1. Field of the Invention
The present invention relates to an abnormality determination apparatus for an internal combustion engine, and particularly to a technique of determining that a passage for allowing exhaust gas discharged from a cylinder to be returned to the cylinder has been clogged.
2. Description of the Background Art
An internal combustion engine provided with an EGR (Engine Gas Recirculation) system is known. With the EGR system, pumping loss and unburned gas can be reduced by means of an external EGR for allowing exhaust gas discharged from a cylinder to an exhaust passage to be recirculated to an intake passage.
The amount of exhaust gas recirculated by the EGR system from the exhaust passage to the intake passage is regulated by an EGR valve provided on an EGR gas passage. When the EGR valve opens, recirculation from the exhaust passage to the intake passage is performed. When the EGR valve closes, recirculation from the exhaust passage to the intake passage is stopped.
When exhaust gas is recirculated into the cylinder, the combustion temperature within the cylinder decreases. Consequently, knocking hardly occurs. Since knocking hardly occurs, the need to retard ignition timing decreases. Thus, with recirculation of exhaust gas into the cylinder, ignition timing is advanced.
At this time, for example, if a passage provided for any of a plurality of cylinders for allowing exhaust gas to be returned into the cylinder is clogged, the advancement of ignition timing could be accompanied by occurrence of knocking. If the ignition timing is retarded in order to reduce knocking, a combustion state could be degraded in cylinders to which exhaust gas is recirculated normally. Therefore, it is necessary to identify the cylinder for which the passage for allowing exhaust gas to be returned into the cylinder is clogged, and to take appropriate measures.
Japanese Patent Laying-Open No. 2010-25059 discloses, in paragraph 34 for example, that when an intake pressure sensor detects a decrease of the intake pressure below a target pressure, and a knock sensor detects occurrence of knocking at any particular cylinder, ignition timing of the particular cylinder is retarded by a spark plug without changing an opening position of an EGR valve.
However, the intake pressure may decrease and knocking may occur even if a passage for allowing exhaust gas to be returned into the cylinder is not clogged. Therefore, with the method described in Japanese Patent Laying-Open No. 2010-25059, there could be a case where clogging of a passage for allowing exhaust gas to be returned into a cylinder is erroneously detected.