By Paul Abelson
senior technical editor
After eight months using ultra-low sulfur diesel, it’s time to review truckers’ experiences and problems, then examine steps to alleviate the problems. We’ll look at detergency, lubricity, maintenance issues, winter operations, fuel economy and cost.
Low-sulfur diesel differs from ultra-low sulfur diesel – ULSD – in sulfur content and molecular structure. Hydrotreating, which is refining with added hydrogen, changes the ratio of aromatic molecules, which are hard to burn, and paraffinic molecules, which are easy to burn. Over time, low-sulfur diesel tends to oxidize and leave deposits inside fuel tanks. With low-sulfur diesel, the process was constant and stable.
However, hydrotreated ULSD acts like a detergent on the low-sulfur diesel deposits. The longer a truck has been in service, the more deposits it will have accumulated. ULSD starts dissolving these deposits as soon as it is introduced into a fuel tank. Filters and screens remove the deposits from most bulk fuel tanks, but the detergent action starts again in trucks’ fuel tanks.
Within weeks of the introduction of ULSD, many engines experienced low power, poor fuel mileage and rough running. All are signs of fuel starvation. In most cases so far, changing the primary fuel filters made the problems disappear. With a few older trucks, which have more deposits, it takes longer, so a second filter change is needed.
Sulfur is a good lubricant. The hydrotreating process that removes it also results in fuel that is “dryer” or has less inherent lubrication. Hearing these facts, shade tree mechanics decided too much lubricity was lost, so they started adding motor oil to ULSD. That fix was even advocated on at least one national radio show for truckers.
Luckily, this trend was nipped in the bud. Not only is oil not needed, it has additives that create sulfated ash and other contaminants when burned. Fuel additives serve many useful purposes, but motor oil is not a fuel additive.
Low-sulfur diesel and ULSD have had to meet minimum standards for lubricity since January 2005. Set by the American Society for Testing and Materials, Standard D975, mandates tests and standards for on-highway diesel fuel lubricity.
Seals in fuel systems absorb some fuel and swell to stay soft and maintain barrier performance. ULSD can dry and shrink some seals, causing small leaks. Good maintenance involves checking seals for leaks at every PM.
Paraffinic ULSD has a naturally higher cloud point, the temperature when wax crystals become visible, than low-sulfur diesel. Gelling and cold filter plugging take place at higher temperatures than with low-sulfur diesel.
Additional quantities of more-costly-to-refine No. 1 diesel are needed to match cold weather performance of old low-sulfur diesel, so winter ULSD prices increased more than expected. But blends may not flow without winter additives.
ULSD has from 1 percent to 3 percent less energy content than low-sulfur diesel. It’s within the variability allowed for diesel, so you may not have noticed any fall off in fuel mileage. Some may have found up to a 3 percent drop, especially in winter. That could mean a loss of 0.1 to 0.2 mpg for most drivers.
EPA originally estimated that ULSD would be about 5 cents per gallon higher than low-sulfur diesel because of additional steps to produce it.
Since the introduction last fall, actual price increases during the winter were closer to 8 cents per gallon, according to the U.S. Energy Information Administration.
Up north, winter blends have added up to 10 cents per gallon. The difference has dropped to right around 5 cents per gallon more for ULSD since the start of spring.
The long and short of it
Correctable problems with filters have likely been corrected by now. Longer term problems with seals need checking on a regular basis.
Additives help keep trucks running in winter. And, the loss of fuel mileage coupled with the increase in fuel cost is just something we can’t control and have to learn to live with.
Paul Abelson may be reached at email@example.com.