# Research on Testing Method of Oil Characteristic Based on Quartz Tuning Fork Sensor
## 2 Fluid Property Sensing
### Engine Oil  
Engine oils are composed of paraffinic, naphthalene, and aromatic hydrocarbons mixed with different additives and elaborated to fulfill several functions: lubrication, sealing, power transmission, engine part cooling, cleaning, and neutralization of acid components. The objective of lubrication is to reduce the friction between sliding surfaces and minimize wear. Insufficient lubrication leads to engine heating, oil degradation, and acceleration and higher risk of engine failures. Sealing between cylinders and piston rings is necessary to avoid blow-by fuel contamination and nitrogen oxide contamination from the combustion gas [30]. Sealing is also required between the valve and valve stem guides to prevent unburned fuel from reaching the oil. Power transmission increases engine efficiency and ensures tappet clearance and camshaft adjustment. Proper lubrication, sealing, and power transmission are ensured by appropriate oil viscosity. The viscosity of a fluid is a measure of its resistance to gradual deformation by shear stress. Dynamic viscosity can be defined by considering a fluid trapped between a fixed plate and a second plate moving at a velocity (u). The friction between each fluid layer of area A leads to a force (F) resisting the relative motion. \[\mathrm{F}\;=\;\eta\cdot\mathrm{A}\cdot\frac{\partial\mathrm{u}}{\partial\gamma} \tag{5}\]  
Dynamic viscosity \(\eta\) is the proportionality factor relating F and the local shear velocity \(\frac{\partial\mathrm{u}}{\partial\gamma}\). Viscosity can also be expressed in other forms such as through kinematic viscosity \(\mu\) (cSt), which is the ratio of dynamic viscosity \(\eta\) (cP) to density \(\rho\) (gm/cc) of the fluid. Viscosity is closely linked to the fluid temperature. Kinematic viscosity \(\mu\) (T) can be modeled with the following expression:  
\[\log(\log\mu+0.7)=\mathrm{B}-\mathrm{C}\cdot\log(\mathrm{T}+273.15) \tag{6}\]  
where T is the temperature in \({}^{\circ}\)C, \(\log\) is the base 10 log, and B and C are oil-dependent constants. Density (\(\rho\)) can be modeled in a simple linear form:  
\[\rho=\mathrm{D}+\mathrm{E}.\mathrm{T} \tag{7}\]  
where D and E are oil dependent constants. Dynamic viscosity \(\eta\) (T) can be deduced from kinematic viscosity \(\mu\) (T) and density \(\rho\) (T). Base oil viscosity has a naturally high temperature dependence and is characteristically low [31]. To ensure efficient engine performance, oil viscosity should be sufficiently high at high temperature but not thick enough at low temperature to prevent easy engine start-up. To increase the viscosity index (VI), engine oils contain additives designed to improve the VI and these are composed of long-chained hydrocarbon polymers tightly folded at low temperatures and unfolded at high temperatures. Lubrication is also improved by anti-wear additives, which prevent friction between metallic surfaces during extreme operating conditions. Zinc dithiophosphate (ZDP) is an example of an anti-wear agent. Incorrect sealing can lead to functional deficiency or foam formation. Foam inhibitor additives containing silicon compounds prevent formation of stable oil foams and improve air removal ability [32; 33; 34]. Oil cleaning is ensured by two types of additives: dispersants and detergents. A dispersant is composed of a long, nonpolar oleophilic tail and a polar head. The function of the dispersant is to maintain solid or liquid contamination particles suspended in the oil and thus avoid any agglomeration that could lead to sludge formation. The two phenomena involved in this process are peptization and solubilization. Peptization consists of wrapping solid oil contamination particles (diameters of 50-150 nm), such as dust and soot, and keeping them in suspension. Solubilization uses the same mechanism, but it is related to liquid contaminants, such as acids or condensates (less than 20 nm). A detergent is a colloidal metal-based additive and has a structure similar to that of the dispersant, but its tail is shorter and the polarity of its head is greater.