LO 10-2 Describe the purpose, properties, function, composition, types, and applications of diesel engine lubricating oils

1.14. Viscosity Index and Shear

Viscosity Index and Shear
Moving oil through pumps and between rotating bearings and shafts at high pressure and scraping oil from cylinder walls literally chops or shears molecules of the oil and its VI improver. When sheared, the smaller oil and VI improver molecules allow a faster temperature-related change in viscosity and loss in viscosity as the oil warms up. The problem with oil when it becomes too thin is that it cannot properly form a hydrodynamic wedge in the oil clearance, which means it cannot properly separate parts or support a load. Higher viscosity enables the oil to support heavy loads without damage. Again, thinner, lower- viscosity oil can leak past piston rings and valve guides and burn, leading to increased oil consumption. It may make sense to use thicker, more viscous oils, but higher-viscosity oils operating with larger oil clearances increase the energy required from the engine to pump oil. To reduce fuel consumption, it is no longer acceptable to use thicker-viscosity oils pushed through large bearing clearances. Newer engine oils with lower viscosity contain additives that prevent metal-to-metal contact, which in turn permit the use of energy-saving, lower-viscosity oil.