Air-fuel ratio control apparatus

An air-fuel ratio control apparatus comprising:

• a catalyst provided in an exhaust passage of an internal combustion engine;
• an upstream gas sensor which is provided in the exhaust passage to be located on the upstream side of the catalyst and which includes a detection element and an element heating heater;
• a downstream gas sensor which is provided in the exhaust passage to be located on the downstream side of the catalyst and which includes a detection element and an element heating heater;
• heater control means for controlling the heating states of the element heating heater of the upstream gas sensor and the element heating heater of the downstream gas sensor;
• and
• air-fuel ratio control means for controlling exhaust air-fuel ratio on the basis of an output from at least one of the upstream gas sensor and the downstream gas sensor,
• the air-fuel ratio control apparatus further comprising:
• start state determination means for determining whether or not the internal combustion engine is in a low-temperature start state, wherein
• the heater control means controls the temperature of the detection element of the downstream gas sensor to a predetermined downstream target temperature using the element heating heater of the downstream gas sensor when the start state determination means determines that the engine is in the low-temperature start state, and the heater control means controls the temperature of the detection element of the upstream gas sensor to a predetermined upstream target temperature using the element heating heater of the upstream gas sensor when the start state determination means determines that the engine is not in the low-temperature start state;
• the air-fuel ratio control means feedback-controls the exhaust air-fuel ratio based on the output of the downstream gas sensor when the start state determination means determines that the engine is in the low-temperature start state and the detection element of the downstream gas sensor becomes active, and the air-fuel ratio control means feedback-controls the exhaust air-fuel ratio based on the output of the upstream gas sensor when the start state determination means determines that the engine is not in the low-temperature start state and the detection element of the upstream gas sensor becomes active;
• and
• the downstream target temperature is a temperature which is lower than the upstream target temperature and at which the detection element of the downstream gas sensor becomes active, and at which the detection element of the downstream gas sensor does not break due to water adhesion.