Predictions of the effects of piston-liner crevices on flow motion and emissions in three-dimensional diesel engine simulations

Abstract

Abstract

Computational fluid dynamics predictions of the flow field and pollutant emissions in a high-speed direct-injection diesel engine operating at high load and rated speed are presented. Piston-liner crevice flows during the expansion stroke can give rise to vortex motions whose characteristics depend on the piston crevice configuration. Both a realistic and a simplified crevice volume that consists of a neck area and a reservoir to keep the surface—volume ratio realistic are considered. In the simplified crevice approach, with moderate intensity of the crevice flow, the vortex enhances mixing and engine-out carbon monoxide (CO) emissions are reduced. However, when the crevice flow intensity is above a certain level, a strong tumble motion is created in the bulk mixture in the squish area, which isolates a region of high CO concentration and prevents oxidation. The results also show that proper timing of the crevice flow into the combustion chamber lowers the mixture temperature and reduces nitrogen oxide emissions. Calculations with a realistic crevice flow geometry and a crevice flow submodel also predict squish region vortex motion.