Short answer
Ultra-low sulfur diesel (ULSD) is diesel fuel with a maximum of 15 parts per million sulfur, the S15 grade in ASTM D975. EPA phased it in for highway diesel from 2006 to 2010, for most nonroad engines in 2010, and for locomotive and marine engines in 2012. It made modern diesel exhaust aftertreatment practical.
Why sulfur was cut
Sulfur in diesel forms sulfur oxides and sulfate particulate when burned, and it poisons the catalysts used in diesel particulate filters and other exhaust aftertreatment. Engines built to the 2007 and later US emission standards rely on that aftertreatment, so they require ULSD.
The US timeline
Beginning June 1, 2006, refiners had to make at least 80 percent of their highway diesel as 15 ppm fuel, and by December 1, 2010 only ULSD was available for highway use nationwide. Nonroad diesel moved to 500 ppm in 2007 and to 15 ppm in June 2010; locomotive and marine fuel followed to 15 ppm in June 2012, with some small-refiner provisions extending the transition to 2014.
What changed in the fuel
The hydrotreating that removes sulfur also removes some of the compounds that gave diesel its natural lubricity. ASTM D975 responded with a lubricity requirement, and refiners commonly add lubricity improvers so the finished fuel meets it. Fuel-injection makers pay close attention to lubricity because pumps and injectors are lubricated by the fuel itself.
ULSD also tends to have lower electrical conductivity, which industry groups have flagged as a static-charge consideration during fast tank filling. Fuel handlers follow grounding and bonding practices for that reason.
ULSD and engine oil
The move to ULSD and aftertreatment came alongside new heavy-duty engine oil categories with limits on sulfated ash, phosphorus and sulfur, starting with API CJ-4 for 2007 engines. The fuel and the oil were designed to work with the same exhaust hardware.
Frequently asked questions
Can older diesel engines run on ULSD?
ULSD is the only highway diesel sold in the US, and older engines run on it. Engine makers issued guidance on seals and lubricity during the transition; check with the engine maker if you have a concern about a specific older engine.
Is ULSD the same as winter diesel?
No. ULSD refers to sulfur content. Winter diesel is a seasonal blend, often with No. 1-D or cold-flow additives, and is also ULSD in on-road use.
General maintenance information. Always follow your vehicle or equipment manufacturer’s requirements and the current product data. Questions? Email [email protected].
