To provide insights into the fundamental characteristics of pre-chamber combustion engines, the ignition of lean premixed CH4/air due to hot gas jets initiated by a passive narrow throated pre-chamber in a heavy-duty engine was studied computationally. A twelve-hole pre-chamber geometry was investigated using CONVERGETM software. The numerical model was validated against the experimental results. To elucidate the main-chamber ignition mechanism, the spark plug location and spark timing were varied, resulting in different pressure gradient during turbulent jet formation. Different ignition mechanisms were observed for turbulent jet ignition of lean premixed CH4/air, based on the geometry effect. Ignition behavior was classified into the flame and jet ignition depending on the significant presence of hot active radicals. The jet ignition, mainly due to hot product gases was found to be advanced by the addition of a small concentration of radicals. In turbulent conditions, the ignition behavior falls between the flame and jet ignition, depending on the pressure gradient and geometry of the pre-chamber. An ignition Damkohler number is identified and used to characterize different ignition behavior. The ignition powers of different pre-chamber jets were analyzed, based on the heat release rate in the main chamber. The influence of spark plug location and spark timing on the ignition power of the jets and its importance in the main chamber combustion characteristics were analyzed.