It has been observed that the swirl characteristics of in-cylinder air flow in a spark ignition direct-injection (SIDI) affect the fuel spray dispersion and flame propagation speed, impacting the fuel mixture formation and combustion process under higher conditions. In addition, the cycle-to-cycle variations of swirl flow often degrade the fuel spray mixing and combustion quality in the cylinder. In this study, the 2D flow structure along a swirl plane at 30 mm below the injector tip was recorded using high-speed particle image velocimetry in a four-valve optical SIDI engine under high swirl condition. Quadruple proper orthogonal decomposition (POD) was used to investigate the cycle-to-cycle variations of 200 consecutive cycles during the intake and compression strokes. The flow fields were analyzed by dividing the swirl plane into four zones along the measured swirl plane according to the positions of intake and exhaust valves in the cylinder head. Experimental results revealed that the coefficient of variation (COV) of the time coefficients of the quadruple POD mode coefficients could be used to estimate the cycle-to-cycle variations at a specific crank angle. The dominant structure was represented by the first POD mode in which its kinetic energy could be correlated with the motions of the intake valve. Moreover, the higher order flow variations were closely related to the flow stability at different zones. In summary, quadruple POD provides another meaningful way to understand the intake swirl impact on the cycle-to-cycle variations of the in-cylinder flow characteristics in SIDI engine.

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