EP1394400A1

EP1394400A1 - Abnormality diagnosis device and abnormality diagnosis method for exhaust gas recirculation unit

Info

Publication number: EP1394400A1
Application number: EP03018994A
Authority: EP
Inventors: Takahiro Uchida; Yoshiyasu Ito; Atsushi Morikawa
Original assignee: Toyota Motor Corp
Current assignee: Toyota Motor Corp
Priority date: 2002-08-23
Filing date: 2003-08-21
Publication date: 2004-03-03
Anticipated expiration: 2023-08-21
Also published as: JP3846381B2; JP2004084492A; EP1394400B1; ES2346415T3; DE60332982D1

Abstract

An electronic control unit (ECU) calculates a reference value relating to an opening of an exhaust gas recirculation (EGR) valve on the basis of an output value of a lift sensor at the time when a command signal for fully closing the EGR valve is output, and stores and updates the reference value. The ECU corrects a relationship between sensor output value and opening of the EGR valve on the basis of this reference value (step 330 and step 360). The ECU calculates a difference between a target opening of the EGR valve corresponding to an operational state of an engine and a post-correction opening of the EGR valve, and determines on the basis of a result of comparison between an absolute value of the difference and a criterial value whether there is an abnormality occurring in an EGR unit (from step 330 to step 370). In making this determination, the criterial value is switched depending on whether an initial value of the reference value has been stored (YES in step 320) or not (NO in step 320) (step 330 and step 360).

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an abnormality diagnosis device and an abnormality diagnosis method for determining whether there is an abnormality occurring in an exhaust gas recirculation unit provided in an internal combustion engine.

2. Description of the Related Art

As an internal combustion engine such as an on-vehicle engine or the like, there has been known one which is provided with an exhaust gas recirculation (EGR) unit that recirculates part of exhaust gas to an intake passage with a view to improving exhaust emission properties. This EGR unit is provided with an EGR passage through which the intake passage communicates with an exhaust passage of the internal combustion engine, and with an EGR valve provided in the EGR passage. By adjusting an opening of the EGR valve, an amount of exhaust gas (an EGR amount) recirculated from the exhaust passage to the intake passage through the EGR passage is adjusted. If part of exhaust gas is returned to the intake passage by the EGR unit as described herein, the exhaust gas lowers a combustion temperature and inhibits nitrogen oxides (NOx) from being produced in combustion chambers. As a result, exhaust emission properties are improved.
If the EGR unit as described herein undergoes some abnormality such as motional stagnation of the EGR valve, inoperativeness of the EGR valve resulting from sticking thereof, clogging of the EGR passage with a foreign matter or a carbide and the like contained in exhaust gas, the EGR amount may deviate from a value suited for an operational state of the engine at the time of the abnormality. In this case, a deterioration in combustion state and an increase in the amount of NOx are caused. Therefore, various abnormality diagnosis devices for determining whether there is an abnormality occurring in an EGR unit have been proposed.
For instance, Japanese Patent Application Laid-Open No. 4-103865 discloses an abnormality diagnosis device that calculates a target opening of an EGR valve corresponding to an operational state of an engine, that detects an actual opening of the EGR valve by means of a lift sensor, and that determines on the basis of a difference between the target opening and the actual opening whether there is an abnormality occurring in an EGR unit.
In the abnormality diagnosis device mentioned above, if an actual (true) opening of the EGR valve does not coincide with an output value of the lift sensor due to differences among individual products of the lift sensor (i.e., dispersion of characteristics among individual products of the lift sensor) or aging, it is difficult to accurately determine whether there is an abnormality occurring in the EGR unit. This causes a problem in that precision in making this determination is adversely affected. As a countermeasure against the problem, an output value of the lift sensor at the time when a command signal for fully closing the EGR valve is output is usually stored (learned) as a reference value, and a relationship between output value of the lift sensor and opening of the EGR valve at each moment is corrected on the basis of the reference value.

SUMMARY OF THE INVENTION

It is an object of the invention to provide such an abnormality diagnosis device or method for an exhaust gas recirculation unit as can inhibit a determination on the occurrence of an abnormality in the exhaust gas recirculation unit from being made erroneously in the case where the determination is made with a reference value relating to an opening of an exhaust gas recirculation valve not having been stored.
As a first aspect of the invention, an abnormality diagnosis device for an exhaust gas recirculation unit that adjusts an amount of exhaust gas recirculated from an exhaust passage to an intake passage in an internal combustion engine by means of an exhaust gas recirculation valve comprises opening detection means, learning means, correction means, determination means, and criterial value switching means. The opening detection means detects an opening of the exhaust gas recirculation valve. The learning means stores as a reference value an opening detected by the opening detection means when a command signal for maintaining the exhaust gas recirculation valve at a predetermined opening is output, and updates the reference value at appropriate timings. The correction means corrects a relationship between output value of the opening detection means and opening of the exhaust gas recirculation valve on the basis of the reference value updated by the learning means. The determination means calculates a difference between a target opening of the exhaust gas recirculation valve corresponding to an operational state of the internal combustion engine and a post-correction opening obtained by the correction means, and determines on the basis of a result of comparison between the difference and a criterial value whether there is an abnormality occurring in the exhaust gas recirculation unit. The criterial value switching means switches the criterial value used by the determination means depending on whether the reference value has been stored in the learning means.
According to the aforementioned first aspect, the learning means stores as a reference value an output value (an opening) of the opening detection means at the time when a command signal for maintaining the exhaust gas recirculation valve of the exhaust gas recirculation unit at the predetermined opening is output, and updates the reference value at appropriate timings. Further, the correction means corrects a relationship between output value of the opening detection means and opening of the exhaust gas recirculation valve on the basis of the reference value obtained by the learning means. Accordingly, even if an output value of the opening detection means has deviated from an actual opening of the exhaust gas recirculation valve, this deviation can be counterbalanced by the aforementioned correction, and the output value (the post-correction opening) can be made to coincide with the actual opening.
In addition, the determination means calculates a difference between a target opening of the exhaust gas recirculation valve corresponding to an operational state of the internal combustion engine and the post-correction opening obtained by the correction means. In this case, for example, if the exhaust gas recirculation unit is in normal operation, the post-correction opening of the exhaust gas recirculation valve is close to the target opening. Therefore, there ought to be a small difference between them. Thus, by comparing the difference with a criterial value, it becomes possible to determine on the basis of a result of comparison whether there is an abnormality occurring in the exhaust gas recirculation unit. For example, if the difference is larger than the criterial value, it can be determined that the exhaust gas recirculation unit is abnormal, whereas if the difference is equal to or smaller than the criterial value, it can be determined that the exhaust gas recirculation unit is normal.
If the stored reference value is cleared, the output value of the opening detection means is relatively unreliable until a reference value is newly calculated and stored after the issuance of a command for maintaining the exhaust gas recirculation valve at a predetermined opening. This is because no correction is made by the correction means. In this case as well, therefore, if a determination on the occurrence of an abnormality in the exhaust gas recirculation unit is made using substantially the same criterial value as in the case where the reference value has been stored and updated, the determination may be made erroneously. In this respect, according to the invention described in the first aspect, the criterial value is switched depending on whether the reference value has been stored. Thus, even in the case where the reference value has not been stored yet, an erroneous determination can be inhibited from being made by using a suitable criterial value that is irrelevant to the case where the reference value has been stored.
As a second aspect of the invention, the aforementioned first aspect is modified as follows. If the difference is larger than the criterial value, the determination means determines that the exhaust gas recirculation unit is abnormal. If the reference value has not been stored, the criterial value switching means switches the criterial value to a value larger than a value that is used when the reference value has been stored.
According to the aforementioned second aspect, if it has turned out as a result of comparison that the difference is larger than the criterial value, the determination means determines that the exhaust gas recirculation unit is abnormal. If the reference value has not been stored under such a circumstance, the criterial value switching means switches the criterial value to a value larger than the criterial value that is used in the case where the reference value has been stored. Because of this switching, the criterion for determination (the criterial value) is less strict in the case where the reference value has not been stored than in the case where the reference value has been stored. In the case where the reference value has not been stored, an output value of the opening detection means is used without being rid of a deviation from an actual (true) opening of the exhaust gas recirculation valve. However, use of a large criterial value as mentioned above makes it possible to inhibit an erroneous determination from being made.
In setting a criterial value in the case where the reference value has been stored, there is no need to consider precision in making a determination in the case where the reference value has not been stored. Thus, by making the criterion for determination (i.e., by setting the criterial value small) in the case where the reference value has been stored, it becomes possible to enhance precision in making a determination on the occurrence of an abnormality.
As a third aspect of the invention, the aforementioned first or second aspect is modified as follows. The learning means stores as a reference value an opening detected by the opening detection means when a command signal for fully closing the exhaust gas recirculation valve is output, and updates the reference value at appropriate timings. The abnormality diagnosis device further comprises updating prohibition means that prohibits the reference value from being updated by the learning means if an output value of the opening detection means has deviated from the reference value toward the opening side by a predetermined value or more.
In the case where the reference value is stored and updated in accordance with a command signal for fully closing the exhaust gas recirculation valve, if the exhaust gas recirculation valve is regarded as having been fully closed when there is actually a foreign matter or the like stuck therein, the reference value may be stored and updated erroneously. In this respect, according to the aforementioned third aspect, it is determined whether the output value of the opening detection means has deviated from the reference value toward the opening side by the predetermined value or more. If it is determined that the output value has deviated from the reference value toward the opening side by the predetermined value or more, the updating prohibition means prohibits the reference value from being updated by the learning means. In this case, the last reference value is held as it is. As a result, even if there is a foreign matter stuck in the exhaust gas recirculation valve, the reference value can be inhibited from being stored and updated erroneously.
As a fourth aspect of the invention, any one of the aforementioned first to third aspects is modified as follows. The abnormality diagnosis device further comprises exhaust gas recirculation forcible stoppage means that stops recirculation of exhaust gas by outputting a command signal for fully closing the exhaust gas recirculation valve for a predetermined period after the start of the internal combustion engine. The learning means sets an initial value of the reference value on the basis of an output value of the opening detection means at the time when a command signal for fully closing the exhaust gas recirculation valve is output while recirculation of exhaust gas has been stopped by the exhaust gas recirculation forcible stoppage means.
According to the aforementioned fourth aspect, recirculation of exhaust gas is forcibly stopped, and an initial value of a reference value relating to full closure of the exhaust gas recirculation valve is set, whereby it becomes possible for the correction means to make a correction at an early stage. As a result, as soon as the internal combustion engine is started, the presence or absence of an abnormality in the exhaust gas recirculation unit can be diagnosed with high precision.
As a fifth aspect of the invention, any one of the first to fourth aspects is modified as follows. The abnormality diagnosis device further comprises second determination means and determination restriction means. The second determination means calculates a difference between a target opening of the exhaust gas recirculation valve corresponding to an operational state of the internal combustion engine and a post-correction opening obtained by the correction means, and determines on the basis of a result of comparison between the difference and a criterial value whether there is an abnormality occurring in the exhaust gas recirculation unit. The determination restriction means allows the second determination means to make a determination on the condition that the reference value be stored by the learning means.
According to the aforementioned fifth aspect, only if the reference value has been stored by the learning means, the second determination means carries out a diagnosis of abnormality. Hence, by correcting the relationship between output value of the opening detection means and opening of the exhaust gas recirculation valve by means of the reference value and using a post-correction opening thus obtained to make a determination, it becomes possible to enhance precision in carrying out a diagnosis of abnormality by means of the second determination means.
As a sixth aspect of the invention, an abnormality diagnosis method for an exhaust gas recirculation unit that adjusts an amount of exhaust gas recirculated from an exhaust passage to an intake passage in an internal combustion engine by means of an exhaust gas recirculation valve comprises the steps of detecting an opening of the exhaust gas recirculation valve, storing as a reference value an opening at the time when a command signal for maintaining the exhaust gas recirculation valve at a predetermined opening is output and updating the reference value at appropriate timings, correcting a relationship between the output value and opening of the exhaust gas recirculation valve on the basis of the reference value, calculating a difference between a target opening of the exhaust gas recirculation valve corresponding to an operational state of the internal combustion engine and a post-correction opening and determining on the basis of a result of comparison between the difference and a criterial value whether there is an abnormality occurring in the exhaust gas recirculation unit, and switching the criterial value depending on whether the reference value has been stored.
According to the aforementioned sixth aspect, an output value (an opening) at the time when a command signal for maintaining the exhaust gas recirculation valve of the exhaust gas recirculation unit at a predetermined opening is output is stored as a reference value, and the reference value is updated at appropriate timings. Further, a relationship between output value of the opening detection means and opening of the exhaust gas recirculation valve is corrected on the basis of the reference value. Accordingly, even if an output value has deviated from an actual opening of the exhaust gas recirculation valve, this deviation can be counterbalanced by the aforementioned correction, and the output value (the post-correction opening) can be made to coincide with the actual opening.
In addition, a difference between a target opening of the exhaust gas recirculation valve corresponding to an operational state of the internal combustion engine and the post-correction opening is calculated. In this case, for example, if the exhaust gas recirculation unit is in normal operation, the post-correction opening of the exhaust gas recirculation valve is close to the target opening. Therefore, there ought to be a small difference between them. Thus, by comparing the difference with a criterial value, it becomes possible to determine on the basis of a result of comparison whether there is an abnormality occurring in the exhaust gas recirculation unit. For example, if the difference is larger than the criterial value, it can be determined that the exhaust gas recirculation unit is abnormal, whereas if the difference is equal to or smaller than the criterial value, it can be determined that the exhaust gas recirculation unit is normal.
If the stored reference value is cleared, the output value is relatively unreliable until a reference value is newly calculated and stored after the issuance of a command for maintaining the exhaust gas recirculation valve at the predetermined opening. This is because no correction is made. In this case as well, therefore, if a determination on the occurrence of an abnormality in the exhaust gas recirculation unit is made using substantially the same criterial value as in the case where the reference value has been stored and updated, the determination may be made erroneously. In this respect, according to the invention described in the sixth aspect, the criterial value is switched depending on whether the reference value has been stored. Thus, even in the case where the reference value has not been stored yet, an erroneous determination can be inhibited from being made by using a suitable criterial value that is irrelevant to the case where the reference value has been stored.
As a seventh aspect of the invention, the aforementioned sixth aspect is modified as follows. If the difference is larger than the criterial value, it is determined that the exhaust gas recirculation unit is abnormal. If the reference value has not been stored, the criterial value is switched to a value larger than a value that is used when the reference value has been stored.
As an eighth aspect of the invention, the aforementioned sixth or seventh aspect is modified as follows. An opening of the exhaust gas recirculation valve at the time when a command signal for fully closing the exhaust gas recirculation valve is output is stored as a reference value, and the reference value is thereby updated at appropriate timings. If the output value has deviated from the reference value toward the opening side by a predetermined value or more, the reference value is prohibited from being updated.
As a ninth aspect of the invention, any one of the aforementioned sixth to eighth aspects is modified as follows. The abnormality diagnosis method further comprises the steps of stopping recirculation of exhaust gas by outputting a command signal for fully closing the exhaust gas recirculation valve for a predetermined period after the start of the internal combustion engine, and setting an initial value of the reference value on the basis of the output value at the time when a command signal for fully closing the exhaust gas recirculation valve is output during the stoppage of recirculation of exhaust gas.
As a tenth aspect of the invention, any one of the aforementioned sixth to ninth aspects is modified as follows. The abnormality diagnosis method further comprises the steps of calculating a difference between a target opening of the exhaust gas recirculation valve corresponding to an operational state of the internal combustion engine and a post-correction opening and determining on the basis of a result of comparison between the difference and a criterial value whether there is an abnormality occurring in the exhaust gas recirculation unit, and making the determination on the condition that the reference value be stored.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further objects, features and advantages of the invention will become apparent from the following description of preferred embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein:
Fig. 1 is a schematic view of the construction of an abnormality diagnosis device of the invention which is applied to a diesel engine in accordance with a first embodiment thereof;
Fig. 2 is a flowchart showing a procedure of setting an initial value of a reference value (an initial reference value) relating to a fully closed position;
Fig. 3 is a flowchart showing a procedure of updating a reference value relating to a fully closed position;
Fig. 4 is a flowchart showing a procedure of diagnosing the presence or absence of an abnormality in an EGR unit; and
Fig. 5 is a flowchart showing a procedure of diagnosing the presence or absence of an abnormality in the EGR unit.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereinafter, the first embodiment in which an abnormality diagnosis device for an exhaust gas recirculation unit in accordance with the invention is applied to a vehicular diesel engine will be described with reference to the drawings.
As shown in Fig. 1, a vehicle is mounted with a diesel engine 11 as an internal combustion engine. The diesel engine 11 is provided with a cylinder head 12, and a cylinder block 14 having a plurality of cylinders 13. Each of pistons 15 is reciprocally movably accommodated in a corresponding one of the cylinders 13. Each of the pistons 15 is coupled to a crank shaft 17 as an output shaft by a connecting rod 16. Reciprocating movements of each of the pistons 15 are converted into rotating movements while being transmitted to the crank shaft 17 through the connecting rod 16.
An intake passage 19 and an exhaust passage 20 are connected to each of combustion chambers 18, each of which is formed in a corresponding one of the cylinders 13. In a manner corresponding to each of the cylinders 13, an intake valve 21 and an exhaust valve 22 are provided in the cylinder head 12. The intake valve 21 and the exhaust valve 22 reciprocally move in association with rotation of the crank shaft 17, and thereby open or close the intake passage 19 and the exhaust passage 20 respectively. If each of the pistons 15 moves downwards with the exhaust valve 22 and the intake valve 21 being closed and open respectively during an intake stroke of the diesel engine 11, an air pressure in a corresponding one of the cylinders 13 assumes a value (a negative pressure) lower than an outside air pressure, and air outside the diesel engine 11 flows through respective portions of the intake passage 19 and is sucked into a corresponding one of the combustion chambers 18.
A throttle valve 23 as an intake throttle valve is rotatably supported in the intake passage 19. The throttle valve 23 is driven by an actuator 24 such as a stepper motor or the like. An intake air amount, which is an amount of air flowing through the intake passage 19, changes in accordance with a throttle opening corresponding to a rotational angle of the throttle valve 23.
Fuel injection valves 25 are attached to the cylinder head 12. Each of the fuel injection valves 25 injects fuel into a corresponding one of the combustion chambers 18 in a corresponding one of the cylinders 13. Each of the fuel injection valves 25 is provided with an electromagnetic valve (not shown), which controls injection of fuel from each of the fuel injection valves 25 to a corresponding one of the combustion chambers 18. The fuel injection valves 25 are connected to a common rail 26 as a common accumulator line. While the electromagnetic valve of each of the fuel injection valves 25 is open, fuel in the common rail 26 is injected from that fuel injection valve 25 into a corresponding one of the combustion chambers 18. A relatively high pressure corresponding to a fuel injection pressure is accumulated in the common rail 26. To realize accumulation of this pressure, the common rail 26 is connected to a supply pump 28 via a feed line 27. The supply pump 28 sucks fuel from a fuel tank 29, reciprocally moves a plunger by means of a cam (not shown) operating in synchronization with rotation of the diesel engine 11, pressurizes fuel to a predetermined pressure, and supplies the fuel to the common rail 26.
Fuel is then injected from each of the fuel injection valves 25 toward high-temperature, high-pressure intake air that has been introduced into a corresponding one of the cylinders 13 through the intake passage 19 and that has been compressed by a corresponding one of the pistons 15. The fuel thus injected spontaneously ignites and bums. Combustion gas generated at this moment reciprocally moves a corresponding one of the pistons 15, whereby the crank shaft 17 is rotated. As a result, a driving force (an output torque) of the diesel engine 11 is obtained. Combustion gas is discharged into the exhaust passage 20 as the exhaust valve 22 is opened.
The diesel engine 11 is provided with an exhaust gas recirculation (hereinafter referred to as "EGR") unit 31 that recirculates part of exhaust gas flowing through the exhaust passage 20 into the intake passage 19. The EGR unit 31 is designed to increase a ratio of inactive gas contained in mixture by means of exhaust gas (EGR gas) that is mixed with intake air in the course of recirculation, to lower a maximum combustion temperature, and to reduce an amount of generation of nitrogen oxides (NOx) as substances causing air pollution.
The EGR unit 31 is provided with an EGR passage 32 and an EGR valve 33. The EGR passage 32 connects the exhaust passage 20 and the intake passage 19 at their portions downstream of the throttle valve 23. The EGR valve 33 is disposed in the EGR passage 32. The flow rate of EGR gas flowing through the EGR passage 32 changes in accordance with the opening of the EGR valve 33 (EGR opening). The EGR opening changes in accordance with the lift amount of a valve body of the EGR valve 33.
The vehicle is provided with various sensors for detecting an operational state of the diesel engine 11. For example, a throttle position sensor 36 that detects a throttle opening on the basis of a rotational angle of the throttle valve 23 is attached thereto. A lift sensor 37 as opening detection means is attached to the EGR valve 33. The lift sensor 37 detects a lift amount of the valve body as an EGR opening. A coolant temperature sensor 38 that detects a coolant temperature as a temperature of engine coolant is attached to the cylinder block 14. A crank position sensor 39 that outputs a pulse signal every time the crank shaft 17 rotates by a predetermined angle is disposed in the vicinity of the crank shaft 17. This pulse signal is used to detect an engine rotational speed, which is a number of revolutions of the crank shaft 17 per unit time. In addition, an accelerator opening sensor 40 that detects an accelerator opening as a depression stroke of an accelerator pedal 34 operated by a driver is disposed in the vicinity of the accelerator pedal 34. Although many more sensors are attached to the diesel engine 11 or the like, they will not be described herein.
In order to control respective portions of the diesel engine 11 on the basis of values detected by the aforementioned sensors 36 to 40 and the like, the vehicle is provided with an electronic control unit (ECU) 41 that is mainly constructed of a microcomputer. In the ECU 41, a central processing unit (CPU) performs calculation processings in accordance with control programs, initial data, control maps, and the like which are stored in a read-only memory (ROM), and performs various control operations on the basis of calculation results. The calculation results obtained by the CPU are temporarily stored in a random access memory (RAM). In addition, the ECU 41 is provided with a backup RAM, which is backed up by a battery. The backup RAM stores and holds various data even after the ECU 41 has been stopped from being supplied with electric power. These data include a reference value, which will be described later.
The aforementioned various control operations include fuel injection control, throttle control, EGR control, learning control of a reference value relating to a predetermined opening of the EGR valve 33, abnormality diagnosis control of the EGR unit 31, and the like. In fuel injection control, for example, amounts of fuel injected from the fuel injection valves 25 and timings for injecting fuel from the fuel injection valves 25 are determined on the basis of an operational state of the diesel engine 11, and supply of electricity to the fuel injection valves 25 is controlled in accordance with the amounts and timings thus determined.
In throttle control, for example, a target throttle opening corresponding to an engine rotational speed and a fuel injection amount is calculated. The actuator 24 is drivingly controlled such that an actual throttle opening detected by the throttle position sensor 36 coincides with the target throttle opening.
In EGR control, for example, it is determined on the basis of an engine rotational speed, a coolant temperature, an accelerator opening, and the like whether a condition for performing EGR control has been fulfilled. If the condition for performing EGR control has not been fulfilled, the EGR valve 33 is held in its fully closed state. On the other hand, if the condition for performing EGR control has been fulfilled, a target opening of the EGR valve 33 corresponding to an engine rotational speed and a fuel injection amount is calculated, for example, by referring to a predetermined control map. The EGR valve 33 is drivingly controlled on the basis of the target opening thus calculated.
Next, learning control of a reference value relating to a predetermined opening of the EGR valve 33 will be described. The gist of this control consists in that a reference value relating to a fully closed state is calculated on the basis of an output value of the lift sensor 37 (a sensor output value) on condition that a command signal for maintaining the EGR valve 33 at a predetermined opening (fully closed in this case) be output, and in that the reference value thus calculated is stored and updated at appropriate timings.
In performing this control, the ECU 41 executes an "initial reference value setting routine" shown in a flowchart of Fig. 2 and a "reference value updating routine" shown in a flowchart of Fig. 3. Each of these routines is repeatedly executed at intervals of a predetermined period. In performing respective processings in these routines, various flags such as an initial reference value setting completion flag, a learning abnormality flag, and the like are used. It is to be noted herein that the initial reference value setting completion flag is designed to make a determination on the learning history on an initial value of a reference value (an initial reference value). The initial reference value setting completion flag is set as "0" when learning is not being carried out, whereas the initial reference value setting completion flag is set as "1" when learning is carried out. The learning abnormality flag is designed to determine whether an updated reference value is a suitable value. The learning abnormality flag is set as "1" if the updated reference value is unsuitable, whereas the learning abnormality flag is set as "0" if the updated reference value is suitable. Incidentally, both the flags mentioned above are initially set as "0".
In the initial reference value setting routine shown in Fig. 2, it is determined first of all in step 110 whether the diesel engine 11 is being started. If this criterial condition has been fulfilled (i.e., if the diesel engine 11 is being started), it is determined in step 120 whether the initial reference value setting completion flag has been set as "0". If this criterial condition has been fulfilled, namely, if a reference value relating to a fully closed position has never been stored in the backup RAM, it is determined in step 130 whether a period that has elapsed since the start of the diesel engine 11 is less than a predetermined period ΔT (e.g., a few milliseconds). For example, both the criterial conditions in step 120 and step 130 are fulfilled, for example, immediately after a reference value stored in the backup RAM has been cleared through shutoff of the supply of electric power resulting from an exchange of batteries. If the criterial condition in step 130 has been fulfilled (i.e., if a period that has elapsed is less than the predetermined period ΔT), a command signal for fully closing the EGR valve 33 is output to forcibly stop EGR in step 140. That is, the EGR valve 33, which is intrinsically not to be closed unless a learning performance condition relating to a fully closed position is fulfilled, is exceptionally fully closed to stop EGR. This forcible closure of the EGR valve 33 basically lasts only for such a period (for the predetermined period ΔT in this case) as does not cause a deterioration in exhaust emission properties.
Then, in step 150, a difference between a reference value and a sensor output value is calculated, and it is determined whether an absolute value of the difference is larger than a predetermined value α. If this criterial condition in step 150 has been fulfilled, namely, if the reference value has greatly deviated from the sensor output value or if there is a substantial difference between the reference value and the sensor output value, a difference between a last-calculated reference value (a last reference value) and the sensor output value is divided, for example, by "2", and a quotient thus obtained is added to the last reference value in step 160. A sum thus obtained is then set as a new reference value (a current reference value) and stored into the backup RAM. A preset value is used as the last reference value when the processings in step 150 and step 160 are performed for the first time after the engine has been started. After the processing in step 160 has been terminated, the control operation returns to step 120. Accordingly, the processings from step 140 to step 160 are repeated until the initial reference value setting completion flag is switched to "1" or until the predetermined period ΔT elapses after the diesel engine 11 has been started. Due to these processings, the current reference value is updated step by step.
If the criterial condition in step 150 becomes unfulfilled, namely, if the last reference value approaches the sensor output so that a difference between them becomes sufficiently small, the initial reference value setting completion flag is switched from "0" to "1" in step 170 on the ground that an initial value of a reference value (an initial reference value) has been calculated. After the processing in step 170 has been terminated, the initial reference value setting routine is temporarily terminated.
If each of the aforementioned criterial conditions in step 110, step 120, and step 130 has not been fulfilled, the initial reference value setting routine is temporarily terminated without performing any of the following processings. Thus, after the initial reference value setting completion flag has been temporarily set as "1" in step 170 mentioned above, the criterial condition in step 120 becomes unfulfilled, whereby the initial reference value setting routine is terminated. In this manner, as long as an initial reference value relating to a fully closed position is not stored, a command signal for fully closing the EGR valve 33 is output when the diesel engine 11 is started, and the processing of calculating an initial reference value is performed. A reference value at the time when a determination result obtained in step 150 is about to switch from "YES" to "NO" is set and stored as an initial reference value. If the predetermined period ΔT has elapsed before the criterial condition in step 150 becomes unfulfilled, the initial reference value setting completion flag remains "0" without being switched.
Next, in the reference value updating routine shown in Fig. 3, it is determined first of all in step 200 whether the predetermined period ΔT has elapsed since the start of the engine. If this criterial condition has not been fulfilled, the reference value updating routine is temporarily terminated. If this criterial condition has been fulfilled, it is determined in step 210 whether the learning performance condition has been fulfilled. The processing in step 200 is performed so as to prevent a reference value before the lapse of the predetermined period ΔT, namely, a reference value during a series of the aforementioned processings from step 120 to step 160 from being used for updating processings starting from step 220.
The learning performance condition is, for example, that a command signal for fully closing the EGR valve 33 be continuously output for a predetermined period. If the criterial condition in step 210 has not been fulfilled, the reference value updating routine is temporarily terminated without performing any of the following processings.
On the other hand, if the aforementioned criterial condition in step 210 has been fulfilled, it is determined in step 220 whether the sensor output value has deviated from the last reference value toward the opening side by a predetermined value or more. If this criterial condition has not been fulfilled (i.e., if the sensor output has deviated from the last reference value by the predetermined value or more), there may be a foreign matter or the like stuck in the EGR valve 33. If the reference value is updated on the assumption that the EGR valve 33 assumes its fully closed state at this moment, the reference value may be updated to a wrong value. In this case, therefore, the last reference value is set as a current reference value in step 240. In other words, the last reference value is held instead of being updated. After the processings in step 240 have been terminated, the reference value updating routine is temporarily terminated.
On the other hand, if the aforementioned criterial condition in step 220 has been fulfilled (i.e., if the sensor output value has deviated from the last reference value by less than the predetermined value), it is determined in step 230 whether the sensor output value has deviated from the last reference value toward the opening side by less than a predetermined value. Alternatively, it is determined in step 230 whether the sensor output value has deviated from the last reference value toward the closing side by the predetermined value or more. If this criterial condition in step 230 has not been fulfilled, the control operation proceeds to step 240 mentioned above, and the last reference value is held. On the other hand, if the criterial condition in step 230 has been fulfilled, a predetermined value is added to the last reference value, and a sum thus obtained is set, stored, and updated as a current reference value in step 250. If the sensor output value has deviated from the last reference value toward the closing side by less than the predetermined value (i.e., if the amount of deviation toward the closing side is small) in the processing of step 230, the reference value is not updated so as to inhibit the reference value from being stored and updated in vain as a result of dispersion of the sensor output value. That is, if the amount of deviation of the sensor output value from the reference value is confined to a certain range (a dead zone), the reference value is not stored or updated.
It is then determined in step 260 whether the reference value is within a prescribed range that has been set in advance. More specifically, it is determined whether the reference value is either equal to one of a lower limit value and an upper limit value or larger than the lower limit value and smaller than the upper limit value. If this criterial condition has been fulfilled, the learning abnormality flag is set as "0" in step 270 on the ground that the reference value is normal. On the other hand, if the criterial condition in step 260 has not been fulfilled, namely, if the reference value is out of the prescribed range, the learning abnormality flag is set as "1" in step 280 on the ground that the reference value is abnormal. A guard processing is thereafter performed in step 290. For example, if the reference value is above the upper limit value, the upper limit value is set as a current reference value. If the reference value is below the lower limit value, the lower limit value is set as a current reference value. After the processing in step 270 or step 290 has been terminated, the reference value updating routine is temporarily terminated.
It is to be noted herein that the respective processings in the aforementioned initial reference value setting routine and the respective processings in the reference value updating routine correspond to the learning means. It is also to be noted that the processings of step 220 and step 240 in the reference value updating routine correspond to the updating prohibition means.
Next, abnormality diagnosis control performed by the EGR unit 31 will be described. In performing this control, the ECU 41 executes a "first EGR abnormality diagnosis routine" shown in a flowchart of Fig. 4 and a "second EGR abnormality diagnosis routine" shown in a flowchart of Fig. 5. These routines are repeatedly performed at intervals of a predetermined period. Although these routines are both designed to determine whether there is an abnormality occurring in the EGR unit 31, determination results obtained from the routines are utilized in different circumstances. The determination result obtained from the former routine is utilized when diagnostic precision higher than that of the determination result obtained from the latter routine is required.
In the first abnormality diagnosis routine shown in Fig. 4, it is determined first of all in step 310 whether an abnormality diagnosis performance condition as a prerequisite condition for carrying out a diagnosis of abnormality has been fulfilled. This abnormality diagnosis performance condition is, for example, that the accelerator opening be 0%, that the fuel injection amount be equal to or smaller than a predetermined value (e.g., an idling injection amount), that the degree of change in engine rotational speed be smaller than a predetermined value, or the like. The abnormality diagnosis performance condition is regarded as having been fulfilled only if all the aforementioned conditions have been fulfilled.
If the aforementioned criterial condition in step 310 has not been fulfilled, the first EGR abnormality diagnosis routine is temporarily terminated. If the aforementioned criterial condition in step 310 has been fulfilled, the control operation proceeds to step 320. It is determined in step 320 whether the initial reference value setting completion flag has been set as "1".
In the case where the stored reference value has been cleared, no correction is made using the initial reference value until a command signal for fully closing the EGR valve 33 is output to newly calculate and store an initial reference value. Thus, the sensor output value and the post-correction opening are relatively unreliable. In this case as well, therefore, if a determination on the occurrence of an abnormality in the EGR unit 31 is made using substantially the same criterial value as in the case where an initial reference value is stored and updated, an erroneous result may be obtained. Hence, in accordance with a result of determination in step 320, it is determined using different criterial values whether there is an abnormality occurring in the EGR unit 31.
More specifically, if the criterial condition in step 320 has been fulfilled, namely, if an initial reference value has already been set and stored, a first criterial value is used. On the other hand, if the criterial condition in step 320 has not been fulfilled, namely, if an initial reference value has not been set or stored yet, a second criterial value is used. The second criterial value is set, for example, as a value including a deviation of an output value of the lift sensor 37 from an actual (true) opening. This deviation results from differences among individual products of the lift sensor 37, aging thereof, or the like. On the other hand, the first criterial value is set as a value that does not include this deviation. Accordingly, the second criterial value is larger than the first criterial value by the deviation.
If the aforementioned criterial condition in step 320 has been fulfilled (i.e., if an initial reference value has been set), a difference between a target opening of the EGR valve 33 and a post-correction opening of the EGR valve 33 is calculated, and it is determined in step 330 whether an absolute value of the difference is equal to or larger than the first criterial value. If the aforementioned criterial condition in step 320 has not been fulfilled (i.e., if an initial reference value has not been set yet), a difference between a target opening and a post-correction opening is calculated, and it is determined in step 360 whether an absolute value of the difference is equal to or larger than the second criterial value.
As described above, a target opening is calculated on the basis of an operational state (an engine rotational speed, a fuel injection amount, or the like) in performing EGR control. A post-correction opening is obtained by correcting a relationship between sensor output value and opening of the EGR valve 33 using the aforementioned reference value relating to a fully closed position. The post-correction opening is closer to an actual (true) opening of the EGR valve 33. This is because, even if the sensor output value has deviated from the actual (true) opening of the EGR valve 33, this deviation is counterbalanced by the aforementioned correction.
For example, if the EGR unit 31 is in normal operation, the post-correction opening approaches the target opening due to the aforementioned EGR control, and the difference between them ought to become small. From this viewpoint, an absolute value of the difference is compared with the criterial values (the first criterial value and the second criterial value) in step 330 and step 360 respectively. It is determined on the basis of a result of this comparison whether there is an abnormality occurring in the EGR unit 31. That is, if the absolute value of the difference is equal to or larger than each of the criterial values, it is determined that the EGR unit 31 is abnormal, whereas if the absolute value of the difference is smaller than each of the criterial values, it is determined that the EGR unit 31 is normal.
More specifically, if the criterial condition in step 330 has been fulfilled, it is considered that the EGR unit 31 is abnormal because the post-correction opening has greatly deviated from the target opening. In this case, therefore, the EGR abnormality flag is set as "1" in step 340. On the other hand, if the criterial condition in step 330 has not been fulfilled, it is considered that the EGR unit 31 is normal because the post-correction opening is close to the target opening. In this case, the EGR abnormality flag is set as "0" in step 350.
By the same token, if the criterial condition in step 360 has been fulfilled, it is considered that the EGR unit 31 is abnormal because the post-correction opening has greatly deviated from the target opening. In this case, therefore, the EGR abnormality flag is set as "1" in step 370. On the other hand, if the criterial condition in step 360 has not been fulfilled, it is considered that the EGR unit 31 is normal because the post-correction opening is close to the target opening. In this case, the EGR abnormality flag is set as "0" in step 350 mentioned above. After the processings in step 340, step 350, and step 370 have been terminated, the first EGR abnormality diagnosis routine is temporarily terminated. A diagnosis result obtained from this routine (a state of the EGR abnormality flag) is used as an indicator for making a determination on the necessity to perform fail-safe processings, for example, in performing fail-safe control.
As for the processings in step 330 and step 360 of the aforementioned first EGR abnormality diagnosis routine, the processings of calculating a post-correction opening correspond to the correction means. In this routine, the processings from step 330 to step 370 correspond to the determination means, and the processings in step 320, step 330, and step 360 correspond to the criterial value switching means.
Next, in the second EGR abnormality diagnosis routine shown in Fig. 5, it is determined first of all in step 410 whether the initial reference value setting completion flag has been set as "1". Further, it is determined in step 420 whether the learning abnormality flag has been set as "0". If both the aforementioned criterial conditions in step 410 and step 420 have been fulfilled, a diagnosis of abnormality is carried out in step 430 using the lift sensor 37. As the contents of this diagnosis, for example, a difference between a target opening and a post-correction opening is calculated, and it is determined whether an absolute value of the difference is equal to or larger than a predetermined criterial value. The target opening and the post-correction opening are substantially the same as those mentioned in step 330 and step 360.
If this criterial condition has been fulfilled, it is determined that the EGR unit 31 is abnormal because the post-correction opening has greatly deviated from the target opening. On the other hand, if the aforementioned criterial condition has not been fulfilled, it is determined that the EGR unit 31 is normal because the post-correction opening is close to the target opening. After the processing in step 430 has been terminated, the second EGR abnormality diagnosis routine is temporarily terminated.
On the other hand, even if one of the criterial conditions in step 410 and step 420 has not been fulfilled, the second EGR abnormality diagnosis routine is temporarily terminated. Accordingly, only if an initial reference value has been set (YES in step 410) while a reference value is within a prescribed range (YES in step 420), a diagnosis of abnormality is carried out using an output value of the lift sensor 37.
Further, if the criterial condition in step 260 of the aforementioned reference value updating routine has not been fulfilled, namely, if the reference value is out of the prescribed range, it is determined that the reference value relating to a fully closed position is abnormal, and a diagnosis of abnormality is not carried out using the lift sensor 37.
The abnormality diagnosis device for the EGR unit 31 is composed of the sensors 36 to 40, the ECU 41 for executing the first and second EGR abnormality diagnosis routines, and the like. As a matter of course, the lift sensor 37 is included in the sensors 36 to 40.
According to the first embodiment of the invention described hitherto in detail, the following effects are achieved.
As the first effect, if the diesel engine 11 is started with an initial value of a reference value relating to a fully closed position (an initial reference value) not having been stored, a command signal for fully closing the EGR valve 33 is output for the predetermined period ΔT immediately after the start of the engine so as to forcibly stop EGR. The initial reference value relating to the fully closed position is set and stored under this circumstance (from step 110 to step 160). Hence, it is possible to correct a relationship between sensor output value and opening of the EGR valve 33 at an early stage. As a result, as soon as the engine is started, it can be determined with high precision whether there is an abnormality occurring in the EGR unit 31.
As the second effect, two criterial values, namely, the first and second criterial values are set and selectively used depending on whether the initial reference value has been stored (step 320, step 380, and step 360). Accordingly, even in the case where a diagnosis of abnormality is carried out on the basis of a relatively unreliable post-correction opening when the initial reference value has not been stored yet, the possibility of making an erroneous determination can be reduced by using a suitable criterial value that is irrelevant to the case where the reference value has been stored.
As the third effect, which is associated with the second effect, if the reference value has not been stored, the criterial value is switched to a value (the second criterial value) larger than the criterial value that is used when the reference value has been stored. Because of this switching operation, the criterion for determination is less strict in the case where the reference value has not been stored than in the case where the reference value has been stored. In the case where the reference value has not been stored, the sensor output value is used to make a determination without being rid of a deviation from an actual (true) opening of the EGR valve 33. The second criterial value, which is larger than the first criterial value, is used as described above, whereby the aforementioned deviation is counterbalanced and an erroneous determination is reliably inhibited from being made.
In setting a criterial value in the case where the reference value has been stored, there is no need to consider precision in making a determination in the case where the reference value has not been stored. Thus, by making a criterion for determination in the case where the reference value has been stored strict (i.e., by setting the first criterial value as a small value), it becomes possible to enhance precision in a diagnosis of abnormality.
As the fourth effect, as regards the updating of the reference value, if the sensor output value has deviated from the last reference value toward the opening side by the predetermined value or more, the last reference value is held instead of being updated (step 220 and step 240). Hence, even if there is a foreign matter or the like stuck in the EGR valve 33, the reference value can be prevented from being updated by mistake.
As the fifth effect, a diagnosis of abnormality is carried out using the lift sensor 37 (step 430) on the condition that the setting of an initial reference value relating to the fully closed position be completed (step 410 and step 430). That is, a restriction is imposed on the implementation of a diagnosis of abnormality. Hence, a relationship between sensor output value and opening of the EGR valve 33 is corrected using the reference value, and a post-correction opening thus obtained is used to make a determination. Thus, a diagnosis of abnormality can be carried out with enhanced precision using the lift sensor 37.
As the sixth effect, if the learning abnormality flag indicates "1" in the second EGR abnormality diagnosis routine (NO in step 420), namely, if the reference value is out of the prescribed range in the reference value updating routine (NO in step 260), a diagnosis of abnormality is not carried out using the lift sensor 37. Hence, the relationship between sensor output value and opening of the EGR valve 33 can be prevented from being corrected on the basis of a wrong reference value, and the post-correction opening can be prevented from being used for a diagnosis of abnormality. As a result, diagnostic precision can be enhanced.
Incidentally, the invention can be embodied as the following embodiments shown below.
The invention is also applicable to an abnormality diagnosis device which stores as a reference value an output value (an opening) of the lift sensor 37 at the time when a command signal for fully opening the EGR valve 33 is output, and which updates the reference value at appropriate timings.
The invention can also be embodied not only in the abnormality diagnosis device for the EGR unit for the diesel engine 11 but also in an abnormality diagnosis device for an EGR unit for other internal combustion engines such as a gasoline engine and the like.
Various types of EGR valves can be employed as the EGR valve of the invention. For instance, an EGR valve whose valve body is operated using a negative pressure, an EGR valve whose valve body is operated by a motor such as a stepper motor or the like, or an EGR valve whose valve body is operated by a linear solenoid can be employed.
The method of calculating a reference value in step 160 of the initial reference value setting routine (Fig. 2) may be altered appropriately. For example, although the difference between the reference value and the sensor output value is divided by "2" in the aforementioned first embodiment, the difference may be divided by a value different from "2".
It is to be noted herein that the processings in step 110, step 130, and step 140 of the aforementioned initial reference value setting routine correspond to the exhaust gas recirculation forcible stoppage means of the invention.
Further, the processing in step 430 of the aforementioned second EGR abnormality diagnosis routine corresponds to the second determination means of the invention, and the processing from step 410 to "RETURN" corresponds to the determination restriction means of the invention.
An electronic control unit (ECU) calculates a reference value relating to an opening of an exhaust gas recirculation (EGR) valve on the basis of an output value of a lift sensor at the time when a command signal for fully closing the EGR valve is output, and stores and updates the reference value. The ECU corrects a relationship between sensor output value and opening of the EGR valve on the basis of this reference value (step 330 and step 360). The ECU calculates a difference between a target opening of the EGR valve corresponding to an operational state of an engine and a post-correction opening of the EGR valve, and determines on the basis of a result of comparison between an absolute value of the difference and a criterial value whether there is an abnormality occurring in an EGR unit (from step 330 to step 370). In making this determination, the criterial value is switched depending on whether an initial value of the reference value has been stored (YES in step 320) or not (NO in step 320) (step 330 and step 360).

Claims

An abnormality diagnosis device for an exhaust gas recirculation unit (31) that adjusts an amount of exhaust gas recirculated from an exhaust passage (20) to an intake passage (19) in an internal combustion engine (11) by use of an exhaust gas recirculation valve (33), characterized by comprising:

opening detection (37) that detects an opening of the exhaust gas recirculation valve (33);

learning means (41) that stores as a reference value an opening detected by the opening detection means (37) when a command signal for maintaining the exhaust gas recirculation valve (33) at a predetermined opening is output, and that updates the reference value at appropriate timings;

correction means (41) that corrects a relationship between output value of the opening detection means (37) and opening of the exhaust gas recirculation valve (33) on the basis of the reference value updated by the learning means (41);

determination means (41) that calculates a difference between a target opening of the exhaust gas recirculation valve (33) corresponding to an operational state of the internal combustion engine (11) and a post-correction opening obtained by the correction means (41), and that determines on the basis of a result of comparison between the difference and a criterial value whether there is an abnormality occurring in the exhaust gas recirculation unit (31); and

criterial value switching means (41) that switches the criterial value used by the determination means (41) depending on whether the reference value has been stored in the learning means (41).
The abnormality diagnosis device according to claim 1, characterized in that if the difference is larger than the criterial value, the determination means (41) determines that the exhaust gas recirculation unit (31) is abnormal, and
if the reference value has not been stored, the criterial value switching means (41) switches the criterial value to a value larger than a value that is used when the reference value has been stored.
The abnormality diagnosis device according to claim 1 or 2, characterized in that the learning means (41) stores as a reference value an opening detected by the opening detection means (37) when a command signal for fully closing the exhaust gas recirculation valve (33) is output, and updates the reference value at appropriate timings, and characterized by further comprising:

updating prohibition means (41) that prohibits the reference value from being updated by the learning means (41) if an output value of the opening detection means (37) has deviated from the reference value toward the opening side by a predetermined value or more.
The abnormality diagnosis device according to any one of claims 1 to 3, characterized by further comprising:

exhaust gas recirculation forcible stoppage means that stops recirculation of exhaust gas by outputting a command signal for fully closing the exhaust gas recirculation valve (33) for a predetermined period after the start of the internal combustion engine (11), and characterized in that the learning means (41) sets an initial value of the reference value on the basis of an output value of the opening detection means (37) at the time when a command signal for fully closing the exhaust gas recirculation valve (33) is output while recirculation of exhaust gas has been stopped by the exhaust gas recirculation forcible stoppage means.
The abnormality diagnosis device according to any one of claims 1 to 4, characterized by further comprising:

second determination means (41) that calculates a difference between a target opening of the exhaust gas recirculation valve (33) corresponding to an operational state of the internal combustion engine (11) and a post-correction opening obtained by the correction means (41), and that determines on the basis of a result of comparison between the difference and a criterial value whether there is an abnormality occurring in the exhaust gas recirculation unit (31); and

determination restriction means that allows the second determination means (41) to make a determination on the condition that the reference value be stored by the learning means (41).
An abnormality diagnosis method for an exhaust gas recirculation unit (31) that adjusts an amount of exhaust gas recirculated from an exhaust passage (20) to an intake passage (19) in an internal combustion engine (11) by use of an exhaust gas recirculation valve (33), characterized by comprising the steps of:

detecting an opening of the exhaust gas recirculation valve (33);

storing as a reference value an opening at the time when a command signal for maintaining the exhaust gas recirculation valve (33) at a predetermined opening is output, and updating the reference value at appropriate timings;

correcting a relationship between the output value and opening of the exhaust gas recirculation valve (33) on the basis of the reference value;

calculating a difference between a target opening of the exhaust gas recirculation valve (33) corresponding to an operational state of the internal combustion engine (11) and a post-correction opening, and determining on the basis of a result of comparison between the difference and a criterial value whether there is an abnormality occurring in the exhaust gas recirculation unit (31); and

switching the criterial value depending on whether the reference value has been stored.
The abnormality diagnosis method according to claim 6, characterized in that if the difference is larger than the criterial value, it is determined that the exhaust gas recirculation unit (31) is abnormal, and if the reference value has not been stored, the criterial value is switched to a value larger than a value that is used when the reference value has been stored.
The abnormality diagnosis method according to claim 6 or 7, characterized in that an opening of the exhaust gas recirculation valve (33) at the time when a command signal for fully closing the exhaust gas recirculation valve (33) is output is stored as a reference value, and the reference value is thereby updated at appropriate timings, and if the output value has deviated from the reference value toward the opening side by a predetermined value or more, the reference value is prohibited from being updated.
The abnormality diagnosis method according to any one of claims 6 to 8, characterized by further comprising the steps of:

stopping recirculation of exhaust gas by outputting a command signal for fully closing the exhaust gas recirculation valve (33) for a predetermined period after the start of the internal combustion engine (11); and

setting an initial value of the reference value on the basis of the output value at the time when a command signal for fully closing the exhaust gas recirculation valve (33) is output during the stoppage of recirculation of exhaust gas.
The abnormality diagnosis method according to any one of claims 6 to 9, characterized by further comprising the steps of:

calculating a difference between a target opening of the exhaust gas recirculation valve (33) corresponding to an operational state of the internal combustion engine (11) and a post-correction opening, and determining on the basis of a result of comparison between the difference and a criterial value whether there is an abnormality occurring in the exhaust gas recirculation unit (31); and

making the determination on the condition that the reference value be stored.