Kickstarting "Clean Diesel"

Many of Europe's top automotive brands have long been pushing the U.S. Environmental Protection Agency (EPA) to allow the new common-rail "clean diesel" technology, which has become so prevalent in Europe, to enter the American market. But the EPA has always been skeptical about the technology, particularly because it requires the consumer to occasionally add a chemical mixture to the vehicle in order to facilitate the special catalysts that make the technology work. Now, in a decision that came at the end of March, the EPA made a move that, it says, "helps pave the way for putting more innovative and fuel-efficient 'clean diesel' cars and trucks on America's roads."

The big first step has come in guidance to auto manufacturers on emission certification procedures for on-road diesels that use "clean diesel," which is referred to in the industry as selective catalyst reduction (SCR) technology. The EPA noted that while SCR has been used successfully in other applications, its guidance enables automakers for the first time to adapt the technology to light- and heavy-duty vehicles on American roads.

SCR reduces emissions of the ozone-forming pollutant nitrogen oxide (NOx), the key ingredient, along with sunlight, in creating smog. It uses a nitrogen-containing "reducing agent" (usually ammonia or urea) that is injected into the exhaust gas upstream of the catalyst. The ammonia or urea injection is very effective in reducing nitrogen oxide, but the systems must be periodically replenished with the agent for the catalyst system to limit NOx production. As the EPA wrote in its letter of guidance, "Without the reducing agent, the efficiency of the SCR catalyst drops to zero and NOx emissions can increase substantially." 

This has been the major obstacle to the approval of the technology, since the EPA has had a time-worn bias against systems that involve significant amounts of maintenance. By its letter, however, EPA has opened the door to examine and potentially approve some "clean diesel" SCR systems.

While the EPA spelled out quite clearly that it would still flunk SCR-equipped vehicles if the vehicles exceed emissions standards because of the lack of "reducing agent," the sentence from the guidance letter that gives manufacturers hope reads: "If the manufacturer can prove to the EPA that their SCR system design will not run out of reducing agent in-use and thus not exceed the emission standards, we may determine that the design is acceptable and approve certification of the vehicle design."

This clears the way for manufacturers to submit SCR systems for testing and potential EPA certification that do not require a great deal of consumer maintenance and refilling of reducing agent tanks. Further, the EPA has spelled out prerequisites for its acceptance of such systems, all designed to make as certain as possible that the reducing agent tanks remain filled during the car's period of use. The agency specified that "clean diesel" systems must include a warning to the driver if the reducing agent tank is nearing empty and it offered other standards as well, including clearly labeled, easy-to-purchase reducing agents and durable, tamper-resistant design of the entire system.

The crack in the EPA's door on this issue was greeted warmly by one big "clean diesel" advocate, Mercedes-Benz. Dr. Dieter Zetsche, chairman of the board of management DaimlerChrysler and head of the Mercedes Car Group, issued a statement saying, "Mercedes-Benz welcomes and supports the EPA's announcement on Selective Catalytic Reduction (SCR) guidelines, which represent a critical next step for the future acceptance of diesel vehicles in the U.S. market. This decision, teamed with the Agency's recent mandate for Ultra Low Sulfur Diesel (ULSD) fuel availability, serves to reinforce diesel's benefit as a viable alternative to help reduce fuel consumption and ultimately, reduce oil imports."

The luxury brand recently announced plans to offer BLUETEC diesel-powered versions of its M-, R- and GL-Class sport-utility vehicles in the United States beginning in 2008. The new BLUETEC SUVs are expected to be the world's first diesel-powered vehicles to meet the EPA's stringent BIN5 emissions standards for all 50 states. The BLUETEC SUVs will offer AdBlue injection, a process that adds precisely measured quantities of a urea-based solution into the exhaust stream to help selectively reduce nitrogen oxide emissions by up to 80 percent.