Evaluation of long-term stability of analysis of fuel dilution for engine oils using a backflush GC system

Significance of the Topic

Accurate measurement of fuel dilution in engine oil is vital for assessing lubrication performance and predicting oil service life. Fuel ingress reduces oil viscosity and impairs engine protection leading to accelerated wear and maintenance costs.

Study Objectives and Overview

This study evaluates the long term analytical stability of a Shimadzu backflush gas chromatography system for determining diesel content in engine oil following ASTM D7593. Two extended repeatability tests were conducted: one comprising three sequential runs of 550 injections and another of three runs of 200 injections.

Methodology and Instrumentation

• Gas chromatograph: Shimadzu Nexis GC-2030 AF with AOC-20i autosampler
• Column: Restek SH-Rxi-1ms (15 m x 0.25 mm i d, 0.25 μm film) with flow restrictor
• Detector: Flame ionization detector at 350 °C
• Carrier gas: Nitrogen at constant pressure (2.3 mL/min)
• Sample preparation: Base oil spiked with 0 to 10 weight percent diesel (light fraction distilled)
• Injection: 0.1 μL split mode and backflush initiated at 1.8 minutes
• Backflush period: 2.2 minutes to remove high boiling components

Main Results and Discussion

Calibration curves showed excellent linearity over 0 to 10 w/w percent diesel with coefficient of determination above 0.9996. Backflushing reduced cycle time to approximately 4 minutes compared to over 30 minutes without backflush. In the 550-sample test the percent relative standard deviation RSD was 2.87, 3.27 and 3.05 across three cycles. The 200-sample test achieved improved precision with RSD values of 1.90, 2.21 and 2.07, all within ASTM D7593 tolerance. No carryover was detected in blank runs following backflush, confirming effective cleaning of residual oils.

Benefits and Practical Applications

This method delivers high throughput and reliable long term performance without frequent maintenance. Minimal sample pretreatment and use of nitrogen make it cost effective for routine quality control of engine oil. It meets standardized accuracy and repeatability requirements, supporting condition based monitoring and oil change scheduling in automotive and industrial settings.

Future Trends and Potential Applications

Emerging developments may include coupling backflush GC with mass spectrometry for detailed hydrocarbon profiling, extending the approach to biodiesel and gasoline per ASTM D7593, and integration with automated data processing for real time monitoring. Advances in column technology and alternative carrier gases could further enhance speed and sensitivity.

Conclusion

The Shimadzu backflush GC system demonstrates robust long term stability and precision for measuring diesel fuel dilution in engine oils. It ensures rapid, accurate analysis with reduced cycle times, offering a practical solution for extended routine testing in research and industrial laboratories.

Reference

• ASTM D7593 2014 Standard Test Method for Determination of Fuel Dilution in Used Lubricating Oils