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Optimizing the Piston/Bore Tribology: The Role of Surface Specifications, Ring Pack, and Lubricant
ISSN: 0148-7191, e-ISSN: 2688-3627
To be published on September 15, 2020 by SAE International in United States
The continuing pursuit for better fuel efficiency stands behind many recent advancements in engine technology. "Downsize and charge" has become the major development paradigm alongside broad acceptance of fuel stratified injection, variable valvetrain, cylinder deactivation, 48V electric auxiliaries, powertrain hybridization, use of low-friction coatings and advanced surface finishing methods in component manufacture, etc. The introduction of higher power densities (in excess of 100 kW/L and 200 Nm/L in modern engines) raises performance requirements for engine oil. At the same time, the introduction of high-speed diesel engines justifies the move towards lower oil viscosities in order to reduce on-road greenhouse gas (GHG) emissions. Advanced surface finishing methods, such as self-lubricated hard coatings, mirror-like thermally sprayed bores, mechanochemical surface finishing, helical slide honing etc. as well as availability of high-sensitivity testing rigs and "digital twin" simulation tools create new opportunities for engine tribology optimization. On the lubricant side, there is an increasing use of synthetics due to their superior performance. New additives types are also coming to the market to address new performance challenges. In particular, superlubricity additives should be mentioned. However, there exists a certain gap, both in attitude and competence, between university researchers and lube industry professionals as their willingness to venture out for new technologies is concerned. A narrow-sighted one-component-at-a-time optimization approach still prevails at many OEMs due to their internal organizational structure. However, there are many examples proving that the one-component optimization is often nothing less than misleading, necessitating the need for a complete system approach. For instance, there is no point in optimizing the friction of the top ring without taking into account the oil control ring, as all rings in the ring pack work together. Also, what is optimal for a high-speed short-stroke engine is not necessarily optimal for a low-speed long-stroke engine. In the present communication, we will demonstrate such a "system approach" in action. We will present experimental data and simulation results demonstrating the role of GD&T, surface specifications, ring pack, and lubricant on the piston/bore tribology. Friction, wear and combustion chamber sealing will be considered and the shortcomings of one-component approach demonstrated. The importance of in-design "pairing" of low-viscosity motor oils with the ring pack and the cylinder bore characteristics in order to achieve maximum reduction in GHG emissions and improvement in fuel economy without sacrificing the endurance will be elucidated. References: E. Tomanik et al. Combined lubricant-surface system approach for potential passenger car CO2 reduction on piston-ring-cylinder bore assembly, in press, Available online 21 December 2018, Tribology International, https://doi.org/10.1016/j.triboint.2018.12.014 B. Zhmud, Low-Friction Powertrains: Current Advances in Lubricants and Coatings, in Proc. Engine Expo 2017, June 20-22, 2017, Stuttgart, Germany. B. Zhmud, In-manufacture Running-in of Engine Components by Using the Triboconditioning® Process, in Proc. the 7th International Conference on Fracture Fatigue and Wear, July 9-10, 2018, Ghent, Belgium, pp. 671-681.