Exhaust gas purifying device for engine

An exhaust gas purifying device for an engine, comprising:

• a NOx trap catalyst disposed in an exhaust gas passage of the engine including a NOx trap material to absorb NOx while oxygen of an exhaust gas is in high concentration and release the absorbed NOx while oxygen is in low concentration, a catalytic metal to purify the NOx released from said NOx trap material, and an O2 storage material to absorb O2 while oxygen of the exhaust gas is in high concentration and release the absorbed O2 while oxygen is in low concentration;
• a NOx sensor disposed in the exhaust gas passage downstream of said NOx trap catalyst;
• an O2 sensor disposed in the exhaust gas passage downstream of said NOx trap catalyst;
• an engine speed sensor operative to detect an engine speed;
• an accelerator opening sensor operative to detect an opening of an accelerator;
• an engine coolant temperature sensor operative to detect a temperature of an engine coolant;
• a control unit operative to receive output values of said respective sensors, and to control a fuel injector and an ignition timing of the engine and execute a diagnosis of deterioration of said NOx trap catalyst, according to said received output values of the sensors;
• wherein said control unit calculates the amount of NOx absorbed in said NOx trap catalyst according to an output value of said NOx sensor;
• controls said fuel injector so as to lower the concentration of oxygen of the exhaust gas in the exhaust gas passage and thereby to cause said NOx trap catalyst to release NOx absorbed when the amount of the NOx absorbed in said NOx trap catalyst reaches a specified amount;
• determines that a specified diagnosis condition to diagnose said NOx trap catalyst is satisfied when the engine is warmed up with the engine coolant temperature that is greater than a specified temperature and the engine is in an ordinary state with a changing rate of the accelerator opening that is lower than a specified rate;
• detects whether or not O2 release from said O2 storage material caused by oxygen concentration lowering of the exhaust gas in the exhaust gas passage has terminated according to the output value of said O2 sensor;
• diagnoses, when said specified diagnosis condition is satisfied, that O2 storage capacity of said O2 storage material included in said NOx trap catalyst has deteriorated according to output values of said NOx sensor that are generated during a term from the beginning of the oxygen concentration lowering until a termination of the O2 release from said O2 storage material and that NOx reduction capacity of said catalytic metal included in said NOx trap catalyst has deteriorated according to an output value of said NOx sensor that is generated after a termination of the O2 release from said O2 storage material;
• and controls the fuel injector such that a speed of lowering oxygen concentration when said specified diagnosis condition is satisfied is slower than that when said specified diagnosis condition is not satisfied.

An exhaust gas purifying device for an engine, comprising:

• a NOx trap catalyst disposed in an exhaust gas passage of the engine including a NOx trap material to absorb NOx while oxygen of an exhaust gas is in high concentration and release the absorbed NOx while oxygen is in low concentration, a catalytic metal to purify the NOx released from said NOx trap material, and an O2 storage material to absorb O2 while oxygen of the exhaust gas is in high concentration and release the absorbed O2 while oxygen is in low concentration;
• a NOx sensor disposed in the exhaust gas passage downstream of said NOx trap catalyst;
• an O2 sensor disposed in the exhaust gas passage downstream of said NOx trap catalyst;
• an engine speed sensor operative to detect an engine speed;
• an accelerator opening sensor operative to detect an opening of an accelerator;
• an engine coolant temperature sensor operative to detect a temperature of an engine coolant;
• a control unit operative to receive output values of said respective sensors, and to control a fuel injector and an ignition timing of the engine and execute a diagnosis of deterioration of said NOx trap catalyst, according to said received output values of the sensors;
• wherein said control unit calculates the amount of NOx absorbed in said NOx trap catalyst according to an output value of said NOx sensor;
• controls said fuel injector so as to lower the concentration of oxygen of the exhaust gas in the exhaust gas passage and thereby to cause said NOx trap catalyst to release NOx absorbed when the amount of the NOx absorbed in said NOx trap catalyst reaches a specified amount;
• determines that a specified diagnosis condition to diagnose said NOx trap catalyst is satisfied when the engine is warmed up with the engine coolant temperature that is greater than a specified temperature and the engine is in an ordinary state with a changing rate of the accelerator opening that is lower than a specified rate;
• detects whether or not O2 release from said O2 storage material caused by oxygen concentration lowering of the exhaust gas in the exhaust gas passage has terminated according to the output value of said O2 sensor;
• diagnoses, when said specified diagnosis condition is satisfied, that O2 storage capacity of said O2 storage material included in said NOx trap catalyst has deteriorated when an integrated value of output values of said NOx sensor with respect to time during a term from the beginning of the oxygen concentration lowering until a termination of the O2 release from said O2 storage material is smaller than a first threshold value and that NOx reduction capacity of said catalytic metal included in said NOx trap catalyst has deteriorated when a maximum value of output values of said NOx sensor that are generated after a termination of the O2 release from said O2 storage material is greater than a second threshold value;
• and controls the fuel injector such that a speed of lowering oxygen concentration when said specified diagnosis condition is satisfied is slower than that when said specified diagnosis condition is not satisfied.