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Mechanics Corner: The truth about diesel fuels

Levi Perkins Published on 09 February 2012
Is diesel fuel the same as it was 50 years ago? Do you receive the same quality of diesel fuel from one fueling station to the next? Is there a difference in some diesel fuels?

Historically speaking, diesel fuel was developed in the mid-1920s to meet the growing demands of the newly introduced diesel engines and vehicles.

Diesel fuel was classified as medium-weight oil in comparison to gasoline being a lightweight oil. Diesel fuel was similar to kerosene in some ways but very different in others.

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Diesel can be classified in several ways. We will discuss three of them in this article. The first way is by its sulfur content. The next difference in diesel fuels can be in it being an on-road or off-road fuel. The last way is by its volatility.

When diesel fuel was first introduced over 80 years ago, it was a very high-sulfur fuel and was primarily designed for the early diesel fuel systems. Higher sulfur content resulted in greater lubricating value but also in higher emissions or particulate matter.

Since the introduction of diesel fuels and now with the current demand for higher emissions standards, diesel fuel has changed.

Since its birth in the mid-1920s, diesel fuel has always had a high sulfur content. In 2006, the Environmental Protection Agency (EPA) mandated that all diesel fuels should be low-sulfur. Low-sulfur would be designated by 500 ppm (parts per million) units of sulfur.

By the end of 2010, EPA again mandated that all diesel fuel intended for use on road should be ultra-low-sulfur diesel fuel.

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Ultra-low-sulfur diesel fuel or ULSD was designated by 15 ppm units of sulfur. Off-road fuel could still be low-sulfur fuel. Then again in 2011, all diesel fuels produced in the U.S. were mandated to be ULSD, whether they were intended for on road or off-road use.

What does all of this mean? It means several things. First, it means diesel fuel contains as much as 100 times less sulfur content than it did 10 years ago. Although it is a much cleaner fuel, this reduction in emissions does come with a cost.

Did you know ULSD contains less heat energy per gallon than high-sulfur fuels due to the refining process? This can constitute for a lack in performance when comparing the two variations of diesel fuel. Another side effect of the lack of sulfur in the fuel is that many of our older fuel injection systems suffer.

Less sulfur indirectly means less lubricating value and less lubricating value equals older fuel systems that fail a lot sooner than intended. Have you ever wondered why 15 years ago you rarely ever lost a fuel injector in your tractor, but now you change or repair them often?

The vast majority of the older fuel systems were designed for high-sulfur fuels – not ULSD. If you are conscious of your older diesel equipment or vehicles, it would pay to put a diesel lubricant additive in with each tank of diesel.

For a long time many diesel consumers believed that the non-taxed off-road fuel was still high-sulfur fuel and only the on-road fuel was ULSD. The only difference currently in the two fuels is the red dye used to distinguish the taxed fuel from the non-taxed fuel.

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Other than that the two fuels are identical in their chemistry and sulfur content. Many diesel consumers believe there are not any differences even in diesel fuel from one pumping station to the next or from one time of year to the next. This brings us to our next point on fuel – volatility.

The volatility of diesel fuel is something very few diesel consumers will believe they have even paid attention to. Have you ever put a winterized blend of diesel fuel in your equipment or diesel vehicle during the winter months?

Winterized fuels can be a blend of No. 1 diesel and No. 2 diesel fuel. No. 1 diesel has a higher volatility than the No. 2 diesel.

This means that not only can it atomize better or turn from liquid to vapor easier, but it also has a lower gelling point. A lower gelling point aids fuel to keep it from gelling or waxing in colder temperatures. This is great for keeping your diesel vehicle running in the winter.

Although winterized fuels are great for cold temperatures, they do actually have one downside. No. 1 diesel fuel has a lower cetane rating than No. 2 diesel fuel. Cetane rating refers directly to the ability of the fuel to combust – in other words, the combustion quality of the fuel.

Cetane ratings in diesel fuel can be compared to octane ratings in gasoline fuel – the higher the number, the better. Most No. 2 diesel has a cetane rating of anywhere from 50 to 55 points.

Most No. 1 diesel fuels have 40 to 50 cetane points. Simply put, there is less heat energy in one gallon of No. 1 diesel fuel than there is in one gallon of No. 2 diesel fuel.

This will explain why your diesel pickup gets 18 miles per gallon in the summer and when winter hits it only gets 15 mpg. It will also explain why your diesel equipment has a harder time starting in the winter months. No, it is not just because it is cold out.

The quality of winterized fuel is less than summer fuel if it is a blend. Likewise, diesel fuels can easily be winterized by additives made by various manufacturers and not lower the cetane levels. They may even boost them.

In short, the next time you pull up to the diesel pump, you may consider asking yourself the following questions:

• Do I know what the cetane rating is on this fuel?

• Is this a winterized blend of No. 1 and No. 2? If it is a blend, what is the percentage?

• Is the fuel I am using ultra-low sulfur diesel fuel? If it is, do I need to put additives in for my older fuel system?

If anything, maybe you’ll think twice before you pull up at the gas station and just look for the green nozzle. PD

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Levi Perkins

Instructor
Diesel Technology Program
College of Southern Idaho

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