(1) Field of the Invention
The present invention relates to a system and method for controlling an air/fuel mixture ratio of an air/fuel mixture supplied to an internal combustion engine in which an operating variable of a feedback correction coefficient (LAMBDA) is corrected in accordance with a degree of deterioration of an oxygen sensor so that the air/fuel mixture ratio of the air/fuel mixture sucked into the engine reaches a target air/fuel mixture ratio.
(2) Background of the Art
A Japanese Patent Application First Publication (Non-examined) Showa 60-240840 published on Dec. 29, 1985 exemplifies one of previously proposed air/fuel mixture ratio controlling systems.
In the above-identified Japanese Patent Application First Publication No. Showa 60-240840, an intake air quantity Q and/or intake air pressure PB is detected as an input variable related to intake air. A basic fuel supply quantity T.sub.p is calculated on the basis of the input variable such as Q and/or PB and another input variable such as an engine revolutional speed N.
The basic fuel supply quantity T.sub.p is corrected with various kinds of correction coefficients COEF set on the basis of each of various engine driving conditions such as engine temperature represented by an engine coolant temperature, air-fuel mixture ratio correction coefficient LAMBDA (.lambda.), and a correction coefficient T.sub.s relative to a variation of a battery voltage to calculate a final fuel supply quantity T.sub.i =(T.sub.p .times.COEF.times.LAMBDA+Ts). The calculated quantity of fuel is supplied to the engine through a fuel injector(s).
The air/fuel mixture ratio feedback correction coefficient LAMDA is set, e.g., in a proportional-integral control (P-I) mode. When the actual air/fuel mixture ratio based on the oxygen concentration in the exhaust gas second by means of the oxygen sensor is rich (or lean) with respect to a stoichiometric air/fuel mixture ratio (target air/fuel mixture ratio), the correction coefficient LAMBDA is initially decreased (or increased) by a proportional constant P and thereafter is gradually decreased (or increased) by an integration constant I in synchronization with time or engine revolutions so that the actual air/mixture ratio is repeatedly reversed in the vicinity of the target air/fuel mixture ratio. When repeating the rich and lean air/fuel mixture ratios for the same time, an average air/fuel mixture ratio is, thus, controlled to the target air/fuel mixture ratio.
For the oxygen sensor used in feedback control of the air/fuel mixture ratio, a sensor utilizing oxygen concentration in the exhaust gas rapidly changed with the stoichiometric air/fuel mixture ratio as a boundary and capable of detecting richness and leaness of the actual air/fuel mixture with respect to the stoichiometric air/fuel mixture ratio has commonly been used. The sensor is so constructed that an electrode is formed on each of inner and outer surfaces of a zirconia tube and an electromotive force is generated between both eletrodes according to a ratio between the oxygen concentration in the air introduced into the inner side of the tube and that in the exhaust gas emitted on the outer side of the tube. If the electromotive force is monitored, the oxygen concentration in the exhaust gas, i.e., the rich and lean in the intake air mixed with fuel sucked into the engine with respect to the stoichiometric air/fuel mixture ratio can indirectly be detected (refer to a Japanese Utility Model Registration First Publication No. Showa 63-51273 published on Apr. 6, 1986).
In the previously proposed air/fuel mixture controlling system in which the air/fuel mixture ratio is controlled in the feedback control mode according to a result of detection of the oxygen sensor, the oxygen sensor deteriorates so that the output characteristic of the detection signal with respect to the stoichiometric air/fuel mixture ratio is, from the intial stage of service, changed. Then, the actual air/fuel mixture ratio obtained by the alternate repetitions of the rich side and lean side of the air/fuel mixture ratio is not controlled in the vicinity to the target ratio (stoichiometric air/fuel mixture ratio).
A three-catalytic converter is installed in the exhaust system of a vehicular engine in order to clarify the exhaust gas. Since the three-catalytic converter exhibits best conversion efficiency when the air/fuel mixture is burned at the stoichiometric air/fuel mixture ratio, the conversion efficiency is reduced by means of the three-catalytic converter so that harmful components of CO, HC, and NO.sub.x are increased in the exhaust gas when the air/fuel mixture ratio controlled in the feedback mode due to the deterioration of the oxygen sensor deviates from the stoichiometric air/fuel mixture ratio.
In the case where almost no change in the static characteristic in the oxygen sensor is found and a response time of the oxygen sensor becomes changed from the initial stage, when, e.g., the actual air/fuel mixture ratio is reversed from the rich side to the lean and vice versa, a control point of the air/fuel mixture ratio intially and thereafter deviates from the stoichiometric air/fuel mixture ratio so that sufficient exhaust purification effect cannot be achieved any more by means of the three-catalytic converter.
Examples of characteristic changes due to the deterioration in the oxygen sensor will be described below (refer to FIGS. 10 to 13).
In a case where a slight thermal deterioration occurs in the zirconia constituting the oxygen sensor of the well known zirconia tubular type oxygen sensor, the characteristic is shifted toward the rich side with respect to the initial output characteristic and the response characteristic is such that the response from the rich state to the lean state becomes fast as compared with that at the initial stage, as shown in Table I, and the control frequency becomes high. Therefore, since the oxygen sensor is used to perform feedback control, the air/fuel mixture ratio is controlled toward the richer air/fuel mixture ratio rather than toward the stoichiometric air/fuel mixture ratio. In addition, as the thermal deterioration proceeds, the output at the rich side is reduced. Consequently, since the characteristic of the output signal is step with the stoichiometric air/fuel mixture ration as the boundary, the control frequency becomes smaller so that the response speed becomes slower.
TABLE I ______________________________________ Output Con. response A/Fr. R L Fre. balance (FIG. 14) C.P. ______________________________________ Small thermal -- -- f. 1, b R. deterioration Inside thermal low low -- 1, a R. deterioration outside -- high s. 1, c or d L. clogging Large thermal low -- s. 2 or 3, a L. deterioration ______________________________________
On the other hand, in a case where the zirconia tube type oxygen sensor is used, the air is introduced toward the inner side of the zirconia tube and the electromotive force is generated according to the ratio between the oxygen concentration in the air and oxygen concentration in the exhaust gas, the electrode installed in the inner side of the tube deteriorates and a clog in a protective layer protecting the zirconia tube from the exhaust gas occurs. At this time, the sensor output characteristic is changed so as to not indicate steep change and so as to have a more flat change. (Refer to FIGS. 12 and 13).
That is to say, if the inner electrode deteriorates and electromotive force cannot be picked up sufficiently, the output voltages at the rich side or at the lean side are reduced so that the control point of the feedback control will be transferred to the rich side (refer to Table I). In addition, when the clog occurs in the protecting layer, the ratio of oxygen concentration does not become large even in the lean state, the lean output voltage becomes high. Consequently, the detection response characteristic from the rich side to the lean side becomes worse and the control point deviates from the lean side (refer to Table 1).