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Study finds advances in engine and aftertreatment technology result in declining benefit from California diesel spec

A team from the University of California Riverside and the California Air Resources Board (ARB) has found a declining emissions (NOx, PM) benefit from the use of California diesel compared to US Federal diesel fuels given the continuing advances in engine and aftertreatment technology. Their study is published in the International Journal of Engine Research.

The California Air Resources Board (CARB) has regulated the properties of diesel fuel sold in California since 1988 to lower emissions of particulate matter (PM) and oxides of nitrogen (NOx). Among the properties of California diesel are ultra-low sulfur (15 ppm max) and an aromatic hydrocarbon context of a maximum of 10%.

Subsequently, the US Environmental Protection Agency (EPA) issued regulations for ultra-low sulfur diesel. California Assembly Bill 679, approved by the Governor in 2006, required ARB to convene a panel of interested parties to develop a test protocol to measure the differences in NOx and particulate matter (PM) emissions between CARB diesel and EPA 2006 ultra-low sulfur highway (Federal) diesel fuel.

Although many studies have shown that reduced levels of aromatics and higher cetane numbers can improve emissions, the actual impact of CARB fuels on in-use diesel emissions has not yet been extensively studied, especially as diesel engine and aftertreatment technology has evolved over the years. This study evaluates the differences between California and Federal diesel fuels with heavy-duty engine and chassis dynamometer tests.

—Hajbabaei et al.

Among their findings:

  • Engine dynamometer results showed that NOx emissions for the Federal fuels ranged from 4.7% to 9.5% higher than the CARB diesel. These NOx reductions are similar to the estimates being used in the latest regulations.

  • Chassis dynamometer test results did not show as consistent trends for NOx as those seen for the engine dynamometer testing. For the chassis dynamometer testing, four out of ten vehicles showed consistent reductions in NOx, with emissions for the Federal fuels ranging from 3.3% to 9.9% higher than the CARB diesel, while the other six vehicles did not show consistent fuel impacts.

On an absolute level, the NOx benefit for CARB diesel shows a decline with continuing advances in engine technology. The results showed that CARB diesel did not show strong benefits for PM. The results also showed that the introduction of aftertreatment systems for PM and NOx will, over time, largely eliminate any potential benefits that might be obtained through the use of CARB diesel, although NOx benefits will persist through to 2020.

—Hajbabaei et al.

The team, led by Dr. Thomas Durbin, had earlier found a relatively clear trend of increasing NOx emissions with increasing biodiesel blend level at levels of B20 and above for CARB-like/high cetane diesel fuels. (Earlier post.)


  • Maryam Hajbabaei, Kent C Johnson, Jim Guthrie, Thomas D Durbin (2013) Assessment of the emissions from the use of California Air Resources Board qualified diesel fuels in comparison with Federal diesel fuels. International Journal of Engine Research vol. 14 no. 2 138-150 doi: 10.1177/1468087412446883



CARB requirements are too lenient.

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