An experimental study investigating the cyclic variation of the internal air motion has been conducted in a motored engine having a cup-in-piston combustion chamber. The engine used for this study has a transparent quartz liner and a transparent acryl piston. The continuous wave beam of a 4 watt argon ion laser was formed into a thin light sheet to define the measurement plane. The scattered light from the particles which were supplied to the intake air stream, were recorded by a NAC E-10 highspeed camera and the bulk flow fields inside the combustion chamber were measured at planes parallel and perpendicular to the piston crown by means of an image processing technique. We reveal, for the first time, continuous cycle-resolved two-dimensional bulk flow fields inside the combustion chamber around the compression TDC. Also, comparison between cycle-resolved and ensemble-averaged flow patterns has shown that the latter may become artificial ones if there exists a strong effect of cyclic variation. Moreover, by examining the rms of the variation component and the ratios of the rms to ensemble-averaged velocity with and without swirl, it is found that the ratios of the rms to ensemble-averaged velocity can be used for evaluating the effect of the cyclic variation on the how fields.
