Modern diesel engines temporarily use a very late post-injection in the combustion cycle to either generate heat for a diesel particulate filter regeneration or purge a lean NOx trap. In some configurations, unburned fuel is left at the cylinder walls and is transported via the piston rings toward the lower crankcase region, where fuel may dilute the oil. Reduced oil lubrication shortens the oil service intervals and increases friction. Beside diesel fuel, this problem may also occur for other types of liquid fuels such as alcohols and e-fuels. The exact transport mechanism of the unburned fuel via the piston ring pack grooves and cylinder wall is hard to measure experimentally, motivating numerical flow simulation in early design stages for an in-depth understanding of the involved processes. A new CFD simulation methodology has been developed to investigate the transient, compressible, multiphase flow around the piston ring pack, through the gap between piston and liner, and its impact on fuel or oil transport. The modern level-set approach is used for the multiphase physics, which directly captures the sharp interface between blow-by gas and fuel or oil. Transient blow-by and two-phase flow simulations have been extensively applied to a Ford 2.0L I4 diesel test engine. The results confirm the validity of the flow compressibility assumption and highlight the sensitivity of the fuel leakage regarding piston sealing ring movement and highly resolved meshes for the multiphase flow. Based on the simulation results, design recommendations for piston and piston ring geometry are provided to reduce the fuel transport toward the crankcase.

翻译：现代柴油发动机在燃烧循环中会采用极晚的后喷射，以产生热量用于柴油颗粒过滤器再生或净化稀燃NOx捕集器。在某些配置中，未燃烧的燃油残留在气缸壁表面，并通过活塞环向曲轴箱下部区域输送，导致燃油稀释机油。机油润滑性能下降会缩短换油周期并增加摩擦。除柴油外，该问题也可能出现在其他液态燃料（如醇类和电子燃料）中。未燃烧燃油通过活塞环组沟槽和气缸壁的精确传输机制难以通过实验测量，这促使在早期设计阶段通过数值流模拟深入理解相关过程。本文开发了一种新的计算流体动力学（CFD）模拟方法，用于研究活塞环组周围、活塞与缸套间隙中的瞬态可压缩多相流及其对燃油或机油传输的影响。采用现代水平集方法处理多相物理过程，该方法能直接捕捉窜气与燃油或机油之间的尖锐界面。瞬态窜气及两相流模拟已广泛应用于福特2.0L I4柴油试验发动机。结果验证了流动可压缩性假设的有效性，并揭示了燃油泄漏对活塞密封环运动及多相流高分辨率网格的敏感性。基于模拟结果，本文提出了活塞及活塞环几何结构的设计建议，以减少燃油向曲轴箱的输运。