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 performance 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 medium-load conventional combustion regime, the results highlight significant differences in the evolution of reacting flow for these bowl geometries. CIV-resolved swirl ratio shows that a re-entrant piston induces a stronger swirl amplification effect than a stepped-lip piston during the late-stage combustion (CA50-CA90). The tumble-plane projection of CIV-resolved velocity fields confirm that the injection-induced redistribution of angular momentum charge is very different between the two piston geometries. The relatively high angular momentum charge is re-distributed towards center at a slower rate with the stepped-lip piston than with the re-entrant piston. During late-stage combustion, reacting flow in the stepped-lip piston reaches “solid-body-like” rotation earlier than in the re-entrant piston. When spray targeting is optimized for the best soot-NOx trade-off , the stepped-lip piston exhibits a long-lasting flow structure with opposing radial velocity directions between the squish-region and stepped-lip-region which are not optically observed with the re-entrant piston geometry.