Environmental Protection Agency The particulate filter traps the particles of soot in the exhaust.

Ultra-Low Sulphur Diesel Fuel Explained

What is Ultra Low Sulphur Diesel (ULSD) fuel? In a nutshell, it's cleaner diesel. Ultra low sulphur diesel fuel has been refined so that its sulphur content is 15 parts per million (ppm) or less. This is 97 percent cleaner than the standard highway-use diesel fuel sold in the US, which contains an average of 500 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 June 2006, when the EPA enacted a mandate requiring 80 percent of the highway diesel fuel produced or imported to meet the 15 ppm standard. The new ULSD fuel went on sale at most stations nationwide in mid-October 2006. Both diesel fuels will be on sale for the next few years, with the goal being a gradual phaseout of 500 ppm diesel. By December 2010, 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.)

ULSD fuel will work in concert with a new generation of diesel engines that will begin arriving for the 2007 model year. Ideally, ULSD will enable the new generation of diesel vehicles to meet the same strict emission standards as gasoline-powered vehicles. The new engines will utilize an emissions-reducing device called a particulate filter. The process is similar to a self-cleaning oven's cycle: a filter traps the tiny particles of soot in the exhaust fumes. The filter uses a sensor that measures back pressure, or the force required to push the exhaust gases out of the engine and through to the tailpipes.

As the soot particles in the particulate filter accumulate, the back pressure in the exhaust system increases. When the pressure builds to a certain point, the sensor tells the engine management computer to inject more fuel into the engine. This causes heat to build up in the front of the filter, which burns up the accumulated soot particles. The entire cycle occurs within a few minutes and is undetectable by the vehicle's driver.

With appropriate vehicle maintenance, ULSD fuel is expected to be fully compatible with 2006 and earlier diesel engines and perform as well as previous diesel fuels in normal operating conditions. Diesel engines manufactured for the 2007 model year and later will utilize some type of particulate filter and will be designed to run solely on ULSD fuel. The use of non-ULSD fuel in these engines is illegal, as the higher sulphur content will prevent the particulate filter and other emissions controls from working properly. And, since non-ULSD fuel will damage the engine over time, its use will likely void the vehicle's warranty.

In the next section, we'll explore the recent changes in the diesel marketplace, and how auto manufacturers are adapting.