Control method of a spark-ignition engine, computer-program product and spark-ignition engine system

In a specified operating area Y including a low engine-speed and low engine-load area, an excess air ratio » is set at a specified ratio which is two or greater and three or smaller and an ignition timing ¸ig of a spark plug 11 is set at a timing of MBT to provide the maximum torque as a normal combustion control (S4). In case a catalyst temperature Tc is lower than a predetermined first temperature T1 in this area Y, a control of retarding the ignition timing ¸ig from the MBT timing and/or a control of decreasing the excess air ratio » to a ratio below the above-described ratio are executed (S6, S8, S9). Accordingly, the temperature of the catalyst can be prevented simply and effectively from decreasing excessively, maintaining combustion conditions to provide a properly high thermal efficiency.