Automakers are considering installing urea injection systems in their vehicles to meet Environmental Protection Agency (EPA) standards. The requirements, which fully go into effect in 2009, will require that diesel-powered vehicles emit almost no NOx (oxides of nitrogen).

Urea injection, also called Selective Catalytic Reduction (SCR), works by an ammonia-like acid (urea) being squirted from a canister into a special catalyst to reduce NOx in diesel emissions. However, SCR is seen as a controversial system because the EPA has concerns as to how drivers will be alerted if the car is running low on urea. If the tank should run dry, the driver would not notice a change in performance, but the vehicle would no longer be meeting the NOx emissions standards.

According to, SCR systems have been shown to reduce NOx by 65–99 percent, but for the EPA to approve them, several issues must first be addressed. First, car manufacturers need to develop a warning system to prompt the driver to refill the urea. One solution could involve a vehicle only being able to operate in a low-speed limp mode until urea tank is refilled. Burkhard Goschel, BMW Group’s board member in charge of product development, said he believes automakers would be able to design such a system.

diesel aftertreatment bosch

SCR or Selective Catalytic Reduction incorporates a second catalyst mounted behind a conventional 3-way catalyst. In the 3-way device, NO is converted to NO2, which then flows to the SCR unit where, under controlled temperatures and in the presence of a catalyst, it is combined with ammonia and reduced to free nitrogen and water. The urea is a convenient carrier for the ammonia molecule, although other ammonia carriers may be developed. Graphic courtesy of The Robert Bosch Corporation

Other issues involve the availability of urea to the public. Some suggestions include having it in dealership service departments, auto parts stores, filling stations, and oil change businesses. In addition, there are concerns about the system’s packaging, durability, and ability to prevent unreacted ammonia from escaping out the tailpipe.

Automakers must make a decision soon on whether they will pursue urea injection because engineers will need two to three years to design, develop, and test the system. General Motors (GM) is considering urea injection and will make a decision to install it in the next twelve months. David Brown, a GM staff engineer, says chances are better than fifty-fifty that they will use it because SCR is cost effective and durable.

Ford is also considering urea injection and NOx traps, and DaimlerChrysler has confirmed it will use urea injection in its diesel vehicles. In addition, BMW wants to introduce its own diesel engine to the U.S. market in 2007, most likely in the form of a sports utility vehicle (SUV). The company and other European automakers have asked potential suppliers to make urea available to U.S. motorists, according to Goschel.

By 2006, the EPA projects its ultra-low-sulfur fuel requirements will result in cleaner-burning gasoline that contains 90 percent less sulfur, highway diesel fuel that contains 97 percent less sulfur, and a reduction in NOx emissions by more than 90 percent. The agency says its standards will make all cars, SUVs, pickups, and vans 77–95 percent cleaner by 2009.

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