In light-duty direct injection (DI) diesel engines, combustion chamber geometry influences the complex interactions between swirl and squish flows, spray-wall interactions as well as late-cycle mixing. Because of these interactions, piston bowl geometry significantly affects fuel efficiency and emissions behavior. However, due to lack of reliable in-cylinder measurements, the mechanisms responsible for piston-induced changes in engine behavior are not well understood. Non-intrusive, in-situ optical measurement techniques are necessary to provide a deeper understanding of the piston geometry effect on in-cylinder processes and to assist in the development of predictive engine simulation models.This study compares two substantially different piston bowls with geometries representative of existing technology: a conventional re-entrant bowl and a stepped-lip bowl. Both pistons are tested in a single-cylinder optical diesel engine under identical boundary conditions. Utilizing high-speed soot natural luminosity (NL) imaging, 20 kHz time-resolved combustion-image-velocimetry (CIV) technique is developed to quantify the macro-scale motions of soot clouds during the mixing-controlled portion of combustion.Under a part-load conventional combustion regime, CIV-resolved swirl ratio and the tumble-plane projection of velocity fields confirm that the injection-induced redistribution of angular momentum, rather than squish/reverse squish flow, is a dominant source for swirl amplification between two piston geometries. A strong connection has been found between the CIV-resolved combusting flow structure and its succeeding enhanced late-stage burn rate. With SOImain shortly after TDC, combustion in stepped-lip piston exhibits shorter late-burn duration (CA50-CA90) and faster burn rate compared to re-entrant piston. In the same boundary condition, a unique combusting flow structure is observed with CIV in the stepped-lip piston: a long-lasting flow structure with opposing radial velocity directions between the squish-region and stepped-lip-region. Interestingly, this flow structure is never optically observed with the re-entrant piston. The best hypothesis is that there exists a long-lasting vertical toroidal vortex on the shoulder of stepped-lip piston crown near CA50. A phenomenological model is proposed to provide a partial, but valuable picture of late-stage combusting flow structure which is a key to understand how piston bowl geometry can influence thermal efficiency for swirl-supported diesel engines.