From the time the Environmental Protection Agency proposed that ultra-low sulfur diesel (ULSD) be run in all on-highway diesel vehicles to when it was fully implemented in late 2006, there has been much debate about this new fuel's effect on older diesel engines. Specifically, the fear that ULSD has a lower lubricity and cetane rating, which could affect pre-2007 diesel engines and fuel systems.
One major oil company has assured us that the proper lubrication formulas are now added and mixed before the fuel we use ever makes it to the pump. Still, many aren't convinced this is the case, as a lot of folks in the pickup segment, and especially those in the trucking industry, noticed a decrease in fuel economy once ULSD became our nation's primary diesel fuel.
The Importance Of Cetane
Diesel engines will operate fine on any cetane rating between 40 to 55, but the higher the number, the shorter the ignition delay is, and the more complete and efficient the combustion cycle becomes. In our research we found that cetane numbers in ULSD fluctuate between 40 to 45. And coinciding with what we've seen, with inconsistencies in cetane ratings at the pump, many diesel owners experience fluctuating fuel mileage.
During the refinement of ULSD, fuel is flooded with hydrogen to remove sulfur, but in the process also removes much of the fuel's lubrication properties, as well as cetane content. This leads to many things, but we're focusing on the fact that cetane numbers aren't consistent. This means each time you fill up at your local station, the quality of fuel is different. Knowing this, we contacted FPPF about using its 8+ Cetane Improver.
The Importance Of Lubricity
Another reason ULSD has been blamed for premature fuel system and engine wear is due to its low sulfur content. At 15 ppm (out of 1 million parts, just 15 can be sulfur), ULSD contains only a fraction of the sulfur concentration found in the previously used low-sulfur diesel (LSD), which was roughly 500 ppm. And since sulfur acts as a lubrication agent in diesel fuel, ultra-low sulfur content in ULSD is thought to be one of the main suspects in causing fuel-related problems in pre-2007 diesels, which were designed to run primarily on LSD.
According to Exxon Mobil, ULSD's refining process can reduce the energy content of the fuel. They say a reduction is minimal, but nevertheless, energy loss occurs. Knowing this, we contacted Opti-Lube about its XPD fuel additive, which is designed to increase diesel fuel's lubricity as well as its cetane rating.
Our Test And Its Parameters
Between ULSD's lower cetane rating and lubricating abilities and the fact that diesel prices in Southern California peaked as high as $5.39 a gallon in the summer of 2008, the idea of testing several fuel additives came to us rather easily. After all, everyone at that time wanted some help in the mileage department, so we set out to bring readers a basic, real-world diesel additive test. The experiment kicked off with us running straight diesel fuel for some baseline fuel economy numbers, followed by a lubricity-only additive (Opti-Lube's XPD), and finally, FPPF's 8+ Cetane Improver.