# Research on Testing Method of Oil Characteristic Based on Quartz Tuning Fork Sensor
## 2 Fluid Property Sensing
### Engine Oil
Detergents are used to remove, or avoid formation of, carbon, varnish, or lacquer deposits and to neutralize acids created from combustion, oxidation, and nitration processes. Indeed, contaminating acids are dangerous because they can cause corrosion and accelerate oil degradation. Engine oil also provides alkaline reserves to prevent acidification. The alkaline reserve can be determined by the value of the total base number (TBN). In the same way, acidity can be determined by the total acid number (TAN). The pH value is not suited for measurement in oil because the medium is not aqueous. The unit for TAN and TBN measurement is mg KOH/g, which corresponds to the equivalent mass of potassium hydroxide required to neutralize one gram of solution. Neutralization is ensured by the use of detergents. Some over-based detergents composed of calcium carbonate or magnesium carbonate achieve a high level of protection against acidity. However, they can lead to ash formation, which is problematic for diesel particulate filters. Manufacturers tend to use engine oils with a relatively low ash content, meaning lower TBN and alkaline reserves; this reduces resistance to chemical degradation. During oil aging, polar compounds and charge carrier degradation products are created. In parallel, base additive compounds are depleted, decreasing protection against acidification. The electrical and chemical behaviors of these substances have a direct impact on the oil dielectric constant (\(\epsilon\)) and resistivity (P). The dielectric constant (\(\epsilon\)) or relative permittivity is the ratio of media permittivity to vacuum permittivity. The dielectric constant represents the capacity of the medium to be polarized under the application of an electric field. Concretely, polarization is the consequence of the reorientation of molecular dipoles in the lubricant. During oil aging, dipole moments change because of microscopic chemical reactions, such as oxidation or nitration, or because of the addition of polar contaminants, such as water (\(\epsilon\) water = 80) or soot. The dielectric constant (\(\epsilon\)) is defined according to the following equation:  
\[\epsilon=\text{F}+\text{G.T} \tag{8}\]  
where T is the temperature in \({}^{\circ}\)C and F and G are oil-dependent constants. Indeed, the dielectric constant (\(\epsilon\)) can be linked with the density (\(\rho\)) using the following relationship (9):  
\[\frac{\epsilon-1}{\epsilon+2}=4.\pi.\rho\.\text{Na.}\frac{\alpha}{3.\text{M}} \tag{9}\]  
where Na is Avogadro's number, \(\alpha\) is the molecular polarizability volume, and M is the molar mass of the substance. The resistivity (P) of a medium measures the drag force encountered by free charge carriers moving through the medium under the application of an electric field. Oil resistivity is dependent on fluid viscosity, the density of free charge carriers and the specific chemical composition of the oil. To a first approximation, resistivity (P) can be modeled in a simple linear form:  
\[\text{P}=\text{H}+\text{LT} \tag{10}\]  
where T is the temperature in \({}^{\circ}\)C and H and I are oil-dependent constants. General measurements of oil temperature are presented in Figure 6. Two engine oil samples (new 15W40 and 1000 h aged) were measured in the standard engine oil application temperature range. Temperature homogeneity was ensured by mechanical stirring. The results demonstrate the ability of the fluid property sensor to measure oil viscosity (\(\eta\)), density (\(\rho\)), dielectric (\(\epsilon\)) and Rp as an indicator of resistivity (P) over the entire range of application temperatures. Viscosity, density, and dielectric constants are increased during aging. Without any contamination, increases in the three parameter can be considered normal, even if we also observe viscosity decreases due to oil shear. The increase in resistivity at high temperatures is due to depletion of free charge carrier additives [35].