A control apparatus is capable of separately detecting an unexpected fluctuation in the amount of NOx discharged from an engine (combustion chamber), separately from deterioration of lean NOx catalyst performance, thereby avoiding incorrect diagnosis of deterioration of the lean NOx catalyst, and ensuring the optimum control of the catalyst.

An exhaust gas cleaning apparatus for an engine has a NOx trapping catalytic converter and a control unit for controlling an intake air throttle valve and a fuel injection timing in accordance with an operating condition of the engine. The opening degree of the intake air throttle valve is reduced to accomplish rich spike control. The fuel injection timing is adjusted according to the boost pressure that develops downstream of the intake air throttle valve during rich spike control. Since the combustion tends to degrade when the boost pressure changes during rich spike control, the fuel injection timing is advanced more as the boost pressure becomes larger. This advancement of the injection timing compensates for the combustion degradation and prevents the occurrence of torque fluctuations when the opening degree of the throttle valve is adjusted during the shift to rich spike control to improve the engine operating performance.

To provide an apparatus for evaluating the deterioration condition of a catalyst of an internal combustion engine that can improve the accuracy of an evaluation of the deterioration condition of a catalyst and can suppress a worsening of emissions. The apparatus forcedly sets the air/fuel ratio upstream of a catalyst provided in an exhaust system of an internal combustion engine at a rich condition or a lean condition on the basis of a detected value of a sub O.sub.2 sensor downstream of the catalyst in the internal combustion engine and evaluates the deterioration condition of the catalyst. The air/fuel ratio control is reversed so that the air/fuel ratio upstream of the catalyst becomes a lean condition or a rich condition when `egasum` has reached a predetermined value Ga, `egasum` being an integrated value of an admitted air volume (an integration count of the amount of exhaust gas passing through the catalyst) in the period after the sub O.sub.2 sensor outputs a detected value, which shows a rich condition or a lean condition, until the reversing of the output of the sub O.sub.2 sensor.

A method for operating an internal combustion engine, in whose exhaust region, in the direction of flow of the exhaust gas, there are situated a first broadband lambda sensor which makes available a first lambda signal, an NOx adsorption catalyst and a second broadband lambda sensor which makes available a second lambda signal, in which the NOx adsorption catalyst cyclically stores the NOx emissions of the internal combustion engine, and is regenerated within the scope of a nominal regeneration by the operation of the internal combustion engine using a rich/stoichiometric air/fuel mixture, and a device for carrying out the method are provided. An intensified regeneration, compared to a nominal regeneration, of the NOx adsorption catalyst is undertaken, which supports a better setting of the thermodynamic equilibrium in the exhaust gas compared to the nominal regeneration. At the end of the intensified regeneration, at a still rich/stoichiometric air/fuel mixture, a plausibility check/correction of the first lambda signal using the second lambda signal is provided.