Abstract: Oscillation of the engine is detected by an oscillation sensor which outputs a signal representing the oscillation of the engine. The maximum value of the output of the oscillation sensor during combustion period in each cycle is detected. The maximum values obtained in a predetermined number of cycles are stored in a memory and knock occurrence is determined on the basis of the predetermined number of maximum values output from the memory.

Abstract: An ignition timing correcting circuit which retards ignition timing when knock occurs and advances ignition timing when knock does not occur. An optimal ignition timing on which the ignition timing is to be converged is set to a latest ignition timing to which the ignition time can be retarded without raising the exhaust gas temperature higher than a predetermined value at the actual air-fuel ratio at that time. The actual ignition timing corrected by the ignition timing correcting circuit is compared with the optimal ignition timing. An air-fuel ratio control circuit corrects the air-fuel ratio toward the lean side when the actual ignition timing is on the advance side of the optimal ignition timing and toward the rich side when the former is on the retard side of the latter.