http://en.wikipedia.org/wiki/Ultra-low_sulfur_diesel
United States
Ultra-low sulfur diesel was proposed by EPA as a new standard for the sulfur content in on-road diesel fuel sold in the United States since October 15, 2006, except for rural Alaska. California required it since September 1, 2006, and rural Alaska will transition all diesel to ULSD in 2010. This new regulation applies to all diesel fuel, diesel fuel additives and distillate fuels blended with diesel for on-road use, such as kerosene, however, it does not yet apply to train locomotives, marine, or off road uses. By December 1, 2010, all highway diesel will be ULSD. Non-road diesel transitioned to 500 ppm sulfur in 2007, and to ULSD in 2010. Locomotive and marine diesel also transitioned to 500 ppm sulfur in 2007, and to ULSD in 2012. There are exemptions for small refiners of nonroad, locomotive and marine diesel that allow for 500 ppm diesel to remain in the system until 2014. After December 1, 2014 all highway, nonroad, locomotive and marine diesel produced and imported will be ULSD.
The allowable sulfur content for ULSD (15 ppm) is much lower than the previous U.S. on-highway standard for low sulfur diesel (LSD, 500 ppm), which not only reduces emissions of sulfur compounds (blamed for acid rain), but also allows advanced emission control systems to be fitted that would otherwise be poisoned by these compounds. These systems can greatly reduce emissions of oxides of nitrogen and particulate matter
Sulfur is not a lubricant, however the process used to reduce the Sulfur also reduces the fuel's lubricating properties. Lubricity is a measure of the fuel's ability to lubricate and protect the various parts of the engine's fuel injection system from wear. The processing required to reduce sulfur to 15 ppm also removes naturally-occurring lubricity agents in diesel fuel. To manage this change ASTM International (formerly the American Society for Testing and Materials) adopted the lubricity specification defined in ASTM D975 for all diesel fuels and this standard went into effect January 1, 2005. [4]
The refining process that removes the sulfur also reduces the aromatic content and density of the fuel, resulting in a minor decrease in the energy content, by about 1%. This decrease in energy content may result in reduced peak power and fuel economy. The reduction is only slight and will likely go unnoticed.