Varying the fuel stratification during gasoline compression ignition (GCI) combustion has been shown to impact important combustion parameters and emissions. The effect of varied injection pressure and injection timing on the fuel stratification and formation of nitric oxide (NOx) emissions was studied at two engine operating conditions. At a 1500 revolutions per minute (rpm) engine condition, a 100 bar increase in injection pressure required a 1.4? crankangle retard of the injection timing to maintain constant NOx emissions. The required injection timing shift to maintain constant NOx emissions at a 1900 rpm condition for a 100 bar increase in injection pressure was 2.5? crankangle. A skip-firing injection strategy illustrated the importance of the second injection in creating fuel stratification and promoting ignition for GCI combustion. The effects of injected fuel mass variability on combustion stability were investigated using a randomized injection strategy. Analysis showed that the injected fuel mass uncertainty required to induce combustion instability was between 3.2-4.8%.
Three-dimensional computational fluid dynamics (CFD) and a one-dimensional (1-D) turbulent jet model were used to analyze the fuel-air mixing. A quasi-steady jet timescale was used to non-dimesionalize the time after start of injection. The ability of the timescale to collapse the jet vapor penetration and fuel-mass-weighted PDF of mixture fraction/equivalence ratio were evaluated for a variety of conditions at times significantly after end of injection. The quasi-steady jet timescale reasonably collapsed jet vapor penetration for various injection pressures but did not collapse the fuel-mass-weighted PDFs of equivalence ratio at times of interest during transient changes to the ambient gas density unless changes in spray spreading angle are accounted for. The 1-D jet model was benchmarked to CFD and evaluated at different conditions to analyze the assumptions of the 1-D model. A sensitivity analysis of the 1-D model was conducted. The 3-D CFD results are utilized to analyze the connection between the fuel-air distribution and the engine-out NOx emissions at the constant-NOx engine operating conditions. Computational fluid dynamics results showed similar equivalence ratio distributions resulted in relatively constant NOx emissions.