Catalyst diagnosis device

A catalyst diagnosis device which is provided with each Air/fuel ratio (AFR) sensor on an upstream side and on a downstream side of a catalyst provided in an exhaust passage of an engine and which determines deterioration of the catalyst on the basis of output of each AFR sensor, the catalyst diagnosis device comprises an electronic control unit (ECU) configured to function as:

• a fuel injection quantity controller which controls a fuel injection quantity in order to effect a state change on the AFR detected on the upstream side of the catalyst from a lean state to a rich state or from the rich state to the lean state across reference AFR set in a stoichiometric area between the lean state and the rich state;
• a timer which counts elapsed time since the state change on the AFR on the upstream side until AFR detected by the AFR sensor on the downstream side meets a predetermined threshold condition;
• an Oxygen Storage Amount (OSA) calculating section which calculates oxygen storage capacity (OSC) of the catalyst on the basis of the elapsed time;
• a deterioration diagnosing section which diagnoses a deterioration of the catalyst on the basis of the oxygen storage capacity;
• and
• an oxygen purge amount (OPA) calculating section which calculates an oxygen purge amount of the catalyst in an oxidizing atmosphere on the basis of the elapsed time;
• wherein the OSA calculating section includes:
• an AFR calculating section which calculates a difference between average values of AFRs detected by each sensor in the elapsed time;
• a fuel weight per cycle of the engine (Mfuel) calculating section which calculates fuel weight per cycle of the engine;
• and
• an engine speed (Ne) calculating section which calculates engine speed,
• wherein the oxygen storage capacity of the catalyst is calculated as a function of the difference between the average values of the AFRs, the fuel weight, the engine speed and the elapsed time, and
• wherein the elapsed time counted by the timer since the state change from the lean state to the rich state on the AFR before the catalyst until AFR detected by the AFR sensor on the downstream side meets the predetermined threshold condition.