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Engine oils formulated using molybdenum dialkyldithiocarbamate, Mo(dtc)2, additives can provide substantial friction reduction under mixed to boundary lubrication conditions. It has been previously shown that the effectiveness of Mo(dtc)2 is significantly affected by the presence of other additives and by additive interaction and depletion processes occurring during use. In this study, ligand exchange reactions in an additive system containing Mo(dtc)2 and zinc dialkyldithiophosphate, Zn(dtp)2, have been investigated during oxidation in hexadecane and various base oils at 160°C. Samples of different composition obtained from these studies were used in investigations of the effects of original additives and ligand exchange products on friction reducing capability at 45 and 105°C.

Meeting upcoming stringent emission standards will require that exhaust gas catalyst systems become active very quickly, function at very high efficiencies and maintain those capabilities at high mileages. This means that contamination of the catalysts by engine oil derived poisons must be minimized. Phosphorus compounds, derived from the zinc dialkyldithio-phosphate (ZDTP) additives that provide antiwear and antioxidant activity, are a principal contaminant that can increase catalyst light off times and reduce catalyst efficiency. Therefore, reducing the concentration of, or eliminating, phosphorus in engine oils is desirable. Doing so, however, requires that oils be reformulated to ensure that wear protection will not be compromised and that oxidation stability will be maintained. To address these concerns, laboratory tests for evaluating oil oxidation and wear performance have been developed and used to evaluate developmental low phosphorus oils.

The relationship between engine oil formulations and catalyst performance was investigated by comparatively testing five engine oils. In addition to one baseline production oil with a calcium plus magnesium detergent system, the remaining four oils were specifically formulated with different additive combinations including: one worst case with no detergent and production level zinc dialkyldithiophosphate (ZDTP), one with calcium-only detergent and two best cases with zero phosphorus. Emissions performance, phosphorus loss from the engine oil, phosphorus-capture on the catalyst and engine wear were evaluated after accumulating 100,000 miles of taxi service in twenty vehicles. The intent of this comparative study was to identify relative trends.

Selected engine oils and zinc dialkyldithiophosphate additive concentrates have been added to used oils in various engines and subjected to engine testing. Oil samples obtained as a function of mileage accumulation have been analyzed using the peroxy radical titration method and 31P NMR. The engine studies were supplemented by laboratory investigations of antioxidant behavior of pure neutral and basic zinc dialkyldithiophosphates, dialkyldithiophosphoric acid, tetraalkylthioperoxydiphosphate (disulfide), and of neutral zinc dialkyldithiophosphate in combination with a hindered phenol antioxidant, 4,4′-methylenebis(2,6-di-tert-butylphenol). These investigations included studies of the effects of hydroperoxides and hydrocarbon oxidation products on the radical scavenging activity of the above compounds.

A reaction scheme depicting engine oil oxidation chemistry occurring in internal combustion engines is proposed. This scheme reflects the idea that hydroperoxides, which are initial oxidation products, are formed continuously in engine oil, regardless of the presence of radical trapping inhibitors, due to a continuous influx of free radicals from the combustion process. Therefore, the antioxidant behavior of a zinc dialkyldithiophosphate (ZDTP) itself and in combination with an ashless phenolic antioxidant has been investigated using a model hydrocarbon oxidation system in the presence of excess hydroperoxides. In order to approximate temperatures existing in critical engine locations, these studies were carried out at 160°C. Results obtained contribute to a better understanding of the antioxidant mechanisms of ZDTP and also provide basic information needed in development of laboratory test procedures for evaluation of engine oil antioxidant systems under realistic conditions.