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Thermal management of the exhaust has become important as exhaust catalyst systems are being added to heavy duty (HD) diesel engines. The HD certification cycle consists of a cold-start transient cycle followed by a twenty minute soak with the engine off and then another ‘hot’ transient cycle. The first half of the HD transient cycle represents urban driving and consequently the engine duty cycle is very low. The low duty cycle results in low exhaust temperatures that can challenge the performance of catalysts. The cycle then transitions to a highway simulation at high loads. The high loads require high catalyst efficiency. If the catalysts are below optimum temperature prior to this transition, they need to warm up very quickly to prevent excessive emissions slip. The twenty minute soak also provides opportunity for catalysts to experience a significant temperature drop prior to the ‘hot’ cycle.

A 5.9 liter medium-heavy-duty diesel engine, equipped with a diesel exhaust aftertreatment system consisting of catalyzed diesel particulate filters and NOx adsorber catalysts arranged in a dual-path configuration was evaluated with the goal of studying the thermal aging characteristics of a number of NOx adsorber formulations and documenting adsorber formulation improvements over time. The performance of 2005 vintage NOx adsorber formulations was compared with the performance of older formulations. The testing was performed in three steps. The first was to run with zero sulfur fuel at a high temperature engine operating mode to characterize performance losses due to exposure to the highest temperatures experienced under normal operation. This was followed by aging without sulfur but with desulfation events to characterize the impact of the higher desulfation temperatures on catalyst deactivation.

Beginning June 1, 2006, 80% of the highway diesel fuel produced in the United States had to contain 15 ppm sulfur or less. To account for sulfur test method variability, the United States Environmental Protection Agency (US EPA) allowed a 2 ppm compliance margin, meaning that in an EPA enforcement action fuel measuring 17 ppm or less would still be deemed compliant since the true sulfur level could still be 15 ppm. Concern was voiced over the appropriateness of the 2 ppm compliance margin, citing recent American Society for Testing and Materials (ASTM) round-robin and crosscheck test program results that showed sulfur test lab-to-lab variability (reproducibility) on the order of 4 to 5 ppm depending on test method.