Measurement of Soot Content in Engine Oil by ATR Method of Fourier Transform Infrared (FTIR) Spectroscopy

Significance of the Topic

Monitoring soot in engine oil is essential to maintain lubrication performance, prevent abrasive wear and extend engine life. Incomplete combustion generates soot particles that, when accumulated in oil, can impair viscosity, increase friction and lead to damage of critical engine components.

Objectives and Study Overview

This study demonstrates a streamlined method for quantifying soot content in used engine oil by employing attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. The goal is to develop a rapid, reliable alternative to traditional transmission cell measurements specified in ASTM E2412-10.

Methodology and Instrumentation

The ATR technique uses a high-refractive-index diamond prism placed in direct contact with the oil sample. Infrared light undergoes total internal reflection within the prism, and evanescent waves interact with the sample’s surface, producing spectra indicative of soot presence as a baseline elevation rather than discrete peaks. Measurement focused on the absorbance at 1,850 cm⁻¹ to avoid overlap with the diamond ATR element’s own absorption.

Instrument Used

• Shimadzu IRSpirit™ FTIR spectrophotometer with KBr window plate
• Shimadzu QATR™-S wide-band diamond ATR attachment
• Resolution set to 4 cm⁻¹, 20 scans per measurement, DLATGS detector, square-triangle apodization

Main Results and Discussion

A calibration curve constructed from standard oil samples with known soot contents (0.2 % to 4 % mass) exhibited excellent linearity (r = 0.998). Repeatability testing showed a coefficient of variation of 1.58 % at 0.20 % soot and 2.64 % at 3.93 % soot over ten consecutive scans without sample replacement. Higher soot levels led to increased baseline drift due to progressive deposition on the prism, and lower levels were more affected by baseline noise.

Benefits and Practical Applications

The ATR-FTIR method simplifies cleaning, reduces sample preparation time and maintains high accuracy, making it suitable for routine oil condition monitoring in automotive and industrial maintenance programs. It provides rapid feedback for optimizing oil change intervals and preventing unscheduled downtimes.

Future Trends and Applications

• Integration of automated sampling and prism cleaning modules to enhance throughput
• Development of on-line ATR-FTIR sensors for real-time engine oil monitoring
• Application of chemometric models to improve sensitivity for low soot concentrations
• Expansion to other lubricant contaminants such as oxidation products and fuel dilution

Conclusion

The ATR-FTIR approach offers a fast, reliable and low-maintenance alternative for soot quantitation in engine oil. By targeting the 1,850 cm⁻¹ absorbance and leveraging a robust calibration curve, this method meets industry needs for efficient condition monitoring while highlighting areas for further optimization.

Reference

1. ASTM E2412-10: Standard Practice for Condition Monitoring of Soot in Used Lubricant Oils by FTIR Spectroscopy.