PURPOSE: To improve the responsiveness in the rich-lean judgment of air-fuel ratio, by setting, as the rich/lean judging value, an upper limit value less than the maximum value of the output signal of an air-fuel ratio sensor and/or an lower limit value greater than the minimum value of the output signal of the air-fuel ratio sensor.
CONSTITUTION: During the operation of an engine 1, a CPU 52 obtains a basic injection quantity on the basis of the outputs of an air flowmeter 12 and a revolution sensor 32, and obtains an execution injection quantity by correcting this with correction factors such as the air-fuel ratio feedback factor FAF determined by a rich/lean judging routine and the warming increase quantity. In this case, in a rich/lean judging routine, the output of an air-fuel ratio sensor 41 is compared with the rich/lean judging value to judge the rich/lean condition of air-fuel ratio, and when it is judged that the air-fuel ratio is on the rich (lean) side, if the value obtained by subtracting (adding) a prescribed value from the maximum value (to the minimum value) of the output of the air-fuel ratio sensor 41 does not exceed (is not less than) the upper limit value (lower limit value), this value is set as the judging value hereafter.
NOROTA KAZUHIKO
KUSHI NAOTO
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