EPA Issues Guidance on Urea SCR for Diesel Emissions Control
30 March 2007
The EPA has issued guidance on emission certification procedures for on-road diesels that use selective catalyst reduction (SCR) technology for NOx reduction. This guidance enables automakers, for the first time, to adapt the technology to light- and heavy-duty vehicles in the US.
The specific area of concern is the use of a reducing agent (e.g., AdBlue) injected into the exhaust gas upstream of the catalyst. Without the reducing agent, the efficiency of the SCR catalyst drops to zero and NOx emissions can increase substantially. Many automakers, however, are looking to such urea SCR solutions as the near-term solution for meeting Tier 2 Bin 5 emissions regulations for diesels.
The guidance letter is not a ruling—it does not establish a final certification process, but rather reflects the agency’s current thinking and direction.
The letter focuses on specific regulatory requirements that can impact the certification and implementation of SCR for light-duty and heavy-duty diesel vehicles and heavy-duty diesel engines: Allowable Maintenance and Adjustable Parameters.
Allowable maintenance. Under existing regulations, emission-related maintenance can not occur before 100,000 miles of use (150,000 miles for medium- and heavy-duty engines) or before 100,000 mile intervals thereafter. Because the SCR catalyst in a urea SCR system does not function without the use of a reducing agent, EPA believes that the 100,000-mile interval applies to the SCR catalyst and all of the associated hardware, including but not limited to, the reducing agent, the reducing agent storage tank, the dosing valve, and all lines and hoses.
The problem with that from the OEM’s point of view is their inability to equip vehicles with storage tanks of sufficient size to allow for that 100,000-mile interval. The regulatory fix to this is for OEMs to request a change to the scheduled maintenance interval, based on the technological necessity of a new maintenance interval. EPA opened the door for that with the guidance letter, and suggested that “It may be appropriate for EPA to approve an industry-wide scheduled maintenance change, as we have done previously in similar situations.”
Adjustable parameters. Emissions certification testing is done across a range of parameters that can be (a) physically adjusted and (b) affect the emissions outcome. EPA is considering an SCR system that requires a reducing agent to meet the definition of an adjustable parameter—i.e., without the reducing agent, the NOx control fails.
This means that we have the authority to test an SCR-equipped vehicle with varying levels of reducing agent in the storage tank, or, theoretically, without any reducing agent at all. If the vehicle is capable of meeting the NOx standard without any reducing agent, we would not consider the SCR system to be in violation of the standard. However, if the vehicle exceeds emissions standards without reducing agent in the tank, we expect that we would deny the certification because the design will be considered unacceptable. If the manufacturer can prove to 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.
EPA then divides its acceptance criteria for urea SCR systems into two categories: vehicle compliance and reducing agent availability and accessibility.
Vehicle compliance. There are five different categories for vehicle compliance, and manufacturers must satisfy all five:
- Driver warning system
- Driver inducement
- Identification of incorrect reducing agent
- Tamper resistant design
- Durable design
Reducing agent availability. EPA will review each manufacturer’s plan for reducing agent availability and accessibility, with particular emphasis on the following procedures:
- Reducing Agent Available at Dealerships
- Reducing Agent Available at Truckstops
- Back-Up Plan
The EPA also wants to see public eduction plans from each manufacturer. The agency also calls for the establishment of an industry-wide reducing agent quality standard and specifications. SCR systems will need to be designed to operate using the general range of commercially available reducing agents. The agency also notes the importance of having a clear and unambiguous industry-wide identifier, regardless of any brand name.
DaimlerChrysler, with its focus on AdBlue urea SCR systems, was very pleased to have the guidance in hand.
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 US market.—Dieter Zetsche, CEO DaimlerChrysler
Mercedes will offer BLUETEC diesel-powered versions of its M-, R- and GL-Class sport- utility vehicles in the United States beginning in CY 2008. The BLUETEC SUVs will use AdBlue injection with SCR.
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