Diesel fuel has been called the lifeblood of American productivity. Due to its high energy content, virtually all engines in public fleets run on it.

But high performance comes with a price, and not just the one you pay at the pump or rack. Sulfur is found in all crude petroleum from which diesel fuel is refined. When burned, the sulfur converts to hydrogen sulfide and sulfuric acid, contributing to acid rain and smog.


Diesel used to contain 3000 to 5000 parts per million (ppm) of sulfur. In 1993, the U.S. Environmental Protection Agency prohibited the use of this “high-sulfur diesel” in on-highway vehicles, but allowed it in non-road applications. On-site generators and portable (but not self-propelled) devices fall into this category. Many agencies maintain separate fuel tanks for non-road applications because high-sulfur diesel, if still available, is less expensive than low-sulfur diesel (LSD), which in 1993 was limited to 500 ppm sulfur (S500) for on-road use.

Since the introduction of LSD, acid rain has become a minor, if non-existent, issue. And although LSD costs more than its high-sulfur counterpart, public fleets have recouped the difference through reduced engine wear and extended preventive maintenance intervals thanks to lower sulfur content.

With less sulfur in the fuel and less acid accumulation in the engine oil, oils designed to neutralize sulfuric acid last longer. Fleet managers who regularly changed oil at less than 200 hours have found, through oil analysis, that oil was remaining serviceable at 250 to 300 hours or longer.

Beginning this year, refiners must reduce sulfur to 500 ppm for non-road use. Small refiners may still provide S500, but it's hard to find. Most refiners have switched all production to lower sulfur levels.


Ultra-low-sulfur diesel (ULSD) with 15 ppm sulfur (S15) is required for all 2007 and later on-highway diesel engines because the new engines use diesel particulate filters with diesel oxidation catalysts (DOC). Sulfur “poisons” the DOCs, interfering with the precious metals so they cannot convert soot to carbon dioxide.

Legally, engines manufactured before 2007 can run on S500 while supplies last. By June 15, though, all diesel engines operating on roads must use S15 diesel. In California, S15 has been required since Sept. 1, 2006, for all applications.

Most experts believe the fuel will have little effect on daily operations, although there might be some loss of efficiency.

Sulfur is a natural lubricant and contributes to the combustion process. ULSD has about 1% less energy content than LSD and less natural lubricity (the property of lubricating an engine's fuel injection system). Since 1% is well within the variability of diesel fuel blends, you may not see lower mileage per gallon.