Heat release analysis of the in-cylinder pressure records and images of the naturally occurring combustion luminosity obtained in an optical engine are used to explore the effect of variable swirl ratio on the diesel combustion process. Swirl ratios Rs at IVC of 1.5, 2.5, and 3.5 were investigated. The engine is equipped with common-rail fuel injection equipment, and the combustion chamber geometry is maintained as close as possible to typical engines intended for automotive applications. The operating condition employed was 2000 rpm, with a gross IMEP of 5.0 bar and 800 bar injection pressure.Swirl ratio is found to exert a measurable influence on most of the combustion process, from ignition to late-cycle oxidation. Ignition delay decreases with increasing Rs, as do the magnitudes of the initial premixed burn, the peak rates of heat release, and the maximum rates of pressure rise. Despite the monotonic behavior of the initial ignition and premixed combustion process, by the end of the fuel injection event the mass of fuel burnt is found to be lower for Rs = 2.5 than for the other swirl ratios. Images of early combustion chemiluminescence indicate that the location of ignition does not change with swirl ratio. During the mixing-controlled portion of combustion, Rs initially has little effect on the heat release rate, as the mixing is likely dominated by the turbulence generated during fuel injection. Later, the heat release rates are seen to increase with increasing Rs. Luminosity images show increased activity in the squish volume and deep in the bowl during this latter time period. Late in the cycle, heat release rates are dominated by the amount of unburned fuel left in the cylinder. Late-cycle oxidation is observed to occur predominantly above the piston and outside the bowl for all swirl ratios.