Patent attributes
An ion current detecting apparatus includes a capacitor C that is charged as a spark plug 3 discharges, a Zener diode ZD that limits a maximum voltage of the capacitor C while charging, a detection resistor R2 through which a discharge current of the capacitor C flows, and a comparator circuit 5 that compares a current value of the detection resistor R2 with a predetermined value. A charge quantity Q of the capacitor C at discharge start is set so that, as a consequence of a discharging operation of the capacitor C, discharge duration required until the current value of the detection resistor R2 falls below the predetermined value becomes longer than time duration in which the ion current generated by fuel combustion in the engine combustion chamber lasts. The ion current detecting apparatus automatically detects degradation in an insulation resistance in the spark plug, ensuring normal operation in engine control.
