Effect of Hydrogen Enhancement on Natural Flame Luminosity of Tri-Fuel Combustion in an Optical Engine

A novel combustion mode, namely tri-fuel (TF) combustion using a diesel pilot to ignite the premixed methane–hydrogen–air (CH₄–H₂–air) mixtures, was experimentally investigated under various H₂ fractions (0%, 10%, 20%, 40%, 60%) and ultra-lean conditions (equivalence ratio of (Formula presented.) 0.5). The overarching objective is to evaluate the effect of H₂ fraction on flame characteristics and engine performance. To visualize the effect of H₂ 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 H₂ into the CH₄–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 H₂ 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 H₂ concentration in the CH₄–air mixture dramatically improves the flame propagation, which might reduce the CH₄ slip. However, higher H₂ concentration in the CH₄–air mixture might lead to a high-temperature reaction that sequentially promotes soot emissions, which emit a bright yellowish flame.