PURPOSE: To detect such a fluctuation as is felt unpleasant in order to prevent a lowering of drivability by sending an ionic current in a combustion chamber for every ignition, measuring a period during which an ionic current is generated, and computing a fluctuation during the period so as to decide a lean limit. ;CONSTITUTION: Both a power source 24 for bias voltage, which is for measuring an ionic current, and an ionic current measuring circuit 25 are connected to a spark plug 18 via a high voltage diode 23. Immediately after the ignition, if the bias voltage is impressed from the bias power source 24 to the spark plug 18, an ionic current suddenly flows. This ionic current decreases once just before the top dead center and then increases again, and the current value takes the peak value in the vicinity of a crank angle, at which the combustion pressure becomes maximum. Therefore, a period of time, during which the ionic current flows, is measured for every ignition, and at the same time a coefficient of fluctuation in the period of time measured is computed based on the data of the period of time, whose number of data is determined from the number of revolutions of an engine. After this, the lean limit is decided, based on the coefficient of fluctuation thus obtained.;COPYRIGHT: (C)1996,JPO
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