Multiple Injection Strategy Investigation for Well-Mixed Operation in an Optical Wall-Guided Spark-Ignition Direct-Injection (WG-SIDI) Engine through Flame Shape Analysis

One major drawback of spark-ignition direct-injection (SIDI) engines is increased particulate matter (PM) and unburned hydrocarbon emissions at high load, due to wall wetting and a reduction in available air/fuel mixing time when compared to port-fuel injection (PFI). It is therefore necessary to understand the mechanics behind injection strategies which are capable of reducing these emissions while also maintaining the performance and efficiency of the engine. This study investigates the effect of varying the number fuel injection events and equivalence ratio on the operation of a wall-guided SIDI (WG-SIDI) engine. Of particular interest is how increased mixture homogeneity achieved by the double injection events impacts in-cylinder conditions and flame development. Performance parameters derived from in-cylinder pressure data are analysed alongside high speed natural flame chemiluminescence images in order to obtain relationships between engine output and the physical properties associated with air/fuel mixing and flame propagation. Compared to the single injection event, the results show that the double injection strategy leads to significantly higher net indicated mean effective pressure (IMEP_n) and total heat release for the same fuel mass, which is consistent with the increased flame propagation speed. The mechanisms behind this trend are then further explored through the analysis of flame shape parameters such as flame boundary eccentricity, orientation and apparent flame boundary deviation (i.e. a wrinkling level) of the line-of-sight integrated image. It is found that single injection strategies tend to result in stretched flame fronts with a high elliptical eccentricity, regardless of equivalence ratio. The orientation of the semi-major axis of this elliptical flame is perpendicular to the injection momentum axis and is aligned closely with the tumble flow axis, indicating that fuel inhomogeneity occurring along the path of the fuel jet is a significant factor for flame distortion. Compared to the single injection strategy, the flame eccentricity is consistently lower for the double injection events, likely due to increased mixture homogeneity. Changes in the deviation in flame boundaries are found to be due to the global eccentric nature of the flame, rather than increased localised wrinkling of the apparent flame front; and, the difference in apparent flame boundary wrinkling across both injection strategies is either non-existent or too negligible to be resolved by the method used in this study, suggesting the wrinkling is not significantly impacted by localised regions of lean or rich mixture but is due entirely to turbulence intensity and length scale.