Viscosity is probably the single most important characteristic of oil that affects the engine. By definition, viscosity is a measure of oil’s resistance to flow, and it is measured at one or more standardized temperatures so that we can define viscosity grades of engine oils. It’s important to understand that the viscosity of engine oil changes continuously as the temperature of the oil changes. A properly formulated engine oil of the proper viscosity grade will provide a lubricant film in-between moving parts in the engine and protect them from wear. Viscosity characteristics of the oil will also affect things like oil consumption rates, low-temperature oil flow to the engine, and the speed at which the engine will crank — especially if the ambient temperatures are cold.
Factors such as application speed, load, and operating temperature are all important factors influencing the choice of the correct oil to apply in any application. Generally, low viscosity oils are preferred for applications where either high speeds or low temperatures and pressures are present. When application speeds are reduced or operating temperatures are increased, the viscosity of the oil required to provide lubrication also increases.
Choosing an oil with the correct viscosity for any given application requires consideration of all the operating and environmental factors that the lubricated surfaces will be subject to in use. Basically, the oil must be thick enough to provide an adequate separation of the lubricated surfaces. That is heavily influenced by the speed, load, and surface temperatures that the surfaces will be exposed to in operation. The ideal oil for a given application will be viscous enough to ensure a proper fluid film under all operating conditions, yet fluid enough to avoid power losses resulting from excessive fluid friction.
Generally, we use the lowest viscosity oil in an application that will support the required loads. Sometimes, all of these criteria can result in a scenario where almost any oil will do, although it may not be optimal. Other times, it can be difficult to identify a single oil that will function adequately in the entire range of operating or environmental conditions that an engine may be subject to. For instance, an aircraft piston engine generally requires a fairly heavy oil to provide good lubrication due to design, cooling, and normal engine operating parameters. But high viscosity oils are usually limited in their ability to provide adequate flow characteristics at very low winter-time ambient temperatures. Therefore, aircraft piston engine designers must resort to the use of supplemental crankcase heaters for aircraft that must start under those cold conditions because using an oil with low enough flow characteristics at those low temperatures that would allow an engine to start when it is cold would not provide adequate protection when the engine gets to normal operating temperatures.
Viscosity measurement systems
Two common viscosity measurement systems are the Saybolt and Kinematic systems. These systems differ in the design used to make the measurement and the way it is calibrated, but the principle is the same. Oil to be measured is contained in a vessel which is immersed in a bath at a constant temperature. Remember viscosity of oil changes as temperature changes. So, if we are going to understand the viscosity of an oil, we need to understand the temperature at which the measurement was taken.
Once the temperature of the sample is allowed to stabilize, the sample is allowed to flow through a calibrated restriction (basically this is a fancy funnel). The time for a measured volume to pass through the restriction is measured. The higher the viscosity of an oil, the longer it will take to flow through the funnel.
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