What is ultra-low sulfur diesel?

Ultra-low sulfur diesel (ULSD) is diesel fuel with 15 parts per million (ppm) or lower sulfur content. The U.S. Environmental Protection Agency requires 80% of the highway diesel fuel refined in or imported into the United States (100% in California) to be ULSD as of 2006. One hundred percent must be ULSD nationwide by 2010. Different requirements apply to non-highway diesel.

Currently, the vast majority of ULSD is produced from petroleum. However, biodiesel; biomass-to-liquids, coal-to-liquids, and gas-to-liquids diesel; and hydrogenation-derived renewable diesel are inherently ultra-low sulfur fuels and could help meet ULSD requirements in the future. Petroleum-based ULSD is not considered an alternative fuel under the Energy Policy Act of 1992 (EPAct), but most ULSD fuels produced from non-petroleum and renewable sources are considered EPAct alternative fuels.

Ultra-Low Sulfur Diesel as a Vehicle Fuel

Ultra-low sulfur content in diesel fuel is beneficial because it enables use of advanced emission control technologies on light-duty and heavy-duty diesel vehicles. The combination of ULSD with advanced emission control technologies is sometimes called Clean Diesel.

Nitrogen oxides (NOx) and particulate matter (PM) are the two most harmful diesel pollutant emissions. These emissions can be controlled with the use of catalytic converters (for NOx) and particulate traps (for PM). However, sulfur—in amounts that used to be allowable in diesel fuel—deactivates these devices and nullifies their emissions control benefits. Using ULSD enables these devices to work properly.

In general, ULSD should cause no noticeable impact on vehicle performance, although fuel economy might be slightly reduced because the process that produces ULSD can also reduce the fuel's energy content. Removing sulfur from diesel reduces lubricity. This issue can be resolved by the addition of additives prior to retail sale that increase lubricity. In addition, blending biodiesel with ULSD also increases lubricity.

Using ULSD in older diesel vehicles might affect fuel system components or loosen deposits in fuel tanks. These vehicles should be monitored closely for fuel system problems and premature fuel filter plugging during the transition to ULSD. New vehicles designed to use ULSD must never be fueled with a higher-sulfur fuel. If kerosene is blended with ULSD for improved cold-weather performance, it must be ultra-low sulfur (15 ppm or lower) kerosene. New engine oils have been developed for use with new diesel vehicles fueled with ULSD.