Numerical study of the effects of throat diameter on ignition and flame propagation in a methanol engine with passive prechamber

Prechamber ignition is an effective approach to achieving high-efficiency lean combustion to improve thermal efficiency of methanol engines. However, the acting mechanism of throat diameter which is a critical structural parameter remains unclear. In this study, the numerical model was established for a methanol engine with a passive prechamber to investigate the effects of throat diameter (4.5, 6, and 7 mm) on ignition and turbulent flame propagation in both the prechamber and the main chamber. Overall, the results reveal that among the three typical throat diameters, the shortest ignition delay (8.51 °CA), the highest peak in-cylinder pressure (3.82 MPa) and the highest IMEP (0.53 MPa) are achieved at the throat diameter of 4.5 mm, indicating superior combustion characteristics. In the prechamber, the high turbulent regions introduced by the circulating vortices are largest at the throat diameter of 4.5 mm, with mean velocity and TKE reaching 14.42 m/s and 28.40 m 2 /s 2 , facilitating the formation of initial flame kernels. At the throat diameter of 7.0 mm, the strengthened recirculation zone weakened and displaced circulating vortices. In the main chamber, high-temperature combustion products of the jet dominate ignition, with the ignition site located in the lateral shear layer of the jet and local quenching occurring near the orifice. The larger proportion of hot jet mass (53.68%) and higher turbulence intensity (1158 m 2 /s 2 ) contribute to improved ignition performance at the throat diameter of 4.5 mm. In addition, the turbulent combustion regime falls into the thin reaction zone in the prechamber, but the combustion regime undergoes multi-stage variations in the main chamber. At the early ignition stage, a higher Ka is obtained at the throat diameter of 4.5 mm. This study uncovers the effects of throat diameter on ignition and turbulent flame propagation in both the prechamber and the main chamber, providing valuable guidance for prechamber design in methanol engines.