Thermal management of hybrid LNT/SCR aftertreatment during desulfation

Abstract

In an exhaust aftertreatment system comprising a NOx adsorber-catalyst followed by an SCR catalyst, means are provided for preventing the SCR catalyst from becoming heated to near the same peak temperatures as the NOx adsorber-catalyst during desulfation. In one embodiment, the means is a thermal mass between the NOx adsorber-catalyst and the SCR catalyst. In another embodiment, the means is a valve configured to selectively divert exhaust leaving the NOx adsorber-catalyst from the SCR catalyst. In a method of the invention, the NOx adsorber-catalyst temperature is cycled during desulfation. The peaks of the cycles are within an appropriate temperature range for desulfating the NOx adsorber-catalyst, but the average temperature is below the temperature range at which the SCR catalyst is damaged. The temperature peaks are damped as they travel from the NOx adsorber-catalyst to the SCR, whereby the SCR experiences much lower peak temperatures than the NOx adsorber-catalyst.

Description

FIELD OF THE INVENTION [0001] The present invention relates to pollution control devices for diesel engines, and lean-burn gasoline engines. BACKGROUND OF THE INVENTION [0002] NOx emissions from diesel engines are an environmental problem. Several countries, including the United States, have long had regulations pending that will limit NOx emissions from trucks and other diesel-powered vehicles. Manufacturers and researchers have already put considerable effort toward meeting those regulations. [0003] In gasoline powered vehicles that use stoichiometric fuel-air mixtures, three-way catalysts have been shown to control NOx emissions. In diesel-powered vehicles, which use compression ignition, the exhaust is generally too oxygen-rich for three-way catalysts to be effective. [0004] Several solutions have been proposed for controlling NOx emissions from diesel-powered vehicles. One set of approaches focuses on the engine. Techniques such as exhaust gas recirculation and partially homoge...

AI Technical Summary

Benefits of technology

[0014] The inventors believe that commonly used SCR catalysts can be damaged by repeated exposure to temperatures used in desulfating NOx adsorber-catalysts. Accordingly, one of the inventors' concepts relates to an exhaust aftertreatment system comprising a NOx adsorber-catalyst, an SCR catalyst, and means for preventing the SCR catalyst from becoming heated to near the same peak temperatures as the NOx adsorber-catalyst during desulfation. The system generally further comprises a particulate filter located either upstream of the NOx adsorber-catalyst or downstream of the SCR catalyst.

[0015] In one embodiment, the means for preventing the SCR catalyst from becoming heated to near the same peak temperatures as the NOx adsorber-catalyst during desulfation comprises a device that provides a thermal mass between the NOx adsorber-catalyst and the SCR catalyst. In this embodiment, the device is not a particulate filter, an oxidation catalyst, a SCR catalyst, nor a NOx adsorber-catalyst. The device is functional to substantially dampen temperature pulses transmitted to the NOx adsorber-catalyst through the exhaust, whereby the temperature pulses are attenuated and have substantially lower peaks when they reach the SCR catalyst.

[0016] In another embodiment, the means for preventing the SCR catalyst from becoming heated to near the same peak temperatures as the NOx adsorber-catalyst during desulfation comprises a flow diverter, such as a valve, configured to selectively divert exhaust leaving the NOx adsorber-catalyst, whereby the exhaust does not enter the SCR catalyst.

Problems solved by technology

The inventors believe that commonly used SCR catalysts can be damaged by repeated exposure to temperatures used in desulfating NOx adsorber-catalysts.

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