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We can measure dynamic viscosity in millipascals-second (mPa⋅s) or with a fancier equivalent called the "centipoise." On the other hand, we can express kinematic viscosity in square millimeters per second (mm 2/s), which also has an equivalent unit called "centistokes." For the simplicity of this text, we will only be using milliPascals-second and square millimeters per second for dynamic viscosity and kinematic viscosity, respectively. On the other hand, the kinematic viscosity tells about the speed the fluid reaches when a particular force is applied to the fluid. When choosing between the two viscosities, it is worth noting that dynamic viscosity tells us about the force required to move the fluid at a certain speed. The larger the force or stress needed to move the plate, the more viscous the fluid is. Dynamic viscosity, or the absolute viscosity of water, or any fluid, is proportional to the tangential shear stress per unit area needed to move one plate at a constant speed over another plate at a maintained fluid thickness between these two plates, like in a Couette flow, as shown below: When dealing with viscosities, when we mention "viscosity," we actually mean dynamic viscosity. In this article, we'll focus more on the viscosity of liquids, specifically on the kinematic viscosity and the dynamic viscosity of water. This weakening results in liquid molecules to move more freely and, therefore, with a lower viscosity. In liquids, when molecules start to move faster, their attraction from each other weakens. As a result, in gases, molecules experience more friction against each other, making them flow slower and become viscous. However, when we apply heat or additional thermal energy to our fluids, their molecules start moving faster.
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The attraction between the molecules of a viscous fluid is much higher than that of a less viscous fluid. We can also express viscosity as the internal friction of a fluid in motion. Maple syrup, a very viscous fluid, would pour slower than when you pour milk on your cereal as milk's viscosity is much lower. Imagine dripping maple syrup on your waffles for your breakfast. The higher the viscosity of a fluid (liquid or gas), the slower it traverses across a surface. The vital roles of the additive package, performance polymer and base oil mean they must work in harmony with each other, while maintaining durability and delivering improved fuel efficiency.įor more information on HTHS engine lubricants, contact your Lubrizol representative.Viscosity is the measure of a fluid's resistance to flow. Lower HTHS viscosity engine lubricants are complex and require formulation changes over higher HTHS viscosity engine lubricants. Lower HTHS viscosity is being pursued by an increasing number of original equipment manufacturers (OEM) to move significantly below this long-established 3.5 cP HTHS limit in conjunction with modern engine hardware designs. Typically, heavy-duty diesel engine lubricants have had a minimum HTHS viscosity of 3.5 cP. HTHS viscosity is measured in cond (mPa.s) but is also commonly referred to in centipoise (cP). The lower the measured number, the lower the HTHS viscosity of the oil and the higher the anticipated fuel efficiency benefits. HTHS viscosity measures the temporary viscosity loss under these conditions: It is designed to ensure engine lubricants maintain sufficient film strength to prevent excessive wear under engine operating conditions, while also allowing engine lubricants to have low enough viscosity to deliver fuel efficiency and emissions benefits. HTHS dynamic viscosity is the current industry standard that best predicts fluid behavior in HTHS environments. The focus on HTHS viscosity is gaining momentum as it is regarded as the parameter of the viscosity classification that more accurately mimics the engine lubricants behavior in increasingly severe operating conditions. Today, HTHS viscosity-or specifically “low” HTHS viscosity-is a phrase fast becoming associated with increased fuel efficiency.
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With such changes in engine hardware design, engine lubricants are increasingly having to perform in high temperature high shear (HTHS) environments within the engine-notably in the camshaft, bearings as well as the piston rings and cylinder liners. The drive to reduce emissions and increase fuel efficiency has led to significant changes in heavy-duty engine hardware design, including downsizing, down speeding and stop/start technology to name a few.