Development and application of a method for characterizing mixture formation in a port-injection natural gas engine

Natural gas has been identified as one of the most promising alternative fuels. Port injection of natural gas, due to its advantages of costs, manufacturing complexity and mixture homogeneity, is relevant to the current and future engine development. The present work aims to provide a comprehensive characterization of gas fuel injection and mixing process, by developing a modeling method for the injector and the engine as a whole serving as a diagnostic tool for further expounding on the basis of experimental results. The injector is modeled by the source cell approach that allows for cost-efficient and physics-consistent description of the underexpanded gas jet, by which a set of fuel injection timings is investigated and then compared with a conventional premixed case. Two mechanisms peculiar to gas port injection are characterized, being firstly the two-stage mixing process that involves immediate induction of the residual fuel from the previous engine cycle and delayed induction of the fuel injected in the current cycle, and secondly the limited fuel penetration speed along the intake ports with associated delay of charge induction. Additional information on volumetric efficiency, mixture quality, coherent flow motion and turbulence level is highlighted. It is concluded that the otherwise intuitive correlation between injection timing and mixture homogeneity for port injection is complicated by those two mechanisms, and, depending on specific engine design and operating point, differences resulted from modeling the engine operation with fuel injection and with premixed charge may prove combustion-significant. The method and the underlying mechanisms found herein are equally applicable to other combustion systems involving port injection of gaseous fuels.