Patent ID: 6990955

Claim:
A spark ignition engine provided with a low RON fuel tank to store a low RON fuel and a high RON fuel tank to store a high RON fuel, in which the fuel in the low RON fuel tank and the fuel in the high RON fuel tank are injected into a combustion chamber, at a mixing ratio in accordance with the state of operation of the engine, comprising fuel mixing means for mixing the fuel from the high RON fuel tank and the fuel from the low RON fuel tank at a predetermined mixing ratio, so that the ratio of the fuel from the high RON fuel tank is increased as the load is increased and the ratio of the fuel from the low RON fuel tank is increased as the load is decreased; practical ignition timing determination means for determining a practical ignition timing based on the state of occurrence of knocking during a practical operation of the engine; and comparing ignition timing memory means for storing a comparing ignition timing with which the practical ignition timing is compared, wherein whether or not the low RON fuel and the high RON fuel are properly stored in the low RON fuel tank and the high RON fuel tank, respectively, is judged based on differences between the comparing ignition timing and the practical ignition timing, at predetermined plural load values.