Device for detecting deterioration of a catalytic converter for an engine

A device for detecting a deterioration of a three-way catalytic converter for an internal combustion engine, said engine comprising, a plurality of cylinders divided into cylinder groups, individual exhaust passages connected to the respective cylinder groups, a common exhaust passage into which said individual exhaust passages are merged, upstream air-fuel ratio sensors disposed in said respective individual exhaust passages for detecting air-fuel ratios of the exhaust gases in said individual exhaust passages, a downstream air-fuel ratio sensor disposed in said common exhaust passage for detecting an air-fuel ratio of the exhaust gas in said common exhaust passage, at least one three-way catalytic converter disposed in exhaust passage between said upstream air-fuel ratio sensors and said downstream air-fuel ratio sensor, and a means for individually controlling the air-fuel ratio of each cylinder group based on outputs of said respective upstream air-fuel ratio sensors, said device comprising:

• an estimating means for estimating the air-fuel ratio of a mixture of the exhaust gases from said respective individual exhaust passages flowing into said common exhaust passage based on the output signals of said upstream air-fuel ratio sensors;
• and
• a determining means for determining whether or not said three-way catalytic converter has deteriorated based on the output signal of said downstream air-fuel ratio sensor and said estimated air-fuel ratio of the mixture of the exhaust gas flowing into said common exhaust passage.