Mercedes-Benz AG has announced details of the emissions system it will be using on diesel cars in the US to meet the 2007 emission limits set by the EPA.

Mercedes will use a SCR (selective catalytic reduction) system, which is a scaled-down version of the system Mercedes will also use on its commercial vehicles.  The SCR system involves introducing urea into the exhaust gases from the engine to react with oxides of nitrogen (NOx).  Most automakers say they should be able to meet particulate matter standards through improvements to the diesel's fuel injection, combustion and exhaust systems. It is the NOx standard that is hard to satisfy. 

In the new Mercedes system, the urea is held in a replaceable canister, from where it is injected into the exhaust system.

Mercedes says the new urea injection system is a cost effective alternative to other NOx limiters, and could be the fillip needed to drive the US diesel market, with other automotive giants also said to be interested.

However, there are some concerns that the emphasis on the consumer to ensure their urea supply is maintained, and hence the emissions technology operates as required under the legislation. Margo Oge, director of the Office of Transportation and Air Quality, part of the Office of Air and Radiation for the EPA told Autoweek: "Companies have to carefully evaluate their business plans and see if this is something that will sell to the consumer."

"The question is whether (drivers) can see urea in a similar fashion like engine oil or transmission fluid," added Bharat Balasubramanian, DaimlerChrysler's vice president of development and engineering technologies and regulatory affairs.  The system will be launched in late 2006 for the 2007 model year.

A spokesman for Volkswagen of America Inc. says that urea is not an option for the New Beetle, Jetta, Touareg, Passat and Golf, the only diesel cars offered in the United States before the Mercedes E320. VW is working on a NOx storage catalyst.