Exhaust gas recirculation device for engine

Abstract

To provide an exhaust gas recirculation device for an engine, which can improve engine performance while avoiding misfire.SOLUTION: An exhaust gas recirculation device comprises an EGR passage 41 and an EGR valve 43 for opening and closing the EGR passage 41 to change an EGR amount that is the amount of EGR gas to be recirculated to an intake passage 20, and when an engine E is operated in an EGR region A1, executes EGR control to open the EGR valve 43 to recirculate the EGR gas. In addition, when an intake air temperature detected by an intake air temperature sensor SN3 is lower than a predetermined first set temperature, even when the engine E is operated in the EGR region A1, the EGR control is prohibited under a condition that an intake air flow rate, which is the flow rate of intake air flowing through the intake passage 20, is less than a predetermined set flow rate.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to an exhaust gas recirculation device for an engine. [Background technology]

[0002] It is known that an engine mounted on a vehicle performs EGR control to recirculate EGR gas, which is a part of exhaust gas flowing through an exhaust passage, to an intake passage, as disclosed in Patent Document 1. Specifically, Patent Document 1 discloses an engine that includes an EGR passage that connects the exhaust passage and the intake passage, and an EGR valve that opens and closes the EGR passage, and the opening degree of the EGR valve is adjusted according to operating conditions. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2021-105352 Summary of the Invention [Problem to be solved by the invention]

[0004] EGR gas is primarily composed of inert gases. Therefore, by implementing EGR control and returning the EGR gas to the intake passage, the heat capacity of the gas in the engine's combustion chamber is increased, lowering the combustion temperature, thereby improving engine performance. For example, lowering the combustion temperature reduces the generation of NOx.

[0005] However, if EGR control is performed when the temperature of the intake air flowing through the intake passage is low, the low intake passage temperature and the low amount of saturated steam in the intake air can cause the moisture in the EGR gas to condense in the intake passage, resulting in the accumulation of condensed water in the intake passage.If condensed water accumulates in the intake passage and flows into the combustion chamber all at once, there is a risk of misfire.

[0006] The present invention has been made in view of the above circumstances, and has as its object to provide an exhaust gas recirculation device for an engine that can improve engine performance while avoiding misfires. [Means for solving the problem]

[0007] In order to solve the above problems, the present invention provides an exhaust gas recirculation device for an engine including an engine body having a combustion chamber formed therein, an exhaust passage connected to the engine body and through which exhaust gas discharged from the engine body flows, and an intake passage connected to the engine body and through which intake air introduced into the engine body flows, the device including an intake air temperature sensor for detecting an intake air temperature, an EGR passage connecting the exhaust passage and the intake passage and for recirculating EGR gas, which is a part of the exhaust gas, into the intake passage, and an EGR passage that opens and closes to allow the EGR gas to flow into the intake passage. The invention is characterized in that it comprises an EGR valve that changes the EGR amount, which is the amount recirculated into the passage, and a control device that performs EGR control by opening the EGR valve to recirculate the EGR gas when the engine is operating in a predetermined EGR range, and when the intake temperature detected by the intake temperature sensor is lower than a predetermined first set temperature, the control device prohibits the EGR control under conditions where the intake flow rate, which is the flow rate of intake air flowing through the intake passage, is less than a predetermined set flow rate, even when the engine is operating in the EGR range (Claim 1).

[0008] According to the present invention, when the engine is operating in the EGR region, EGR control is performed, and the EGR gas, which is an inert gas, is recirculated to the intake passage and ultimately to the combustion chamber. Therefore, the EGR control reduces the combustion temperature, thereby improving engine performance. For example, when the present invention is applied to an engine with a spark plug, even if the ignition timing is advanced, the EGR control can suppress the increase in combustion temperature that accompanies the advance of the ignition. Therefore, it is possible to prevent an excessive increase in the exhaust gas temperature while maintaining engine output.

[0009] Furthermore, EGR control is prohibited under conditions where the intake temperature is lower than the first set temperature and the intake air flow rate is less than the set flow rate. Under these conditions, condensation due to EGR gas is likely to occur in the intake passage due to the low intake temperature, but the intake air flow rate is small, so the condensed water cannot be sufficiently blown toward the combustion chamber by the intake air. Therefore, under these conditions, condensed water is likely to accumulate in the intake passage. In contrast, in the present invention, the recirculation of EGR gas is stopped under these conditions, so condensed water can be prevented from accumulating in the intake passage. Therefore, it is possible to prevent a large amount of condensed water accumulated in the intake passage from being introduced into the combustion chamber when the intake air flow rate increases, and ultimately to prevent misfires from occurring.

[0010] In the above configuration, preferably, when the intake air temperature is lower than the first set temperature, the control device determines whether the engine is operating in a predetermined low-temperature EGR region that is preset in the EGR region where the intake air flow rate is equal to or higher than the set flow rate, and performs the EGR control when the engine is operating in the low-temperature EGR region (claim 2).

[0011] According to this configuration, when the intake air temperature is lower than the first set temperature, it is possible to determine whether or not to perform EGR control by determining whether or not the engine is being operated in the low-temperature EGR range.

[0012] For example, the low-temperature EGR region is set to satisfy at least one of the following requirements: the lower limit value of the engine speed in the low-temperature EGR region is greater than the lower limit value of the engine speed in the EGR region; and the lower limit value of the engine load in the low-temperature EGR region is greater than the lower limit value of the engine load in the EGR region (Claim 3).

[0013] In the above configuration, preferably, when the EGR control is performed, the control device controls the EGR valve so that the maximum value of the EGR amount is smaller when the intake air temperature is lower than the first set temperature than when the intake air temperature is higher (claim 4).

[0014] With this configuration, when the intake air temperature is lower than the first set temperature and moisture in the EGR gas is likely to condense, the amount of EGR gas recirculated to the intake passage is kept low, thereby keeping the amount of condensed water in the intake passage low and reliably preventing misfires caused by the condensed water.

[0015] In the above configuration, preferably, when the EGR control is performed, when the intake air temperature is lower than a predetermined second set temperature that is lower than the first set temperature, the control device controls the EGR valve so that the EGR amount is smaller than when the intake air temperature is equal to or higher than the second set temperature (claim 5).

[0016] With this configuration, when the intake air temperature is lower than the second set temperature and moisture in the EGR gas is more likely to condense in the intake passage, the amount of EGR gas recirculated to the intake passage is kept low, thereby keeping the amount of condensed water in the intake passage low and more reliably preventing misfires caused by the condensed water.

[0017] In the above configuration, preferably, the control device prohibits the EGR control when the intake air temperature is lower than a predetermined third set temperature that is lower than the second set temperature (claim 6).

[0018] With this configuration, when the intake air temperature is lower than the third set temperature and moisture in the EGR gas is more likely to condense in the intake passage, the recirculation of the EGR gas is stopped, thereby preventing condensation in the intake passage due to the EGR gas and more reliably suppressing misfires. [Effects of the Invention]

[0019] As described above, the exhaust gas recirculation system for an engine according to the present invention can improve engine performance while avoiding misfires. [Brief explanation of the drawings]

[0020] [Figure 1] 1 is a diagram showing a configuration of an engine according to an embodiment of the present invention. [Figure 2] FIG. 2 is a diagram showing a control block of the engine. [Figure 3] FIG. 4 is a diagram showing a control map for intake air temperature and engine water temperature. [Figure 4] 3 is a flowchart showing the contents of control related to the EGR system. [Figure 5] 3 is a flowchart showing the contents of control related to the EGR system. [Figure 6] FIG. 2 is a diagram showing an EGR region. [Figure 7] 1 is a graph showing an example of a normal EGR rate. [Figure 8] FIG. 2 is a diagram showing a low-temperature EGR region. [Figure 9] 4 is a graph showing an example of a low-temperature EGR rate. [Figure 10] 1 is a graph showing a normal EGR amount relative to engine speed and engine load. [Figure 11] 1 is a graph showing the amount of low-temperature EGR with respect to the engine speed and the engine load. [Figure 12] FIG. 4 is a diagram showing the relationship between the engine load and the low-temperature EGR amount and the normal EGR amount. [Figure 13] FIG. 4 is a diagram showing the relationship between intake air temperature and an EGR rate for extremely low temperatures. DETAILED DESCRIPTION OF THE INVENTION

[0021] (Overall engine configuration) FIG. 1 is a schematic system diagram showing a preferred embodiment of an engine E to which an exhaust gas recirculation device 100 according to an embodiment of the present invention is applied. The engine E includes an engine body 1 that is driven by a supply of fuel, and an intake passage 20 and an exhaust passage 30 connected to the engine body 1. The intake passage 20 is a passage through which intake air, which is air introduced into the engine body 1, flows. The exhaust passage 30 is a passage through which exhaust gas discharged from the engine body 1 flows. The engine E is mounted, for example, in a vehicle such as an automobile as a power source for driving the vehicle. In this embodiment, the engine E is a gasoline engine, and the engine body 1 is driven by a fuel containing gasoline as a main component.

[0022] The engine body 1 is a multi-cylinder engine having a plurality of cylinders 2a (only one of which is shown in FIG. 1). In this embodiment, the engine body 1 is a four-cylinder in-line engine, with the four cylinders 2a aligned in a direction perpendicular to the plane of the paper in FIG. 1. The engine body 1 includes a cylinder block 2 having the plurality of cylinders 2a formed therein, a cylinder head 3 attached to the upper surface of the cylinder block 2 so as to close the upper end openings of each cylinder 2a, and a plurality of pistons 4 housed in each cylinder 2a so as to be able to slide back and forth.

[0023] A combustion chamber 5 is defined above the piston 4 of each cylinder 2a. Fuel is supplied to the combustion chamber 5 by injection from an injector 10, which will be described later. The mixture of the supplied fuel and air is combusted in the combustion chamber 5, and the expansion force caused by the combustion causes the piston 4 to reciprocate up and down.

[0024] A crankshaft 13, which is the output shaft of the engine body 1, is provided at the bottom of the cylinder block 2 (below the pistons 4). The crankshaft 13 is connected to the pistons 4 of each cylinder 2a via connecting rods. The crankshaft 13 rotates around its central axis in response to the reciprocating motion (up and down movement) of the pistons 4. A crank angle sensor SN1 is attached to the cylinder block 2. The crank angle sensor SN1 detects the crank angle, which is the rotation angle of the crankshaft 13, and the engine speed, which is the rotation speed of the crankshaft 13.

[0025] A water jacket 15 through which coolant flows to cool the engine body 1 is formed in the cylinder block 2 and the cylinder head 3. A water temperature sensor SN2 is attached to the cylinder block 2. The engine water temperature sensor SN2 detects the temperature of the coolant flowing through the water jacket 15, i.e., the engine water temperature.

[0026] An intake port 6 and an exhaust port 7 that communicate with the combustion chamber 5 are formed in the cylinder head 3 for each cylinder 2a. The cylinder head 3 is equipped with an intake valve 8 that opens and closes the opening of the intake port 6 on the combustion chamber 5 side, and an exhaust valve 9 that opens and closes the opening of the exhaust port 7 on the combustion chamber 5 side, for each cylinder 2a.

[0027] The cylinder head 3 is equipped with an injector 10 and a spark plug 11, each for each cylinder 2a. One injector 10 and one spark plug 11 are provided for each cylinder 2a. The injector 10 is a fuel injection valve that injects fuel into the combustion chamber 5. In this embodiment, the injector 10 is attached so that its tip faces the combustion chamber 5 from near the center of the ceiling surface of the combustion chamber 5. The spark plug 11 is an ignition device that ignites the fuel-air mixture formed in the combustion chamber 5. In this embodiment, the spark plug 11 is disposed so that its tip, including the spark plug, faces the interior of the combustion chamber 5 from near the center of the ceiling surface of the combustion chamber 5.

[0028] The intake passage 20 is connected to the cylinder head 3 so as to communicate with the intake ports 6 of each cylinder 2a. In the intake passage 20, an air cleaner 21, a throttle valve 22, and a surge tank 23 are arranged in this order from the upstream side in the flow direction of the intake air.

[0029] The air cleaner 21 is a filter that removes foreign matter from the intake air. The throttle valve 22 is a valve that opens and closes the intake passage 20. The amount of intake air flowing through the intake passage 20, and therefore the amount of air introduced into the engine body 1, is changed depending on the opening of the throttle valve 22. The surge tank 23 is a tank that provides space for evenly distributing the intake air to each cylinder 2a.

[0030] An intake air temperature sensor SN3 is disposed in the intake passage 20. The intake air temperature sensor SN3 is disposed near the air cleaner 21, between the air cleaner 21 and the throttle valve 22. The intake air temperature sensor SN3 detects the intake air temperature, which is the temperature of the intake air passing near the air cleaner 21.

[0031] The exhaust passage 30 is connected to the cylinder head 3 so as to communicate with the exhaust ports 7 of each cylinder 2a. A catalytic converter 31 is provided in the exhaust passage 30. The catalytic converter 31 is a device that purifies exhaust gas. The catalytic converter 31 has a built-in catalyst 31A, and purifies the exhaust gas by the function of the catalyst 31A. For example, a three-way catalyst is used as the catalyst 31A.

[0032] The engine E is provided with an EGR system 40. The EGR system 40 includes an EGR passage 41, an EGR cooler 42 provided in the EGR passage 41, and an EGR valve 43. The EGR passage 41 connects the exhaust passage 30 and the intake passage 20 and is a passage for recirculating EGR gas, which is a part of the exhaust gas, to the intake passage 20. The EGR passage 41 connects a portion of the exhaust passage 30 downstream of the catalytic converter 31 in the exhaust flow direction with a portion of the intake passage 20 between the throttle valve 22 and the surge tank 23. The EGR cooler 42 is a device that cools the EGR gas by heat exchange while passing through the EGR passage 41. Cooling water circulating through the water jacket 15 is introduced into the EGR cooler 42. The EGR gas is cooled by heat exchange with the cooling water in the EGR cooler 42. The EGR valve 43 is provided in the EGR passage 41 downstream of the EGR cooler 42 in the flow direction of the EGR gas (closer to the intake passage 20). The EGR valve 43 opens and closes the EGR passage 41. The EGR amount, which is the amount of EGR gas that passes through the EGR passage 41 and flows back to the intake passage 20, is changed according to the opening degree of the EGR valve 43.

[0033] (Control system) 2 is a functional block diagram showing a control system for engine E. The ECU 80 shown in this diagram is a device for overall control of engine E. The ECU 80 is configured by a microcomputer including a processor (CPU) that performs various arithmetic processing, memories such as ROM and RAM, and various input / output buses. The ECU 80 corresponds to the "control device" in this disclosure.

[0034] The ECU 80 is electrically connected to a crank angle sensor SN1, an engine water temperature sensor SN2, and an intake air temperature sensor SN3. The vehicle is equipped with an accelerator sensor SN4 that detects the accelerator position, which is the opening of an accelerator pedal provided on the vehicle. The ECU 80 is also electrically connected to the accelerator sensor SN4. Information detected by each of the sensors SN1 to SN4, i.e., information on the crank angle, engine speed, engine water temperature, intake air temperature, and accelerator position, is sequentially input to the ECU 80.

[0035] The ECU 80 controls each part of the engine E while executing various determinations and calculations based on input information from the sensors SN1 to SN4. The ECU 80 is electrically connected to the injector 10, the spark plug 11, the throttle valve 22, and the EGR valve 43, and outputs control signals to these devices based on the results of the calculations.

[0036] (EGR system control) The control of the EGR system 40, which is a feature of the present invention, will now be described.

[0037] FIG. 3 is a diagram showing a control map for intake air temperature and engine water temperature. Hereinafter, in the control areas for intake air temperature and engine water temperature, an area in the high water temperature area where the engine water temperature is higher than a predetermined first water temperature RW1 and where the intake air temperature is higher than a predetermined first intake air temperature RA1 is referred to as a first area X1. An area in the high water temperature area where the intake air temperature is equal to or lower than the first intake air temperature RA1 and higher than a predetermined second intake air temperature RA2 is referred to as a second area X2. An area in the high water temperature area where the intake air temperature is equal to or lower than the second intake air temperature RA2 and higher than a predetermined third intake air temperature RA3 is referred to as a third area X3. An area in the high water temperature area where the intake air temperature is equal to or lower than the third intake air temperature RA3 is referred to as a fourth area X4. The first intake air temperature RA1 corresponds to the "first set temperature" in this invention. The second intake air temperature RA2 corresponds to the "second set temperature" in this invention. The third intake air temperature RA3 corresponds to the "third set temperature" in this invention.

[0038] The first water temperature RW1 is set in advance and stored in the ECU 80. For example, the first water temperature RW1 is set to about 60°C. The second intake air temperature RA2 is a temperature higher than the third intake air temperature RA3. The first intake air temperature RA1 is a temperature higher than the second intake air temperature RA2. The third intake air temperature RA3, the second intake air temperature RA2, and the first intake air temperature RA1 are each set in advance and stored in the ECU 80. For example, the third intake air temperature RA3 is set to about −15°C, the second intake air temperature RA2 is set to about −10°C, and the first intake air temperature RA1 is set to about −3°C.

[0039] Additionally, within the low water temperature area where the engine water temperature is equal to or lower than the first water temperature RW1 and higher than a predetermined second water temperature RW2, an area where the intake air temperature is higher than the first intake air temperature RA1 is referred to as a fifth area X5. Within the low water temperature area, an area where the intake air temperature is equal to or lower than the first intake air temperature RA1 and higher than the second intake air temperature RA2 is referred to as a sixth area X6. Within the low water temperature area, an area where the intake air temperature is equal to or lower than the second intake air temperature RA2 and higher than the third intake air temperature RA3 is referred to as a seventh area X7. Within the low water temperature area, an area where the intake air temperature is equal to or lower than the third intake air temperature RA3 is referred to as an eighth area X8.

[0040] Furthermore, within the extremely low water temperature area where the engine water temperature is equal to or lower than the second water temperature RW2, an area where the intake air temperature is higher than the second intake air temperature RA2 is referred to as a ninth area X9. Within the extremely low water temperature area, an area where the intake air temperature is equal to or lower than the second intake air temperature RA2 is referred to as a tenth area X10. The second water temperature RW2 is set in advance and stored in the ECU 80. For example, the second water temperature RW2 is set to about 30°C.

[0041] 4 and 5 are flowcharts showing the contents of the control relating to the EGR system 40 performed by the ECU 80. Steps S1 to S25 shown in Fig. 4 and Fig. 5 are repeatedly performed at predetermined intervals while the engine body 1 is running.

[0042] First, the ECU 80 reads various information detected by the sensors SN1 to SN4 etc. (Step S1) In Step S1, the ECU 80 reads at least the engine speed, engine water temperature, intake air flow rate, intake air temperature, and accelerator opening.

[0043] Next, the ECU 80 determines whether the engine water temperature read in step S1 is higher than the first water temperature RW1 (step S2). If the determination in step S2 is NO, that is, the engine water temperature is equal to or lower than the first water temperature RW1, the ECU 80 proceeds to step S20 shown in FIG.

[0044] On the other hand, if the determination in step S2 is YES and the engine water temperature is higher than the first water temperature RW1, the ECU 80 determines whether the intake air temperature read in step S1 is higher than the third intake air temperature RA3 (step S3).

[0045] If the determination in step S3 is NO and the intake air temperature is equal to or lower than the third intake air temperature RA3, that is, if the intake air temperature and the engine water temperature are within the fourth area X4, the ECU 80 prohibits EGR control (step S11). EGR control is control that opens the EGR valve 43 to recirculate EGR gas to the intake passage 20. In step S11, the ECU 80 prohibits the EGR valve 43 from opening and closes the EGR valve 43. Note that if the EGR valve 43 is already closed, the valve remains closed. After step S11, the ECU 80 returns to step S1.

[0046] In this way, when the intake air temperature and engine water temperature are within the fourth area X4, EGR control is prohibited.

[0047] Returning to step S3, if the determination in step S3 is YES and the intake air temperature is higher than the third intake air temperature RA3, the ECU 80 determines whether or not the intake air temperature is higher than the first intake air temperature RA1 (step S4).

[0048] If the determination in step S4 is YES and the intake air temperature is higher than the first intake air temperature RA1, that is, when the engine water temperature and the intake air temperature are within the first area X1, ECU 80 next determines whether engine E is operating in EGR area A1 (step S5).

[0049] The EGR range A1 is preset for the engine speed and engine load and stored in the ECU 80. FIG. 6 is a diagram showing the operating range of the engine E and the EGR range A1. The EGR range A1 is set to a range in which the engine E can operate where the engine load is equal to or greater than a predetermined first load Q1, and includes a high-load range near the maximum load QM. The first load Q1 is set to a relatively low value, and the EGR range A1 occupies most of the operating range of the engine E. In step S5, the ECU 80 compares the engine load with the first load Q1 and determines that the engine E is operating in the EGR range A1 if the engine load is equal to or greater than the first load Q1. The ECU 80 also separately calculates the engine load based on the accelerator opening, engine speed, etc. read in step S1.

[0050] If the determination in step S5 is NO and the engine E is not operating in the EGR region A1, the ECU 80 proceeds to step S11, prohibits EGR control, and closes the EGR valve 43. After step S11, the ECU 80 returns to step S1.

[0051] In this way, when the intake air temperature and engine water temperature are within the first area X1 and the engine E is operating in a region outside the EGR region A1, EGR control is prohibited.

[0052] On the other hand, if the determination in step S5 is YES and the engine E is operating in the EGR region A1, the ECU 80 proceeds to step S6. In step S6, the ECU 80 performs EGR control to open the EGR valve 43 and sets a target EGR rate, which is a target value of the EGR rate, to a normal EGR rate. The ECU 80 also adjusts the opening of the EGR valve 43 so as to achieve the target EGR rate. The EGR rate is the weight ratio of EGR gas to all gases in the combustion chamber 5. The normal EGR rate is set in advance and stored in the ECU 80. In this embodiment, the normal EGR rate is stored in the ECU 80 as a map relating to engine speed and engine load. In step S6, the ECU 80 extracts a value corresponding to the engine speed and engine load read in step S1 from the map and sets the extracted value as the target EGR rate. For example, as shown in FIG. 7, the normal EGR rate is set to a higher value overall when the engine speed is low. The normal EGR rate is set to a value greater than 0.

[0053] In this way, when the intake air temperature and engine water temperature are within the first area X1 and the engine E is operating within the EGR area A1, EGR control is performed and the EGR valve 43 is controlled so that the EGR rate becomes the normal EGR rate.

[0054] Returning to step S4, if the determination in step S4 is NO and the intake air temperature is lower than the first intake air temperature RA1, that is, if the engine water temperature and intake air temperature are within the second area X2 or the third area X3, ECU 80 determines whether engine E is operating in the low-temperature EGR area A2 (step S7).

[0055] The low-temperature EGR area A2 is set in advance for the engine speed and engine load and stored in the ECU 80. Fig. 8 is a diagram showing the low-temperature EGR area A2. The low-temperature EGR area A2 is set within the EGR area A1. The low-temperature EGR area A2 is set to satisfy the requirement that the lower limit value of the engine speed in the low-temperature EGR area A2 is higher than the lower limit value of the engine speed in the EGR area A1. In addition, the low-temperature EGR area A2 is set on the higher engine speed and engine load side of the line L1 shown in Fig. 8.

[0056] Line L1 is a line at which the intake air flow rate, more specifically, the weight of intake air flowing into the combustion chamber 5 per unit time, is a predetermined set flow rate. The higher the engine speed, the greater the weight of intake air flowing into the combustion chamber 5 per unit time. Furthermore, the higher the engine load, the greater the weight of intake air introduced into the combustion chamber 5 to achieve this. Therefore, at operating points on line L1 and at operating points in regions where the engine speed and engine load are higher than those on line L1, the intake air flow rate is equal to or greater than the set flow rate.

[0057] As shown in FIG. 8, in this embodiment, the low-temperature EGR region A2 is set to a region A1_x in the EGR region A1 where the engine speed and engine load are higher than the line L1, and where the engine speed is higher than a predetermined low-temperature EGR start speed N1. The low-temperature EGR start speed N1 corresponds to the lower limit of the low-temperature EGR region A2. As shown in FIG. 8, the low-temperature EGR start speed N1 is higher than the lower limit N0 of the engine speed in the EGR region A1. In this embodiment, the lower limit of the engine load in the low-temperature EGR region A2 is set to a first load Q1, which is the same as the lower limit of the engine load in the EGR region A1. For example, the set flow rate is set to approximately 30 g / s, and the low-temperature EGR start speed N1 is set to approximately 3000 rpm.

[0058] 4, if the determination in step S7 is NO and the engine E is operating outside the low-temperature EGR region A2, the ECU 80 proceeds to step S11 to prohibit EGR control and close the EGR valve 43. After step S11, the ECU 80 returns to step S1.

[0059] Thus, when the intake air temperature and engine water temperature are within the second area X2 or the third area X3 and the engine E is operating outside the low-temperature EGR region A2, EGR control is prohibited. That is, when the engine water temperature is higher than the first water temperature RW1 and the intake air temperature is equal to or lower than the first intake air temperature RA1, EGR control is prohibited even if the engine E is operating in the EGR region A1 under the condition that the engine speed and engine load are lower than line L1 and the intake flow rate is less than the set flow rate, or under the condition that the engine speed is equal to or lower than the low-temperature EGR start speed N1.

[0060] On the other hand, if the determination in step S7 is YES and the engine E is operating in the low-temperature EGR range A2, the ECU 80 next determines whether the intake air temperature is higher than the second intake air temperature RA2 (step S8).

[0061] If the determination in step S8 is YES and the intake air temperature is higher than the second intake air temperature RA2, that is, if the engine E is operating within the low-temperature EGR area A2 and the intake air temperature and the engine water temperature are within the second area X2, the ECU 80 proceeds to step S9. In step S9, the ECU 80 performs EGR control to open the EGR valve 43 and sets a target EGR rate, which is a target value of the EGR rate, to the low-temperature EGR rate. The ECU 80 also adjusts the opening of the EGR valve 43 so that the target EGR rate is achieved. The low-temperature EGR rate is set in advance and stored in the ECU 80. In this embodiment, the low-temperature EGR rate is stored in the ECU 80 as a map relating to engine speed and engine load. In step S9, the ECU 80 extracts a value corresponding to the engine speed and engine load read in step S1 from the map and sets the extracted value as the target EGR rate. For example, the low-temperature EGR rate is set as shown in FIG. 9. The "Regr" values ​​in the graphs of Figures 7 and 9 are the same, and as is clear from a comparison of these graphs, the low-temperature EGR rate is set to a value that is generally smaller than the normal EGR rate for each engine speed and engine load.

[0062] Here, the low-temperature EGR rate is set so that the EGR amount when the low-temperature EGR rate is realized is equal to or less than a predetermined low-temperature upper limit EGR amount. Hereinafter, the EGR amount when step S6 is performed and the normal EGR rate is realized will be referred to as the normal EGR amount, as appropriate. Furthermore, the EGR amount when step S9 is performed and the low-temperature EGR rate is realized will be referred to as the low-temperature EGR amount. The low-temperature upper limit EGR amount is a value smaller than the maximum value of the normal EGR amount, and the low-temperature EGR amount is smaller than the maximum value of the normal EGR amount. Furthermore, the low-temperature EGR rate is set so that the low-temperature EGR amount is equal to or less than the normal EGR amount when the low-temperature EGR amount and the normal EGR amount are compared at the same operating point where the engine speed and engine load are the same.

[0063] FIG. 10 is a graph showing the normal EGR amount versus engine speed and engine load. FIG. 11 is a graph showing the low-temperature EGR amount versus engine speed and engine load. The "Megr" values ​​in both FIG. 10 and FIG. 11 are the same. FIG. 12 is a graph showing the relationship between the engine load, the low-temperature EGR amount, and the normal EGR amount at a predetermined engine speed included in the low-temperature EGR region A2, i.e., a predetermined engine speed higher than the low-temperature EGR start speed N1. As can be seen from comparing FIG. 10 with FIG. 11 and from FIG. 12, the low-temperature EGR amount is less than the normal EGR amount. This relationship holds true at any operating point (any engine speed and engine load). That is, the low-temperature EGR amount is less than the normal EGR amount at any operating point (any engine speed and engine load). Furthermore, as can be seen from comparing FIG. 10 with FIG. 11 and from FIG. 12, the maximum low-temperature EGR amount is less than the maximum normal EGR amount. 11 and 12, the low-temperature EGR amount is the upper limit, i.e., the low-temperature upper limit EGR amount, except for a part of the low-temperature EGR range A2 where the engine speed and engine load are low. In other words, the low-temperature EGR rate is set so that the EGR amount is the low-temperature upper limit EGR amount in most of the low-temperature EGR range A2.

[0064] In this way, when the intake air temperature and engine water temperature are within the second area X2 and the engine E is operating within the low-temperature EGR area A2, EGR control is performed to control the EGR valve 43 so that the EGR rate becomes the low-temperature EGR rate. The EGR amount at this time is set to a value equal to or less than the low-temperature upper limit EGR amount and a value equal to or less than the normal EGR amount at the same operating point, i.e., the EGR amount when the intake air temperature and engine water temperature are within the first area X1 and the engine speed and engine load are the same. The EGR amount at this time is set to a value smaller than the maximum value of the normal EGR amount, i.e., the maximum value of the EGR amount when the intake air temperature and engine water temperature are within the first area X1.

[0065] Returning to step S8, if the determination in step S8 is NO and the intake air temperature is equal to or lower than the second intake air temperature RA2, that is, if the engine E is operating within the low-temperature EGR area A2 and the intake air temperature and the engine water temperature are within the third area X3, the ECU 80 proceeds to step S10. In step S10, the ECU 80 performs EGR control to open the EGR valve 43, sets the target EGR rate to the extremely low-temperature EGR rate, and adjusts the opening of the EGR valve 43 so that this is achieved.

[0066] When the intake temperature is the third intake temperature RA3, the EGR rate for extremely low temperatures is set to 0 (zero). When the intake temperature is the second intake temperature RA2, the EGR rate for extremely low temperatures is set to the same value as the EGR rate for low temperatures. When the intake temperature is between the third intake temperature RA3 and the second intake temperature RA2, the EGR rate for extremely low temperatures is set to a value obtained by linearly interpolating 0 (zero) and the value when the intake temperature is the second intake temperature RA2 for the intake temperature. As a result, as shown in Figure 13 which shows the relationship between the intake temperature and the EGR rate for extremely low temperatures, the EGR rate for extremely low temperatures is set to a larger value as the intake temperature is higher.

[0067] Specifically, the ECU 80 first sets the low-temperature EGR rate value corresponding to the engine speed and engine load read in step S1 as an interpolation reference value. Next, the ECU 80 calculates the extremely low-temperature EGR rate XXEGR by the formula XXEGR=(TA-RA1) / (RA2-RA1)×XEGR, where XEGR is the interpolation reference value and TA is the intake air temperature.

[0068] As described above, when the intake air temperature and engine water temperature are within the third area X3 and the engine E is operating within the low-temperature EGR region A2, EGR control is performed. The extremely low-temperature EGR rate calculated as described above is smaller than the low-temperature EGR rate and the normal EGR rate at the same operating point. As a result, when the intake air temperature and engine water temperature are within the third area X3 and the engine E is operating within the low-temperature EGR region A2, the EGR amount becomes equal to or less than the low-temperature EGR amount, and therefore equal to or less than the low-temperature upper limit EGR amount, and equal to or less than the normal EGR amount at the same operating point.

[0069] Returning to step S2, if the determination in step S2 is NO and the engine water temperature is equal to or lower than the first water temperature RW1, the ECU 80 proceeds to step S20 shown in Fig. 5. In step S20, the ECU 80 determines whether the engine water temperature read in step S1 is higher than the second water temperature RW2.

[0070] If the determination in step S20 is NO and the engine water temperature is equal to or lower than the second water temperature RW2, that is, if the intake air temperature and the engine water temperature are within the ninth area X9 or the tenth area X10, the ECU 80 proceeds to step S11, prohibits EGR control, and closes the EGR valve 43. After step S11, the ECU 80 returns to step S1.

[0071] In this way, when the intake air temperature and engine water temperature are within the ninth area X9 or the tenth area X10, EGR control is prohibited.

[0072] On the other hand, if the determination in step S20 is YES and the engine water temperature is higher than the second water temperature RW2, the ECU 80 determines whether the intake air temperature read in step S1 is higher than the second intake air temperature RA2 (step S21).

[0073] If the determination in step S21 is NO and the intake air temperature is equal to or lower than the second intake air temperature RA2, that is, if the intake air temperature and the engine water temperature are within the seventh area X7 or the eighth area X8, the ECU 80 proceeds to step S11, prohibits EGR control, and closes the EGR valve 43. After step S11, the ECU 80 returns to step S1.

[0074] In this way, when the intake air temperature and engine water temperature are within the seventh area X7 or the eighth area X8, EGR control is prohibited.

[0075] On the other hand, if the determination in step S21 is YES and the intake air temperature is higher than the second intake air temperature RA2, that is, if the engine water temperature and the intake air temperature are within the fifth area X5 or the sixth area X6, the ECU 80 determines whether the engine E is operating in the low-temperature EGR range A2 (step S22), similar to step S7. The determination in step S22 is the same as step S7, and therefore will not be described here.

[0076] If the determination in step S22 is NO and the engine E is operating outside the low-temperature EGR region A2, the ECU 80 proceeds to step S11 to prohibit EGR control and close the EGR valve 43. After step S11, the ECU 80 returns to step S1.

[0077] In this way, when the intake air temperature and engine water temperature are within the fifth area X5 or the sixth area X6 and the engine E is operating outside the low-temperature EGR area A2, EGR control is prohibited.

[0078] On the other hand, if the determination in step S22 is YES and the engine E is operating in the low-temperature EGR range A2, the ECU 80 next determines whether the intake air temperature is higher than the first intake air temperature RA1 (step S23).

[0079] If the determination in step S23 is YES and the intake air temperature is higher than the first intake air temperature RA1, that is, if the engine E is operating within the low-temperature EGR area A2 and the intake air temperature and the engine water temperature are within the fifth area X5, the ECU 80 proceeds to step S24. In step S24, the ECU 80 performs the same control as in step S9. That is, in step S24, the ECU 80 performs EGR control to open the EGR valve 43 and sets the target EGR rate to the low-temperature EGR rate. In addition, the ECU 80 adjusts the opening of the EGR valve 43 so as to achieve the set target EGR rate.

[0080] In this way, when the intake air temperature and the engine water temperature are within the fifth area X5 and the engine E is operating within the low-temperature EGR area A2, EGR control is performed to control the EGR valve 43 so that the EGR rate becomes the low-temperature EGR rate. Also, as in the case of performing step S9, the EGR amount at this time is set to be equal to or less than the low-temperature upper limit EGR amount and equal to or less than the normal EGR amount at the same operating point.

[0081] Returning to step S23, if the determination in step S23 is NO and the intake air temperature is equal to or lower than the first intake air temperature RA1, that is, if the engine E is operating within the low-temperature EGR area A2 and the intake air temperature and the engine water temperature are within the sixth area X6, the ECU 80 proceeds to step S25. In step S25, the ECU 80 performs EGR control to open the EGR valve 43. The ECU 80 also sets the target EGR rate to the extremely low water temperature EGR rate and adjusts the opening of the EGR valve 43 so that this target EGR rate is achieved.

[0082] The extremely low water temperature EGR rate is set to 0 (zero) when the intake air temperature is the second intake air temperature RA2. The extremely low temperature EGR rate is set to the same value as the low temperature EGR rate when the intake air temperature is the first intake air temperature RA1. The extremely low temperature EGR rate when the intake air temperature is between the second intake air temperature RA2 and the first intake air temperature RA1 is set to a value obtained by linearly interpolating 0 (zero) and the value when the intake air temperature is the first intake air temperature RA1 for the intake air temperature. Thus, like the extremely low water temperature EGR rate, the higher the intake air temperature, the larger the value set to.

[0083] As described above, when the intake air temperature and engine water temperature are within the sixth area X6 and the engine E is operating within the low-temperature EGR region A2, EGR control is performed. Here, the extremely low water temperature EGR rate calculated as described above is smaller than the low-temperature EGR rate and the normal EGR rate at the same operating point. As a result, when the intake air temperature and engine water temperature are within the sixth area X6 and the engine E is operating within the low-temperature EGR region A2, the EGR amount becomes equal to or less than the low-temperature EGR amount, and therefore equal to or less than the low-temperature upper limit EGR amount, and also equal to or less than the normal EGR amount at the same operating point.

[0084] Although not shown in the flowchart, in this embodiment, near the boundary between the sixth area X6 and the second area X2, near the boundary between the seventh area X7 and the third area X3, and near the boundaries between the ninth area X9 and the fifth area X5 and the sixth area X6, the target EGR rate is calculated by linear interpolation of the engine water temperature, and the opening of the EGR valve 43 is adjusted so that this target EGR rate is realized.

[0085] Specifically, when the intake air temperature and engine water temperature are near the boundary between the sixth area X6 and the second area X2, the intake air temperature is higher than the second intake air temperature RA2 and equal to or lower than the first intake air temperature RA1, and the engine water temperature is lower than the first water temperature RW1 and higher than the interpolation first water temperature RW1', the target EGR rate is set as follows: First, the ECU 80 sets the low-temperature EGR rate based on the engine speed and the engine load. Then, the ECU 80 sets the extremely low-water-temperature EGR rate based on the engine speed, the engine load, and the intake air temperature. The ECU 80 then sets the set low-temperature EGR rate as the target EGR rate when the engine water temperature is the first water temperature RW1, sets the set extremely low water temperature EGR rate as the target EGR rate when the engine water temperature is the first interpolation water temperature RW1', calculates a value by linearly interpolating these two values ​​for the engine water temperature, and sets the calculated value as the target EGR rate when the engine water temperature is a temperature between the first interpolation water temperature RW1' and the first water temperature RW1. The first interpolation water temperature RW1' is preset to a value that is very small in difference from the first water temperature RW1 and is stored in the ECU 80. For example, the first interpolation water temperature RW1' is set to a value that is approximately 1°C lower than the first water temperature RW1.

[0086] When the intake air temperature and engine water temperature are in the boundary area between the seventh area X7 and the third area X3, the intake air temperature is higher than the third intake air temperature RA3 and equal to or lower than the second intake air temperature RA2, and the engine water temperature is lower than the first water temperature RW1 and higher than the first interpolation water temperature RW1', the ECU 80 sets the target EGR rate as follows: That is, the ECU 80 sets the target EGR rate when the engine water temperature is the first water temperature RW1 to the EGR rate for extremely low water temperature, sets the target EGR rate when the engine water temperature is the first interpolation water temperature RW1' to 0 (zero), and sets the target EGR rate to a value obtained by linearly interpolating these two values ​​with respect to the engine water temperature. Furthermore, when the intake air temperature and engine water temperature are in the boundary area between the ninth area X9 and the sixth area X6, where the intake air temperature is higher than the second intake air temperature RA2 and is equal to or lower than the first intake air temperature RA1, and the engine water temperature is lower than the second water temperature RW2 and higher than the second water temperature RW2' for interpolation, the ECU 80 sets the target EGR rate as follows: That is, the ECU 80 sets the target EGR rate when the engine water temperature is the second water temperature RW2 to the EGR rate for extremely low water temperature, sets the target EGR rate when the engine water temperature is the second water temperature RW2' to 0 (zero), and sets the target EGR rate to a value obtained by linearly interpolating these two values ​​for the engine water temperature. Furthermore, when the intake air temperature and engine water temperature are in the boundary area between the ninth area X9 and the fifth area X5, where the intake air temperature is higher than the first intake air temperature RA1 and the engine water temperature is lower than the second water temperature RW2 and higher than the second interpolation water temperature RW2', the ECU 80 sets the target EGR rate as follows. That is, the ECU 80 sets the target EGR rate when the engine water temperature is the second water temperature RW2 to the low-temperature EGR rate, sets the target EGR rate when the engine water temperature is the second interpolation water temperature RW2' to 0 (zero), and sets the target EGR rate to a value obtained by linearly interpolating these two values ​​with respect to the engine water temperature. The first interpolation water temperature RW1' is preset to a value whose difference from the second water temperature RW2 is very small and is stored in the ECU 80. For example, the second interpolation water temperature RW2' is set to a value approximately 1°C lower than the second water temperature RW2.

[0087] (effect, etc.) In the above embodiment, the EGR region A1 in which EGR control is performed when the intake air temperature and engine water temperature are within the first area X1 is set over a relatively wide range. Also, when the intake air temperature and engine water temperature are within the second area X2, the third area X3, the fifth area X5, and the sixth area X6, EGR control is performed when the engine E is operating in the low-temperature EGR region A2. This allows for more opportunities to introduce EGR gas, which is an inert gas, into the combustion chamber 5 to lower the combustion temperature and improve engine performance.

[0088] In particular, in the above embodiment, the EGR region A1 and the low-temperature EGR region A2 include regions where the engine speed and engine load are high, and EGR control is performed even when the engine speed and engine load are high. This prevents excessive exhaust gas temperature rise while maintaining engine output. Specifically, when the engine speed and engine load are high, the combustion temperature tends to increase. As the combustion temperature increases, the temperature of the exhaust gas, and therefore the temperature of the catalyst 31A provided in the exhaust passage 30, increases, which may accelerate deterioration of the catalyst 31A. Therefore, it is particularly necessary to lower the combustion temperature when the engine speed and engine load are high. Methods for lowering the combustion temperature include increasing the amount of fuel supplied to the combustion chamber 5 to lower the temperature in the combustion chamber 5 using the latent heat of vaporization of the fuel, and delaying the ignition timing at which the air-fuel mixture in the combustion chamber 5 is ignited to slow combustion. However, these methods may increase the amount of unburned fuel contained in the exhaust gas and reduce engine output. In contrast to this, in the above embodiment, EGR gas is introduced into the combustion chamber 5 even when the engine speed and engine load are high, thereby lowering the combustion temperature. Therefore, it is possible to prevent the exhaust gas from excessively heating up without supplying excessive fuel or retarding the ignition timing, that is, while ensuring engine output and improving exhaust performance.

[0089] However, when the intake air temperature is low, moisture in the EGR gas may condense in the intake passage 20, causing condensed water to form in the intake passage 20. Also, when the engine water temperature is low, the EGR gas may be excessively cooled by low-temperature cooling water in the EGR cooler 42, causing condensed water to form in the EGR passage 41 and ultimately the intake passage 20. A small amount of condensed water has little effect on the combustion state in the combustion chamber 5. However, if a large amount of condensed water flows into the combustion chamber 5 all at once, the air-fuel mixture may not burn properly in the combustion chamber 5, causing a misfire. Also, if a large amount of condensed water accumulates around the throttle valve 22 and freezes, it will affect the operation of the throttle valve 22.

[0090] In contrast, in the above embodiment, when the intake air temperature and engine water temperature are within the second area X2, the third area X3, the fifth area X5, and the sixth area X6—that is, when the intake air temperature or engine water temperature is low—EGR control is performed only when the engine E is operating within the low-temperature EGR region A2. This low-temperature EGR region A2 is set to a region on the high engine speed and engine load side of the line L1, where the intake air flow rate is the set flow rate, and is set to a region where the intake air flow rate is equal to or greater than the set flow rate. That is, when the intake air temperature or engine water temperature is low and the intake air flow rate is less than the set flow rate, preventing the intake air from blowing the condensed water into the combustion chamber 5, the recirculation of EGR gas to the intake passage 20 is prohibited. This prevents condensed water from accumulating in the intake passage 20, thereby avoiding misfires due to condensed water. Furthermore, good drive of the throttle valve 22 is ensured.

[0091] Therefore, according to the above embodiment, it is possible to avoid misfires caused by condensed water while ensuring opportunities to perform EGR control, thereby improving engine performance.

[0092] In the above embodiment, the low-temperature upper limit EGR amount is set to be equal to or less than the maximum value of the normal EGR amount. That is, when the intake air temperature and engine water temperature are within the second area X2, the third area X3, the fifth area X5, and the sixth area X6, the maximum value of the EGR amount is set to be smaller than when the intake air temperature and engine water temperature are within the first area X1. Therefore, when the intake air temperature or engine water temperature is low, the amount of condensed water generated in the intake passage 20 due to condensation of water in the EGR gas can be reliably reduced, and misfires caused by the condensed water can be reliably prevented.

[0093] Furthermore, switching between performing and stopping EGR control involves switching between opening and closing the EGR valve 43, which may cause fluctuations in the flow of EGR gas and intake air, resulting in unstable engine behavior. Furthermore, when the engine E is mounted on a vehicle and the vehicle is traveling in mountainous areas, the intake air temperature may frequently vary above the first intake air temperature RA1. In contrast, according to the above embodiment, when the engine E is operating within the low-temperature EGR range A2, EGR control is continuously performed even if the intake air temperature varies above the first intake air temperature RA1. This stabilizes engine behavior, thereby improving the ride comfort of the vehicle.

[0094] In the above embodiment, when the intake air temperature and engine water temperature are within the fourth area X4, the seventh area X7, the eighth area X8, the ninth area X9, or the tenth area X10 and the intake air temperature or the engine water temperature is extremely low, EGR control is prohibited. In other words, when there is a risk that an amount of condensed water that cannot be blown into the combustion chamber 5 by the intake air is generated, EGR control is prohibited. Therefore, as described above, it is possible to reliably prevent condensed water from accumulating in the intake passage 20 while ensuring opportunities to perform EGR control. Furthermore, when the intake air temperature or the engine water temperature is extremely low, there is a risk that condensed water will freeze in the EGR passage 41 and cause the EGR valve 43 to freeze. However, by prohibiting EGR control as described above, it is possible to prevent the EGR valve 43 from freezing.

[0095] In the above embodiment, the target EGR rate when the intake air temperature and engine water temperature are within the third area X3 is set to a value obtained by linearly interpolating the low-temperature EGR rate and 0 (zero) using the intake air temperature. Accordingly, the target EGR rate and thus the EGR amount when the intake air temperature and engine water temperature are within the third area X3 are lower than the target EGR rate and thus the EGR amount when the intake air temperature and engine water temperature are within the second area X2. Therefore, when condensation is more likely to occur than when the intake air temperature and engine water temperature are within the second area X2, the amount of EGR gas can be reduced to keep the amount of condensed water that is generated low.

[0096] Furthermore, the linear interpolation described above prohibits EGR control when the intake air temperature and engine water temperature are within the fourth area X4, while preventing the EGR rate from changing significantly and the flow of intake air and EGR gas from fluctuating when the intake air temperature changes between the fourth area X4 and the second area X2.

[0097] Similarly, in the above embodiment, the target EGR rate when the intake air temperature and engine water temperature are within the sixth area X6 is set to a value obtained by linearly interpolating the low-temperature EGR rate and 0 (zero) using the intake air temperature. This makes it possible to avoid a large amount of EGR gas from flowing back into the intake passage 20 in a state where condensation is more likely to occur, thereby minimizing the amount of condensed water generated in the intake passage 20. Furthermore, while prohibiting EGR control when the intake air temperature and engine water temperature are within the seventh area X7, it is possible to prevent fluctuations in the flow of intake air and EGR gas caused by a significant change in the EGR rate when the intake air temperature changes between the fifth area X5 and the seventh area X7.

[0098] In the above embodiment, the low-temperature EGR range A2 is preset and stored in the ECU 80, and the condition that the engine E is operating in the low-temperature EGR range A2 is set as the condition for performing EGR control when the intake air temperature and engine water temperature are within the second area X2, the third area X3, the fifth area X5, or the sixth area X6. Therefore, the ECU 80 can easily determine whether to perform EGR control by determining whether the engine E is operating in the low-temperature EGR range A2.

[0099] (Variation) In the above embodiment, a case has been described in which it is determined in step S5 whether the engine E is operating in the EGR region A1, and EGR control is switched between being performed and being stopped (step S6 / step S11) based on this determination, but this determination step may be omitted, and the target EGR rate outside the EGR region A1 may be set to 0 (zero), and EGR control may be prohibited by controlling the EGR valve 43 so as to realize this target EGR rate. Similarly, it is also possible to omit the determinations in steps S7 and S22, and the target EGR rate outside the low-temperature EGR region A2 may be set to 0 (zero), and EGR control may be prohibited by controlling the EGR valve 43 so as to realize this target EGR rate.

[0100] In the above embodiment, the low-temperature EGR region A2 is described as being set to the region of the EGR region A1 on the side of the line L1 where the engine speed and engine load are high, in the region A1_x where the intake flow rate is higher than the set flow rate, and further in the region where the engine speed is higher than a predetermined low-temperature EGR start speed N1. However, the low-temperature EGR region A2 may also be set to the entire region of the EGR region A1 on the side of the line L1 where the engine speed and engine load are high.

[0101] Here, the higher the engine speed, the larger the intake air flow rate. Furthermore, the higher the engine load, the larger the intake air flow rate. Therefore, even if the low-temperature EGR area A2 is set to satisfy the requirement that its lower limit value of the engine load be greater than the lower limit value of the EGR area A1, the low-temperature EGR area A2 will be set to a region of the EGR area A1 where the intake air flow rate is greater. Therefore, in the above embodiment, the low-temperature EGR area A2 is set to satisfy the requirement that the lower limit value of the engine speed of the low-temperature EGR area A2 be greater than the lower limit value of the engine speed of the EGR area A1. However, the low-temperature EGR area A2 may be set to satisfy the requirement that the lower limit value of the engine load be greater than the lower limit value of the EGR area A1, or to satisfy the requirement that the lower limit values ​​of both the engine load and the engine speed be greater than the lower limits of the EGR area A1.

[0102] In the above embodiment, the switching between performing and stopping EGR control when the intake air temperature and engine water temperature are within the second area X2, the third area X3, the fifth area X5, or the sixth area X6 is described as being performed based on the result of determining whether the engine E is operating in the low-temperature EGR area A2. However, instead of this determination, the intake air flow rate may be calculated, and the switching may be performed based on whether the calculated intake air flow rate is equal to or greater than a set flow rate.

[0103] Furthermore, the specific structure of the engine body 1, such as the number of cylinders, and the specific values ​​of the temperatures are not limited to those described above. [Explanation of symbols]

[0104] 1 Engine body 2a cylinder 5 Combustion chamber 20 Intake passage 30 Exhaust passage 41 EGR passage 42 EGR cooler 43 EGR valve 80 ECU (control unit) SN3 Intake air temperature sensor

Claims

1. An exhaust gas recirculation device for an engine including an engine body having a combustion chamber, an exhaust passage connected to the engine body and through which exhaust gas discharged from the engine body flows, and an intake passage connected to the engine body and through which intake air introduced into the engine body flows, an intake air temperature sensor that detects an intake air temperature, which is the temperature of the intake air; an EGR passage that connects the exhaust passage and the intake passage and recirculates EGR gas, which is a part of the exhaust gas, to the intake passage; an EGR valve that opens and closes the EGR passage to change an EGR amount, which is the amount of EGR gas that is recirculated to the intake passage; a control device that performs EGR control to open the EGR valve and recirculate the EGR gas when the engine is operated in a preset EGR region, The control device is characterized in that when the intake temperature detected by the intake temperature sensor is lower than a predetermined first set temperature, the control device prohibits the EGR control under conditions where the intake flow rate, which is the flow rate of intake air flowing through the intake passage, is less than a predetermined set flow rate, even when the engine is operating in the EGR region.

2. 2. The engine exhaust gas recirculation system according to claim 1, The control device determines whether the engine is operating in a predetermined low-temperature EGR region that is preset in the EGR region where the intake air flow rate is equal to or greater than the set flow rate when the intake air temperature is lower than the first set temperature, and performs the EGR control when the engine is operating in the low-temperature EGR region.

3. 3. The engine exhaust gas recirculation system according to claim 2, An exhaust gas recirculation device for an engine, characterized in that the low-temperature EGR region is set so as to satisfy at least one of the following requirements: a lower limit value of the engine speed in the low-temperature EGR region is greater than a lower limit value of the engine speed in the EGR region; and a lower limit value of the engine load in the low-temperature EGR region is greater than a lower limit value of the engine load in the EGR region.

4. The exhaust gas recirculation device for an engine according to any one of claims 1 to 3, The control device controls the EGR valve so that, when the EGR control is performed, the maximum EGR amount is smaller when the intake air temperature is lower than the first set temperature than when the intake air temperature is higher.

5. 2. The engine exhaust gas recirculation system according to claim 1, An exhaust gas recirculation device for an engine, characterized in that, when the EGR control is performed, when the intake air temperature is lower than a predetermined second set temperature that is lower than the first set temperature, the control device controls the EGR valve so that the EGR amount is less than when the intake air temperature is equal to or higher than the second set temperature.

6. 6. The engine exhaust gas recirculation system according to claim 5, The exhaust gas recirculation device for an engine, wherein the control device prohibits the EGR control when the intake air temperature is lower than a predetermined third set temperature that is lower than the second set temperature.

Images