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Deactivation of the diesel catalyst causes gradual deterioration of its performance resulting in increased emission levels from the vehicle. Most emission standards specify “emission durability” requirements, defined as the minimum mileage and/or time period after which the engine is still expected to conform with applicable emission limits. New emission standards typically increase the required emission durability targets, putting higher demands on emission catalysts and emission control systems in general.
Most U.S. emission standards adopt a two-tier approach to emission durability, which involves (1) emission warranty and (2) full life durability demonstration. Engine manufacturers warrant to the purchaser that the engine is designed and built to conform with applicable emissions standards. The following are the warranty periods for different classes of vehicles through the model year 2003:
As part of the engine emission certification process, manufacturers are required to demonstrate compliance with emission standards over the “full useful life” of the engine. Useful engine lives are defined as follows:
Beginning in model year 2004, the EPA established increased useful life periods for all pollutants. The revised requirements for the heavy duty diesel engine service class are 435,000 miles, 22,000 hours, or 10 years, whichever occurs first. Very durable catalyst technologies will be needed to meet these increased requirements.