Malfunction monitoring apparatus and method for secondary air supply

Abstract

A malfunction monitoring apparatus for a secondary air supply system of an internal combustion engine includes an air flow sensor for measuring the flow rate of an air flowing into an intake air path of the engine, a secondary air introducing path disposed in the intake air path on the downstream side of a measuring position of the air flow rate for taking in a part of the air through the intake air path as a secondary air, a secondary air supply device for supplying the secondary air to an exhaust gas path, and a control unit. The apparatus may further include a pressure sensor for detecting the internal pressure of an intake manifold. The control unit determines a secondary air flow rate from a difference between a first air flow rate measured by the air flow sensor when the secondary air is not being supplied from the secondary air introducing path to the exhaust gas path and a second air flow rate measured by the air flow sensor when the secondary air is being supplied, or a difference between an intake air flow rate of the engine determined on the basis of the internal pressure of the intake manifold detected by the pressure sensor and an air flow rate measured by the air flow sensor when the secondary air is being supplied. In the case where the determined secondary air flow rate is out of a predetermined allowable range, the secondary air supply device is judged as being abnormal.