Date of Award


Degree Name

Doctor of Philosophy


Agricultural Sciences

First Advisor

Watson, Dennis


Engine oil is critical to tractor engine performance. Engine designers recommend farmers change engine oil depending on recommendations by engine manufacturers. Engine manufacturers did not take into account different tasks often performed by tractors in fields, like tillage or seeding. Farmers do not have certain criteria to determine when engine oil must change. The only criteria to change engine oil is the physical /chemical method, which takes at least one week to obtain oil results. It is a waste of time to wait one week to get the results. There will be a lot of mechanical engine problems if oil is not changed. These engine oil problems cost farmers a lot of money. The aim of this research is to use new technology that could be contributed to solving these technical difficulties. Terahertz technology was used to determine engine oil characteristics by measuring refractive index and absorption coefficient on different conditions. Four experiments were performed to identify the ability of terahertz technology on various engine oil grades, engine oil types, and engine oil contaminants by using terahertz time-domain spectroscopy (THz-TDS). The first experiment was classifying gasoline engine oils of various viscosities by using THz-TDS. The range of 0.5–2.0 THz was evaluated for distinguishing among gasoline engine oils of three different grades (SAE 5W-20, 10W-40, and 20W-50) from the same manufacturer. ANOVA results confirmed a highly significant difference (p<0.0001) in refractive index among each of the three oils across the 0.5–2.0-THz range. Linear regression was applied to refractive index data at 0.25-THz intervals from 0.5 to 2.0 THz to predict kinematic viscosity. The refractive indices of these oil samples were promising for identification and distinction of oil grades. The second research identified three levels of water contaminants 0.0%, 0.1% and 0.2% inside diesel engine oils, grade SAE 15W-40, by utilizing THz-TDS in the range of 0.5 to 2.0 THz. The 0% water contamination level had the lowest absorption coefficient, while 0.2% water had the highest absorption coefficient. The refractive index of 0% water was the lowest and 0.2% water was the highest across the THz range. The refractive indices of these oil samples were promising for discrimination of water contamination. The third experiment demonstrated the possibility of identifying gasoline in engine oil (SAE 5W-20) which was contaminated with four rates (0%, 4%, 8% and 12%) of gasoline fuel and were measured by using THz-TDS. For both refractive index and absorption coefficient of the single cuvette method, ANOVA and Fisher results illustrated that there were highly significant differences (p < 0.0001) among each of the gasoline contaminant levels across the 0.5-2.5 THz range. The 2.5 THz frequency was the best to predict fuel contamination based on refractive index, and 0.5 THz was the best frequency for absorption coefficient. The fourth experiment illustrated the potential of THz-TDS to detect viscosity at 40 °C and TBN changes in gasoline engine oil (SAE 5W-20) due to thermal oxidation (TO). For refractive index, ANOVA and Fisher results showed that there were highly significant differences (p < 0.0001) among each of the TO times across the 0.51 - 2.48 THz range. Refractive index was used to predict TO time, and the 1.25 THz frequency was best to predict viscosity at 40°; for TBN, 2.25 THz was best.

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