PSAA Curriculum Unit Physical Science Systems Problem Area Energy and Power Systems Lubricating Oils: Viscosity and Temperature Lesson Oil Comparison What is the difference in the pour rate? Why does a difference occur? What implications are there for the selection of the proper viscosity rated oil? Learning Objectives Define friction and explain the types of friction that affect internal combustion engines. Define viscosity as it relates to single and multiviscosity lubricating oils. Explain the effects of temperature on the viscosity of single and multiviscosity oils. Define synthetic oil and explain its advantages and disadvantages compared to petroleum-based oils. Terms Adhesion Cohesion Flow rate Friction Lubricant S.A.E. Sliding friction Starting friction Synthetic oil Temperature Viscosity Viscosity index What is friction and how does it affect moving objects such as engines? The force that acts to resist the rotation of objects that are in contact is friction. Friction can be desirable and even necessary in some situations and clearly undesirable in others. What is friction and how does it affect moving objects such as engines? In an internal combustion engine friction leads to reduced power, efficiency and wear on the engine parts. Force of friction between two objects depends upon the surface finish, materials used and the pressure forcing them together. What is friction and how does it affect moving objects such as engines? The material used affect the cohesion (attraction of like substances) and adhesion (attraction of unlike substances) between the surfaces. Starting friction is the resistance to movement between two solids. Sliding friction is the friction between two moving objects. What is viscosity and how does viscosity relate to single and multiviscosity lubricating oils? In an engine, moving parts are lubricated to reduce wear, friction and heat. The role of a lubricant is to provide a thin film of oil between moving parts that reduces the cohesive forces between the two metallic objects. Viscosity Flow rate is an excellent test of viscosity under different temperatures. Flow rate measures the amount of time a given amount of oil takes to pass a certain point at a given temperature. Viscosity (cont.) Viscosity refers to the internal friction of a fluid. This friction is created by molecular attraction or cohesion, which makes the fluid resistant to flow. A viscous fluid has a cohesive and sticky fluid consistency. Engine oils have to be viscous to be able to adhere better to moving engine parts. Viscosity (cont.) Multi viscosity oils contain polymers added to a light base (5W, 10W, 20W) which prevent the oil from thinning as much as increases in temperature. A 20W50 oil is a 20 weight oil that will not thin more than a 50 weight at high temperatures. Viscosity (cont.) Higher viscosity numbers usually mean a thicker oil. Lower viscosity means thinner oil. The weight given to oils are numbers assigned the S.A.E. (Society of Automotive Engineers). Viscosity (cont.) These numbers correspond to “real” viscosity measured by several techniques. Oils that fall into a certain range are designated 5, 10, 20, 30, 40, 50 by the S.A.E. The “w” means the oil meets specifications for viscosity at 0 degrees Fahrenheit and is suitable for winter use. How do various temperatures affect the viscosity of single viscosity and multiviscosity lubricating oils? Viscosity index is an empirical number indicating the rate of change in viscosity of an oil within a given temperature range. Higher numbers indicate a low change, lower numbers indicate a relatively large change. The higher the number, the better the oil performs within a viscosity range. Temperature affects… At cold temperatures the polymers in multi viscosity oils coil up and allow the oil to flow as the low numbers indicate. As the oil warms up the polymers begin to unwind into long chains that prevent the oil from thinning as much as it normally would. What advantages and disadvantages do synthetic oils have compared to naturally occurring petroleum-based oils? Synthetic oil has been defined as any lubricant that does not have a naturally occurring mineral oil base-stock. True synthetic lubricants were defined as those which do not rely on petro-chemicals to be produced. Synthetic Oils General characteristics for synthetic oils are that far greater loads and temperatures can be withstood than the equivalent mineral or vegetable based lubricant. Synthetics have been developed as lubricating oils for extreme conditions such as truck engines and extra pressure gear trains. Synthetic Oils Synthetics offer superior high temperature oxidation resistance, high film strength, very low tendency to form deposits, stable viscosity base and low temperature flow characteristics. Synthetics are superior lubricants compared to traditional petroleum oils. The consumer must decide if the high cost is justified in the application. Review/Summary What is friction and how does it affect moving objects such as engines? What is viscosity and how does viscosity relate to single and multiviscosity lubricating oils? Review/Summary How do various temperatures affect the viscosity of single viscosity and multiviscosity lubricating oils? What advantages and disadvantages do synthetic oils have compared to naturally occurring petroleum-based oils?