# Research on Testing Method of Oil Characteristic Based on Quartz Tuning Fork Sensor
## 3 Oil Ageing Processes and Sensors for Fluid Property Detection
### Coolant Contamination  
Engine coolant contamination comes from leakage from the engine cooling system. Coolant is composed of ethylene glycol or propylene glycol (40 to 60%), water (60 to 40%) and several additives at relatively low concentrations (approximately 5%). Ethylene glycol and propylene glycol are miscible with water and with most organic solvents, but are not miscible with oil. Common coolant additives include sodium silicate, disodium phosphate, sodium molybate, and sodium borate, and these function as lubricants, buffers, and corrosion inhibitors. Coolant contamination in engine oil can be dangerous for an engine, and have several negative impacts. Low levels of coolant contamination can cause heavy sludge deposits, the formation of acidic compounds, such as glycolic or methanolic acids, and acceleration of oil oxidation. When mixed with oil, high coolant contamination could lead to emulsions or gels and induce occlusions of walls and oil passageways, risks of filter blocking, reductions in oil flow, and a serious general decrease of lubrication efficiency [41].  
Coolants in oil are typically measured in the laboratory using FTIR spectroscopy (ASTM D7418) or ICP-AES spectrometry (ASTM D7151), mainly through quantification of the coolant additive elements Na, B, and/or K within the oil.  
The fluid property sensor measurement of coolant (50% water) dilution is presented in Figure 13. Engine oil (15W40) was stabilized at 50 \({}^{\circ}\)C and diluted with coolant. Emulsion homogeneity was ensured by magnetic stirring. The results demonstrate the capability of the fluid property sensor to detect and measure coolant contamination, mainly with dielectric and resistivity measurements.  
Viscosity and density did not show any changes. The dielectric increased linearly. With the coolant dilution, when the coolant dilution rate was lower than 2000, the linear decrease of resistivity was not obvious, but it did decrease linearly at a coolant dilution rate above 2000. These results are perfectly consistent with the water dilution results described in the previous section. Indeed, the tested coolant was composed of 50% water, and the dielectric and resistivity slopes are half as high as those in the water dilution results. The changes can be explained by the high dielectric constant of water (\(\epsilon_{\text{Water}}\)= 80) and the autoionization of water, which can release free charge carriers and protons into the oil [42].  
The fluid property sensor measures dissolved and emulsified water coming from the coolant blend, which guarantees good compatibility with the application. Indeed, engine operation ensures emulsion by establishing a sufficiently high mixing ratio. When engine oil reaches a high temperature, water may evaporate, so the detection has to be performed before evaporation, preferably just after the engine start-up during the oil temperature increase.  
The coolant in engine oil can be modeled using the following equations at a constant temperature:  
\[\eta_{\text{th}}=\eta_{\text{i}} \tag{20}\]  
\[\rho_{\text{th}}=\rho_{\text{i}} \tag{21}\]  
\[\varepsilon_{\text{th}}=\varepsilon_{\text{i}}\cdot\left(1+10\times 10^{-6}. \text{Coo}\text{Dil}_{\text{(ppm)}}\right) \tag{22}\]  
\[\text{Rp}_{\text{th}}=\text{Rp}_{\text{i}}\cdot\left(1-3\times 10^{-6}. \text{Coo}\text{Dil}_{\text{(ppm)}}\right) \tag{23}\]  
Figure 13: Coolant dilution in engine oil (15W40) at 50 \({}^{\circ}\)C: (**a**) viscosity; (**b**) density; (**c**) dielectric; and (**d**) Rp.  
These models are plotted in Figure 13 with the related accuracy specifications given as full blue lines.