Patent ID: 8341937

Claim:
An apparatus for diagnosing an exhaust gas purification system for an internal combustion engine, the apparatus comprising: a main exhaust gas passage; a main catalytic converter disposed in the main exhaust gas passage; a bypass exhaust gas passage in fluid communication with the main exhaust gas passage so as to bypass a section thereof between a branch point at which the bypass exhaust gas passage branches out of the main exhaust gas passage and a junction at which the bypass exhaust gas passage merges into the main exhaust gas passage at a upstream side of the main catalytic converter; a bypass catalytic converter disposed in the bypass exhaust gas passage; a valve provided in the section of the main exhaust gas passage and configured to open or close the section of the main exhaust gas passage; a first sensor disposed in the bypass exhaust gas passage to output a first signal indicative of a first air-fuel ratio of an exhaust gas flowing in the bypass exhaust gas passage; a second sensor disposed in the main exhaust gas passage to output a second signal indicative of a second air-fuel ratio of an exhaust as flowing into the main catalytic converter; and a controller receiving the first and second signals, the controller determining whether the valve in the closed configuration leaks exhaust gas through the section of the main exhaust gas passage based on a comparison of the temporal responses of the first and second signals to a change in an engine air-fuel ratio supplied to the internal combustion engine; wherein the first sensor detects the first air-fuel ratio of exhaust gas upstream of the bypass catalytic converter; wherein the first signal indicates changes between rich and lean states of the first air-fuel ratio of exhaust gas passing through the bypass exhaust gas passage, and the second signal indicates changes between rich and lean states of the second air-fuel ratio of exhaust gas passing through the main exhaust gas passage; and wherein the determining whether the valve in the closed configuration is leaking comprises measuring a time delay between the first signal indicating the changes between rich and lean states of the first air-fuel ratio and the second signal indicating the changes between rich and lean states of the second air-fuel ratio, the time delay being in response to periodic changes of an engine air-fuel ratio supplied to the internal combustion engine.