Viscosity meaning in engineering1/16/2024 However, there is no way that the kinematic viscosity of two different fluids can be the same due to the differences in their density. The dynamic viscosity of two different fluids can be the same. The kinematic viscosity gives the inherent resistance of the fluid to flow when there is no external force applied to it except gravity. When the resistance to flow is measured under the weight of gravity, then the viscosity is called kinematic viscosity. ![]() The dynamic viscosity measures the resistance offered by the fluid to the flow when an external force is applied. The dynamic viscosity divided by the density of the fluid gives kinematic viscosity. The absolute or dynamic viscosity is the ratio of shear stress to shear strain. In this article, we will compare the dynamic vs. kinematic viscosity, it is important to note that no two fluids can have the same kinematic viscosity, but they can have the same dynamic viscosity. ![]() The viscosity of fluids can be expressed in terms of either dynamic viscosity or kinematic viscosity. Fluids differ in the internal resistance that is exerted, causing differences in viscosity as well. In fluids, the resistance to flow or to deform is known as viscosity. In fluids, the resistance to flow or to deform is known as viscosity Whenever a fluid is not subjected to physical forces other than gravitational force, kinematic viscosity is the right measurement. When external forces are applied to a fluid, dynamic viscosity measurements are the best choice. Lower viscosity motor oils also result in better fuel economy.No two fluids can have the same kinematic viscosity, but they can have the same dynamic viscosity. These lower viscosity motor oils are needed to move through the thin engine pathways to protect and clean metal surfaces. This has led over time to motor oil viscosity decreasing – currently 5W-30 and 5W-20 are the most popular grade and 0W-20 is the fastest growing grade. But as engine technology has advanced over the years, the size of engines and pathways in engine that oil flows through have gotten smaller and thinner. In the past, it was common for vehicles to require higher viscosity oils such as 20W-50’s, 10W-40’s and 10W-30’s. The 5W-20 will have a lower viscosity and provide less resistance than the 10W-30 at both start-up and normal engine operating temperatures. ![]() If we compare a 5W-30 and a 10W-30, they will behave very similarly as the engine heats up, but at start-up, the 5W-30 will provide less resistance and start easier than the 10W-30. Less resistance results in improved fuel economy but the oil is not as thick and forms a smaller layer of protection between metal surfaces. But as the engine heats up, the 5W-20 will move with less resistance than an 5W-30. Again, the lower the number, the lower the viscosity of the oil and the faster the oil will move around the engine.Īs an example, let’s compare 5W-20, 5W-30 and 10W-30 motor oils. A 5W-20 and 5W-30 will have very similar if not equal viscosity at lower start-up temperatures. In the 5W-30 example, the 30 defines the viscosity of the oil at normal engine temperatures. The high temperature viscosity is the number after the dash and is related to the viscosity of the oil as it is moving around your engine after the car has warmed up and is at normal engine temperature. The lower the number, the faster the oil will flow at vehicle start up. For example, if the oil is a 5W-30, the 5W part describes the viscosity of the oil at low temperatures. That value has the letter “W” after the number and has a dash after the W. The low temperature viscosity of the oil is a measurement that simulates starting a car on a cold winter day. But as motor oil technology advanced, additives such as viscosity index improvers allowed for the use of the same grade of oil year-round. Years ago, most vehicles used one viscosity grade oil in the summer and a different viscosity grade oil in the winter. The measurement of viscosity at high and low temperatures are properties of multi-grade oils. ![]() Motor oil viscosity grades are based on a scale developed by the API (American Petroleum Institute) lubricant organization. The values are defined in a specification known as API 1509 and are based on the resistance the oil gives to flowing at two different temperatures – cold and high temperature. Viscosity is the resistance to flow of a fluid.
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