1. Field of the Invention
The present invention relates to an air/fuel ratio control system for an internal combustion engine, and more particularly to an air/fuel ratio control system which uses an oxygen concentration sensor for sensing an oxygen concentration in the exhaust gas of the internal combustion engine.
2. Description of Background Information
In order to accelerate the purification of the exhaust gas and to improve the fuel economy of an internal combustion engine, a feedback type air/fuel ratio control system is used, in which oxygen concentration in the exhaust gas is detected and air/fuel ratio of the mixture supplied to the engine is controlled to a target air/fuel ratio by a feedback control operation in accordance with an output signal of the oxygen concentration sensor.
As an oxygen concentration sensor for use in such an air/fuel ratio control system, there is a type which is capable of producing an output signal whose level is proportional to the oxygen concentration in the exhaust gas of the engine. For example, a critical current type oxygen concentration sensor which includes a flat oxygen ion conductive solid electrolyte member provided with a pair of electrodes on the main surfaces thereof, and the surface of one of the electrodes forms a part of a gas retaining chamber which gas retaining chamber communicates with a flow of measuring gas such as the exhaust gas through a communication hole, is disclosed in Japanese Patent Application laid open No. 52-72286. In the case of this oxygen concentration sensor, the oxygen ion conductive solid electrolyte member and the electrode pair serve as an oxygen pump element. If a current is supplied across the electrodes in such a manner that the electrodes facing the gas retaining chamber operates as a negative electrode, the oxygen component of the gas in the gas retaining chamber is ionized on the surface of the negative electrode of the oxygen pump element, and migrates through the inside of the oxygen pump element to the positive electrode, where the oxygen ions are released from the surface thereof in the form of the oxygen gas.
Under this condition, the magnitude of the critical current flowing across the electrodes becomes constant irrespectively of the applied voltage, and varies substantially in proportion to the oxygen concentration in the measuring gas. Therefore, by detecting the critical current value, the oxygen concentration in the measuring gas can be measured. However, if the air/fuel ratio is controlled by using this oxygen concentration sensor, the output signal of the oxygen concentration sensor becomes proportional to the oxygen concentration only when the air/fuel ratio of the mixture supplied to the engine is leaner than the stoichiometric air/fuel ratio. Therefore, an air/fuel ratio control operation using a target air/fuel ratio which is set in a rich range was not possible. As an example of oxygen concentration sensor capable of producing an output signal which is proportional to the oxygen concentration in the exhaust gas in both of the lean and rich ranges, an oxygen concentration sensor which includes a pair of flat oxygen ion conductive solid electrolyte members each of which is provided with a pair of electrodes, and the surface of one electrode of each solid electrolyte member forms a part of a gas retaining chamber which gas retaining chamber communicates with a flow of measuring gas through a communication hole, and the surface of the other electrode of one of two solid electrolyte members faces an atmospheric chanber, is disclosed in Japanese Patent Application laid open No. 59-192955. In this oxygen concentration sensor, one of two oxygen ion conductive solid electrolyte members and the electrode pair serve as an oxygen concentration ratio detection sensor cell element, and the other one of two oxygen ion conductive solid electrolyte members and the electrode pair serve as the oxygen pump element. By supplying a current so that the oxygen ions moves through the inside of the oxygen pump element toward the electrode located on the gas retaining chamber's side when the voltage generated across the electrodes of the oxygen concentration ratio detection sensor cell element is higher than a reference voltage, and so that the oxygen ions move through the inside of the oxygen pump element toward the electrode located on the other side of the gas retaining chamber when the voltage generated across the oxygen concentration ratio detection sensor cell element is equal to or lower than the reference voltage, the current value becomes proportional to the oxygen concentration both in the lean and rich regions.
When this oxygen concentration proportional type oxygen concentration sensor is used in an exhaust pipe of the internal combustion engine, it is recognized that the diameter of the gas introduction hole reduces gradually because of the adhesion of substances such as oxide compounds contained in the exhaust gas. If the diameter of the gas introduction hole is reduced, the output signal characteristic of the oxygen concentration sensor will be changed and a desirable output signal characteristic can not be obtained. Therefore, it becomes difficult to accurately determine the air/fuel ratio of the mixture from the output signal level of the oxygen concentration sensor. Thus, there was a problem that the change in the output signal characteristic of the oxygen concentration sensor causes a reduction of the efficiency of the purification of the exhaust gas of the engine.