Peak-Aligned Spatiotemporal Evolution and Local Thermal Contrast of High-KL Regions in a Marine Diesel Engine

High-soot optical-loading structures in a large-bore marine diesel engine were investigated using endoscopic flame images acquired under four operating conditions. Apparent temperature and soot optical thickness, represented by the KL factor (the product of extinction coefficient K and optical path length L), were reconstructed by two-color pyrometry from the experimental images. A peak-aligned image-analysis framework was then used to separate high-KL formation extent from post-peak persistence, quantify peripheral and azimuthal organization, and evaluate local thermal contrast after radial–azimuthal spatial matching. The 25% nominal load condition produced the highest peak high-KL coverage (17.24%), whereas the 50% condition exhibited the longest coverage half-decay interval (8.20 °CA). The peak of azimuthal nonuniformity shifted from 4 and 3 °CA before the coverage peak at the 10% and 15% conditions to 4 and 11 °CA after the peak at the 25% and 50% conditions, respectively. After spatial matching, the pre-to-post-peak thermal-contrast transition remained negative at 10% and 15%, positive at 50%, and close to zero with greater grid sensitivity at 25%. These results show that high-KL coverage, spatial reorganization, and local thermal response evolve on distinct timescales and cannot be represented adequately by a single field-averaged KL metric.