Fuel efficiency is now the over-riding engine development objective. With approximately 50–60% of the input fuel energy in an internal combustion engine lost through thermal and mechanical inefficiencies, friction has been targeted as the arch nemesis in any engine development program. A significant portion of the parasitic frictional losses is due to the top compression ring. This suggests that optimization of tribological performance of the compression ring conjunction warrants much more attention that it has been hitherto afforded. Studies reported thus far take into account ring-bore conformance, based on static fitment of the ring within an out-of-round bore, whose out-of-circularity is affected by manufacturing processes, surface treatment and assembly. The various static fitment analyses presume quasi-static equilibrium between ring tension and gas pressure loading with the generated conjunctional pressures. This is an implicit assumption of ring rigidity whilst in situ, which is in fact not the case in practice. The transient nature of combustion variation means that mere static or quasi-static force balance is inappropriate. Furthermore, the bore is not a right circular cylinder. Thus, its radial cross-sectional out-of-roundness and its axial variation introduce further transience in the ring-bore conformance. Consequently, the net force applied to the ring induces its modal behaviour, which accounts for its instantaneous in situ shape within the bore. These considerations are not taken into account in the often idealized ring-bore tribology. The paper provides transient solution of ring-bore conjunction, when subjected to ring in-plane modal behavior, when the conjunction is subjected to a mixed regime of lubrication, comprising hydrodynamic viscous action and boundary interactions.

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