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Newly developed, low volatility zinc dialkyldithiophosphate (ZDDP) technology significantly reduces the amounts of volatile phosphorus (P) species while retaining their antiwear protective behavior and antioxidant performances. In the past researchers gathered a variety of engine data supporting the hypothesis that P volatility can have a significant impact on catalytic converter efficiency, but corresponding field performance results were limited. A recent field trial was conducted with New York City taxi cabs that provided extended sets of engine and drain performance parameters data. This trial compared side-by-side performance of oils formulated with conventional and low volatility ZDDPs. Following completion of the test, the extensive engine inspections and used oil analysis revealed excellent antiwear control and oxidation inhibition with both types of ZDDP technologies.

Four 1998 Mitsubishi Carismas, two equipped with direct injection (GDI) and two with port fuel injection engines (PFI) were tested in a designed experiment to determine the effect of mileage accumulation cycle, engine type, fuel and lubricant type on engine wear and engine oil performance parameters. Fuel types were represented by an unadditised base fuel meeting EEC year 2000 specifications and the same base fuel plus synthetic deposit control additive packages. Crankcase oils were represented by two types (1) a 5W-30 API SJ/ILSAC GF-2 type engine oil and (2) a 10W-40 API SH/CF ACEA A3/ B3-96 engine oil. The program showed that specific selection of oil additive chemistry may reduce formation of intake valve deposits in GDI cars.. In general, G-DI engines produced more soot and more pentane insolubles and were found to be more prone to what appears to be soot induced wear than PFI engines.

One of the key functions of lubricating oil additives in diesel engines is to control oil thickening caused by soot accumulation. Over the last several years, it has become apparent that the composition of the base oil used within the lubricant plays an extremely important role in the oil thickening phenomenon. In particular, oil thickening observed in the Mack T-8 test is significantly affected by the aromatic content of the base oil. We have found that the Mack T-8 thickening phenomenon is associated with high electrical activity, i.e., engine drain oils which exhibit high levels of viscosity increase show significantly higher conductivities. These findings suggest that electrical interactions are involved in soot-induced oil thickening.

Our basic understanding of the chemical and physical nature of soot, its interaction with lubricant components and its role in promoting wear and oil thickening in heavy duty diesel engines continues to grow. Our current study in the GM 6.5L engine focuses on examining the effects of variations in base stock type (Group I vs. Group II), viscosity index improver or viscosity modifier (VM) chemistry (OCP vs. dispersant OCP), zinc dithiophosphate (ZDP) type and dispersant type (low MW vs. high MW) on roller follower wear, viscosity growth and other measured responses. In this study, more robust fluids were tested producing very low wear results and minimal viscosity increase of the lubricant. Fluids containing dispersant OCP (DOCP) and high MW dispersant produced a lower degree of wear, whereas varying the ZDP type (1° vs. 2°) showed no effect on wear. The use of Group II base stocks was associated with significantly lower viscosity increases.

Fundamental knowledge investigations of soot-lubricant interactions continue. In earlier work [1-2], we examined the impact of formulation variables, engine type and mode of engine operation on the formation and nature of diesel soot and its interactions with the crankcase lubricant. Three types of North American heavy duty diesel engines were utilized: Mack EM6-285, GM 6.2L and GM 6.5L. Experiments identified additive compositions capable of providing good viscosity and wear control. Furthermore, we identified soot agglomeration, rather than amount of soot, as the phenomenon responsible for roller follower wear at low dispersant levels. Oil thickening results from the level of soot contamination, in combination with the “state” of the soot. The latter is noticeably affected by the lubricant dispersant level. Part 4 of our studies examines the impact of oil composition on a fluid's ability to handle soot in the Mack T-8 Test.

Statistically designed experiments were developed to investigate the nature of soot, to understand its role in oil viscosity growth, and to study the interactions involved with additives that inhibit viscosity growth. The matrix was designed to examine effects of engine type, mode of operation, and the oil formulations. Mack EM6-285 and GM 6.2L engines operating under both high speed and high torque conditions were used in this study. An API CE\SG quality lubricant was used as the baseline. The detergent sulfonate substrate was varied from standard to three-fold levels; the dispersant TBN contribution ranged from 1.1 to over 3.0. The surface and bulk exhaust soot properties were determined. Colloidal suspension stability and rheology were measured to evaluate the design factor effects on the formation of soot and subsequent effects on oil thickening. The Mack EM6-285 engine produced less soot, less oil viscosity growth, and less oxidation than the GM 6.2L engine.

A new field test procedure for evaluation of the sludge formation tendencies of lubricants has been developed. The procedure has the benefits of short running time, reduced variability, and dramatic separation of API SF vs API SG oils. This paper discusses development of the operational procedure and evaluation of four lubricants, including commercial-type API SF and API SG oils as well as experimental future oils. Significantly improved sludge ratings were obtained with an experimental API SG oil. The sludge formation process was studied using infrared spectroscopy, TAN, dielectric measurements, viscosity, quasielastic light scattering particle size, and transmission electron microscopy techniques. These analyses show production of contaminants which form insoluble particles that build up and precipitate out of suspension as sludge. Certain drain analyses can be used as tools for predicting field sludge deposition time.

Polyol ester-based diesel engine lubricants which achieve maximum theoretical high-temperature performance have been developed in our laboratories during the past three years. New lubricant basestocks and additives are currently being developed to perform under more severe thermal conditions, anticipated in low heat rejection diesel engines at the turn of the century. In this paper, the status of our current laboratory development and evaluation of new diesel engine lubricants, with high-temperature applicability beyond polyol esters, is summarized. Our final work in the polyol ester class of lubricants, through single-cylinder engine tests, is also presented.