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US EPA publishes guidance document for using urea-SCR

30 March 2007

The US Environmental Protection Agency (EPA) has finalized its guidance document for using SCR technology for NOx control in light- and heavy-duty diesel vehicles and engines. The document opens the door for the introduction of SCR technology in Tier 2 light-duty vehicles, 2010 heavy-duty engines, and in other future diesel engine applications in the United States. Compared with the draft version of the document, the EPA compromised on some of the most controversial issues, including the responsibilities of vehicle manufacturers in ensuring urea availability.

In the SCR technology, a NOx reductant—such as a water-based urea solution—is stored in an onboard tank and injected upstream of the SCR catalyst. If the vehicle is operated without urea, the SCR catalyst becomes inactive, which may result in significantly increased NOx emissions. The guidance document outlines the intended EPA approach for emission certification of SCR-equipped vehicles.

The EPA derives its legal authority to impose conditions on the use of SCR technology from two groups of provisions in vehicle emission regulations, with regards to: (1) allowable maintenance, and (2) adjustable parameters. EPA considers urea replenishment to be a scheduled maintenance item, which is allowed to occur at an interval of no less than 100,000 miles in light-duty vehicles and 150,000 miles in heavy-duty vehicles. Since urea in SCR systems would have to be replenished at intervals on the order of 10,000 miles, a special permission would have to be granted by the EPA to allow the increased maintenance frequency.

In vehicles with adjustable parameters that can affect emissions (for example the idle air-to-fuel ratio in spark ignited engines) the EPA is allowed to test the vehicle over the full range of the adjustable parameter. In SCR-equipped vehicles, the EPA considers urea level to be an adjustable parameter. Thus, manufacturers must ensure in their SCR system design that the operation of the vehicle with empty reducing agent tank be impossible. The EPA acceptance criteria for the adjustable parameter include vehicle compliance (vehicles must not be operated without urea) and urea availability (drivers must be able to find urea when they need it). The EPA interpretation of adjustable parameter was contested by some manufacturers in their comments on the proposal, as it remains unclear if a fluid level indeed can be considered an adjustable parameter.

The vehicle compliance criteria are divided into: (1) driver warning system, (2) driver inducement, (3) identification of incorrect reducing agent, (4) tamper resistant design, and (5) durable design. The driver warning system must include a dashboard indicator (separate from the existing “check engine” light) which is activated and gradually escalates before the urea tank becomes empty. In case the urea tank becomes empty, the driver inducement system will ensure that users replenish urea. An example driver inducement strategy is to limit the number of engine restarts once urea reaches a certain minimum level. Some form of vehicle performance degradation can be also used as a driver inducement strategy. The identification of incorrect reducing agent must ensure that the urea tank is filled with the correct urea solution. This can be achieved through the use of NOx emission sensors or urea quality sensors.

To address urea availability, the EPA requires that manufacturers prepare plans for urea availability and accessibility. The reducing agent should be available (1) at dealerships, (2) at truckstops, and (3) a back-up must exist such as a toll-free number that the customers can call if they are unable to locate a source of urea. Compared to the proposal, the EPA has relaxed its urea availability requirements. The final guidance calls for the manufacturers to “use best efforts”—as opposed to “guarantee” in the proposed language—to ensure dealers have adequate supplies of urea. The requirement to guarantee urea availability was rejected by most manufacturers in their comments on the proposal.

Source: US EPA