Patent Publication Number: US-7721711-B2

Title: Engine control system including means for learning characteristics of individual fuel injectors

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is based upon and claims benefit of priority of Japanese Patent Application No. 2006-316505 filed on Nov. 24, 2006, the content of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an engine control system, in which injection characteristics of individual fuel injectors are learned and an amount of fuel injected from individual fuel injector is controlled based on the injection characteristics. 
   2. Description of Related Art 
   Recently, some proposals have been made for improving detection accuracy of an air-fuel ratio in an internal combustion engine. For example, JP-A-2005-207405 proposes the following system: an air-fuel ratio of each cylinder is estimated based on output signals of an air-fuel ratio sensor disposed in an exhaust pipe at a position where exhaust gas streams from plural cylinders merge; an air-fuel ratio deviation among cylinders is calculated; an mount for adjusting the air-fuel ratio of each cylinder is calculated to minimize the air-fuel ratio deviation among cylinders; and the air-fuel ratio of each cylinder is controlled using the calculated amount for adjusting the air-fuel ratio. 
   On the other hand, JP-A-2-78750 proposes the following system: a target amount of fuel injection for each cylinder and an average amount of fuel injection among all cylinders are calculated based on operating conditions of an engine when the engine is idling; the injection amount for each cylinder is adjusted using a difference between the target amount and the average amount; and thus the average amount of fuel injection for all cylinders converges to the target amount. 
   As shown in  FIG. 3  attached hereto, an injection characteristic (an amount of injected fuel versus time period in which fuel is injected) of an individual injector is not the same as the standard injection characteristic. This means that a certain error (deviation) in the injection amount relative to the standard amount cannot be avoided. The error (deviation) may depend on original individuality of each injector, or it may be caused in a course of actual usage. 
   Since the deviation from the standard characteristic is unavoidable for each injector, the air-fuel ratio deviation among cylinders cannot be precisely detected in the system disclosed in JP-A-2005-207405. This is because an influence of the injection amount error of each injector is included in the deviation of air-fuel ratio among cylinders. Accordingly, the air-fuel ratio deviation among cylinders due to external disturbances, such as introduction of evaporated gas or a blow-by gas into an intake system, is not accurately detected. 
   In the system disclosed in JP-A-2-78750, the average amount of fuel injection for all cylinders is converged to a target amount if there is a deviation in the fuel amount injected from each injector in the idling state. However, the error in the injection amount of each injector due to the injection characteristic deviation cannot be adjusted. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide an improved system for controlling operation of an internal combustion engine, in which an error in an amount of fuel injected from each injector due to an injection characteristic deviation among individual injectors is effectively adjusted. 
   The engine control system of the present invention includes a fuel injector for each cylinder of the engine, an air-fuel ratio sensor disposed in an exhaust manifold at a position where exhaust pipes of all cylinders merge, and an electronic control unit that controls operation of the engine based on signals inputted from various sensors. 
   An air-fuel ratio deviation among cylinders is detected based on output signals of the air-fuel ratio sensor in reference to a model for estimating an air-fuel ratio of each cylinder. An injection amount error (a deviation from a standard amount) of each fuel injector is detected based on the air-fuel ratio deviation among cylinders. The injection amount errors are detected when the engine is stably operating under a heavy load and under a light load (such as idling). An injection characteristic (i.e., a relation between an injection amount and a injection time period) of each fuel injector is learned from the injection amount errors of each fuel injector. 
   The injection amount errors are accurately adjusted based on the leaned injection characteristic of each fuel injector, and a deviation of air-fuel ratio among cylinders caused by external disturbances, such as introduction of evaporated gas or blow-by gas into an intake system, is effectively adjusted. 
   Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiment described below with reference to the following drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a drawing showing an entire structure of an engine control system according to the present invention; 
       FIG. 2  is a time-chart for explaining an air-fuel ratio adjustment factor that changes according to operating conditions of an engine; 
       FIG. 3  is a graph showing an injection characteristic (relation between injection time and an amount of injected fuel) of a standard injector and an actual injector; 
       FIG. 4  is a flowchart showing a process of controlling an air-fuel ratio of each cylinder; 
       FIG. 5  is a flowchart showing a process of controlling an amount of injected fuel; and 
       FIG. 6  is a flowchart showing a process of learning an injection characteristic. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A preferred embodiment of the present invention will be described with reference to accompanying drawings. First, an entire structure of an engine control system according to the present invention will be described with reference to  FIG. 1 . An internal combustion engine  11  having four cylinders in line is shown in  FIG. 1  as an example. An air cleaner  13  is disposed at an upstream end of an intake pipe  12  of the engine  11 . An airflow meter  14  for detecting an amount of intake air is disposed downstream of the air cleaner  13  in the intake pipe  12 . A throttle valve  15  driven by an actuator such as a motor and a throttle sensor  16  for detecting an opening degree of the throttle valve  15  are disposed downstream of the airflow meter  14 . 
   A surge tank  17  is connected to a downstream end of the intake pipe  12 , and an intake air pressure sensor  10  for detecting a pressure in the intake pipe  12  is mounted to the surge tank  17 . An intake manifold  19  for supplying air to each cylinder of the engine  11  is connected to the surge tank  17 . A fuel injector  20  is disposed in each branch of the intake manifold  19  at a position close to an intake port. When the engine is in operation, fuel contained in a fuel tank  21  is fed into a delivery pipe  23 , and fuel in the delivery pipe  23  is injected from the injector  20  into each cylinder of the engine  11  in a controlled manner. A fuel pressure sensor  24  is installed to the delivery pipe  23 . 
   A mechanism  27  for varying opening and closing timing of an intake valve  25  and a mechanism  28  for varying opening and closing timing of an exhaust valve  26  are installed to the engine  11 . An intake camshaft  29  driving the intake valves  25  and an exhaust camshaft  30  driving the exhaust valves  26  are also installed to the engine  11 . A sensor  31  for detecting a rotational angle of the intake camshaft  29  and a sensor  32  for detecting a rotational angle of the exhaust camshaft  30  are installed to the engine  11 . A crank angle sensor  33  is also installed to the engine  11 . The crank angle sensor  33  outputs a pulse signal every predetermined rotational angle (for example, 30 degrees) of a crankshaft of the engine  11 . 
   An air-fuel ratio sensor  37  is disposed in an exhaust manifold  35  at a position  36  where exhaust pipes connected to respective cylinders merge. A three-way catalyst for purifying exhaust gas components such as CO, HC and NOx is disposed downstream of the air-fuel ratio sensor  37 . 
   Output signals of the air-fuel ratio sensor  37  and other sensors mentioned above are inputted to an engine control unit  40  (referred to as ECU). The ECU  40  including a microcomputer performs engine control programs stored in a ROM in the ECU  40 , and an amount of fuel supplied to each cylinder and ignition timing is controlled according to operating conditions of the engine. 
   The ECU  40  also performs a process of controlling an air-fuel ratio of each cylinder shown in  FIG. 4  (which will be explained later in detail). In this process, the air-fuel ratio of each cylinder is estimated based on output signals of the air-fuel ratio sensor  37  and a model for estimating an air-fuel ratio of each cylinder. In this model, a relation between the air-fuel ratio of each cylinder and the output signal of the air-flow sensor  37  is defined. A deviation of the estimated air-fuel ratio of each cylinder from a standard air-fuel ratio is calculated, and an air-fuel ratio deviation among cylinders is calculated. An air-fuel ratio adjustment factor for each cylinder is calculated so that the air-fuel ration deviation among cylinders is minimized. An amount of fuel supplied to each cylinder is adjusted using the air-fuel ratio adjustment factor, and thus the air-fuel ratio deviation among cylinders is controlled to minimize the same. 
   The air-fuel ratio deviation among cylinders is detected when the operating conditions of the engine is steady and transient. It may be also detected when evaporated gas or blow-by gas is being introduced into the engine or when other adjustment operation is being performed if the influence of such operations on the air-fuel ration deviation is detectable. 
     FIG. 3  shows an injection characteristic of a standard fuel injector (referred to as a standard injection characteristic) and an injection characteristic of an actual injector. The standard injection characteristic is shown with a solid line and that of an actual injector with a dotted line. As seen in the graph, an amount of injected fuel of the actual injector differs from that of the standard injector even when a period of time in which fuel is injected (referred to as injection time period) is equal for both injectors. Such a difference, or a deviation, is caused by individuality of the injectors (i.e., an original difference among individual injectors), or actual use of the injector. 
   Since an individual injector includes a deviation from the standard injection characteristic, it is difficult to accurately calculate an air-fuel ratio deviation among cylinders based on output signals of the air-fuel ratio sensor  37 . This is because the deviation in injection amount of each injector is included in the air-fuel ratio deviation among cylinders. For example, influence of an external disturbance, such as introduction of evaporated gas or blow-by gas into the cylinder, on the air-fuel ratio deviation among cylinders cannot be detected. Accordingly, the air-fuel ratio adjustment factor for each cylinder cannot be accurately calculated, and therefore the air-fuel ratio deviation among cylinders caused by the external disturbance cannot be accurately adjusted. 
   To cope with the problem caused by the injection characteristic difference among injectors, a process of learning the injection characteristic shown in  FIG. 6  is employed in the present invention. More particularly, the air-fuel ratio deviation among cylinders is detected based on the output signals of the air-fuel ratio sensor  37  when the engine is stably operated under a heavy load and a low load. The air-fuel ratio adjustment factor for each cylinder is calculated to minimize the air-fuel ratio deviation among cylinders, and an injection amount error (a deviation from a standard amount) of each injector is adjusted in the following manner. 
   If an injection amount error (or a deviation from the standard amount) is at a plus side (+X %) as shown at “A” in  FIG. 3 , the air-fuel ratio adjustment factor has to be at an minus side (−X % from a standard level set to 1.0) as shown at “A” in  FIG. 2 . If an injection amount error is at a minus side (−Y %) as shown at “B” in  FIG. 3 , the air-fuel ratio adjustment factor has to be at a plus side (+Y % from a standard level set to 1.0) as shown at “B” in  FIG. 2 . In other words, when the air-fuel ratio adjustment factor decreases by X %, the injection amount error is calculated as +X %. When the air-fuel ratio adjustment factor increases by Y %, the injection amount error is calculated as Y %. Since the injection characteristic is substantially linear as shown in  FIG. 3 , a whole characteristic can be estimated if the injection amount errors are determined at two points, as shown with “A” and “B” in  FIG. 3 . 
   After estimating the injection characteristic of each injector  20  in the manner described above, it is memorized in a non-volatile rewritable memory such as a backup RAM in the ECU  40 . Thus, the injection characteristic of each injector  20  is learned. An injection time period of each injector corresponding to a required injection amount is set in reference to the injection characteristic learned and stored in the memory. In this manner, the injection amount error of each injector due to the individuality of the injection characteristic can be adjusted in an almost entire region of the engine operating conditions. 
   The air-fuel ratio deviation among cylinders is detected only when the engine is stably operated. It is also possible to detect the air-fuel ratio deviation among cylinders under special conditions, i.e., when evaporated gas or blow-by gas is being introduced into the intake system or other adjusting control is being performed, if an amount of changes in the air-fuel ratio due to such special conditions is detectable. 
   With reference to  FIG. 4 , a process of controlling the air-fuel ratio of each cylinder will be described. This process is performed periodically when power is supplied to the ECU  40 . At step S 101 , the output signals of the air-fuel ratio sensor  37  are read. At step S 102 , the air-fuel ratio of each cylinder is estimated based on the output signals of the air-fuel ratio sensor  37  and in reference to the model for estimating the air-fuel ratio of each cylinder. Then, at step S 103 , a difference between the estimated air-fuel ratio of each cylinder and an average air-fuel ration of all cylinders or a target air-fuel ratio is calculated, and thereby the air-fuel ratio deviation among cylinders is calculated. Then, at step S 104 , the air-fuel ratio adjusting factor for each cylinder is calculated so that the air-fuel ratio among cylinders is minimized. At step S 105 , the injection amount of each cylinder is adjusted using the calculated air-fuel ratio adjustment factor. Thus, the air-fuel ratio deviation among cylinders is decreased. 
   With reference to  FIG. 5 , a process of controlling the injection amount (an amount of fuel injected from an injector) will be described. This process is performed periodically when the ECU is in operation. At step S 201 , whether the injection characteristic of each injector is memorized or not is determined. If the injection characteristic of each injector is not memorized, the process proceeds to step S 202 , where the injection characteristic of each injector is learned in a process shown in  FIG. 6  (which will be explained later in detail). If the injection characteristic is memorized, the process proceeds to step S 203 , where the injection time period corresponding to a required amount of fuel for each cylinder is set in reference to the injection characteristic. Each injector is controlled using the injection time period thus set. Then, the process proceeds to step S 204 , where the air-fuel ratio of each cylinder is estimated based on the output signals of the air-fuel ratio sensor  37  in reference to the model for estimating an air-fuel ratio of each cylinder, and the air-fuel ratio among cylinders is calculated. 
   The process of learning the injection characteristic will be described with reference to  FIG. 6 . This process is performed as a step S 202  shown in  FIG. 5  as explained above. At step S 301 , whether the engine is stably operated or not is determined based on rotational speed of the engine and an engine load. If it is determined at step S 301  that the engine is not stably operated, the process directly comes to the end without performing other steps in this process. If the engine is stably operated, the process proceeds to step S 302 , where whether the engine load is heavy or not is determined. For example, it is determined that the engine load is heavy if the engine load k is equal to or higher than a predetermined load Hk. The engine load may be represented by an amount of intake air or a pressure in the intake pipe. 
   If the engine load is heavy, the process proceeds to step S 303 , where the air-fuel ratio deviation among cylinders is calculated in the manner described above. Then, at step S 304 , the air-fuel ratio adjustment factor for each cylinder under the heavy load condition is calculated so that the air-fuel ratio deviation among cylinders is minimized. The injection amount error of each injector is calculated based on the air-fuel ratio adjustment factor of each injector. On the other hand, if it is determined that the engine load is low (e.g., under an idling condition), the process proceeds to step S 305 , where the air-fuel ratio deviation among cylinders is calculated. Then, at step S 306 , the air-fuel ratio adjustment factor for each cylinder under the low load condition is calculated so that the air-fuel ratio deviation among cylinders is minimized. The injection amount error of each injector is calculated based on the air-fuel ratio adjusting factor of each injector. 
   Then, the process proceeds to step S 307 , where whether the injection amount errors under both of the heavy load condition and the low load condition are detected or not is determined. If it is determined that the injection amount errors under both conditions are detected, the process proceeds step S 308 , the injection characteristic of each injector is determined from the injection amount errors detected and the injection time period corresponding to such injection amount errors (refer to  FIG. 3 ). The injection characteristic of each injector is memorized in a memory such as a backup RAM in the ECU  40 . Thus, the process of learning the injection characteristic is completed. 
   As described above, the injection amount error of each cylinder (or each injector) is calculated based on the air-fuel ratio deviation among cylinders. The injection amount errors are detected under both of the heavy and low load conditions. The injection characteristic of each injector is learned based on the detected injection amount errors. The fuel injectors are controlled based on the learned injection characteristics. Therefore, the injection errors are corrected in an almost entire region of operating conditions of the engine. The influence of the external disturbances, such as introduction of evaporated gas or blow-by gas into the intake system, on the air-fuel ratio deviation among cylinders is accurately detected. Accordingly, the air-fuel ratio adjustment factor is accurately calculated, and changes in the air-fuel ratio among cylinders due to the external disturbance are precisely adjusted. 
   When the engine is stably operated, the air-fuel ratio of each cylinder is stable and the air-fuel ratio deviation among cylinders precisely reflects the injection amount errors of each injector. Based on this fact, the injection characteristic of each injector is learned under the stable operating conditions of the engine. In addition, in learning the injection characteristic, injection amount errors detected at two points (heavy load and low load conditions of the engine), which are apart certain distance from each other, are used. Therefore, the injection characteristic can be learned with a high accuracy. 
   The present invention is not limited to the embodiment described above, but it may be variously modified. For example, the injection characteristic may be learned by using the injection amount errors under three or more operating conditions of the engine. Though the air-fuel ratio of each cylinder is estimated based on the output signals of the air-fuel ratio sensor  37  in reference to the model for estimating the air-fuel ratio in the embodiment described above, the air-fuel ratio of each cylinder may be estimated or detected by other methods. For example, it may be estimated based on the outputs of the air-fuel ratio sensor  37  when a dither control of the air-fuel ratio is performed, i.e., when the air-fuel ratio is forcibly changed. Though a four-cylinder engine is controlled in the embodiment described above, other engines such as two-cylinder engine, three-cylinder engine, or engines having five or more cylinders may be controlled according to the present invention. 
   While the present invention has been shown and described with reference to the foregoing preferred embodiment, it will be apparent to those skilled in the art that changes in form and detail may be made therein without departing from the scope of the invention as defined in the appended claims.