An experimental study on aging effects of fuel-cut events including sound optimized torque reduction on modern three-way catalysts

Abstract

Continuously increasing country-specific emission standards for passenger vehicles demand additional development steps in complex exhaust gas aftertreatment systems for a more effective reduction of harmful gases. The efficiency of the aftertreatment system declines over its lifecycle due to specific load profiles and other boundary conditions, resulting in increasing emissions over vehicle mileage. As a result, the durability of the exhaust system becomes more important. Fuel cut is an intended, temporary interruption of the fuel supply of modern combustion engines of passenger cars to reduce fuel consumption and emissions. During this fuel reduction event, unconsumed oxygen of rich and cool fresh air is introduced to the combustion chamber and to the exhaust system. Besides a catalyst cool down, oxidation effects are provoked by enhanced oxygen reactions processes within the catalyst’s washcoat and surface layers. In this publication, specific aging cycles with a variation of fuel-cut programs were examined on their effects on a modified modern eight-cylinder turbo-charged engine, especially on their oxidation effects correlating to catalyst aging. Light-off curves and conversion heat maps were used to evaluate possible damaging impacts of fuel-cut events on the aging behavior of the catalyst systems. Results indicate that higher frequency of fuel-cut events results in a reduced catalyst conversion efficiency, whereas thermal sintering might be reduced due to overall lower temperature load. Temporary oxidation processes on catalyst’s surfaces occur only in a short timespan after changing the air fuel ratio to lean, resulting in a weaker thermal sintering during longer fuel-cut events. Sound optimized torque reduction functions, used in sports cars for a better drivability and powertrain response, exhibit a stronger aging influence due to additional thermal shock effects on the catalysts front face, as demonstrated via optical infrared sensor technology during catalyst aging cycles on the modified engine bench setup. A theoretical calculation of aging processes as a combination of thermal sintering by Arrhenius equation as well as oxidation process is discussed to describe an aging classification induced via oxidation processes. Additional physisorption measurements of different catalyst probes support the results which indicate the direction of future experimental approaches.