Coupled thermal-geometric effects on glow-plug-assisted natural gas jet ignition in high-pressure direct-injection engines

Natural gas is a highly promising low-carbon alternative fuel for internal combustion engines, but its high-octane number and elevated auto-ignition temperature limit direct application in conventional compression-ignition (CI) engines. Glow plug (GP) assisted ignition, relying on high-temperature surfaces, provides an effective thermal strategy to overcome these challenges. In this study, numerical simulations using KIVA-3V systematically investigate GP-assisted natural gas ignition in a high-pressure direct-injection (HPDI) engine. The effects of GP surface temperature, fuel injection angle, and shield design (including the number and spatial arrangement of openings) were examined for three key ignition characteristic parameters: ignition delay by pressure (IDP), ignition delay by temperature (IDT), and ignition delay difference (IDD). The study presents a comprehensive multi-parameter analysis of GP ignition, revealing how shield-opening geometry influences the coordination between ignition onset and flame propagation. For single- and square-opening shields, IDP exhibits angular symmetry with respect to injection angle, primarily determined by initial flame propagation characterized by IDD, with the square-opening design showing the largest IDP fluctuations. In contrast, the diamond-opening shield achieves the closest correlation between IDT-IDP and IDD-IDP, reflecting an effective balance between fuel inflow and flame propagation and yielding the most stable and optimal ignition. Increasing the number of openings without proper geometric arrangement can exacerbate asynchrony between IDT and IDD effects on IDP, thereby reducing ignition reliability. Overall, GP ignition performance is controlled by the complex interaction of thermal, flow, and geometric factors. These findings provide insights into the thermal and geometric design of GP-assisted ignition systems, and offer practical guidance for improving ignition stability and performance in low-carbon engine applications.