Reversibility of Sulfur Effects on Emissions of California Low Emission Vehicles

The Coordinating Research Council conducted a program to measure the reversibility of fuel sulfur effects on emissions from California Low Emission Vehicles (LEVs). Six LEV models were tested using two non-oxygenated conventional Federal fuels with 30 and 630 ppm sulfur. The following emission test sequence was used: 30 ppm fuel to establish a baseline, 630 ppm fuel, and return to 30 ppm fuel. A series of emission tests were run after return to 30 ppm to ensure that emissions had stabilized. The effect of the driving cycle on reversibility was evaluated by using both the LA4 and US06 driving cycles for mileage accumulation between emission tests after return to 30-ppm sulfur fuel. The reversibility of sulfur effects was dependent on the vehicle, driving cycle, and the pollutant. For the test fleet as a whole most but not all of the sulfur effects were reversible. With the LA4 driving cycle, sulfur effects on CO and NOx were partially irreversible (approximately 79% and 84% recovery, respectively); for NMHC there was no evidence of any irreversibility (approximately 100% recovery). With the US06 driving cycle, sulfur effects on NOx were partially irreversible (approximately 95% recovery); for NMHC and CO there was no evidence of any irreversibility (approximately 100% recovery). Partial irreversibility, where detected, was statistically significant.