23rd International Colloquium Tribology - January 2022 393 Study of the Capacity of Spectroscopy UV-Vis and NIR to Quantify Fuel Dilution on used engine oil Bernardo Tormos CMT - Motores Térmicos, Universitat Politècnica de València, Valencia, Spain. Vicente Macián CMT - Motores Térmicos, Universitat Politècnica de València, Valencia, Spain. Benjamín Pla CMT - Motores Térmicos, Universitat Politècnica de València, Valencia, Spain. Adbeel Balaguer CMT - Motores Térmicos, Universitat Politècnica de València, Valencia, Spain. Corresponding author: abalrey@mot.upv.es. 1. Introduction Fuel dilution of engine oil is a common problem for both diesel and gasoline internal combustion engines: when the fuel contaminates the oil, it modifies its properties, mainly viscosity, reducing lubrication film strength. The quantification of fuel in oil (FiO) is an important parameter in the ICE’s condition monitoring, as it allows performance prediction and to take proper actions. The standard method (e.g., ASTM D3524 - 14) for assessing the content of fuel in oil require high-cost equipment, as chromatographer and solvents, while this work evaluates the capacity of UV-Vis and FT-NIR spectroscopy to quantify fuel dilution, considering them as techniques with simple operation and reduced cost, able to produce accurate and significant results. This work evaluated both spectroscopic techniques and the results proved that is possible to quantify diesel dilution by FT-NIR spectroscopy using the spectral peak area at 4600 cm -1 (BL: 4635-4558 cm -1 ). 2. Methods and Results To study the capability of the spectroscopy for FiO quantification, the first step in this work was to prepare diluted samples using fresh oil for analysis, in order to avoid interference from contaminants, as soot and other typical impurities, in the spectrum. The first test samples were prepared using SAE 5W-30 fresh engine oil contaminated with diesel in a concentration ranging from 0-10.0% (w/ w) and then analyzed in both, UV-Vis and NIR. The second phase of the work was to use the spectroscopic method that have provided the best results in previous step, to analyze used oil samples with different fuel dilution rate. 2.1 Fresh oil samples analyzed in UV-Vis The UV-Vis data (700-200nm) for the fresh oil samples contaminated with diesel was acquired in a Perkin Elmer UV-Vis Lambda 365 using an optical path length of 10 mm. Due to the similarities between the spectra of the oil and the fuel it was not possible to differentiate a specific peak or area where the quantification could be done directly, therefore the possibility of using a fuel dye to improve quantification in this region was studied as in the work of Neupane et al when using laser induced florescence spectroscopy. The technique consisted in dying the fuel used to prepare the samples at different concentration rates and then obtaining the spectra, eight samples were used, and the spectra were collected by triplicate. The use of the dye in the fuel (600 ppm) allowed to identify an area that could be used for quantification, between 415-435 nm, after preforming a spectral peak area analysis for the collected spectra the coefficient of linear correlation obtained was R² = 0.8240. 2.2 Fresh oil samples analyzed in FT-NIR The NIR spectra for the fresh oil samples contaminated with diesel was acquired in a Perkin Elmer FT-NIR Spectrum Two N between 11500-4500 cm -1 also using an optical path length of 10 mm. The first step was to obtain and compare the spectra of the fresh oil and the diesel fuel, it was possible to identify a specific peak, as shown in Figure 1, at the wave number 4600 cm -1 where the diesel could be quantified. 394 23rd International Colloquium Tribology - January 2022 Study of the Capacity of Spectroscopy UV-Vis and NIR to Quantify Fuel Dilution on used engine oil Figure 1: Fresh engine oil and diesel NIR spectra Following, the spectra of the prepared samples were collected by triplicate. The quantitative analysis technique chosen was peak height at 4600 cm -1 , it proved the existence of a linear relationship according to the Beer-Lamber Law, between the peak height at 4600 cm -1 and the concentration of diesel in the sample, the fitting equation displayed a good regression coefficient of R 2 =0.98368, in the studied range, as shown in Figure 2. Comparing the results of both methods, the FT-NIR let to the obtention of significantly better results for quantification in the fresh oil samples whit a simpler procedure, therefore it was the chosen spectroscopic alternative to continue the analysis in used oil samples. Figure 2: Linear fitting for fresh oil samples 2.3 Used oil samples analyzed in FT-NIR Samples for this stage were prepared gravimetrically from used oil from diesel engines and contaminated with fuel, 9 samples were used to build the model, in a concentration ranging from 0-10.0% (w/ w). The first attempt to record the spectra of the used samples was not effective because the soot content generated noise in all the spectrum range, therefore it was required to dilute the samples. Various solvents were put on trial and finally the heptane was chosen, as well, several dilution ratios were evaluated, for this group of samples that was prepared from a used oil containing 0.067% of soot, the selected ratio was 3.4 g of heptane per each gram of sample since it produced a clean spectrum to work on the analysis stage. To record the spectra, the reference cell was filled with heptane, triplicate measurements were made, and the spectra were collected between 11500- 4500 cm -1 . For this case the quantitative analysis measurement technique chosen was spectral peak area analysis at the peak previously identified in the fresh samples, the area was measured over the range of 4635-4558 cm -1 . The areas were calculated using the own Perkin Elmer equipment software. For the construction of the calibration model, the area corresponding to each sample was the difference after subtracting the area of the sample without added diesel from the area of the contaminated sample. Using that peak area of the samples over the range of 4635 cm -1 to 4558 cm -1 as the arguments, and the fuel concentration as the dependent variable, the fitting equation of peak area with fuel concentration is obtained. The equation presented a regression coefficient was R 2 = 0.92068, which indicates a good correlation between predicted value and known value, therefore this method could be used to assess the content of fuel as a first approximation. The same procedure was applied on another group of samples prepared from the same type of engine oil but having a higher content of soot, and it was noticed that it required a higher dilution ratio to eliminate the noise in the recorded spectra. The results obtained were analogues to the previous one, for this the fitting equation presented a regression coefficient R 2 = 0.93240, similar to the previous one. Figure 3: Linear fitting for used oil samples 2.4 Determination of the dilution factor To apply this method is required to dilute the used oil sample with heptane in a proportion that varies according 23rd International Colloquium Tribology - January 2022 395 Study of the Capacity of Spectroscopy UV-Vis and NIR to Quantify Fuel Dilution on used engine oil to the sample’s soot content, towards reducing the noise in the measured spectrum. This work studied used oils in a range of soot concentration from 0.067% to 0.282%, that concentration was measured with FTIR. The following table presents the dilution ratios used in this work: Soot (%) g heptane/ g sample 0.067 3.40 0.115 6.33 0.174 11.11 0.282 13.33 3. Conclusion This work proposes a methodology for quantifying fuel dilution particularly focused on diesel engines using spectral peak area analysis that has been shown as a viable method in the near-infrared region. Although full spectrum features can be used when working with multivariate methods, it is important to remark the identification of a unique spectral region that describes accurately the fuel dilution concentration providing a simple method for measuring it. References [1] ASTM Standard D3524 - 14, 2020, “Standard Test Method for Diesel Fuel Diluent in Used Diesel Engine Oils by Gas Chromatography”, ASTM International, DOI: 10.1520/ D3524-14R20, www.astm. org. [2] S. Neupane, V. Boronat Colomer, D. Splitter, et al. “Measurement of Engine-Oil Fuel Dilution Using Laser Induced Fluorescence Spectroscopy”. 2020 Spring Technical Meeting of the Central States Section of the Combustion Institute Proceedings. Center States Combustion Institute (CSSCI) 2020. Acknowledgments This work has been partially supported by grant PID2020-11969RB-100 funded by MCIN/ AEI/ 10.13039/ 501100011033.