Experimental and numerical insights into the narrow-throat pre-chamber design criterion and the ignition mechanism

The narrow-throat pre-chamber (PC) exhibits high pressure buildup and high jet velocity, resulting in superior ignition performance for lean combustion engines. Its design criterion was proposed in our previous numerical study based on a dimensionless parameter: throat diameter ratio (Rt). However, the design criterion has not been validated through experiments, and the detailed PC ignition mechanisms remain to be clarified. In this study, a narrow-throat PC and a wide-throat PC were designed for experiments based on the criterion, which were optimized numerically to maintain a similar peak pressure difference (ΔP) for the two PCs. The results reveal that the narrow-throat PC has superior ignition performance under various conditions, validating our previous design criterion. It owes to three main factors: less flame quenching, higher jet velocity, and earlier combustion phases. First, the narrow-throat PC shows more uniform jet flames while the wide-throat PC suffers from fewer ignition points due to severe flame quenching. Second, the narrow-throat PC presents stronger jets owing to the higher average mass flow discharged during the hot jet stage. Third, a stronger inflow velocity of the narrow-throat PC facilitates its earlier hot jet discharge and earlier combustion phases. Differences in ignition duration, number of ignition points, and ignition distance between the two PCs lead to distinct overall combustion patterns. Since the wide-throat PC is more restricted by quenching with smaller orifices, the disadvantages of quenching under excess air ratios (λ) of 0.8 and 0.9 outweigh the advantages of the faster flame propagation than λs of 1.2 and 1.0, respectively.