Failure detection device for exhaust gas recirculation device

Abstract

This failure detection device for an exhaust gas recirculation device comprises: a failure type estimation unit (31) for estimating the type of failure that has occurred in an exhaust gas recirculation device (20); and a determination unit (32) for determining the presence or absence of a failure according to an intake pressure change when the opening degree of an exhaust gas recirculation valve (22) is switched during a fuel cut operation. The determination unit (32) determines the presence or absence of a failure by changing a valve opening degree set for the exhaust gas recirculation valve (22) according to the type of failure estimated by the failure type estimation unit (31) during failure diagnosis.

Description

Fault detection device for exhaust gas recirculation systems

[0001] The present invention relates to a fault detection device for an exhaust gas recirculation system.

[0002] Typically, an exhaust gas recirculation system is provided between the exhaust passage and the intake passage of an engine, introducing a portion of the exhaust gas into the intake air as exhaust gas recirculation gas. The exhaust gas recirculation system includes an exhaust gas recirculation passage that connects the exhaust passage and the intake passage, and an exhaust gas recirculation valve provided in the exhaust gas recirculation passage. Depending on the pressure state in the intake passage caused by the opening and closing of the exhaust gas recirculation valve and the throttle valve, an appropriate amount of exhaust gas recirculation gas is introduced into the intake air. Introducing exhaust gas recirculation gas into the intake air lowers the combustion temperature in the combustion chamber, which in turn reduces nitrogen oxides and other emissions, improves fuel efficiency, and suppresses knocking.

[0003] However, exhaust gas recirculation systems can sometimes develop abnormalities (failures) over time, such as a stuck exhaust gas recirculation valve, foreign matter getting caught in the exhaust gas recirculation valve, or foreign matter clogging the exhaust gas recirculation passage.

[0004] If the malfunction is due to the exhaust gas recirculation valve being stuck open or being unable to close from a certain position due to foreign matter lodged in it, an excessive amount of exhaust gas recirculation gas may be introduced into the intake, resulting in unstable combustion.Also, if the malfunction is due to the exhaust gas recirculation valve being stuck closed or due to foreign matter clogging the exhaust gas recirculation passage, which reduces the flow rate of exhaust gas recirculation gas below its intended (desired) value, the amount of exhaust gas introduced may be too low, resulting in an increase in combustion temperature depending on the operating conditions.

[0005] To detect such a failure, for example, there is a failure detection device described in Patent Document 1. In this failure detection device, as a failure diagnosis operation, if the difference between the intake pressure (P1) when a predetermined time has elapsed with the exhaust gas recirculation valve fully closed during fuel cut and the intake pressure (P2) when a predetermined time has elapsed with the exhaust gas recirculation valve then opened becomes equal to or exceeds a predetermined value, it is determined that the exhaust gas recirculation device has failed.

[0006] Japanese Patent Application Laid-Open No. 2022-128831

[0007] In conventional fault detection devices, the valve opening of the exhaust gas recirculation valve is always set to a constant value when fault detection is performed. This results in a problem of variations in the accuracy of fault detection depending on the type of fault. For example, in the case of a fault that causes the flow rate of exhaust gas recirculation gas to increase above its normal flow rate, if the valve opening set for fault detection is large (larger than the engagement opening), the intake differential pressure (P2 - P1) may exceed a predetermined value, even though a fault has occurred, and the fault may not be detected.

[0008] Therefore, an object of the present invention is to improve the accuracy of failure determination in an exhaust gas recirculation system and to make a substantial failure determination according to the type of failure.

[0009] In order to solve the above problems, the present invention provides a fault detection device for an exhaust gas recirculation system that includes an exhaust gas recirculation passage that connects the exhaust passage and intake passage of an engine, and an exhaust gas recirculation valve provided in the exhaust gas recirculation passage, and the device includes a fault type estimation unit that estimates the type of fault that has occurred in the exhaust gas recirculation system, and a judgment unit that determines the presence or absence of a fault from a change in intake pressure when the opening of the exhaust gas recirculation valve is switched during fuel cut operation, and the judgment unit adopts a fault detection device for an exhaust gas recirculation system that determines the presence or absence of a fault by changing the valve opening set for the exhaust gas recirculation valve in accordance with the type of fault estimated by the fault type estimation unit when diagnosing the presence or absence of a fault (Configuration 1).

[0010] As in the configuration 1, by changing the opening degree of the exhaust gas recirculation valve at the time of failure determination depending on the type of failure, the accuracy of failure determination can be improved.

[0011] In an aspect having configuration 1, the failure type estimation unit estimates that a first failure has occurred, in which the flow rate of exhaust recirculation gas increases from the original flow rate, when the difference between the intake air amount upstream of the connection point of the intake passage with the exhaust recirculation passage and the intake air amount downstream is equal to or greater than a first threshold value with the exhaust recirculation valve closed, and the determination unit can adopt a configuration in which, when the first failure is estimated, the valve opening set for the exhaust recirculation valve is set smaller than when the first failure is not estimated (configuration 2).

[0012] In this configuration 2, if a first fault (i.e., a fault in which the exhaust recirculation gas flow rate increases above the intended flow rate due to a foreign object getting caught in the exhaust recirculation valve, etc.) is suspected, the opening of the exhaust recirculation valve during fault diagnosis is made smaller than when another fault (i.e., a fault in which the exhaust recirculation gas flow rate decreases below the intended flow rate, etc.) is suspected. This improves the accuracy of fault determination in the case of a fault in which the exhaust recirculation gas flow rate increases above the intended flow rate.

[0013] In the aspect having the configuration 2, the failure type estimation unit can be configured to estimate a first failure when a state in which a difference between the intake air amount upstream of a connection portion of the intake passage with the exhaust gas recirculation passage and the intake air amount downstream of the connection portion is equal to or greater than a first threshold value and an engine rotation fluctuation is equal to or greater than a second threshold value is measured a predetermined number of times (configuration 3). This configuration 3 enables more accurate estimation of the first failure.

[0014] In an aspect having configuration 2, the failure type estimation unit may be configured to estimate a second failure in which the flow rate of exhaust recirculation gas is reduced from its original flow rate when the difference between the intake air amount upstream of the connection point of the intake passage with the exhaust recirculation passage and the intake air amount downstream of the connection point is less than a third threshold value with the exhaust recirculation valve open, and when the second failure is estimated, the determination unit may be configured to set the exhaust recirculation valve opening degree set at the time of failure determination to be larger than when a failure other than the second failure is estimated (configuration 4). This configuration 4 may also be employed in the aspect of configuration 1 that does not have configuration 2.

[0015] In this configuration 4, if a second fault (i.e., a fault in which the exhaust recirculation gas flow rate is reduced below its intended flow rate due to clogging of the exhaust recirculation passage, etc.) is suspected, the opening of the exhaust recirculation valve during fault diagnosis is made larger than when another fault (i.e., a fault in which the exhaust recirculation gas flow rate is increased above its intended flow rate, etc.) is suspected. As a result, in the case of a fault in which the exhaust recirculation gas flow rate is reduced below its intended flow rate, a fault is more unlikely to be determined than in the case of other fault types, and a diagnosis is made according to the urgency of the fault.

[0016] In the aspect having the configuration 4, the failure type estimation unit can be configured to estimate a second failure when a state in which a difference between the intake air amount on the upstream side and the intake air amount on the downstream side of a connection portion of the intake passage with the exhaust gas recirculation passage is less than a third threshold and an engine rotation fluctuation is equal to or greater than a fourth threshold is measured a predetermined number of times (configuration 5). This allows for more accurate estimation of the second failure.

[0017] In the aspect having configuration 4, when the fault type estimation unit estimates both the first fault and the second fault, the determination unit can employ a configuration in which the opening degree of the exhaust gas recirculation valve set at the time of fault determination is reduced (configuration 6). This configuration 6 aims to prioritize the first fault determination in anticipation of a risk of both the first fault and the second fault. That is, the first fault is prioritized over the second fault as the fault type estimated by the fault type estimation unit.

[0018] In an aspect having the configuration 4, the determination unit can be configured to notify the occupant of the occurrence of a malfunction only when the first malfunction is determined (Configuration 7). This configuration 7 aims to prevent unnecessary notifications to the occupant by making it relatively more difficult to determine a malfunction in the case of a malfunction that reduces the flow rate of exhaust gas recirculation gas, since the amount of harmful components in the exhaust gas does not increase.

[0019] According to the present invention, it is possible to further improve the accuracy of failure determination in the exhaust gas recirculation device, and also to realize a substantial failure determination according to the type of failure.

[0020] Fig. 1 is a schematic diagram showing an embodiment of the present invention, illustrating the configuration of an engine equipped with an exhaust gas recirculation system and a failure detection device therefor. Fig. 2 is a graph showing an example of control according to the present invention. Fig. 3 is a graph showing an example of control according to the present invention. Fig. 4 is a graph showing an example of control according to the present invention. Fig. 5 is a graph showing an example of control according to the present invention. Fig. 6 is a graph for explaining the control according to the present invention. Fig. 7 is a graph for explaining the control according to the present invention. Fig. 8 is a flowchart showing the control according to the present invention.

[0021] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described with reference to the accompanying drawings, in which: Fig. 1 is a schematic diagram showing the configuration of an engine 1 equipped with an exhaust gas recirculation system and a failure detection device therefor according to the present invention;

[0022] The engine 1 is a four-cylinder engine for automobiles. As shown in FIG. 1 , the engine has four cylinders (combustion chambers) 2, each housing a piston, arranged in parallel. It also includes an intake passage 4 connected to an intake port that feeds air into each cylinder 2, an exhaust passage 5 connected to an exhaust port through which exhaust gas from each cylinder 2 is discharged, and an injector 10 that supplies fuel to each cylinder 2. Note that FIG. 1 only shows components and means directly related to the present invention; other components are omitted. Although the drawing shows an example with four cylinders 2, the number of cylinders 2 can be any number, and the cylinders do not need to be arranged in series. Furthermore, although the injector 10 injects fuel directly into each cylinder 2, it may also inject fuel into the intake port.

[0023] A throttle valve 3 for adjusting the flow area is provided upstream of the intake passage 4, making it possible to adjust the amount of intake air. In addition, various sensors (not shown) such as an air-fuel ratio sensor for detecting the air-fuel ratio in the exhaust passage 5 are provided in the exhaust passage 5. Furthermore, an exhaust purification device 13 equipped with a catalyst or the like for removing harmful substances in the exhaust is provided downstream in the exhaust passage 5, and further downstream of that, a muffler 15 and the like are provided.

[0024] Between the intake passage 4 and the exhaust passage 5 is provided an exhaust gas recirculation system 20 which includes an exhaust gas recirculation passage 21 which connects the intake passage 4 and the exhaust passage 5, and an exhaust gas recirculation valve 22 which opens and closes the exhaust gas recirculation passage 21. The exhaust gas recirculation passage 21 is connected to the intake passage 4 downstream of the throttle valve 3. Depending on the pressure state in the intake passage 4 caused by the opening and closing of the exhaust gas recirculation valve 22 and the throttle valve 3, a portion of the exhaust gas is introduced into the intake air as exhaust gas recirculation gas.

[0025] The devices necessary for the operation of the engine, including the throttle valve 3, the injector 10, and the exhaust gas recirculation valve 22, are each controlled by an electronic control unit 30 provided in the vehicle on which the engine 1 is mounted. Information from various sensors is also transmitted to the electronic control unit 30.

[0026] The electronic control unit 30 controls the throttle valve 3 to adjust the amount of intake air in the intake passage 4 and the amount of fuel injected into the cylinder 2 from the injector 10 through feedback control, thereby controlling the air-fuel ratio in the cylinder 2. The exhaust gas from each cylinder 2 is discharged into the exhaust passage 5, purified by the exhaust purification device 13, and then discharged through the muffler 15. The electronic control unit 30 also controls the opening and closing of the exhaust recirculation valve 22 and the pressure state in the intake passage 4 according to the driving conditions, thereby adjusting the amount of exhaust recirculation gas introduced into the intake air. The amount of exhaust recirculation gas introduced into the intake air may be adjusted through feedback control, which brings the actual exhaust recirculation gas ratio closer to a target exhaust recirculation gas ratio.

[0027] The electronic control unit 30 also has a function (device) for detecting a malfunction of the exhaust gas recirculation device 20. Specifically, the electronic control unit 30 has a malfunction type estimation unit 31 that estimates the type of malfunction occurring in the exhaust gas recirculation device 20, and a determination unit 32 that determines the presence or absence of a malfunction based on a change in the intake air amount when the opening degree of the exhaust gas recirculation valve 22 is changed during fuel cut operation. The information on the change in the intake air amount here also includes information on changes in the intake pressure and the charging efficiency in the intake passage 4. The intake air amount in the intake passage 4 can be determined, for example, from information from an intake air amount sensor 7 provided in the intake passage 4 upstream of the throttle valve 3 and downstream of the air cleaner 6, or a pressure sensor 8 provided in the intake manifold, as shown in FIG. 1 . Note that the sensors for determining the intake air amount in the intake passage 4 are not limited to the intake air amount sensor 7 and the pressure sensor 8, and their locations and number are not limited to those described above.

[0028] The fault type estimation unit 31 estimates a first fault, in which the flow rate of exhaust recirculation gas increases from the original flow rate, when the difference between the intake air amount upstream of the connection of the intake passage 4 with the exhaust recirculation passage 21 (information acquired by the intake air amount sensor 7) and the intake air amount downstream of the connection of the intake passage 4 with the exhaust recirculation passage 21 (information acquired by the pressure sensor 8) with the exhaust recirculation valve 22 closed is equal to or greater than a predetermined first threshold.Furthermore, the fault type estimation unit 31 estimates a second fault, in which the flow rate of exhaust recirculation gas decreases from the original flow rate, when the difference between the intake air amount upstream of the connection of the intake passage 4 with the exhaust recirculation passage 21 and the intake air amount downstream of the connection of the intake passage 4 with the exhaust recirculation passage 21 with the exhaust recirculation valve 22 open is less than a predetermined third threshold.

[0029] That is, the fault type estimation unit 31 acquires information on the difference in the intake air volume before and after the exhaust recirculation gas is mixed in the intake passage 4 in both a state where an instruction to control the introduction of exhaust recirculation gas is being issued (when the exhaust recirculation valve 22 is instructed to be open) and a state where such an instruction is not being issued (when the exhaust recirculation valve 22 is instructed to be fully closed), and estimates the type of fault that has occurred in the exhaust gas recirculation device 20. If the difference is large (equal to or greater than a first threshold value), a first fault is estimated, and if the difference is small (less than a third threshold value), a second fault is estimated.

[0030] Here, when estimating that a first failure has occurred in the exhaust gas recirculation device 20, if the difference between the intake air volume upstream of the connection point of the intake passage 4 with the exhaust gas recirculation passage 21 and the intake air volume downstream thereof is equal to or greater than a first threshold value, and in addition, if the rotation fluctuation of the engine 1 is measured a predetermined number of times to be equal to or greater than a predetermined second threshold value, then the first failure can be estimated more accurately.

[0031] Specifically, when the fault type estimation unit 31 estimates a first fault in the exhaust gas recirculation device 20, the fault diagnosis is first performed under the condition that the air-fuel ratio is stable near stoichiometry (theoretical air-fuel ratio) during stoichiometry feedback control in an idling state after warm-up is complete (hot idling). Then, the fault diagnosis estimates that a first fault has occurred in the exhaust gas recirculation device 20 if the following requirements (a) "Ec (charging efficiency) deviation determination" and (b) "poor combustion determination" are simultaneously met when a command to fully close the exhaust gas recirculation valve 22 is issued continuously for a predetermined period (e.g., 5 seconds): (a) Ec deviation amount = MAPS calculated Ec - AFS calculated Ec > first threshold value; (b) combustion index ΔNe < second threshold value.

[0032] The Ec deviation determination determines the difference between the intake air volume upstream and downstream of the connection between the intake passage 4 and the exhaust gas recirculation passage 21. The MAPS-calculated Ec is the charging efficiency calculated based on information from the pressure sensor 8. The AFS-calculated Ec is the charging efficiency calculated based on information from the intake air volume sensor 7 (see FIG. 3A). The Ec deviation is the difference between the MAPS-calculated Ec and the AFS-calculated Ec (see FIG. 3B). As shown in FIGS. 3A and 3B, the Ec deviation increases as the opening of the exhaust gas recirculation valve 22 increases and the amount of exhaust gas recirculation gas increases. In other words, if the Ec deviation is greater than the first threshold, it can be inferred that the exhaust gas recirculation valve 22 is wide open despite a command to fully close the exhaust gas recirculation valve 22 being issued. Here, the combustion index ΔNe is the variation in the engine speed of the engine 1, which corresponds to the rotational fluctuation of the engine 1 (see FIG. 3C). When the combustion state of the engine 1 deteriorates, the engine speed of the engine 1 decreases significantly (the rotational fluctuation increases), and the combustion index ΔNe falls below the second threshold. In this embodiment, as shown in FIG. 3C , the number of times the combustion index ΔNe falls below the second threshold is also counted, and if the number of times the combustion index ΔNe falls below the second threshold exceeds a predetermined number, it is determined that the requirement for the "poor combustion determination" in (b) is met. As the opening of the exhaust gas recirculation valve 22 increases and the amount of exhaust gas recirculation gas increases, the combustion state of the engine 1 deteriorates and the engine speed decreases significantly. In other words, when the combustion index ΔNe falls below the second threshold (the engine speed decreases significantly), it can be inferred that the exhaust gas recirculation valve 22 is wide open despite a command to fully close the exhaust gas recirculation valve 22 being issued. There are various methods for calculating the combustion index ΔNe. For example, the rotational fluctuation may be calculated from the output of a rotational speed sensor that acquires the engine speed of the engine 1.

[0033] Furthermore, when inferring that a second failure has occurred in the exhaust gas recirculation device 20, if the difference between the intake air amount upstream and downstream of the connection point of the intake passage 4 with the exhaust gas recirculation passage 21 is less than a third threshold and in addition the rotation fluctuation of the engine 1 is measured a predetermined number of times to be equal to or greater than a preset fourth threshold, the second failure can be inferred more accurately. Here, the third threshold may be the same value as the first threshold. Also, the fourth threshold may be the same value as the second threshold.

[0034] Specifically, when the fault type estimation unit 31 estimates a second fault in the exhaust gas recirculation device 20, the fault diagnosis is performed using, as a diagnostic start condition, that the air-fuel ratio is stable near stoichiometry during stoichiometric feedback control at hot idling. Then, the fault diagnosis estimates that a second fault has occurred in the exhaust gas recirculation device 20 if the following conditions (c) "Ec (charging efficiency) deviation judgment" and (d) "poor combustion judgment" are both not met when a command to open the exhaust gas recirculation valve 22 is issued continuously for a predetermined period (for example, 5 seconds). (c) Ec deviation amount = MAPS calculated Ec - AFS calculated Ec > third threshold (d) combustion index ΔNe < fourth threshold

[0035] In other words, if neither of requirements (c) nor (d) is met, a predetermined amount of exhaust gas recirculation gas should be introduced, but since no deviation in Ec has occurred and no fluctuation in the rotation of engine 1 has occurred, it is assumed that sufficient exhaust gas recirculation gas is not being introduced, and a second failure has occurred.

[0036] The method of estimating a failure by the failure type estimation unit 31 is not limited to the above example. For example, although the accuracy of the diagnosis is somewhat lower, a method of estimating whether the failure in the exhaust gas recirculation device 20 is a first failure or a second failure based only on the information on fluctuations in the intake air amount (intake manifold pressure) obtained by the pressure sensor 8 can also be considered.

[0037] Based on the result of the fault estimation by the fault type estimation unit 31, the determination unit 32 performs fault diagnosis by changing the valve opening set for the exhaust gas recirculation valve 22 in accordance with the estimated fault type. By changing the opening of the exhaust gas recirculation valve 22 during fault diagnosis in accordance with the fault type, the accuracy of fault determination can be improved.

[0038] Here, the judgment unit 32 judges that the difference ΔP = P' - P between the intake manifold pressure (P) when the exhaust gas recirculation valve 22 is instructed to be fully closed, i.e., the output value of the pressure sensor 8, and the intake manifold pressure (P') when the exhaust gas recirculation valve 22 is instructed to be open, is normal if it is equal to or greater than a predetermined value, and judges that there is a malfunction if the difference ΔP = P' - P is less than the predetermined value.

[0039] Specifically, when a first fault is estimated, the determination unit 32 reduces the opening of the exhaust gas recirculation valve 22 set during fault diagnosis, i.e., the opening of the exhaust gas recirculation valve 22 when acquiring the intake manifold pressure (P′), compared to when a first fault is not estimated. The opening (basic opening) of the exhaust gas recirculation valve 22 when acquiring the intake manifold pressure (P′) is determined based on a map including the relationship between the engine speed and the opening of the exhaust gas recirculation valve 22. In the event of a first fault, a correction is made to reduce the basic opening by reducing the opening of the exhaust gas recirculation valve 22 relative to the engine speed on the map. FIG. 2A shows an example in which the basic opening is reduced (an example in which the basic opening is smaller than the trapped opening) when the first fault is foreign matter lodged in the exhaust gas recirculation valve 22 (a fault in which the valve does not fully close). FIG. 2B shows an example in which the basic opening is increased (an example in which the basic opening is larger than the trapped opening) when the first fault is foreign matter lodged in the exhaust gas recirculation valve 22.

[0040] In FIG. 2A , when foreign matter is lodged (dashed line), the difference ΔP = P' - P between the intake manifold pressure (P) when the valve is fully closed and the intake manifold pressure (P') when the valve is open is minimal (almost zero), enabling a malfunction to be determined. On the other hand, in FIG. 2B , even when foreign matter is lodged (dashed line), the difference ΔP = P' - P exceeds a predetermined value, preventing a malfunction from being determined. Thus, when foreign matter is lodged, the smaller the base opening, the more likely a malfunction is to be determined. Furthermore, when the exhaust gas recirculation valve 22 is stuck open, regardless of the base opening setting, the difference ΔP = P' - P between the intake manifold pressure P when the exhaust gas recirculation valve 22 is instructed to be fully closed and the intake manifold pressure P' when the exhaust gas recirculation valve 22 is instructed to be open is minimal (almost zero). This allows a malfunction to be determined with high accuracy regardless of the base opening.

[0041] Furthermore, when a second fault is estimated, the determination unit 32 increases the opening of the exhaust gas recirculation valve 22 during fault diagnosis compared to when a second fault is not estimated. That is, when a second fault occurs, the determination unit 32 increases the opening of the exhaust gas recirculation valve 22 relative to the engine speed on the map as an increase correction of the basic opening. FIG. 2C shows an example in which the basic opening is reduced (e.g., exhaust gas recirculation gas does not flow even at the basic opening) due to clogging of the exhaust gas recirculation passage 21 by foreign matter (a fault in which foreign matter obstructs the passage of exhaust gas recirculation gas). FIG. 2D shows an example in which the basic opening is increased (e.g., exhaust gas recirculation gas flows at the basic opening) due to clogging of the exhaust gas recirculation passage 21 by foreign matter (dashed line). In FIG. 2C, when a foreign matter is clogged, the difference ΔP = P' - P becomes extremely small (almost zero), and a fault is determined. On the other hand, in FIG. 2D, even when a foreign matter is clogged (dashed line), the difference ΔP = P' - P exceeds a predetermined value, and a fault is not determined.

[0042] This allows for a diagnosis according to the degree of urgency of the failure. In other words, whether a first failure or a second failure is suspected, a smaller basic opening makes it easier to determine that there is a failure, but in the case of a second failure in which the flow rate of exhaust gas recirculation gas decreases, the degree of urgency of the failure is low because the exhaust gas does not deteriorate due to this alone, and the requirement for failure determination is deliberately set higher by increasing the basic opening.

[0043] In contrast, a first failure may cause the exhaust gas recirculation gas flow rate to increase above the normal flow rate, which may lead to unstable combustion and deterioration of exhaust gas. For this reason, when a first failure is suspected, a correction is made to reduce the basic opening, making it easier to determine whether a failure has occurred.

[0044] Furthermore, the determination unit 32 notifies the occupant of the occurrence of a malfunction only when it determines that a first malfunction has occurred, and does not notify the occupant when it determines that a second malfunction has occurred, thereby preventing unnecessary notifications from being made to the occupant.

[0045] The fault diagnosis is performed while the fuel to the engine 1 is being cut off (while the vehicle is decelerating), but according to the above method, the opening of the exhaust gas recirculation valve 22 is reduced during the fault diagnosis, which increases the rotational resistance of the engine 1 and ensures a sense of deceleration of the vehicle due to the fuel cut.

[0046] 4 is a flowchart relating to this control. Control is started in step S1, and the counter is reset (Nm=0) in step S2. In step S3, it is determined whether or not fault diagnosis is possible. The conditions for determining that fault diagnosis is possible are that the engine 1 is in a fuel cut-off state and that the charging efficiency and rotation speed of the engine 1 are within a predetermined range. If fault diagnosis is possible, the process proceeds to step S4; if not, step S3 is repeatedly applied.

[0047] In step S4, an instruction is issued to fully close the exhaust gas recirculation valve 22, and the process proceeds to step S5, where the intake manifold pressure (P) is measured. The intake manifold pressure (P) is obtained by the pressure sensor 8 while the exhaust gas recirculation valve 22 is in the fully closed state. Then, in step S6, it is determined whether or not a first or second fault has already been estimated in the previous stage of this control, and whether or not the first or second fault has occurred.

[0048] If either the first or second fault has already been estimated at step S6, the process proceeds to step S8, where the opening of the exhaust gas recirculation valve 22 during fault diagnosis is corrected (exhaust gas recirculation valve fault opening). As described above, the opening is decreased or increased in the case of the first fault, and increased in the case of the second fault. If the fault type estimation unit 31 estimates both the first fault and the second fault, the determination unit 32 reduces the opening of the exhaust gas recirculation valve 22 set during fault diagnosis. That is, when there is a concern about both the first fault and the second fault, the determination unit 32 prioritizes making it easier to determine the first fault, taking a safer approach. If it is estimated at step S6 that neither the first fault nor the second fault has occurred, the process proceeds to step S7, where fault diagnosis is performed at the normal opening (normal diagnostic opening). Note that even if neither the first fault nor the second fault has been estimated, the process proceeds to step S7, where fault diagnosis is performed at the normal opening.

[0049] Next, the process proceeds to step S9, where the intake manifold pressure (P') is measured. Then, in step S10, the difference ΔP between the intake manifold pressure (P) and the intake manifold pressure (P') is calculated, ΔP=P'-P.

[0050] In step S11, if the difference ΔP = P' - P is equal to or greater than a predetermined value, the process proceeds to step S15, where the counter value is maintained at its current value. In the following step S16, it is determined that the exhaust gas recirculation device 20 is normal, and control ends in step S17. Also, in step S11, if the difference ΔP = P' - P is less than a predetermined value, the process proceeds to step S12, where the counter value is incremented by one (Nm = Nm + 1). Then, steps S3 to S12 are repeated, and if the counter value reaches 3 (Nm = 3) in step S13, it is determined that a failure determination has been performed three times, and a failure (first failure or second failure) is determined to have occurred in the following step S14, and control ends in step S17. The counter value for determining that a failure has occurred is not limited to 3 and may be any suitable value.

[0051] In the above embodiment, the configuration of the present invention has been described using an engine 1 without a turbocharger as an example, but the present invention may also be applied to an engine 1 with a turbocharger. Furthermore, the engine 1 may be provided with, as the exhaust gas recirculation device 20, a selective high-pressure exhaust gas recirculation device that includes an exhaust gas recirculation passage 21 connecting the downstream side of the turbocharger compressor and the upstream side of the turbine and treats relatively high-pressure exhaust gas, or a low-pressure exhaust gas recirculation device that includes an exhaust gas recirculation passage 21 connecting the upstream side of the turbocharger compressor and the downstream side of the turbine and treats relatively low-pressure exhaust gas, or both. Furthermore, the engine 1 to which the present invention can be applied may be a gasoline engine, a diesel engine, or the like.

[0052] REFERENCE SIGNS LIST 1 Engine 2 Combustion chamber (cylinder) 3 Throttle valve 4 Intake passage 5 Exhaust passage 6 Air cleaner 7 Intake pressure sensor 8 Pressure sensor 10 Injection device (in-cylinder injection device) 13 Exhaust purification device 20 Exhaust gas recirculation device 21 Exhaust gas recirculation passage 22 Exhaust gas recirculation valve 30 Electronic control unit 31 Failure type estimation section 32 Determination section

Claims

1. A fault detection device for an exhaust gas recirculation system comprising an exhaust gas recirculation passage communicating between an exhaust passage and an intake passage of an engine, and an exhaust gas recirculation valve provided in the exhaust gas recirculation passage, the fault detection device comprising: a fault type estimation unit which estimates a type of fault that has occurred in the exhaust gas recirculation system, and a judgment unit which judges the presence or absence of a fault from a change in intake pressure when the opening degree of the exhaust gas recirculation valve is switched during fuel cut operation, the judgment unit determining the presence or absence of a fault by changing the valve opening degree set for the exhaust gas recirculation valve in accordance with the type of fault estimated by the fault type estimation unit during fault diagnosis.

2. A fault detection device for an exhaust gas recirculation system as described in claim 1, wherein the fault type estimation unit estimates a first fault in which the flow rate of exhaust gas recirculation gas increases above its original flow rate when the difference between the intake air volume upstream and downstream of the connection of the intake passage with the exhaust gas recirculation passage is equal to or greater than a first threshold value with the exhaust gas recirculation valve closed, and wherein the judgment unit, when it estimates the first fault, sets the valve opening set for the exhaust gas recirculation valve to a smaller value compared to when the first fault is not estimated.

3. A fault detection device for an exhaust gas recirculation system as described in claim 2, wherein the fault type estimation unit estimates a first fault to have occurred when a state in which the difference between the intake air volume upstream and downstream of the connection of the intake passage with the exhaust gas recirculation passage is equal to or greater than a first threshold value and engine rotation fluctuation is equal to or greater than a second threshold value is measured a predetermined number of times.

4. A fault detection device for an exhaust gas recirculation system as described in claim 2 or 3, wherein the fault type estimation unit estimates a second fault in which the flow rate of exhaust recirculation gas is reduced from the original flow rate when the difference between the intake air volume upstream and downstream of the connection of the intake passage with the exhaust recirculation passage is less than a third threshold value with the exhaust gas recirculation valve open, and when the second fault is estimated, the judgment unit sets the opening degree of the exhaust gas recirculation valve, which is set during fault diagnosis, to a larger value compared to when a fault other than the second fault is estimated.

5. A fault detection device for an exhaust gas recirculation system as described in claim 4, wherein the fault type estimation unit estimates a second fault to have occurred when a state in which the difference between the intake air volume upstream and downstream of the connection of the intake passage with the exhaust gas recirculation passage is less than a third threshold value and the engine rotation fluctuation is equal to or greater than a fourth threshold value is measured a predetermined number of times.

6. A fault detection device for an exhaust gas recirculation system as described in claim 4 or 5, wherein when the fault type estimation unit estimates both the first fault and the second fault, the judgment unit reduces the opening degree of the exhaust gas recirculation valve that is set during fault diagnosis.

7. A fault detection device for an exhaust gas recirculation system as set forth in any one of claims 4 to 6, wherein the determination unit notifies an occupant of the occurrence of a fault only when the first fault is determined.

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