ATRI: ULSD Has More Energy Than Engineers First Expected
By Jonathan S. Reiskin, Associate News Editor
This story appears in the Jan. 19 print edition of Transport Topics. Click here to subscribe today.
Ultra-low-sulfur diesel, the standard fuel of heavy-duty trucking since late 2006, has more energy content than engineers originally had expected, a welcome surprise, according to a study from the American Transportation Research Institute.
“We didn’t see the loss [of en-ergy] that many had expected,” said Mike Tunnell, principal investigator of the Jan. 15 study and the institute’s director of environmental research.
Tunnell said it would be inappropriate to conclude that the reduced-sulfur fuel definitely packs more potential energy, leading to better fuel mileage. But he said it was comforting to find that the fuel is at least roughly similar in its energy content to the substance it replaced.
ATRI hired a laboratory that found a 0.4% increase in the energy content of diesel samples taken early in 2007, compared with samples taken a year earlier. After October 2006, maximum permissible sulfur content dropped to 15 parts per million from 500 ppm.
The study also said 2007 ULSD is more similar to California and Texas specialty diesels than was 2006 diesel.
Al Mannato, fuels issues manager for the American Petroleum Institute, described the ATRI study as “solid” in its formulation, but not nearly broad enough to characterize conclusively the nation’s diesel supply.
“Our [initial] general assessment, from industry and government, was a 1% reduction in energy density. That’s what we thought would happen, going in,” Mannato said.
Although Mannato said he would like to see a more extensive sampling before reaching a final conclusion, he said ATRI’s study is the first research he has seen on the subject and he does not know of anyone else working on the topic.
Tunnell said that ATRI, an affiliate of American Trucking Associations, commissioned the study to see how large the supposed drop in energy content might be.
ATRI had the lab analyze the fuels’ British thermal unit content, or heat, per gallon in accordance with methods developed by ASTM International, the Pennsylvania-based engineering standards organization.
The ATRI study said that, on average, the 2007 ULSD had about 138,370 Btu per gallon, slightly more than the 137,840 Btu in the 2006 sample. The Environmental Protection Agency ordered the decrease in sulfur content ahead of its January 2007 mandate of diesel particulate filters for truck engines. Truck makers put DPFs on their engines to meet tighter federal regulations on particulate matter emissions.
Before the 2006 switch to ULSD, engineers for truck and engine makers and fleet maintenance managers discussed at industry meetings what might be the unintended consequences of the installation of DPFs and the switch to ultra-low-sulfur diesel.
The study found that the sulfur content of ULSD sold in 2007 was well below EPA’s 15 ppm limit, with an average content rating of 6 ppm.
Tunnell said ATRI’s work is not the final word on the subject. “We’re anxious to get the discussion going and to hear more on this,” he said.
The study — “Energy and Other Fuel-Property Changes with On-Road, Ultra-Low-Sulfur Diesel Fuel” — also looked at what it called “boutique” diesel blends required by California, Minnesota and Texas.
Minnesota’s rule requires mixing of 2% biodiesel (fatty acids derived from vegetable oils or animal fats) into petroleum, but the California and Texas rules seek to control emissions by setting standards for cetane and aromatic compounds. Cetane in diesel is somewhat analogous to octane in gasoline, with higher amounts required in certain engines.
Aromatics contain benzene rings and are to be minimized.
The cetane rating in the ATRI samples averaged 49.8 in the 2007 ULSD, up from 48.6 in the prior year. Meanwhile, the volume of aromatic compounds declined to 22.9% from 32.2% in the 2006 regular diesel.
The ATRI study said California and Texas want 10% caps on aromatics or a demonstration of emissions equivalency. California does not have a cetane standard, ATRI said, but Texas seeks a minimum rating of 48, which ATRI’s ULSD average finding exceeds.
