# Research on Testing Method of Oil Characteristic Based on Quartz Tuning Fork Sensor
## 3 Oil Ageing Processes and Sensors for Fluid Property Detection
### Water Contamination  
Water contamination comes from the fuel combustion process or from the exterior engine system. Water in oil can be dangerous for an engine because it could induce excessive wear, for example, by cavitation or corrosion. Water also promotes oil oxidation and the formation of acid byproducts and sludge, and thus poor engine reliability. Water has three different states in oil: dissolved water, emulsified water, and free water. Dissolved water is characterized by dispersed individual water molecules chemically linked to the oil. Above this level, called solubility, the excess water is saturated and will not be chemically linked to the oil; free water is formed, leading to a two-phase system. Because the density of water is higher than that of oil, free water typically accumulates at the bottom of the oil pan. When the mixing ratio is sufficiently high, or in the presence of surfactant additives, an emulsion can be formed. Emulsified water is homogeneously dispersed into the oil [40]. Figure 9: Engine oil (15W40) oxidation at 100 \({}^{\circ}\)C: (**a**) viscosity; (**b**) density; (**c**) dielectric; and (**d**) Rp. Solubility depends on temperature, oil additives, contamination, and oxidation [9]. Water has low solubility in pure oils and hydraulic oils. Typical solubility in engine oil is approximately 2000 ppm at 50 \({}^{\circ}\)C.  
The solubilities of water in hydraulic oil and fresh and aged 15W40 engine oils, as measured by a water content sensor, are presented in Figure 11.  
Water in oil is typically measured in the laboratory with Karl Fisher titration (ISO12937 or ASTM D1533). The Karl Fisher method measures total absolute water in ppm with high accuracy. However, this method is not totally appropriate with engine oil because the measurement could be impacted by specific engine oil additives. The Aqua test (ISO/DIS 9114 or ASTM D7358) and FTIR spectroscopy (ASTM D7418) are alternative methods for measuring total absolute water down to 100 ppm and with 100 ppm accuracy. These three  
Figure 11: Water solubility in hydraulic and engine oil measured by a water content sensor. Figure 10: Typical water solubility in engine oil and associated water states. methods do not differentiate dissolved or free water. A water content sensor is able to measure water activity and then to deduce solubility, independent of oil aging. Water activity can be defined by the following formula:  
\[\text{a}_{\text{Water}}=\frac{\left[\text{H}_{2}\text{O}\right]}{\left[\text{H}_ {2}\text{O}\right]_{\text{Saturation}}} \tag{15}\]  
where 0 \(<\) a\({}_{\text{Water}}\) \(<\) 1, and \(\left[\text{H}_{2}\text{O}\right]\) is the concentration of dissolved water in oil (ppm) and \(\left[\text{H}_{2}\text{O}\right]_{\text{Saturation}}\) is the concentration of dissolved water in oil (ppm) at the saturation point. The water content sensor and its water activity measurements would be the relevant sensor solution for measuring water content at the saturation point with the highest accuracy. The fluid property sensor water dilution measurement is presented in Figure 12. Engine oil (15W40) was stabilized at 50 \({}^{\circ}\)C and diluted with deionized water. Emulsion homogeneity was ensured by magnetic stirring. The results demonstrate the ability of fluid property sensors to detect and measure water contamination, mainly with dielectric and resistivity measurements. Figure 12: Water dilution in engine oil (15W40) at 50 \({}^{\circ}\)C: (**a**) viscosity; (**b**) density; (**c**) dielectric; and (**d**) Rp. Viscosity and density do not show any changes. The dielectric constant is linearly increased and the resistivity is linearly decreased with water dilution. These noticeable changes have been described in the literature and can be explained by the high dielectric constant of water (\(\epsilon_{\text{Water}}\)= 80) and the autoionization of water that can release free charge carriers, e.g., protons, into the oil.