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Engine oils contain additives that provide wear protection to prolong engine life. In a previous study using direct acting mechanical bucket valve train components, we found that aged oil provided better wear protection and friction reduction under certain circumstances. To understand this effect further, friction and wear performance of fresh and laboratory-aged oils with 0.1% phosphorus was studied with ball-on-flat and cylinder-on-flat rigs. Test durations were chosen according to the electrical contact resistance (ECR) values observed between the contacting surfaces. Anti-wear films were characterized primarily by UV and visible Raman spectroscopy, and results were corroborated by Auger electron and infrared spectroscopies. The greatest compositional differences occurred between films formed by fresh and aged oils. The degree of ECR response or the length of oil aging generally did not affect the type of component observed in the films.

Future vehicle emission regulations both in the US and Europe will require maintaining catalyst efficiency for longer mileage intervals. In order to achieve this requirement, chemical restrictions are being placed on elements in engine oil that can poison catalysts. Most of phosphorus and a significant amount of sulfur in current engine oils come from zinc dialkyldithiophosphates, ZDDPs, which are a class of cost-effective multifunctional additives providing wear, oxidation and corrosion protection. Reducing ZDDP concentrations raises oxidation and wear concerns. The overall purpose of this research is to look at the antioxidation and antiwear capability of low phosphorus engine oils containing 0.05 wt% phosphorus and the potential of engine oils formulated without phosphorus. In addition to fresh oils, used oils drained from fleet vehicles were also analyzed and evaluated.

The overall purpose of this research is to determine the antiwear capability of low phosphorus engine oils containing 0.05 wt% phosphorus. The antiwear performance of 0.05 wt% phosphorus engine oils was evaluated using a laboratory valvetrain bench test rig coupled with an on-line wear measurement technique and a high frequency reciprocating rig (HFRR). Low phosphorus engine oils were compared with GF-3 engine oils containing 0.1 wt% phosphorus. In addition to fresh oils, long drain used oils from fleet vehicles were also analyzed and investigated. This information is important to develop engine oil formulations to meet the latest government emission and fuel economy requirements. The results indicate that by appropriately selecting and balancing supplemental antiwear and/or antioxidation additives the wear loss due to the reduction of zinc dialkyldithiophosphate (ZDDP) may be compensated or even reduced.

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 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.

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.

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 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.

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.