The Slippery Slope: Thoughts on Oil Viscosity

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Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

Viscosity is what allows the relative motion of parts to drag a fluid lubricant, which for us usually means oil, into the loaded zones between those parts. A crank journal rotating within a plain bearing or a piston sliding in its cylinder uses the oil’s viscosity to drag it to where it is needed. Early lubrication researchers were astounded to see that the oil pressure in a plain bearing’s loaded zone could be hundreds or even thousands of pounds per square inch. Such pressure is what supports the load, and it comes not from the engine’s 40-60 psi oil pump but from the oil’s viscosity.

Viscosity is a fluid’s internal friction, the force required to shear the fluid, as it slides over itself or across other surfaces. Oil’s viscosity is responsible for most of an engine’s friction loss; these losses arise from the force required to shear the oil films separating the moving parts. That internal friction arises because a fluid’s molecules have an attraction for each other, the same attraction that eventually causes a sufficiently cooled fluid to solidify or freeze.

Related: How Thinner Oils Make Modern Engines More Efficient

In some cases we can reduce friction by reducing oil viscosity, but we must take care that the minimum oil-film thickness doesn’t drop so low that the irregularities on the mating surfaces come into contact with each other. During the early 1990s lower-viscosity oils began to appear, such as 0w-15, but they could be used only in engines with suitably refined surface finishes.

Measuring Viscosity

A fluid’s viscosity is measured in a variety of ways; by letting a column of that fluid held at a specified temperature run through an orifice, or by immersing closely concentric cylinders in the fluid and then measuring the torque required to rotate one cylinder with respect to the other.

In the liquid state (above the fluid’s melting point) there is a constant “battle” between the thermal agitation of the molecules (which is their temperature, as they collide with one another, rotate, or vibrate within themselves) and their mutual attraction. The higher a fluid’s temperature, the more easily thermal agitation overcomes the mutual attraction. For this reason an oil’s viscosity decreases with increasing temperature. Because of this variation, we must always state the viscosity of a fluid at a particular temperature.

If we measure an oil’s viscosity over a range of temperatures, then enter those measurements on a graph with viscosity on one axis and temperature on the other, we get a sloping line. That slope is defined as the fluid’s viscosity index, or VI.

Related: How Does The Lubrication System Work?

In the lubrication business, viscosity index is very important because machines such as internal-combustion engines contain parts operating at widely different temperatures. It would be too complicated to provide different oils for the different engine parts, each tailored to provide adequate viscosity for its application. Therefore to lubricate such parts engineers seek oils whose viscosity changes least with temperature—oils with a high VI. To continue with our “tailoring” analogy, they’re looking for one size that fits most.

Early 20th-century research showed that oils made from Pennsylvania crude had a higher VI than did oils from many other fields. In more recent times it has become possible to chemically alter, or reform, lower grade oils to give them a higher VI.

Ideally we would like engine oils to have a viscosity that remains constant over all operating temperatures, a viscosity low enough to permit winter cold starting yet high enough to carry the loads in a warmed-up engine. That is not possible because of the battle between the thermal agitation of temperature and the affinity oil molecules have for each other: At a sufficiently low temperature the latter “wins” when the oil congeals into a solid.

Since that early research, oil chemists knew that such differences in VI were caused by temperature-driven changes in the shapes of oil molecules. Oil molecular structures are often illustrated as Tinkertoy-like structures of knobs (individual atoms) and sticks (the electrical bonds that hold molecules together). These are of three general types—long, straight-but-flexible chains of carbon atoms with attached hydrogen atoms; branched chains; and ring structures. As it turns out, at lower temperatures straight-chain oil molecules are able to assume a more compact form, as this allows their atoms to stick to each other. This consequently reduces viscosity, because instead of a long chain whose atoms are attracted to many nearby molecules (and resisting motion), they are instead attracted to each other, assuming a compact form. This behavior improves the VI of straight-chain oils.

Related: Oils Well That Ends Well, Part 1

Today’s high-quality petroleum oils are chemically reformed to give them the desired straight-chain structure. Each time I boil spaghetti, the writhing mass of long strands suggests to me the behavior of long-chain oil molecules.

Chemists then realized they could use this concept of molecular folding to create oils with two “personalities.” They added molecules consisting of very long polymer chains (such as methyl methacrylate, the material which gives us Plexiglas) to a thin base-stock oil such as a 10 weight. When cold, the additive molecules folded into compact form, contributing little to viscosity, and leaving the cold oil thin enough to make winter-morning start-ups possible. But as the oil temperature rose during engine operation the additive molecules unfolded, becoming longer and contributing more and more to viscosity, allowing the thin base stock plus additive to behave like a heavier weight oil when hot.

High-quality petroleum oils are chemically reformed to give them the desired molecular structure, thus multigrade oil can function at hot and cold temperatures. (Sport Rider Archives/)

Multigrade Oils

This is not to say that the thin oil gained viscosity as its temperature rose, only that it lost less viscosity than it would have had it been a thin oil without additives. Such oils are called multigrades. The two numbers on the container specifying viscosity, such as 15w-30, are determined by the oil’s behavior at two different temperatures: zero degrees Fahrenheit and 200 degrees Fahrenheit. In our example, 15w, which stands for “15 winter,” describes the oil’s viscosity at the lower temperature, and the 30 on the right describes how it behaves at the higher temperature. In the early days of multigrades, less-durable polymer additives were more easily sheared by passing between gear teeth or between cam lobes and tappets, breaking the long chains and so gradually resulting in loss of viscosity. This caused old-timers to shy away from “new-fangled” oils.

Related: Finding the Best Motorcycle Oil: Part 2

At zero degrees, such multigrade oils behave like their thin base stocks. This is useful because it greatly reduces the torque that the starter motor must produce to rotate the engine during cold start. Then at operating temperature (roughly that of engine coolant in a warmed-up engine) the oil behaves as a heavier grade (having enough viscosity to protect moving parts with complete oil films). The base stock by itself would have been too thin to adequately lubricate engine parts at operating temperature, but its viscosity when cold does a fine job of protecting parts immediately after cold start. As the engine warms up, the VI-improver additive molecules in the oil gradually unroll, changing the oil’s behavior to that of a heavier grade.

If instead we had provided an additive-free straight oil of the heavier grade, starter torque in winter cold starting might be excessive (I remember it well from -30 nights in the Adirondacks), while warmed-up operation would have been normal.

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[fastbob]

Oh Kevin , go and walk the dog or something , get some fresh air .

[fullscreenaging]

1 hour ago, fastbob said:

Oh Kevin , go and walk the dog or something , get some fresh air .

I think this is exactly the scenario he comes up with this drivel. 

[Gerontious]

22 hours ago, fastbob said:

Oh Kevin , go and walk the dog or something , get some fresh air .

If you ever have trouble sleeping… 

 

 

[fastbob]

1 hour ago, Gerontious said:

If you ever have trouble sleeping… 

 

 

Oh my goodness . There's definitely something wrong with the poor old bugger AND he's American .