Ignition timing control system for internal combustion engine

An ignition timing control system for an internal combustion engine, comprising:

• EGR controlling means for controlling a valve opening of an EGR valve in accordance with a target EGR rate varying depending on an operating state of the internal combustion engine, the EGR valve being provided in an EGR passage that connects an exhaust system of the internal combustion engine to an intake system thereof;
• EGR rate estimating means for estimating an EGR rate of the internal combustion engine in accordance with the operating state thereof;
• target ignition timing setting means for setting a target ignition timing, corresponding to the EGR rate estimated by said EGR rate estimating means, based on an ignition timing for EGR operation and an ignition timing for non-EGR operation that are individually determined from first and second maps set in advance;
• retard amount setting means for setting a retard amount based on a ratio between the estimated EGR rate and the target EGR rate;
• ignition timing correcting means for correcting the target ignition timing, set by said target ignition timing setting means, in accordance with the retard amount set by said retard amount setting means;
• and
• ignition timing controlling means for controlling an ignition timing of the internal combustion engine in accordance with the target ignition timing corrected by said ignition timing correcting means.