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UHC and CO Emissions Sources from a Light-Duty Diesel Engine Undergoing Dilution-Controlled Low-Temperature Combustion
Abstract:
Unburned hydrocarbon (UHC) and carbon monoxide (CO) emission sources are examined in an optical, light-duty diesel engine operating under low load and engine speed, while employing a highly dilute, partially premixed low-temperature combustion (LTC) strategy. The impact of engine load and charge dilution on the UHC and CO sources is also evaluated. The progression of in-cylinder mixing and combustion processes is studied using ultraviolet planar laser-induced fluorescence (UV PLIF) to measure the spatial distributions of liquid- and vapor-phase hydrocarbon. A separate, deep-UV LIF technique is used to examine the clearance volume spatial distribution and composition of late-cycle UHC and CO. Homogeneous reactor simulations, utilizing detailed chemical kinetics and constrained by the measured cylinder pressure, are used to examine the impact of charge dilution and initial stoichiometry on oxidation behavior. The measured distributions are also compared to multidimensional simulation results and with engine-out emissions measurements.
Homogeneous reactor simulations show that increased dilution leads to a narrower equivalence ratio (ϕ) range that allows for acceptable UHC and CO oxidation. As dilution increases, the increased charge-fuel mass ratio for a given ϕ amplifies the impact of a reduced rich-limit ϕ for acceptable UHC oxidation, since a greater fraction of the fuel is embedded in rich mixture. In-cylinder UHC and CO imaging highlights the differences that changes in dilution and load have on the three main UHC source regions: 1) The cylinder center region contains intense near-injector fluorescence indicative of late-cycle fuel addition, while diffuse fluorescence is present from UHC and CO that is embedded in the surrounding fuel-lean bulk gases; 2) Within the bowl and central clearance volume, a rich mixture plume that exits the piston bowl is the dominant source of predicted UHC and CO, but is not observed experimentally; 3) Squish volume UHC and CO principally results from the partial oxidation of lean mixture, although UHC from piston top fuel films and crevice flows is also observed.