Abstract:
An air/fuel ratio control system is provided for an internal combustion engine having a plurality of cylinders and an air/fuel ratio sensor. The air/fuel ratio sensor is disposed at an exhaust gas merging portion of an exhaust passage where exhaust gas discharged from the plurality of cylinders merges together. The system includes an air/fuel ratio control device, an abnormality diagnosis device, and an enabling device. The air/fuel ratio control device individually controls an air/fuel ratio of each of the plurality of cylinders based on an output of the air/fuel ratio sensor. The abnormality diagnosis device determines whether abnormality of the air/fuel ratio sensor exists. The enabling device enables the air/fuel ratio control device to execute the controlling of the air/fuel ratio of each of the plurality of cylinders when the abnormality diagnosis device determines that the abnormality of the air/fuel ratio sensor does not exist after starting of the engine.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is based on and incorporates herein by reference Japanese Patent Application No. 2007-90429 filed on Mar. 30, 2007. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an air/fuel ratio control system for an internal combustion engine having a function of controlling an air/fuel ratio in each cylinder based on output of an air/fuel ratio sensor disposed in an exhaust air merging portion of the engine. 
   2. Description of Related Art 
   In most recent electronically controlled internal combustion engines, an air/fuel ratio sensor is disposed in an exhaust gas passage for detecting an air/fuel ratio of exhaust gas, and air/fuel ratio F/B control, whereby an air/fuel ratio (e.g., fuel injection quantity) in each cylinder is equally F/B (feedback) controlled such that an air/fuel ratio detected in the air/fuel ratio sensor accords with a target air/fuel ratio, is performed. 
   Furthermore, as described in JP2005-337194A, for example, the air/fuel ratio in each cylinder is estimated using a model, in which a detection value (air/fuel ratio at the exhaust air merging portion) of an air/fuel ratio sensor disposed in an exhaust air merging portion where exhaust gases from cylinders merge together is related with the air/fuel ratio in each cylinder. Based on the estimation result, each-cylinder air/fuel ratio control, whereby the air/fuel ratio (e.g., fuel injection quantity) in each cylinder is controlled so that a variation of the air/fuel ratios among the cylinders is small, is performed in order to improve air/fuel ratio control accuracy. 
   Also, as described, for example, in JP 2004-3513A corresponding to U.S. Pat. No. 5,672,817, in order to make an abnormal diagnosis of the air/fuel ratio sensor, an output change rate of the air/fuel ratio sensor in a predetermined period after the fuel injection cut-off in the engine is started is calculated as a responsivity detection value. Then, the output change rate of the air/fuel ratio sensor is compared with an abnormity determination value, to determine whether the air/fuel ratio sensor is abnormal (deterioration in responsivity). 
   Generally, although an abnormal electrical connection (e.g., a broken wire and a short circuit) in the air/fuel ratio sensor can be determined immediately after the engine is started (e.g., after an ignition switch is turned on), it cannot be determined, for example, whether the responsivity of the air/fuel ratio sensor is abnormal until the engine is in a predetermined operating condition (e.g., fuel injection cut-off state). In a system in which the air/fuel ratio F/B control, whereby the air/fuel ratio in each cylinder is equally controlled based on output of the air/fuel ratio sensor, is executed, the air/fuel ratio F/B control is started early on after the engine is started to reduce exhaust gas emission. Therefore, the air/fuel ratio F/B control is started at the time that a predetermined execution condition for the air/fuel ratio F/B control (e.g., the air/fuel ratio sensor is in an active state) is satisfied even before it is determined whether the responsivity of the air/fuel ratio sensor is abnormal. Then, if it is determined that the responsivity of the air/fuel ratio sensor is abnormal, the air/fuel ratio F/B control is forbidden at that point. 
   However, in the each-cylinder air/fuel ratio control, whereby the air/fuel ratio in each cylinder is controlled based on the output of the air/fuel ratio sensor, the air/fuel ratio in each cylinder is accurately estimated from the output of the air/fuel ratio sensor through the inverse operation, for example. Accordingly, the air/fuel ratio at the exhaust air merging portion varying with combustion in each cylinder needs to be detected in fast response in the air/fuel ratio sensor. As a result, higher-level responsivity of the air/fuel ratio sensor than general air/fuel ratio F/B control is required. When the each-cylinder air/fuel ratio control is started before it is determined whether the responsivity of the air/fuel ratio sensor is abnormal similar to the general air/fuel ratio F/B control, the each-cylinder air/fuel ratio control may be performed with the responsivity of the air/fuel ratio sensor deteriorated below the required level. In consequence, control accuracy in the each-cylinder air/fuel ratio control is deteriorated, and thereby the variation of the air/fuel ratios among the cylinders is large. Thus, a problem that exhaust gas emission is deteriorated is created. 
   SUMMARY OF THE INVENTION 
   The present invention addresses the above disadvantages. Thus, it is an objective of the present invention to provide an air/fuel ratio control system for an internal combustion engine, which prevents execution of each-cylinder air/fuel ratio control when an air/fuel ratio sensor is in an abnormal condition and thereby performs the each-cylinder air/fuel ratio control accurately. 
   To achieve the objective of the present invention, there is provided an air/fuel ratio control system for an internal combustion engine, which has a plurality of cylinders and an air/fuel ratio sensor. The air/fuel ratio sensor is disposed at an exhaust gas merging portion of an exhaust passage where exhaust gas discharged from the plurality of cylinders merges together. The air/fuel ratio control system includes an air/fuel ratio control means, an abnormality diagnosis means, and an enabling means. The air/fuel ratio control means is for individually controlling an air/fuel ratio of each of the plurality of cylinders based on an output of the air/fuel ratio sensor. The abnormality diagnosis means is for determining whether an abnormality of the air/fuel ratio sensor exists. The enabling means is for enabling the air/fuel ratio control means to execute the controlling of the air/fuel ratio of each of the plurality of cylinders when the abnormality diagnosis means determines that the abnormality of the air/fuel ratio sensor does not exist after starting of the engine. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, together with additional objectives, features and advantages thereof, will be best understood from the following description, the appended claims and the accompanying drawings in which: 
       FIG. 1  is a schematic view illustrating a configuration of an overall engine control system according to an embodiment of the invention; and 
       FIG. 2  is a flowchart illustrating a processing flow in an air/fuel ratio control routine according to the embodiment. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   An embodiment of the invention is described below. Firstly, a schematic configuration of an overall engine control system is described with reference to  FIG. 1 . 
   An air cleaner  13  is disposed in an uppermost stream portion of an intake pipe  12  of an inline four-cylinder engine  11 , which is an internal-combustion engine. An air flow meter  14  is disposed on a downstream side of the air cleaner  13  for detecting an amount of intake air. A throttle valve  15  and a throttle opening degree sensor  16  are disposed on a downstream side of the air flow meter  14 . An opening degree of the throttle valve  15  is regulated by a motor or the like. The throttle opening degree sensor  16  detects opening degree (throttle opening degree) of the throttle valve  15 . 
   A surge tank  17  is disposed on a downstream side of the throttle valve  15 . An intake pipe pressure sensor  18  is disposed on the surge tank  17  for detecting intake pipe pressure. An intake manifold  19  is formed from the surge tank  17  for introducing air into each cylinder of the engine  11 . A fuel injection valve  20  is attached near an intake port of the intake manifold  19  of each cylinder for injecting fuel. Fuel in a fuel tank  21  is delivered to a delivery pipe  23  by a fuel pump  22  when the engine  11  is in operation. Fuel is injected from the fuel injection valve  20  of each cylinder every injection timing for each cylinder. A fuel pressure sensor  24  is attached to the delivery pipe  23  for detecting pressure of fuel (fuel pressure). 
   Variable valve timing mechanisms  27 ,  28  are disposed in the engine  11  for varying opening/closing timings of an intake valve  25  and an exhaust valve  26 , respectively. An intake cam angle sensor  31  and an exhaust cam angle sensor  32 , and a crank angle sensor  33  are disposed in the engine  11 . The intake cam angle sensor  31  and the exhaust cam angle sensor  32  output cam angle signals in synchronization with respective rotations of an intake cam shaft  29  and an exhaust cam shaft  30 . The crank angle sensor  33  outputs a pulse of a crank angle signal at every predetermined crank angle (e.g., 30° CA) in synchronization with rotation of a crankshaft of the engine  11 . 
   An air/fuel ratio sensor  37  is disposed at an exhaust air merging portion  36  where an exhaust manifold  35  for each cylinder of the engine  11  merges together. The air/fuel ratio sensor  37  detects an air/fuel ratio of exhaust gas. A catalyst  38  such as a three-way catalyst is disposed on a downstream side of the air/fuel ratio sensor  37 . The catalyst  38  purifies carbon monoxide (CO), hydrocarbon (HC), and nitrogen oxide (NOx) in exhaust gas. 
   Output of various sensors such as the air/fuel ratio sensor  37  is inputted into an engine control circuit (hereinafter referred to as ECU)  40 . The ECU  40  mainly includes a microcomputer, and controls a fuel injection quantity or ignition timing of the fuel injection valve  20  of each cylinder according to an operating condition of the engine  11  by executing various engine control programs stored in a read-only memory (storage medium) integrated into the ECU  40 . 
   The ECU  40  serves as an abnormality diagnosis means by executing various air/fuel ratio sensor abnormal diagnosis routines (not shown). The ECU  40  determines whether the air/fuel ratio sensor  37  (a sensor element and a heater) has an abnormal electrical connection (e.g., a broken wire and a short circuit), and whether the air/fuel ratio sensor  37  has an abnormal responsivity and active time (time it takes for the air/fuel ratio sensor  37  to go into an active state). 
   The abnormal diagnosis of responsivity of the air/fuel ratio sensor  37  may be made in the following manner. For example, when the air/fuel ratio sensor  37  is in an idle state after it has gone into the active state, lean control whereby an air/fuel ratio of exhaust gas is varied in a lean direction and rich control whereby the air/fuel ratio of exhaust gas is varied in a rich direction are alternately performed. Then, an output variation of the air/fuel ratio sensor  37  in a predetermined period during the lean control and an output variation of the air/fuel ratio sensor  37  in a predetermined period during the rich control are respectively compared with an abnormity determination value, to determine whether the responsivity of the air/fuel ratio sensor  37  is abnormal. 
   Alternatively, when fuel injection is cut off after the air/fuel ratio sensor  37  has gone into the active state, an output change rate of the air/fuel ratio sensor  37  in a predetermined period after the fuel injection cut-off is started is calculated. Then, the output change rate of the air/fuel ratio sensor  37  is compared with an abnormity determination value, to determine whether the responsivity of the air/fuel ratio sensor  37  is abnormal. In addition, a response time after the fuel injection cut-off is started until an output of the air/fuel ratio sensor  37  reaches a predetermined value may be measured. Then, the response time is compared with an abnormity determination value to determine whether the responsivity of the air/fuel ratio sensor  37  is abnormal. 
   Furthermore, the ECU  40  performs the following air/fuel ratio control by executing an air/fuel ratio control routine (to be described in greater detail hereinafter) shown in  FIG. 2 . 
   When it is determined that the air/fuel ratio sensor  37  does not have the abnormal electrical connection, air/fuel ratio F/B (feedback) control is started at the time when a predetermined execution condition for the air/fuel ratio F/B control is satisfied, even before the abnormal diagnosis of the active time of the air/fuel ratio sensor  37  and the abnormal diagnosis of responsivity of the air/fuel ratio sensor  37  are ended (before it is determined whether the active time of the air/fuel ratio sensor  37  is abnormal and whether the responsivity of the air/fuel ratio sensor  37  is abnormal). 
   According to the air/fuel ratio F/B control, an air/fuel ratio F/B correction amount is calculated such that an air/fuel ratio detected in the air/fuel ratio sensor  37  when the engine  11  is in operation accords with a target air/fuel ratio. Then, by equally correcting a fuel injection quantity in each cylinder using the air/fuel ratio F/B correction amount, an air/fuel ratio of an air-fuel mixture supplied to each cylinder is equally corrected. 
   After this, the abnormal diagnosis of the active time of the air/fuel ratio sensor  37  and the abnormal diagnosis of responsivity of the air/fuel ratio sensor  37  are ended, and accordingly when it is determined that the air/fuel ratio sensor  37  is normal (the active time of the air/fuel ratio sensor  37  is not abnormal and the responsivity of the air/fuel ratio sensor  37  is not abnormal), air/fuel ratio F/B control for each cylinder is started at the time when a predetermined execution condition for the air/fuel ratio F/B control for each cylinder is satisfied. 
   According to the air/fuel ratio F/B control for each cylinder, an air/fuel ratio in each cylinder is estimated based on a detection value in the air/fuel ratio sensor  37  when the engine  11  is in operation using a model, in which the detection value in the air/fuel ratio sensor  37  (an air/fuel ratio of exhaust gas flowing at the exhaust air merging portion  36 ) is related with the air/fuel ratio in each cylinder. By calculating a difference between an estimated air/fuel ratio in each cylinder and a reference air/fuel ratio (an average value of estimated air/fuel ratios for all cylinders or a control target value), a variation of the air/fuel ratios among the cylinders is calculated. Then, the air/fuel ratio F/B correction amount is calculated for each cylinder such that the variation of the air/fuel ratios among the cylinders is small. Based on the calculated air/fuel ratio F/B correction amount, the fuel injection quantity in each cylinder is corrected for each cylinder. Accordingly, the variation of the air/fuel ratios among the cylinders is controlled to be small by correcting the air/fuel ratio of the air-fuel mixture supplied to each cylinder for each cylinder. 
   After the air/fuel ratio F/B control for each cylinder is started, both the air/fuel ratio F/B control and the air/fuel ratio F/B control for each cylinder may be executed. Alternatively, the air/fuel ratio F/B control is stopped so that only the air/fuel ratio F/B control for each cylinder may be executed. 
   The air/fuel ratio control in the present embodiment is performed in the ECU  40  according to the air/fuel ratio control routine shown in  FIG. 2 . Processing in the routine is described below. 
   The air/fuel ratio control routine shown in  FIG. 2  is executed at predetermined intervals while the ECU  40  is turned on, and serves as a second air/fuel ratio control means and an air/fuel ratio control means (first air/fuel ratio control means). When the routine is started, a basic fuel injection quantity is calculated at step  101  based on an operating condition of the engine  11  (e.g., a rotational speed of the engine  11  and a load). 
   After this, control proceeds to step  102 , where it is determined whether the air/fuel ratio sensor  37  has the abnormal electrical connection. If it is determined that the air/fuel ratio sensor  37  has the abnormal electrical connection, the air/fuel ratio F/B control is forbidden, and the air/fuel ratio F/B control for each cylinder is forbidden (steps  107 ,  112 ). 
   If it is determined at step  102  that the air/fuel ratio sensor  37  does not have the abnormal electrical connection, control proceeds to step  103 , where it is determined whether the abnormal diagnosis of the active time of the air/fuel ratio sensor  37  and the abnormal diagnosis of responsivity of the air/fuel ratio sensor  37  are ended. If it is determined that these abnormal diagnoses are ended, control proceeds to step  104 , where it is determined whether the air/fuel ratio sensor  37  is normal (the active time of the air/fuel ratio sensor  37  is not abnormal and the responsivity of the air/fuel ratio sensor  37  is not abnormal). 
   If it is determined at step  103  that the abnormal diagnosis of the active time of the air/fuel ratio sensor  37  and the abnormal diagnosis of responsivity of the air/fuel ratio sensor  37  are not ended (it is not yet determined whether the active time of the air/fuel ratio sensor  37  is abnormal and whether the responsivity of the air/fuel ratio sensor  37  is abnormal), or if it is determined at step  104  that the air/fuel ratio sensor  37  is normal, control proceeds to step  105 . At step  105 , it is determined whether the execution condition for the air/fuel ratio F/B control is satisfied. If it is determined at step  105  that the execution condition for the air/fuel ratio F/B control is satisfied, control proceeds to step  106 , where the air/fuel ratio F/B control is executed. 
   If it is determined at step  103  that the abnormal diagnosis of the active time of the air/fuel ratio sensor  37  and the abnormal diagnosis of responsivity of the air/fuel ratio sensor  37  are ended, and if it is determined at step  104  that the air/fuel ratio sensor  37  is abnormal (at least one of the active time and responsivity of the air/fuel ratio sensor  37  is abnormal), the air/fuel ratio F/B control is forbidden, and the air/fuel ratio F/B control for each cylinder is forbidden (steps  107 ,  112 ). 
   At step  108 , it is determined whether the abnormal diagnosis of the active time of the air/fuel ratio sensor  37  and the abnormal diagnosis of responsivity of the air/fuel ratio sensor  37  are ended. If it is determined that the abnormal diagnosis of the active time of the air/fuel ratio sensor  37  and the abnormal diagnosis of responsivity of the air/fuel ratio sensor  37  are ended, control proceeds to step  109 , where it is determined whether the air/fuel ratio sensor  37  is normal. 
   If it is determined at step  108  that the abnormal diagnosis of the active time of the air/fuel ratio sensor  37  and the abnormal diagnosis of responsivity of the air/fuel ratio sensor  37  are not ended, or if it is determined at step  109  that the air/fuel ratio sensor  37  is abnormal, control proceeds to step  112 , where the air/fuel ratio F/B control for each cylinder is forbidden. 
   If it is determined at step  108  that the abnormal diagnosis of the active time of the air/fuel ratio sensor  37  and the abnormal diagnosis of responsivity of the air/fuel ratio sensor  37  are ended, and if it is determined at step  109  that the air/fuel ratio sensor  37  is normal, control proceeds to step  110 , where it is determined whether the execution condition for the air/fuel ratio F/B control for each cylinder is satisfied. If it is determined that the execution condition for the air/fuel ratio F/B control for each cylinder is satisfied, control proceeds to step  111 , where the air/fuel ratio f/B control for each cylinder is executed. In this case, both the air/fuel ratio F/B control and the air/fuel ratio F/B control for each cylinder may be executed. Alternatively, the air/fuel ratio F/B control is stopped so that only the air/fuel ratio F/B control for each cylinder may be executed. 
   After this, control proceeds to step  113 , the basic fuel injection quantity for each cylinder is equally corrected using the air/fuel ratio F/B correction amount in the air/fuel ratio F/B control, and the basic fuel injection quantity for each cylinder is corrected using the air/fuel ratio F/B correction amount calculated for each cylinder in the air/fuel ratio F/B control for each cylinder, to calculate a final fuel injection quantity for each cylinder. 
   In the present embodiment, the air/fuel ratio F/B control for each cylinder is started at the time when the execution condition for the air/fuel ratio F/B control for each cylinder is satisfied after the determinations are made that the air/fuel ratio sensor  37  does not have the abnormal electrical connection, the abnormal diagnosis of the active time of the air/fuel ratio sensor  37  and the abnormal diagnosis of responsivity of the air/fuel ratio sensor  37  are ended, and that the air/fuel ratio sensor  37  is normal. A flow from step  109  (Yes) to step  111  via step  110  corresponds to an enabling means (first enabling means). Accordingly, execution of the air/fuel ratio F/B control for each cylinder when the air/fuel ratio sensor  37  is abnormal is prevented, and the air/fuel ratio F/B control for each cylinder is started after it is confirmed that the air/fuel ratio sensor  37  is normal. As a result, the air/fuel ratio F/B control for each cylinder is accurately executed. 
   Furthermore, when it is determined that the air/fuel ratio sensor  37  does not have the abnormal electrical connection, air/fuel ratio F/B control is started at the time when the execution condition for the air/fuel ratio F/B control is satisfied, even before the abnormal diagnosis of the active time of the air/fuel ratio sensor  37  and the abnormal diagnosis of responsivity of the air/fuel ratio sensor  37  are ended. A flow from step  103  (No) to step  106  via step  105  corresponds to a second enabling means. Accordingly, before the air/fuel ratio F/B control for each cylinder is started, exhaust gas emission is reduced by controlling the air/fuel ratio in each cylinder by the air/fuel ratio F/B control. 
   In addition, if it is determined that at least one of the active time and responsivity of the air/fuel ratio sensor  37  is abnormal, the air/fuel ratio F/B control and the air/fuel ratio F/B control for each cylinder are forbidden. However, even though the heater of the air/fuel ratio sensor  37  breaks down and thereby the active time of the air/fuel ratio sensor  37  becomes abnormal, for example, the air/fuel ratio F/B control and the air/fuel ratio F/B control for each cylinder are accurately executed after the air/fuel ratio sensor  37  is activated by exhaust heat as long as the responsivity of the air/fuel ratio sensor  37  is normal. Therefore, in such a case, the air/fuel ratio F/B control and the air/fuel ratio F/B control for each cylinder may be executed when it is determined that the responsivity of the activated air/fuel ratio sensor  37  is normal, regardless of whether the active time of the air/fuel ratio sensor  37  is abnormal. 
   Additional advantages and modifications will readily occur to those skilled in the art. The invention in its broader terms is therefore not limited to the specific details, representative apparatus, and illustrative examples shown and described.