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This paper provides an overview of driveline fluids, in particular automatic transmission fluids (ATFs), and is intended to be a general reference for those working with such fluids. Included are an introduction to driveline fluids, highlighting what sets them apart from other lubricants, a history of ATF development, a description of key physical ATF properties and a comparison of ATF fluid specifications. Also included are descriptions of the chemical composition of such fluids and the commonly used basestocks. A section is included on how to evaluate used driveline oils, describing common test methods and some comments on interpreting the test results. Finally the future direction of driveline fluid development is discussed. A glossary of terms is included at the end.

Biodiesel fuel is a promising new renewable, alternate fuel source. However, its effect on diesel engine oil lubrication is largely untested at present. There is some indication that the use of biodiesel fuel can degrade diesel engine oil performance to such an extent that shortening of oil drain intervals is required. Oil which is fuel-diluted with biodiesel, which is known to contain unsaturated hydrocarbon bonds, would be expected to be more prone to oxidation. Current diesel engines designed to meet environmental standards tend to introduce more soot into the crankcase oil. The new diesel engine oils for use with biodiesel fuel must be capable of dispersing soot to minimize soot-induced viscosity increase of the oil and prevent engine wear. Oils will also need improved oxidation and corrosion inhibition. To examine soot-handling, ASTM D 7156 Mack T-11 engine test results with 20 wt% soy methyl ester in ultra-low sulfur diesel fuel (B20) were employed.

In the latest passenger car motor oil specifications the Sequence IIIG engine test is used to determine the ability of lubricants to control piston deposits. We have analyzed the chemical composition of Sequence IIIG deposits in order to determine the source of the piston deposits and determine if the mechanism for deposit formation in the Sequence IIIG engine test is similar to previously published mechanisms for formation of high temperature engine deposits. These previous mechanisms show that combustion by-products react with lubricant in the piston ring zone. The mixture of combustion by-products and lubricant are oxidized to form deposit precursors which are further oxidized to form deposits. Since the Sequence IIIG engine test uses lead-free fuel it is important to reexamine the nature of piston deposits formed in gasoline engines and in particular in the Sequence IIIG engine test.

In the next ILSAC passenger car motor oil specification the Sequence IIIG engine test, as well as two versions of the Thermo-Oxidation Engine Oil Simulation Test (TEOST) have been proposed as tests to determine the ability of crankcase oils to control engine deposits. The Sequence IIIG engine test and the TEOST MHT test are designed to assess the ability of lubricants to control piston deposits and the TEOST 33 test is designed to assess the ability of lubricants to control turbocharger deposits. We have previously characterized the chemical composition of Sequence IIIG piston deposits using thermogravimetric, infrared and SEM/EDS analyses. Sequence IIIG piston deposits contain a significant amount of carbonaceous material and the carbonaceous material is more prevalent on sections of the pistons that should encounter higher temperatures. Furthermore, the carbonaceous material appears to be a deposit formed by the Sequence IIIG fuel.

The low-temperature pumpability of engine oil throughout the engine at startup is an important property. Insuring that fresh oils can be pumped at low temperatures has been a requirement of crankcase lubricants for approximately two decades. Extending the assurance of the oil's low temperature pumpability as it ages under engine operation has been the concern of car manufacturers and lubricant marketers for some time. In order to determine the factors influencing the aged oil's low temperature pumpability, we have undertaken a fleet test. We found that as lubricants are aged, excellent low temperature pumping properties can be maintained if lubricants are formulated with viscosity-index improvers incapable of forming polymer networks, base oils with a low tendency to form wax networks, effective pour-point depressants, and if oil drain intervals are not extended beyond the performance limitations of the specific lubricant category.

A taxi field test in 1999 resulted in unusually high used oil MRV TP-1 viscosity in the first 16,000-kilometer drain oil. A subsequent root cause investigation revealed that contamination of the test oil by carry-over of the factory-fill oil followed by oil aging in the vehicle was responsible for the unusual high MRV TP-1 viscosity. Contamination by the factory-fill oil alone cannot account for the high MRV TP-1 viscosity; oil aging in vehicles is an essential co-factor. While the precise mechanism has not been determined, high MRV TP-1 viscosity and yield stress appear to be the consequence of reduction in PPD effectiveness but not PPD degradation. However, the MRV TP-1 viscosity and yield stress of such used oil can be restored to acceptable levels by an optimized PPD system. The study found that used oil MRV TP-1 pumping viscosity and yield stress can be highly dependent on the viscosity index improvers used in the oils involved.

Currently there is no axle test aimed specifically at bearing durability in automotive hypoid axles. Existing axle tests are primarily focused on gear distress and lubricant protection of gears. In light of the new test information showing axle bearing distress, there is a need to develop a new bearing durability test for automotive and truck axle lubricants. To fulfill this need, a new bearing durability test has been developed to better assess lubricant requirements for rolling element bearing durability. Although the final test of an axle lubricant is in a driven automobile or truck, an effective screening test based on actual light duty truck conditions can be used to accelerate lubricant development to enhance bearing performance in hypoid axles. This new test simulates actual road durability tests in the lab. A specific load cycle which retains the critical road test loading conditions reduces test time and helps speed up lubricant development.