24th International Colloquium Tribology - January 2024 61 Base Oil Solvency and High Temperature Deposit Formation in Gas Engine Oils - a Model Study - Thomas Norrby 1* , Marcella Frauscher 2 , Christoph Schneidhofer 3 , Frans Nowotny-Farkas 4 1 Nynas AB, MS Technical Support & Development, Nynashamn, Sweden, 2 AC2T, Lubricants & Interface Mechanisms, Wiener Neustadt, Austria 3 AC2T, Oil Condition Sensors, Wiener Neustadt, Austria 4 LubEx Consulting, Schwechat, Austria * Corresponding author: E-mail thomas.norrby@nynas.com 1. Introduction The ever-growing demand for electric power results in the need to further improve engine efficiencies by increasing the power density of stationary gas engines. Thus, newer generations of gas engines feature an innovative design, e.g., including steel pistons, and operate at higher BMEP (Break Mean Effective Pressure). Due to the resulting higher operating temperatures, gas engine oils are subjected to higher stress, leading to increased deposit formation and more intense thermo-oxidative degradation. Both factors, among others, promote undesirable consequences like shorter oil change intervals, more engine downtime, and even engine failure [1, 2]. Recent trends in engine oil formulation, i.e., the use of highly refined paraffinic base oils also in gas engine oil formulation have not always been successful, if care is not taken to ensure sufficient solvency of the base oil blends employed. This study focuses on investigating the tendency of various blends of paraffinic and naphthenic base oils to form high-temperature deposits using a novel test rig design that we have called the „oil chute“ laboratory test for deposit formation tendency [3]. The laboratory investigations give clear indications on how base oil solvency, as described by standard test methods e.g., Aniline Point (ASTM D 611), correlated to deposit formation when the oil passes over hot metal. 1.1 Experimental methods 1.1.1 The Oil Chute The Oil Chute setup, Figure 1, circulates oil through two temperature zones, where deposit formation is triggered in a heated aluminium metal U-shaped profile, the “Oil Chute”. A pump circulates the oil which flows under gravity down the slope of the U-profile. The mass of deposits formed was determined gravimetrically. The detailed test parameters are given in the following: • U-channel temperature: 300-°C • Water cooling temperature: 40-°C • Sample amount: 85 g • Oil flow rate: ~ 7 mL/ min • Test duration: 21 h The temperatures of the hot and cold zone were chosen following the real-life conditions in a gas engine. Figure 1: Oil chute setup Typical sump and hot zone temperatures of gas engines with aluminium pistons are in the range of around 85-°C and 260- 275-°C, respectively. Modern gas engines with steel pistons reach temperatures of approximately 95-°C in the sump and about 265-290-°C in the upper piston ring area. 1.1.2 Oil test matrix A selection of base oil properties is given in Table 1. P-1 is a Group I paraffinic base oil; P2and P-3 are Group II; P-4 is a Group III; and N 1 and N-2 are Group V Naphthenic base oils. Note the very significant differences is VI, and solvency as represented by the Aniline Point. Table 1: Base oils used for model blends Designation KV 40 (cSt) ASTM D 7042 VI (-) ASTM D 7042 Aniline Point (°C) ASTM D 611 P-1 340 91 118 P-2 42.2 107 116 P-3 104.1 106 127 P-4 48.2 135 129 N-1 358 40 98 N-2 607 -20 92 In Tabe 2, a selection from the full test matrix is reported. Note that pure Paraffinic blends, or Naphthenic/ Paraffinic 62 24th International Colloquium Tribology - January 2024 Base Oil Solvency and High Temperature Deposit Formation in Gas Engine Oils - a Model Study - bends, and varying treat rates of the gas engine oil additive package (GEO-AP) have been utilized for the model gas engine oil candidates. Table 2: Base oil and additive selection for GEOs 1.1.3 Thermo-oxidative stability Utilizing the Oil Chute, the thermo-oxidative stability of the oil blends was examined using an artificial alteration method. Oxidation stability (acidity, viscosity change) varies, with the more highly paraffinic blend having an edge, but where the paraffinic/ naphthenic blends still show acceptable behaviour. A correlation was found between solvency, or oxidation stability, and the resulting deposit formation at end of test. Regarding high-temperature deposit formation, Figure 2 shows photos from above of the oil chute profiles at end of test. The oil blends containing naphthenic components (GEO Exp 11 & 12) yielded very low deposit levels and, thus, represent a significant improvement compared with purely paraffinic oil blends (GEO Exp 5). Moreover, their deposit-formation tendency accords with that of highly rated reference gas engine oil (GEO REF 7), meaning that they offer an advantage over many conventional gas engine oils. Figure 2: Photos showing extent and nature of deposits formed at end of test. 2. Conclusion Thus, adding selected high viscosity, high solvency naphthenic base oils to a formulation offers a direct route to gas engine oils with improved technical performance. It is also cost-effective in terms of addressing the cleanliness and deposit formation issues via base oil selection compared with using higher additive pack treat rates or boosters. However, the model blends comprising a naphthenic base oil exhibits somewhat faster degradation under thermo-oxidative stress. Consequently, achieving an optimal balance between paraffinic and naphthenic base oil components will enable the formulation of gas engine oils that possess the necessary properties to withstand the elevated stress levels experienced in modern gas engines. Funding: This research was funded by the “Austrian COM- ET-Program” (project InTribology1, no.-872176) via the Austrian Research Promotion Agency (FFG) and the federal states of Niederösterreichand Vorarlberg. References [1] Garcia, L.; Reher, J. Latest Generation of High Performance Gas Engine Oils - Tackling Reliability Challenges and Extending Oil Life in Modern Highly Efficient Gas Engines. In Proceedings of the CIMAC Congress; Vancouver, Canada, June 2019. [2] Hughes, J. Development of a New Lubricating Oil for Use in Modern High Efficiency Gas Engines. In Proceedings of the CIMAC Congress; Vancouver, Canada, June 2019. [3] Ronai, B.; Schneidhofer, C.; Novotny-Farkas, F.; Norrby, T.; Li, J.; Pichler, J.; Frauscher, M. ‘Assessing the High-Temperature Deposit Formation of Paraffinic and Naphthenic Oil Blends Using the Oil Chute Method‘. Lubricants 2022, 10, 327. https: / / doi.org/ 10.3390/ lubricants10120327