MACHINERY LUBRICATION- INDIA SEPTEMBER-OCTOBER 2019 | Page 10

AS I SEE IT equal from the standpoint of volatility (risk of evaporative loss). Some lubricants may exhibit as much as a 50-percent greater loss from volatility than others. This is influenced by the base oil’s molecular weight distribution. Of course, temperature plays a key role. A low liner temperature translates to a low evaporation rate. Liner temperature is influenced by load, combustion efficiency and cooling. Approximately 74 percent of vaporization occurs during intake and compression strokes (no speed effects have been found). High Ring Float Conditions Researchers have found that lower oil viscosity can reduce the oil control ring’s “float” conditions. “Float” basically means there is too much film thickness between the oil control ring and the cylinder wall. Consequently, this excessive viscosity fights the ring’s ability to squeegee (downscrape) the oil sufficiently from the cylinder wall and return it to the sump. As a result, too much oil is left on the cylinder wall that then can move toward the compression rings or remain adherent to the liner, increasing oil loss through misting and evaporation. Blow-by from Ovaloid Cylinder Bores Ovaloid cylinder bores are usually caused by machining issues as well as thermal and pressure distortions. Piston rings can conform to out-of- roundness cylinders to a certain extent. Still, reverse blow-by gases and oil mist can follow the pathway across these cylinder bore distortions by moving more easily against the ring’s running face. Oil mist is carried with reverse blow-by gases into the combustion chamber and outward with the exhaust. 8 | September - October 2019 | It is worth noting that too little viscosity induces a plethora of dangers as well. The optimum reference viscosity (not too low or high) is always desired. This “optimum” is pushed and pulled by numerous engine design and operation factors, including the desire to mitigate oil consumption. Oil Change Interval Effect Extended oil drains are an ever-growing trend. While there are clear advantages (lower oil change costs, higher productivity, environmental benefits, etc.), there are also engine life risks, fuel economy risks and oil economy penalties. A recent study on the effects of the oil change interval on miles per quart of oil is shown in Figure 3. Three different engines (Class 8, long-haul service) at different oil change intervals show a clear relationship between oil health and oil consumption. One can conclude that as oil ages, the effects of aging (high soot, loss of dispersancy, additive depletion, insolubles, viscosity- index shear, dirt load, etc.) impair the ability of the engine to retain the oil during service. www.machinerylubricationindia.com Oil Consumption Issues Revealed by Oil Analysis Monitoring oil levels and makeup rates offers a reliable indication of oil consumption and relative oil economy. If oil consumption is low, it can be assumed that while many things could be going wrong, they are not going wrong simply because engine oil consumption is within a normal and safe range. Therefore, it is logical to track oil levels and makeup oil consumed between scheduled oil changes. The table above not only details how high oil consumption might accompany certain reportable oil analysis conditions but also provides examples of what these conditions may mean. Understanding how engines consume oil is still a work in progress and is the subject of ongoing research by many organizations. It is important to slow down or arrest the problem as much as possible. Undoubtedly, much progress will be made in the years to come. In the meantime, it will be beneficial to use the current knowledge to its fullest extent. The strategies described in this article offer several plausible ways that this can be achieved. About the Author Jim Fitch has a wealth of “in the trenches” experience in lubrication, oil analysis, tribology and machinery failure investigations. Over the past two decades, he has presented hundreds of courses on these subjects. Jim has published more than 200 technical articles, papers and publications. He serves as a U.S. delegate to the ISO tribology and oil analysis working group. Since 2002, he has been director and board member of the International Council for Machinery Lubrication. He is the CEO and a co-founder of Noria Corporation. Contact Jim at [email protected].