Modeling of Blow-by in a Small-Bore High-Speed Direct-Injection Optically Accessible Diesel Engine 2006-01-0649
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. For the combustion cases, chemical reaction is calculated using the Shell ignition model and KIVA's combustion subroutine. Pressure traces for each case show dramatic improvement with the use of the crevice flow model and reported values around 4% of blow-by was seen. One of the main causes for such high blow-by values found is attributed to the lack of an oil film layer on the surface of the cylinder wall. Without the oil film, a “wet seal” cannot be established, allowing flow to pass through the circumferential gap between the piston ring contact surface and cylinder wall liner.