Oil consumption and Friction in a Piston Ring-Cylinder Liner assembly.-A multifunctional Surface Approach

Doctoral thesis, 2014

About a half of the frictional losses in an internal combustion engine come from the interactions between the piston assembly and cylinder liner surface. The tribological considerations in the contact between the piston ring and cylinder liner have attracted much attention over the past few decades. Many non-conventional cylinder liner finishes have been, and are being, developed with the aim to reduce friction losses and oil consumption, but the effects of the surface finish on piston ring pack performance is not well understood. One way of reducing friction in the cylinder system is to reduce the tangential load from the piston ring pack, focusing on the oil control ring that is the largest contributor to friction losses among the three piston rings. However, the side effect of this is disappointingly increased oil consumption. To counteract the increase of oil consumption a number of different cylinder liner surface specifications were developed and implemented in test engines with the aim of maintaining a sufficiently low level for oil consumption when decreasing the tangential load for the piston ring pack. The cylinder liner surface and piston ring profile interaction has a function working in the micro scale area and therefore manufacturing of the surfaces is a delicate task. By choosing wrong characterization parameters or wrong manufacturing methods the delicate equilibrium in the interaction between liner surface and ring surface can be broken. An algorithm to assess and quantify the presence of blechmantel was developed. The relative success of the Rq and Rk families of parameters in quantifying liner surfaces were compared and several parameters were found to be so highly correlated as to be redundant. No convincing correlation could be obtained with honing parameters. Topological diagrams helped to distinguish the robustness of parameters. Simulation suggested that low roughness reduces oil consumption but increases friction, while higher roughness produces the opposite effects, and this finding was confirmed by engine tests over a range of speeds. Finally a area scale fractal analysis was performed to investigate the correlation between liner surface areal scale and oil consumption results in order to investigate the scale of functional occurrence. The analysis has given some promising results in finding scale range with good correlations with functional data and ability to discriminate properties in surface geometries.