Lean combustion is one of the most applied methods to increase engine efficiency and maintain a good trade-off with engine emissions. The pre-chamber combustion (PCC) is one of the most promising combustion concepts to extend the lean operating limits of the engine. The Narrow throat pre-chamber has shown better lean limit extension compared to other ignition sources. The pre-chamber jets and the main-chamber combustion were studied in a Heavy-Duty optical engine using methane fuel. The tested conditions covered global excess air ratios (λ), between 1.9 to 2.3. The combustion process was recorded using three collection systems: (a) Natural Flame Luminosity (NFL) with a temporal resolution of 0.1 CAD; (b) OH* Chemiluminescence, and (c) CH* Chemiluminescence with a temporal resolution of 0.2 CAD for both. The propagating velocity of the reacting jets was studied using Combustion Image Velocimetry (CIV) based on bottom view images of the main chamber. After the pre-chamber jets ignite the main-chamber charge, a two-stage heat release rate (HRR) was observed for all the tested conditions. None of the three combustion visualizations exhibited a linear correlation with the HRR. The CIV reported high-speed jets penetrating around 400 m/s into the main chamber. The richest case of λ 1.9 presented a higher axial and radial propagation speed than the other cases. The individual vector fields have significant differences with the ensemble average fields, behavior characteristic of a high turbulent process. Finally, the ultra-lean case showed higher vorticity during the pre-chamber discharge, a synonym of high air-fuel entrainment that generates heavy waviness on the flame front. This heavy waviness generates local/partial or total extinction, resulting in abnormal combustion cycles.
