Brian Fitzsimons

Transient elastohydrodynamic lubrication of rough new or worn piston compression ring conjunction with an out-of-round cylinder bore

Real cylinder bores are out-of-round and axially asymmetrical. The top compression ring, nominally an incomplete circle, is subjected to ring tension and cyclic combustion force in order to conform to the bore surface. The bounding surfaces are rough and their conjunction is subject to a transient tribological state. Therefore, the ring–bore conjunction is only partially conforming for most of the engine cycle. The conjunction may be viewed as a problem of scale, depending on the analysis carried out at a certain bore order (out-of-roundness). Therefore, the contact may be viewed as a multi-lobed rough conjunction, where the regime of lubrication may vary from hydrodynamics to mix with dominant asperity friction at piston reversals. Measured bores and ring profiles are used to predict conjunctional power loss and percentage fuel energy consumed. Furthermore, lubricant’s flow through the ring is predicted throughout the engine cycle. These measures are key industrial design drivers for fuel efficiency and reduction of emissions. The results show that the effect of bore out-of-roundness can be even more significant than the surface topography.

In-cycle and life-time friction transience in piston ring–liner conjunction under mixed regime of lubrication

The piston ring/cylinder liner conjunction can experience various regimes of lubrication during piston strokes inside the engine cylinder. In the current engines, the nature of lubrication usually remains hydrodynamic at mid-stroke, while a mixed regime of lubrication may be experienced at and near reversals. The direct contact between the tips of some of the asperities of opposing surfaces leads to mixed (partial) regime of lubrication. A model proposed by Greenwood and Tripp can be used to predict asperity-level contribution to the total piston friction. At the same time, Reynolds equation can be employed to predict the portion of load carried by the lubricant trapped between the asperities. Friction between the asperity tips is usually proportional to the load that they support, stated in terms of a proportionality factor, that is, coefficient of friction. The surfaces are usually furnished with hard wear-resistant coatings and in parts by solid lubricants. Both the piston rings and cylinder liner surfaces are usually coated. These coatings change the friction characteristics of the counterfaces because of their surface topography as well as material mechanical properties. Atomic force microscope is used to obtain surface topographical parameters in contact tapping mode. The corresponding surface topographical parameters are obtained from representative regional areas of the contacting solid surfaces, using a Talysurf. The combination of topography and coating characteristics are used to develop the necessary parameters for a boundary friction model. A numerical model of the top compression ring to cylinder liner is developed based on mixed-hydrodynamic regime of lubrication. The results for friction and the effect of coating on the power loss and wear of the conjunction are discussed in this article.

The influence of piston ring geometry and topography on friction

This article provides solution for isothermal mixed hydrodynamic conjunction of the compression ring to cylinder liner. This is obtained using the average flow model representation of Reynolds equation based on pressure- and shear-induced flow factors. In particular, the effects of compression ring axial profile along its face-width and surface topography of contiguous solids are investigated. It is shown that ring geometry may be optimized to improve lubrication, whilst care should be taken in order to avoid oil loss or degradation resulting from any loss of sealing. In predicting friction, it is shown that appropriate surface parameters should be used in-line with the state of wear of the ring. For a new ring against a plateau honed liner, boundary friction contribution during the initial running-in wear phase should be predicted according to the average asperity peak heights protruding above the plateau, whilst the plateau height also takes into account the valleys within the surface roughness or grooves created by any cross-hatch honing would be the appropriate measure of topography for worn rings. The main contributions of the article are in providing an analytic solution as well investigation of ring face-width geometry and effect of wear upon friction. However, it is acknowledged that generated heat, inlet boundary starvation and circumferential non-conformity of ring to the bore surface would affect the film thickness and exacerbate generated friction accordingly. These further considerations would require a numerical solution, rather than an analytical one presented here.

Influence of In-Plane Dynamics of Thin Compression Rings on Friction in Internal Combustion Engines

The compression ring-bore conjunction accounts for significant frictional parasitic losses relative to its size. The prerequisite to improving the tribological performance of this contact is a fundamental understanding of ring dynamics within the prevailing transient nature of regime of lubrication. 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 static fitment analyses presume quasi-static equilibrium between ring tension and gas pressure loading with generated conjunctional pressures. This is an implicit assumption of ring rigidity whilst in situ. The current analysis considers the global modal behaviour of the ring as an eigenvalue problem, thus including its dynamic in-plane behaviour in the tribological study of a mixed-hydrodynamic regime of lubrication. The results show that the contact transit time is shorter than that required for the ring to reach steady state condition. Hence, the conjunction is not only subject to transience on account of changing contact kinematics and varied combustion loading, but also subject to perpetual ring transient dynamics. This renders the ring-bore friction a more complex problem than usually assumed in idealised ring fitment analyses. An interesting finding of the analysis is increased ring-bore clearance at and in the vicinity of top dead centre, which reduces the ring sealing effect and suggests a possible increase in blow-by. The current analysis, integrating ring in-plane modal dynamics and mixed regime of lubrication includes salient features which are closer representation of practice, an approach which has not hitherto been reported in literature.

On the Effect of Transient In-Plane Dynamics of the Compression Ring Upon Its Tribological Performance

Energy losses in an internal combustion engine are either thermal or parasitic. The latter are the mechanical inefficiencies, chiefly as the result of generated friction. Nearly half of these losses are attributed to the piston–cylinder system. During idle and at low engine speeds, friction is the major contributor to the overall engine losses. In particular, the rather small top compression ring accounts for a disproportionate share. Therefore, detailed understanding of compression ring tribology/dynamics (referred to as tribodynamics) is essential. Moreover, the ring’s primary sealing function may be breached by its elastodynamic behavior. The reported analyses in literature do not account for the transient nature of ring elastodynamics, as an essential feature of ring–bore tribology. The transient in-plane dynamics of incomplete rings are introduced in the analysis and verified using a finite element analysis (FEA) model, in order to address this shortcoming. The methodology is then coupled with the tribological analysis of the top compression ring. Comparison is made with experimental measurements which show the validity of the proposed method. The radial in-plane elastodynamic response of the ring improves the accuracy of the frictional power loss calculations.

Performance Evaluation of Piston Compression Ring Through Accelerated Wear in Engine Tests

Both the piston ring and cylinder bore experience wear throughout their life cycle. Therefore change occurs in the geometrical profile and topography of the ring. In addition, coating on the ring is also subject to wear, thus altering the physical/mechanical property of the contacting surface. These changes affect the tribological performance of the ring-bore conjunction. Geometrical changes often alter the ring contacting profile, which affects the entrainment of the lubricant into the conjunction. This can affect the regime of lubrication, thus mechanisms that contribute to friction. Wear of surfaces also plays some role in boundary friction in terms of topographical changes as well as surface properties such as hardness and asperity shear strength. Most analysis does not take into account changes in tribological conditions which occur as the result of these salient changes in practice. The paper intends to bridge this gap in the fundamental knowledge and provide explanations for some in-field experiences noted with wear of a compression ring in a typical engine test. The method of solution used is based on the average flow factors which are indicative of entrainment of the lubricant through the rough ring-bore conjunction. This approach was initiated by Patir and Cheng, for which realistic topographical parameters according to the stage of the wear process is included. Changes in friction and fuel energy consumed are predicted.