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Diesel Digest: Understanding ultra low-sulfur diesel standards


Diesel Digest: by Capt. Jeff Werner

In 1990, the U.S. Congress amended the Clean Air Act to require stricter reductions of air pollutants. These amendments included more stringent tailpipe emission standards and stricter emission testing procedures for cars and trucks. At about the same time, the Environmental Protection Agency started imposing limits on diesel fuel sulfur content and was responsible for the enforcement of these new regulations. Over the past three decades, the EPA has mandated an overall 99.7 percent reduction in diesel fuel sulfur content when compared with the high-sulfur diesel fuels used prior to the 1990s. This newer type of fuel is called ultra low-sulfur diesel (ULSD).

In 1997, the International Convention for the Prevention of Pollution from Ships (MARPOL) added Annex VI to address air pollution from marine engines on ocean-going vessels. This international convention on pollution from ships limits the amount of emissions from nitrogen oxides (NOx), sulfur oxides (SOx) and particulate matter (PM), and required the use of low-sulfur diesel fuel. In the U.S., the U.S. Coast Guard enforces the MARPOL Annex VI guidelines.

Sulfur oxides, specifically sulfur dioxide (SO2), cause both health and environmental harm. Health concerns related to exposure from SOx include respiratory problems and lung damage. SOx causes haze and environmental harm in the form of tree, plant and stone damage by acid rain. The less sulfur content in fuel, the less SOx emissions that fuel will release. Therefore, a reduction in fuel sulfur content means less pollution.

In 2001, the EPA finalized a federally mandated program called the 2007 Heavy-Duty Highway Diesel Program, which was created to lower emissions from highway diesel engines. This program, effective June 2006, decreased the maximum allowed diesel sulfur level from 500 to 15 parts per million (ppm), essentially mandating the switch from low-sulfur diesel to ultra low-sulfur diesel.

To manage the stricter pollutant limits for marine engines, generators, locomotives, cars and trucks, a phased in approach was developed using a series of steps, which were designated tiers 1 through 4. The higher tier number corresponded to greater pollution reduction requirements.

The EPA then implemented the Clean Air Non-Road Diesel – Tier 4 Final Rule in steps. This rule required sulfur reductions for non-road diesel engines, which includes those used aboard yachts. The first step, in 2007, lowered the maximum diesel fuel sulfur levels from 3,000 to 500 ppm, and the final step from 500 to 15 ppm beginning in 2010. The Tier 4 Final emission standards will be fully implemented at the end of this year.

The use of ULSD is a major component of meeting the SOx reduction required for compliance with MARPOL Annex VI. It is so important that in 2011 “the EPA and USCG entered into a Memorandum of Understanding to enforce Annex VI. The EPA and USCG will jointly and cooperatively enforce the provisions of Annex VI. Efforts to be conducted by USCG and EPA include inspections, investigations and enforcement actions if a violation is detected.”  

Reducing the sulfur content greatly alters the overall chemical composition of diesel fuel. Done at the refinery with a process known as hydrotreating, not only is the sulfur removed, but also the natural lubricity compounds that are needed for fuel injectors to operate without undue wear. Additionally, the energy density and fuel economy is lowered, while the cost of the diesel fuel increases.

Recently, diesel fuel tank corrosion damage hit an all-time high. This is caused by fuel-hauling tanker trucks using a technique called switch loading. That means one day a truck will haul ethanol-based gasoline, and the next day haul ULSD. Initial tank corrosion occurs when ULSD is exposed to small quantities of biofuel. For example, when ULSD contains ethanol due to switch loading, microbial growth increases, and so does the acid level in the diesel – and that acidic contamination is key to continued tank corrosion and rust.

To effectively combat the potential problems associated with ULSD, the use of a broad-spectrum fuel additive, such as AXI International’s AFC 710, when topping off a yacht’s fuel tanks is recommended. This Tier-4 compliant fuel additive will not only add lubricity to the fuel, it will clean out the fuel system and tank, increase fuel economy, increase filter life and increase horsepower.

Capt. Jeff Werner is a 25-year veteran of the yachting industry as a captain on private and charter yachts, both sail and power, and a certified instructor for the RYA, MCA, USCG and US Sailing. He also owns Diesel Doctor ( Comments are welcome below.


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