One of the most important lessons learned in the approximately 50-year history of vehicle pollution control worldwide is that vehicles and fuels must be treated as a system. Improvements in vehicles and fuels must proceed in parallel if significant improvements in vehicle related air pollution are to occur. A program that focuses on vehicles alone is doomed to failure; conversely, a program designed to improve fuel quality alone also will not be successful.
Reformulated diesel fuels can reduce particulate emissions from all diesel vehicles, as discussed earlier. [Approximately 70-80% of diesel PM is composed of elemental/black carbon. Gasoline PM contains only about 25% elemental/black carbon. Controls on diesel PM, especially catalyzed PM filters, greatly reduce the elemental carbon both in mass and fraction. For example, a 2007 HDD with a catalyzed PM trap has lower PM with only ~10% as elemental carbon.]. Especially low sulfur fuels reduce the sulfate contribution. Certain after-treatment technologies are especially sensitive to the sulfur content of the fuel. Therefore if very stringent control of NOX and PM was needed, sulfur levels will need to be reduced to 50 ppm or less and Euro 4 vehicle standards introduced. Euro 5 or U.S. Tier 2 standards include a fuel sulfur limit of 10-15 ppm. Technologies to achieve these levels already exist and even more advanced technologies are being introduced for new vehicles.
With regard to gasoline-fueled vehicles, the use of catalyst exhaust gas treatment requires the elimination of lead from gasoline. This change, which has occurred throughout most of the world, has resulted in a dramatic reduction of ambient lead levels. Other gasoline properties that can be adjusted to reduce emissions include, roughly in order of effectiveness, sulfur level, vapor pressure, distillation characteristics, light olefin content, and aromatic content . Catalyst technology is emerging for two to three wheeled vehicles and, therefore, lead free and lower sulfur gasoline will be important for these vehicles as well.
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