1. Field of the Invention
The present invention relates to an air-fuel ratio feedback control system in an internal combustion engine having air-fuel ratio sensors upstream and downstream of a three-way reducing and oxidizing catalyst converter in an exhaust gas passage.
2. Description of the Related Art
Among known air-fuel ratio feedback control systems using air-fuel ratio sensors (O.sub.2 sensors), there exists a single O.sub.2 sensor system, i.e., having only one O.sub.2 sensor. Note, in this system the O.sub.2 sensor is disposed either upstream or downstream of the catalyst converter.
In a single O.sub.2 sensor system having an O.sub.2 sensor upstream of the catalyst converter, the O.sub.2 sensor is disposed in the exhaust gas passage near to a combustion chamber, i.e., near the concentration portion of an exhaust manifold. In this system, however, the output characteristics of the O.sub.2 sensor are directly affected by a non-uniformity or non-equilibrium state of the exhaust gas. For example, when the air-fuel ratio actually indicates a rich state, but oxygen is still present, the output characteristics of the O.sub.2 sensor fluctuate. Also, in an internal combustion engine having a plurality of cylinders, the output characteristics of the O.sub.2 sensor are also directly affected by differences in individual cylinders, and accordingly, it is impossible to detect the mean air-fuel ratio for the entire engine, and thus the accuracy of the control of the air-fuel ratio is low.
On the other hand, in a single O.sub.2 sensor system having an O.sub.2 sensor downstream of the catalyst converter, the non-uniformity or non-equilibrium state of the detected exhaust gas has little or no effect, and thus the mean air-fuel ratio for the engine can be detected. In this system, however, due to the capacity of the catalyst converter, the response characteristics of the O.sub.2 sensor are lowered, and as a result, the efficiency of the catalyst converter cannot be properly exhibited, and thus the HC, CO and NO.sub.x emissions are increased.
To solve the above problems, the following method, for example, is known. Namely, the actual air-fuel ratio is adjusted by a self-oscillating term, and a mean value thereof, i.e., a coarse-adjusting term, is controlled in accordance with the output of the O.sub.2 sensor disposed downstream of the catalyst converter.
Nevertheless, this method cannot eliminate the increase of HC, CO and NO.sub.x emissions occurring when the actual air-fuel ratio deviates from the stoichiometric air-fuel ratio, because the integral speed for the coarse-adjusting term is set at a small value, and it takes a long time to correct the air-fuel ratio so that the efficiency of the catalyst converter is properly exhibited.
To solve the above problem, the present inventor has already suggested a method of using a proportional O.sub.2 storage term and an integral O.sub.2 storage term, to forcibly shift the coarse-adjusting term when the output of the O.sub.2 sensor disposed downstream of the catalyst converter is outside a pre-determined region, or when the actual air-fuel ratio has become rich after the completion of the warming-up of the catalyst converter.
The above method, however, cannot eliminate the increase of HC, CO and NO.sub.x emissions, when the catalyst converter is cold because the functioning of the O.sub.2 sensor is delayed, even if it is equipped with a heater, and the start of the air-fuel ratio feedback control is also delayed because the O.sub.2 sensor is located at a position where the exhaust gas temperature is low.