Device for detecting deterioration of a catalytic converter for an engine

Abstract

In an engine having two or more cylinder groups and a common exhaust passage with which exhaust passages individually connected to the respective cylinder groups are merged, the exhaust gases in the respective individual exhaust passage mix each other in the common exhaust passage before flowing into catalytic converter disposed in the common exhaust gas passage, therefore, when the air-fuel ratios of the respective cylinder groups are individually controlled, the air-fuel ratio of the exhaust gas in the common exhaust passage does not correspond to air-fuel ratio of the exhaust gas in any of the individual exhaust passages, and this makes it difficult to detect the deterioration of the catalyst based on the output of the upstream O.sub.2 sensor disposed in the individual exhaust passage and the downstream O.sub.2 sensor disposed in the common exhaust passage. In the present invention, the control circuit estimates the air-fuel ratio of the exhaust gas after mixed by averaging the outputs of the upstream air-fuel ratio sensors disposed in the respective individual exhaust passages, and detects the catalyst deterioration using the average output of the upstream air-fuel ratio sensor and the output of the downstream O.sub.2 sensor.