Study on the lean limit and thermal efficiency of an SI engine equipped with newly developed in-cylinder flow-control adapter

Lean combustion technology has been one of the promising approaches for developing next-generation engines with high thermal efficiency and low environmental impact. However, the practical lean limit of such engines is typically constrained by combustion stability. To address this issue, the present study introduces a newly designed intake adapter that can effectively generate in-cylinder tumble flow during the engine’s intake stroke, which is expected to promote ignition and flame propagation. The effect of this adapter was evaluated using a single-cylinder spark-ignition (SI) engine, with a particular focus on thermal efficiency and cycle-to-cycle variation. The results show that, with the adapter, the engine achieved a significant extension of the lean limit compared to the baseline case without flow control. Specifically, under operating conditions of IMEP=1.15 MPa and a compression ratio (CR) of 17, stable combustion was sustained at an air–fuel equivalence ratio of λ=2.26. Meanwhile, thermal efficiency improved by up to 3% relative to the no-adapter case. Furthermore, under lean conditions, the concentrations of unburned hydrocarbons (HC) and carbon monoxide (CO) in the exhaust were significantly reduced, indicating that the adapter enables stable combustion even under ultra-lean conditions. Finally, it was found that adjustments to the geometric parameters of the intake adapter can further enhance both thermal efficiency and lean limit performance under high-load and high-compression-ratio operations.