Sources of UHC Emissions from a Light-Duty Diesel Engine Operating in a Partially Premixed Combustion Regime

Sources of unburned hydrocarbon (UHC) emissions are examined for a highly dilute (10% oxygen concentration), moderately boosted (1.5 bar), low load (3.0 bar IMEP) operating condition in a single-cylinder, light-duty, optically accessible diesel engine undergoing partially-premixed low-temperature combustion (LTC). The evolution of the in-cylinder spatial distribution of UHC is observed throughout the combustion event through measurement of liquid fuel distributions via elastic light scattering, vapor and liquid fuel distributions via laser-induced fluorescence, and velocity fields via particle image velocimetry (PIV). The measurements are complemented by and contrasted with the predictions of multi-dimensional simulations employing a realistic, though reduced, chemical mechanism to describe the combustion process. Homogeneous reactor simulations also employed to clarify the influence of chemistry (vs. mixing) on UHC oxidation, and to compare the behavior of the reduced chemical mechanism with a more detailed mechanism. The impact of injection timing on the UHC distributions is also examined. At the completion of heat-release, UHC is found in three regions: near the injector tip and cylinder centerline, the bowl and central clearance volume, and the squishvolume. The centerline UHC is composed of partiallyreacted fuel that leaks from the injector late in the cycle and of discrete liquid droplets, embedded within diffuse UHC associated with overly lean mixture. Within the bowl and central clearance volume, UHC stemming from both rich and lean mixtures is observed. With advanced injection, UHC from lean mixtures dominate; with retarded injection rich-mixtures are dominant. In the squish-volume, UHC originating from piston-top films and ring-land crevice flows is observed with advanced and MBT injection timing, while with retarded timing, UHC from lean mixtures is dominant.