Shear Thinning and Hydrodynamic Friction of Viscosity Modifier-Containing Oils. Part II: Impact of Shear Thinning on Journal Bearing Friction

Abstract

In a companion paper, the temporary shear thinning behaviour of a series of viscosity-modifier (VM)-containing blends was studied over a wide shear rate and temperature range [Marx et al. in Tribol Lett, https ://doi.org/10.1007/s1124 9-018-1039-5]. It was found that for almost all VMs the resulting data could be collapsed on a single viscosity versus reduced strain rate curve using time-temperature superposition. This made it possible to derive a single equation to describe the viscosityshear rate behaviour for each VM blend. In the current paper, these shear thinning equations are used in a Reynolds-based hydrodynamic lubrication model to explore and compare the impact of different VMs on the film thickness and friction of a lubricated, isothermal journal bearing. It is found that VMs reduce friction and especially power loss markedly at high shaft speeds, while still contributing to increased hydrodynamic film thickness at low speeds. The model indicates that VMs can contribute to reducing friction in two separate ways. One is via shear thinning. This occurs especially at high bearing speeds when shear rates are large and can result in a 50% friction reduction compared to the equivalent isoviscous oil at low temperatures for the blends studied. The second is via their impact on viscosity index, which means that for a set viscosity at high temperature the low-shear-rate (and thus the high shear rate) viscosity of a high-VI oil, and consequently its hydrodynamic friction, will be lower at low temperatures than that of a low-VI oil. The identification and quantification of these two alternative ways to reduce friction should assist in the design of new, fuel-efficient VMs.

Shear Thinning and Hydrodynamic Friction of Viscosity Modifier-Containing Oils. Part I: Shear Thinning Behaviour

“…The ability to derive a single equation (Eq. 12) to describe the shear thinning of a given VM blend is important since it enables quantification of the impact of VM on shear thinning over a wide range of conditions and can also be used in modelling of the hydrodynamic behaviour of VM blends in hydrodynamic lubricated components, as described in the companion paper, Part 2 [1].…”

“…In a companion paper, the master shear thinning curves obtained in this paper are used to explore how VMs impact film thickness and friction in a steadily loaded, isothermal journal bearing [1].…”

“…This paper therefore describes a systematic study of the temporary shear thinning properties of a range of commercial VM blends, both in simple solutions and in fully formulated engine oils. The measured shear thinning behaviour is then used in conjunction with a conventional isothermal hydrodynamic lubrication model in the companion paper to explore how and the extent to which shear thinning influences film thickness and friction in a journal bearing [1].…”

Hydrodynamic Friction of Viscosity-Modified Oils in a Journal Bearing Machine

“…In the current work, a subset of these oils is studied, as listed in Table 1. The original oil codes used in [1,2] have been retained. Oils #1, #4, #5, #7, and #10 are simple solutions of different VMs in the same 6 cSt KinVis Group II base oil.…”

“…It was found that for all but one of the VMs, the viscosity of a given blend at all shear rates and temperatures could be described by a single equation based on time-temperature superposition. The resulting shear thinning equations were then used to explore the impact of the VMs on plain bearing friction using an isothermal, 3D hydrodynamic lubrication model [2]. This showed that VMs can reduce friction in two ways, by increasing viscosity index, leading to a low viscosity at low temperatures, and via shear thinning which reduces effective viscosity in high sliding speed, high shear rate lubricated contact conditions [2].…”