The viscosity index describes the change in viscosity with temperature and is calculated from the kinematic viscosity at 40☌ and 100☌. Engine manufacturers specify oils that behave consistently to ensure consistent lubrication performance, permitting only minimal wear under normal operating conditions. Quality oils that have a high viscosity index (VI) remain stable with minimal variation in viscosity over the entire temperature range. Automotive lubricants are required to work at cold ambient temperatures i.e., down to -40☌ and up to 200☌. Many lubricant applications, for example engine oil, require the lubricant to perform near uniformly across a wide range of operating conditions. The viscosity index is a crucial metric, which can be used to assess the viscosity stability of an oil. Modern automotive engine oils are almost exclusively multigrade oils. The 5W part refers to the low-temperature viscosity or how the oil behaves in winter, 30 refers to the high-temperature viscosity (summer). Multigrade oils must fulfil two viscosity specifications, their viscosity grade consists of two numbers such as 5W-30, for example.
#KINEMATIC VISCOSITY OF ENGINE OIL ISO#
Under the ISO VG system, oils are generally separated into two main groups, multigrade oils and monograde oils. The resulting system is known as the International Standards Organization Viscosity Grade, ISO VG. A group of renowned standardisation institutes consisting of the International Standards Organization (ISO), American Society for Testing and Materials (ASTM), Society for Tribologists and Lubrication Engineers (STLE), British Standards Institute (BSI) and Deutsches Institute for Normung (DIN) created a common viscosity classification. In 1975 a common system of grading engine oils according to their viscosity was agreed. Low viscosity = fluid = fast flowing = less stable film High viscosity = viscous = slow flowing = more stable film In order to achieve a good flow of oil around the engine and good flow properties at low temperatures, the oil must be low-viscosity. The faster the oil flows around the engine, the better, i.e., the faster it can provide protection and prevent wear. The time it takes for an oil to travel around an engine, especially at low temperatures, depends on the oil’s viscosity. How does viscosity affect engine oil selection? The technical definition of viscosity is the measure of a fluid’s internal flow resistance. Fuel consumption should be lower as a result of the reduced HTHS index.Put simply, viscosity indicates how thick a fluid is and how easily it flows. Engine oil qualities in category ACEA A1/B1 have a reduced HTHS of up to 2.9 mPas. The limit value for engine oils with specification ACEA A2/A3 and ACEA B2/B3 is to be found at an HTHS of 3.5 mPas. HTHS limit values are defined to ensure that even in bearings (where both shear forces and oil temperatures are high), engine oils can be relied upon to provide the necessary lubrication. It is expressed in millipascal seconds (mPas). It describes dynamic viscosity measured at 150☌ and under higher shear forces. HTHS stands for High Temperature High Shear. The viscosity classes referred to above (winter and summer) are supplemented by what is known as HTHS viscosity. The larger the number in front of the W, the higher the viscosity of the oil at 100☌. Typical viscosity classes are 20, 30, 40, 50 and 60. SAE summer viscosity classes are classified at a test temperature of 100☌. Kinematic viscosity describes the ratio between dynamic viscosity and the thickness of the engine oil at a certain temperature. The lower the cold viscosity index, the easier the cold engine will turn over on starting. Dynamic viscosity affects the starter speed when the engine is cold, for example. The smaller the number in front of the W, the lower the viscosity of the oil at cold temperatures. Oils are divided into the winter viscosity classes 0W, 5W, 10W, 15W, 20W, 25W. This describes the engine oil's resistance to flow at low temperatures. Measures of viscosity are based on two variables: SAE 5W30 is an example of a viscosity designation of a multi-grade oil. Most of the oils used today are multi-grade oils. As long ago as 1911, viscosity provided the basis for the first engine oil classification system and was defined in the Society of Automotive Engineers (SAE) classification system.
It is determined by the internal friction which resists the flow of adjacent particles in the fluid. Viscosity is measure of a fluid's resistance to flow. The viscosity of an oil is always marked on its barrel. Viscosity is one of the most important properties of engine oil.
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