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Flow and flame structure visualization and modeling were performed to clarify the characteristics of bulk flow, turbulence and mixing in a four-valve engine to adopt the lean combustion concept named “Barrel-Stratification” to the larger displacement center-spark four-valve engine. It was found that the partitions provided in the intake port and the tumble-control piston with a curved-top configuration were effective to enhance the lean combustion of such an engine. By these methods, the fuel distribution in the intake port and the in-cylinder bulk flow structure are optimized, so that the relatively rich mixture zone is arranged around the spark plug. The tumble-control piston also contributes to optimize the flow field structure after the distortion of tumble and to enable stable lean combustion.

LDV measurements, have been conducted to determine the in-cylinder gas flow produced by intake ports of various designs. The analysis of these measurements revealed the formation and development processes of tumble and clarified the requirements of the tumble formation by intake ports. The air flow from the intake port upper section toward the combustion chamber on the exhaust side has been found a governing factor for the increase of tumble intensity. By tuning the intake flow to have an appropriate direction in the earlier stage of the intake process, a directed flow ascending the cylinder wall is generated in the later stage of the intake process. At an early stage of the compression process, the structure of the tumbling vortex becomes clear. As an index of the tumble intensity, the instantaneous kinetic moment of air around the geometric center of in-cylinder space was proposed.