The Amazing Modern-Oil Cocktail

[Hugh Janus]

This piston covered with oil does not occur in reality, but here symbolizes the fact that when adequate cooled and filtered oil of the correct viscosity and type is supplied to every point in the engine that needs it, life can be long and uneventful. (Jeff Allen /)

In a celebrated case, a vertical shaft hydroelectric turbine ran continuously for 40 years without shutdown. When it was finally replaced, its thrust-bearing surfaces still bore the tool marks of their manufacture. How was that possible? It was because the bearing surfaces—one rotating, the other stationary—had never touched. They had been completely separated by a thin and continuously replaced film of oil.

This is the ideal to which all surface friction in internal-combustion engines aspires: When surfaces are separated by a continuous oil film, wear from metal-to-metal contact is impossible.

Why isn’t oil immediately squeezed out from between crank journals and bearings, between cam lobes and tappets, between pistons and piston rings, and the cylinders in which they slide? The internal friction of oil, called viscosity, strongly resists such squeezing out; it also allows the relative motion of the surfaces to continuously drag more oil between them. An engine’s oil pump does not generate the pressure that separates moving parts from each other. It is the motion of those parts, acting through the property of viscosity, that drags oil between them as fast as the applied load can squeeze it out.

The amber color indicates the thin cylindrical oil film that separates the connecting rod’s big-end bearing from the crankpin. The piston, attached to the other end of the rod, is driven by high-pressure combustion gas. (Jeff Allen /)

This can only work when an oil wedge is formed. Fresh oil is dragged in from the thick end of the wedge so forcibly that pressures of thousands of pounds per square inch have been measured in the loaded zones of crankshaft bearings. The wedge is formed as applied load pushes the journal slightly off-center within the bearing’s clearance. Oil is pumped into the bearing on its less-loaded side and is dragged into the loaded zone by the rotation of the journal. A piston and its rings tilt ever so slightly in the cylinder, forming an oil wedge between them.

Engine-friction loss is mainly the force required to shear the oil films that separate its parts. As you reduce oil viscosity, friction loss drops, but oil films also become thinner—possibly thin enough that surface irregularities begin to touch each other. Viscosity is a compromise.

An oil’s viscosity falls as its temperature rises, reducing its load-carrying ability. Hot zones such as the top of the cylinder, the top piston-ring groove, and exhaust-valve guides are therefore lubricated by hot oil that has lost much of its viscosity. To slow this high-temperature viscosity loss, so-called multigrade oils have been developed. Chemists found that some long-chain hydrocarbon molecules become more compact at low temperature—adding little or no viscosity—and extend or “unroll” at higher temperatures, which contributes some viscosity.

This shows that load (in this drawing upward), applied to an oil-lubricated plain bearing, transforms the oil film into a crescent-shaped and extremely thin wedge that dynamically supports that load. The viscosity of the oil—its internal friction—causes it to be dragged by the rotation of the bearing into the very thin film in the loaded zone. There, the pressure generated by this dragging process can be thousands of pounds per square inch, and the minimum oil-film thickness is as little as 1.5 microns (0.00006 inch). (Jeff Allen /)

When such molecules are added to base oils, they slow the rate of loss of viscosity with temperature, which is called the Viscosity Index. Thus, a multigrade 5W-30 oil is made from a 5W base oil—measured at zero degrees Fahrenheit—but the added long-chain molecules slow its viscosity loss to that of a 30-weight oil when at 212 degrees Fahrenheit. The oil does not become more viscous as it heats up—it just loses less of its viscosity. This allows use of engine oils thin enough to cold-start in winter, which retain enough viscosity when hot to do a good job of lubricating the hottest parts of a warmed-up engine.

To handle the heat-driven sludging of oil when hot, detergents are added. These surround sludge particles, allowing them to be swept to the filter by oil flow. To protect oil molecules against the sludging effects of high-temperature oxidation, antioxidant is added.

Most wear occurs during cold-start and warmup, when instead of full-film oil-wedge lubrication, mixed lubrication occurs—part oil film, part surface contact. To reduce damage, surface-active anti-wear additives are developed, which form a self-healing solid lubricant layer on surfaces at points of contact. Because water is a product of combustion, oils contain emulsifiers to prevent water films from rusting or corroding parts. To help make oil flow in winter cold-starting, pour-point depressants surround waxy components of the oil before they can clump together. Lubricity agents—long-chain molecules terminating in functional groups—adhere to metal surfaces at all times, reducing the friction between them.

If you disassemble a modern motorcycle engine, you will find that its oil system is very comprehensive. Here we have a camshaft, whose rotating lobes press against the valve tappets to open and close the engine’s valves. It is normal for oil under pressure from the engine’s oil pump to be supplied to the hollow interior of the shafts, emerging through drilled holes to lubricate every camshaft bearing, and every cam lobe and tappet. Thanks to 130 years of engineering, it all works very nicely. (Jeff Allen /)

Oil-additive chemistry is a work in progress, as conditions in today’s smaller, harder-working but more fuel-economical engines become ever more severe.

Modern premium petroleum oils consist of hydrocarbon molecules that have been reshaped to give them desirable structures. Synthetic oils are made by linking together hydrocarbon elements such as ethylene gas to form desirable oil structures. Makers of reformed- petroleum oils and of synthetics are working toward a common ideal. Consult your owner’s manual to find out which oil-viscosity grade—such as 5W-30—and service category—SN, etc.—have been found to perform best in your engine. This information is printed on the oil container itself. In a few cases, synthetic oil only is specified by a manufacturer.

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