Machinery Lubrication magazine published by Noria Corporation
Issue link: http://www.e-digitaleditions.com/i/601893
12 November - December 2015 | www.machinerylubrication.com in dynamometer tests or field experience, and an understanding of the relationship between the engine's needs and the oil's phys- ical and/or chemical properties. Engine Oil Properties To ser ve the engine, oil must possess certain physical and chemical properties. During the oil's ser vice, the engine gener- ates a number of operating stresses that adversely affect the long-term ability of the oil to function at a consistently high level. Ser vice conditions may also var y widely depending on the environment and the way the vehicle is used. Consequently, choosing an engine oil to meet particular ser vice needs and conditions requires knowledge of several important oil proper- ties, including viscosity. Viscosity Viscosity may be defined as a fluid's resistance to flow. Because a fluid's molecules are somewhat attracted to one another, energy is required to pull them apart and create flow. In general, larger molecules have more attraction between them and a higher viscosity. The energy required to overcome this molecule-to- molecule attraction and produce fluid flow can be considered a form of friction. Therefore, viscosity can be defined as a form of molecular friction. Of all the engine oil's physical and chemical qualities, its viscosity and viscometric behavior during use are often considered the most important. Viscosity and Wear Prevention This same molecular friction prevents the oil from escaping too quickly when two engine surfaces in relative motion are brought closely together under pressure. This inability of the inter vening oil to escape quickly and its level of incompressibility hold the two surfaces apart and prevent wear, a process that is termed hydrodynamic lubrication. The higher the viscosity, the greater the attraction of the oil molecules and the greater the wear protection. Viscosity Classification A lubricant's viscosity has always been associated with wear protection. Early in its history, SAE recognized viscosity as important to engine function and instituted the J300 classifica- tion system, which establishes viscosity levels for engines by a series of grades. These grades are defined by viscosity levels in one or two temperature zones. Today, the grades are set for engine operating temperatures and for winter temperatures at which the oil affects starting and pumping. Viscosity at Operating Conditions In the early years of automotive engines, oils were simply formulated and obeyed Newton's equation for viscosity — the more force used to make the fluid flow (shear stress), the faster it would flow (shear rate). Essentially, the ratio of shear stress to shear rate — the viscosity — remained constant at all shear rates. The engine oils of that time were all essentially single grade and carried no SAE "W" classification. This viscometric relationship changed in the 1940s when it was discovered that adding small amounts of high-molecu- lar-weight polymers appeared to give the oil the desired flow characteristics for both low-temperature starting and high-tem- perature engine operation. Accordingly, these polymer-containing oils were listed by the SAE viscosity classification system as multi- grade engine oils, as they met the requirements of both viscosity temperature zones. Since that time, multigrade oils (e.g., SAE 10W-40, 5W-30, 0W-20, etc.) have become very popular. However, they were no longer It is essentially impossible to establish the quality of an engine oil by appearance. This determination can only be made by using the oil or pre-testing it.