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Anti-wear additives are far more than just hype, but the quest for improvement continues.

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Kevin Cameron has been writing about motorcycles for nearly 50 years, first for Cycle magazine and, since 1992, for Cycle World. (Robert Martin/)When I was still in the public school system, my uncle laid out $40 to bring home a worn-out 1940 Chevy coupe for my mechanical education. I slowly fought my way through inches of accumulated sludge; detergent oils were still years away. Having been supine under the car for quite some time, I reached the point where I was able to remove the connecting-rod caps. Now I could remove the pistons, or so I thought, pushing them up to the tops of their bores, as the cylinder head was already off. But try as I might they could not be pushed, bumped, or otherwise persuaded to leave the cylinders.

Pushing any piston down an inch revealed the reason: There was a prominent, feel-it-with-your-fingernail ridge at the top of each cylinder, right where the top piston ring stopped at TDC.



On his following visit my uncle took the next step; he bought a cylinder-ridge reamer. Cylinder ridge? Turns out that most of the wear between the piston rings and the bore is right at the top. This makes sense because:

1. Pistons and rings are moving so slowly around TDC that there’s extra time for oil to squeeze out from between the rings and cylinder wall, allowing metal-to-metal contact.
2. Because the top of the bore is the part closest to the fire, temperature is high, which reduces oil viscosity.
3. Because ignition is timed to allow combustion to reach peak pressure just as the piston begins significant downward motion (10–12 degrees ATDC), maximum pressure between the top piston ring (exerted by combustion gas getting atop and behind the ring) and cylinder wall occurs at the top.

The Sears ridge reamer made chips and allowed me to push the pistons out. Once removed, my uncle pointed out a second but smaller ridge where the rings stopped at BDC.

Related: How Does The Lubrication System Work?

Anti-Wear Additives

Yet high-mileage engines I’ve examined since the 1970s show practically no such wear ridges. The major reason is the adoption of anti-wear additives in lubricating oil. They are not just more empty acronyms flowing from an advertising copywriter’s pen—they actually work. A little careful reading reveals that lubricating oils are roughly 20 percent additives. A little more Google time turns up a claim that machinery wear consumes 6 percent of Gross Domestic Product (GDP).

The three classic regimes of lubrication are:

1. Full hydrodynamic lubrication, resulting in a full film of oil between moving parts, and preventing surface-to-surface contact.
2. Mixed lubrication, in which the load is shared between patches of hydrodynamic lubrication and areas of surface-to-surface contact.
3. Surface-to-surface contact, sometimes called boundary lubrication.

In the case of piston-ring to cylinder-wall sliding, at TDC and to a lesser extent at BDC, the friction is mixed. Tiny local welds form where metal-to-metal contact occurs, and when they are broken by continued ring motion, wear particles may detach. Over years of operation this added up to what I saw in that 1940 Chevy engine.

Chemical engineers are always at work developing potential products, and in the 1920s they discovered the value of oleic acid in reducing friction between surfaces. In effect, such molecules were able to bond to surfaces tightly enough (at least at moderate temperatures) to give useful protection against scuffing, scoring, and seizure. Indeed, whole classes of molecules have this ability, called polarity, to attach themselves fairly strongly to surfaces, rather like the shoreline seaweeds that sometimes cover the surfaces of submerged stones.

Engine-Oil Sulfur Content

It was also known that sulfur “did something” to surfaces in hard rubbing contact. In the early days of steamships, the ever-present “ship’s boy” might be assigned the task of feeling his way back into a dark propeller-shaft tunnel (“Mind the shaft doesn’t get hold of your clothing…”) to a recently replaced intermediate bearing that was running hot. He sits there, occasionally brushing on a slurry of sulfur in oil.



Chemists discovered that where the temperature was high enough, sulfur could react with the hot metal to form metal sulfides, solids less strong than either the steel shaft or bronze journal supporting it, and therefore able to act as a kind of sacrificial solid lubricant. As continuing friction wore away the sulfide layer, it reformed as the lad dutifully applied his brush. Over time, the process gradually wore away the regions of high-pressure contact and bearing temperature declined. The boy was recalled from the darkness for other duties.

Continuing investigation revealed a range of chemistries useful in this way, particularly compounds of phosphorus. When I was a teen, one of the oil companies pushed TCP as a gasoline additive. This was tricresyl phosphate, which could at points of high-temperature friction release its phosphorus to form a layer of metal phosphides. Instead of the high friction and heat generation of metal-to-metal contact, the solid lubricant layer dropped the local friction coefficient to 0.1 or even lower, reducing heat and making such contact survivable.

Related: The Amazing Modern-Oil Cocktail

Other Anti-Wear Additives

Since then, TCP has been set aside as an “aggressive anti-wear compound,” evidently meaning that its ability to polish away areas of hard contact was too much for casual use. The modern world soon settled upon ZDDP, zinc dialkyl dithiophosphate, as the anti-wear most suited for use in automotive lube oils.

The most powerful anti-wears are now termed “Extreme Pressure additives” (EP) used mainly in gear oils. Tooth-to-tooth pressures in heavy-duty gearing can range up to 100,000 psi, so the bad-smelling chlorine- and sulfur-bearing EPs serve in such applications.

Nothing is simple today. Phosphorus from ZDDP could, during combustion, form ash—pyrophosphorus compounds that interfered with catalytic converters. Because demand for anti-wear chemistry is strong, the hunt continues. Alternatives like ionic liquids, emissions catalysts less sensitive to phosphorus, suspended nanoparticles of solid lubricants, or coating pistons and cam lobes directly with solid lubricants are under development. This endless chemical quest reminds me of the lyric from Chuck Berry’s classic “Roll Over Beethoven”: “Long as she got a dime/The music won’t never stop.”