JP4605510B2 - Exhaust gas recirculation device for internal combustion engine - Google Patents

Description

  The present invention relates to an exhaust gas recirculation (EGR) device for an internal combustion engine, and more particularly to an exhaust gas recirculation device for an internal combustion engine equipped with an EGR cooler.

  In general, in an EGR device used to reduce NOx in exhaust gas of an internal combustion engine, introduction of recirculated exhaust gas (EGR gas) reduces the amount of fresh air, so that smoke is likely to be generated particularly in diesel engines. It is known. Therefore, in order to cope with this, an EGR cooler is provided, and the hot EGR gas is cooled to reduce the volume, thereby increasing the amount of fresh air and suppressing the generation of smoke.

  As an EGR device provided with such an EGR cooler, for example, as disclosed in Patent Document 1, an exhaust gas recirculation passage that connects an exhaust passage and an intake passage of an engine is provided with a cooling passage and a cooling passage provided with an EGR cooler. 2. Description of the Related Art A bypass passage that bypasses and an opening / closing valve for regulating the inflow of EGR gas to the EGR cooler is known on the upstream side of the EGR cooler in the cooling passage.

  By the way, in such an EGR cooler, “soot” in the EGR gas, unburned hydrocarbon (unburned HC), etc. adheres to or accumulates on the heat exchange part due to its long-term use, and cooling performance is improved. It is known that a decrease, in other words, a cooling abnormality occurs. Therefore, Patent Document 2 and Patent Document 3 disclose an exhaust gas recirculation device for an internal combustion engine provided with a device for detecting such a cooling abnormality.

  Here, what is disclosed in Patent Document 2 is provided with first and second temperature sensors or pressure sensors on the upstream side and the downstream side of the EGR cooler, respectively, and the difference between the detection values of both sensors is a predetermined value or more. At this time, an alarm is given as an abnormal cooling. However, the exhaust gas recirculation device disclosed in Patent Document 2 does not include a bypass passage that bypasses the EGR cooler.

  Moreover, what was disclosed by patent document 3 is equipped with the temperature sensor which measures the outgas temperature of an EGR cooler, and the temperature measured from the EGR cooler outgas temperature in the normal time estimated from an engine operating state is constant. When the value is higher than the value, it is determined that the cooling is abnormal.

JP-A-11-280565 JP 2001-289125 A JP 2003-336549 A

  However, the one disclosed in Patent Document 2 described above is provided with two first and second temperature sensors or pressure sensors on the upstream side and the downstream side of the EGR cooler, respectively. Have the problem of up.

  On the other hand, what is disclosed in Patent Document 3 is only one temperature sensor for measuring the temperature of the gas discharged from the EGR cooler, which is effective in terms of avoiding the cost increase described above, but when the gas discharged from the EGR cooler is normal. Since this temperature depends on the estimated value from the operating state of the engine, there is a risk that the determination is not reliable and a misdiagnosis may be performed. This is because the temperature of the EGR gas fluctuates due to not only the operating state of the engine but also the influence of the traveling speed of the vehicle and the engine temperature.

  The present invention has been made in view of the above-described problems, and an object thereof is to provide an exhaust gas recirculation device for an internal combustion engine that can more accurately diagnose an EGR cooler cooling abnormality.

  An exhaust gas recirculation device for an internal combustion engine according to an embodiment of the present invention for achieving the above object includes an EGR control valve that controls an exhaust gas circulation amount in an exhaust gas recirculation passage that connects an exhaust passage and an intake passage of the engine. And an EGR cooler that cools the recirculated exhaust gas, and a bypass passage that bypasses the EGR cooler and a switching control valve that controls the inflow into the bypass passage. Temperature detecting means provided on the downstream side of the switch, switching control valve operating means for switching the switching control valve at a predetermined time, and the temperature detecting means before and after switching operation of the switching control valve by the switching control valve operating means Diagnostic means for diagnosing a cooling abnormality of the EGR cooler based on the temperature difference detected by

  According to the exhaust gas recirculation device for an internal combustion engine according to an aspect of the present invention, when the switching control valve is switched at a predetermined time by the switching control valve operating means, the EGR cooler provided in the exhaust gas recirculation passage is passed. The recirculated exhaust gas is switched so as to flow into a bypass passage that bypasses the EGR cooler. Then, the temperature of the recirculated exhaust gas before and after the switching operation is detected by temperature detecting means provided on the downstream side of the switching control valve. Specifically, the temperature of the recirculated exhaust gas that has passed through the EGR cooler and the temperature of the recirculated exhaust gas that has passed through the bypass passage and does not pass through the EGR cooler are detected. Furthermore, the cooling abnormality of the EGR cooler is diagnosed by the diagnostic means based on the temperature difference between the two. Specifically, when the temperature difference between before and after the switching operation of the switching control valve is large, it is a result of cooling the recirculated exhaust gas through the EGR cooler, and it is diagnosed that the EGR cooler is functioning normally. . On the contrary, when the temperature difference before and after that is small, it is a result that the recirculated exhaust gas has not been cooled despite passing through the EGR cooler, and it is diagnosed that the EGR cooler is abnormal in cooling.

  Thus, according to the above configuration, the cooling abnormality of the EGR cooler can be accurately diagnosed while only one temperature detecting means is provided on the downstream side of the switching control valve.

  Here, the predetermined time is preferably when the engine is in a steady state.

  According to this aspect, since the amount and temperature of the recirculated exhaust gas are substantially constant as compared with when the engine is in a transient state, it is possible to more accurately diagnose the cooling abnormality of the EGR cooler.

  An exhaust gas recirculation device for an internal combustion engine according to another embodiment of the present invention for achieving the above object controls an exhaust gas circulation amount in an exhaust gas recirculation passage that connects the exhaust passage and the intake passage of the engine. In an exhaust gas recirculation device for an internal combustion engine, which includes an EGR control valve and an EGR cooler that cools the recirculated exhaust gas, a bypass passage that bypasses the EGR cooler, and a switching control valve that controls the inflow to the bypass passage. An intake air amount measuring means for measuring an air amount, an EGR control valve fully opening means for fully opening the EGR control valve when the vehicle decelerates, and a switching control valve operating means for switching the switching control valve at a predetermined time during vehicle deceleration The intake air amount measured by the intake air amount measuring means before and after the switching operation of the switching control valve by the switching control valve operating means. Based on, characterized in that it comprises a diagnostic means for diagnosing a cooling abnormality of the EGR cooler.

  According to the exhaust gas recirculation device for an internal combustion engine according to another aspect of the present invention, the EGR control valve is fully opened by the EGR control valve fully opening means when the vehicle is decelerated, and the switching control valve operating means is at a predetermined time during the vehicle deceleration. When the switching control valve is switched, the recirculated exhaust gas that has passed through the EGR cooler provided in the exhaust gas recirculation passage is switched so as to flow into the bypass passage that bypasses the EGR cooler. Then, the intake air amount before and after the switching operation is measured by the intake air amount measuring means. Specifically, the intake air amount when the recirculated exhaust gas passes through the EGR cooler and the intake air amount when the recirculated exhaust gas passes through the bypass passage and does not pass through the EGR cooler are detected. Furthermore, the cooling abnormality of the EGR cooler is diagnosed by the diagnostic means based on the difference between the intake air amounts. Specifically, when the difference in the intake air amount before and after the switching operation of the switching control valve is large, the result is that the recirculated exhaust gas passes through the EGR cooler and is cooled to increase the density, that is, the volume decreases and the intake air amount increases. It is diagnosed that the EGR cooler is functioning normally. On the contrary, when the difference in the intake air amount before and after that is small, there is no change in density or volume due to the recirculated exhaust gas passing through the EGR cooler, and there is almost no change in the intake air amount. Yes, the EGR cooler is diagnosed as having a cooling abnormality.

  Thus, according to the above configuration, it is possible to accurately diagnose the cooling abnormality of the EGR cooler while using an existing apparatus without using the temperature detecting means.

  The intake air amount measured by the intake air amount measuring means is preferably the intake air amount per one rotation of the engine.

  According to this aspect, it is possible to make a comparison with the intake air amount that does not depend on the engine speed when the vehicle is decelerated, and it is possible to more accurately diagnose the cooling abnormality of the EGR cooler.

  Hereinafter, specific embodiments of an exhaust gas recirculation device for an internal combustion engine according to the present invention will be described with reference to the drawings.

  FIG. 1 is a side cross-sectional schematic diagram showing a schematic configuration of an engine 100 to which an exhaust gas recirculation apparatus for an internal combustion engine according to an embodiment of the present invention is applied and an intake / exhaust system thereof. The engine 100 shown in FIG. 1 is a water-cooled four-cycle diesel engine.

  The engine 100 is provided with a fuel injection valve 104 that injects light oil of fuel into the cylinder 102. Further, an intake passage 106 communicates with the engine 100. The intake passage 106 is provided with an intake throttle valve 108 that adjusts the flow rate of intake air flowing through the intake passage 106. An air flow meter 110 as an intake air amount measuring means for outputting a signal corresponding to the intake air amount passing through the intake passage 106 is attached in the middle of the intake passage 106 upstream of the intake throttle valve 108. . The intake fresh air amount of the engine 100 can be obtained from the output signal of the air flow meter 110.

  On the other hand, an exhaust passage 112 is communicated with the engine 100. The exhaust passage 112 communicates with the atmosphere downstream.

  Further, the engine 100 is provided with an exhaust gas recirculation device 120 (hereinafter referred to as an EGR device 120). The EGR device 120 is connected to an exhaust gas recirculation passage 122 (hereinafter referred to as an EGR passage 122) that communicates the exhaust passage 112 and the intake passage 106 of the engine 100 with an EGR control valve 124 and a recirculation exhaust. An EGR cooler 126 that cools the gas is provided, and a bypass passage 128 that bypasses the EGR cooler 126 and a switching control valve 129 that controls inflow into the bypass passage 128 are provided. In the present embodiment, the switching control valve 129 is provided at the junction of the bypass passage 128 and the EGR passage 122 on the downstream side of the EGR cooler 126, and further, the EGR control valve 124 on the downstream side of the switching control valve 129. Is provided with a temperature sensor 130 as temperature detecting means. The EGR passage 122 communicates with the exhaust passage 112 and the intake passage 106, and a part of the exhaust gas (hereinafter also referred to as EGR gas) flowing through the exhaust passage 112 through the EGR passage 122 is referred to as an intake passage. Reflux or recycle to 106.

  In the EGR passage 122, an EGR cooler 126, a switching control valve 129, a temperature sensor 130, and an EGR control valve 124 are arranged in this order from the exhaust passage 112 side. One end of the bypass passage 128 communicates with the EGR passage 122 upstream of the EGR cooler 126, and the other end communicates with the EGR passage 122 downstream of the EGR cooler 126 where the switching control valve 129 is disposed.

  For example, cooling water for the engine 100 circulates in the EGR cooler 126, and heat exchange is performed between the EGR gas and the cooling water for the engine 100 in the EGR cooler 126 to cool the EGR gas. The switching control valve 129 is a valve that operates according to a signal from the ECU 200 described later, and changes the ratio of the amount of EGR gas that passes through the EGR cooler 126 and the bypass passage 128 with respect to the total amount of EGR gas. Here, the change in the ratio of the EGR gas amount includes the case of complete switching. The temperature sensor 130 outputs a signal corresponding to the temperature of the EGR gas flowing through the EGR passage 122. The EGR control valve 124 is opened and closed by a signal from the ECU 200 described later, and adjusts the flow rate of the EGR gas flowing through the EGR passage 122.

  The engine 100 is also provided with an ECU 200 that is an electronic control unit for controlling the engine 100. The ECU 200 is a unit that controls the operating state of the engine 100 in accordance with the operating conditions of the engine 100 and the driver's request.

  In addition to the air flow meter 110 and the EGR gas temperature sensor 130, the ECU 200 includes an accelerator opening sensor 140 that outputs a signal corresponding to the accelerator opening, an engine speed sensor (not shown), a vehicle speed sensor, a cooling water temperature sensor, and the like. Connected via wiring, an output signal from the sensor or the like is input. The required load given to the engine 100 can be obtained by the accelerator opening sensor 140. It is possible to obtain a load applied to engine 100 based on the amount of fuel injected from fuel injection valve 104.

  On the other hand, the fuel injection valve 104, the intake throttle valve 108, the switching control valve 129, and the EGR control valve 124 are connected to the ECU 200 via electric wiring, and these can be controlled by the ECU 200. Then, the intake air amount provided for engine combustion including the EGR gas amount can be adjusted by opening / closing the intake throttle valve 108.

  In the present embodiment configured as described above, an example of a control procedure for diagnosing a cooling abnormality of the EGR cooler 126 will be described with reference to the flowchart of FIG. Note that this diagnosis is executed at least once in a so-called one trip from when the engine 100 is started to when it is stopped, for example, at a relatively early time after completion of warm-up of the engine 100 or several times at a predetermined cycle. It only has to be done. Therefore, when diagnosis is normally completed, useless diagnosis during one trip may be avoided by setting a diagnosis completion flag. The timing or number of times of the diagnosis can be set in advance in a program in the ECU 200.

  Therefore, when the diagnosis starts at the time set in the ECU 200, the switching control valve 129 is controlled in step S201 so that the EGR gas flows to the EGR cooler 126 in the normal running state. Then, in the next step S202, it is determined whether or not the EGR gas flow rate is sufficient. If it is not sufficient, this diagnostic routine is once terminated. The determination of whether or not this is sufficient can be made based on a preset EGR gas amount corresponding to the operating state or the like. For example, the opening degree of the EGR control valve 124 or the fuel injection correlated with the EGR gas amount. This can be done on a quantity basis. Further, the process proceeds when the EGR gas flow rate is sufficient. In step S203, it is determined whether or not the engine is in a steady state. If not, the diagnosis routine is once terminated. The determination as to whether or not the engine is in a steady state depends on fluctuations in the load of the engine 100, for example, whether or not the difference between the fuel injection amount in the previous routine cycle and the fuel injection amount in the current routine cycle is equal to or less than a predetermined value. Can be done.

  When the engine is in a steady state, the process proceeds to step S204, and the temperature TA of the EGR gas that has passed through the EGR cooler 126 is measured and stored by the temperature sensor 130.

  Next, the process proceeds to step S205, and the switching control valve 129 is controlled to be switched so that the entire amount of EGR gas that has flowed to the EGR cooler 126 flows to the bypass passage 128. That is, EGR gas is prevented from passing through the EGR cooler 126. Then, in the next step S206, it is determined whether or not the engine state has changed. When there is a change, that is, when the steady state obtained in step S203 no longer exists, this diagnostic routine is temporarily terminated.

  When there is no change in the state of the engine, the process proceeds to step S207, and the temperature TB of the EGR gas that passes through the bypass passage 128 and does not pass through the EGR cooler 126 is measured and stored by the temperature sensor 130.

  Further, in the next step S208, the temperature difference (ΔT = | TA−TB |) between the temperature TA of the EGR gas measured and stored in step S204 and the temperature TB of the EGR gas measured and stored in step S207. ) Is obtained and compared with a predetermined threshold value α to diagnose whether or not there is a cooling abnormality. That is, when the temperature difference ΔT is small without exceeding the predetermined threshold value α, it is a result that the EGR gas has not been cooled even though it has passed through the EGR cooler 126, and it is assumed that the EGR cooler 126 is abnormal in cooling. Proceed to, and diagnosis of abnormality is performed. On the other hand, when the temperature difference ΔT is larger than the predetermined threshold value α, it is a result of the EGR gas passing through the EGR cooler 126 and being cooled, and it is determined that the EGR cooler 126 is functioning normally, and the process proceeds to step S210. Diagnosed. Note that an alarm may be issued to perform a predetermined treatment when an abnormality is diagnosed.

  Here, FIG. 3 is a time chart showing a state of change in the control amount of the related part and a state of change in the EGR gas temperature measured by the temperature sensor 130 in the above-described diagnosis procedure. 3A shows the fuel injection amount from the fuel injection valve 104, FIG. 3B shows the opening of the EGR control valve 124, FIG. 3C shows the operating position of the switching control valve 129, and FIG. The temperature T of the EGR gas is shown.

  As is apparent from FIG. 3, the diagnosis routine is executed in a steady engine state in which the fuel injection amount from the fuel injection valve 104 and the opening degree of the EGR control valve 124 are constant, and the EGR gas flows through the EGR cooler 126 side. At a predetermined time ta while the switching control valve 129 is positioned, the temperature TA of the EGR gas that has passed through the EGR cooler 126 is measured by the temperature sensor 130 and stored. Further, at a predetermined time tb while the switching control valve 129 is positioned so that the EGR gas flows on the bypass passage 128 side, the temperature TB of the EGR gas that has passed through the bypass passage 128 without passing through the EGR cooler 126 is the temperature. Measured by the sensor 130 and stored. Then, although not shown in FIG. 3, a diagnosis is made as to whether or not the above-described EGR cooler 126 is functioning normally.

  In FIG. 3, a state in which the EGR cooler 126 in which the temperature difference ΔT = | TA−TB | has a predetermined magnitude is functioning normally is indicated by a solid line (a broken line when cooling is abnormal). ing.

  Next, another embodiment of the present invention will be described with reference to FIG. 1, FIG. 4, and FIG. In the previous embodiment, the cooling abnormality of the EGR cooler 126 is diagnosed using the temperature sensor 130, but in this embodiment, the diagnosis is performed without using such a temperature sensor. Since the hardware is the same as that of the previous embodiment, a duplicate description is avoided.

  Therefore, as in the previous embodiment, when the diagnosis starts at the time set in the ECU 200, the switching control valve 129 is controlled so that the EGR gas flows through the EGR cooler 126 in the normal running state in step S401. In the next step S402, it is determined whether or not the vehicle is in a decelerating state. If the vehicle is not in a decelerating state, the diagnosis routine is temporarily terminated. As is well known, the determination as to whether or not the vehicle is in a deceleration state can be made based on the accelerator opening, the engine speed, the vehicle speed, the fuel injection amount, and the like. Further, when the vehicle is decelerating, the process proceeds to step S403, and the EGR control valve 124 is fully opened. Then, the intake air amount GnA when in this deceleration state is measured and stored based on the output value of the air flow meter 110.

  Thereafter, in step S405, the switching control valve 129 is temporarily t0 so that the EGR gas that has passed through the EGR cooler 126 provided in the exhaust gas recirculation passage 122 flows into the bypass passage 128 that bypasses the EGR cooler 126. In the meantime, the switching operation is performed. In the next step S406, the intake air amount GnB after the switching operation of the switching control valve 129 is measured by the air flow meter 110 and stored.

  Further, in the next step S407, the difference in intake air amount (ΔGn = | GnA−GnB |) between the intake air amount GnA measured and stored in step S404 and the intake air amount GnB measured and stored in step S406. ) Is obtained and compared with a predetermined threshold value β to diagnose whether or not there is a cooling abnormality. That is, when the intake air amount difference ΔGn is small without exceeding the predetermined threshold value β, it is a result that the EGR gas has not been cooled despite passing through the EGR cooler 126, and the EGR cooler 126 is abnormal in cooling. In step S408, an abnormality is diagnosed. On the other hand, when the intake air amount difference ΔGn exceeds the predetermined threshold β and is large, this is a result of the EGR gas passing through the EGR cooler 126 and being cooled, and the process proceeds to step S409 assuming that the EGR cooler 126 is functioning normally. Diagnosed as normal.

  Here, FIG. 5 is a time chart showing the state of change in the control amount of the relevant part and the state of change in the intake air amount Gn measured by the air flow meter 110 in the above-described diagnosis procedure. 5A shows the fuel injection amount from the fuel injection valve 104, FIG. 5B shows the opening degree of the EGR control valve 124, FIG. 5C shows the operating position of the switching control valve 129, and FIG. The temperature T, (E) of the EGR gas to be shown indicates the intake air amount Gn measured by the air flow meter 110.

  As is apparent from FIG. 5, the diagnosis routine according to the present embodiment is executed in a deceleration state of the vehicle in which the fuel injection amount from the fuel injection valve 104 is stopped and the opening degree of the EGR control valve 124 is fully open. The intake air amount GnA is measured by the air flow meter 110 at a predetermined time tc while the switching control valve 129 is positioned so that the EGR gas flows on the EGR cooler 126 side. Then, at a subsequent predetermined time td, the switching control valve 129 is temporarily set so that the EGR gas that has passed through the EGR cooler 126 provided in the exhaust gas recirculation passage 122 flows into the bypass passage 128 that bypasses the EGR cooler 126. Therefore, the switching operation is performed for t0. Further, the intake air amount GnB is measured by the air flow meter 110 at a predetermined time te during the temporary switching operation. Then, although not shown in FIG. 5, a diagnosis is made as to whether or not the above-described EGR cooler 126 is functioning normally.

  Here, a supplementary explanation will be given as to why the cooling abnormality of the EGR cooler 126 can be diagnosed based on whether or not the intake air amount difference ΔGn exceeds the predetermined threshold value β. In the present embodiment, in the deceleration state of the vehicle, In addition, since the opening of the EGR control valve 124 is executed in a fully opened state, the intake air amount sucked into the cylinder 102 of the engine 100 at this time is determined by the flow resistance of the exhaust gas recirculation passage 122 or the like. And the amount of fresh air passing through the air flow meter 110 is added. Accordingly, the fresh air amount decreases as the EGR gas amount sucked into the cylinder 102 of the engine 100 increases, and the fresh air amount increases as the EGR gas amount decreases.

  By the way, when the EGR gas is caused to flow through the EGR cooler 126 and the cooling function of the EGR cooler 126 is normally performed, the EGR gas has a high density, that is, its volume is reduced by the cooling action. At this time, the amount of intake air sucked into the cylinder 102 of the engine 100 is constant per one rotation of the engine 100 in the deceleration state of the vehicle, so that the amount of fresh air increases by the amount of decrease in the volume of the EGR gas. On the other hand, when the EGR gas is not flowing through the bypass passage 128 and when the cooling function of the EGR cooler 126 is not exerted, the density of the EGR gas remains small and the volume remains large. It will decrease.

  Here, since the amount of fresh air passing through the air flow meter 110 is measured by the air flow meter 110 as the intake air amount Gn, as described above, there is a cooling abnormality of the EGR cooler 126 based on the change in the intake air amount Gn. It can be diagnosed. According to the present embodiment in which the intake air amount that is the fresh air amount measured by the air flow meter 110 is the intake air amount Gn per one rotation of the engine 100, the intake air that does not depend on the engine speed during vehicle deceleration. Comparison with the amount of air is possible, and a cooling abnormality of the EGR cooler 126 can be diagnosed more accurately.

  As a modification of the other embodiment described above, the intake air amount measured by the air flow meter 110 as shown in steps S404, S406 and S407 of the flowchart of FIG. 4 and FIG. 5D. Diagnosing a cooling abnormality of the EGR cooler 126 using the temperature sensor 130 that measures the temperature T of the EGR gas (TA and TA before and after switching) without using Gn or in combination with Gn. You can also When measuring the temperature T of the EGR gas, the temperature TA of the EGR gas that has passed through the EGR cooler 126 at a predetermined time tf while the switching control valve 129 is positioned so that the EGR gas flows on the EGR cooler 126 side. Is measured by the temperature sensor 130, and the EGR that has passed through the bypass passage 128 at a predetermined time te during which the switching control valve 129 is temporarily switched for t0 so that EGR gas flows through the bypass passage 128 side. The gas temperature TB may be measured by the temperature sensor 130. The temperature difference ΔT = | TA−TB | is compared with the predetermined threshold value γ, which is the same as in the above-described embodiment. In this way, a more accurate diagnosis can be made.

1 is a schematic side sectional view showing an embodiment of an exhaust gas recirculation device for an internal combustion engine according to the present invention. It is a flowchart which shows an example of the diagnostic procedure in embodiment of this invention. It is a time chart for demonstrating the effect | action of embodiment of this invention. It is a flowchart which shows an example of the diagnostic procedure in other embodiment of this invention. It is a time chart for demonstrating the effect | action of other embodiment of this invention.

Explanation of symbols

100 Engine 106 Intake passage 110 Air flow meter (intake air amount measuring means)
112 Exhaust passage 120 Exhaust gas recirculation device (EGR device)
122 Exhaust gas recirculation passage (EGR passage)
124 EGR control valve 126 EGR cooler 128 Bypass passage 129 Switching control valve 130 Temperature sensor (temperature detection means)
140 Accelerator position sensor 140

Claims (4)

An exhaust gas recirculation passage communicating the exhaust passage and the intake passage of the engine is provided with an EGR control valve for controlling the exhaust gas circulation amount and an EGR cooler for cooling the recirculated exhaust gas, and a bypass passage for bypassing the EGR cooler; In an exhaust gas recirculation device for an internal combustion engine comprising a switching control valve for controlling the inflow into the bypass passage,
Temperature detecting means provided downstream of the switching control valve;
Switching control valve actuating means for switching the switching control valve at a predetermined time; and
Based on a difference in temperature detected by the temperature detecting means before and after the switching operation of the switching control valve by the switching control valve operating means and a predetermined threshold value that can determine that the EGR cooler is functioning normally. Diagnostic means for diagnosing cooling abnormality of the EGR cooler,
An exhaust gas recirculation device for an internal combustion engine, comprising:   2. The exhaust gas recirculation device for an internal combustion engine according to claim 1, wherein the predetermined time is when the engine is in a steady state. An exhaust gas recirculation passage communicating the exhaust passage and the intake passage of the engine is provided with an EGR control valve for controlling the exhaust gas circulation amount and an EGR cooler for cooling the recirculated exhaust gas, and a bypass passage for bypassing the EGR cooler; In an exhaust gas recirculation device for an internal combustion engine comprising a switching control valve for controlling the inflow into the bypass passage,
An intake air amount measuring means for measuring the intake air amount;
EGR control valve fully opening means for fully opening the EGR control valve when the vehicle decelerates;
Switching control valve operating means for switching the switching control valve at a predetermined time during vehicle deceleration;
Diagnosing means for diagnosing abnormal cooling of the EGR cooler based on a difference in intake air amount measured by the intake air amount measuring means before and after switching operation of the switching control valve by the switching control valve operating means;
An exhaust gas recirculation device for an internal combustion engine, comprising:   The exhaust gas recirculation device for an internal combustion engine according to claim 3, wherein the intake air amount measured by the intake air amount measuring means is an intake air amount per one rotation of the engine.

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