Importance

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Abstract

A method and apparatus for on-board monitoring of NO.sub.x trap performance is proposed that uses two HEGO sensors, one positioned upstream of the NO.sub.x trap, and the other positioned downstream of the NO.sub.x trap. When the engine A/F is reduced from lean to stoichiometric or rich operation to regenerate or purge the NO.sub.x trap the difference in the time it takes for the upstream and downstream HEGO sensors to switch from a lean to a rich indication provides a quantitative measure of the amount of NO.sub.x that was stored on the NO.sub.x trap during the previous lean period of operation. This measure is related to an estimated amount of NO.sub.x produced by the engine to infer the operating performance or efficiency of the NO.sub.x trap. The difference in the output voltage of the two sensors is compared with a predetermined value to determined when to terminate the NO.sub.x purge. When the efficiency drops below a predetermined value the time that the engine is run in a lean cruise mode is reduced. If the time is reduced below a minimum time interval, a sulfur purge is performed. If sulfur purges are required more often than a predetermined repetition time, the lean cruise mode is terminated and an indicator lamp is energized.

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First Claim