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Modern Passenger Car Motor Oils (PCMOs) are formulated to provide superior wear, oxidation, and deposit control under the most demanding driving conditions. In most PCMOs, zinc dialkyldithiophosphate (ZnDTP) has been the dominant antiwear and antioxidant agent for well over 50 years. Unfortunately, the phosphorus in ZnDTP may partially deactivate the exhaust emission catalyst. To ensure that the catalysts function for at least 120,000 miles, engine manufacturers are exploring phosphorus limitations for upcoming PCMO performance categories. This paper explores the antiwear film formation of low phosphorus engine oils using the Electrical Contact Resistance bench test. A prototype ILSAC GF-4 formulation blended with varying amounts and types of ZnDTP was tested at temperatures typical of operating engines. Secondary ZnDTP was found to produce the best films under the broadest temperature range.

Because of the rising costs of engine tests, bench testing is a necessity in engine oil development. Which bench test to use remains a problem. Recently, we have reported on the use of electrical contact resistance (ECR) coupled with a ball-on-disk tribometer to study the formation and the durability of antiwear films from binary additive mixtures. This paper extends the ECR study to fully formulated fresh oils run in both fired gasoline engines and the ECR bench test. X-ray Photoelectron Spectroscopy (XPS) analyses of used Sequence VE engine parts from highash fully formulated lubricants are shown and the relationship of ECR film formation to fired-engine test performance is discussed.

The authors measured camshaft surface temperatures in two different gasoline engines: a Ford 2.3-liter overhead-camshaft engine with finger-follower and an Oldsmobile V-8 5.7-liter engine with rotating tappets and pushrods. Using unique surface thermocouples in the cam-lobes, we found that maximum cam-lobe temperatures occur at the cam-nose and increase linearly with speed and oil temperature. At high speed, the rotating tappet produced lower temperatures than the finger-follower. In addition, at maximum speed the cam-lobe temperatures in the ASTM Sequence V-D and IIID tests were similar--200°C. The similarity in these surface temperatures explains why both engines require similar zinc dithiophosphates (ZnDTP) for wear control. The surface temperature controls the surface chemistry.

We were able to identify engine blow-by as a primary factor affecting camshaft wear in gasoline engines. Using a 2.3-liter overhead-camshaft engine, we isolated the valve-train oil from the crankcase oil and its blow-by using a separated oil sump. We find that: with engine blow-by, the camshaft wear was high. without blow-by, the camshaft wear was low. with blow-by piped into the isolated camshaft sump, the wear was high again. Later studies identified nitric acid as a primary cause of camshaft wear. It is derived from nitrogen oxides reacting with water in the blow-by. But even in the presence of blow-by, camshaft wear can be controlled by the proper selection of zinc dithiophosphates (ZnDTP) and detergent type.

The modern crankcase oil should prevent wear in both gasoline and diesel engines. This paper addresses the effects of zinc dithiophosphates (ZnDTP) and detergents on wear control in both applications. The authors find a need to properly balance detergent and ZnDTP types in order to obtain optimum wear performance for gasoline valve train wear. In addition, high levels of magnesium sulfonate produce higher bore polishing and/or ring wear than calcium detergents in three different diesel engine tests. Finally, a proper balance of sulfur-containing components and ZnDTP is necessary to prevent corrosive attack of the bronze pins in some diesel camshaft roller followers. Film and metallurgical analyses of used engine test parts are presented.