Shear stability of engine oils
The change of oil viscosity as a function of shear has been a key discussion topic for researchers across the globe. It has attracted a great deal of interest recently with the introduction of the 0W lower viscosity grades. Oil-soluble polymers known as viscosity modifiers (VMs) have been used in the SAE multi-grade automotive lubricants to minimize the oil’s adverse dependence on temperature and enable development of oils that meet both the high- and low-temperature performance requirements.
Regarding the change in viscous effects the polymer may experience during operation, it has been shown to be dependent on polymer chemistry, molecular geometry and molecular weight. The balance between viscosity improvement and shear stability is critical in achieving optimum performance from an engine oil. Consequently, various types of VMs with different response levels to shear stability have been developed and utilized over the years. Recent efforts are underway to develop relevant shear stability specifications across the globe, with several OEMs leading the initiative. Using the data from the Institute of Materials Database, examples will be provided on engine oils available in the market that demonstrated an increase in viscosity at operating temperatures and flow conditions.
The performance of the oils (using HTLS, HTHS, and kinematic viscosity measurements) will be compared with samples collected over several years, as well as between the various geographies.