This study applies high-speed particle image velocimetry (HS-PIV) and flame image velocimetry (HS-FIV) to show flow fields under the effect of varied swirl ratios in a small-bore optical compression-ignition engine. The base swirl ratio and maximum swirl ratio conditions were applied to investigate structures, magnitude and turbulence distribution of the in-cylinder flow as well as the flow within the flame. For each swirl ratio, 100 individual cycles were measured for PIV analysis at motoring conditions and then another 100 cycles for FIV analysis at firing conditions. The derived flow fields were ensemble averaged to show flow structure evolution while the spatial filtering method was applied to extract high-frequency flow component for the analysis of turbulence distributions. The results showed that the intake air flow generates undefined, chaotic flow fields, which are followed by a gradual production of an asymmetric swirl flow. This is due to uneven intake port shape with one spiraling passage and the other straight passage. Upon the formation of a clearly defined swirl flow structure, its centre position rotates in the counter-clockwise direction within the measurement field of view, which impacts the reacting jet flame development with the down-swirl side of the wall-interacting jet penetrating faster. At the maximum swirl ratio realised by blocking the straight port, the intake air flow vectors become more chaotic and stronger, resulting in increased bulk flow magnitude and turbulence. Also, a ring vortex of the flame head on the up-swirl side appears closer to the jet axis while the vectors travelling faster on the down-swirl side than that of the base swirl ratio. Notably, the cyclic variation of in-flame vector magnitude is observed to decrease for the higher swirl ratio, which is attributed to faster mixing to eliminate the high cyclic variation zone formed within the jet-jet interaction region.
