The Effect of Swirl Ratio and Fuel Injection Parameters on CO Emission and Fuel Conversion Efficiency for High-Dilution, Low-Temperature Combustion in an Automotive Diesel Engine

Engine-out CO emission and fuel conversion efficiency were measured in a highly-dilute, low-temperature diesel combustion regime over a swirl ratio range of 1.44-7.12 and a wide range of injection timing. At fixed injection timing, an optimal swirl ratio for minimum CO emission and fuel consumption was found. At fixed swirl ratio, CO emission and fuel consumption generally decreased as injection timing was advanced. Moreover, a sudden decrease in CO emission was observed at early injection timings. Multi-dimensional numerical simulations, pressure-based measurements of ignition delay and apparent heat release, estimates of peak flame temperature, imaging of natural combustion luminosity and spray/wall interactions, and Laser Doppler Velocimeter (LDV) measurements of in-cylinder turbulence levels are employed to clarify the sources of the observed behavior. Mixing processes occurring after the pre-mixed burn are found to be the likely source of the optimal swirl ratio, while enhanced pre-combustion mixing dominates the reduction in CO with earlier injection. Liquid fuel films formed on the bowl lip are not found to significantly impact CO emissions, and increased injection pressure typically reduces CO emissions at this high dilution rate. Fuel conversion efficiency is examined in terms of component efficiencies related to combustion phasing, heat loss, and combustion efficiency. The influence of swirl and injection timing on each of these efficiencies is discussed.