Their primary role is to enhance the lubricant viscosity index of their blends. Viscosity index improver polymers or viscosity modifiers (VMs) have been used as engine oil additives since the mid-1930s and in recent years their application has extended to gear oils, automatic transmission fluids, greases and some hydraulic fluids. This enables the influence of the VM additive on both hydrodynamic film thickness and hydrodynamic friction to be predicted for the oils studied in, and the ways that VMs contribute to friction reduction to be determined. This paper described the application of a generalised Reynolds equation to model hydrodynamic lubrication behaviour of a steadily loaded, isothermal engine journal bearing incorporating temporary shear thinning of the lubricant. Such an equation in conjunction with a hydrodynamic lubrication model makes it possible to explore the impact of VM additives on film thickness and friction in lubricated machine components. 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. 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. It is well known that their blends exhibit shear thinning at the high shear rates present in lubricated contacts and that the resulting reduction in viscosity leads to thinner lubricant films and lower hydrodynamic friction than predicted in the absence of shear thinning. Viscosity modifier additives (VMs) are used to increase the viscosity index of lubricants and are key components of most crankcase engine oils. The identification and quantification of these two alternative ways to reduce friction should assist in the design of new, fuel-efficient VMs. 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. ![]() 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 model indicates that VMs can contribute to reducing friction in two separate ways. ![]() 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. 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. This made it possible to derive a single equation to describe the viscosity–shear rate behaviour for each VM blend. 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. 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.
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