9.6 bar, respectively. Knocking combustion was occurred when the micro-pilot injection was retarded under the intake air temperature of 55 °C. This phenomenon increased the standard deviation of the peak cylinder pressure to 6.9 bar. Premixed ignition in the end-gas region combustion corresponded to the optimal engine operating conditions for marine engines because of the low combustion variation even under high cylinder pressures of approximately 170 bar to 220 bar. However, high NOx emissions occurred under these conditions.