Flow enhancement strategies for reducing flow interference and improving in-cylinder flow in multi-cylinder GDI engines under low-speed low-temperature operating condition

Abstract

Gasoline direct injection (GDI) engines face challenges during low-temperature conditions, primarily due to high hydrocarbon and particulate emissions and unstable combustion. The objective of this work is to investigate in-cylinder flow variations for enhancing combustion stability in multi-cylinder GDI engines under low-temperature operation conditions. First, the analysis revealed that cylinder-to-cylinder flow variations arise from interference between the back flow of a preceding cylinder and the intake flow of the subsequent cylinder, with the extent of interference governed by the distance between firing-adjacent cylinders. Second, optimization of injection timing demonstrated that early injection at BTDC 300° effectively sustained tumble flow, leading to a 17% increase in TKE (turbulent kinetic energy) and a 7% reduction in wall film accumulation. Third, the introduction of an intake port tumble insert effectively suppressed flow structures that disrupt tumble development (counter-flow) while reinforcing those that sustain rotational structure (co-flow), thereby enhancing in-cylinder tumble flow. As a result, TKE increased by 32%, and wall film formation was reduced by 11%. Finally, modifying the firing order to eliminate long-distance cylinder pairs mitigated in-cylinder flow imbalances and improved the overall uniformity of tumble intensity. These findings highlight that appropriate design and operational strategies, such as injection strategy, tumble insert in intake port, and firing order adjustments, can substantially improve flow dynamics, mixture preparation, and ultimately, performance of low-speed low-temperature conditions in multi-cylinder GDI engines.