Air/fuel ratio control apparatus for general-purpose engine

An apparatus for controlling an air/fuel ratio of a general-purpose internal combustion engine that is supplied with mixed fuel containing alcohol and gasoline and stored in a fuel tank and that is connectable to a load, comprising

• a desired engine speed input switch adapted for an operator to input a desired speed of the engine,
• an engine speed detector that detects a speed of the engine,
• a throttle opening regulator that regulates an opening of a throttle valve installed in an air intake pipe of the engine such that the detected engine speed converges to the inputted desired engine speed,
• a map switch that selects one of a plurality of maps corresponding to respective rates of alcohol contained in the mixed fuel stored in the fuel tank, the respective rates of alcohol being greater than 0%, the map switch having a plurality of selectable positions corresponding to the respective rates of alcohol and selecting the one of the plurality of maps in response to a selected position,
• a fuel injection amount calculator that calculates a fuel injection amount of the engine based on the detected engine speed and the regulated throttle opening in accordance with the rate of the alcohol corresponding to the map selected by the map switch, and
• an injector that injects the fuel based on the calculated fuel injection amount,
• wherein the apparatus comprises:
• a load determiner that determines whether the load connected to the engine is kept constant;
• a fuel injection amount injecting increasing or decreasing unit that increases or decreases the calculated fuel injection amount to be injected, when the load connected to the engine is determined to be kept constant;
• an air/fuel ratio estimator that determines a minimum value of the throttle opening using the change of the throttle opening changed by the throttle opening regulator in accordance with the increase or decrease of the calculated fuel injection amount and estimates an air/fuel ratio at which an output of the engine becomes maximum based on the determined minimum value;
• a fuel injection amount corrector that calculates a correction coefficient based on the estimated air/fuel ratio, corrects the map by multiplying each fuel injection amount in the map corresponding to the selected position of the map switch by the calculated correction coefficient and corrects the fuel injection amount in accordance with the corrected map;
• and
• a correction coefficient storing unit that stores the calculated correction coefficient,
• wherein the fuel injection amount corrector determines whether the map switch has been switched before starting the engine, and corrects the map using the correction coefficient stored in the correction coefficient storing unit when the map switch has not been switched before starting the engine, whereas the fuel injection amount corrector sets the correction coefficient stored in the correction coefficient storing unit to 1 to initialize the correction coefficient when the map switch has been switched before starting the engine.