Engine control apparatus

An engine control apparatus, comprising:

• an oxygen sensor that outputs an oxygen sensor output value corresponding to an oxygen concentration of exhaust gas exhausted from an engine;
• and
• an air-fuel ratio feedback control unit that performs air-fuel ratio feedback control on the basis of the oxygen sensor output value in order to adjust an amount of fuel injected into the engine,
• the air-fuel ratio feedback control unit including:
• an air-fuel ratio region detection unit that detects an air-fuel ratio region, among four or more preset air-fuel ratio regions, to which an air-fuel ratio of the engine belongs on the basis of the oxygen sensor output value;
• and
• an air-fuel ratio feedback control correction amount calculation unit that calculates a first feedback control correction amount for use during the air-fuel ratio feedback control in accordance with the air-fuel ratio region detected by the air-fuel ratio region detection unit,
• wherein the four or more regions include at least a first rich region and a second rich region set on a rich side of a stoichiometric air-fuel ratio in ascending order of a value of the air-fuel ratio, and a first lean region and a second lean region set on a lean side of the stoichiometric air-fuel ratio in descending order of the value of the air-fuel ratio, and
• the air-fuel ratio region detection unit:
• includes a first determination voltage set at a higher value than a target voltage value that is a voltage value indicating the stoichiometric air-fuel ratio, and a second determination voltage set at a lower value than the target voltage value;
• compares the oxygen sensor output value respectively with the first determination voltage and the second determination voltage;
• determines that the air-fuel ratio of the engine is within the first rich region when the oxygen sensor output value equals or exceeds the first determination voltage;
• determines that the air-fuel ratio of the engine is within the second rich region when the oxygen sensor output value equals or exceeds the target voltage value but is lower than the first determination voltage;
• determines that the air-fuel ratio of the engine is within the second lean region when the oxygen sensor output value equals or exceeds the second determination voltage but is lower than the target voltage value;
• and
• determines that the air-fuel ratio of the engine is within the first lean region when the oxygen sensor output value is lower than the second determination voltage,
• wherein a rate of change of the oxygen sensor output value varies relative to the air-fuel ratio of the engine, and
• wherein the first determination voltage and the second determination voltage are set such that, at a predetermined temperature of the oxygen sensor;
• when the oxygen sensor output value exceeds the first determination voltage, the rate of change of the oxygen sensor output value varies at a first rate relative to the air-fuel ratio of the engine,
• when the oxygen sensor output value is between the first determination voltage and the second determination voltage, the rate of change of the oxygen sensor output value varies at a second rate relative to the air-fuel ratio of the engine, and
• when the oxygen sensor output value is lower than the second determination voltage, the rate of change of the oxygen sensor output value varies at a third rate relative to the air-fuel ratio of the engine,
• wherein the second rate is greater than the first rate and the third rate.