Numerical analysis of performance and soot emissions of a natural gas engine operating in HPDI and SPC combustion modes

High-pressure direct injection (HPDI) natural gas (NG) engines can help significantly improve thermal efficiency and reduce harmful emissions. In this study, we established a three-dimensional (3D) model combined with a multicomponent reduced chemical kinetic model and a phenomenological soot model. Using the 3D simulation model, we investigated the influence of pilot diesel mass on the combustion and emission characteristics of an HPDI NG engine at different start of injection of NG (NSOI). Subsequently, we evaluated the influence of diesel and NG injection interval (DNT) on the combustion characteristics and soot emissions in the HPDI and slightly premixed combustion (SPC) combustion modes at different NSOI. The simulation results indicated that with an increase in the pilot diesel mass at different NSOI, the peak values of cylinder pressure (P-max) and nitrogen oxides (NOx) emissions increase, but carbon monoxide (CO) and soot emissions decrease simultaneously. Advancing NSOI leads to higher NOx emissions and increased P-max and maximum pressure rise rate (MPRR) at different pilot diesel masses; by contrast, delaying NSOI leads to decreased indicated thermal efficiency (ITE). In the HPDI mode, combustion parameters such as P-max and CA50% are not sensitive to variations in the DNT at different NSOI. Furthermore, the variations in the DNT do not notably influence the reduction of soot emissions. In the SPC mode, the ignition delay significantly increases with the decrease in the DNT, whereas CA50% moves away from the top dead center (TDC) and P-max decreases. Meanwhile, the peak values of pyrene (A(4)) and acetylene (C2H2) simultaneously decrease, and the OH radicals are more extensively distributed in the cylinder, thereby resulting in a considerable reduction in final soot emissions. Considering the combined influence of P-max, MPRR, ITE, and soot emissions, the operating point with the NSOI of -12 degrees CA ATDC and DNT of -4 degrees CA is considered the optimized point.