| Summary: | A novel combustion mode, namely tri-fuel (TF) combustion using a diesel pilot to ignite the premixed methane–hydrogen–air (CH<sub>4</sub>–H<sub>2</sub>–air) mixtures, was experimentally investigated under various H<sub>2</sub> fractions (0%, 10%, 20%, 40%, 60%) and ultra-lean conditions (equivalence ratio of <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>φ</mi><mo>=</mo></mrow></semantics></math></inline-formula> 0.5). The overarching objective is to evaluate the effect of H<sub>2</sub> fraction on flame characteristics and engine performance. To visualize the effect of H<sub>2</sub> fraction on the combustion process and flame characteristics, a high-speed color camera (Photron SA-Z) was employed for natural flame luminosity (NFL) imaging to visualize the instantaneous TF combustion process. The engine performance, flame characteristics, and flame stability are characterized based on cylinder pressure and color natural flame images. Both pressure-based and optical imaging-based analyses indicate that adding H<sub>2</sub> into the CH<sub>4</sub>–air mixture can dramatically improve engine performance, such as combustion efficiency, flame speed, and flame stability. The visualization results of NFL show that the addition of H<sub>2</sub> promotes the high-temperature reaction, which exhibits a brighter bluish flame during the start of combustion and main combustion, however, a brighter orangish flame during the end of combustion. Since the combustion is ultra-lean, increasing the H<sub>2</sub> concentration in the CH<sub>4</sub>–air mixture dramatically improves the flame propagation, which might reduce the CH<sub>4</sub> slip. However, higher H<sub>2</sub> concentration in the CH<sub>4</sub>–air mixture might lead to a high-temperature reaction that sequentially promotes soot emissions, which emit a bright yellowish flame.
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