Method for monitoring the performance of a NO.sub.x trap

A system for monitoring performance of a NO.sub.x trap disposed in an exhaust passage of an internal combustion engine, comprising:

• an exhaust gas sensor disposed downstream from said trap;
• means for generating a command to switch the operation of said engine from a relatively lean A/F to a relatively rich A/F to purge said NO.sub.x trap;
• means for detecting the switching of said sensor from a first to a second state in response to the exhaust gas content;
• means for measuring the time interval between the initiation of said command and the detection of said switching;
• means for determining the sorption of said NO.sub.x trap as a function of said time interval;
• means for estimating the amount of NO.sub.x produced by the engine;
• and
• means for determining the sorption efficiency of said NO.sub.x trap based on the ratio of said sorption to said amount.

A system for monitoring performance of a NO.sub.x trap disposed in an exhaust passage of an internal combustion engine, comprising:

• a first exhaust gas oxygen sensor disposed upstream from said trap;
• a second exhaust gas oxygen sensor disposed downstream from said trap;
• said first and second sensor producing respective outputs indicative of the oxygen content of the exhaust gas;
• air fuel ratio control means for switching the operation of said engine from a relatively lean mode of operation to a relatively rich mode of operation to initiate a purge of said NO.sub.x trap;
• said control means detecting the switching of said first and second sensors from a relatively high exhaust gas oxygen content state to a relatively low exhaust gas oxygen content state following switching to said relatively rich mode of operation;
• said control means determining the sorption of said NO.sub.x trap as a function of the temperature of said trap and the time interval between the switching of said first and second sensors.