Gasoline engine oil (SAE 5W20) was contaminated with four levels (0%, 4%, 8% and 12%) of gasoline fuel and submitted to terahertz time-domain spectroscopy (THz-TDS). Three sampling methods were used to compare measurement variations. For all sampling methods, refractive index decreased with increased fuel contamination and absorption coefficient increased with increased fuel contamination. Absorption coefficients were significantly different for each fuel contamination level for each sampling method across the entire 0.5–2.5 THz range. The frequency of 0.5 THz produced the best model of absorption coefficient predicting fuel contamination with a rootmean- square error of 0.21% points. THz-TDS demonstrated high potential for estimating gasoline fuel contamination in gasoline engine oil.
Abdul-Munaim, Ali M., Mendez Aller, Mario, Preu, Sascha and Watson, Dennis G. "Discriminating gasoline fuel contamination in engine oil by terahertz time-domain spectroscopy." Tribology International 119 (Mar 2018): 123-130. doi:10.1016/j.triboint.2017.10.026.