Optical diagnostics of premixed energy ratio effects on RCCI combustion and soot formation characteristics under low load conditions

Abstract

The study aims to explore the feasibility of using alternative fuels to achieve a high premixed energy ratio in reactivity-controlled compression ignition (RCCI) engine and reduce combustion difficulties at low loads. Specifically, the effects of premixed energy ratios (70%, 80%, and 90%) on combustion and soot formation characteristics were studied using ethanol-gasoline fuel blend (E10) and hydrogenated catalytic biodiesel (HCB) fuels in an optical engine under light load conditions. To investigate the flame characteristics and soot distributions, natural flame luminosity and two-color methods by wavelength integration were employed. The experimental results demonstrate that the RCCI engine, when fueled with E10 and HCB, achieved reliable ignition under the given test conditions. The study revealed that an increase in the premixed energy ratio results in a longer ignition delay, lower in-cylinder pressure, and heat release rate. This promotes a more homogeneous combustion process, which in turn leads to a slower combustion rate. Additionally, chemiluminescence imaging revealed that variations in the premixed energy ratio had a significant impact on the generation of flame kernels, the properties of flame propagation, and the evolution and distribution of soot. The areas where flame kernels are generated gradually shifted from the near-wall region towards the spray region. The premixed flame front transitioned from propagating from the near-wall region towards the center to propagating outward along the spray direction. Increasing the premixed energy from 70% to 90% significantly reduced the high-temperature and soot concentration regions, particularly at the intersection of the flame fronts, resulting in a significant decrease in the integrated KL factor by 75.69% and 96.41%, respectively. Based on an analysis of combustion and soot formation characteristics, a higher premixed ratio (90%) results in a more favorable flame propagation, cleaner combustion process, and a higher IMEP at low load conditions. These findings offer valuable insights for optimizing RCCI combustion, aiming to enhance engine performance under low load conditions with higher premixed energy ratios.