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Valve seat wear or “sinkage” or “recession” was observed in eight engines run on unleaded fuel. Detailed studies of valve seat wear were undertaken in a 302 cu in. V-8 engine run for 10-1/4 hr at WOT on a dynamometer stand. The seat was oxidized and flaky, and the valve face was spotted with hard (45-55 Rc) oxide nodules. It was concluded that during operation the oxide flakes adhere to the valve face to form nodules which become embedded in the metal (43 Rc). With the embedded nodules, the valve wears the cast iron seat (40 Rc) by abrasion. Loads, rotation, and temperatures are often influencing factors. The use of leaded fuel in another engine was found to form PbO · PbSO4 on the hot valve face and Pb (Cl0.75 Br0.25)2 on the cooler seat. These compounds are high-temperature solid film lubricants which, by coating the surface, inhibit cast iron oxidation and also prevent material transfer.

Three cars have been operated for approximately 8000 miles on each of four unleaded gasolines. These gasolines encompassed varied levels of C6-C8 aromatics and varied polynuclear aromatic (PNA) content. Exhaust PNA emissions and lube oil PNA content were measured periodically during the course of mileage accumulation. Results of this study show that increases in both light C6-C8 fuel aromatics and fuel-contained PNAs can result in significantly increased exhaust PNAs. Vehicles meeting increasingly stringent hydrocarbon (HC) and carbon monoxide (CO) emissions standards emit greatly reduced quantities of exhaust PNAs, though the rate of lube oil PNA accumulation appears to be unaffected by emissions control systems. Accumulated lubricating oil mileage was found to correlate with increased PNA emissions with a high level of statistical significance. This may be the result of an observed increase in lube oil PNA content with mileage accumulation.

Unleaded gasoline was used in a 30 car, 48,000 mile taxicab field test to study the effects of a deposit control (DC) gasoline additive and lubricating oil additives on engine deposits, wear, and other variables. The unleaded gasoline performed as well as or better than a leaded gasoline in this test when both gasolines contained the DC additive. Engine deposits were comparable, and wear and corrosion were reduced when unleaded gasoline was used. With unleaded gasoline, we found motor oils need a high level of ashless dispersant to obtain maximum varnish control. Also, in these test engines, an ashless oil gave the same wear protection with unleaded gasoline that an SE-type oil gave with leaded gasoline. A highly effective DC gasoline additive minimized carburetor, intake valve, and crankcase deposits with unleaded gasoline. This agreed with the effective performance that this additive had previously shown in leaded gasoline service.