An Investigation of Tribological Characteristics of Energy-Conserving Engine Oils Using a Reciprocating Bench Test

Engine design and tribology engineers are constantly challenged to develop advanced products with reduced weight, reduced friction, longer life, and higher engine operating temperatures. The resulting engine systems must also meet more demanding emissions and fuel economy targets. Advanced energy-conserving lubricants and surface coatings are concurrently evolving to meet the needs of new engine materials. Because of the enormous cost and time associated with engine testing, much interest is being focused on the development of representative and repeatable bench tests for evaluation of engine materials and lubricants. The authors have developed a bench test employing reciprocating motion for evaluating friction and energy-conserving characteristics of lubricants. The main advantage of this developed bench test is that parts of real components are tested (cylinder liners and piston rings) so that geometry and metallurgy of the engine are preserved and representative surface finishes can be evaluated.

Tribological properties of several energy-conserving engine oils were evaluated using a laboratory bench test. Real engine components (3.8L cast iron bore and Mo-coated piston ring segments) have been used in measuring friction under conditions typical of an operating engine. The fuel economy characteristics of the oils in vehicle tests were correlated with the friction coefficient from the bench tests and viscometric properties of the oils. The influence of friction modifiers, including organo-molybdenum additives, and various base oils was investigated. Electron Probe Microanalysis (EPMA) and Scanning Electron Microscopy (SEM) analyses were performed to identify the formation of surface-active layers on the sliding components and their influence on performance. Friction-reducing mechanisms are proposed for explaining the observed friction characteristics. In addition, the friction-reducing and wear mechanisms observed in the bench tests will be compared with the engine test results, reinforcing the validity of the bench test.