A late-injection strategy is typically adopted in stratified-charge direct injection spark ignition (DISI) engines to improve combustion stability for lean operation, but this may induce wall wetting on the piston surface and result in high soot emissions. E30 fuel, i.e., gasoline with 30% ethanol, is a potential alternative fuel that can offer a high Research Octane Number. However, the relatively high ethanol content increases the heat of vaporization, potentially exacerbating wall-wetting issues in DISI engines. In this study, the Refractive Index Matching (RIM) technique is used to measure fuel wall films in the piston bowl. The RIM implementation uses a novel LED illumination, integrated in the piston assembly and providing side illumination of the piston-bowl window. This RIM diagnostics in combination with high-speed imaging was used to investigate the impact of coolant temperature on the characteristics of wall wetting and combustion in an optical DISI engine fueled with E30. The experiments reveal that the smoke emissions increase drastically from 0.068 FSN to 1.14 FSN when the coolant temperature is reduced from 90 °C to 45 °C. Consistent with this finding, natural flame luminosity imaging reveals elevated soot incandescence with a reduction of the coolant temperature, indicative of pool fires. The RIM diagnostics show that a lower coolant temperature also leads to increased fuel film thickness, area, and volume, explaining the onset of pool fires and smoke.

分层装料直喷火花点火（DISI）发动机通常采用后期喷射策略，以提高稀薄运行的燃烧稳定性，但是这可能会导致活塞表面的壁变湿并导致高烟尘排放。E30燃料，即，具有30％乙醇的汽油是一种潜在的替代燃料，可提供较高的研究辛烷值。但是，相对较高的乙醇含量会增加汽化热，可能会加剧DISI发动机的壁润湿问题。在这项研究中，折射率匹配（RIM）技术用于测量活塞碗中的燃料壁膜。RIM实施使用新颖的LED照明，该照明集成在活塞组件中，并提供活塞碗窗的侧面照明。该RIM诊断程序与高速成像相结合，用于研究冷却液温度对装有E30的光学DISI发动机中壁润湿和燃烧特性的影响。实验表明，烟雾排放量从0.068 FSN急剧增加到1。当冷却液温度从90°C降低到45°C时为14 FSN。与该发现一致的是，自然火焰发光度成像显示烟灰白炽度升高，冷却剂温度降低，表明发生了池火。RIM诊断表明，较低的冷却液温度也会导致燃料膜厚度，面积和体积增加，从而解释了池火和烟气的发生。