“It’s got three parameters, so it’s a better fit, but it requires more data. The Vogel equation 2 is used in numerical solutions of lubricated flow, Martini says. For example, the Roelands equation 1 is designed so that all of the normal paraffins have a viscosity versus temperature slope of one. Today’s low-viscosity oils are off the VI scale at higher temperatures, Martini says. However, with newer lubricant formulations and hotter-running engines (e.g., for electric vehicles), the limitations of VI calculations are beginning to become more apparent. Viscosity index (VI) is a common method for comparing the temperature-related viscosity changes for various oils, and it requires just a few data points. This is becoming more of a problem as the demand for low-viscosity lubricants, designed for high-speed engines, increases. Calculations that use only a few data points are convenient, but straying too far from these data points for either extrapolation or interpolation can give inaccurate results, she says. “One reason there are so many approaches is that none of them are quite right,” says STLE member Ashlie Martini, professor of mechanical engineering at the University of California, Merced, and editor of STLE-affiliated journal “Tribology Letters.” All of them have their limitations, she says, noting that history and familiarity play a part in which calculations are used most frequently. Thus, Zakarian says, researchers have come up with “hundreds of different equations” that allow users to measure a few data points and extrapolate an oil’s performance under a specific set of conditions. This works well as long as an application falls within the parameters under which the profile was created, but it offers limited knowledge of how the oil will perform under different conditions. The next best solution after “test everything” is to include a viscosity versus temperature profile in an oil’s specification. Some mechanical parts are very large or otherwise unwieldy, such that conducting standard rig tests is impractical. However, that approach requires a significant investment of time and money. Testing specific oils with specific engine components would be the most direct way to find this out. Specifically, it is important to know how a specific lubricant formulation will behave over the entire range of temperatures it might encounter during operation. “Pretty much anybody who works in the oil industry at some point or other has probably had to do the calculations,” he adds-parts designers, OEMs, lubricant formulators and even end-users.ĭespite the difficulties in untangling these interactions, trends toward faster, hotter engines and tighter tolerances require a more sophisticated understanding. “Most people end up having to know something about viscosity,” says lubricant formulation specialist and STLE member Jack Zakarian, principal at JAZTech Consulting. For example, thicker lubricant films keep contacting parts well separated, but they also can increase friction and drag, which increases temperature in the contact area, which thins the lubricant film. Lubricant viscosity, temperature and operating conditions all interact in complex ways, some of which are poorly understood. Though it is widely known that oil-based lubricants are less viscous at higher temperatures and more viscous at lower temperatures, calculating a precise viscosity presents a formidable challenge.
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