Method for controlling engine and engine control apparatus

A method for controlling an engine using a cylinder, an injector that injects fuel into the cylinder, a spark plug that ignites an air-fuel mixture containing the fuel injected from the injector, and a controller that is electrically connected to the injector and the spark plug and controls each of the injector and the spark plug, the method comprising:

• selecting with the controller a control point consisting of a combination of an injection timing at which the injector injects the fuel and an ignition timing at which the spark plug ignites the air-fuel mixture, corresponding to a selection on a parameter map defined by a first axis of the injection timing and a second axis of the ignition timing that is orthogonal to the first axis of the injection timing, wherein
• a plurality of limit lines are provided on the parameter map, each of the limit lines varying in accordance with an operating state of the engine,
• the plurality of limit lines include
• a first limit line indicating an advance limit of the injection timing for restraining preignition,
• a second limit line indicating an advance limit of the ignition timing for restraining smoke,
• a third limit line indicating an advance limit of the ignition timing for restraining knocking, and
• a fourth limit line indicating a retard limit of the ignition timing for ensuring combustion stability, and
• the plurality of limit lines define a parameter region surrounded by the plurality of limit lines on the parameter map;
• and
• selecting with the controller the control point so that the ignition timing is brought to maximum advance within a range of the parameter region.