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

Abstract

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.

Influence of Injection Timing for Split-Injection Strategies on Well-Mixed High-Load Combustion Performance in an Optically Accessible Spark-Ignition Direct-Injection (SIDI) Engine

“…It was also reported that the flame front grew in a more spherical manner, although the extent of this phenomenon was not quantified. Previous work conducted by the present authors found that a double injection strategy resulted in higher indicated performance and flame propagation speeds when compared to an equivalent single injection strategy [29]. The application of a best-fit ellipse algorithm to the flame boundary points revealed that double injections also gave rise to more circular flames, thought to be due to increased charge homogeneity resulting in less variation in laminar burn speed throughout the combustion chamber.…”

Cyclic variations of in-cylinder pressure and turbulent flame propagation in an optical direct-injection spark-ignition engine

“…The selection of recording 105 individual cycles is based on careful trial-and error testing such that the processed flame and in-cylinder pressure parameters are effectively isolated from the cyclic variation. A similar approach is found in the previous work conducted in the same optical engine [13]. The flame images were processed by in-house developed Matlab codes, which employ contrast adjustment, imaging binarization and boundary detection processes.…”

“…The flame images were processed by in-house developed Matlab codes, which employ contrast adjustment, imaging binarization and boundary detection processes. The area of the flame was calculated from the binarized image in each frame, which was used to estimate the flame growth rate [13].…”

Simultaneous In-Cylinder Flow Measurement and Flame Imaging in a Realistic Operating Engine Environment Using High-Speed PIV

“…PFI: port fuel injection; CA aTDC: Crank-Angle There are a number of diagnostics that one can use to observe the flame, ranging from simple natural emission based such as chemiluminescence or natural-flame luminosity to more complicated techniques such as tomography of hydroxyl planar laser-induced fluorescence (OH PLIF). Clark et al investigated the effect of the timing of the first and second injection for an evenly split dual injection strategy in an optical engine [10,11]. Performance parameters derived from in-cylinder pressure data are analyzed alongside high-speed natural flame luminescence images in order to obtain relationships between engine output and the physical properties associated with flame propagation.…”