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EPA Issues Guidance on Urea SCR for Diesel Emissions Control

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:

  1. Driver warning system
  2. Driver inducement
  3. Identification of incorrect reducing agent
  4. Tamper resistant design
  5. 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:

  1. Reducing Agent Available at Dealerships
  2. Reducing Agent Available at Truckstops
  3. 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.




Ewww why would anyone buy a diesel car? Dumbest thing I've ever read. Only thing diesel is good for is trucks and school buses. Even those stink. They should ban all diesel trucks. Force them to convert to clean gasoline. Diesel Mercedes. LOL!!!!!!


Theres a simple solution to all this. Urinal on the side of the car.



Lev II and Tier 2 bin 5 regs require diesel exhaust to be as clean as gasoline for passenger cars. So your arguments are outdated.


No cervus. Your face is outdated. Get a life.


No go, Bob. Discriminating to women.

Rafael Seidl

The wording on this guideline appears muddled. On the one hand, EPA accepts that it is neither possible nor sensible to lug around enough reducing agent to last 100,000 - 150,000 miles. It also accepts that manufacturers can only do so much to prompt drivers to avoid running out of reducing agent.

At the same time, the agency asserts it is free to conduct certification testing with an empty reducing agent tank and, to deny certification if the vehicle then fails to pass. Of course no manufacturer is going to install an expensive SCR system if the vehicle could pass even without any reducing agent!

EPA needs to clarify what actions manufacturers are supposed to implement if the reducing agent level is (a) low and (b) zero. The options range from various warnings to the driver to a limp-home mode to stranding the vehicle. A progression of responses is also conceivable.

Similarly, EPA needs to clarify the appropriate response to adulterated or otherwise unacceptable liquids in the reducing agent tank and, at what level of confidence this response must occur.

Wrt the the industry-wide indentifier, I'd suggest doing what the pharmaceutical industry does: label product with both the branded product name and the active ingredients.

@ Frank, Bob, sadlkf;jghh:
Do you get paid for making such ignorant and asinine comments? Kindly bring yourselves up to speed on the state of the art, e.g. starting here, or take your prejudices elsewhere.


As Rafael has pointed out, maybe a way to prevent people from riding around without the reducing agent is to enforce some kind of retribution system. For example, the ECU only frees 80% of power if you’re running low and 50% if you ran out. Most people (especially males) wouldn’t appreciate to have their ride so severally “castrated” and would act in advance.

Rafael Seidl

MH -

EPA would not allow you to drive around on even 50% of rated power forever. Rather, if the industry can persuade them to allow a limp-home mode at all, it will most likely be limited in some way - e.g. by distance driven since you ran out. Other ideas have been proposed as well.

Of course, the driver will be warned multiple times before that happens. EPA is also "recommending" that manufacturers offering vehicles with SCR technology offer a 24/7 1-800 number to help motorists find the nearest source of reducing agent. Btw, a full tank of the stuff will see you through 15,000+ miles of normal operation.

The agency is struggling with the concept of an emissions technology that requires regular co-operation by the customer. The default assumption appears to be that all drivers of all vehicles are basically crooks to begin with.


Why is it that new diesel emission systems are so important and that the EPA has so much control over the manufactures. Adding the ADblue liquid is a maintenance issue, simple as that and the owner is responsible for maintaining the motor. If you do not maintain any engine, such as fill the oil, the engine will stop running. There is no US agency regulating an oil tank/filter for life or a 800 number for the motorist. Or if you do not change the spark plugs an engine will eventually run very poor, increasing the emissions as well. There is EPA standing in the way for gas fueled motors due to people not changing there plugs per the manufactures guidelines.

If emissions are SO critical, then the EPA should be checking ALL cars to ensure they meet the standards at the time of manufacture and fining the owners that do not comply.


You tell 'em BillW,
New cars should constantly test their own emissions, telephone the EPA when they're out of tune, starting the process which ends in a mailed citation arriving at the doorstep of the owner.

Now lets extend this process to transportation where it truly matters, to retrofit particulate and NOx traps with sensors and cell phone into older cars, trucks, buses, trains, boats, ships and planes.

But let's not stop there. Extend the system to all smokestacks, big and small. Think of the revenue! This will make parking tickets and moving violations seem like tiddly-winks.

Now let's write-up people and companies for having uninsulated homes and businesses. Make them all board-up their shiny glass office towers with insulation. Won't our cities look swell.


Why not make it clear to owners of such diesel cars that they will not start/run without the urea, just as they will not start/run without the fuel? Such a simple solution...


Hold on there JC, My point was that the EPA seems to be unfairly targeting new diesel cars and that they dont apply the same logic to other type of cars.

I understand that they require all automotive emissions equipment to last 100k miles, but requiring a consumable be part of the 100k requirement seems unreasonable.

Unreasonable because every car has consumables that effect emissions, for example, spark plugs and O2 sensors. The EPA is not preventing the construction of any car with these items, although they have as large an impact on emissions as adding urea, probably greater. The manufacture publishes a maintenance schedule and then it is up to the operator to follow the schedule. Why does this not apply to diesel urea systems?

My last statement was an exaggeration to point out how the EPA is ignoring the millions of cars on the road and some of them producing emissions well beyond there model year limits do to failed components.


Exactly, they dont run without fuel either, nobody complains about that, well maybe they do... :>

Alot of people change thier oil much more often than required (3000 miles!!!) so adding urea should not be an issue, right?


...a lot of people do the opposite. So, expect the worst!

Jim from Virginia

Remember, urea is a very cheap chemical. Why not have dealerships that sell diesels just offer urea refills for free? Customers could just do a self-serve fill-up whenever necessary.

Perhaps the vehicle could start playing only rap music, the same song over and over, until you did your refill. Seriously though, you could have an annoying blinking warning light until the refill is done.


Many States/Provinces/Cities/Municipalities require vehicle emission testing on a yearly (or otherwise)basis. This is especially true in areas with the worst air quality. Eventually, those that don't maintain their vehicles to meet the required emission figures will pay the price, one way or the other


Comments by dim-wits like Frank are part of the problem! The other problem is the EPA's moronic approach to emissions! The emission standards were written for gasoline engines! They do not translate to diesel engine technology, nor should they be applied to diesels. The oil companies admit that if one-third of all light-duty vehicles in the U.S. were operated with modern diesel engines, the U.S. would save 1.4 million barrels of oil a day -- equal to the amount of crude oil the country imports daily from Saudi Arabia! Also, diesels are not "dirtier" then gasoline engines, their exhaust is just different. Diesels produce less CO2, less CO, and less Hydrocarbons then gasoline engines, and these are the more dangerous of the pollutants. They produce about the same amount of NOx. They do produce more particulates, but this is a more easily solved problem then the others. The EPA needs to look at the big picture, and write standards that make sense for diesels, and not try to apply inappropriate gasoline standards to them. It's like using standards for milk to package and distribute orange juice!

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