Investigation of in-cylinder flow and cycle-to-cycle variations in small-bore optical spark-ignition engines with different displacement volumes

Small-bore engines are widely used in two- and three-wheelers due to their compactness, low cost, and favorable balance between power output and fuel economy. However, they often exhibit significant cycle-to-cycle variations (CCV) in fuel–air mixing and turbulence generation, especially under low-speed and low-throttle conditions, which can degrade combustion stability and overall performance. This variability is primarily driven by the unsteady evolution of in-cylinder flow structures. To address this, the present study utilizes particle image velocimetry to capture flow fields from two small-bore engines (with displacement volumes of 110 and 200 cm3) operating under part-open (25%) and wide-open (100%) throttle conditions. Proper orthogonal decomposition-based quadruple decomposition is employed to assess CCV with flow components—mean, coherent, transition, and turbulent—identified based on cumulative energy contributions from reconstructed modes. Additionally, the relevance index metric is used to quantify the similarity between individual cycles and ensemble-averaged flow fields. The findings of the present study indicate that during the early intake stroke, both engines exhibit nearly identical flow structures with high similarity among the collected snapshots and a dominant contribution from the mean mode. As the intake progresses, the mean energy share decreases due to the intensification of anticlockwise vortices and the onset of backflow, particularly under wide-open throttle conditions. In the compression stroke, the merging of piston-driven upward motion with residual tumble strengthens the mean flow and reduces the CCV in the flow fields. These findings provide insight into the dynamic flow behavior contributing to CCV in small-bore engines and underscore the importance of coherent structure evolution in shaping in-cylinder flow stability.