23rd International Colloquium Tribology - January 2022 209 Thermal expansion influence on the scuffing initiation in a piston ring cylinder liner contact Simona Dahdah Univ Lyon, INSA-Lyon, CNRS UMR5259, LaMCoS, F-69621, France. Groupe PSA, Velizy Villacoublay, France. Corresponding author: simona.dahdah@insa-lyon.fr Nans Biboulet Univ Lyon, INSA-Lyon, CNRS UMR5259, LaMCoS, F-69621, France. Antonius Lubrecht Univ Lyon, INSA-Lyon, CNRS UMR5259, LaMCoS, F-69621, France. Pierre Charles Groupe PSA, Velizy Villacoublay, France. 1. Introduction The piston-ring/ cylinder-liner (PRCL) contact influ-ences the overall performance of internal combustion engine (ICE) as it causes half of the engine losses [1]. Hence, the importance to study the phenomena acting in this contact as well as its lubrication system. In addi-tion, the PRCL pack may be exposed to scuffing. Scuff-ing is a failure mode that can appear in a lubricated contact with sliding bodies [2]. It is accompanied by temperature and friction increase. Its occurrence in the ICE is rare but once it initiates, the damage is cata-strophic and permanent, thus the importance to predict its initiation. Several factors contribute to scuffing initiation. Oil quantity and contact temperature as well as the contact load are crucial parameters [3]. In the current study scuffing initiation is predicted through a thermal approach. The reduction of the lubricant film thickness increases the friction and the temperature leading to a thermal expansion. The latter in turn in-creases the contact load and the contact temperature [4]. This is the thermo-mechanical effect. Moreover, the temperature gradient generates a surface tension gradient and a lubricant convection can take place leading to a temporarily local lack in lubricant. These two effects accentuate the scuffing initiation risk in the PRCL contact. On the contrary, the ring passage along the liner redistributes the oil in the contact and helps to avoid starvation [5]. This effect delays the scuffing initiation. This study aims to model the contribution of the mentioned effects to scuffing occurrence. 2. Model 2.1 Marangoni effect Figure 1: Marangoni effect model Figure 1 describes the Marangoni effect, the lubricant flows from the low surface tension zone (γ low) to the high surface tension zone (γ high). The oil convection leads to the appearance of a localized lack of lubricant. The oil quantity moved due to the Marangoni effect is quantified as a surface as a function of the temperature and surface tension gradients. In addition, the lubricant redistributed due to the ring passage is also quantified as a moved surface. Both surfaces are compared is order to predict the scuffing initiation limit. 210 23rd International Colloquium Tribology - January 2022 Thermal expansion influence on the scuffing initiation in a piston ring cylinder liner contact 2.2 Thermo-mechanical effect Figure 2: Thermo-mechanical effect loop The thermo-mechanical effect is modelled via a loop shown in figure 2. Assuming that a temporarily local lack of lubricant occurs in the contact, the coefficient of friction increases and an additional heat flux is generat-ed that causes an increase in the contact temperature. The temperature gradient leads to a ring deformation, and an additional load is generated. The oil film thick-ness is then calculated and the coefficient of friction is defined via the Stribeck curve. The loop runs until the contact temperature re-stabilizes to the nominal tem-perature or it exceeds the lubricant additive desorption limit. 3. Main results Different cases are given in table 1. The red color in the table indicates the scuffing initiation risk whilst the green one refers to the safe zone where no risk of scuffing exists. When the contact temperature exceeds the limit temperature of the oil additive system desorption (Tlimit=180°C) or when the surface displaced due to the Marangoni effect is larger than the redistribution due to the ring passage, scuffing risk exists. Otherwise, scuff-ing initiation is prevented. The correct way to read the table is to compare the cells with the same color of the input parameter, which is the only parameter varied between the two cases. By comparing case 2 to case 1, the initial temperature increase varies. In case 1, the increase of 15°C did not lead to scuffing initiation, and the contact opeartes in the safe zone. Whilst, in case 2, the temperature increase is suffisient to cause scuffing and the contact no longer operates in the safe zone. Case 3 is also compared to case 1, the oil supply regime carries the difference out. When the lubricant film thickness decreases, the coefficient of friction in-creases, as well as the generated heat. A transition from the safe zone tot he risk zone occurs. To study the effect of the heated zone width, case 4 is compared to case 2. For the same temperature increase, scuffing is delayed when the width of the heated zone increases. In case 4, scuffing is prevented and the contact operates in the safe zone. A transition from the safe zone to the risk zone takes place between case 4 and case 5 when the lubricant supply regime becomes more severe. 4. Notation Table 2: Problem parameters Symbol Parameter T Temperature [°C] ΔT Temperature increase [°C] p contact Contact pressure [pa] p additional Additional pressure [pa] μ Coefficient of friction [-] u m Mean velocity [m/ s] v Lubricant velocity [m/ s] q Heat flux [w/ m2] h 0 Oil film thickness [m] h mean Mean oil thickness [m] s Heated zone width [mm] λ Oil distribution wavelength [mm] σ Surface roughness [m] γ Surface tension [N/ m] η Oil viscosity [pa.s] 5. Conclusion This study models the interaction between the tempera-ture, the oil film thickness and the contact load. The scuffing initiation limit is predicted as a function of three effects: the thermo-mechanical effect, the Marangoni effect and the ring displacement effect. The first two effects 23rd International Colloquium Tribology - January 2022 211 Thermal expansion influence on the scuffing initiation in a piston ring cylinder liner contact accentuate the scuffing initiation whilst the third one contributes to delay the scuffing risk. The three of them depend on the problem operating parameters. References [1] S.C.Tung, and M.L.McMillan, “Automotive tribology overview of current advances and challenges for the he future,” Tribology International, 37, 3, 2004, 517-536. [2] F.Saedi, S.Shevchik, and K.Wasmer, “Automatic detection of scuffing using acoustic emission,” Tribo-logy International, 94, 2016, 112-117. [3] P.Obert, T.Mlüler, H.J.Füßer, and D.Bartel, “The influence of oil supply and cylinder liner temperature on friction, wear and scuffing behavior of piston ring cylinder liner contacts-a new model test,” Tribology International, 94, 2016, 306-314. [4] C.Fang, X.Meng, X.King, B.Zhao, and H.Huang, “Transient tribo-dynamics analysis and friction loss evaluation of piston during cold-and warm-start of a SI engine,” International Journal of Mechanical Sciences, 133, 2017, 767-787. [5] M.Organisciak, G.Cavallaro, and A.Lubrecht, “Variable lubricant supply of a starved hydrodynamic line-ar contact: lubricant lateral flow for smooth and laser textured surfaces,” Proceedings of the Institution of Mechanical Engineers, Journal of Engineering Triboln ogy, 221, 3, 2007, 247-258. Table 1: Different cases Parameter Unit Case 1 Case 2 Case 3 Case 4 Case 5 s=λ mm 4.4 4.4 4.4 6.25 6.25 T 0 °C 90 90 90 90 90 ΔT °C 15 20 15 20 20 Oil supply regime mean oil supply regime mean oil supply regime mean oil supply regime mean oil supply regime mean oil supply regime h 0 =h mean m 8e -7 4.3e -8 3.4e -7 7e -7 4e -8 T °C 90 182 182 90 182 Surface Marangoni mm 2 2e -7 1.6e -6 9.5e -7 1.4e -7 7e -7 Surface ring passage mm 2 5e -9 1.2e -5 8e -6 3.4e -9 4e -6 Operating condition no risk of scuffing risk of scuffing risk of scuffing no risk of scuffing risk of scuffing