RESEARCH ARTICLE

Abstract PIV data obtained in a transparent motored Diesel engine are presented and discussed in this paper. These data are obtained in severe thermodynamical conditions in the cylinder during the cycle and in the bowl at top dead center (TDC). Moreover, they are obtained at consecutive cycles. We first consider the flow in the middle of the compression phase. In particular, instantaneous velocity fields are analyzed and classified by computing circulation statistics. We show that the structure of the swirling motion is varying very signifi-cantly from ‘‘vortex type’ ’ to ‘‘annular type’ ’ from one cycle to another. In-cycle and inter-cycle statistics of these structure fluctuations are discussed. For example, we do not find any cycle-to-cycle correlation. The cir-culation data are then decomposed by using a proper orthogonal decomposition of the statistical set. The possibility of low order description of this kind of flow is considered. Finally, we study the in-bowl flow at the very end of the compression phase. The quantitative analysis based on circulation statistics show that we still detect strong structure fluctuations of the swirling flow at TDC and that the intensity of these fluctuations have not decreased after the squish phase.