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Under idle operations of a spark-ignition direct-injection engine, issues such as misfire, unstable combustion, and power imbalance between individual cylinders are often encountered, which worsen the fuel economy and tailpipe emissions. These undesired phenomena have close relations with cyclic variations of the fuel sprays in the cylinder. In this article, the spray cyclic variations under idle operations have been investigated at a constant volume chamber using ultraviolet/visible laser absorption/scattering imaging technique and Mie scattering optical diagnostics combined with different statistical methods such as probability presence image, intersection over union, and edge fluctuation length. The variations in spray morphology of liquid/vapor phases and vapor mass distributions have been characterized. It was found that the cyclic spray variation after the end of injection is too large to ignore, implying that this cyclic variation should be taken into consideration when matching the spray to a combustion chamber or numerical modeling. The effects of injection pressure and fuel temperature on spray cyclic variations have been quantitatively examined. The results show that the higher injection pressure or the higher fuel temperature is, the larger variation in spray morphology and vapor mass distributions was observed, indicating that adopting an appropriately lower injection pressure or lower fuel temperature is helpful to a stable ignition and combustion under idle conditions for a non-homogeneous spark-ignition direct-injection engine.

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