Effects of Mileage on Engine Oil of an Aged Spark Ignition Retrofitted Engine Fueled by Gasoline and Compressed Natural Gas

This study examines the influence of engine oil on friction reduction in spark ignition (SI) engines, specifically analyzing the effects of mileage on oil viscosity, contaminants, and overall oil quality when employing gasoline versus compressed natural gas (CNG) as fuels. The maintenance of engine oil quality is essential for optimal performance, necessitating rigorous monitoring of parameters in accordance with manufacturer specifications. Although prior research has investigated CNG as a vehicle fuel, its impact on the performance of aged engine oil remains inadequately addressed. A 4-cylinder, 1.5 L SI engine, exhibiting 161,000 km of mileage, underwent 100 h of testing on gasoline followed by 100 h on CNG. Oil samples were collected at 20-h intervals for analysis of both local and foreign contaminants, oil performance metrics (total base number (TBN), total acid number (TAN), key additives (calcium, zinc, phosphorus), and viscosity at 40 and 100°C. Results revealed significant changes in oil properties: Oil viscosity decreased more with gasoline than CNG after 100 h at 40°C, silicon (Si) contaminants rose from 0 to 13 ± 1 ppm with gasoline compared to an increase from 0 to 4 ± 1 ppm with CNG; fuel dilution reached 1.5 ± 0.87% with gasoline, while CNG showed a 0.5% increase. Iron (Fe) content surged from 0 to 21 ± 0.5 ppm with gasoline and 11 ppm with CNG, while aluminum (Al) levels fluctuated minimally. TBN and TAN values demonstrated slight variations, indicating that CNG generally outperformed gasoline, with minimal differences in TBN relative to mileage.