Machinery Lubrication magazine published by Noria Corporation
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Figure 3. Effects of the oil change interval on miles per quart of oil (Ref. Carver, Exxon) 4 | January - February 2016 | www.machinerylubrication.com AS I SEE IT 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 influ- enced by load, combustion efficiency and cooling. Approximately 74 percent of vapor- ization occurs during intake and compression strokes (no speed effects have been found). 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 distor- tions 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. 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. Conse- quently, 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. 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, environ- mental 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. Oil Consumption Issues Revealed by Oil Analysis Monitoring oil levels and makeup rates offers a reliable indication of oil consump- tion 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 5,000 Engine C Engine B Engine A 4,500 4,000 3,500 3,000 2,000 2,500 1,500 1,000 500 0 Long-haul Class 8 Truck 1 5 9 13 17 21 25 29 33 37 41 45 49 THOUSANDS OF MILES SINCE OIL CHANGE OIL CONSUMPTION:MILES PER QUART How Oil Consumption Influences Tailpipe Emissions and Health As engines age and wear, they become greater consumers of crankcase oil. Solid contam- inants combined with soot and other oil suspensions influence engine wear, deposits and oil economy (oil consumption rate). When oil is consumed, it enters the combustion chamber, burns with the fuel and is pushed out with exhaust gases as particles and volatile hydrocarbons. Fresh new lubricants have more volatile light-end molecules and are more prone to hydrocarbon emissions. As the oil ages, the hydrocarbon emission levels off but can pick up again if the oil becomes contaminated with fuel (fuel dilution), such as from short run times or long idles. However, in general, the service life of the oil has no significant influence on carbon monoxide and nitric-oxide emissions. The level of exhaust emissions can increase considerably over time, corresponding to engine wear and deposit formation. This leads not only to greater exhaust particulates but also to a higher percentage that are hydrocarbon, which is a byproduct of oil consumption. It has been observed that lubricating oil is a significant contributor to the particulate emissions signature as the engine ages, especially with diesel engines. The obvious strategy to control/reduce hydrocarbon emissions is to decrease oil consump- tion. This, in large part, is accomplished only by controlling combustion efficiency, wear and deposits (especially through good lubrication and filtration practices). Nitrogen oxides (NOx) consist of nitric oxide (NO) and nitrogen dioxide (NO2). These ozone precursors also lead to smog when exposed to hydrocarbon gases and sunlight. As a health hazard, NOx can potentially cause irritation and damage to lung tissue as well as paralysis. Because of regulatory requirements and environmental protection pressures to lower both particulates and NO2, increased pressure has been placed on lubricant formulation, engine design and filter performance.