  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.
