No abstract

No abstract

Stringent particulate emission regulations are applied to spark-ignition direct-injection (SIDI) engines, calling for a significant in-cylinder reduction of soot particles. To enhance fundamental knowledge of the soot formation and oxidation process inside the cylinder of the engine, a new in-flame particle sampling system has been developed and implemented in a working optical SIDI engine with a side-mounted, wall-guided injection system. Using the sampling probes installed on the piston top, the soot particles are directly sampled from the petrol flame for detailed analysis of particle size distribution, structure and shape. At the probe tip, a transmission electron microscope (TEM) grid is stored for the soot collection via thermophoresis, which is imaged and post-processed for statistical analysis. Simultaneously, the flame development was recorded using two highspeed cameras to evidence the direct exposure of the sampling grids to the soot-laden diffusion flames and pool fires. The focus of the present study is the uncertainty analysis of this newly developed technique through variation of the number of injection cycles, cyclic variations, and sampling locations at fixed fuel injection and firing conditions. From the engine runs of 3, 5 and 7 injection cycles, it was found that the number of sampled soot aggregates increases with increasing injection cycles but the soot morphology does not change significantly. However, the cyclic variations make a significant impact such that the size of soot aggregates increases with higher peak in-cylinder pressure and earlier combustion phasing, which was observed from 6 different engine runs at fixed 5 injection cycle tests. The sampling experiment was also performed with the probes installed at four different locations across the piston top, which showed significant variations in morphology such that soot primary particles and aggregates become larger due to longer soot residence time within the wall wetting-induced pool fire.

One major drawback of spark-ignition direct-injection (SIDI) engines is increased particulate matter (PM) emissions at high load, due to increased wall wetting and a reduction in available mixture preparation 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. Splitting the fuel delivery into two or more injections is a proven way of working towards this goal, however, many different injection permutations are possible and as such there is no clear consensus on what constitutes an ideal strategy for any given objective. In this study, the effect of the timing of the first and second injections for an evenly split dual injection strategy are investigated in an optical SIDI engine running at 1200 RPM with an unthrottled intake. Performance parameters derived from in-cylinder pressure data are analysed alongside high-speed natural flame luminescence images in order to obtain relationships between engine output and the physical properties associated with flame propagation. It was found that the best performing injection strategy consisted of one injection relatively early in the intake stroke and one injection towards the middle of the compression stroke, likely due to the combination of both ample mixture formation time and a high level of spray induced turbulence. Interestingly the trends found in the results are significantly non-monotonic, as there are a series of peaks and troughs in indicated mean effective pressure (IMEP) and combustion phasing (CA50) as the injection timings are varied. The relationships between these indicated performance parameters and flame propagation were also analysed, where it was discovered that flame speed is very closely correlated with CA50, regardless of the value of IMEP. Additionally, an elliptical approximation was calculated for each flame in an attempt to establish a link between injection timing and flame shape. It was found that retarded second injections lead to a less circular flame, potentially as a result of decreased charge homogeneity causing local regions of varied laminar flame speed throughout the combustion chamber. Furthermore, analysis of eccentricity and CA50 graphs suggest that there is a link between retarded combustion phasing and non-circular flame fronts.

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 incylinder 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.