ELECTRONIC AIR-FUEL RATIO CONTROL APPARATUS IN INTERNAL COMBUSTION ENGINE

Abstract: An electronic air-fuel ratio control apparatus in an internal combustion engine provided with an oxygen sensor emitting an output voltage in response to an oxygen concentration including oxygen in nitrogen oxides in an exhaust gas from the engine controls an air-fuel ratio of an air-fuel mixture by a feedback correction-control based on a fuel injection quantity in an on-off manner. By using the oxygen sensor having the nitrogen oxides-reducing catalytic layer, the detection of a theoretical air-fuel ratio is performed on a richer side comparing with the output on the detection of a theoretical air-fuel ratio by an oxygen sensor without the nitrogen oxides-reducing function and is not changed even though the nitrogen oxides concentration changes. Accordingly the feedback air-fuel ratio control effects to decrease the amount of nitrogen oxides so as to stabilize the air-fuel ratio control. A first target air-­ fuel ratio for the air-fuel ratio feedback control is changed to a second target air-fuel ratio which is richer than the first target air-fuel ratio when the high nitrogen oxide concentration in the exhaust gas is detected or which is leaner than the first target air-fuel ratio when the incompletely burnt component concentration in the exhaust gas is detected.

Description: Background of the Invention (1) Industrial Application Field of the Invention The present invention relates to an air-fuel ratio control apparatus in which a fuel injection valve arranged in an intake passage of an internal combustion engine is pulse-controlled in an on-off manner and an optimum air-fuel ratio in an air-fuel mixture sucked in the engine is obtained by electronic feedback control correction. More particularly, the present invention relates to an air-fuel ratio control apparatus in which the amounts discharged of nitrogen oxides (NO x ) and incompletely burnt components (CO, HC and the like) are reduced. (2) Description of the Related Art As the conventional air-fuel ratio electronic control apparatus in an internal combustion engine, there can be mentioned a control apparatus as disclosed in Japanese Patent Application Laid-Open Specification No. 240840/85. This apparatus is now summarized. A flow quantity Q of air sucked in the engine and the revolution number N of the engine are detected and the basic fuel supply quantity Tp (= K.Q/N: K is a constant) corresponding to the quantity of air sucked in a cylinder is computed. This basic fuel injection quantity is corrected according to the engine driving states, for example the engine temperature and the like and the air-fuel ratio feedback correction coefficient LAMBDA is performed based on a signal from an oxygen sensor for detecting the air-fuel ratio of the air-fuel mixture by detecting the oxygen concentration in the exhaust gas, and correction based on a battery voltage or the like is carried out and a fuel injection quantity Ti (=TP x C0EF x LAMBDA + Ts) is finally set. By putting out a driving pulse signal of a pulse width corresponding to the thus set fuel supply quantity Ti to an electromagnetic fuel injection valve at a predetermined timing, a predetermined quantity of a fuel is injected and supplied to the engine. The air-fuel ratio feedback correction coefficient LAMBDA is set to control an air-fuel ratio in an air-fuel mixture sucked into the engine to a target air-fuel ratio (the theoretical air-fuel ratio). The LAMBDA is gradually changed in the manner of proportion and integration controls to attain a stable and smooth control for the air-fuel ratio feedback. Incidentally the proportion control is generally recognized to belonged to the integration control. The reason why the air-fuel ratio in the mixture is controlled to a value close to the theoretical air-fuel ratio is that the conversion efficiency (purging efficiency) of a ternary catalyst disposed in the exhaust system to oxidize CO and HC (hydrocarbon) in the exhaust gas and reduce NO x for purging the exhaust gas is set so that a highest effect is attained for an exhaust gas discharged when combustion is performed at the theoretical air-fuel ratio. Accordingly, a system having a known sensor portion structure as disclosed in Japanese Patent Application Laid-Open Specification No. 204365/83 is used for the oxygen sensor. This system comprises a ceramic tube having an oxygen ion-conducting property and a platinum catalyst layer for promoting the oxidation reaction of CO and HC in the exhaust gas, which is laminated on the outer surface of the ceramic tube. O₂ left at a low concentration in the vicinity of the platinum catalyst layer on combustion of an air-fuel mixture richer than the theoretical air-fuel ratio is reacted in a good condition with CO and HC to lower the O₂ concentration substantially to zero and increase the difference between this reduced O₂ concentration and the O₂ concentration in the open air brought into contact with the inner surface of the ceramic tube, whereby a large electromotive force is produced between the inner and outer surfaces of the ceramic tube. On the other hand, when an air-fuel mixture leaner than the theoretical air-fuel ratio is burnt, since high-concentration O₂ and low-concentration CO and HC are present in the exhaust gas, even after by the reaction of O₂ with CO and HC, excessive O₂ is still present and the difference of the O₂ concentration between the inner and outer surfaces of the ceramic tube is small, and no substantial voltage is generated. The generated electromotive force (output voltage) of the oxygen sensor has such a characteristic that the electromotive force abruptly changes in the vicinity of the theoretical air-fuel ratio, as pointed out above. This output voltage V₀₂ is compared with the reference voltage (slice level SL) to judge whether the air-fuel ratio of the air-fuel mixture is richer or leaner than the theoretical air-fuel ratio. For example, in the case where the air-fuel ratio is lean (rich), the air-fuel ratio feedback correction coefficient LAMBDA to be multiplied to the above-mentioned basic fuel injection quantity Ti is gradually increased (decreased) by predetermined integration constant, i.e. the feedback control correction constant, whereby the air-fuel ratio is controlled to a value close to the theoretical air-fuel ratio. From the comprehensive viewpoint, although the oxygen component in NO x should be detected as a part of the oxygen concentration in the exhaust gas, this oxygen cannot be grasped by the oxygen sensor, reversion of the electromotive force tends to occur at the air-fuel ratio leaner by the oxygen component in NO x than the theoretical air-fuel ratio and the air-fuel ratio is controlled to a too much lean value, whereby reduction of the conversion of NO x in the ternary catalyst is promoted. Therefore, reduction of NO x tried by performing EGR (exhaust gas recycle) control in combination. However, mounting of an EGR apparatus results in increase of the cost, and the fuel rating is drastically reduced by reduction of the combustion efficiency by introduction of the exhaust gas. Under this background, there has been proposed an oxygen sensor in which an NO x -reducing catalyst layer containing rhodium or the like capable of promoting the reduction reaction of NO x in the exhaust gas is arranged and NO x is thus reduced, whereby oxygen in NO x can be detected (see E. P. 0. 267,764 A2 and E. P. 0. 267.765 A2). If this oxygen sensor is used, the electromotive force of the oxygen sensor is reversed at the true air-fuel ratio. This true air-fuel ratio is a value shifted to a rich side by the oxygen component in NO x from the theoretical air-fuel ratio at which the electromotive force is reversed when the oxygen sensor having no capacity of reducing NO x . Accordingly, if this oxygen sensor is used, the air-fuel ratio is shifted to a rich side and controlled to a value close to the true theoretical air-fuel ratio. Furthermore, since the air-fuel ratio is controlled to a substantially constant level irrespectively of the value of the NO x concentration, the conversions of CO, HC and NO x are sufficiently increased in the ternary catalyst, and the amounts discharged of CO and HC can be most effectively reduced and the NO x content can be effectively lowered, with the result that omission of the EGR apparatus becomes possible. However, even in the case where the air-fuel ratio is thus controlled to the vicinity of the true theoretical air-fuel ratio, since the NO x , CO and HC (especially NO x and CO) conversions of the ternary catalyst abruptly change in the vicinity of this value because of the above-mentioned characteristic of the ternary catalyst and the conversion is unstable because of the dispersion and the deterioration of parts and since the air-fuel ratio is temporarily made much leaner or richer in the manner of frequency with respect to the theoretical air-fuel ratio, it is actually difficult to obtain the high and stable conversions of the catalyst. From the above-mentioned view point, setting the target air-fuel ratio to a slightly leaner value than the theoretical air-fuel ratio is desirably expected in case of an engine in which the combustion performance is inherently poor and incompletely burnt components CO and HC are easily formed by incomplete combustion. This is because the high and stable conversions of CO and HC in the catalyst can be positively attained while forming of NO x component in the engine is reduced. On the contrary, in case of an engine in which the combustion performance is inherently good and the NO x component is easily formed from the engine while poor CO and HC components are formed, it is desirably expected to set the target air-fuel ratio to a value slightly richer than the theoretical air-fuel ratio for attaining the high and stable conversion of NO x in the ternary catalyst. Further, even a same engine has different driving states where CO and HC component is easily formed or where NO x component is easily formed, therefore as same as the afore-discussion, it is preferable to reset the target air-fuel ratio corresponding to the difference of the engine driving states. Setting the target air-fuel ratio to slightly richer or leaner value in the air-fuel ratio feedback control should be carried out within the predetermined zone with the theoretical air-fuel ratio for effectively reducing the CO, HC and HO x component in the exhaust gas. If the target air-fuel ratio is set to the extremely lean air-fuel ratio, the amount of the CO component exhaust from the engine is reduced with the result that the reduction reaction between NO x and CO can hardly be performed. Based on this, the reversing point of the output voltage from the oxygen sensor cannot be shifted enough to richer air-fuel ratio than the oxygen sensor without the NO x educing capacity and then the function of reducing the NO x component amount by the air-fuel ratio feedback control using the oxygen sensor with NO x reducing capacity is no longer effectively performed. While, if the target air-fuel ratio is set to an extremely rich air-fuel ratio beyond the predetermined zone, only the amount of CO and HC components are increased and the NO x reducing reaction in the NO x reducing oxygen sensor and the ternary catalyst is saturated. Consequently, the target air-fuel ratio in the air-fuel ratio feedback control apparatus is necessary to be set to the optimum value within the predetermined air-fuel ratio zone in order to reduce the CO and HC components and also NO x component when the air-fuel ratio feedback control apparatus comprises the NO x reducing oxygen sensor. The earlier, non-prepublished European patent application EP-A-308870 discloses an electronic air-fuel ratio control apparatus comprising an engine driving state detecting means, a nitrogen oxides concentration detecting means, an oxygen sensor, an air-fuel ratio feedback control means, a fuel injecting means and a fuel injection quantity computing means. The air-fuel ratio feedback control means selects different target values for the feedback control. The respective target values are set according to specific table entry values according to the actual engine driving state. However, this reference fails to disclose the setting of the second air-fuel ratio to a value thereof which is leaner than the theoretical air-fuel ratio by up to 5% when a high incompletely burnt component concentration is detected. The present invention is based on the object of providing an electronic air-fuel ratio control apparatus and a method for controlling the air-fuel ratio of an air-fuel mixture in which a target air-fuel ratio is set to a value near the vicinity of the true theoretical air-fuel ratio such that the total amount of discharge of CO, HC and NO x can be reduced whilst maintaining a good balance there among under the NO x reducing performance of the oxygen sensor with an NO x reducing capacity. This object is achieved by an electronic air-fuel ratio control apparatus in accordance with claim 1 and by a method for controlling the air-fuel ratio in accordance with claim 14. A preferred embodiment of the invention will now be described in detail with reference to the accompanying drawings, in which: Fig. 1 is a block diagram illustrating the structure of the present invention. Fig. 2 is a sectional view illustrating the main part of an oxygen sensor used in one embodiment of the present invention. Fig. 3 is a diagram illustrating the system of the embodiment shown in Fig. 2. Fig. 4 is a flow chart showing a fuel injection quantity control routine in the embodiment shown in Fig. 2. Fig. 5 is a flow chart showing a feedback correction coefficient-setting routine in the embodiment shown in Fig. 2. Fig. 6 is a diagram illustrating the characteristics of the oxygen sensor in the embodiment shown in Fig. 2. Fig. 7 is a diagram illustrating the characteristics of a ternary catalyst used in the embodiment shown in Fig. 2. Fig. 8 is a diagram illustrating the concentration characteristics of various exhaust gas components. Figs. 9 and 10 are time charts respectively illustrating the changes of the feedback correction coefficient and the output voltage of the oxygen sensor at the time of the control in the embodiment of the present invention. Detailed Description of the Preferred Embodiments Fig. 2 illustrates the structure of a sensor portion of an oxygen sensor used in one embodiment of the present invention. Referring to Fig. 2, inner and outer electrodes 2 and 3 composed of platinum are formed on parts of the inner and outer surfaces of a ceramic tube 1, as the substrate, which is composed mainly of zirconium oxide (ZrO₂) which is a solid electrolyte having an oxygen ion-conducting property and has a closed top end portion. Furthermore, a platinum catalyst layer 4 is formed on the surface of the ceramic tube 1 by vacuum deposition of platinum. The platinum catalyst layer 4 is an oxidation catalyst layer for promoting the oxidation reaction of CO and HC in the exhaust gas. A NO x -reducing catalyst layer 5 (having, for example, a thickness of 0.1 to 5 µm) is formed on the outer surface of the platinum catalyst layer 4 by incorporating particles of a catalyst for promoting the reduction reaction of nitrogen oxides NO x , such as rhodium Rh or ruthenium Ru (in an amount of, for example, 1 to 10%), into a carrier such as titanium oxide TiO₂ or lanthanum oxide La₂O₃. A metal oxide such as magnesium spinel is flame-sprayed on the outer surface of the NO x -reducing catalyst layer 5 to form a protecting layer 6 for protecting the platinum catalyst layer 4 and the NO x reducing catalyst layer 5. Rhodium Rh and ruthenium Ru are publicly known as catalysts for reducing nitrogen oxides NO x , and it has been experimentally confirmed that if titanium oxide TiO₂ or lanthanum oxide La₂O₃ is used as the carrier for this catalyst, the reduction reaction of NO x can be performed much more efficiently than in the case where γ-alumina or the like is used as the carrier. Incidentally, in the oxygen sensor shown in Fig. 2, the protecting layer 6 is formed on the outer surface of the reducing catalyst layer 5, but there may be adopted a modification in which the protecting layer 6 is formed between the platinum catalyst layer 4 and the NO x -reducing catalyst layer 5. In the above-mentioned structure, when nitrogen oxides NO x contained in the exhaust gas arrive at the NO x -reducing catalyst layer 5, the NO x -reducing catalyst layer 5 promotes the following reactions of NO x with unburnt components CO and HC contained in the exhaust gas: NO x + CO → N₂ + CO₂ NO x + HC → N₂ + H₂O + CO₂ As the result, the amounts of the unburnt components CO and HC to be reacted with O₂ arriving at the platinum catalyst layer 4 located on the inner side of the NO x -reducing layer 5 are reduced by the above reactions in the NO x -reducing catalyst layer 5, and the O₂ concentration is accordingly increased. Therefore, the concentration difference between the O₂ concentration on the inner side of the ceramic tube 1 falling in contact with the open air and the O₂ concentration on the exhaust gas side is reduced, therefore, the electromotive force of the oxygen sensor is reversed below the reference value (slice level) and reduced on the side richer than in the conventional oxygen sensor in which the NO x components in the exhaust gas are not reduced, with the result that lean detection can be performed. Accordingly, if the feedback control of the air-fuel ratio is carried out based on the detection results (the results of the judgement as to whether the air-fuel mixture is rich or lean) of this oxygen sensor, the air-fuel ratio is controlled to a rich level closer to the true theoretical air-fuel ratio, obtained by detecting the oxygen concentration while taking the oxygen component of NO x into account. Incidentally, the NO x -reducing catalyst layer 5 has also a function of promoting the reaction of the unburnt components CO and HC with O₂. However, since this function is substituted for the function of the platinum catalyst layer 4, the O₂ concentration on the exhaust gas side is not reduced. An embodiment of the apparatus of the present invention for controlling the air-fuel ratio in an internal combustion engine by using the above-mentioned oxygen sensor provided with the NO x -reducing catalyst will now be described. Referring to Fig. 3, an air flow meter 13 for detecting the sucked air flow quantity Q and a throttle valve 14 for controlling the sucked air flow quantity Q co-operatively with an accelerator pedal are arranged on an intake passage 12 of an engine 11, and electromagnetic fuel injection valves 15 for respective cylinders are arranged in a manifold portion located downstream. Each fuel injection valve 15 is opened and driven by an injection pulse signal from a control unit 16 having a microcomputer built therein to inject and supply a fuel fed under a pressure from a fuel pump not shown in the drawings and maintained under a predetermined pressure controlled by a pressure regulator. Moreover, a water temperature sensor 17 for detecting the cooling water temperature Tw in a cooling jacket of the engine 11 is arranged, and an oxygen sensor 19 (see Fig. 2 with respect to the structure of the sensor portion) for detecting an air-fuel ratio in a sucked air-fuel mixture by detecting the oxygen concentration in an exhaust gas in an exhaust passage 18 is disposed. Furthermore, there is arranged a ternary catalyst 20 for purging the exhaust gas by performing oxidation of CO and HC and reduction of NO x in the exhaust gas on the downstream side. A crank angle sensor 21 is built in a distributor not shown in the drawings, and the revolution number of the engine is detected by counting for a predetermined time crank unit angle signals put out from the crank angle sensor 21 synchronously with the revolution of the engine or by measuring the frequency of crank reference angle signals. The routine of the control of the air-fuel ratio by the control unit 16 will now be described with reference to the flow chart shown in Fig. 4, which illustrates the fuel injection quantity-computing routine. This routine is carried out at a predetermined frequency (for example, 10 ms). At step (indicated by "S" in the drawings) 1, the basic fuel injection quantity Tp corresponding to the flow quantity Q of sucked air per unit revolution is computed from the sucked air flow quantity Q detected by the air flow meter 13 and the engine revolution number N calculated from the signal from the crank angle sensor 21 according to the following formula: Tp = K x Q/N (K is a constant) At step 2, various correction coefficients COEF are set based on the cooling water temperature Tw detected by the water temperature sensor 17 and other factors. At step 3, the feedback correction coefficient LAMBDA set based on the signal from the oxygen sensor 19 by the feedback correction coefficient-setting routine, described hereinafter, is read in. At step 4, the voltage correction portion Ts is set based on the voltage value of the battery. This is to correct the change of the injection quantity in the fuel injection valve 15 by the change of the battery voltage. At step 5, the final fuel injection quantity Ti is computed according to the following formula: Ti = Tp x C0EF x LAMBDA + Ts At step 6, the computed fuel injection quantity Ti is set at the output register. The portion including steps 5 and 6 shows a fuel injection quantity computing means. The engine driving state detecting means includes the air flow meter 13, the crank angle sensor 21, the water temperature sensor 17 and others. According to the above-mentioned routine, a driving pulse signal having a pulse width of the computed fuel injection quantity Ti is given to the fuel injection valve 15 at the predetermined timing synchronous with the revolution of the engine to effect injection of the fuel. The air-fuel ratio feedback control correction coefficient LAMBDA-setting routine having the feedback control constant-setting function according to the present invention will now be described with reference to Fig. 5. This routine is carried out synchronously with the revolution of the engine and shows an air-fuel ratio feedback control means by incorporated with the routine shown in Fig. 4. At step 11, the signal voltage V₀₂ from the oxygen sensor 19 is read in. At step 12, the feedback control constant is retrieved from the map stored in ROM based on newest data of the present engine revolution number N and basic fuel injection quantity Tp. As described below in Figs. 9 and 10, the feedback control constant comprises the first proportion constant P R to be added for correction of increase of the fuel injection quantity just after the rich air-fuel ratio has been reversed to the lean air-fuel ratio and the first integration constant I R to be added for correction of increase of the fuel injection quantity at the time other than the point just after the above-mentioned reversion of the air-fuel ratio. Furthermore, the feedback control constant comprises the second proportion constant P L to be subtracted for correction of decrease of the fuel injection quantity just after the lean air-fuel ratio has been reversed to the rich air-fuel ratio and the second integration constant I L to be subtracted for correction of decrease of the fuel injection quantity at the time other than the point just after the above-mentioned reversion of the air-fuel ratio. In short, the feedback control constant includes two kinds of constants, each of which has the integration constant and the proportion constant. The proportion constant is generally deemed as a kind of the integration constant. Feedback control constants P R , P L , I R and I L are rewritably stored in driving state regions which are arranged on the map in a manner of a grid based on N and TP. In the region among them where the high combustion temperature in cylinders of the engine and hence the high concentration of NO x in the exhaust gas are experimentally detected, first feedback control constants P R and I R for increasing the fuel injection quantity are set at larger value than second feedback control constants P L and I L for decreasing the fuel injection quantity respectively or set so that P R /P L and I R /I L are larger than 1 and have a tendency of increasing. While in the region where the combustion performance in the engine is not good and hence the high concentration of the incompletely burnt components CO and HC are experimentally emitted, first feedback control constants P R and I R are set at smaller value than second feedback control constants P L and I L respectively or set so that P R /P L and I R /I L are larger than 1 and have a tendency of decreasing. In each of the other driving state regions, P R and I R are mutually set at even values and also P L and I L are set at even values. Then the routine goes into step 13. As is apparent from the explanation of step 12, it is understood that the step 12 corresponds to a nitrogen oxides concentration detecting means and an incompletely burnt component concentration detecting means of the present invention as same as step 13, which is hereinafter explained. At step 13, the reference value SL (slice level), with which the signal voltage V₀₂ from the oxygen sensor is to be compared, is retrieved from the map stored in ROM based on newest data of the present engine revolution number N and the basic fuel injection quantity TP. This step 13 corresponds to a first target air-fuel ratio setting means according to the present invention. In this map, the driving region is finely divided by N and TP, and in the region where the combustion temperature is high and the NO x discharge concentration is increased (experimentally determined and retrieving these region corresponds to a nitrogen oxides concentration detecting means according to the present invention as same as in step 12), the second reference value SL H of a relatively high voltage corresponding to the air-fuel ratio richer up to 5% than the true theoretical air-fuel ratio is set while in the region where the combustion performance in the engine is not good and hence the high concentration of the incompletely burnt components CO and HC are emitted in the experimentally determination a second slice level SL L is set at a lower level than the value corresponding to the theoretical air-fuel ratio so that the second slice level SL L corresponds to the air-fuel ratio leaner by up to 5% than the theoretical air-fuel ratio (these functions correspond to a second target air-fuel setting means according to the present invention). In the other region where the NO x , CO and HC concentrations are relatively low, the first reference value SL O of a voltage corresponding to the true theoretical air-fuel ratio is set. Instead of this two-staged setting, other setting can be optionally set according to the NO x concentration. Then, the routine goes into step 14, and the signal voltage V₀₂ read in at step 11 is compared with the reference value SL (SL O , SL H or SL L ) retrieved at step 13. In the case where the air-fuel ratio is rich (V₀₂ > SL), the routine goes into step 15, and it is judged whether or not the lean air-fuel ratio has been reversed to the rich air-fuel ratio. When the reversion is judged, the feedback correction coefficient LAMBDA is decreased at step 16 by a predetermined proportion constant P L . When the non-reversion is judged, the routine goes into step 17 and the precedent value of the feedback correction coefficient LAMBDA is decreased by a predetermined integration constant I L . When it is judged at step 14 that the air-fuel ratio is lean (V₀₂ < SL), the routine goes into step 18 and it is similarly judged whether or not the rich air-fuel ratio has been reversed to the lean air-fuel ratio. When the reversion is judged, the routine goes into step 19 and the feedback correction coefficient LAMBDA is increased by a predetermined proportion P R . When the non-reversion is judged, the routine goes into step 20 and the precedent value is increased by a predetermined integration constant I R . Thus, the feedback correction coefficient LAMBDA is increased or decreased at a certain gradient. Incidentally, the relation of I « P is established. (In general, the proportion constant P is included in the integration constant I.) Incidentally the step 14 corresponds to an air-fuel ratio judging means according to the present invention. When P R and I R are even and P L and I L are even, maps of feedback control constants P R , I R , P L and I L stored in ROM at step 12 and of the slice levels SL O stored in ROM at step 13 and the functions of retrieving and setting the slice level SL O at step 13, retrieving feedback control constants P R , I R , P L and I L , and setting feedback correction coefficient LAMBDA at steps 12, 16, 17, 19 and 20 correspond to a first target air-fuel ratio setting means according to the present invention. When P R and I R are different and P L and I L are different each other, maps at step 12 and step 13, and functions of retrieving and setting the slice levels SL H and SL L at step 13, retrieving P R , I R , P L and I L , and setting feedback correction coefficient LAMBDA at steps 12, 16, 17, 19 and 20 correspond to a second air-fuel ratio setting means according to the present invention. If the arrangement in this embodiment is adopted, in the region where the NO x concentration in the exhaust gas is high, the ubrupt output reversion characteristic of the oxygen sensor 19 between the high and low levels is shifted to the richer side by the NO x -reducing catalyst layer 5 than that in the conventional oxygen sensor without NO x -reducing catalyst layer and in addition, the reference value is shifted to a level SL H corresponding to a richer air-fuel ratio than the theoretical air-fuel ratio. Furthermore, since first feedback control constants P R and I R for increasing the fuel injection quantity for correction are set at values larger than the second feedback control constants P L and I L for decreasing the fuel quantity for correction respectively, the ratio of the air-fuel ratio-rich period in the air-fuel ratio feedback control is increased (see Fig. 9). Accordingly, the driving state region of maps in steps 12 and 13 where the conversion of NO x is sufficiently high in the ternary catalyst 20 is used, as shown in Fig. 7, and therefore, a good NO x -reducing function can be maintained stably even if there is a dispersion in parts or the like. Since the second slice level SL H is controlled to a level corresponding to an air-fuel ratio richer by up to 5% than the theoretical air-fuel ratio, the trouble of increase of the amounts of discharged CO and HC by too rich air-fuel ratio can be prevented. On the other hand, in the region where the CO and HC concentrations are high, as shown in Fig. 8, the ubrupt output reversion characteristic of the oxygen sensor 19 between the high and low levels is shifted to the leaner side because the second slice level SL L is shifted to a level corresponding to an air-fuel ratio leaner than the theoretical air-fuel ratio as shown in Fig. 6. Moreover. the second feedback control constant P L and I L are set at levels larger than the first feedback control constant P R and I R . Accordingly, the ratio of the air-fuel ratio-lean time is increased (see Fig. 10). As the result, the region where the conversions of CO and HC are sufficiently high in the ternary catalyst 20 is used, as shown in Fig. 7, and a good CO- and HC-reducing function can be maintained stably even if there is a dispersion in parts or the like. Also in this case, if the slice level SL L is set at a level corresponding to an air-fuel ratio unnecessarily shifted to the lean side, since the air-fuel ratio is made too lean, decrease of the NO x -reducing reaction in the NO x -reducing catalyst layer by decrease of the amounts of formed CO and HC which can react to reduce NO x becomes conspicuous and the rich-shifting effect of the oxygen sensor with the NO x reducing capacity is lost. According to the present invention, however, this trouble can be obviated by setting the second reference value SL L at a level corresponding to an air-fuel ratio leaner by up to 5% than the theoretical air-fuel ratio, and the amount of NO x can be controlled below the allowable level. More specifically, by setting the second slice levels SL H and SL L at a level corresponding to an air-fuel ratio richer or leaner by up to 5% than the theoretical air-fuel ratio, the NO x -reducing reaction by the NO x -reducing catalyst layer is promoted, and therefore, even if an EGR apparatus or the like is not disposed, the function of reducing the amounts of CO and HC can be enhanced while maintaining a good NO x -reducing function. Accordingly, the amounts of CO, HC and NO x can be reduced with a good balance over the entire driving region and the overall exhaust gas emission performance can be highly improved. Incidentally, as easily understood from the foregoing description, only each one of setting feedback control constants P R , P L , I R and I L at different values respectively and setting the slice levels SL H and SL L is sufficient for effectively setting the second target air-fuel ratio instead of both set. As means for improving the fuel consumption characteristic, there is known a method in which the ignition timing is controlled to the advance side in the normal driving region. In this method, however, the amount of NO x increases with elevation of the combustion temperature. If the control is carried out according to the present invention, the amount of NO x can be reduced and the present invention makes contributions to the improvement of the fuel consumption characteristic. In an engine in which surging (longitudinal vibration of a car body) is often caused and the combustion stability is bad, surging can be controlled by advancing the ignition timing. Also in this case, the amount of NO x is increased, but if the present invention is adopted, the amount of NO x can be reduced by the above-mentioned control. Accordingly, the present invention makes contributions to the control of surging.

Claims: Electronic air-fuel ratio control apparatus in an internal combustion engine with a ternary catalyst disposed in an exhaust system which is effective in oxidation reaction of carbon oxide and hydro carbon and in reduction reaction of nitrogen oxides when an air-fuel mixture sucked into the engine is in a theoretical air-fuel ratio, which comprises: an engine driving state-detecting means (S1) for detecting a driving state of the engine; a nitrogen oxides concentration detecting means (S12) for detecting a nitrogen oxides concentration in the exhaust gas; an incompletely burnt component concentration detecting means (S12) for detecting an incompletely burnt component concentration including carbon oxide CO or hydro carbons CH in the exhaust gas; an oxygen sensor (1-6) disposed in the exhaust system of the engine to detect the air-fuel ratio of the air-fuel mixture through the oxygen concentration in the exhaust gas, said oxygen sensor comprising an oxidizing catalyst layer (4) and a nitrogen oxides-reducing catalyst layer (5) for promoting the reaction of reducing nitrogen oxides and emitting a voltage signal (V₀₂) with the point of the theoretical air-fuel ratio corresponding to the oxygen concentration in the exhaust gas including the oxygen in the nitrogen oxides; an air-fuel ratio feedback control means (S11, S20) for controlling the air-fuel ratio of the air-fuel mixture by increasing or decreasing a fuel injection quantity (Ti) to be supplied to the engine based on the engine driving state detected by said engine driving state-detecting means and the air-fuel ratio detected by said oxygen sensor so as to eliminate the deviation of the air-fuel ratio detected by said oxygen sensor from a target air-fuel ratio; a fuel-injecting means (15, 16) for injecting and supplying fuel to the engine in an on-off manner according to a driving pulse signal emitted from said air-fuel feedback control means; wherein the target air-fuel ratio in said air-fuel ratio feedback control means (S11-S20) has first and second air-fuel ratios and comprises: a first target air-fuel ratio setting means (S13) for setting the first target air-fuel ratio based on the engine driving state detected by said engine driving state detecting means and the air-fuel ratio detected by said oxygen sensor; a second target air-fuel ratio setting means (S17-S20) for changing the first air-fuel ratio to set the second target air-fuel ratio which is richer than the first air-fuel ratio when the high nitrogen oxides concentration is detected by said high nitrogen oxides concentration detecting means or which is leaner than the first air-fuel ratio when the high incompletely burnt component concentration is detected by said incompletely burnt component concentration detecting means; wherein said second target air-fuel ratio setting means sets the second air-fuel ratio to a value thereof which is leaner than the theoretical air-fuel ratio by up to 5% when the high incompletely burnt component concentration is detected; and a fuel injection quantity computing means (S1, S5) for computing and setting a fuel injection quantity to be injected from said fuel-injecting means (15, 16) to the engine to attain the first target air-fuel ratio or the second target air-fuel ratio of the air-fuel mixture based on the engine driving state, the air-fuel ratio of the air-fuel mixture, the nitrogen oxide concentration and the incompletely burnt component concentration. Electronic air-fuel ratio control apparatus as set forth in Claim 1 wherein said second target air-fuel ratio setting means (S17-S20) sets the second air-fuel ratio to a value thereof which is richer than the theoretical air-fuel ratio by up to 5% when the high nitrogen oxides concentration is detected. Electronic air-fuel ratio control apparatus as set forth in Claim 1 or 2 wherein said air-fuel ratio feedback control means further comprises an air-fuel ratio judging means (S14) for comparing the voltage signal (V₀₂) from said oxygen sensor (1-6) with a slice level (SL) as a referene value to judge the air-fuel ratio of the air-fuel mixture richer or leaner than the slice level (SL) and an air-fuel ratio feedback control correction coefficient setting means for setting an air-fuel ratio feedback control correction coefficient (LAMBDA) so as to eliminate the deviation of the air-fuel ratio detected by said oxygen sensor from the target air-fuel ratio in a manner of an integration control. Electronic air-fuel ratio control apparatus as set forth in one of the Claims 1 to 3 wherein said fuel injection quantity computing means computes the fuel injection quantity (Ti) according to the following formula: Tp = K·Q/N Ti = Tp·COEF·LAMBDA + Ts where Ti stands for the fuel injection quantity, K stands for a constant, Q stands for a quantity of air sucked into the engine and detected by said engine driving state detecting means, N stands for an engine revolution number detected by said engine driving state detecting means, Tp stands for a basic fuel injection quantity, COEF stands for a various correction coefficients of engine driving states and Ts stands for a correction quantity pertaining to a fluction of a battery voltage for the engine. Electronic air-fuel ratio control apparatus as set forth in Claim 3 wherein the slice level (SL) has first and second slice levels and said first target air-fuel ratio setting means is means for setting first slice level (SL₀) and said second target air-fuel ratio setting means is means for setting the second slice level (SL H ) higher than the first slice level (SL₀) so that the second target air-fuel ratio is set in a side richer than the theoretical air-fuel ratio. Electronic air-fuel ratio control apparatus as set forth in Claim 5 wherein the second slice level (SL H ) is changeably set in accordance with the nitrogen oxides concentration. Electronic air-fuel ratio control apparatus as set forth in Claim 3 wherein the slice level (SL) has first and second slice levels and said first target air-fuel ratio setting means is means for setting slice level (SL₀) and said second target air-fuel ratio setting means is means for setting the second slice level (SL L ) lower than the first slice level (SL₀) so that the second target air-fuel ratio is set in a side leaner than the theoretical air-fuel ratio. Electronic air-fuel ratio control apparatus as set forth in Claim 7 wherein the second slice level (SL L ) is changeably set in accordance with the concentration of the incompletely burnt component. Electronic air-fuel ratio control apparatus as set forth in Claim 3 wherein the air-fuel ratio feedback control correction coefficient (LAMBDA) has first and second coefficients, said first target air-fuel ratio setting means (S13) is means for setting the first air-fuel ratio feedback control correction coefficient (LAMBDA) which is increased or decreased in a manner of integration feedback control in every air-fuel ratio feedback control routine and said second air-fuel ratio setting means (S17-S20) is means for setting the second air-fuel ratio feedback control correction coefficient (LAMBDA) in every air-fuel ratio feedback control routine, which is increased or decreased by first and second feedback control constants, the first feedback control constant being set to a larger value when the high nitrogen oxides concentration is detected and when the air-fuel ratio feedback control is performed in the direction of increasing the fuel injection quantity rather than the second feedback control constant set when the air-fuel ratio feedback control is performed in the direction of decreasing the fuel injection quantity. Electronic air-fuel ratio control apparatus as set forth in one of the Claims 3 to 9 wherein the air-fuel ratio feedback control correction coefficient (LAMBDA) has first and second coefficients, said first target air-fuel ratio setting means (S13) is means for setting the first air-fuel ratio feedback control correction coefficient (LAMBDA) which is increased or decreased in a manner of integration feedback control in every air-fuel ratio feedback control routine and said second air-fuel ratio setting means (S17-S20) is means for setting the second air-fuel ratio feedback control correction coefficient (LAMBDA) in every air-fuel ratio feedback control routine, which is increased or decreased by first and second feedback control constants, the first feedback control constant being set to a larger value when the incompletely burnt component concentration is detected and when the air-fuel ratio feedback control is performed in the direction of decreasing the fuel injection quantity rather than the second feedback control constant set when the air-fuel ratio feedback control is performed in the direction of increasing the fuel injection quantity. Electronic air-fuel ratio control apparatus as set forth in one of the Claims 1 to 10 wherein said nitrogen oxides concentration detecting means (S12) is means for detecting predetermined engine driving regions where high nitrogen oxides concentration is emitted in the exhaust gas from the engine. Electronic air-fuel ratio control apparatus as set forth in one of the Claims 1 to 11 wherein said incompletely burnt component concentration detecting means (S12) is means for detecting predetermined engine driving regions where high incompletely burnt component concentration is emitted in the exhaust gas from the engine. Electronic air-fuel ratio control apparatus as set forth in one of the Claims 1 to 12 wherein said oxygen sensor comprises a substrate (1) composed of a solid electrolyte having an oxygen ion-conducting property, an oxidation catalyst layer (4) for promoting the oxidation reaction of the incompletely burnt component such as carbon oxide and hydrocarbons in the exhaust gas, which is formed on the exhaust gas-contacting outer surface of the substrate and a NO x -reducing catalyst layer (5) for promoting the reduction reaction of NO x in the exhaust gas, which is laminated on the oxidation catalyst layer, and the oxygen sensor has such a structure that the electromotive force generated between the exhaust gas-contacting outer surface (3) of the substrate (1) and the air-contacting inner surface (2) of the substrate (1) is taken out as the output value. Method for controlling the air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine with a ternary catalyst disposed in an exhaust system which is effective in oxidation reaction of carbon oxide and hydro carbon and in reduction reaction of nitrogen oxides when an air-fuel mixture sucked into the engine is in a theoretical air-fuel ratio, comprising the steps of: detecting (S1) a driving state of the engine; detecting (S12) a nitrogen oxides concentration in the exhaust gas; detecting (S12) an incompletely burnt component concentration including carbon oxide CO or hydro carbons CH in the exhaust gas; detecting the air-fuel ratio of the air-fuel mixture through the oxygen concentration in the exhaust gas by means of an oxygen sensor comprising an oxidizing catalyst layer and a nitrogen oxides-reducing catalyst layer for promoting the reaction of reducing nitrogen oxides and emitting a voltage signal with the point of the theoretical air-fuel ratio corresponding to the oxygen concentration in the exhaust gas including the oxygen in the nitrogen oxides; feedback-controlling (S11-S20) the air-fuel ratio of the air-fuel mixture by increasing or decreasing a fuel injection quantity to be supplied to the engine based on the engine driving state and the air-fuel ratio so as to eliminate the deviation of the air-fuel ratio from a target air-fuel ratio; injecting fuel to the engine in an on-off manner according to a driving pulse signal; wherein a target air-fuel ratio has first and second target air-fuel ratios and wherein the feedback-controlling step comprises: setting (S13) the first target air-fuel ratio based on the engine driving state and the air-fuel ratio; changing (S17-S20) the first air-fuel ratio to set the second target air-fuel ratio which is richer than the first air-fuel ratio when the high nitrogen oxides concentration is detected or which is leaner than the first air-fuel ratio when the high incompletely burnt component concentration is detected; wherein the second air-fuel ratio is set to a value thereof which is leaner than the theoretical air-fuel ratio by up to 5% when the high incompletely burnt component concentration is detected; and setting a fuel injection quantity to attain the first target air-fuel ratio or the second target air-fuel ratio of the air-fuel mixture based on the engine driving state, the air-fuel ratio of the air-fuel mixture, the nitrogen oxide concentration and the incompletely burnt component concentration.

Inventors: NAKANIWA, SHINPEI JAPAN ELEC.CONTROL SYST.CO. LTD., UCHIKAWA, AKIRA JAPAN ELEC. CONTROL SYST. CO. LTD.
Applicants: JAPAN ELECTRONIC CONTROL SYSTEMS CO., LTD.
Application year: 1988
Main classification: F02D41
IPC codes: F02D41