Development of laboratory engine simulation tests for the evaluation of lubricants

Engine testing of new lubricants and modifications of lubricant formulations is expensive and time consuming. The screening of lubricants and the reduction of the number of tests through bench testing has always been controversial. However, bench tests that help understand the mechanism of various aspects of oil performance are valuable, especially if it can be show that they are correlatable with engine oil performance tests. This paper reviews some of the technical issues involved in how well an oil performs in an engine. Oxidation, thermal stability, deposit formation, viscometrics and friction and wear are key issues. The differences in the mechanisms involved in gasoline and diesel engine tests are compared. Simulation tests described include one bench test, TFOUT, that has had considerable success in correlating with III-D gasoline engine oil performance tests. This has recently been updated to enable evaluation of V-D engine oils. The use of thermal gravimetric analysis (TGA) and differential scanning calorimeter (PDSC) methods to evaluate volatility, oxidation and deposit formation in diesel engines is described. Simulation models led to the development or refinement of these bench tests. Cooperative studies with industry show correlation between a 'two-peak' PDSC deposit formation simulation test with diesel engine oil performance tests. The methodology described is useful in evaluating the relative contribution of the lubricant to engine emissions.