efficiency, and emissions. These characteristics are cetane number (a measure of fuel self-ignition in the CI cycle—important in cycle efficiency, but also in low-temperature operation), density/heating value (a measure of volumetric energy content), lubricity (important for fuel system wear and durability), and sulfur level (important for proper operation of the engine exhaust aftertreatment system).
In the U.S. market, there is only one diesel fuel suited for on-road transportation; its characteristics are specified by the ASTM Standard D975. Most state regulations require the enforcement of these specifications. In the EU, where light-duty CI diesel passenger cars are widespread and about half the new cars are powered by diesel engines, the diesel fuel is specified by the EN590 standard. There are significant differences between the EU and the ASTM standards. The EU fuel has much higher cetane (e.g., 52 versus 40-48), the fuel density is limited to a minimum to assure adequate energy density (no limit exists in the ASTM standard), and the lubricity is better. In terms of fuel sulfur, European fuel has similar levels to U.S. fuels, for which sulfur level is regulated by the 2006 EPA standards to 15 ppm or less.
In the near future, most diesel passenger cars in the United States will be imports from Europe. Their engines have been adapted for use of U.S. diesel fuel, and the manufacturers do not expect to encounter performance and emission issues connected with the fuel, as long as fuel specifications are enforced and quality is adequate. Cylinder-pressure-based closed-loop control, as discussed earlier and utilized in one of the new 2009 CI diesel vehicles, can adjust for market variability in the cetane number of the fuel and provide compensation over the entire operating engine map. The lower lubricity of the U.S. diesel fuel requires protective coatings for the high-pressure pump in the fuel injection system. As noted earlier, the ultralow level of sulfur in the fuel regulated to less than 15 ppm is a necessary enabler for the efficient and durable operation of the exhaust aftertreatment system. Nonetheless, all OEMs marketing CI diesel vehicles in the North American (NA) market have concerns over the seasonal and regional variability of diesel fuel as well as the enforcement of fuel quality.
At present, the ASTM D975 fuel standard allows up to 5 percent biodiesel blend stock in the fuel provided the blend stock meets the characteristics of the ASTM standard. The European OEMs exporting diesel vehicles to the United States have stated that their engines are robust to this fuel blend and that performance and emissions are not affected as long as the blend is at or under 5 percent. For the European market, the manufacturers may allow up to 7 percent FAME (fatty acid methyl ester), plus up to an additional 3 percent hydrogenated biofuel. The difference in the proportion allowed by the European OEMs for the U.S. market versus for the European market is due to their concern over the qual-