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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.

Improvement of engine fuel efficiency through the use of low friction engine oils is a major task in engine lubrication research. This friction reduction can be achieved by improving the rheological characteristics and elastohydrodynamic (EHD) properties of engine oils, and by controlling boundary chemical interactions between oil-based additives and lubricated components in the engine. In order to achieve minimal frictional power loss under all lubrication regimes, engine tribological systems must be designed to effectively use advanced lubricant technology, material and surface modifications. This paper presents results of cooperative research addressing opportunities for minimizing friction through extension of hydrodynamic lubrication regime in lubricated components using various formulation approaches. A set of experimental oils has been evaluated using laboratory test rigs that simulate hydrodynamic, EHD, mixed and boundary lubrication.

The friction reducing capability of engine oils in the piston ring/cylinder bore contact was investigated under fully-flooded and starved lubrication conditions at 100° C using a laboratory piston ring/cylinder bore friction rig. The rig is designed to acquire instantaneous transient measurements of applied loads and friction forces at the ring/bore interface in reciprocating motion over a 50.8 mm stroke. The effects of increasing load and speed on the friction coefficient have been compared with new and used engine oils of different viscosity that were formulated with and without friction modifying additives. Test results with fully formulated engine oils containing molybdenum dithiocarbamate (MoDTC) show that friction is always lower than that obtained with non-friction modified oils but in regions of persistent starvation the coefficient of friction can increase significantly, approaching levels equivalent to fully-flooded non-friction modified formulations.

Development of the Sequence VIB dynamometer engine test procedure for evaluating the fuel efficiency benefits of engine oils has recently been completed. This test was designed as an improvement over its predecessor, the Sequence VIA test. It evaluates fuel economy using a range of boundary/mixed and hydrodynamic lubrication stages selected to better represent a wider range of engines. In addition to determining “fresh oil” fuel economy, the new test determines fuel efficiency retention after a second oil aging stage that corresponds to 6437 - 9674 km (4,000 - 6,000 miles) of pre-certification aging of engine oils in vehicles and is representative of customer use. This paper describes the selection of aging conditions and length.

The newly developed Sequence VIB engine dynamometer test for measuring the ability of engine oils to improve engine fuel efficiency was designed as an improvement on its predecessor, the Sequence VIA test. The Sequence VIB test features an additional, extended oil aging to correspond to aging of engine oils in certification vehicles and in customer use, and a new set of boundary/mixed and hydrodynamic lubrication stages to better represent a wider range of engines. Five fuel economy measurement stages were chosen for the Sequence VIB test from a larger set of prototype stages, based on extensive friction modeling of engines, analysis of Sequence VIA data on reference oils, and operational considerations. Time factors for these stages were derived based on a mini-mapping of engines considering engine operating conditions in the Metro/Highway Federal fuel economy test procedure (FTP M/H) and the estimated market volume of each engine-vehicle.

Lubrication conditions in journal bearings lubricated with low friction engine oils have been investigated using two complementary experimental techniques. Load supporting capacity under conditions ranging from fully flooded to mixed lubrication was measured for several candidate oils using a bench test that simulates the dynamic motion of a journal bearing at fixed, measurable eccentricities. The performance of these oils was also assessed using a bearing test rig in which journal friction is measured under typical engine conditions of speed, load and temperature. Significant mixed lubrication conditions were shown to exist at low speeds in heavily loaded journal bearings. Under such conditions, oil with friction reducing additives exhibit higher load supporting capacity, distinct separation of moving parts, and reduced friction relative to oils without such additives.

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.

Fuel efficient engine oils containing molybdenum dialkyldithiocarbamate, MoDTC, friction modifiers can lose their ability to reduce friction during service prematurely. Depletion processes involving antioxidant reactions and interactions with other additives play important roles in determining the performance of these formulations. This paper describes results from investigations of the antioxidant reactions of MoDTC alone and in combinations with zinc dialkyldithiophosphates and a phenolic antioxidant. The effect of supplementary ashless antioxidant on retention of friction reducing capability is described.

The High Frequency Reciprocating Rig (HFRR) commonly used to measure the friction and wear properties of diesel fuels has been modified to study gasoline lubricity. Wear tests have been carried out on a range of gasoline and diesel fuels. The non-additised gasolines tested all give higher wear than severely-refined Class I diesel fuels. The effect of relative humidity on the wear properties of both gasoline and diesel fuels has been compared. Both types of fuel give wear behavior which is almost independent of water vapour pressure down to 0 8 kPa, but show a reduction of wear below this humidity level. In practice most gasoline fuels contain detergent additives. The influence of two commercial gasoline detergent additives of different structure on gasoline lubricity has been studied. Both additives reduce wear, to an extent which is dependent upon additive concentration and also upon the base fuel.

The friction reducing potential of low viscosity engine oils in bearings has been investigated using a journal bearing test rig. Friction was measured on both the journal and bearing of a production connecting rod and compared to results from a simple analytical model over a range of speeds and loads. Hydrodynamic lubrication conditions existed over most test conditions, but mixed lubrication conditions were shown to exist at low speeds in heavily loaded journal bearings. Results indicate significant friction reduction through the use of low viscosity oils under hydrodynamic lubrication conditions. In mixed lubrication conditions, oils with friction reducing additives exhibit lower friction than those without such additives. These findings suggest the possibility that a low viscosity oil with effective friction reducing additives can extend low friction benefits over the entire range of operation of engine bearings.

The main factors influencing the development of engine oils for the future are environmental protection, resource utilization and customer satisfaction. Improving engine oil no longer means just providing adequate durability but also maximizing fuel efficiency, minimizing detrimental effects on emission systems and maximizing useful life. Opportunities for improvements in these areas, discussed in detail in this paper, will be considered by ILSAC (International Lubricant Standardization and Approval Committee formed by the American Automobile Manufacturers Association, AAMA, and Japan Automobile Manufacturers Association, JAMA) in developing the ILSAC GF-3 standard to be introduced around the year 2000.

Obtaining the maximum benefits from advanced fuel efficient engine oils will require that those oils not only provide fuel efficiency when new but also throughout the service interval for the oil. The effects of laboratory and engine aging on the ability of an oil containing a molybdenum dialkyldithiocarbamate (MoDTC) additive to provide reduced friction have been investigated. Results of these studies show that the friction reducing capability of this oil, as measured in a laboratory test, was lost prior to depletion of the MoDTC. Interactions between MoDTC and other additives were found to be important with regard to providing friction reduction. Implications for development of advanced oils that will provide lasting fuel efficiency benefits are discussed.

A vehicle system to monitor the actual condition of engine oil in service would provide the customer with the opportunity to utilize the full useful life of the oil and would minimize problems which can occur when oils remain in the engine too long and are excessively degraded and/or contaminated. This paper describes limitations of some systems which have been proposed, outlines the requirements and potential difficulties related to development of sensors designed to monitor changes in chemical properties of the oil, and describes laboratory and vehicle evaluations of a candidate sensor.

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.

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 new method for the determination of antioxidant capacity in lubricants has been utilized for the analyses of new and used engine oil samples. The results derived from these analyses were used for monitoring the antioxidant decay during laboratory and long drain fleet testing and in various correlation studies. The application of the method provided new insights into the nature of the oxidative deterioration of engine oils as a function of type and initial concentration of antioxidant additives, of type of base oil and of test or engine severity.