Reversibility of Gasoline Sulfur Effects on the Exhaust Emissions of Late Model Vehicles 2014-01-1624

The U.S. EPA has proposed a Tier 3 rule to lower average NMOG+NOx emissions from new light duty vehicles by approximately 80% from 2017 to 2025. Early in this time period, gasoline-fueled vehicles are expected to use technologies similar to California SULEV-II/PZEV certified models currently in limited production. These late model vehicles feature engine control systems that promote rapid catalyst light-off and are designed for ultra-high catalyst conversion efficiency.

To enable the use of advanced catalyst coatings and materials, the EPA is also proposing to limit the sulfur content of gasoline to an annual average of 10 ppm while optionally maintaining the current maximum cap of 80 ppm. Fuel sulfur is known to poison precious metal-based catalysts, and the impact on emissions is well understood for older technology vehicles. However, there is a lack of test data on the sensitivity and reversibility of late model vehicle emissions to sulfur.

This study evaluated six late model vehicles to determine if the exhaust emissions effects caused by exposure to 80 ppm high sulfur fuel were reversible, after the vehicles were refueled with 10 ppm sulfur fuel. Catalysts and sensors were aged to full useful life in the laboratory and then installed on the vehicles for testing. A statistical analysis concluded that there was no difference in the mean emissions of NMOG, NOx, CO, Soot and PN measured before and after the high sulfur fuel exposure. The emissions effects caused by high sulfur fuel exposure were reversible for all vehicles with 95% confidence.