# Research on Testing Method of Oil Characteristic Based on Quartz Tuning Fork Sensor
## 3 Oil Ageing Processes and Sensors for Fluid Property Detection
### Fuel Contamination  
Fuel contamination in engines is caused by imperfect fuel combustion and imperfect sealing. Unburned fuel reaches the crankcase through the space between the piston and cylinder and between the piston groove and the ring. Fuel dilution can be enhanced under particular conditions during the DPF regeneration process. Internal combustion engines use fuels such as diesel or gasoline, which are mostly hydrocarbon compounds mixed with a variety of additives. Generally obtained as a specific fractional distillate of petroleum, they are now increasingly blended with biofuels such as biodiesel and ethanol. Blended with petroleum diesel, biodiesel is a vegetable oil or animal fat-based fuel composed of long chain alkyl esters. In the US, fuel standard ASTM D7467 specifies blends from B6 to B20, which corresponds to blends with 6% to 20% biodiesel. In Europe, fuel standard EN 590 specifies blends up to B7. In gasoline fuels, ethanol is blended up to 10% (E10) in common markets and can be blended up to 85% (E85) in flexible fuel vehicle applications (ASTM 5798). In the future, biofuel blending rates are expected to increase in order to meet environmental protection rules. Fuel physical properties can be modeled as oil physical properties:  
\[\log(\log(\mu+0.7)) =\mathrm{B-C.\log(T+273.15)} \tag{24}\] \[\rho =\mathrm{D+E.T}\] (25) \[\epsilon =\mathrm{F+G.T}\] (26) \[\mathrm{R_{P}} =\mathrm{H+I.T} \tag{27}\]  
where T is the temperature in \({}^{\circ}\)C and B, C, D, E, F, G, H, and I are fuel-dependent constants. Dynamic viscosity \(\eta\) (T) is deduced from kinematic viscosity \(\mu\) (T) and density \(\rho\) (T). Fluid property sensor measurements of different fuels are presented in Figure 14. Fuel temperature homogeneity was ensured by magnetic stirring. The results demonstrate the fluid property sensor's capability to accurately measure fuel properties and allow robust fuel type differentiation. B100 biodiesel has a higher viscosity, density, and dielectric value than pure petroleum fuels. Gasoline shows a much lower viscosity than diesel fluids. The lower resistivity of biodiesel is explained by its hygroscopic characteristics. Fuel property measurements allow differentiation between gasoline, B0 and B100 but also with alternative fuels such as kerosene or hydrotreated vegetable oil (HVO). Fuel contamination in engine oil can be dangerous for the engine because it can promote oxidation of oil, formation of acidic compounds and formation of soot. These degradations are even more prevalent with biofuels because of the hygroscopic characteristics of biodiesel or ethanol. Fuel dilution will also degrade oil lubrication properties and amplify risks of wear. Fuel contamination in oil is typically estimated in the laboratory with a method using flash point (ISO 2719) and viscosity measurements (ASTM D445) or with a more accurate and more expensive method using gas chromatography measurements [43]. The fluid property sensor fuel dilution measurement is presented in Figure 15. Engine oil (15W40) was stabilized at 100 \({}^{\circ}\)C and diluted with diesel fuel. Blending temperature homogeneity was ensured by mechanical stirring. The results demonstrate the capability of the fluid property sensor to detect and measure fuel contamination, mainly with viscosity measurements. Resistivity does not show any consequent change. Density and dielectric are linearly related to fuel dilution. Slopes depend directly on the density and dielectric properties of the two fluids because they are miscible. The most obvious change is measured with viscosity, which decreases exponentially with fuel dilution. As engine oil and fuel are miscible fluids, the Refutes method can be used to calculate the theoretical kinematic viscosity (\(\mu\)) of the blend.