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

Abstract

Viscosity versus shear rate curves have been measured up to 10 7 s −1 for a range of VM solutions and fully formulated oils of known composition at several temperatures. This shows large differences in the shear thinning tendencies of different engine oil VMs. It has been found that viscosity versus shear rate data at different temperatures can be collapsed onto a single master curve using time-temperature superposition based on a shear rate shift factor. This enables shear thinning equations to be derived that are able to predict the viscosity of a given oil at any shear rate and temperature within the range originally tested. One of the tested lubricants does not show this time temperature superposition collapse. This fluid also exhibits extremely high viscosity index and shear thins more easily at high than at low temperature, unlike all the other solutions tested. This unusual response may originate from the presence on the VM molecules of two structurally and chemically different components. 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].

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

“…In a companion paper, the temporary shear thinning behaviour of a series of VM-containing blends was investigated, including both simple solutions in base oil and full engine oil formulations [1]. It was found that, with one exception, time-temperature superposition could be used to derive a single equation to describe the viscosity of the VM blend at any shear rate and temperature for a given blend.…”

“…This enables the influence of the VM additive on both hydrodynamic film thickness and hydrodynamic friction to be predicted for the oils studied in [1], and the ways that VMs contribute to friction reduction to be determined.…”

“…In a companion paper, Part I [1], the temporary shear thinning properties of a series of 17 VM blends were studied, all having the same HTHS viscosity of 3.7 cP (at 10 6 s −1…”

“…Also studied was an additive-free Group IV base oil, #18, having the same HTHS as that of the VM blends. Further details of all of these oils are provided in [1].…”

“…, as described in [1], and it was found that their viscosity versus shear rate behaviour at a given temperature could be described by the Carreau-Yasuda equation [10]:…”