Ignition timing control apparatus for an internal combustion engine

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

• ignition unit means for supplying an ignition signal to the engine at a predetermined ignition timing;
• knocking sensor means, attached to the engine, for detecting the vibration of the engine and producing a knocking detection signal indicating the degree of occurring knocking;
• control unit means including a processor, which has plural tables for providing basic ignition timings and is programed so as to execute the following steps:
• selecting one of the plural tables on the basis of the knocking detection signal;
• and
• determining, in accordance with the selected ignition timing table, an actual ignition timing to be supplied for said ignition unit as the predetermined ignition timing,
• characterized in that
• the plural ignition timing tables includes a first ignition timing table providing a basic ignition timing for high-octane fuel and a second ignition timing table providing a basic ignition timing for low-octane fuel, and
• when one of the first and second ignition timing tables is switched over to the other ignition timing table, the processor included in said control unit means further executes the processing of the following steps:
• setting the other ignition timing table;
• obtaining a difference between the basic ignition timings according to the first and second ignition timing tables;
• reducing the difference with time at a predetermined constant rate;
• correcting the basic ignition timing according to the one ignition timing table by the reducing difference so that when the reducing difference becomes zero, the actual ignition timing becomes equal to the basic ignition timing according to the other ignition timing table;
• comparing the reducing difference with a certain threshold provided in advance;
• and
• determining whether the one ignition timing table is to be maintained or switched over to the other ignition timing table in accordance with the comparison result of the preceding step.