Control system for internal combustion engines

In a control system for an internal combustion engine having at least one cylinder, and a spark plug provided for each of said at least one cylinder, said control system including control means for controlling operation of said engine according to an air-fuel ratio of an air-fuel mixture supplied to said engine, said control means including signal-generating means for determining ignition timing of said engine and for generating an ignition command signal indicative of the determined ignition timing ,and igniting means responsive to said ignition command signal for generating high voltage for causing generation of sparking voltage across said at least one spark plug,

• the improvement comprising:
• voltage value-detecting means coupled to said igniting means for detecting said sparking voltage generated across said at least one spark plug when said high voltage is generated;
• duration-measuring means coupled to said voltage value-detecting means for measuring a duration over which said sparking voltage across said spark plug exceeds a predetermined reference value, after generation of said ignition command signal;
• and
• air-fuel ratio-detecting means coupled to said duration-measuring means and said control means for detecting said air-fuel ratio of said fuel-fuel mixture supplied to said engine, based on said duration measured.