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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.

The preparation of fuel-air mixture and its efficient, clean, and reliable combustion in spark-ignition direct-injection (SIDI) engines depend to a large extend on the complex in-cylinder air flow. It has been widely recognized that the ensemble-averaged flow field provides rather limited understanding of in-cylinder air motion due to the strong cycle-to-cycle variations. In this study, time-resolved particle image velocimetry (PIV) is utilized to measure the in-cylinder air motion in a motored single-cylinder optical engine. Then, the velocity fields from different phases (crank-angle positions during intake and compression strokes) of 200 engine cycles are analyzed using phase-invariant proper orthogonal decomposition (POD) technique. With the phase-invariant POD method, the velocity fields from different phases are decomposed into a single set of POD modes. In this manner, the POD modes can be used to represent any phase of the flow.