JP2006316706A - Exhaust gas recirculation device and control method of internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To minimize an emission of NOx and soot in exhaust gas even when the operation state of an internal combustion engine is varied. <P>SOLUTION: In an exhaust gas recirculation device equipped with an EGR valve and an exhaust oxygen concentration sensor, an EGR gas flow rate sensor comprised of a heating resistance is provided in the recirculation flow passage, an opening of the EGR valve is adjusted in accordance with a detected oxygen concentration of the exhaust oxygen concentration sensor in a predetermined operation state 1, and the opening of the EGR valve is adjusted in accordance with the detected gas flow rate of the EGR gas flow rate sensor in a predetermined operation state 2 wherein the exhaust oxygen concentration is higher as compared with the predetermined operation state 1. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a configuration and a control method of an exhaust gas recirculation device for an internal combustion engine that recirculates a part of exhaust gas to intake air.

  In an internal combustion engine, exhaust gas recirculation (EGR), which recirculates part of the exhaust discharged to the intake side and lowers the combustion temperature of the air-fuel mixture to reduce NOx in the exhaust, has been widely used.

  Since the EGR gas flow rate affects the exhaust amount of NOx and soot in the exhaust gas, the EGR flow rate is controlled by detecting the intake pipe pressure and the fresh air flow rate. There are descriptions such as those in which the EGR gas flow rate is directly detected and controlled by a gas flow rate sensor.

  It is generally known that the NOx emission amount and the soot emission amount in the exhaust gas largely depend on the oxygen concentration in the entire intake gas of the cylinder. Here, in general, the oxygen concentration of the intake gas is controlled by changing the ratio (EGR rate) of the EGR gas to the total intake gas mainly by adjusting the opening of the EGR valve.

  A method of controlling the EGR valve opening for accurately controlling the oxygen concentration of the intake gas is described in Patent Document 2 and the like.

  Further, an oxygen concentration sensor (λ sensor) is provided in the exhaust pipe in order to accurately control the oxygen concentration of the intake gas, and the EGR valve opening, fuel injection amount, fresh air is set so that the output of the λ sensor becomes a target value. What controls the amount is described in Patent Document 3 and the like. If the fuel injection amount and the fresh air amount are set as predetermined target values, the oxygen concentration of the intake gas can be obtained from the oxygen concentration of the exhaust gas.

  Generally, as described in the above patent document, when the oxygen concentration of the intake gas is set to the optimum value, the exhaust oxygen concentration is stored as a target value, and the exhaust oxygen concentration detected by the λ sensor is higher than the target value. Increases the EGR valve opening to lower the oxygen concentration of the intake gas. If the exhaust oxygen concentration detected by the λ sensor is lower than the target value, the EGR valve opening is decreased to increase the oxygen concentration of the intake gas. Let That is, by adjusting the EGR valve opening degree or the like so as to match the exhaust oxygen concentration with the target value, control is performed so that the exhaust gas NOx and soot discharge amount becomes a small intake gas oxygen concentration.

JP-A-6-74100 JP 9-1206060 A Japanese Patent Laying-Open No. 2005-30407

  A λ sensor composed of a solid electrolyte such as zirconia generates an electromotive force according to the difference in oxygen concentration between the atmosphere and exhaust gas, and the electromotive force is generated under conditions where the exhaust oxygen concentration is low and the oxygen concentration difference between the atmosphere and exhaust gas is large. Oxygen concentration detection error is small because the change in the output current of the λ sensor with respect to the change in exhaust oxygen concentration is large, but the electromotive force is small under conditions where the exhaust oxygen concentration is high and the oxygen concentration difference between the atmosphere and exhaust gas is small. Since the change amount of the output current of the λ sensor with respect to the change of the exhaust oxygen concentration is small, the sensitivity is lowered. This relatively increases the influence of variations in output current due to variations in the characteristics of individual sensor circuits and elements with respect to the output current of the λ sensor. As a result, the variation in oxygen concentration detection increases and the detection accuracy decreases.

For this reason, in the case where the oxygen concentration of the intake gas is controlled using the λ sensor provided in the exhaust pipe as in Patent Document 3, the oxygen concentration of the intake gas is controlled in an operating state where the oxygen concentration of the exhaust gas is high. Accuracy is reduced. When the detected oxygen concentration of the λ sensor becomes higher than the actual oxygen concentration due to the detection error of the λ sensor, the EGR valve opening becomes larger than the optimum value, so that oxygen is insufficient and soot discharge increases. Further, when the detected oxygen concentration of the λ sensor is lower than the actual oxygen concentration, the EGR valve opening becomes smaller than the optimum value, so that there is a problem that the amount of NOx is increased due to excess oxygen.

  An object of the present invention is to minimize the discharge amount of NOx soot even when the operating state changes.

In order to solve the above problems, an exhaust gas recirculation device according to the present invention is attached to a recirculation path for introducing exhaust gas from an exhaust pipe of an internal combustion engine to an intake pipe, an EGR valve attached in the recirculation path, and the exhaust pipe. An exhaust gas oxygen concentration sensor, an EGR gas flow sensor mounted in the reflux path and configured by a heating resistor, and a control means for adjusting the opening degree of the EGR valve according to the detected oxygen concentration of the exhaust gas oxygen concentration sensor; With
The control means adjusts the opening degree of the EGR valve in accordance with the detected oxygen concentration of the exhaust oxygen concentration sensor in a predetermined first operating state, and has a predetermined exhaust oxygen concentration higher than that in the predetermined first operating state. In the second operation state, the opening degree of the EGR valve is adjusted according to the gas flow rate detected by the EGR gas flow rate sensor.

  As described above, according to the exhaust gas recirculation device of the present invention, even if the operating state of the internal combustion engine fluctuates, the EGR valve is used with the sensor that can control the oxygen concentration of the intake gas with the highest accuracy according to the operating state. Therefore, the amount of NOx and soot discharged in the exhaust gas can be reduced.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  Next, a representative example of the engine system configuration according to the present embodiment will be described with reference to FIG.

  FIG. 1 shows a configuration diagram of an engine system according to the present embodiment. Here, an exhaust gas recirculation system for a diesel engine will be described in particular. First, in such a diesel engine system, the injector 5 or turbocharger 6 for injecting fuel pressurized to a high pressure into the combustion chamber, the post-processing system 7 as a DPF or NOx for oxidizing and removing particulate matter contained in the exhaust gas. A catalyst or the like is mounted, and the system is controlled by an ECU 8 mainly composed of a computer including a CPU, a ROM, and a RAM. In an EGR system that recirculates part of the exhaust discharged from the engine to the intake air to reduce NOx, an EGR cooler 10 for lowering the EGR gas temperature is disposed upstream of the reflux path 9. The flow rate of the cooled EGR gas is controlled by the EGR valve 11 provided downstream. The exhaust pipe 19 is provided with a λ sensor 18 for detecting the oxygen concentration of the exhaust gas.

  In the configuration to which the present invention is applied, an EGR gas flow rate sensor 12 that directly measures the mass flow rate of EGR gas is provided in the reflux path 9.

In addition, an air flow sensor 15 that detects the amount of fresh air is provided downstream of the air cleaner 14 in the intake pipe 13.

  Here, the intake pipe pressure sensor 16 and the intake air temperature sensor 17 may be provided without providing the air flow sensor 15, and the fresh air flow rate may be obtained from the detected values of these sensors and the EGR gas flow rate sensor 12.

  FIG. 2 shows a configuration diagram of the EGR gas flow rate sensor 12. The flow rate is measured by at least two resistors, the heating resistor 1 for measuring the flow rate and the temperature measuring resistor 2 for detecting the temperature of the gas to be measured, which is arranged upstream or downstream of the heating resistor. The principle of flow rate detection is that the temperature difference is always kept constant by electrically connecting the heating resistor 1 and the resistance temperature detector 2 in the control circuit 3 provided outside, so that the gas to be measured is kept constant. By eliminating the resistance value change of the heating resistor 1 caused by the temperature change, the flow rate is calculated by setting the change in the resistance value as the flow rate change only. The EGR gas flow sensor 12 outputs an electrical signal corresponding to the flow rate to the ECU via the connector 4 and converts the electrical signal input from the flow meter into a flow rate in the ECU. Further, the present invention is not limited to the above-described method for calculating the flow rate by keeping the temperature difference constant, but may be a method for keeping the heating temperature of the heating element constant.

  As described above, the discharge amount of NOx and soot depends on the oxygen concentration in the intake gas, but the λ sensor generally used for controlling the oxygen concentration of the intake gas has the following problems.

  A λ sensor composed of a solid electrolyte such as zirconia generates an electromotive force according to the difference in oxygen concentration between the atmosphere and the exhaust gas. As shown in FIG. 5, the oxygen concentration difference between the atmosphere and the exhaust gas is low as shown in FIG. Under large conditions, the electromotive force increases, and the change in the output current of the λ sensor with respect to the change in exhaust oxygen concentration is large and the oxygen concentration detection error is small. The amount of change in the output current of the λ sensor with respect to the change in exhaust oxygen concentration is small and the sensitivity is lowered.

  The solid line is the representative characteristic, and the dotted line shows the output current variation of each λ sensor. The variation in the output current of the λ sensor is caused by the variation in the characteristics of the element and the detection circuit.

  As can be seen from the figure, under the condition where the oxygen concentration is high, the influence of the output current variation of each λ sensor becomes relatively large with respect to the change of the output current with respect to the change of the oxygen concentration. As a result, when the oxygen concentration of the exhaust gas is high, the detection variation of the oxygen concentration increases, the detection accuracy decreases, and the oxygen concentration control accuracy of the intake gas decreases.

  Next, a control error when the EGR gas flow rate sensor 12 is provided in the reflux path 9 and the oxygen concentration of the intake gas is controlled from the mass flow rate of the EGR gas detected by the EGR gas flow rate sensor 12 will be described.

  The actual oxygen concentration DO2 of the intake gas is expressed by the equation (1) from the oxygen amount O2a in the fresh air, the oxygen amount O2e in the EGR gas, the fresh air amount Qa, and the EGR gas flow rate Qe.

  In this example, a configuration including the EGR gas flow sensor 12 and the air flow sensor 15 will be described.

  The actual oxygen concentration DO2 of the intake gas when complete combustion is assumed in the steady state from the equation (1) is obtained from the equation (2) from the oxygen concentration a in the fresh air, the EGR rate R, the fuel injection amount Qf, and the theoretical air-fuel ratio k. Indicated by Here, the EGR rate R is defined by R = Qe / (Qa + Qe).

  That is, the oxygen concentration of the intake gas can be obtained from the mass flow rate Qe of the EGR gas detected by the EGR gas flow rate sensor 12, the fresh air amount Qa detected by the air flow sensor 15, and the fuel injection amount Qf. It is possible to control the oxygen concentration of the intake gas from the signal.

  Next, a control error when the oxygen concentration of the intake gas is controlled by the EGR gas flow sensor 12 and the air flow sensor 15 is obtained.

  Here, the detection error of the EGR gas flow sensor 12 and the air flow sensor 15 that detect the mass flow rate of the gas based on the amount of heat released from the heating resistor is a value determined by a predetermined ratio (ratio) with respect to the detected flow rate. That is, when the detected flow rate of the EGR gas flow sensor is Qes and the detection error ratio is εe, Qes = (1 + εe) Qe. Here, the absolute value of the detection error is εe · Qe, and the absolute value of the detection error of the EGR gas flow rate sensor 12 constituted by the heating resistor decreases as the gas flow rate Qe decreases.

Similarly, when the detection flow rate of the air flow sensor 15 is Qas and the detection error ratio is εa, Qas = (1 + εa) Qa. Therefore, the oxygen concentration calculation value DO2S of the intake gas obtained from the sensor signal from the equation (1) is expressed by the equation (3).

  Here, the calculated oxygen amount value O2as in the fresh air of the molecule is obtained from the air flow sensor 15, and is a value obtained by multiplying the actual oxygen amount O2a in the fresh air by the error (1 + εa) of the air flow sensor 15. Similarly, the calculated amount of oxygen O2es in the molecular EGR gas is a value obtained by multiplying the actual oxygen amount O2e in the EGR gas by the error (1 + εe) of the EGR gas flow sensor 12.

  Here, since the oxygen amount O2e in the EGR gas is smaller than the oxygen amount O2a in the fresh air, it can be neglected approximately. At this time, the oxygen concentration calculation value DO2S of the intake gas is expressed by the equation (4).

  The actual oxygen concentration of the intake gas is expressed by equation (5) when εe = 0 and εa = 0 in equation (4).

  Therefore, the error ratio εde of the intake gas oxygen concentration DO2S to the actual oxygen concentration DO2 calculated from the EGR gas flow rate sensor 12 and the airflow sensor 15 is expressed by Expression (6).

  Here, when there is a variation in the injection amount of the injector 5, the optimum oxygen concentration of the suction gas shifts accordingly, so the shift due to the variation in the injection amount is added as an error of the oxygen concentration DO2S of the suction gas. is doing.

  The detection error of the intake gas oxygen concentration εde is a control error when the intake gas oxygen concentration is adjusted using the EGR gas flow rate sensor 12.

  From the equations (4) and (5), the control error εde of the intake gas oxygen concentration by the EGR gas flow rate sensor 12 when the EGR rate changes depending on the operating state can be obtained from the equation (6).

FIG. 3 shows an example of the relationship between the EGR rate with respect to the exhaust oxygen concentration and the calculation result of the control error εde of the intake gas oxygen concentration according to the equation (6). In comparison with this, an example of a control error when the intake gas oxygen concentration is controlled by the λ sensor 18 is also shown.

  Here, εe = + 0.05 (+ 5%), εa = −0.03 (−3%) and εF as −0.04 (−4%) were calculated as conditions for the maximum absolute value of εde.

  It has been found that the exhaust gas oxygen concentration increases as the EGR rate decreases, but the intake gas oxygen concentration control error εde by the EGR gas flow rate sensor 12 decreases as the exhaust oxygen concentration increases.

  This is because the absolute amount of detection error of the EGR gas flow rate sensor 12 formed of the heating resistor as shown in the equation (3) is indicated by εe · Qe and is determined by the relative ratio to the detected flow rate, and the EGR flow rate (EGR rate) This is because the absolute amount of the EGR flow rate detection error decreases and the ratio of the EGR flow rate detection error to the total intake gas amount decreases.

  Specifically, when the EGR rate (EGR flow rate) decreases in the calculated intake gas oxygen concentration value of Equation (3), the ratio of the EGR flow rate detection error εe · Qe to the total intake gas amount Qa + Qe in the denominator decreases, and EGR flow rate detection The control error of the intake gas oxygen concentration due to the error is reduced.

  Here, regarding the influence of the error εa of the air flow sensor, the oxygen mass of the numerator of equation (3) is obtained from the detected value of the new air amount by the air flow sensor, and when the EGR flow rate decreases, the new air amount is detected by the denominator air flow sensor. Since the ratio of the value to the total amount of inhaled gas increases, the influence of εa becomes almost equal in the numerator and denominator, and the influence of the error of the air flow sensor decreases as the EGR rate decreases.

  On the other hand, regarding the control error of the intake gas oxygen concentration by the λ sensor 18 that detects the exhaust oxygen concentration as described above, the exhaust oxygen concentration is low because the λ sensor generates a current corresponding to the oxygen concentration difference between the atmosphere and the exhaust gas. Under conditions where the oxygen concentration difference between the atmosphere and exhaust gas is large, the change in the current of the λ sensor is large and the control error is small with respect to the change in exhaust oxygen concentration, but under conditions where the exhaust oxygen concentration is high and the oxygen concentration difference between the atmosphere and exhaust gas is small Since the current change amount of the λ sensor with respect to the concentration change is small and the sensitivity is lowered, the ratio of the influence of the current variation of the λ sensor increases, and the control error of the intake gas oxygen concentration increases.

  Comparing the control error of the intake gas oxygen concentration by the EGR gas flow rate sensor 12 and the control error of the intake gas oxygen concentration by the λ sensor 18, the EGR gas is controlled by the λ sensor when the exhaust oxygen concentration is low. It can be seen that the control error is smaller than when the control is performed with the flow sensor, and that the control error is smaller when the intake gas oxygen concentration is controlled with the EGR gas flow sensor than when the control is performed with the λ sensor. It was.

  That is, in the configuration in which the heating resistor type EGR gas flow rate sensor 12 is arranged in the reflux path 9 and the EGR gas flow rate is directly detected, the EGR gas flow rate sensor 12 increases as the EGR rate decreases (the exhaust oxygen concentration increases). Since the ratio of the detection error to the total intake gas amount decreases, the error in the calculated oxygen concentration value of the intake gas by the EGR gas flow rate sensor 12 has a characteristic that decreases as the exhaust oxygen concentration increases, which has a high exhaust oxygen concentration. As a result, it has been found that there is a contradictory error characteristic with respect to the λ sensor 18 in which the error of the oxygen concentration calculation value of the intake gas increases.

  In the present invention, based on the evaluation results of the error characteristics described above, in the exhaust gas recirculation device provided with the λ sensor 18, the EGR gas flow rate sensor 12 composed of a heating resistor is provided in the recirculation path 9, and the exhaust oxygen concentration is a predetermined value. When the exhaust gas concentration is higher than the predetermined value, the EGR valve opening degree is controlled using the EGR gas flow rate sensor 12 when the exhaust gas oxygen concentration is higher than the predetermined value. did. As a result, even if the exhaust oxygen concentration varies due to changes in the operating state, the oxygen concentration control error of the intake gas can always be minimized, and the discharge amount of NOx and soot in the exhaust gas can be minimized.

  In the above-described example, the configuration including the λ sensor, the EGR gas flow rate sensor, and the air flow sensor has been described. However, the configuration may be applied to a configuration in which the intake pipe pressure sensor 16 and the intake temperature sensor 17 are provided instead of the air flow sensor.

In the configuration including the intake pipe pressure sensor 16 and the intake air temperature sensor 17, the total intake gas amount Qg of the cylinder is generally obtained from these sensor signals. The oxygen concentration DO2 of the intake gas is expressed by equation (7) from Qg and the EGR gas flow rate Qe. Here, a is the oxygen concentration of inhaled fresh air (atmosphere).

  If the error of the total intake gas amount Qg calculated by the intake pipe pressure sensor 16 and the intake air temperature sensor 17 is εg, and the detection error of the EGR gas flow rate sensor is εe, the oxygen concentration DO2S of the intake gas calculated from these sensor signals. Is represented by equation (8).

  If O2e is ignored with respect to O2a, DO2S is approximated by equation (9).

  From Equation (9), DO2S can be expressed by Equation (10) using the EGR rate R.

  The actual oxygen concentration DO2 of the intake gas is expressed by equation (11) with εg = 0 and εe = 0 in equation (10).

  Accordingly, the calculated error rate εde of the oxygen concentration DO2S of the intake gas with respect to the actual oxygen concentration DO2 is expressed by Equation (12).

  Here, the error rate εde is a control error of the intake gas oxygen concentration as it is.

  FIG. 4 shows an example of the relationship between the EGR rate, the calculation result of the intake gas oxygen concentration control error εde according to the equation (12), and the exhaust oxygen concentration. Further, in comparison with this, a control error when the intake gas oxygen concentration is controlled by the λ sensor 18 is also shown.

As in the case where the intake air oxygen concentration is calculated by obtaining the fresh air amount by the air flow sensor 15 described above, the control error εde of the intake gas oxygen concentration by the EGR gas flow rate sensor 12 is reduced when the EGR rate (EGR gas flow rate) decreases. It becomes smaller as the oxygen concentration increases.

  Therefore, the control error of the intake gas oxygen concentration by the EGR gas flow rate sensor 12 and the control error of the intake gas oxygen concentration by the λ sensor 18 are compared in the same manner as when the intake gas oxygen concentration is calculated by obtaining the fresh air amount by the air flow sensor 15. When the exhaust gas oxygen concentration is low, the control error is smaller when the intake gas oxygen concentration is controlled by the λ sensor 18 than when the exhaust gas oxygen concentration is controlled by the EGR gas flow rate sensor. Control error is smaller when the oxygen concentration is controlled than when the oxygen concentration is controlled.

  Therefore, when the intake gas oxygen concentration is calculated by obtaining the fresh air amount by the air flow sensor, similarly, when the exhaust oxygen concentration is lower than the predetermined value, the EGR valve opening is controlled using the λ sensor, and the exhaust oxygen concentration is controlled. Is higher than the predetermined value, the control error of the intake gas oxygen concentration can be reduced by controlling the EGR valve opening using the EGR gas flow rate sensor.

  Next, a control flow by the ECU 8 of the present invention will be described with reference to FIG.

  In step 100, signals from the λ sensor 18, the EGR gas flow rate sensor 12, the air flow sensor 15, the intake pipe pressure sensor 16, and the intake air temperature sensor 17 are read.

  In step 110, it is determined whether the λ sensor 18 is activated. When the λ sensor 18 is not activated, the EGR valve opening degree is controlled according to step 140 using the EGR gas flow rate sensor 12.

If the λ sensor 18 is activated, an EGR control sensor (λ sensor or EGR gas flow rate sensor) is selected in step 120. As an example of the selection method of the sensor for EGR control,
The ECU 8 sequentially calculates the control error εde of the intake gas oxygen concentration when the EGR valve is controlled by the EGR gas flow sensor 12, and the control error εdλ () of the intake gas oxygen concentration when the EGR valve is controlled by this and the λ sensor 18 and is stored in the ROM of the ECU as table data as shown in FIG. 8 as a function of the exhaust oxygen concentration detected in advance by the λ sensor), and when εdλ <εde The λ sensor 18 is selected as the EGR control sensor. When εdλ ≧ εde, the EGR gas flow rate sensor 12 is selected as the EGR control sensor.

  Here, when εdλ <εde, the λ sensor 18 is selected as a basic EGR control sensor. However, since the EGR gas flow rate sensor 12 is excellent in response to the λ sensor 18, the λ sensor 18 is used as the EGR valve in a steady state. The EGR valve may be temporarily controlled using the EGR gas flow rate sensor 12 in a transient state such as acceleration / deceleration. When εdλ ≧ εde, the EGR gas flow rate sensor 12 is used as a sensor for controlling the EGR valve. At this time, the detection signal of the λ sensor 18 is not used for controlling the EGR valve.

The εde is calculated from the EGR gas flow rate detected by the EGR gas flow rate sensor 12 and the fresh air amount by the airflow sensor 15 or the total intake gas amount calculated from the intake pipe pressure sensor 16 and the intake air temperature sensor 17. The EGR rate R and εe, εa, εF determined from the characteristic variation of each sensor and injector are calculated using the equation (6), or εde for these sensor signals is obtained in advance in the memory (ROM) of the ECU. It may be stored as map data as shown in FIG. 7 corresponding to the EGR rate R calculated from each sensor signal, and the map data may be obtained by sequentially referring to each sensor signal.

  Further, the exhaust gas oxygen concentration DEO2λ detected by the λ sensor is compared with a predetermined threshold value SL. When DEO2λ is lower than SL, the λ sensor is selected as an EGR control sensor, and when DEO2λ is higher than SL, EGR gas is selected. The flow sensor 12 may be selected as an EGR control sensor. Here, SL is set in advance to an upper limit value that can ensure the detection accuracy of the λ sensor, or, as described above, since the accuracy of oxygen concentration control of the intake gas by the EGR gas flow rate sensor 12 depends on the EGR rate, SL is set in advance. You may make it set with the table data memorize | stored as a function of an EGR rate as shown in FIG.

Further, the control error εdλ and εde of the intake oxygen concentration by each sensor depends on the exhaust oxygen concentration and the EGR rate, but the exhaust oxygen concentration depends on the EGR rate and the fuel injection amount. Generally, the EGR rate depends on the fuel injection amount (load) and the rotation. Therefore, the EGR control sensor may be selected according to the fuel injection amount (load) and the rotation. In this case, the control error of the intake gas oxygen concentration by each sensor described above is obtained in advance for the fuel injection amount (load) and rotation, and the sensor having the smaller control error is stored in the memory of the ECU 8 with respect to the load and rotation. You may make it memorize.

  Further, since the exhaust oxygen concentration mainly depends on the EGR rate at low to medium loads, the EGR rate calculated from the detection value of the EGR gas flow rate sensor 12 or the like or the EGR flow rate detection value has a predetermined EGR rate. When the value is higher than the threshold value, the λ sensor 18 may be used as the EGR valve control sensor, and when the EGR rate is lower than the threshold value, the EGR gas flow rate sensor 12 may be used. The threshold value may be corrected by a fuel injection amount or the like.

  When the λ sensor 18 is selected as the EGR control sensor in step 120, the detection value DEO2λ of the exhaust oxygen concentration detected by the λ sensor 18 is compared with the target value DEO2T of the exhaust oxygen concentration in step 130, and DEO2λ> DEO2T is satisfied. To increase the EGR valve opening θEGR and increase the EGR rate to decrease the exhaust oxygen concentration, and when DEO2λ <DEO2T, decrease the EGR valve opening θEGR to decrease the EGR rate and increase the oxygen concentration of the intake gas Thus, θEGR is controlled so that DEO2λ is set to DEO2T. Here, the target value DEO2T of the exhaust oxygen concentration is stored in advance in the memory of the ECU 8 in accordance with the operating state so that the exhaust gas NOx and the soot discharge amount are minimized.

  When the EGR gas flow rate sensor 12 is selected as the EGR control sensor in step 120, the detected gas flow rate Qes of the EGR gas flow rate sensor 12 is compared with the target value QeT of the gas flow rate in step 140, and when Qes> QeT Decreases θEGR, and when Qes <QeT, controls θEGR to increase θEGR and to set Qes to QeT. Here, the target value QeT of the EGR gas flow rate is stored in advance in the memory of the ECU 8 in accordance with the operating state so that the exhaust gas NOx and the soot discharge amount are minimized. As a result, the oxygen concentration of the intake gas can be accurately controlled to the optimum value by the EGR gas flow rate sensor 12 even in an operating state where the detection accuracy of the exhaust oxygen concentration cannot be ensured by the λ sensor 18.

The block diagram of the engine system which concerns on this embodiment. The block diagram of the EGR gas flow rate sensor which concerns on this embodiment. Relationship between exhaust oxygen concentration and control error of each sensor (system with air flow sensor). Relationship between exhaust oxygen concentration and control error of each sensor (system with intake pipe pressure sensor). Relationship between exhaust oxygen concentration and λ sensor output current. The control flowchart which concerns on this embodiment. The control error characteristic table of the intake gas oxygen concentration by an EGR gas flow sensor. The control error characteristic table of the intake gas oxygen concentration by a lambda sensor. Threshold table of exhaust oxygen concentration for selecting a control sensor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Heating resistor, 2 ... Resistance temperature detector, 3 ... Control circuit, 5 ... Injector, 8 ... ECU, 9 ... Recirculation path, 10 ... EGR cooler, 11 ... EGR valve, 12 ... EGR gas flow sensor,
DESCRIPTION OF SYMBOLS 13 ... Intake pipe, 15 ... Air flow sensor, 16 ... Intake pipe pressure sensor, 17 ... Intake temperature sensor, 18 ... Lambda sensor, 19 ... Exhaust pipe

Claims (5)

A recirculation path for introducing exhaust gas from an exhaust pipe of an internal combustion engine to an intake pipe, an EGR valve attached to the recirculation path, an exhaust oxygen concentration sensor attached to the exhaust pipe, and a heating resistance attached to the recirculation path An EGR gas flow sensor composed of a body, and control means for adjusting the opening of the EGR valve based on the detected oxygen concentration of the exhaust oxygen concentration sensor,
The control means adjusts the opening degree of the EGR valve in accordance with the detected oxygen concentration of the exhaust oxygen concentration sensor in a predetermined first operating state, and has a predetermined exhaust oxygen concentration higher than that in the predetermined first operating state. An exhaust gas recirculation device and control method for an internal combustion engine, wherein the opening degree of the EGR valve is adjusted based on a gas flow rate detected by the EGR gas flow rate sensor in the second operating state.   The control means decreases the opening of the EGR valve when the detected oxygen concentration of the exhaust oxygen concentration sensor is lower than a predetermined first target value in the predetermined first operating state, and the exhaust oxygen concentration sensor When the detected oxygen concentration is higher than a predetermined first target value, the opening degree of the EGR valve is increased, and the detected gas flow rate of the EGR gas flow sensor is set to a predetermined second in the predetermined second operating state. When the detected gas flow rate of the EGR gas flow sensor is higher than a predetermined target value 2, the EGR valve opening is decreased when the EGR valve opening is lower than the target value. The exhaust gas recirculation apparatus and control method according to claim 1.   The control means determines the first operating state and the second operating state based on any one of a detected oxygen concentration of the exhaust oxygen concentration sensor, a detected gas flow rate of the EGR gas flow rate sensor, and a fuel injection amount. The exhaust gas recirculation apparatus and control method according to claim 1.   The control means adjusts the opening degree of the EGR valve according to the detected oxygen concentration of the exhaust oxygen concentration sensor when the detected oxygen concentration of the exhaust oxygen concentration sensor is lower than a predetermined threshold, and the exhaust oxygen concentration 2. The exhaust gas recirculation according to claim 1, wherein when the detected oxygen concentration of the sensor is higher than a predetermined threshold value, the opening degree of the EGR valve is adjusted according to the detected gas flow rate of the EGR gas flow rate sensor. Apparatus and control method. The control means adjusts the opening degree of the EGR valve according to the detected oxygen concentration of the exhaust oxygen concentration sensor when the detected gas flow rate of the EGR gas flow rate sensor is higher than a predetermined threshold value, and the EGR gas flow rate 2. The exhaust gas recirculation according to claim 1, wherein when the detected gas flow rate of the sensor is lower than a predetermined threshold value, the opening degree of the EGR valve is adjusted according to the detected gas flow rate of the EGR gas flow rate sensor. Apparatus and control method.

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