C.M. Taylor

Automobile engine tribology : approaching the surface

There has been relentless pressure in the second half of the twentieth century to develop ever more fuel efficient and compact automobile engines with reduced environmental impact. From the viewpoint of the tribologist this means increasing specific loads, speeds and temperatures for the major frictional components of the engine, namely the piston assembly, the valve train and the journal bearings, and lower viscosity engine oils with which to lubricate them. Inevitably, this leads to decreasing oil film thicknesses between the interacting surfaces of these components and a more crucial role for the topography and surface profile of the two surfaces in determining tribological performance. This paper will review the nature of the surfaces encountered in the piston assembly, valve train and journal bearings of the internal combustion engine and how mathematical models of engine tribology are endeavouring to cope with the extreme complexities the incorporation of surface topography potentially brings. Key areas for future research and the implications for design will be highlighted.

Automobile engine tribology—design considerations for efficiency and durability

Abstract In the United Kingdom, the Technology Foresight Programme [HMSO, Progress Through Partnership 1 (1995) 126 pp.], through its Transport Panel, revealed the requirement for key generic technologies and scientific research in relation to automobiles. One of the `three major development opportunities which will help accommodate increased demand in a sustainable way was identified as `vehicles with greater efficiency and reduced environmental impact. In addition, further studies in `fuel efficiency and `simulation and modelling were recommended. The total scope of tribological considerations with regard to the above prospective research themes is immense and the present paper will focus upon the major frictional components of the automobile engine, that is, the bearings, the valve train and the piston assembly. In particular, the current position surrounding the modelling of these components will be reviewed and future possibilities identified. Prediction of overall engine friction will be addressed and the specific issues of modelling of lubricant behaviour and the role of surface topography touched upon.

A theoretical analysis of the isothermal elastohydrodynamic lubrication of concentrated contacts II. General case, with lubricant entrainment along either principal axis of the Hertzian contact ellipse or at some intermediate angle

The initial objective of the work reported in this paper was the development of generalized representations of film thickness results for elastohydrodynamic conjunctions in which lubricant entrainment coincided with one of the principal axes of the Hertzian conjunction. Some 106 solutions have been considered, including 33 presented in part I for entrainment along the major axis, four further solutions of a similar kind, the 34 solutions presented by Hamrock & Dowson (J. lubr. Technol. 98, 264-276 (1977)) for entrainment along the minor axis and 35 new solutions for similar geometries. It has been shown that normalization of the principal parameters in terms of the curvature in the direction of lubricant entrainment, 1/Re, permits the display of both central and minimum film thickness values as functions of the ratio of the radii of the solids normal to, and in the direction of, lubricant entrainment. These continuous curves enable film thickness to be predicted over a very wide range of geometrical configurations, but valid empirical expressions for both central and minimum dimensionless film thickness have also been developed. The second major feature of the study was to develop elastohydrodynamic solutions for the non-symmetrical conditions encountered when the lubricant entraining vector did not coincide with either of the principal axes of the conjunction. Such solutions are more representative of the conditions encountered in certain machine elements than the symmetrical solutions already reported. Examples of the resulting nonsymmetrical pressure distributions, elastic deformations and film shapes are presented. It is shown that normalization in terms of the curvature in the direction of lubricant entrainment, and the use of a simple trigonometric function, enables both the central and minimum film thicknesses to be predicted for any entrainment angle. It is demonstrated that this comprehensive and generalized presentation of new and previous solutions to the elastohydrodynamic lubrication problem for elliptical conjunctions yields film thickness predictions that compare very well indeed with specific solutions reported earlier. It is further shown that the central film thickness is little affected by the orientation of the lubricant entraining vector for many ellipsoidal solids, but that the minimum film thicknesses encountered cover a much wider range of values. In many cases the minimum film thicknesses occur in side-lobes located near the lateral boundaries of the Hertzian conjunction, which perform a sealing role and thus permit the generation of near-Hertzian hydrodynamic pressures in the central regions of the conjunction. The results are expected to provide a basis for the analysis and design of a wide range of machine elements operating in the elastohydrodynamic régime of lubrication.

Predictive wear modelling of lubricated piston rings in a diesel engine

The tribological performance of piston rings in reciprocating internal combustion engines can only be fully understood when both lubrication and wear are considered in combination. To this end, a numerical model has been developed that predicts the dynamics, lubrication and wear of piston rings interactively for the first time. This paper reports the application of this new model to the piston ring pack of a diesel engine. With the overall aim of evaluating the correlation between theory and experiments, this analysis is divided into two discrete parts. First, the model is used to predict the lubrication performance of measured ring packs before and after periods of running, at constant speed and load, in a Caterpillar 1Y73 single-cylinder diesel engine: the objective being to establish the change in tribological behaviour with observed wear in the engine. Secondly, the model is used interactively to predict the lubrication and wear of the top compression ring from the same engine. This research advances the understanding of piston ring profile evolution with time and its dependence on complex interactions between lubrication and wear.

Theoretical modelling of cavitation in piston ring lubrication

Abstract Mathematical models of piston ring dynamics and lubrication are sensitive to the boundary conditions adopted to describe the cavitation occurring in the diverging outlet region of the lubricant film between the piston ring and cylinder wall. In this paper, such sensitivity is investigated by applying different models of gaseous cavitation, flow separation and fluid film reformation to the analysis of a single compression ring from a diesel engine. Significant differences are predicted in hydrodynamic pressure profiles, lubricant film boundaries, lubricant film thickness, oil flow and friction. Such indications of substantial differences in piston ring operating characteristics associated with the distinct cavitation boundary conditions considered highlights the need for further research in this field. However, the lack of detailed experimental data to validate the predictive models suggests that future progress must be based upon combined theoretical and experimental approaches to the problem. It is postulated that boundary conditions based upon Reynolds cavitation and fluid film reformation may be applicable at high loads, and fluid film separation of a form proposed by Coyne and Elrod at low loads.

Detailed Analysis of Oil Transport in the Piston Assembly of a Gasoline Engine

More realistic and useful models of piston ring lubrication can only be achieved if there is a better understanding of the complex mechanisms by which oil flows in this region of the engine. The volume of oil in the piston assembly and its residence time in this high-temperature environment are crucial in determining the quantity and quality of oil available to lubricate the piston rings. Typically models of piston ring pack lubrication focus upon the oil flowing through the piston ring/cylinder interface. However, a number of additional oil flow paths and interactions with gas blow-by have been observed in the piston assembly. This paper presents a model that includes a number of such mechanisms and evaluates their influence on the lubrication of a piston ring pack from a typical automotive gasoline engine. The results indicate that such additional mechanisms are needed to give improved predictions of oil transport they highlight the relative importance of several of these mechanisms and help guide future research.

A comparative thermal analysis of the static performance of different fixed profile bore plain bearings

Abstract Profile bore bearings are important components in high-speed rotating machinery. In order to provide bearing designers with some useful information on the performances of different bearing profiles, this paper presents a theoretical evaluation of five commonly used types of bearing through a comparison of their steady state performance characteristics. The bearings investigated were two-lobe circular, elliptical (lemon-bore), offset-half, three-lobe and four-lobe types. Their operating characteristics, including the minimum film thickness, maximum temperature, power loss and flowrate, have been calculated using a computerized detailed thermohydrodynamic model developed by the authors. The influences of the shaft speed, specific load, bearing pre-load ratio and lubricant viscosity index on the performance of the bearings have been investigated. In addition, the effect of oil groove angle and load orientation on a four-lobe bearing is briefly examined. Results show that, in general, the performance of the non-circular bearings is inferior to that of the circular bearing; of the non-circular types, the offset-half and elliptical bearings have better overall performances. Also, it is shown that reasonably increasing the angular size of an oil groove has a favourable effect on the static performance of a multi-lobe bearing.

Waviness Orientation in EHL Point Contact

In recent years, EHL point contact analysis has greatly benefited from the development of new numerical techniques. Amongst these, the multigrid multi-integration method has opened real perspectives not only in simulations of smooth surface contacts, but also in the consideration of rough surfaces and transient effects. Brandt [1], Lubrecht [2] and Vernier [3] have demonstrated the quality of such a solver, which allows high levels of discretisation, and enhances stability and robustness. Using this technique, an investigation on the effect of waviness orientation in EHL point contacts under high load situations has been carried out. Pure sliding is considered and the waviness is placed on the stationary surface. Under severe loading conditions the maxima of waviness are largely flattened and large pressure ripples are produced. The orientation of the waviness strongly influences the behaviour of the flow at the entrance of the contact, which in turn considerably effects the deformation of the surfaces inside the contact area. Leakage flows and accumulation of lubricant at the inlet introduce surface constrictions and grooves, which propagate all along the contact in the direction of the flow. Results show that the transverse waviness presents the best lubrication capability. The lowest minimum film thickness is obtained when the waviness is orientated at about 60° compared to the direction of the surface velocity.

Lubrication Regimes and the Internal Combustion Engine

Publisher Summary This chapter highlights the significance of a lubricant as an engineering design material. If the lubricant film is sufficiently thick to prevent the opposing solids from coming into contact, the condition is referred to as “fluid film lubrication.” This condition is often considered as the ideal form of lubrication since it provides low friction and a high resistance to wear. The behavior of the contact is governed by the bulk physical properties of the lubricant, and the frictional characteristics arise purely from the shearing of the viscous lubricant. Different modes of lubrication are significant in the alternative tribological components of the engine and a component may undergo a wide range of lubrication regimes during a cycle. The physical and chemical actions associated with the modes of lubrication known as hydrodynamic, elastohydrodynamic, mixed, and boundary are discussed in the chapter.

Time-Dependent Solutions with Waviness and Asperities in EHL Point Contacts

The effects of various surface textures, described either by an orientated waviness or an uniform distribution of asperities, are compared under pure rolling conditions for elastohydrodynamic lubricated point contacts. Time-dependent solutions, obtained with a Multigrid Multi-Integration method, reveal that the orientation of the waviness, in these conditions, has only a moderate effect on the values of minimum film thickness and maximum pressure. These results are in contrast to those obtained under pure sliding conditions, where the orientation of the surface texture leads to important differences in terms of minimum film thickness. In pure sliding, it is shown that the best lubrication condition is produced for the transverse waviness.