Multiphase Flow Simulation of Blow-by and Fuel-in-Oil Dilution via the Piston Ring Pack Using the CFD Level-Set Method

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.