JP5038992B2 - Diesel engine exhaust gas recirculation system - Google Patents

Description

  The present invention relates to an exhaust gas recirculation device for a diesel engine that recirculates a part of exhaust gas in an exhaust passage to an intake passage.

Conventionally, a diesel engine, for the purpose of reducing emissions of NO _x, exhaust gas recirculation (EGR: Exhaust Gas Recirculation) device is provided. The EGR device increases an inert gas component such as CO _{2 in} the intake air by recirculating a part of the exhaust gas in the exhaust passage to the intake passage as EGR gas. Accordingly, EGR device reduces the combustion temperature of the engine, reducing the emissions of NO _x. Here, in the case where the ratio of the EGR gas occupying the intake (EGR rate) is less than the predetermined, the amount of the NO _x and soot in the exhaust gas (SOOT) are in a trade-off relationship, the amount of SOOT is , It increases as the EGR rate increases. This is presumably because the oxygen concentration in the engine cylinder decreases, the fuel rich region increases, and the oxidation reaction of SOOT during combustion becomes slow. On the other hand, increasing the EGR rate to a predetermined or more (e.g., when a large quantity EGR of the EGR rate of 50% or more), the combustion temperature can be lowered below SOOT formation temperature, the occurrence of the NO _x and SOOT It can be expected to reduce at the same time.

  By the way, as an EGR device used for an engine with a supercharger, a high-pressure EGR device that recirculates EGR gas downstream of a compressor is often used.

  However, since the high-pressure EGR device is configured to recirculate EGR gas downstream of the compressor, a large amount of EGR can be achieved at low loads when the exhaust pressure is sufficiently higher than the intake pressure. It was difficult to achieve a large amount of EGR at medium and high loads when the pressure was increased.

  In recent years, therefore, a low-pressure EGR device has been proposed that recirculates a part of exhaust gas after passing through a diesel particulate filter (DPF) on the downstream side of the turbine as EGR gas upstream of the compressor. Yes. Since this type of low-pressure EGR device introduces EGR gas upstream of the compressor, sufficient EGR gas can be recirculated even at medium and high loads of the engine where the supercharging pressure increases. Further, since SOOT is removed from the gas after passing through the DPF and the EGR gas can be introduced into the engine in a clean state, the SOOT discharged from the engine is also reduced.

  On the other hand, since the EGR gas contains water vapor, when a large amount of EGR is performed, a large amount of water vapor is refluxed through the EGR passage. This water vapor does not cause a particular problem when the exhaust temperature is high, but for example, it contains soot and sulfur components in a low temperature range where the exhaust temperature is 100 ° C. or lower (that is, a low load range such as when idling). It may generate condensed water. Therefore, in the low-pressure EGR apparatus that can recirculate a large amount of EGR gas in a wide operation range, a countermeasure against condensed water is required.

To cope with this, for example, in Patent Document 1, an EGR gas compressor is provided on the upstream side of the EGR cooler interposed in the EGR passage, an EGR gas turbine is provided on the downstream side, and EGR before flowing into the EGR cooler. A technique for preventing generation of condensed water in the EGR cooler by compressing the gas to raise the temperature and expanding and cooling the EGR gas after passing through the EGR cooler is disclosed.
JP 2002-89375 A

  However, although the technique disclosed in Patent Document 1 described above solves the problem of condensed water in the EGR cooler to some extent, no measures are taken against other auxiliary machines. Here, the low pressure type EGR device generally has a tendency that the flow path of the EGR gas is longer than that of the high pressure EGR, and the heat flow of the EGR gas is caused by the extended flow path when the engine is cold. Tend to be robbed. In addition, when the low-pressure EGR device is employed, the number of accessories exposed to the EGR gas tends to increase, for example, the compressor of the supercharger is exposed to the EGR gas. Therefore, in the low pressure type EGR apparatus, further countermeasures are required against clogging or corrosion of various auxiliary machines due to condensed water.

  The present invention has been made in view of the above circumstances, and provides an exhaust gas recirculation device for a diesel engine that can prevent the generation of condensed water in the EGR gas flow path and can accurately protect auxiliary equipment. With the goal.

An exhaust gas recirculation device for a diesel engine according to an aspect of the present invention communicates an exhaust passage downstream of a diesel particulate filter with an intake passage upstream of a compressor of a supercharger. An EGR passage that recirculates a part of the exhaust gas as EGR gas to the intake passage, a cooling means that cools the EGR gas that flows through the EGR passage, a bypass passage that bypasses the cooling means, and the cooling means side; A switching valve that adjusts the flow rate ratio of the EGR gas flowing through the bypass passage, a heater that raises the temperature of the EGR gas flowing through the bypass passage, and an EGR gas that controls the flow rate of the EGR gas flowing through the EGR passage A flow rate control means, wherein the EGR gas flow rate control means has an intake air temperature immediately upstream of the compressor at a set temperature or less. Rutoki, and controls the switching valve so as to reduce the flow rate of the EGR gas flowing through the cooling means side.

  According to the exhaust gas recirculation device for a diesel engine of the present invention, it is possible to prevent the generation of condensate in the EGR gas flow path and protect the auxiliary machinery accurately.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The drawings relate to one embodiment of the present invention, FIG. 1 is a schematic configuration diagram of an engine provided with an exhaust gas recirculation device, and FIG. 2 is a flowchart showing an EGR temperature control routine.

  An engine 1 shown in FIG. 1 is, for example, a diesel engine with a supercharger mounted on a vehicle such as an automobile. In the present embodiment, a horizontally opposed four-cylinder engine is shown. Cylinder heads 3 are crowned on both the left and right banks of the cylinder block 2 of the engine 1, and each cylinder head 3 has an intake port 3a and an exhaust port 3b for each cylinder.

  Each intake port 3a is gathered upstream via an intake manifold 5, and is connected to an intake passage 7 via an air chamber 6 formed in the gathered portion. An intercooler 10 is interposed in the intake passage 7, and a compressor 11 a of the turbocharger 11 is interposed on the upstream side of the intercooler 10. Further, an air cleaner 12 is interposed in the intake passage 7 on the upstream side of the compressor 11 a, and an intake chamber 13 is communicated with the upstream side of the air cleaner 12.

  On the other hand, the intake ports 3 b are gathered downstream via the exhaust manifold 15 and connected to the exhaust passage 17. A turbine 11b of the turbocharger 11 is interposed in the exhaust passage 17, and a diesel particulate filter (DPF: Diesel Particulate Filter) 18 is interposed downstream of the turbine 11b. Further, a muffler 19 is interposed in the exhaust passage 17 on the downstream side of the DPF 18. Here, the DPF 18 is, as is well known, PM (particulate matter: particulate matter = mainly black smoke (SOOT)) in the exhaust gas, unburned fuel or lubricant component called SOF, sulfate (Including components generated from the sulfur content in light oil fuel called)) to suppress these emissions into the atmosphere.

  In such an intake / exhaust system of the engine 1, an exhaust gas recirculation device (EGR device) 30 is provided between the intake passage 7 and the exhaust passage 17. In the present embodiment, the EGR device 30 is constituted by a low-pressure EGR device that recirculates a part of the exhaust gas in the exhaust passage 17 to the upstream side of the compressor 11 a as EGR gas, and the EGR device 30 has an EGR passage 31. . One end side of the EGR passage 31 is connected to the exhaust passage 17 on the downstream side of the DPF 18, and the other end side is connected to the intake passage 7 on the upstream side of the compressor 11 a of the turbocharger 11. As a result, the EGR passage 31 communicates between the exhaust passage 17 and the intake passage 7, and a part of the exhaust gas after the soot and the like is collected by the DPF 18 is returned to the upstream side of the compressor 11a as EGR gas. It is possible to do.

  An EGR control valve 32 that opens and closes the EGR passage 31 is interposed in the middle of the EGR passage 31. The EGR control valve 32 has, for example, a butterfly valve body (not shown), and the opening degree of the valve body is controlled by the engine control unit (ECU) 50 according to the operating state of the engine 1. Thus, the flow rate of the EGR gas flowing through the EGR passage 31 is adjusted.

  Further, for example, an EGR cooler 33 as a cooling means is interposed in the middle of the EGR passage 31 on the upstream side (exhaust passage 17 side) of the EGR control valve 32. In the present embodiment, the EGR cooler 33 is constituted by, for example, a water-cooled cooler, and the cooling water of the engine 1 is circulated through the EGR cooler 33. And the EGR cooler 33 cools the EGR gas which distribute | circulates an inside by heat exchange with this cooling water.

  A bypass passage 34 is interposed in the middle of the EGR passage 31. This bypass passage 34 is a bypass route interposed in parallel with the EGR passage 31, branches from the EGR passage 31 on the upstream side (exhaust passage 17 side) from the EGR cooler 33, and downstream from the EGR cooler 33. The EGR passage 31 joins the EGR passage 31 upstream of the EGR control valve 32.

  On the EGR passage 31, a switching valve 35 is provided at the upstream branch portion between the EGR cooler 33 and the bypass passage 34. The switching valve 35 is constituted by, for example, a three-way valve, and adjusts the flow rate ratio between the EGR gas flowing through the EGR cooler 33 and the EGR gas flowing through the bypass passage 34. That is, the switching valve 35 changes the flow rate ratio of the EGR gas to the EGR cooler 33 and the bypass passage 34 by switching the communication amount between the EGR cooler 33 and the bypass passage 34 with respect to the EGR passage 31 under the control of the ECU 50 described later. Variable adjustment is performed between 1: 0 and 0: 1.

  Further, the bypass passage 34 is provided with a heater 36 as a temperature raising means for appropriately raising the temperature of the EGR gas flowing inside. The heater 36 is constituted by, for example, an electric resistance heater that is energized and controlled by the ECU 50, and is provided around the outer periphery of the bypass passage 34.

  Here, the EGR passage 31 is provided with a temperature sensor 38 for detecting an EGR gas temperature (hereinafter referred to as pre-cooler gas temperature) Tef immediately upstream of the EGR cooler 33, and the intake passage 7 is provided with a compressor 11 a. A temperature sensor 39 is provided for detecting an intake air temperature (intake air temperature including EGR gas; hereinafter referred to as pre-compressor gas temperature) Tcf immediately upstream. The ECU 50 controls the temperature of the EGR gas by controlling the switching valve 35 and the heater 36 based on the detected temperatures Tef and Tcf detected by the temperature sensors 38 and 39, and suppresses the generation of condensed water.

  In this case, as will be described later, the ECU 50 basically controls the switching valve 35 and the heater 36 based on the pre-compressor gas temperature Tcf. The ECU 50 first starts the pre-compressor gas temperature Tcf. Is lower than the set temperature T2, the flow rate ratio of the EGR gas flowing through the EGR cooler 33 side is decreased through the control of the switching valve 35 (the flow rate ratio of the EGR gas flowing through the bypass passage 34 side is increased). . Further, even when the flow of EGR gas to the EGR cooler 33 is interrupted, the ECU 50 passes through the bypass passage 34 through the energization control of the heater 36 when the pre-compressor gas temperature Tcf is equal to or lower than the set temperature T2. The temperature rise control of the circulating EGR gas is performed. However, in order to prevent the generation of condensed water in the EGR cooler 33, even if the pre-compressor gas temperature Tcf is higher than the set temperature T2, if the pre-cooler gas temperature Tef is equal to or lower than the set temperature T2, the ECU 50 The flow of EGR gas to the EGR cooler 33 side is shut off. Here, the set temperature T2 is set on the basis of, for example, a temperature (condensation temperature) T1 at which water vapor in the EGR gas starts condensing, and specifically, a condensation temperature T1 (for example, T1 = 100 ° C.). ) Is set to a temperature that is higher than the predetermined temperature.

  Next, the EGR gas temperature control executed by the ECU 50 will be described in accordance with the EGR gas temperature control routine shown in FIG. This routine, for example, is repeatedly executed at set times during EGR control when the EGR control valve 32 is opened (at the time of EGR gas recirculation control). When the routine starts, the ECU 50 first performs a step. In S101, it is checked whether or not the pre-cooler gas temperature Tef is equal to or lower than the set temperature T2.

  When it is determined in step S101 that the pre-cooler gas temperature Tef is equal to or lower than the set temperature T2, the ECU 50 proceeds to step S102 and sets a flag Fc that prohibits the flow of EGR gas into the EGR cooler 33 (Fc After 1), the process proceeds to step S104. That is, when the pre-cooler gas temperature Tef is equal to or lower than the set temperature T2, when the EGR gas is circulated in the EGR cooler 33, the probability that condensed water is generated inside becomes extremely high. And the condensed water which generate | occur | produced in the EGR cooler 33 can become a factor which causes internal clogging, damage, etc. with the soot, sulfur component, etc. which are contained. Therefore, when the pre-cooler gas temperature Tef is equal to or lower than the set temperature T2, the ECU 50 sets the flag Fc to “1” and inhibits the EGR gas from flowing into the EGR cooler 33.

  On the other hand, if it is determined in step S101 that the pre-cooler gas temperature Tef is higher than the set temperature T2, the ECU 50 proceeds to step S103, clears the flag Fc (Fc ← 0), and then proceeds to step S104. That is, when the pre-cooler gas temperature Tef is higher than the set temperature T2, the ECU 50 clears the flag Fc to “0” to allow the EGR gas to flow into the EGR cooler 33.

  When the process proceeds from step S102 or step S103 to step S104, the ECU 50 checks whether or not the pre-compressor gas temperature Tcf is equal to or lower than the set temperature T2.

  If it is determined in step S104 that the pre-compressor gas temperature Tcf is equal to or lower than the set temperature T2, the ECU 50 proceeds to step S105 in order to perform temperature rise control of the EGR gas.

  In step S105, the ECU 50 checks whether or not the EGR cooler 33 side is fully closed with respect to the EGR passage 31 by the switching valve 35 (whether or not the bypass passage 34 side is fully opened with respect to the EGR passage 31). .

  If it is determined in step S105 that the EGR cooler 33 is not in the fully closed state, the ECU 50 proceeds to step S106 in order to increase the temperature by adjusting the flow rate of the EGR gas to the EGR cooler 33.

  In step S106, the ECU 50 checks whether or not the flag Fc is set to “1”. If the ECU 50 determines that the flag Fc is cleared to “0”, the ECU 50 proceeds to step S107 and controls the switching valve 35. Then, after controlling the opening of the EGR cooler 33 side with respect to the EGR passage 31 in a direction to close the set amount with respect to the current opening, the routine is exited. As a result, the flow rate of the EGR gas flowing through the EGR cooler 33 is limited, and as a result, the temperature of the EGR gas is increased.

  On the other hand, when it is determined in step S106 that the flag Fc is set to “1”, the ECU 50 proceeds to step S108 and controls the EGR cooler 33 to be fully closed with respect to the EGR passage 31 through the control of the switching valve 35. Then exit the routine. Thereby, the circulation of the EGR gas into the EGR cooler 33 is prohibited, and as a result, the temperature of the EGR gas is increased and the generation of condensed water in the EGR cooler 33 is prevented.

  If it is determined in step S105 that the EGR cooler 33 is in the fully closed state, the ECU 50 proceeds to step S109, performs temperature increase control of the EGR gas through energization control for the heater 36, and then exits the routine.

  If it is determined in step S104 that the pre-compressor gas temperature Tcf is higher than the set temperature T2, the ECU 50 proceeds to step S110 in order to perform temperature reduction control of the EGR gas.

  In step S110, the ECU 50 checks whether or not the heater 36 is currently turned on, that is, whether or not energization control is being performed on the heater 36. If it is determined that energization control is being performed, the ECU 50 proceeds to step S111. After proceeding and turning off the power to the heater 36, the routine is exited. Thereby, the temperature rise of the EGR gas flowing through the bypass passage 34 through the heater 36 is interrupted, and the temperature of the EGR gas is lowered.

  On the other hand, if it is determined in step S110 that the energization control for the heater 36 is not currently performed, the ECU 50 proceeds to step S112 and checks whether or not the flag Fc is set to “1”.

  If it is determined in step S112 that the flag Fc is set to “1”, the ECU 50 proceeds to step S113 and fully closes the EGR cooler 33 side with respect to the EGR passage 31 through the control of the switching valve 35. After controlling, exit the routine. That is, when the pre-cooler gas temperature Tef is equal to or lower than the set temperature T2, priority is given to preventing the generation of condensed water in the EGR cooler 33 rather than the temperature drop of the EGR gas.

  On the other hand, if it is determined in step S112 that the flag Fc is cleared to “0”, the ECU 50 proceeds to step S114 and determines whether or not the EGR cooler 33 side is currently fully open with respect to the EGR passage 31. Investigate.

  If it is determined in step S114 that the EGR cooler 33 side is fully open, the ECU 50 directly exits the routine.

  On the other hand, if it is determined in step S114 that the EGR cooler 33 side is not fully opened, the ECU 50 proceeds to step S115, and the opening on the EGR cooler 33 side with respect to the EGR passage 31 is set to the current opening through the switching valve 35. After controlling the opening in the direction to open the set amount, exit the routine. Thereby, the flow rate of the EGR gas flowing through the EGR cooler 33 is increased, and as a result, the temperature of the EGR gas is lowered.

  According to such an embodiment, the bypass passage 34 is provided in the EGR cooler 33 interposed in the EGR passage 31, and the flow rate ratio of the EGR gas flowing through the EGR cooler 33 side and the bypass passage 34 side is controlled by the control valve 35. Can be adjusted as needed, and a heater 36 is provided in the bypass passage 34 so that the temperature of the EGR gas flowing in the bypass passage 34 can be raised appropriately, thereby allowing a large amount of EGR gas to recirculate when the engine 1 is idle or the like. Even in this case, not only the EGR cooler 33 but also the auxiliary machines such as the compressor 11a of the turbocharger 11 can be protected accurately by preventing the generation of condensed water.

  In particular, in the present embodiment, the temperature of the EGR gas can be increased even by energization control of the heater 36. Therefore, even in the low-pressure EGR device 30 that tends to increase the flow path of the EGR gas, The temperature of the gas can be accurately raised.

Schematic configuration diagram of an engine equipped with an exhaust gas recirculation device Flowchart showing EGR gas temperature control routine

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Engine 2 ... Cylinder block 3 ... Cylinder head 3a ... Intake port 3b ... Exhaust port 5 ... Intake manifold 6 ... Air chamber 7 ... Intake passage 10 ... Intercooler 11 ... Turbocharger (supercharger)
DESCRIPTION OF SYMBOLS 11a ... Compressor 11b ... Turbine 12 ... Air cleaner 13 ... Intake chamber 15 ... Exhaust manifold 17 ... Exhaust passage 18 ... Diesel particulate filter 19 ... Muffler 30 ... Exhaust gas recirculation device 31 ... EGR passage 32 ... Control valve 33 ... EGR cooler ( Cooling means)
34 ... Bypass passage 35 ... Switching valve 36 ... Heater 38 ... Temperature sensor 39 ... Temperature sensor 50 ... Engine control unit (EGR gas flow rate control means)
T1 ... Condensation temperature T2 ... Set temperature Tcf ... Gas temperature before compressor Tef ... Gas temperature before cooler

Claims (3)

An EGR that connects the exhaust passage downstream of the diesel particulate filter and the intake passage upstream of the compressor of the supercharger to recirculate a part of the exhaust gas in the exhaust passage to the intake passage as EGR gas. A passage,
Cooling means for cooling the EGR gas flowing through the EGR passage;
A bypass passage that bypasses the cooling means;
A switching valve for adjusting a flow rate ratio of the EGR gas flowing through the cooling means side and the bypass passage side;
A heater for raising the temperature of EGR gas flowing through the bypass passage ;
EGR gas flow rate control means for controlling the flow rate of EGR gas flowing through the EGR passage,
The EGR gas flow rate control unit controls the switching valve so as to reduce a flow rate ratio of the EGR gas flowing through the cooling unit side when an intake air temperature immediately upstream of the compressor is equal to or lower than a set temperature. Diesel engine exhaust gas recirculation device.   2. The exhaust gas recirculation device for a diesel engine according to claim 1, wherein the EGR gas flow rate control unit performs temperature rise control of the EGR gas by the heater when the intake air temperature is equal to or lower than the set temperature. .   Even if the intake air temperature is higher than the set temperature, the EGR gas flow rate control means is configured to control the EGR gas flow rate when the temperature of the EGR gas immediately upstream of the cooling means is equal to or lower than the set temperature. The exhaust gas recirculation device for a diesel engine according to claim 2, wherein the switching valve is controlled so as to block the flow to the cooling means.

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