![]() Essentially, the ratio of shear stress to shear rate - the viscosity - remained constant at all shear rates. 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). Today, the grades are set for engine operating temperatures and for winter temperatures at which the oil affects starting and pumping. These grades are defined by viscosity levels in one or two temperature zones. #Fluid extractor for car ennigh oil seriesEarly in its history, SAE recognized viscosity as important to engine function and instituted the J300 classification system, which establishes viscosity levels for engines by a series of grades. The higher the viscosity, the greater the attraction of the oil molecules and the greater the wear protection.Ī lubricant’s viscosity has always been associated with wear protection. This inability of the intervening oil to escape quickly and its level of incompressibility hold the two surfaces apart and prevent wear, a process that is termed hydrodynamic lubrication. This same molecular friction prevents the oil from escaping too quickly when two engine surfaces in relative motion are brought closely together under pressure. Of all the engine oil’s physical and chemical qualities, its viscosity and viscometric behavior during use are often considered the most important. Therefore, viscosity can be defined as a form of molecular friction. The energy required to overcome this molecule-to-molecule attraction and produce fluid flow can be considered a form of friction. In general, larger molecules have more attraction between them and a higher viscosity. Because a fluid’s molecules are somewhat attracted to one another, energy is required to pull them apart and create flow. Viscosity may be defined as a fluid’s resistance to flow. Consequently, choosing an engine oil to meet particular service needs and conditions requires knowledge of several important oil properties, including viscosity. Service conditions may also vary widely depending on the environment and the way the vehicle is used. During the oil’s service, the engine generates a number of operating stresses that adversely affect the long-term ability of the oil to function at a consistently high level. To serve the engine, oil must possess certain physical and chemical properties. However, the value and significance of this type of test is dependent on a number of factors, including identification of the engine’s specific needs, clear and consistent information from the engine either in dynamometer tests or field experience, and an understanding of the relationship between the engine’s needs and the oil’s physical and/or chemical properties. ![]() These bench tests have the capability of providing a relatively inexpensive measure of oil quality. ![]() This involves using laboratory bench tests designed to correlate closely with engine dynamometer tests or field experience. Fortunately, once the quality level has been determined on the dynamometer or in the field, there is a much less expensive approach that can be applied to more precisely appraise the oil quality. As engine design has progressively increased power from smaller engines, the difficulty of establishing repeatable dynamometer tests has grown even more rapidly. ![]() Even so, to help control warranty costs, the development and use of engine tests is unavoidable for engine manufacturers when determining the oil quality needed for a particular design or component.Īlthough necessary, generating repeatable dynamometer tests for an engine can be challenging. However, using the engine to measure oil quality in dynamometer tests can be an expensive proposition. Even as engine design has changed to meet performance, fuel efficiency and environmental standards, the engine continues to be the ultimate arbiter of oil quality. The engine has always been the ultimate platform for identifying the required quality of its oil. Unfortunately, for the uninformed automobile owner, a high-quality engine oil and one of poor quality will look and feel the same. Poor-quality engine oils are often put on the market due to ignorance or greed. While most engine oils are made to acceptable standards, their general and specific qualities can vary widely. ![]()
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