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The blow-by phenomenon is seldom acquainted with diesel engines, but for a small bore HSDI optical diesel engine, the effects are significant. A difference in peak pressure up to 25% can be observed near top-dead-center. To account for the pressure differences, a 0-D crevice flow model with a dynamic ring pack model was incorporated into the KIVA code to determine the amount of blow-by. The ring pack model will take into account the forces acting on the piston rings, the position of the piston rings, and the pressure located at each region of the crevice volume at every time step. The crevice flow model takes into consideration the flow through the circumferential gap, ring gap, and the ring side clearance. As a result, the cylinder mass, trapped mass in the crevice regions, and the blow-by values are known. Validation of the crevice model is accomplished by comparing the in-cylinder motoring pressure trace with the experimental motoring data.