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 - Approaching the Surface

The development and understanding of elastohydrodynamic lubrication (EHL) can be traced back to the beginning of the previous century. However, it was not until 1949 that the first real solution of the problem was published. Since then, the technology has evolved enormously. In the current article a summary of these developments is given. Smooth surface EHL has become well established. Numerical methods, analytical solutions, and experimental techniques have become mature. Focus areas of research today are thermal EHL, starved EHL, friction (non-Newtonian lubricants), roughness, and grease. The scope of EHL is so wide that the authors needed select the topics of focus in this article. Therefore, in addition to the general overview of the areas of friction, analytical methods, starved EHL, and grease EHL are highlighted in this article.

A Review of Elasto-Hydrodynamic Lubrication Theory

In this study, a transient, non-Newtonian, mixed elastohydrodynamic lubrication (EHL) model of involute spur gear tooth contacts is proposed. Unlike the contact between two cylindrical rollers, spur gear contacts experience a number of time-varying contact parameters including the normal load, radii of curvature, surface velocities, and slide-toroll ratio. The proposed EHL model is designed to continuously follow the contact of a tooth pair from the root to the tip to capture the transient characteristics of lubricated spur gear contacts due to these parameter variations, instead of analyzing the contact at discrete positions assuming time-invariant parameters. The normal tooth force along the line of action is predicted by using a gear load distribution formulation and the contact radii and tangential surface velocities are computed from the kinematics and geometry of involute profiles. A unified numerical approach is adapted for handling asperity interaction in mixed EHL conditions. The differences between the transient and discrete EHL analyses are shown for a spur gear pair having smooth surfaces and different tooth profile modifications. The transient behavior predicted by the proposed model is found to be mainly due to the squeezing and pumping effects caused by sudden load changes. The lubrication behavior under rough conditions is also investigated at different operating conditions. DOI: 10.1115/1.4000270

A Transient Mixed Elastohydrodynamic Lubrication Model for Spur Gear Pairs

We present a realistic elastohydrodynamic lubrication (EHL) simulation in point contact using a Carreau-like model for the shear-thinning response and the Doolittle-Tait free-volume viscosity model for the piezoviscous response. The liquid lubricant modeled is a high-viscosity polyalphaolefin which has been shown by high-pressure viscometry to possess a relatively low threshold for shear-thinning as a single-component liquid lubricant. As a result, the measured EHL film thickness is about one-half of the Newtonian prediction. We derived and numerically solved the two-dimensional generalized Reynolds equation for the modified Carreau model based on Greenwood. In this simulation, viscosity was not treated as an adjustable parameter; the models used for the pressure and shear dependence of viscosity were obtained entirely from viscometer measurements. Truly remarkable agreement is found in the comparisons of simulation and experiment for traction coefficient and for film thickness in both pure rolling and sliding cases.

A Quantitative Solution for the Full Shear-Thinning EHL Point Contact Problem Including Traction

New numerical results, based upon the concept of solidification, are produced which match the intriguing dimple observed by Kaneta in elastohydrodynamic lubrication of point contacts under pure sli...

On lubricant transport conditions in elastohydrodynamic conjuctions

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.

P. Ehret

Papers

On lubricant transport conditions in elastohydrodynamic conjuctions

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