Swirl in diesel engines is known to be an important parameter that affects the mixing of fuel and air, heat release, emissions, and overall engine performance. In this study, a multidimensional model for flows, sprays, and combustion in engines is employed to study the interactions between swirl and the fuel spray in a diesel engine. It is generally believed that swirl enhances mixing in a diesel engine. Previous studies have suggested that there may be an optimal level of swirl at which the mixing in the chamber is maximized for a given set of engine operating conditions. This optimal level may result from a balance between increased jet surface area and utilization of air in the chamber. In this work, those trends are clarified through a detailed evaluation of the velocity flow patterns in the chamber and by relating such patterns to those published in the literature for jets in cross-flow. It is also shown that the optimal swirl ratio for enhanced mixing may be characterized by a nondimensional parameter expressed as a ratio of the jet momentum of the azimuthal momentum.