Numerical analysis on the influence of nozzles geometrical features on the combustion process of passive pre-chamber turbulent jet ignition engine

Engines equipped with pre-chamber technology offer a viable solution for extending the lean ignition limit, thanks to the high ignition energy delivered by the turbulent jets. Multidimensional numerical simulations are a valuable tool to speed up and support the development process of the system, by providing details that cannot be gathered solely from experimental tests. In this study, a 3D-CFD model was validated against experimental data under both stoichiometric and lean conditions in a gasoline engine equipped with a passive pre-chamber, featuring 4 nozzles, each with a 1.0 mm diameter. Afterward, three pre-chamber configurations with variable nozzle cross-sectional area and three geometries with constant nozzle area were analyzed. Among the nozzle configurations with variable cross-sectional area, the pre-chamber with an intermediate nozzle area (4 nozzles, 1.2 mm diameter) leads to the fastest combustion process, reducing combustion duration by 25% compared to the baseline pre-chamber. This is attributed to the improved pre-chamber scavenging, which increases the energy released in the pre-chamber by nearly 25%. This allows the generation of the most intense turbulence, hence increasing the charge entrained by the jets, and finally reducing combustion duration. Conversely, for excessively large total nozzle area (4 nozzles, 1.4 mm diameter), the enhanced scavenging effect is partially offset by the energy losses associated with the ejection of the cold jets, resulting in only a 10% increase in energy released in the pre-chamber compared to the baseline configuration. For the same total nozzle area, when a different number of pre-chamber nozzles is considered, the spatial distribution of the jets plays a crucial role in determining the combustion rate. Indeed, if the number of nozzles is excessively reduced (3 nozzles, 1.4 mm diameter), the consumption of the mixture located between two adjacent jets is delayed, increasing the combustion duration by almost 5% compared to the baseline and 40% compared to the optimal pre-chamber geometry (i.e., 4 nozzles, 1.2 mm).