20 November 2006

The US Environmental Protection Agency (EPA) published a draft guidance document outlining the intended approach for the certification of light-duty diesel vehicles and heavy-duty diesel engines using selective catalytic reduction (SCR) systems for NOx control. As manufacturers intend to use urea-SCR technology for meeting the light-duty Tier 2 and the heavy-duty 2010 diesel emission standards, the EPA has been developing criteria for accepting the SCR technology and ensuring compliance with NOx emission standards.

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In the SCR technology, a NOx reductant such as water-based urea solution is stored in an onboard tank and injected upstream of the SCR catalyst. As urea is consumed in the SCR reaction, it must be replenished at an interval of about 6,000 - 12,000 miles. If the vehicle is operated without urea, the SCR catalyst becomes inactive, resulting in significantly increased NOx emissions. The EPA guidelines address the issues of vehicle compliance (to ensure that vehicles are not operated without urea) and urea availability (to ensure that drivers will find urea when they need it) in vehicles equipped with SCR systems.

The EPA will accept comments on the draft guidance by December 15, 2006.

Source: US EPA