AM-00-27 THE ROLE OF SULFUR IN ENGINE EXHAUST EMISSIONS CONTROL

Description:

During the operation of an internal combustion engine, fuel sulfur is converted to sulfur compounds - mainly sulfur dioxide. Exhaust catalysts have been integrated within spark-ignited engine emission control systems. Therefore it is inevitable that exhaust catalysts will encounter sulfur dioxides. To meet California's very strict LEV-2 emission standards and the new US Environmental Protection Agency (EPA) Tier 2 regulations for the remaining States, exhaust catalysts have to perform and maintain high levels of reduction performance for over 100,000 miles. For these strict emission standards, only slight catalyst performance deterioration can be tolerated. Additionally, the catalysts have to maintain sufficient performance level to meet the emission standards in actual on-road use, where they are exposed to the full operating range of the engine and vehicle operating environments. Fuel sulfur is known to impede catalyst performance in several ways. Recognizing the negative impact on catalyst performance, the US EPA set a lower gasoline sulfur specification as part of the Tier 2 emission regulation. The California Air Resources Board (CA ARB) had already included a low sulfur limit for gasoline. When the new low sulfur gasoline fuels are introduced, they are expected to improve the emission control performance of most vehicles currently on-the-road. In addition, both California and the US EPA are considering the next step for strict on-road heavy-duty engine emission standards - called Phase 2 (or the '2007 rule'). Diesel fuel sulfur is known to impede the performance of several types of diesel emission control catalyst technologies. In fact, diesel sulfur is also considered a barrier to a promising NOx control catalyst system. The role of sulfur and the mechanisms of interaction with exhaust catalysts is discussed. With respect to most exhaust emission catalyst technology for either spark-ignited gasoline engines or compression-ignition diesel engines, catalyst performance improves linearly with lower sulfur content - the lower the better.