Initiative on the Potential Impact of Sulphur in Gasoline on Motor Vehicle Pollution Control and Monitoring Technologies

Executive Summary

July 30, 1997

In fulfilling its mandate, the Task Group assembled and reviewed a comprehensive bibliography of the available technical literature regarding gasoline sulphur impacts on vehicle emissions, emission control systems, and monitoring systems, focusing particularly on LEV vehicles (the term LEV refers to a vehicle certified by the state of California as meeting the LEV emission standard level incorporated in that state's Low Emission Vehicle and Clean Fuels (CAL LEV) program). The Task Group also conducted two workshops at which experts presented the latest available data and described on-going and future studies that will investigate gasoline sulphur impacts on LEV vehicles.

The following findings have been reached by the Task Group regarding the effect of sulphur in gasoline on the emission control and monitoring technologies of LEVs, including OBD II and the ability of LEVs to meet in-use compliance standards.

1. Sulphur in gasoline reduces catalyst efficiency on existing Tier 0, Tier 1, and TLEV vehicles. The limited available data indicate that sulphur also reduces the efficiency of catalysts used on LEV vehicles. Sulphur impacts on catalyst efficiency lead to higher emissions of all regulated pollutants. The effects of sulphur on catalyst efficiency increase as the sulphur content of gasoline increases. Therefore, vehicles operating on higher sulphur gasolines have higher emissions than vehicles operating on lower sulphur gasolines.
2. Available data indicate that, in some instances, sulphur in gasoline may degrade the performance of oxygen sensors on Tier I and TLEV vehicles, resulting in higher emissions. The limited data available indicate that there are also some instances in which sulphur may degrade the performance of oxygen sensors on LEV vehicles.
3. Available data indicate that, in some instances, sulphur in gasoline may impair the performance of OBD II systems on Tier I and TLEV vehicles. The limited data available indicate that sulphur may also impair the performance of LEV OBD II systems. However, neither CARB nor the EPA believes that sulphur by itself will lead to a widespread failure of OBD II systems to properly identify emission-related malfunctions on either existing or LEV vehicles. Although neither agency believes a general relaxation of OBD II requirements is warranted based on sulphur impacts, both agencies have agreed to review manufacturer concerns regarding sulphur effects on a vehicle model by vehicle model basis.
4. Although sulphur in gasoline reduces catalyst efficiency and may impair the performance of oxygen sensors and OBD II systems, the magnitude of sulphur effects is variable. The magnitude of sulphur impacts on in-use emissions depends on a number of factors, including catalyst formulation, catalyst location, emission control and OBD II system design, vehicle calibration, and the sulphur level of the gasoline. However, no systems have been identified that are completely immune to sulphur effects.
5. Available data for existing vehicles indicate that the effects of high sulphur gasolines on catalysts, oxygen sensors, and OBD II systems are reversible upon return to operation on low sulphur gasolines, provided the vehicle is operated at a rich air/fuel ratio (excess fuel) at high temperatures for a sufficient period of time. The limited available data for LEVs suggest that because of more exacting requirements for air/fuel ratio control, sulphur effects may not be completely reversible.

The Task Group has reviewed the currently available technical literature and the results of LEV emissions testing performed to date and has determined that the bulk of the available data were obtained using California Phase 2 gasoline, the fuel that is used for both the emissions certification and in-use operation of LEVs in California. LEV vehicles are designed by vehicle manufacturers to meet in-use compliance requirements on the California fuel, which has an average sulphur content of 30 ppm and a maximum sulphur content of 80 ppm (California Phase 2 gasoline, which is also referred to as Cleaner Burning or California Reformulated gasoline, sets specifications for olefin, aromatic, benzene, and oxygenate content as well as T50 and T90 distillation temperatures and RVP in addition to specifying sulfur content).

The Task Group has also found that there are only limited data available addressing the impact of fuels other than California Phase 2 Reformulated gasoline on the performance of LEV emission control and monitoring systems or on LEV emissions. Specifically, with respect to sulphur impacts, the data are both limited and incomplete in that there are significant gaps related to the impact of sulphur levels greater than 80 ppm on LEV catalysts, LEV OBD II systems, and LEV emissions. At this time, there are insufficient data to determine, with any reasonable degree of certainty, whether a fuel sulphur level above 80 ppm would be compatible with LEV vehicles. As a result, it cannot be reasonably determined what reduction in the sulphur content of current Canadian gasolines is required for LEV technology. This finding is consistent with the findings of the U.S. EPA, which, at present, is still studying the sulphur issue as it relates to LEVs.