Measurements of turbulence sources in a swirl-supported diesel engine

Abstract

The combustion efficiency and pollutant formation in swirl-supported diesel engines strongly depend on the in-cylinder turbulent flows. But the physical mechanism of turbulence generation remains inadequately understood. To identify the sources of turbulence at various spatial locations and to understand the relationship between the flow structures and turbulence production, planar particle image velocimetry experiments are performed in this study to characterize the evolution of turbulent flow in a motored optical diesel engine with a compression ratio of 13:1 and a swirl ratio of 1.5. The temporal and spatial distributions of turbulent kinetic energy ( k) and the turbulence sources are correlated. An extremely high k generated by the intake jet during the intake process was observed, and it remains only 1.6%–3.3% during the later compression stroke. Near the cylinder center, k changes significantly with the crank angle and the higher k was always observed near the swirl center. The production via shear stresses correlates with k well. In other regions, k almost keeps unchanged with the crank angle because of the rigid-body-like swirl flow, which results in a negligible net production by flow deformation. In addition, in the middle region, the evolution of radial angular momentum gradient matches with the k evolution reasonably well, which indicates that k may depend on the transport by the squish flow although the underlying physics deserves further exploration.