Multi-Dimensional Modeling of Direct Natural-Gas Injection and Mixture Formation in a Stratified-Charge SI Engine with Centrally Mounted Injector

Direct injection (DI) of natural gas (NG) at high pressure conditions has emerged as a high-potential strategy for improving SI engine performance. Besides, DI allows an increase in the fuel economy, due to the possibility of a significant engine dethrottling at partial load. The high-pressure gas injection can also increase the turbulence level of mixture and thus the overall fuel-air mixing. Since direct NG injection is an emerging technology, there is a lack of experience on the optimum configuration of the injection system and the associated combustion chamber design. In the last few years, some numerical investigations of gas injection have been made, mainly oriented at the development of reliable numerical investigation tools. The present paper is concerned with the development and application of a numerical Star-CD based model for the investigation of the direct NG injection process from a poppet-valve injector into a bowl-piston engine combustion chamber. Different aspects related to the reliability of prediction models are examined, such as: structure and size of the mesh, flow initial and boundary conditions. In this work, the multidimensional simulation tool is applied to the whole relevant volume, including the injector-nozzle internal path. A challenging outlook, concerning the best compromise between computational cost and accuracy, is the meshing strategy of the moving injector, mainly at the needle-seat passage. The process of model development can be split in three steps. First, a basic numerical study was carried out, in order to characterize the fuel jet properties into an open environment, as functions of both the adopted meshing strategy and flow initialization. Second, an axisymmetric injector-nozzle configuration was designed, with the same jet-formation properties as the starting available design, in order to allow the moving needle simulation with a handmade, high-quality, fully hexahedral computational mesh. Third, the numerical procedure was validated through the comparison of numerical results with Schlieren images of the jet profile in a constant-volume bomb. Finally, the results of model application for simulating the methane direct-injection into the combustion chamber of an engine with centrally mounted injector are presented, in the cases of different injection strategies, injector-tip protrusions, piston and cylinder-head shapes. Indications concerning mixture-formation capabilities of the injector and chamber are analyzed.