# Research on Testing Method of Oil Characteristic Based on Quartz Tuning Fork Sensor
## 3 Oil Ageing Processes and Sensors for Fluid Property Detection
### Oxidation  
When exposed to high temperature, engine oil reacts with oxygen and this reaction is called oxidation. Oil molecules chemically combine with oxygen to form carbonyl compounds (C=O), such as aldehydes, ketones, esters, and carboxylic acids, in different concentrations. The process actually corresponds to two degradations: the evaporation of minor oil components and the incorporation of oxygen elements in oil components. This oxidation induces the polymerization of oil molecules and the formation of insoluble products that lead to an increase in viscosity. Oxidation also induces the formation and depletion of various polar and conductive compounds, which will impact the oil dielectric constant and resistivity [36]. Figure 6: New and aged engine oil (15W40): (**a**) viscosity; (**b**) density; (**c**) dielectric; and (**d**) Rp. The oxidation process is accelerated by heat, light, and metal catalysts, such as copper or iron, and the presence of water, acids, or solid contaminants that could cause deposit formation. Oxidation is also favored by oil shear. Hydrocarbon compounds and polymer viscosity additives are degraded by shearing, creating carboxylic acids and carbon active sites that can be oxidized to induce a new polymerization. Eventually, oxidation is accelerated by biodiesel or ethanol fuels due to their hygroscopic properties. Oxidation was measured in the laboratory using Fourier transform infrared (FTIR) spectroscopy as specified by the standard test methods ASTM D7418 or ASTM D7214. The measurement is based on quantification of the carbonyl functional group (C=O), using the 1700 cm\({}^{-1}\) frequency peak in the absorbance spectrum. Oxidation measurement is expressed in A/cm. The measurement of fluid oxidation with the property sensor is presented in Figure 7. Engine oil (15W40) was stored in the laboratory at 120 \({}^{\circ}\)C for 3000 h and regularly sampled and analyzed until the point of complete degradation. The results demonstrate the ability of the fluid property sensor to detect and measure oil oxidation, mainly with viscosity, dielectric, and resistivity measurements. Figure 7: Engine oil (15W40) oxidation at 120 \({}^{\circ}\)C: (**a**) viscosity; (**b**) density; (**c**) dielectric; and (**d**) Rp. Viscosity is highly increased during all oxidation processes. Resistivity is a very useful parameter because it can be directly linked to the four different phases of the oil oxidation process (see Table 2). During Phase 1, the tested oil resistivity is increased by up to 10% at 120 \({}^{\circ}\)C. This phenomenon can be explained by the depletion of some free charge carrier additives. The viscosity starts its continuous increase, and the dielectric shows a very slight drop. Phase 1 corresponds to the predominance of a fast chemical process involving very reactive additives. During Phase 2, the tested oil resistivity is decreased up to \(-\)10% at 120 \({}^{\circ}\)C, relative to the initial value, and the dielectric showed a significant increase. Phase 2 corresponds to a predominance of a slow chemical process that releases charge carriers and polar compounds. During Phase 3, the resistivity shows a new increase linked to a new depletion of charge carriers, and the dielectric is stabilized. Phase 4 corresponds to the dramatic increase in viscosity before complete oil degradation and potential jelling [37]. Even if the density changes remain relatively slight, they can be linked to the four different oxidation phases. The four different oil degradation phases and corresponding viscosity and resistivity behaviors are described in the literature. Temperature has a relatively low impact on the four oxidation behaviors, and the relative changes, except those for resistivity, are similar for each temperature. The phase 1 resistivity increase is amplified at high temperatures and moderated at lower temperatures. During Phase 1 and Phase 2, the oil condition is considered good, and the level of degradation is quite low. Phase 3 corresponds to a consequent level of oil degradation.