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Abstract
Flame propagation and flow field analyses have been performed in an optical spark-ignition direct-injection (SIDI) engine to improve the understanding of the effects of fuel-injection events on bulk in-cylinder flow, turbulence intensity, burn rate, and indicated output. Initially, an investigation into the benefits of a double injection strategy over a single injection strategy was conducted, where it was found that a double injection strategy leads to significantly higher indicated mean effective pressure (IMEP) and total heat release, which is consistent with an increase in flame propagation speed. To further investigate the potential of double injection strategies, a study on the effect of injection timing was completed. It was found that the best performing 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. A high-speed particle image velocimetry (HSPIV) imaging system was developed to quantify large-scale flow structures and turbulence intensity, in order to aid the interpretation of the fuel-injection timing variation results. HSPIV images were first acquired on both a vertical (tumble) plane and a horizontal (swirl) plane during motored operation (i.e. no fuel injection or spark). The flow was decomposed into mean and fluctuating components via three different methods - ensemble averaging, spatial filtering, and temporal filtering. Relationships between spatial and temporal filtering were investigated, resulting in the creation of a spatial filter which utilises a mean flow speed scaled cut-off length - tuned in order to match the turbulent kinetic energy (TKE) profile of a 300 Hz temporal filter. The spatial filtering techniques were applied to five selected fuel-injected operating conditions, ranging from two early injections to two late injections. The bulk flow was found to be extremely sensitive to relatively small changes in injection timing, although an expected trend of increasing turbulence intensity with retarded injection timing was clearly observed. Relationships between TKE and burn rate were present, but not as obvious as anticipated, as total kinetic energy and cycle-to-cycle variation in large-scale flow structures were also found to play a significant role.