Patent ID: 7746079

Claim:
An ion current detecting apparatus for an internal combustion engine, comprising: a capacitor that is charged as a spark plug discharges; a voltage limiting element that limits a maximum voltage of the capacitor while charging; a detection resistor through which a discharge current of the capacitor flows; and a comparator circuit that compares a current value of the detection resistor with a predetermined value, wherein a stage prior to the comparator circuit is provided with an amplifying element including an inverting input terminal, a non-inverting input terminal, and an output terminal, and the comparator circuit compares a voltage value of the output terminal of the amplifying element with a fixed reference voltage value, and wherein the amplifying element is connected with an input resistor provided between the capacitor and the inverting input terminal, and a feedback resistor provided between the inverting input terminal and the output terminal, and the feedback resistor functions as the detection resistor wherein degradation of an insulation resistance in an ion current path through which an ion current flows is determined either by: providing a prohibiting step in which fuel supply to a combustion chamber of an internal combustion engine is prohibited, and determining an output from the comparator circuit in the prohibiting step, or setting a charge quantity of the capacitor at discharge start so that, as a consequence of a discharging operation of the capacitor, discharge duration required until the current value of the detection resistor falls below the predetermined value becomes longer than time duration in which the ion current generated by fuel combustion in the combustion chamber in the internal combustion engine lasts.