PURPOSE: To improve the judging accuracy of a lean limit by outputting an output signal only during the ion current in a combustion room exceeds a prescribed current value, outputting a time count signal produced at the point of the time in the input term of this output signal and judging a lean limit based on this time count signal. ;CONSTITUTION: When a bias voltage is impressed from an electric power supply 24 for a bias to an ignition plug 18 just after ignition, this ion current is reduced in front of an upper dead point after the ion current is flowed suddenly once and then increased again and flowed in a combustion room 10 while repeating increase/decrease. Then in ECU 6 for inputting the output signal of an ion current measuring circuit 25, a time in which the ion current is flowed at every ignition in a prescribed cylinder, namely a lasting time is measured. The ion current is compared with the prescribed current value and an output signal is outputted only during the ion current exceeds the prescribed current value and also a time count signal is generated at nearly same time as the generation of the ion current and a lean limit is judged based on this time count signal.;COPYRIGHT: (C)1996,JPO
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