Control apparatus for internal combustion engine

A control apparatus for a spark ignition type of internal combustion engine, comprising:

• a sensor configured to detect a rotation speed and a load of the engine;
• and
• an electronic control unit configured to control an ignition timing of the engine based on the rotation speed and the load detected by the sensor and characteristic of a reference ignition timing determined as later one of an optimum ignition timing and a knock ignition timing which have been determined beforehand in accordance with the rotation speed and the load, wherein
• the optimum ignition timing is determined beforehand so that a torque of the engine is maximized in accordance with the rotation speed and the load, wherein
• the knock ignition timing is determined beforehand so that no knocking occurs when fuel of a reference octane number is supplied to the engine, wherein
• the electronic control unit:
• calculates a first retard amount by multiplying a difference between an octane number of fuel currently supplied to the engine and the reference octane number by a predetermined retard amount;
• compensates the knock ignition timing to be retarded by the first retard amount;
• and
• calculates a second retard amount based on the predetermined retard amount and compensates the reference ignition timing to be retarded by the second retard amount when an operation region of the engine defined by the rotation speed and the load based on the rotation speed and the load detected by the sensor becomes either one of a predetermined load region higher in load than an optimum operation line in which a net fuel consumption rate of the engine is highest or a maximum output region in which an output of the engine is maximized.