Investigation of in-cylinder flows and cycle-to-cycle variations in a small-bore gasoline direct-injection optical engine at different throttle openings and speeds

Small-bore engines are globally in high demand, particularly for two- and three-wheeler applications. Implementing direct-injection technology in these engines enhances performance, but it also poses challenges such as inhomogeneous mixture formation and fuel impingement. These issues can degrade performance, especially at lower speeds and throttle openings. To address these limitations, a thorough investigation of in-cylinder flow fields and their cycle-to-cycle variations (CCVs) is necessary. Therefore, in this study, particle image velocimetry is used to measure flow fields in a small-bore optical engine at different throttle openings (25% and 100% throttle at 600 rpm) and speeds (600 and 1200 rpm at 25% throttle). Ensemble-averaged flow fields are obtained and used to compare the temporal evolution of the flow fields across intake and compression strokes. Flow fields are found to be similar in structures for different speeds and, however, differ for different throttle openings. The kinetic energy and vorticity distributions, along with their cycle-averaged values, are subsequently analysed. CCV in the flow fields is quantified using the relevance index and proper orthogonal decomposition-based mean energy share metrics. CCV obtained from the two different metrics showed similar trends and complex dependence on throttle opening and speed. The CCV is observed to be higher during early intake under part throttle condition compared to full throttle condition. However, it becomes lower under part throttle in comparison with full throttle from the late intake to mid-compression, attributed to the increased flow field intensities under part throttle. For different speeds, the CCV is found to be comparable till the intake stroke; however, differences are observed from early compression and onwards.