# Research on Testing Method of Oil Characteristic Based on Quartz Tuning Fork Sensor
## 3 Oil Ageing Processes and Sensors for Fluid Property Detection
### TBN, TAN and Ageing  
Regular, punctual TBN and TAN measurements are used in reference laboratories to externally monitor oil conditions. Aging and contamination processes induce the creation of different acidic compounds until oil degradation is complete. These acidic compounds accelerate oil degradation via different chemical reactions. For example, metal contamination can be increased by corrosion induced by acidic compounds. To counter the creation of acidic compounds and delay oil degradation, an alkaline reserve is added to the chemical composition of the oil. Thus, fresh oil has a high TBN value, typically ranging from TBN = 8 to TBN = 14, and a low TAN value, typically ranging from TAN = 1 to TAN = 4. During oil life, creation of acidic compounds leads to a continuous decrease in TBN and a continuous increase in TAN. When alkaline species are completely depleted, levels of acidic compounds increase dramatically, and general oil degradation is highly accelerated until the point of complete deterioration and jelling. Depending  
Figure 17: Fe contamination in engine oil (15W40) at 100 \({}^{\circ}\)C: (**a**) viscosity; (**b**) density; (**c**) dielectric; and (**d**) Rp. on oil composition and application characteristics, limits on TBN and TAN values can be considered to optimize oil changes. For example, some manufacturers recommend performing oil changes when TBN = TAN, thereby decreasing the risks of dramatic oil degradation and engine failure [46; 47]. Typical TBN and TAN dynamic behaviors are presented in Figure 18. TBN and TAN measurements are plotted as a function of oxidation. Engine oil (15W40) was stored at 150 \({}^{\circ}\)C for 3000 h and regularly sampled and analyzed until complete degradation. TBN and TAN were measured by an external reference laboratory. The oil samples correspond to the same samples used to determine the oxidation data presented in Figure 7.  
The results show a continuous decrease in TBN and a continuous increase in TAN as oxidation increases. The four different phases of oil degradation described in Figure 18 can also be considered. During Phase 1 and Phase 2, the TBN value was higher than the TAN value. At the end of Phase 2, the TAN value reached the TBN value and surpassed the TBN value. TBN and TAN measurements corresponding to the 38 A/cm sample from Figure 18 were not performed because of the excessive degree of oil degradation, from which the oil was indeed completely jelled. The correlation between the absolute value of the difference \(|\)TBN-TAN\(|\) and the resistivity measurement is presented in Figure 18. The results demonstrate the ability of the fluid property sensor to measure TBN and TAN values through resistivity measurements. As described in Figure 19, resistivity increased during Phase 1. Phase 1 is relatively short-lived, and the level of resistivity increase depends on the oil chemical composition and application characteristics. Then, the resistivity decreases during Phase 2 until it reaches a minimum and increases again during Phase 3. \(|\)TBN-TAN\(|\) has similar behavior, starting from a high value (because fresh oil has alkaline reserve and low acid concentration) and decreasing during Phase 2 until a minimum is reached, and then increasing during Phase 3 due to creation of acidic compounds and neutralization of alkaline species. A clear correlation (R\({}^{2}\) = 0.83) is demonstrated between resistivity and \(|\)TBN-TAN\(|\). A similar correlation has been described in the literature. \(|\)TBN-TAN\(|\) can be modeled using Equation (44) at a constant temperature:  
Figure 18: Engine oil (15W40) TBN and TAN were measured in a reference laboratory. Simultaneous resistivity measurements do not allow a precise direct measurement of TBN and TAN values. However, resistivity monitoring allows a prediction of \(|\)TBN\(-\)TAN\(|\) and detection of Phase 2 (when \(\frac{\partial\text{RP}}{\partial\text{t}}<0\), TBN > TAN) and Phase 3 (when \(\frac{\partial\text{RP}}{\partial\text{t}}>0\), TAN > TBN).