Lean-burn spark ignition engines can reduce emissions, increase efficiencies, and mitigate knocking conditions. Several factors can affect the lean flammability limit of natural gas engines, including the fuel composition, temperature, pressure, and spark characteristics. It has recently been shown that spark plugs with a nanostructured central electrode, treated using pulsed laser irradiation and effectively increasing the surface area, extend the lean flammability limit (LFL) of methane/air mixtures in a constant volume combustion chamber (CVCC). In this study, the effect of varying levels of surface modifications is experimentally examined for two different power configurations of femtosecond laser. These spark plugs are tested by igniting methane/air mixtures at different equivalence ratios in a CVCC coupled with high-speed Z-type Schlieren visualization. The durability of the nanostructures on the electrode surfaces is tested by repeating the evaluations after 6,000, 66,000 and 666,000 spark events. Scanning Electron Microscope (SEM) images at different magnification rates and the root mean square (RMS) surface roughness derived from optical profilometry are used to examine the degradation of the electrode surfaces. The results point towards the existence of an optimized value of surface roughness in terms of the LFL (phi = 0.55 for 5.89 μm and phi = 0.58 for 13.68 μm). Performance degradation was particularly pronounced for electrodes with a high level of initial surface roughness (13.68 μm) whereas the electrode with a lower initial surface roughness (5.89 μm) held a superior LFL (phi = 0.57) compared to the standard spark plug (phi = 0.61) even after going through 666,000 sparks.

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