![]() ![]() Different sized capillaries are available to support fluids of varying viscosity. The orifice of the u-tube viscometer produces a fixed resistance to flow. Kinematic viscosity is measured by timing how long it takes oil to pass through the orifice of a capillary when only subjected to the force of gravity see Figure 2. Kinematic Viscosity describes the resistance of a liquid to flow under the influence of gravity. HTHS viscosity is measured at 150☌, to simulate realistic oil shearing conditions in an engine using a tapered bearing simulator viscometer. HTHS dynamic viscosity is designed to ensure engine lubricants maintain sufficient film strength to prevent excessive wear under engine operating conditions, whilst maintaining a viscosity low enough to improve fuel efficiency and emissions. HTHS dynamic viscosity is the industry standard that best replicates engine oil behaviour. HTHS viscosity is a measure of lubricant flow resistance under dynamic conditions. CCS viscosity is one of the factors used to define the “ W” or “winter” part of a multigrade oil. The cold-cranking simulator is a requirement which is used to define the viscosity grade of the oil. The procedure simulates the viscosity of oil in crankshaft bearings when starting the engine at low-temperatures. Figure 1 – Viscosity index comparisonĬold Cranking (viscosity) Simulator (CCS)Ī cold cranking (viscosity) simulator or CCS, is a device used to determine the low temperature performance of lubricants, when starting a cold engine i.e., during the cold-cranking phase. The aim is to keep the viscosity as stable as possible and keep the line (see fig.1), as flat as possible. Most modern hydrocraked oils have a very high viscosity index, meaning that a fully-synthetic oil no longer has this advantage over HC oils. Traditionally, synthetic oils were out on their own in terms of viscosity stability, but nowadays a high-quality hydrocracked oil can easily compete with synthetics. As a rule, the higher the viscosity index, the lower the change in viscosity with increasing temperature i.e., the oil viscosity behaves in a more stable manner for longer. 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. 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. Put simply, viscosity indicates how thick a fluid is and how easily it flows. ![]()
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