Development of Single Cam Rig for Accurate Simulation of Valve Train Tribochemistry

The study of cam-tappet tribochemistry is on the rise due to the need for a better understanding of how nanoscopic tribofilms reduce friction (improved engine efficiency) and wear (durability) in internal combustion engine. Environmental legislation on exhaust gas emissions have further stimulated research on the use of less phosphorus and sulphur containing additives because phosphorus clogs the catalytic converters in an engine exhaust system.
Current tests evaluate the resultant surface films formed on the contact by the additive package which has made understanding of the test conditions crucial due to the increased complexity of tribochemistry. Diamond Like Carbon (DLC) surface coatings are also receiving significant attention even though their interaction with conventional lubricants additives is still unclear.
A vast majority of published studies look at these systems under steady-state conditions whereas, dynamic conditions are predominant. In this work, a newly modified ‘SLICE’ single cam tribometer, which has incorporated a programmable dynamic speed and lubricant supply system, was designed and employed for the study of cam-follower tribochemistry. Information such as frictional torque and lubricant film thickness were obtained using a torque transducer and the Dowson mathematical model respectively.
XPS, RAMAN/FTIR, SEM/EDX surface analytical techniques are used to study the tribofilms. Properties of the tribofilms are evaluated with the aid of AFM, Nano-indenter and Surface Profilometry. Comparison of data with laboratory/conventional tribometers showed that the films had similar characteristics in the boundary lubrication regime and the friction data in a single cam rig closely mimics those in reciprocating pin-on-plate tribometers. The tribofilm was mapped in a unique spot wise manner on the cam and had similar trends with those of the plate in a PoP tribometer. A unique observation in this study was the effect of coating on cam wear. These values closely support those in the PoP reciprocating tribometer. This illustrates that reciprocating laboratory tribometers and bench test data can be used to establish how components in real engines may behave. The rig is capable of ranking candidate materials, surface coatings and fully formulated lubricants for valve train applications.