The Evolution of Ultra-Low Sulfur Diesel (ULSD) and Its Environmental Impact

Ultra-low sulfur diesel (ULSD) significantly reduces emissions and environmental impact by lowering sulfur content and enabling advanced emission control technologies.

• By Ellie Gabel
• May 24, 2024

Mitigating the environmental impact of fossil fuels has been a longstanding endeavor. To that end, the EPA has mandated several new standards to curb pollution and improve air quality. These led to the advent of ultra-low sulfur diesel — a cleaner fuel with significantly lower emissions. Explore the fascinating history behind the emergence of ULSD, its impact on combustion engine operations and how it stacks up against other fuels.

What Is ULSD and Why Does it Exist?

Ultra-low sulfur diesel is a diesel fuel with a significantly lower sulfur content of 15 parts per million (ppm) or less. This makes it around 97% cleaner than conventional on-road low-sulfur diesel (LSD), which contains an average of 500 ppm sulfur.

ULSD results from numerous EPA regulatory actions aimed at decreasing the ecological impact of burning diesel for energy in highway and non-highway applications. Sulfur causes soot or particulate matter buildup — the primary culprit of the toxic black fumes from diesel-powered engines. The move toward ULSD stemmed from the need to limit these harmful exhaust emissions and ensure better air quality.

The Evolution of Ultra-Low Sulfur Diesel

ULSD’s history can be traced back to the Clean Air Act of 1970, which sanctioned the development of regulations to curb the rate of noxious pollution from industrial and automobile sources. These encompassed compressive standards with expanded enforcement authority at the federal and state levels.

Furthermore, this legislation coincided with the enactment of the National Environmental Policy Act, which established the EPA. From then on, the EPA would become the primary federal agency responsible for creating and enforcing the various requirements stipulated in the Acts.

Clean Air Act Amendments of 1990

In 1989, President Bush proposed several extensive revisions to the Clean Air Act to increase the EPA's authority and responsibility in curtailing major environmental threats, such as urban air pollution, acid rain and toxic air emissions. One of the amendments specifically called for using low-sulfur fuels to achieve set air quality goals. Other revisions included provisions for a framework to promote alternative clean fuels derived from grain and natural gas to minimize reliance on oil imports.

Concurrently, the EPA started imposing stricter emission reductions of hazardous chemicals that deplete the ozone layer, such as carbon monoxide, nitrogen oxides (NOx) and particulate matter. This led to the agency introducing more stringent auto and other mobile emission standards.

In 1991, the EPA established the final rule on the first federal “Tier 1” standards, which focused on capping emissions from diesel engines. It set goals to reduce hydrocarbons by 40% and NOx by 50%. These measures would apply to light-duty vehicles in the 1994–1996 model years.

Tier 2 and 3 Standards 

In 1998, the EPA submitted a report to Congress highlighting the need for further emission reductions. This resulted in the Tier 2 standards, which mandated cars and light trucks to reduce emissions by 77% to 95%. These regulations were phased in for covered engines over the 2004–2009 model years. The standards also required removing over 90% of sulfur content in gasoline by the start of 2004.

Tier 3 standards imposed even stricter controls, requiring a 70%–80% decrease in harmful emissions and a two-third reduction in fuel sulfuric content. Many manufacturers introduced advanced engine exhaust treatment systems to meet these requirements.

Highway Diesel Program 

The highway diesel fuel sulfur program of 2001 set the foundation for the nationwide transition from LSD to ULSD, though the change officially began in 2006. At its establishment, the program enabled advanced emission control technologies for newer on-road diesel engines.

However, sulfur buildup easily damaged these components, requiring a more dramatic reduction from 500 ppm to 15 ppm in the sulfur-diesel content ratio. As a result of these changes, most of the diesel fuel sold in the U.S. for on-road use is ULSD.

What About Industrial Diesel Fuel?

At the Tier 1–3 stages, environmental regulations regarding sulfur content did not apply to nonroad diesel fuel. These are fuels consumed in construction, mining, agriculture and other industries, from small skid steer loaders to large forklifts and cranes.

The Clean Air Off-Road Diesel Rule under the Tier 4 emission standards addressed these requirements. It mandated sulfur reductions for nonroad diesel engines from LSD in 2007, followed by a transition to ULSD in 2010. The rule also required fleets to retire older vehicles or repower engines with emission control strategies before deployment.

ULSD used for industrial applications may be exempt from federal and state taxes. As such, fuel suppliers often add a few drops of red dye to differentiate it from on-road diesel.

How Eco-Friendly Is ULSD? 

Before the EPA began its sulfur regulation programs, diesel fuel in the U.S. contained as much as 5,000 ppm of sulfur. With the emergence of ULSD, there has been a 90% reduction in vehicle particulate emissions, while other pollutants like NOx have decreased by 25% to 50%.

However, what makes ULSD truly eco-friendly is its massive reduction of sulfur content in diesel fuel, which is directly responsible for a decrease in sulfur dioxide (SO2) emissions. They can cause considerable damage to vegetation and heavily pollute water bodies. SO2 can also lead to health problems, impairing lung functions and affecting the respiratory system.

The marked reduction of sulfur in diesel fuel has also facilitated the industry-wide adoption of advanced emission control devices, such as diesel particulate filters and exhaust gas recirculation systems. In addition to enabling enhanced pollutant removal, these innovations have improved fuel efficiency in diesel engines. As a result, diesel is 20%–30% more efficient than gasoline, and produces lower levels of carbon monoxide and dioxide.

Due to these environmental benefits, all diesel-powered engines made in 2007 or later must use ULSD. High-sulfur fuel will damage the emissions control systems and void the engine warranty. Additionally, using such fuel is against federal law and will likely incur prosecution.

The Drawbacks of Ultra-Low Sulfur Diesel

While highly beneficial, ULSD presents some drawbacks, with increased pump prices being the most significant. Removing sulfur from diesel adds a step to the fuel production process, raising the cost of the end product. That’s one of the reasons diesel is more expensive than gasoline today.

The sulfur reduction process also strips ULSD of its lubricity, which is essential for keeping the engine's pumps and injectors lubricated. Lower lubricity levels mean the engine will likely experience more wear over time, resulting in higher maintenance costs.

What’s Next for ULSD?

Diesel fuel has come a long way, especially from an environmental standpoint. Only time will tell if the EPA will further scrutinize diesel emissions, which may lead to additional tier standards. If it does, the mandates will likely involve a stronger push toward transitioning to biodiesel and other plant-based biofuels. These alternatives are renewable and biodegradable, making them a viable option for fuels in the future.