Combined measurements of flow structure, partially oxidized fuel, and soot in a high-speed, direct-injection diesel engine

Abstract

The evolution of bulk flow structures and their influence on the spatial distribution of heat release zones and of partially oxidized fuel and particulate matter (soot) is examined experimentally in a swirl-supported, direct-injection diesel engine. Vector fields describing the bulk flow structures are measured with particle image velocimetry (PIV), while complementary scalar field measurements of partially oxidized fuel and soot are obtained in the same vertical plane using broadband laser-induced fluorescence (LIF) and laser-induced incandescence (LII) techniques, respectively. The two-dimensional divergence of the mean velocity fields is also employed to provide information on the mean locations of heat release. Measurements are performed at a highly dilute, 12% O2, operating condition characteristic of low-NOx, low-temperature diesel combustion systems. The spatial distributions of unburned fuel rapidly develop a structure characterized by two separate zones of high fuel concentration, an inner zone in the cylinder center and an outer zone in the squish volume. Single-cycle measurements show that this two-zotie structure is present on an individual cycle basis, and is not an artifact of averaging distinct, single-zone distributions. For this engine build, the mean flow structures developed do not actively promote mixing of either zone, although bulk flow structures in the upper-central region of the cylinder vary significantly on a cycle-by-cycle basis. The measured spatial distributions of particulates indicate that particulates are formed primarily in the inner zone-and remain un-oxidized late in the cycle.