Engine air-fuel ratio controller

Claim 1

An air-fuel ratio controller for an engine provided with a fuel injector, an exhaust gas passage, and a catalytic converter using a three-way catalyst installed in an exhaust gas passage, said controller comprising: a sensor for detecting engine running conditions, means for computing a basic fuel injection amount for the fuel injector based on said engine running conditions detected by said sensor, a first oxygen sensor installed upstream of said catalytic converter in the exhaust passage, means for determining from the output of said first oxygen sensor whether the air-fuel ratio has varied beyond a predetermined theoretical air-fuel ratio, means for computing a basic control constant of air-fuel ratio feedback based on the determining results, a memory for storing correction values to be used for the correction of the basic control constant, each of said correction values being stored in each area defined in the memory according to said engine running conditions, means for determining whether said engine running conditions detected by said sensor are in any of the defined areas, means for retrieving from said memory a correction value corresponding to the defined area which covers said engine running conditions, means for computing an air-fuel ratio feedback correction coefficient from the retrieved correction value and said basic control constant, means for computing a target fuel injection amount from said basic fuel injection amount and said air-fuel feedback correction coefficient, means for controlling the fuel injection amount supplied by the fuel injector to the target amount, a second oxygen sensor installed downstream of the catalytic converter in the exhaust passage, means for setting an updating amount of said correction value such that it becomes larger when the output of said second oxygen sensor deviates a predetermined amount from a value corresponding to said predetermined theoretical air-fuel ratio, and becomes smaller in the region of a predetermined intermediate value, and means for updating said correction value stored in said memory corresponding to said defined area covering said engine running conditions detected by said sensor, by said set updating amount.