Abstract
The friction properties of a range of viscosity modifier-containing oils in an engine bearing have been studied in the hydrodynamic regime using a combined experimental and modelling approach. The viscometric properties of these oils were previously measured and single equations derived to describe how their viscosities vary with temperature and shear rate (Marx et al. Tribol Lett 66:92, 2018). A journal bearing machine has been used to measure the friction properties of the test oils at various oil supply temperatures, while simultaneously measuring bearing temperature using an embedded thermocouple. This shows the importance of taking account of thermal response in journal bearings since the operating oil film temperature is often considerably higher than the oil supply temperature. For Newtonian oils, friction coefficient measurements made over a wide range of speeds, loads and oil supply temperatures collapse onto a single Stribeck curve when the viscosity used in determining the Stribeck number is based on an effective oil film temperature. Journal bearing machine measurements on VM-containing oils show that these give lower friction than a Newtonian reference oil. A thermo-hydrodynamic model incorporating shear thinning has been used to explore further the frictional properties of the VM-containing oils. These confirm the findings of the journal bearing experiments and show that two key factors determine the friction of the engine bearing; (i) the low shear rate viscosity of the oil at the effective bearing temperature and (ii) the extent to which the blend shear thins at the high shear rate present in the bearing.
Highlights
In previous work, the temporary shear thinning behaviour of lubricant blends containing various commercial viscosity modifier additives (VMs) was measured over a wide shear rate range at several temperatures [1]
The ability of VMs to confer temporary shear thinning response and reduce hydrodynamic friction has become an important tool in the formulation of fuel-efficient engine oils [5, 6]
Oils #1 and #4 have relatively high low shear rate viscosities and these negate to some extent the contribution of shear thinning to their friction reduction
Summary
The temporary shear thinning behaviour of lubricant blends containing various commercial viscosity modifier additives (VMs) was measured over a wide shear rate range at several temperatures [1]. The resulting shear thinning equations were 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, The current paper describes a combined experimental and modelling study of the impact of temporary shear thinning of VM blends on the friction of a plain journal bearing. The ability of VMs to confer temporary shear thinning response and reduce hydrodynamic friction has become an important tool in the formulation of fuel-efficient engine oils [5, 6]. The presence of shear thinning led to a reduction in friction compared to a base oil of comparable viscosity
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