Combustion air-fuel ratio control system for an internal combustion engine

A combustion air-fuel ratio control system for an internal combustion engine comprising:

• an exhaust passage where exhaust from the internal combustion engine is guided;
• an exhaust purification catalyst arranged downstream of the exhaust passage;
• a fresh air supplying device arranged upstream of the exhaust purification catalyst to supply fresh air to the exhaust passage;
• an exhaust air-fuel ratio sensor disposed between the exhaust purification catalyst and the fresh air supplying device and arranged to detect an exhaust air-fuel ratio of the exhaust;
• an exhaust air-fuel ratio sensor activation determining unit that determines whether or not the exhaust air-fuel ratio sensor is active;
• and
• a combustion air-fuel ratio control unit configured to control a combustion air-fuel ratio of the internal combustion engine based on the exhaust air-fuel ratio that is detected by the exhaust air-fuel ratio sensor by controlling the fresh air from the fresh air supplying device to the exhaust passage when warming up of the exhaust purification catalyst is requested,
• the combustion air-fuel ratio control unit open controlling the combustion air-fuel ratio to a primary combustion air-fuel ratio which is between a theoretical air-fuel ratio and a combustion limit air-fuel ratio that is richer than the theoretical air-fuel ratio upon the exhaust air-fuel ratio sensor activation determining unit determining the exhaust air-fuel ratio sensor to be inactive, and
• the combustion air-fuel ratio control unit feedback controlling the combustion air-fuel ratio to a secondary combustion air-fuel ratio that is richer than the primary combustion air-fuel ratio upon the exhaust air-fuel ratio sensor activation determining unit determining the exhaust air-fuel ratio sensor to be active.

An air-fuel ratio controlling method for an internal combustion engine, including an exhaust purification catalyst that is arranged in an exhaust passage where exhaust from the internal combustion engine flows, a fresh air supplying device that supplies fresh air upstream of the exhaust purification catalyst, an exhaust air-fuel ratio sensor that detects an exhaust air-fuel ratio between the exhaust purification catalyst and the fresh air supplying device in the exhaust passage, the air-fuel ratio controlling method comprising:

• determining whether or not the exhaust air-fuel ratio sensor is active;
• determining timing of a request for warming up the exhaust purification catalyst;
• setting a combustion air-fuel ratio to a primary combustion air-fuel ratio, which is between a theoretical air-fuel ratio and a combustion limit air-fuel ratio, which is richer than the theoretical air-fuel ratio and allows stable combustion in an open control, while supplying fresh air from the fresh air supplying device to the exhaust passage, when warming up of the exhaust purification catalyst is required, and the exhaust air-fuel ratio sensor is inactive;
• and
• setting the combustion air-fuel ratio to a secondary combustion air-fuel ratio, which is richer than the primary combustion air-fuel ratio and close to the combustion limit air-fuel ratio in a feedback control, while supplying fresh air from the fresh air supplying device to the exhaust passage, when the warming up of the exhaust purification catalyst is required, and the exhaust air-fuel ratio sensor is active.