Parametric Analysis of Injector Operational Parameters, Charge Dilution, and Intake Valve Opening on In-Cylinder Motion and Flame Development in an Automotive, Lean-Burn, Gasoline Engine with High-Tumble

The effect of injection pressure, start of injection (SOI) timing, charge dilution, and valve timing on charge motion and early flame development was investigated for a pre-production automotive gasoline engine. Experiments were performed in a single-cycle optical engine designed to represent the high-tumble (Tumble ratio = 1.8), lean-burn engine. Time-resolved particle image velocimetry (PIV) was used to characterize velocity flow fields throughout the swept volume during the intake and compression strokes. Diffuse back illuminated imaging allowed for visualization and quantification of the injected liquid fuel spray and its interactions with the tumble vortex. Hydroxyl (OH*) chemiluminescence imaging was performed to image spark channel elongation and early flame kernel development. It was observed that an optimal injection timing of 320° before top dead center (bTDC) resulted in attenuation of the tumble motion and an associated reduction in compression flows that shifted the tumble vortex toward the intake valves. These effects were amplified with higher injection pressures or advanced injection timings but muted for lower injection pressures or retarded injection timings. This differs from previous observations with a moderate tumble head (tumble ratio = 1.5), where the optimal injection timing resulted in augmentation of the tumble motion due to the momentum of the injected fuel spray, increasing the compression flows. Chemiluminescence imaging and visualization of the spark behavior of the high-tumble engine revealed that the optimal injection timing produced a highly stretched spark channel that consistently deflected toward the exhaust valves, with the flame kernel rapidly expanding in the recirculation zone at the time of spark, leading to a faster overall flame development in the combustion chamber. Variations of the parameters that increased flow velocities (lower injection pressures and retarded injection timings) resulted in less consistent spark positioning and flame kernel development, as well as a higher chance of observing restrikes of the spark. This is suspected to be why the optimal injection timing for the high-tumble head resulted in the attenuation of the tumble motion and reduced flow velocities compared to the moderate tumble head. These results suggest that it is crucial to balance the interactions between the tumble motion and the fuel spray with the with how the flow affects the quality of the spark channel and early kernel development at the time of spark. The interplay between the bulk flow motion and electrode-generated wake turbulence will be discussed, with experimental data compared to companion large eddy simulation results.