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The deterioration of the friction reducing properties of engine oils containing molybdenum dithiocarbamates (MoDTCs) in service was studied. A quantitative analysis of MoDTCs and zinc dithiophosphates (ZDTPs) remaining in aged oils revealed that ZDTPs were consumed faster than MoDTCs. The consumption rate of ZDTPs was slow in the presence of MoDTCs and peroxide-decomposing antioxidants. The frictional properties of aged oils were evaluated with a reciprocating friction tester (SRV tester). The friction coefficient measured with the SRV tester was correlated to the properties of the aged oils, such as the TAN increase, TBN, and concentration of remaining ZDTPs.

This paper describes lubricant technology to enhance the durability of the low friction performance of gasoline engine oils which were formulated with molybdenum dithiodicarbamates (MoDTCs) as friction modifiers. This paper also describes an evaluation method which consists of three tests: (1) Our in-house rig test to simulate oil deterioration in an engine stand; (2) Quantitative analysis of MoDTC and ZnDTP in oils and; (3) A friction test (SRV). It was found that the low friction performance of fuel economy engine oils deteriorated primarily due to the consumption of MoDTC and ZnDTP. Calcium salicylates had better durability of low friction performance than calcium sulfonates. Furthermore, sulfurized compounds enhanced the durability. Based on these findings, an experimental oil was formulated.

The influence of engine oil viscosity on the wear of piston rings and cam faces has been investigated by fired engine tests using a radioisotope (RI) tracer technique. High-temperature and high-shear-rate (HTHS; 150°C, 1O6 s-1) viscosities of the experimental oils prepared are 2.2, 2.4, 2.6 and 3.1 mPa•s. At an oil temperature of 90°C the wear of piston rings and cam faces did not increase, even if the HTHS viscosity was lowered down to 2.2 mPa•s. However, both piston rings and cam faces exhibited an increase in wear below 2.4 mPa•s at 130°C. It was also recognized that valve train wear did not significantly increase with reducing viscosity in the motored engine tests at a temperature of 50°C. From these test results, it was suggested that the oil with the HTHS viscosity of 2.6 mPa•s sufficiently demonstrates the antiwear performance equivalent to that with around 3.0 mPa•s for application to piston rings and cam faces.

The effect of phosphorus concentration in gasoline engine oils on the valve train wear was experimentally investigated by using the JASO M328-91 3A valve train wear (3A-VTW) test method. The phosphorus concentration is determined proportionally to the amount of zinc dithiophosphate (ZDDP), which is formulated as both antiwear agent and antioxidant. Lower concentrations of ZDDP generally bring about larger wear in the valve train. However, it was found from the experiments that valve train wear remained low despite a decrease of phosphorus concentration when secondary ZDDPs with short alkyl chain together with appropriate ashless dispersants were selected. Since adsorptivity of secondary ZDDPs with short alkyl chain lengths onto rubbing metal surfaces is higher than that of primary types, the secondary types give excellent antiwear characteristics.

The relative oxidation stability of two fully formulated engine oils was compared in three testing methods by following the increase in kinematic viscosity of the oil. The purpose of the study was to determine the cause of the completely opposite ranking of the oxidation stability of the two oils that was observed in the ASTM Sequence IIIE engine test and the JASO M333 93 engine test and to determine the degree of correlation the two engine tests had with the field. The study consisted of laboratory oxidation testing, engine testing and taxi field testing to cover the range of conditions from controlled oxidation to actual driving conditions.