23rd International Colloquium Tribology - January 2022 97 Oil nitration in a large-scale device for artificial alteration Adam Agocs AC²T research GmbH, Wiener Neustadt, Austria Corresponding author: adam.agocs@ac2t.at Charlotte Besser AC²T research GmbH, Wiener Neustadt, Austria Marcella Frauscher AC²T research GmbH, Wiener Neustadt, Austria Nicole Dörr AC²T research GmbH, Wiener Neustadt, Austria 1. Introduction Lubricant condition and tribological performance in an internal combustion engine are strongly interrelated. Trends in engine development are predominantly focusing on lower emissions, which are often achieved by higher compression ratios and turbocharged engines. This, combined with larger mileage oil change intervals, requires increasing stability of engine oils. Several artificial alteration methods are available to simulate lubricant degradation [1], however, nitration of the engine oils is often overlooked in standardized tests. Nitration is of great interest, as petrol and diesel passenger vehicles show significant differences in this regard [2-3], which might be tied to emission values. Diesel engine oils generally exhibit lower nitration [2-3], possibly due to the higher combustion temperature in compression ignition (Diesel) engines, as nitration products show a rapid decomposition above 160 °C [2]. Furthermore, large quantities of test oils with defined and reproducible degrees of degradation are necessary to make reliable predictions on performance and lifetime of engine components. Hence, the provision of test oils by artificial simulation including nitration is required [1]. The study at hand presents a novel alteration method, where up to 200 liters of ‘used’ oil can be produced for the purpose of engine bench testing or component testing in the development phase. This method is capable of close-to-reality simulation of nitration, oxidation, and additive degradation of engine oils. Comparability regarding relevant chemical degradation parameters of the altered oils with used oils from a modern turbocharged petrol vehicle is presented. 2. Materials and methods 2.1 Engine oil A commercially available 5W-30 engine oil suitable for both diesel and petrol engines with approvals for ACEA C3, API SN, BMW longlife-04, MB 229.51, MB 229.52, VW 502.00/ 505.00/ 505.01, and GM dexos2™ was selected. The engine oil formulation is relatively conservative, it contains calcium carbonate as base reserve, both phenolic and aminic antioxidants (AOs) and zinc dialkyldithiophosphate (ZDDP) as an antiwear additive (AW), but no boron-based AW or molybdenum-based friction modifiers, such as molybdenum dithiocarbamate (MoD- TC) can be detected. 2.2 Field test A field test using a conventional passenger car was conducted, where the corresponding results are described in detail in [4]. The selected vehicle was equipped with a modern, turbocharged petrol engine with 1.4 L displacement and 88 kW maximal power output. The car was operated under real driving conditions, predominantly at high-speed (freeway) utilization, which resulted in approx. 20,000 km total mileage in 8 months, accordingly, comparable to a typical oil change interval. 2.3 Artificial alteration The artificial alteration was conducted similarly to the methodology presented in [1]. In doing so, 180 L fresh oil was inserted in a jacketed steel reactor. The oil sample was kept under constant stirring, tempered between 110 °C and 140 °C and subjected to a metered air flow containing 0 ppm to 3000 ppm NO 2 . The experimental parameters were continuously adjusted to achieve the 98 23rd International Colloquium Tribology - January 2022 Oil nitration in a large-scale device for artificial alteration desired ratio of oxidation and nitration in the alteration system. Samples were taken at regular intervals for subsequent oil analysis, where the results were continuously used to modify the experiment parameters (feedback). The goal of the study was to reproduce the condition of the final used oil sample from the field test, corresponding to approx. 20,000 km mileage in a timely and cost-efficient manner. 2.4 Oil condition monitoring The following oil parameters were determined for both the field test as well as the artificially altered samples: • Oxidation and nitration via Fourier-transformed infrared spectroscopy (FT-IR) according to the methods described in [2-4] • Residual amounts of phenolic and aminic AOs as well as ZDDP compared to the fresh oil, based on the FT-IR spectra, using the methods described in detail in [2-4] • Neutralization number (NN) according to DIN ISO 6618 [5] and total base number (TBN) according to DIN ISO 3771 [6] 3. Results and discussion Figure 1 displays the oxidation and nitration of the final used oil sample (field test) as well as the final artificially altered product. Both parameters are showing a very close agreement, with differences in the range of 0.1 - 0.2 A/ cm only. Figure 1: Oxidation and nitration of the final used and artificially altered oil samples The residual additive content of the final field test sample and the artificially altered sample are presented in figure 2. ZDDP and both phenolic and aminic AOs show severe degradation during the lifetime of the lubricant. Comparable to oxidation and nitration, the used and artificially altered samples are showing very similar final values, which supports the analogous oil condition in both samples. Figure 2: Residual ZDDP as well as phenolic and aminic AO content compared to the fresh oil in the final used and artificially altered samples. Figure 3 shows the NN and TBN of the fresh oil, as well as of the final used and altered oil samples. The NN increases during both the field test and the artificial alteration, generally due to the production of organic acids due to oxidation reactions [2; 4]. Subsequently, the TBN decreases as the base reserve partially neutralizes said organic acids. [2; 4]. The field test and the artificial alteration are showing good comparability, once again highlighting the similarity of the artificial alteration with the real field samples. Figure 3: NN and TBN of the fresh as well as the final used and artificially altered oil samples Furthermore, it has to be highlighted that the artificial alteration only required approx. 10 days to simulate the degradation occurring during 20,000 km utilization. 4. Conclusion The obtained final artificially altered oil sample showed very good comparability to the used oil sample selected as reference in regard of oxidation, nitration, ZDDP and AO depletion, NN accumulation and TBN reduction which suggests a very good overall chemical comparability. Accordingly, it was proven that the presented artificial alteration method is highly suitable for the rapid production of test oils in large quantities. 23rd International Colloquium Tribology - January 2022 99 Oil nitration in a large-scale device for artificial alteration References [1] Besser C. et al.: Generation of engine oils with defined degree of degradation by means of a large scale artificial alteration method. Tribol Int 2019; 132: 39-49. https: / / doi.org/ 10.1016/ j.triboint. 2018.12.003. [2] Agocs A. et al.: Comparing oil condition in diesel and gasoline engines. Ind Lubr Tribol 2020; 72: 1033-9. https: / / doi.org/ 10.1108/ ILT-10- 2019-0457. [3] Agocs A. et al.: Comprehensive assessment of oil degradation patterns in petrol and diesel engines observed in a field test with passenger cars - Conventional oil analysis and fuel dilution, Tribol Int 2021; 161. https: / / doi.org/ 10.1016/ j.triboint. 2021.107079. [4] Dörr N. et al.: Engine oils in the field: a comprehensive chemical assessment of engine oil degradation in a passenger car. Tribol Lett 2019; 67: 68. https: / / doi.org/ 10.1007/ s11249-019-1182-7. [5] DIN ISO 6618: 2015-07. Petroleum products and lubricants - Determination of acid or base number - Colour-indicator titration method. Berlin: Deutsches Institut für Normung; 2015. [6] DIN ISO 3771: 1985-04. Petroleum products; total base number; perchloric acid potentiometric titration method. Berlin: Deutsches Institut für Normung; 1985. [7] Agocs A. et al.: Production of used engine oils with defined degree of degradation in a large-scale device. Acta Tech Jaurinensis 2020; 13(2): 131-50. https: / / doi.org/ 10.14513/ actatechjaur.v13.n2.546.