Optical and Numerical Study on Micropilot Diesel Fuel-Ignited Methane Combustion Process under Different Diesel Fuel Injection Strategies

With increasingly stringent emissions regulations, the trend toward using low-carbon fuels in internal combustion engines is unstoppable. Achieving higher rates of methane substitution is a crucial direction for future diesel-methane engines. However, the details of the combustion process involving the auto-ignition of micro pilot diesel fuel and ignition of methane main fuel to form premixed flames are still not clear. Therefore, the study employed visualization technique using a rapid compression machine and numerical study to analyze the process of igniting methane main fuel using various injection strategies for micro pilot diesel fuel. The results indicate that before the onset of the second diesel injection, a pale blue premixed flame had already formed within the cylinder, and the ignition delay of the second diesel injection was influenced by the presence of this premixed flame. The ignition delay of the diesel droplets from the second injection decreases gradually as SOI1 advances. Compared to the single injection strategy, the split injection strategy forms smaller high-temperature regions during the combustion process, thereby displaying a discernible trend toward reducing NOx emissions. Since the energy contribution of the second diesel injection is only 2%, the turbulence and diffusion flames formed by the second diesel injection have a relatively low impact on the propagation of the methane premixed flame. The flame front for single injection strategy is closer to the thickened flames region, suggesting a thicker flame surface due to the concentrated distribution of diesel.