1：Experimental Design and Method Selection:

The study employed optical diagnostics using a high-speed CMOS camera to capture flame luminosity and in-cylinder pressure measurements to analyze combustion phenomena in a converted engine. The rationale was to provide in-cylinder flow and combustion details not obtainable with traditional pressure-based methods.

2：Sample Selection and Data Sources:

Experiments were conducted on a single-cylinder research engine based on a commercial heavy-duty diesel engine, converted to spark-ignition natural gas operation. Data from 21 engine cycles were analyzed under lean-mixture conditions (equivalence ratio

3：66), low speed (900 r/min), and medium load (6 bar IMEP). List of Experimental Equipment and Materials:

Key equipment included a single-cylinder research engine (Model Proteus; Ricardo/Cussons, UK), high-speed CMOS camera (Model FASTCAM SA5; Photron, USA), pressure transducer (Kistler, Model 6011), laminar flow element (Model Z50MC2-2; Meriam, USA), spark plug (Model SRSGN40XLBEX8.4-2; Stitt, USA), gas injector (Model IG7 Navajo; Rail Spa, Italy), and dynamometer (McClure, Model 4999). Materials included

4：4-2; Stitt, USA), gas injector (Model IG7 Navajo; Rail Spa, Italy), and dynamometer (McClure, Model 4999). Materials included 5% methane as a natural gas surrogate. Experimental Procedures and Operational Workflow:

99.5% methane as a natural gas surrogate. 4. Experimental Procedures and Operational Workflow: The engine was operated in skip-fired mode (one fired cycle followed by five motored cycles) to manage heat. Flame images were recorded at 15,000 frames per second, synchronized with spark timing. In-cylinder pressure was measured, and image processing was performed using a custom MATLAB algorithm to determine flame area, radius, and propagation.

5：Data Analysis Methods:

Data analysis involved calculating equivalent flame radius, flame expansion speed, mass fraction burned, and apparent heat release rate from pressure traces and flame images. Statistical analysis included coefficients of variation and standard deviations for key parameters.