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/s11249-018-1039-5]. 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 viscosity–shear 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.
Highlights
Viscosity modifier additives (VMs) are used to increase the viscosity index of lubricants and are key components of most crankcase engine oils
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
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
Summary
Viscosity modifier additives (VMs) are used to increase the viscosity index of lubricants and are key components of most crankcase engine oils. 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 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. Engine oil VMs can exhibit temporary shear thinning at the high shear rates present in engine components, including the ring pack and the journal bearings, and for many years this was considered undesirable since it leads to a reduction in hydrodynamic film thickness [6]. Further details of all of these oils are provided in [1]
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