JP2005127137A - Egr system of engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To change quantity of heat that engine cooling water must absorb by an EGR cooler and temperature of exhaust gas after passing through a turbocharger. <P>SOLUTION: This EGR system of the engine is provided with the EGR cooler 16 cooling second exhaust gas of the engine 1 having the turbocharger 6 driven by first exhaust gas by engine cooling water and absorbing again and a heat exchange means 15 for heat-exchanging the second exhaust gas before passing through the EGR cooler and the first exhaust gas after passing through the turbocharger. Preferably, this system is provided with control means 11, 12, 27 for controlling operation of the heat exchange means, a first temperature detection means 19 for detecting temperature of the first exhaust gas after passing through the turbocharger, and a second temperature detection means 20 for detecting temperature of the second exhaust gas before passing through the heat exchange means. The control means perform heat exchange when temperature detected by the first temperature detection means is less than temperature detected by the second temperature detection means. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an EGR system for an engine suitable for use in a vehicle such as a truck.

2. Description of the Related Art Conventionally, for example, an EGR system for a vehicle engine such as a truck mainly reduces the combustion temperature of the engine by lowering the oxygen concentration in the intake air by partially sucking the engine exhaust gas into the engine, mainly NO. _This is intended to reduce _X. In particular, the NO _X reduction measures of the diesel engine in recent years, is a very important system.

  For example, as shown in FIG. 6, this engine EGR system extracts part of the exhaust gas of the engine 101 from the exhaust manifold 102, and cools the exhaust gas through the EGR cooler 103 with engine cooling water indicated by a broken line. After the temperature is lowered, the intake manifold 104 is supplied. In general, the flow rate control valve 105 performs control to decrease the EGR amount when the engine is high and increase the EGR amount when the engine is low.

  An EGR system similar to this is disclosed, in particular, in which a sufficient flow rate of EGR gas at a high load is ensured (see, for example, Patent Document 1).

On the other hand, the PM (particulate-like emissions) in the exhaust gas not alone can reduce the EGR system of the engine and NO _X are exhaust DPF, an oxidation catalyst, a selective reduction catalyst, the post-processing apparatus 106 with a built-in storage-reduction catalyst, etc. There is an example in which these harmful substances are further reduced by being attached to the road 107.
JP 2002-285854 A (page 2-3, FIG. 1)

As described above, EGR system of a conventional engine, a portion of the exhaust gas of the engine is an intake again to the engine, lowering the combustion temperature of the engine by lowering the oxygen concentration in the intake air, mainly NO _X This is intended for reduction, and is an extremely important system in an engine whose combustion temperature is increasing, especially in a diesel engine.

  However, in an engine equipped with such an EGR system, when a large amount of EGR is performed, the engine cooling water absorbs a large amount of heat from the exhaust gas via the EGR cooler, and the temperature of the engine cooling water increases significantly. May exceed the cooling capacity of the radiator. In such a case, the temperature of the cooling water does not decrease, and problems such as engine overheating occur. On the other hand, there is a problem that it is difficult to increase the size of the radiator in order to cope with the engine overheating and the like because a large change in the vehicle structure is required.

As for the PM and NO _X in the exhaust gas of the engine, in addition to the EGR system, DPF, the oxidation catalyst, by selective reduction catalyst, the post-processing apparatus having a built-in storage-reduction catalyst or the like is mounted in an exhaust path, There is an example of further reducing these harmful substances.

  However, the exhaust gas purification performance of the aftertreatment device greatly depends on the exhaust gas temperature passing therethrough, and there is a problem that when the exhaust gas temperature falls below a certain value, the exhaust gas purification performance deteriorates rapidly. In particular, in an engine equipped with an exhaust gas-driven turbocharger, the exhaust gas is adiabatically expanded by a turbine, and the exhaust gas temperature is greatly reduced, for example, by a large engine, which is lowered by 100 ° C. or more. It may be difficult to maintain gas purification performance. Further, when a heating device is attached to increase the temperature of exhaust gas passing through the aftertreatment device, there arises a problem that fuel efficiency is deteriorated.

  The present invention has been made to solve such a problem, and it is possible to change variously the amount of heat that the engine cooling water must absorb in the EGR cooler and the exhaust gas temperature that has passed through the turbocharger. Even if a large amount of EGR is performed, overheating of the engine can be reliably prevented without making a significant change in the vehicle structure, etc., and for vehicles equipped with exhaust aftertreatment devices, It is an object of the present invention to provide an engine EGR system capable of maintaining high purification performance.

  In order to solve the above-mentioned problem, the means employed by the present invention is an EGR cooler that cools the second exhaust gas of an engine having a turbocharger driven by the first exhaust gas with engine cooling water and re-intakes the air. In the engine EGR system having the above, there is provided a heat exchanging means for exchanging heat between the second exhaust gas before passing through the EGR cooler and the first exhaust gas after passing through the turbocharger.

  As described above, the EGR system of the engine includes heat exchange means for exchanging heat between the second exhaust gas before passing through the EGR cooler and the first exhaust gas after passing through the turbocharger, and the second exhaust gas. Can be exchanged with the first exhaust gas whose temperature has changed after passing through the turbocharger, the amount of heat of the second exhaust gas is changed, and the engine cooling water is absorbed by the EGR cooler. The amount of heat that must be changed can be varied. Further, the temperature of the first exhaust gas after passing through the turbocharger can also be changed.

  Preferably, the engine EGR system includes a control means for controlling the operation of the heat exchange means, a first temperature detection means for detecting the temperature of the first exhaust gas after passing through the turbocharger, and before passing through the heat exchange means. Second temperature detection means for detecting the temperature of the second exhaust gas, and the control means is when the temperature detected by the first temperature detection means is lower than the temperature detected by the second temperature detection means. The heat exchange is performed.

  By doing so, the second exhaust gas before passing through the EGR cooler can be reliably cooled by the first exhaust gas that has passed through the turbocharger and adiabatically expanded and lowered in temperature by the heat exchange means. Therefore, the amount of heat that the engine coolant must absorb in the EGR cooler can be greatly reduced. On the other hand, the first exhaust gas whose temperature has been reduced by the turbocharger is heated by the second exhaust gas by passing through the heat exchanging means. Therefore, a DPF, an oxidation catalyst, a selective reduction catalyst, an occlusion reduction catalyst, etc. are incorporated. The exhaust gas temperature of the engine passing through the aftertreatment device can be raised.

  Preferably, the engine EGR system includes third temperature detection means for detecting a temperature of engine cooling water, and the control means is configured to detect when the temperature detected by the third temperature detection means is equal to or higher than a predetermined overheat temperature. The heat exchange is performed.

  If the engine cooling water exceeds a predetermined overheat temperature, there is a risk of malfunction or damage to parts in various parts of the engine. However, by performing the heat exchange in such an overheated state and cooling the second exhaust gas with the first exhaust gas, the engine cooling water exchanges heat with the second exhaust gas in the EGR cooler. The temperature is lowered, so that malfunction of each part of the engine and parts damage can be prevented.

  Preferably, the predetermined overheat temperature is a predetermined overheat temperature of the EGR cooler. The EGR cooler has a brazed portion inside, and if the internal temperature becomes a certain level or more and becomes overheated, the brazed portion may be damaged, and engine coolant leakage may occur. However, by cooling the second exhaust gas with the first exhaust gas, damage to the brazed portion of the EGR cooler can be prevented in advance.

  Preferably, the EGR system of the engine includes fourth temperature detection means for detecting the temperature of the second exhaust gas after passing through the EGR cooler, and the control means has a temperature detected by the fourth temperature detection means as EGR. The heat exchange is performed when a predetermined dew condensation temperature of the cooler is exceeded.

When the temperature in the EGR cooler drops below a certain level, condensation occurs in the EGR cooler, and SO ₂ in the exhaust gas changes to H ₂ SO ₃ , which may cause corrosion inside. However, only when the temperature of the second exhaust gas after passing through the EGR cooler is equal to or higher than the predetermined dew condensation temperature, the second exhaust gas is cooled by the first exhaust gas, so that dew condensation is generated in the EGR cooler. It can be surely prevented.

  Preferably, the engine EGR system includes a control means for controlling the operation of the heat exchange means, a first temperature detection means for detecting the temperature of the first exhaust gas after passing through the turbocharger, and before passing through the heat exchange means. The second temperature detecting means for detecting the temperature of the second exhaust gas and the third temperature detecting means for detecting the temperature of the engine cooling water are provided, and the control means is detected by the first temperature detecting means. The heat exchange is performed when the temperature exceeds the temperature detected by the second temperature detecting means and the temperature detected by the third temperature detecting means is less than a predetermined overheat temperature.

  As described above, the temperature of the first exhaust gas after passing through the turbocharger is usually lower than the temperature of the second exhaust gas before passing through the EGR cooler due to adiabatic expansion in the turbine section of the turbocharger. However, the mass of the first exhaust gas is overwhelmingly larger than that of the second exhaust gas, and depending on the operating state of the engine, the temperature of the first exhaust gas after passing through the turbocharger may be higher than that before passing through the EGR cooler. The temperature of the exhaust gas 2 may be higher.

  Therefore, when there is no risk of overheating in various parts of the engine, as described above, the second exhaust gas is heated by the first exhaust gas that has passed through the turbocharger by the heat exchange means. As a result, for example, when the engine is started in a cold region, the temperature of the engine cooling water can be increased quickly and the warm-up operation can be promoted.

  Similarly to the above, preferably, the predetermined overheat temperature is a predetermined overheat temperature of the EGR cooler.

  Preferably, the EGR system of the engine includes fourth temperature detection means for detecting the temperature of the second exhaust gas after passing through the EGR cooler, and the control means is configured such that the temperature detected by the fourth temperature detection means is equal to the temperature detected by the fourth temperature detection means. The heat exchange is performed when the temperature is equal to or lower than a predetermined dew condensation temperature of the EGR cooler. By doing so, it is possible to reliably prevent the occurrence of condensation in the EGR cooler.

  The engine EGR system of the present invention is an engine EGR system including an EGR cooler that cools the second exhaust gas of an engine having a turbocharger driven by the first exhaust gas with engine coolant and re-intakes the air. , Since it has heat exchange means for exchanging heat between the second exhaust gas before passing through the EGR cooler and the first exhaust gas after passing through the turbocharger, the amount of heat that the engine cooling water must absorb in the EGR cooler, In addition, there is an excellent effect that the temperature of the first exhaust gas that has passed through the turbocharger can be changed variously.

  Preferably, the engine EGR system of the present invention includes a control means for controlling the operation of the heat exchange means, a first temperature detection means for detecting the temperature of the first exhaust gas after passing through the turbocharger, and a heat exchange means. Second temperature detection means for detecting the temperature of the second exhaust gas before passing, and the control means is such that the temperature detected by the first temperature detection means is less than the temperature detected by the second temperature detection means. Sometimes the above heat exchange is performed, so that overheating of the engine when a large amount of EGR is performed can be surely prevented without significantly changing the vehicle structure, and an exhaust gas aftertreatment device is provided. The vehicle has an excellent effect that its purification performance can be kept high.

  The best mode for carrying out the invention of an engine EGR system according to the present invention will be described in detail with reference to FIGS.

  As shown in FIG. 1, an air cleaner 5, a turbocharger 6, and an intercooler 9 are disposed on the intake side of the engine 1, and the intake passage 4 passes through the compressor 7 and the intercooler 9 of the turbocharger 6 from the air cleaner 5. And connected to the intake manifold 2 of the engine 1.

  On the exhaust side of the engine 1, in addition to the turbocharger 6 described above, a first switching valve (control means) 11, a second switching valve (control means) 12, a DPF, an oxidation catalyst, a selective reduction catalyst, an occlusion reduction catalyst, etc. A built-in post-processing device 13 and a muffler (not shown) are disposed. A flow control valve 14, a first EGR cooler (heat exchange means) 15, and a second EGR cooler 16 are disposed on the EGR passage side of the engine 1.

  An exhaust passage 10 is connected from the exhaust manifold 3 of the engine 1 to a post-processing device 13 and a muffler (not shown) via a turbine 8 of the turbocharger 6 and a first switching valve 11. An exhaust passage 17 that branches from the upstream side of the first switching valve 11 is connected between the first switching valve 11 of the exhaust passage 10 and the aftertreatment device 13 via the second switching valve 12 and the first EGR cooler 15. The In addition, it can replace with the 1st switching valve 11 and the 2nd switching valve 12, and can also use one three-way switching valve.

  Further, the EGR passage 18 is connected from the exhaust manifold 3 of the engine 1 to the intake manifold 2 of the engine 1 via the first EGR cooler 15, the second EGR cooler 16, and the flow rate control valve 14.

  Between the turbine 8 of the turbocharger 6 in the exhaust passage 10 and the branch portion of the exhaust passage 17, between the exhaust manifold 3 of the engine 1 in the EGR passage 18 and the first EGR cooler 15, and the second EGR cooler 16 in the EGR passage 18. And an intake manifold 2 of the engine 1, a turbo exhaust gas temperature sensor (first temperature detecting means) 19 for detecting the temperature T 1 of exhaust gas (first exhaust gas) after passing through the turbocharger 6, respectively. 1 EGR cooler inlet temperature sensor (second temperature detecting means) 20 for detecting EGR cooler inlet temperature T2, which is the temperature of EGR gas (second exhaust gas) before passing through EGR cooler 15, and EGR after passing through second EGR cooler 16 EGR cooler outlet temperature sensor (fourth temperature detecting means) 2 for detecting the EGR cooler outlet temperature T4 which is the gas temperature There is disposed.

  As shown by a broken line in FIG. 1, a radiator 22 and a pump 23 are provided as a cooling system for the engine 1, and the cooling path 24 is connected to the engine 1 again from the engine 1 via the radiator 22 and the pump 23. . Further, the EGR cooling path 26 is connected from the pump 23 to the radiator 22 via the second EGR cooler 16. A cooling water temperature sensor (third temperature detecting means) 25 for detecting the engine cooling water temperature T3 is disposed in the cooling path 26 in the immediate vicinity of the outlet of the second EGR cooler 16.

  As shown in FIG. 2, a controller (control means) 27 is disposed, and the controller 27, turbo exhaust gas temperature sensor 19, EGR cooler inlet temperature sensor 20, EGR cooler outlet temperature sensor 21, cooling water temperature sensor 25, first The switching valve 11 and the second switching valve 12 are electrically connected. Further, the above-described flow control valve 14 is also connected to the controller 27 and adjusts the amount of EGR gas re-intaken into the engine 1.

  Next, the operation of the EGR system of this engine will be described. As shown in FIG. 3, the controller 27 includes an exhaust gas temperature T1 detected by the turbo exhaust gas temperature sensor 19, an EGR cooler inlet temperature T2 detected by the EGR cooler inlet temperature sensor 20, and an EGR cooler detected by the EGR cooler outlet temperature sensor 21. The outlet temperature T4 is read (steps S2 to S6).

  It is determined whether or not the EGR cooler outlet temperature T4 read in step S6 exceeds a predetermined dew condensation temperature Td of the EGR cooler (step S8). When the determination result of step S8 is affirmative (Yes), normal mode control described below is executed (step S10). If the determination result in step S8 is negative (No), that is, if the EGR cooler outlet temperature T4 of the EGR gas is equal to or lower than the predetermined dew condensation temperature Td of the EGR cooler, the dew condensation prevention mode control described below is executed ( Step S12).

  After executing the normal mode control in step S10 or the dew condensation prevention mode control in step S12, it is determined whether or not the engine 1 is stopped (step S14). If the determination result of step S14 is affirmative, the present EGR control is terminated. Further, when the determination result of step S14 is negative, that is, when the engine 1 is operating, the above step S2 and subsequent steps are repeated.

  As shown in FIG. 4, the normal mode control (step S10) in FIG. 3 is executed as follows. It is determined whether or not the exhaust gas temperature T1 read in step S2 is lower than the EGR cooler inlet temperature T2 of the EGR gas read in step S4 (step S100). If the determination result in step S100 is affirmative, that is, if the exhaust gas temperature T1 is lower than the EGR cooler inlet temperature T2 of the EGR gas, the first switching valve 11 is closed and the second switching valve 12 is opened. (Step S102).

  As a result, the exhaust gas of the engine 1 exhausted from the exhaust manifold 3 of the engine 1 is adiabatically expanded by the turbine 8 of the turbocharger 6 to decrease in temperature, and then passes through the second switching valve 12 to enter the first EGR cooler 15. Flow into. In the first EGR cooler 15, the high-temperature EGR gas supplied from the exhaust manifold 3 of the engine 1 is cooled by exchanging heat with the exhaust gas of the engine 1 having a lower temperature. The exhaust gas of the engine 1 that has passed through the first EGR cooler 15 is exhausted to the atmosphere through the post-processing device 13 and a muffler (not shown). On the other hand, after the temperature of the EGR gas is reduced by the first EGR cooler 15, the EGR gas flows into the second EGR cooler 16 and is further cooled by exchanging heat with the engine coolant.

  As described above, the EGR system of the present engine has a high-temperature EGR gas in which the exhaust gas that has been adiabatically expanded by the first EGR cooler 15 in the turbine 8 of the turbocharger 6 and whose temperature has been reduced is supplied from the exhaust manifold 3 of the engine 1. Therefore, the amount of heat that the engine cooling water must absorb in the first EGR cooler 15 is greatly reduced.

  Therefore, overheating of the engine 1 can be reliably prevented even when a large amount of EGR is performed. In addition, the engine EGR system only needs to be additionally equipped with two switching valves 11, 12, one EGR cooler 15, and one controller 27 to the existing EGR system. There is no need to make significant changes to the vehicle structure.

On the other hand, when condensation occurs in the second EGR cooler 16, SO ₂ in the EGR gas that is the exhaust gas changes to H ₂ SO ₃ , and corrosion may occur in the second EGR cooler 16. However, the above-described normal mode control (step S10) is executed only when the EGR cooler outlet temperature T4 read in step S6 exceeds the predetermined dew condensation temperature Td of the second EGR cooler 16, so that the inside of the second EGR cooler 16 Thus, no condensation occurs, and corrosion in the second EGR cooler 16 can be reliably prevented.

  Further, the exhaust gas of the engine 1 whose temperature has dropped after passing through the turbocharger 6 is heated by the first EGR cooler 15 by the EGR gas having a higher temperature, so that the DPF, oxidation catalyst, selective reduction catalyst, occlusion reduction The exhaust gas purification performance of the aftertreatment device 13 incorporating a catalyst or the like can be maintained high.

  Next, when the determination result of step S100 is negative, that is, when the exhaust gas temperature T1 is equal to or higher than the EGR cooler inlet temperature T2 of the EGR gas, the first switching valve 11 is opened and the second switching valve 12 is closed. (Step S104). In this case, the exhaust gas exhausted from the turbine 8 of the turbocharger 6 is exhausted directly into the atmosphere through the aftertreatment device 13, a muffler (not shown), and the like.

  The temperature of the exhaust gas of the engine 1 that has passed through the turbocharger 6 is usually higher than the temperature of the EGR gas supplied from the exhaust manifold 3 of the engine 1 due to adiabatic expansion in the turbine 8 of the turbocharger 6 as described above. Significantly, for example, in large engines, the rating is lower than 100 ° C. However, the exhaust gas amount of the engine 1 is overwhelmingly larger than the EGR gas amount, and depending on the operating state of the engine 1, the exhaust gas temperature T1 of the engine 1 that has passed through the turbocharger 6 becomes the EGR cooler inlet temperature T2 of the EGR gas. May be higher.

  In such a case, the EGR gas supplied from the exhaust manifold 3 of the engine 1 is heated by the exhaust gas that has passed through the turbine 8 of the turbocharger 6 having a higher temperature than in step S104 described above. To prevent.

  As shown in FIG. 5, the dew condensation prevention mode control (step S12) in FIG. 3 is executed as follows. The controller 27 reads the engine coolant temperature T3 detected by the coolant temperature sensor 25 (step S120), and determines whether or not the engine coolant temperature T3 is lower than a predetermined overheat temperature Tb set for the second EGR cooler 16. Is determined (step S122). If the determination result in step S122 is affirmative, that is, if the engine coolant temperature T3 is lower than the predetermined overheat temperature Tb, then the exhaust gas temperature T1 read in step S2 is the EGR gas read in step S4. It is determined whether or not the EGR cooler inlet temperature T2 is exceeded (step S124).

  If the determination result in step S124 is affirmative, if the exhaust gas temperature T1 is higher than the EGR cooler inlet temperature T2, the first switching valve 11 is closed and the second switching valve 12 is opened (step S126). . If the determination result in step S124 is negative, that is, if the exhaust gas temperature T1 is equal to or lower than the EGR cooler inlet temperature T2, the first switching valve 11 is opened and the second switching valve 12 is closed (step) S126). Thus, on the assumption that the second EGR cooler 16 is not in an overheated state, when the exhaust gas temperature T1 of the engine 1 that has passed through the turbocharger 6 is higher than the EGR cooler inlet temperature T2 of the EGR gas, the engine 1 The exhaust gas is guided to the first EGR cooler 15 to heat the EGR gas supplied from the exhaust manifold 3 of the engine 1. In this case, the first EGR cooler 15 operates as a heater.

  Thus, for example, when the engine is started in a cold region, the temperature of the engine cooling water is increased quickly, the warm-up operation is promoted, and the EGR cooler outlet temperature T4 of the EGR gas is changed to a predetermined condensation temperature of the second EGR cooler 16. The temperature can be raised to exceed Td, and condensation in the second EGR cooler 16 is prevented. In addition, since the dew condensation prevention mode control is executed in a range in which the second EGR cooler 16 does not overheat, damage to the brazed portion in the second EGR cooler 16 and accompanying cooling water leakage of the engine are surely prevented.

  Next, when the determination result in step S122 is negative, that is, when the engine coolant temperature T3 is equal to or higher than the predetermined overheat temperature Tb, the lowering of the engine coolant temperature T3 is prioritized and the normal mode shown in FIG. Control similar to the control (step S100 and subsequent steps) is performed.

  As described above, the EGR system of the present engine can reliably prevent engine overheating even when a large amount of EGR is performed without significantly changing the vehicle structure. About the vehicle equipped with the device, the purification performance can be kept high, and further, when starting the engine in a cold region, the temperature of the engine cooling water can be accelerated to promote the warm-up operation. It is an excellent EGR system.

  Note that the EGR system of the present engine is not limited to the control shown in the flowcharts shown in FIGS. 3 to 5, and may be implemented by only a part of the control or by various other controls. The exhaust gas amount of the engine 1 that passes through the turbocharger 6 and flows into the first EGR cooler 15 is not controlled by the first switching valve 11 and the second switching valve 12 or by the three-way switching valve. A flow control valve may be provided so that a part of the exhaust gas of the engine 1 that has passed through the turbocharger 6 flows to the first EGR cooler 15 and the rest flows to the post-processing device 13.

  The controller 27 replaces or together with the engine coolant temperature T3 detected by the coolant temperature sensor 25 disposed in the immediate vicinity of the outlet of the second EGR cooler 16 with other parts of the engine 1, such as an engine such as the radiator 22. The coolant temperature may be detected, and heat exchange in the first EGR cooler 15 may be performed based on the detected temperature.

1 is a schematic diagram showing an EGR system for an engine according to the present invention. It is a block diagram which shows a controller. It is a flowchart which shows the action | operation of the EGR system of the engine of FIG. It is a flowchart which shows normal mode control of step S10 of FIG. It is a flowchart which shows step S12 dew condensation prevention mode control of FIG. It is a trial figure which shows the EGR system of the conventional engine.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Engine 2 Intake manifold 3 Exhaust manifold 4 Intake path 5 Air cleaner 6 Turbocharger 7 Compressor 8 Turbine 9 Intercooler 10 Exhaust path 11 1st switching valve 12 2nd switching valve 13 Aftertreatment device 14 Flow control valve 15 1st EGR cooler 16 1st 2 EGR cooler 17 Exhaust passage 18 EGR passage 19 Turbo exhaust gas temperature sensor 20 EGR cooler inlet temperature sensor 21 EGR cooler outlet temperature sensor 22 Radiator 23 Pump 24 Cooling passage 25 Cooling water temperature sensor 26 EGR cooling passage 27 Controller 101 Engine 102 Exhaust manifold 103 EGR Cooler 104 Intake manifold 105 Flow control valve 106 Aftertreatment device 107 Exhaust passage T1 Exhaust gas temperature T2 EGR cooler inlet temperature T3 Engine coolant temperature T4 EG Cooler outlet temperature Tb predetermined overheating temperature Td predetermined condensation temperature

Claims (8)

  In an engine EGR system including an EGR cooler (16) that cools the second exhaust gas of an engine (1) having a turbocharger (6) driven by the first exhaust gas with engine cooling water and re-intakes the air. An engine EGR comprising heat exchange means (15) for exchanging heat between the second exhaust gas before passing through the EGR cooler and the first exhaust gas after passing through the turbocharger. system.   Control means (11, 12, 27) for controlling the operation of the heat exchange means (15), and a first temperature for detecting the temperature (T1) of the first exhaust gas after passing through the turbocharger (6) Detection means (19) and second temperature detection means (20) for detecting the temperature (T2) of the second exhaust gas before passing through the heat exchange means, and the control means includes the first temperature detection means (20). 2. The engine EGR system according to claim 1, wherein the heat exchange is performed when the temperature detected by the temperature detecting unit is lower than the temperature detected by the second temperature detecting unit. 3.   Third temperature detection means (25) for detecting the temperature (T3) of the engine cooling water is provided, and the control means (11, 12, 27) has a predetermined temperature detected by the third temperature detection means. The engine EGR system according to claim 2, wherein the heat exchange is performed when the temperature is equal to or higher than an overheat temperature of the engine.   4. The engine EGR system according to claim 3, wherein the predetermined overheat temperature is a predetermined overheat temperature (Tb) of the EGR cooler (16).   A fourth temperature detecting means (21) for detecting the temperature (T4) of the second exhaust gas after passing through the EGR cooler (16) is provided, and the control means (11, 12, 27) is provided with the fourth temperature detecting means. The engine EGR system according to any one of claims 2 to 4, wherein the heat exchange is performed when the temperature detected by the temperature detecting means exceeds a predetermined dew condensation temperature (Td) of the EGR cooler. .   Control means (11, 12, 27) for controlling the operation of the heat exchange means (15), and a first temperature for detecting the temperature (T1) of the first exhaust gas after passing through the turbocharger (6) Detection means (19), second temperature detection means (20) for detecting the temperature (T2) of the second exhaust gas before passing through the heat exchange means, and detection of the temperature (T3) of the engine cooling water And a third temperature detecting means (25), wherein the control means exceeds the temperature detected by the second temperature detecting means when the temperature detected by the first temperature detecting means exceeds the third temperature. 2. The engine EGR system according to claim 1, wherein the heat exchange is performed when the temperature detected by the detection unit is lower than a predetermined overheat temperature. 3.   The engine EGR system according to claim 6, wherein the predetermined overheat temperature is a predetermined overheat temperature (Tb) of the EGR cooler (16).   A fourth temperature detecting means (21) for detecting the temperature (T4) of the second exhaust gas after passing through the EGR cooler (16) is provided, and the control means (11, 12, 27) is provided with the fourth temperature detecting means. The engine EGR system according to claim 6 or 7, wherein the heat exchange is performed when the temperature detected by the temperature detecting means is equal to or lower than a predetermined dew condensation temperature (Td) of the EGR cooler.

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