23rd International Colloquium Tribology - January 2022 213 A fast Piston-Ring/ Cylinder-Liner friction prediction based on a semi-analytical hydrodynamic model and real measured surface topography Thomas Lubrecht Univ Lyon, INSA-Lyon, CNRS UMR5259, LaMCoS, F-69621, France. IREIS, HEF GROUPE, Andrézieux-Bouthéon F42160, France. Corresponding author: thomas.lubrecht@insa-lyon.fr Nans Biboulet Univ Lyon, INSA-Lyon, CNRS UMR5259, LaMCoS, F-69621, France. Antonius Adrianus Lubrecht Univ Lyon, INSA-Lyon, CNRS UMR5259, LaMCoS, F-69621, France. Johnny Dufils IREIS, HEF GROUPE, Andrézieux-Bouthéon F42160, France. 1. Introduction It is no longer possible to ignore the impact of the human activity on the earth global warming. In 2018, transport sectors relying on Internal Combustion Engines (ICE) technologies have emitted about 6.8 Gt of CO2 (about 18% of worldwide CO2 emissions) [1-2]. For decades, tribologist have worked to reduce ICEs friction and thus improve their efficiency and reduce their polluting emissions [3]. Regarding todays context, industrials and laboratories have shown even more interest in improving the tribological performance of the piston assembly which accounts for about 50% of the total friction losses in ICEs [3]. Within the piston assembly, the Piston-Ring / Cylinder- Liner (PRCL) contact is the most susceptible to generate significant friction losses because of its tough operating conditions. Numerical simulations have the advantage of being cheap and fast compared to full engine tests. For these reasons many PRCL friction prediction models have been elaborated [4-6]. However, full engine tests give global results while numerical methods mainly focused on one physical phenomenon. For example, complex solvers have been developed trying to understand the hydrodynamic contact physics [7]. Whereas simplified stochastic methods have been used to solve the dry contact challenge [8-10]. According to the authors, in order to be attractive for industrials a simulation tool has to be easy, fast, robust while being consistent with the physics involved. Therefore, a new approach to PRCL friction prediction has been computed. 2. Method It is well known that the PRCL contact operates in the mixed and hydrodynamic lubrication regime [11]. Hence, to correctly simulate the contact a combined hydrodynamic/ dry solver model has been elaborated. 2.1 Hydrodynamic contact Based on the Iso-Viscous-Rigid transient Reynolds equation for smooth starved contacts, the ring force balance and the oil flow balance at the contact, N. Biboulet et al. [12] developed a system of four equations and four unknows. Using a non-linear solver, they managed to compute the ring flying height for a variety of operating conditions. The main benefits of this method are the direct consideration of the oil starvation, the oil transport (i.e. oil accumulation) and the squeeze effect. 2.2 Dry contact Usually, PRCL friction simulation relies on stochastic dry contact theories such as developed by Greenwood & Williamson [8] or Greenwood & Tripp [9]. These theories are easy and quick to compute but they are based on non-measurable surface parameters. In order to avoid this, a different approach is suggested. First, a surface topography is measured as shown in Figure 1. Secondly, the measured topography is used as an input in a highperformance numerical tool [13] computing the load / separation-height curve shown in Figure 2. Lastly, the previous curve is interpolated allowing a direct assessment of the load carried by the solid asperities at a given flying height. Only a few iterations are required to com- 214 23rd International Colloquium Tribology - January 2022 A fast Piston-Ring/ Cylinder-Liner friction prediction based on a semi-analytical hydrodynamic model and real measured surface topography pute the ring minimum film thickness fulfilling the ring force balance, considering hydrodynamic and solid contact physics. Figure 1: Measured liner topography Figure 2: Load-separation curve (circle: load, quares: real contact area ratio) 3. Results Figure 3 shows typical results computed using the previously described method, highlighting a continuous transition through lubrication regimes. One can observe the film squeeze effect at the bottom and top dead centre where the film thickness is not zero. At mid-stroke, oil starvation due to limited oil supply is studied and the ring flying height is steady. The friction is mainly generated by viscous forces. During the combustion stroke (at about +10° crankshaft angle) solid contact between the ring and liner occurs due to the incylinder pressure rise. The friction is dominated by solid contact. Figure 3: Compression ring friction (solid line) and minimum oil film thickness (dashed line). 4. Conclusion Based on a combined approach, a PRCL friction prediction model dedicated to industrial handling has been developed. Key model outcomes are listed below: • Simulations allow a fast and reliable prediction of • PRCL friction for various operating conditions. • Coupling between the semi-analytical hydrodynamic • model and the dry contact model relying on • measured surface topography shows a coherent • transition throughout the different lubrication regimes. • The solid contact model presented offers a fast and • physical meaning alternative to usual stochastic • contact theories. References [1] IEA, “Tracking Transport 2020”, IEA, Paris, 2020. [2] Friedlingstein et al., “Global Carbon Budget 2021”, Earth Syst. Sci. Data Discuss, [preprint], in review, 2021. [3] Holmberg, Kenneth, Peter Andersson, and Ali Erdemir, “Global Energy Consumption Due to Friction in Passenger Cars.”, Tribology International 47, 2012, 221-34. [4] T. Tian, V. W. Wong, and J. Heywood, “A piston ring-pack film thickness and friction model for multigrade oils and rough surfaces”, SAE Technical Paper, vol. 962032, 1996. [5] E. Tomanik, “Modelling the hydrodynamic support of the cylinder bore and piston rings with laser textured surfaces”, Tribology international, 59, 90-96, 2013. 23rd International Colloquium Tribology - January 2022 215 A fast Piston-Ring/ Cylinder-Liner friction prediction based on a semi-analytical hydrodynamic model and real measured surface topography [6] R. I. Taylor, “Squeeze film lubrication in piston rings and reciprocating contacts”, Proc IMechE Part J: J Engineering Tribology, 229, 8, 977-988, 2015. [7] Noutary, M.-P., N. Biboulet, and A.A. Lubrecht, “A Robust Piston Ring Lubrication Solver: Influence of Liner Groove Shape, Depth and Density.”, Tribology International 100, 2016, 35-40. [8] J. A. Greenwood, and J. P Williamson, “Contact of nominally flat surfaces.”, Proceedings of the royal society of London. Series A. Mathematical and physical sciences, 295, 1966, 300-319. [9] J. A Greenwood, and J. H. Tripp, “The Contact of Two Nominally Flat Rough Surfaces.”, Proceedings of the Institution of Mechanical Engineers 185, 1, 1970, 625-33. [10] B.N.J Persson, “Contact Mechanics for Randomly Rough Surfaces.”, Surface Science Reports 61, 2006, 201-27. [11] Heywood, John B, “Internal Combustion Engine Fundamentals.”, New York: McGraw-Hill, 1988. [12] N. Biboulet and A.A. Lubrecht, to be published, 2021. [13] Sainsot, P, and A. A. Lubrecht, “Efficient Solution of the Dry Contact of Rough Surfaces: A Comparison of Fast Fourier Transform and Multigrid Methods.”, J 225, 8, 2011.