What do you understand by the term ULSD (Ultra Low Sulfur Diesel)?

In a general, it's cleaner diesel. Ultra low sulphur diesel fuel has been refined so that its sulphur content is15 parts per million (ppm) or less. This is97 percent cleaner than the standard highway-use diesel fuel sold in the US, which contains an average of500 ppm of sulphur. Sulphur, a natural part of the crude oil from which diesel fuel is derived, is one of the key causes of particulates or soot in diesel. Soot is the main culprit of diesel engines' noxious black exhaust fumes, and is among the prime contributors to air pollution. The move toward ULSD is aimed at lowering diesel engines' harmful exhaust emissions and improving air quality.

ULSD fuel has been the standard in Europe for several years. In the U.S., the changeover process began in June2006, when the EPA enacted a mandate requiring80 percent of the highway diesel fuel produced or imported to meet the15 ppm standard. The new ULSD fuel went on sale at most stations nationwide in mid-October2006. Both diesel fuels will be on sale for the next few years, with the goal being a gradual phaseout of500 ppm diesel. By December2010, all highway-use diesel fuel offered for sale in the U.S. must be ULSD fuel. (ULSD is not to be confused with biodiesel which is diesel derived from biological ingredients such as plant oil or animal fat and usually blended with standard diesel fuel.)