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Proper Orthogonal Decomposition (POD) is a useful statistical tool for analyzing the cycle-to-cycle variation of internal combustion engine in-cylinder flow field. Given a set of flow fields (also known as snapshots) recorded over multiple engine cycles, the POD analysis optimally decomposes the snapshots into a series of flow patterns (known as POD modes) and corresponding coefficients of successively maximum flow kinetic energy content. These POD results are therefore strongly dependent on the kinetic energy content of the individual snapshots, which may vary over a wide range. However, there is as yet no algorithm in the literature to define, detect, and then remove outlier snapshots from the dataset in a systematic manner to ensure reliable POD results.

The cycle-to-cycle variations of in-cylinder flow field represent a significant challenge which influence the stability, fuel economy, and emissions of engine performance. In this experimental investigation, the high-speed time-resolved particle image velocimetry (PIV) is applied to reveal the flow field variations of a specific swirl plane in a spark-ignition direct-injection engine running under two different swirl air flow conditions. The swirl flow is created by controlling the opening of a control valve mounted in one of the two intake ports. The objective is to quantify the cycle-to-cycle variation of in-cylinder flow field at different crank angles of the engine cycle. Four zones along the measured swirl plane are divided according to the positions of four valves in the cylinder head. The relevance index is used to evaluate the cycle-to-cycle variation of the velocity flow field for each zone.