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The demand for internal combustion engines remains high for mobile power sources in all fields due to their low costs, running distance capacity, charging reliability, and heavy driving durability. However, air pollution, efficiency, and environmental factors make this more challenging. According to recent research, using a fueled prechamber can lead to lean combustion in the main chamber, resulting in increased efficiency, reduced fuel consumption, and reduced toxic emissions. However, difficulties in producing a fueled prechamber for commercial engines include mixture and soot formation problems in the limited space of the prechamber, and limited research on the charging possibility of the unfueled prechamber. A removable prechamber is advantageous for used vehicles because an engine redesign is not required. Therefore, we proposed to use an unfueled prechamber to enhance the lean burning efficiency of the spark ignited (SI) engine and explore the possibility of charging an unfueled, unscavenged prechamber with a fuel-rich mixture. Consequently, investigating the possibility of filling an unfueled prechamber with a fuel-rich mixture without additional fuel delivery or an air control system was the aim of this study. For this purpose, the charge flowrate of the centrally located unfueled prechamber is extensively investigated by using Computational Fluid Dynamics (CFD), through its design. As a result, a realizable charge flow was detected for the unfueled prechamber in two periods in the inlet and compression strokes. Most importantly, we found fluctuation phenomena in mass flow rates at the inlet stroke directing a charge flow of the richer mixture into an unfueled prechamber without additional systems. Moreover, keeping the charged rich mixture inside the prechamber during the compression stroke is as important as charging the prechamber with the fuel-rich mixture. The study will enable us to produce a removable prechamber to improve the combustion efficiency of port injected engines.

A third order accurate cellwise relaxation implicit Discontinuous Galerkin (DG) scheme for RANS simulations using unstructured hybrid meshes is presented. A scalar parabolic equation is first examined to clarify what is really important in construction of implicit matrix to keep its diagonal dominance for the third and fourth order cellwise relaxation implicit DG schemes. In addition, discussions are given to approximated construction of implicit matrix for reducing computational cost. Then, the third order accurate cellwise relaxation implicit DG scheme for RANS simulations is successfully developed utilizing the expertise learned in the study of solving the parabolic equation. Superior spatial accuracy of the third order accurate cellwise relaxation implicit DG scheme for RANS simulations, while retaining reasonable convergence properties, is demonstrated for typical aerospace applications.

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