Ultra Low Sulfur Diesel (ULSD)

Sulfur is a natural component of crude oil and products refined from it. Sulfur dioxide (SO2) is produced when fuels containing sulfur are com- busted. Sulfur dioxide is a major precursor to acid rain. It is also a lung irritant. Furthermore, it can reduce the effectiveness of exhaust after-treatment devices. In 1993, regulations from the EPA and the CARB went into effect. The EPA set a maxi- mum of 500 ppm for on-road diesel fuel, while the CARB required the same level for both on-road and off-road diesel fuels. This fuel was referred to as low-sulfur diesel.

The CARB also limited the maximum aromatic content to 10 volume percent, but provided an alternative compliance mechanism which allows refiners to formulate fuels that, though higher in aromatics, would meet the same or lower emissions of a fuel containing 10 volume percent aromatics. However, for the newest technology exhaust after-treatment devices to operate effectively, sulfur must be kept to much lower levels.

In mid-2006, the EPA required that the majority of diesel fuel produced or imported for on-road use contain no more than 15 ppm sulfur, and this fuel is referred to as Ultra-Low Sulfur Diesel (ULSD). All on-road diesel and most off-road diesel will be required to meet a 15 ppm standard beginning in 2010, with locomotive and marine fuels produced by most refiners required to meet the standard in 2012.

Owners of 2007 model year or newer vehicles must use ULSD, because fuels with higher sulfur levels than ULSD can render their advanced exhaust after-treatment devices ineffective. Model year 2006 and earlier models of on-road vehicles may be fueled with low sulfur diesel where it is still available. Currently 80 percent of on-road diesel fuel must be ULSD. By December 1, 2010 all on-road diesel must be ULSD.

The hydro-treating process that is used to reduce sulfur levels in diesel will also reduce the level of naturally occurring components that provide lubrication properties to diesel fuel. Diesel fuel manufacturers will add the necessary additives to ensure that ULSD meets the lubrication requirements of ASTM D 975. The hydro-treating process can also reduce the energy content of the fuel by 1 to 2 percent. Under normal operating conditions, this small change should not affect overall power, but may reduce fuel economy slightly.