The physical properties of diesel engine oil were quantified through direct, real-time measurements using an onboard sensor. The sensor measures the lubricant temperature, density, dynamic viscosity, and dielectric constant. Bench-top experiments were conducted to validate the accuracy of sensor outputs against results from ASTM test methods or reference instruments over a specific temperature range. Bench-top experiments were also used to establish correlations between fuel contamination levels and changes in lubricant properties. Measurements were then conducted in a diesel engine using the onboard sensor to quantify changes in the lubricant physical properties with respect to engine operating time.Through bench-top testing, it was determined that the onboard sensor's viscosity output is accurate to within 6% of results obtained through the ASTM standard method, whereas dielectric constant readings displayed a 7% systematic error with respect to reference values. The oil viscosity was found to decrease by 20% when fuel contamination increased by 10.5% with respect to the baseline value. The dielectric constant showed marginal sensitivity to fuel contamination but significant dependence on the oil additive package. Data measured during engine operation demonstrated a significant, simultaneous increase in viscosity and a decrease in dielectric constant during the first 73 h of testing. The change in the lubricant properties might be attributed to incipient consumption of the additive package and accumulation of oxidation by-products.
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