Development of a Methodology to Separate Thermal From Oil Aging of a Catalyst Using a Gasoline-Fueled Burner System, 08-9217

Principal Investigator
Cynthia C. Webb

Inclusive Dates: 10/01/00 - 05/15/02

Background - Catalytic converters became part of the vehicle exhaust system in the mid-1970s to meet Environmental Protection Agency-mandated emission regulations. Since that time, engineers and scientists have worked to optimize the performance and durability of these devices to meet increasingly stringent emissions and durability standards. As catalysts age, their ability to convert exhaust pollutants decreases. Research continues to focus on understanding thermal deactivation; formulate improvements to increase the stabilization of the washcoat and the substrate at higher temperatures; and improve long-term durability.

The current vehicle catalyst durability requirement is 100,000 miles, with future proposed durability extending to 150,000 miles. To expose a component to actual engine aging would be expensive, time consuming, and not very practical during product design and development. Thermal aging can be efficiently accelerated, however, because the rate of the mechanism by which thermal deactivation occurs can be increased by operating at higher catalyst temperature. Accelerated thermal aging is generally performed using an engine/dynamometer test stand.

Approach - Typically, an engine/dynamometer thermal aging cycle contains combinations of elevated catalyst inlet temperatures, chemical reaction-induced thermal excursions (simulating misfire events), and average air/fuel ratio (AFR) to create a condition that accelerates the aging of the test part. Engine/dynamometer systems are expensive, exhibit variation in operation due to a number of factors, consume oil at an inconsistent rate, and require a substantial amount of maintenance. An equivalent alternative that is less costly, potentially more accurate and repeatable, does not confound the thermal aging with oil poisoning, and requires less maintenance would be very attractive. The objective of this work is to develop a control method for a burner system that would allow the burner to simulate exhaust temperature, flow, and AFR created by an engine during accelerated thermal aging. Validation of the methodology will be achieved through comparison of the differences (or similarities) between engine- and burner-induced accelerated thermal aging. If the burner system provides thermal aging comparable to an engine, an alternative oil-free (or oil consumption controlled, if desired) aging system that is less expensive, and requires less maintenance, will be identified.

Accomplishments - The burner system used in this program was an Institute-designed and built, gasoline-fueled burner (FOCAS® rig) designed to operate around a stoichiometric AFR (a chemically balanced mix of fuel and air). During this work, the burner operation was modified to simulate the temperature and flow of an engine running a thermal aging cycle. The cycle that was simulated was the RAT-A (Rapid Aging Test - A), a published aging protocol. Six similar catalysts were aged during this program. Three catalysts were aged using a gasoline-fueled engine-aging stand; the other three were aged using a computer-controlled burner system. Both systems were programmed to run aging cycles that provided the same inlet temperature and AFR profiles, and space velocity conditions. Each catalyst was evaluated using a vehicle over the FTP emissions test cycle and an AFR sweep test using an engine test stand before and after aging. Finally, the catalysts were cored and analyzed to provide a composition and surface area comparison.

Overall, it was determined that the FOCAS® burner system provided a flexible means for simulating an engine aging cycle and produced thermal aging results similar to the engine cycle. The post-test analysis showed that the FOCAS® aging procedure provides thermal aging in the absence of nonthermal aging (i.e. oil deposits), thereby creating a means for the definitive isolation of thermal and nonthermal aging effects.

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