Effect of Flowfield Non-Uniformities on Emissions Predictions in HSDI Engines 2011-01-0821

The role of the fluid motion in a diesel engine on mixing and combustion was investigated using the CFD code Kiva-3v. The study considered pre-mixed charge compression ignition (PCCI) combustion that is a hybrid combustion system characterized by early injection timings and high amounts of EGR dilution to delay the start and lower the temperature of combustion. The fuel oxidizer mixture is not homogeneous at the start of combustion and therefore requires further mixing for complete combustion. PCCI combustion systems are characterized by relatively high CO and UHC emissions. This work investigates attenuating CO emissions by enhancing mixing processes through non-uniform flowfield motions. The fluid motion was characterized by the amount of average angular rotation about the cylindrical axis (swirl ratio) and the amount of non-uniform motion imparted by the relative amounts of mass inducted through tangential and helical intake ports in a 0.5L HSDI diesel engine. Non-uniform fluid motion, characterized by counter-rotating vortices, was found to contribute to a decrease in the amount of pollutant formation at varying levels of swirl ratio. The attenuation of CO could be significant (~15%) but insufficient to decrease the pollutant levels to desired levels without further after-treatment.