Engine control system

An engine control system comprising:

• an exhaust gas sensor having a sensor element that is provided with an electric heater for controlling temperature near the sensor element and appropriately operating at a predetermined activation temperature, measuring oxygen concentration of exhaust gas of the engine and outputting oxygen-concentration data, and outputting predetermined oxygen-concentration data when said exhaust gas is in an atmospheric air exchange state and the temperature near said sensor element is at the predetermined activation temperature;
• a standard characteristic storage memory in which a functional expression or a data table showing a relation between said oxygen-concentration data and an air-fuel ratio of said engine at said predetermined activation temperature and oxygen-concentration data in the mentioned atmospheric air exchange state are stored;
• atmospheric air judging means for judging an atmospheric air state to determine that said exhaust gas is in an atmospheric air exchange state that the exhaust gas has been exchanged with said atmospheric air when fuel supply to the mentioned engine has been stopped exceeding a predetermined time;
• first heater control means that controls said electric heater so that said oxygen-concentration data coincide with the oxygen-concentration data stored in said standard characteristic storage memory when said atmospheric air state judging means judges that the exhaust gas is in the atmospheric air state;
• calibration signal reading means that stores internal resistance of said exhaust gas sensor or internal resistance of said electric heater at the point of time when the oxygen-concentration data outputted by said exhaust gas sensor being controlled by said first heater control means comes to coincide to said standard value data as a target internal resistance;
• second heater control means that operates when fuel is supplied to said engine to control the foregoing electric heater so that a current value of said internal resistance of said exhaust gas sensor or the electric heater coincides to said target internal resistance;
• and
• air-fuel ratio calculating means that has a microprocessor and calculates the current air-fuel ratio on the basis of the current oxygen-concentration data of said exhaust gas sensor controlled by said second heater control means and said functional expression or data table stored in said standard characteristic storage memory.