Clean Diesel: Ultra Low Sulfur Diesel

The US EPA 2007 clean air regulations are fast approaching. Diesel users prepare for ULSD; but first, they need a few answers, writes Richard George.


ULSD is coming. Many users of diesel fuel have known about this ultra low sulfur diesel for sometime. It’s the details that aren’t so well-known. Why are we doing this? What are the benefits? Are there drawbacks? What do I need to do to be ready? When will it be available? Must I use it?

To begin, diesel fuel quality in the United States is regulated at the state level, and most states have adopted the latest version of ASTM D 975 Standard Specification for Diesel Fuel as their standard, which defines diesel fuel into various grades. On-road vehicles use grades No. 1 and No. 2 and each grade is split into three sulfur levels, S5000, S500 and S15. The number refers to the maximum amount of sulfur allowed in parts per million, or ppm. The highest level, 5,000 ppm, is equivalent to 0.5 percent by mass. S5000 is commonly known as high sulfur (HS), S500 is low sulfur (LS) and the newest designation, S15, is ultra low sulfur (ULS). Currently, the most common grade is low sulfur No. 2.

Distillates and Diesel 101

Diesel fuel is just one of a group of fuels collectively known as “distillates.” Other well-known distillates are kerosene, jet fuel and fuel oil (for home heating). Each has its own ASTM specification. A refinery produces No. 1 and No. 2 fuels at various sulfur levels and it’s common for one refined product to be sold as multiple retail products, provided that the applicable ASTM specs are met. This sometimes requires special testing or the addition of fuel additives.

For example, a LS No. 1 fuel may be sold from a single tank as Jet A, K-1 kerosene, diesel and fuel oil, but this doesn’t mean that they are interchangeable. Kerosene and jet fuel require very special certification testing and some fuels require special additives before use. The best bet is if you want diesel fuel, purchase “diesel fuel.” More importantly, don’t use anything but ASTM D 3699 K-1 kerosene in your indoor space heater.

Diesel fuel is the most common distillate and LS No. 2 is the most commonly used grade of diesel. Nearly all diesel sold at retail outlets is LS No. 2 diesel, which is also referred to as “Low Sulfur No. 2,” “D2,” “LS2,” “LSD” or any other combination of LS, D and No. 2. Unless otherwise specified, “diesel” to most people means LS No. 2 diesel.

So what is No. 1 diesel? Grade No. 1 fuels are very similar, but lighter or less dense than No. 2 fuels. They have lower viscosity, lower boiling points, lower flash points, and typically have better cold flow properties. They are excellent for use in cold climates and in the winter. The drawback is they usually cost more and also have lower energy content, so there is a trade-off to using a No. 1 fuel—better cold flow, but higher costs.

In transit, bus fleets tend to use No. 1 diesel fuels because they usually have less smoke, less NVH (noise, vibration, harshness) and won’t gel as easily in the winter. This makes the bus more “rider-friendly,” especially the part about not stalling in the cold. It is also very common to “winter blend” a No. 1 diesel with a No. 2 diesel in order to get the cold flow needed without losing too much fuel economy. Typically these blends are between 10 to 30 percent No. 1 into No. 2.

Benefits of ULSD

So where does ULSD fit into this? With all this diesel fuel being combusted in engines, it leads to a lot of airborne emissions, most notably, nitrogen oxides (NOx) and particulate matter (PM). NOx contributes to ozone formation and PM is the black smoke pouring out of the older trucks going down the highway. In 1993, the U.S. Environmental Protection Agency (EPA) lowered maximum sulfur levels in diesel fuel from 5,000 ppm to 500 ppm and created high sulfur and low sulfur diesels. The diesels of today have much less smoke than they did when CB radios and the song “Convoy” were popular. The next step to lowering emissions further is even less sulfur. What’s lower than low? Ultra low of course. If you are looking for less than 15 ppm diesel fuel, make sure you ask for “ultra low,” because simply saying “low” will get you the current on-road 500-ppm diesel fuel.

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