Control method and device for exhaust recirculation device

Abstract

This control device for an exhaust gas recirculation device provided with an EGR valve for controlling an exhaust gas recirculation amount includes: a target EGR rate setting unit (51); a target EGR valve passing flow rate calculation unit (52); an EGR valve upstream-side pressure estimation unit (53); an EGR valve downstream-side pressure estimation unit (54); an EGR valve gas temperature estimation unit (55); a basic target EGR valve opening area calculation unit (56); a differential pressure sensor (25); an actual EGR valve opening area calculation unit (57); an opening area ratio calculation unit (58); a limiter (59); a steady state determination unit (60); a final target EGR valve opening area calculation unit (61); and an EGR valve control unit (62). An actual EGR valve opening area based on the front-rear differential pressure (ΔP) detected by the differential pressure sensor (25) is used to correct the basic target EGR valve opening area based on the estimated value.

Description

Control method and device for exhaust gas recirculation system

[0001] The present invention relates to the control of an exhaust gas recirculation device that recirculates part of the exhaust gas from the exhaust system of an internal combustion engine to the intake system.

[0002] In an exhaust gas recirculation system that recirculates a portion of the exhaust gas from the exhaust system to the intake system of an internal combustion engine, to achieve a high exhaust gas recirculation rate close to the combustion limit in order to improve fuel economy, it is necessary to control the flow rate of the exhaust gas actually flowing into the combustion chamber with high precision. The exhaust gas recirculation rate, i.e., the exhaust gas flow rate, is adjusted by the opening of an exhaust gas recirculation control valve (EGR valve) provided in the exhaust gas recirculation passage. If the EGR valve is considered a type of orifice, the flow rate is determined by the opening area of ​​the EGR valve, the differential pressure across the EGR valve, and the exhaust gas temperature.

[0003] Patent Document 1 discloses an exhaust gas recirculation system for a turbocharged diesel engine, which includes a differential pressure sensor that detects the differential pressure across an EGR valve. In this system, the boost pressure sensor detects the boost pressure in the intake manifold, and the opening of the EGR valve is set based on the detected boost pressure and the differential pressure across the exhaust gas recirculation system, using a map that uses these values ​​as parameters.

[0004] However, if the opening of the EGR valve is controlled using the differential pressure across the EGR valve measured by a differential pressure sensor as the main parameter, the pulsating components of the exhaust and intake air will have a large effect, making the opening of the EGR valve and therefore the control of the exhaust gas recirculation amount unstable.

[0005] Japanese Patent Application Laid-Open No. 2004-278421

[0006] This invention is a control method for an exhaust gas recirculation device equipped with an exhaust gas recirculation passage that recirculates a portion of the exhaust gas from the exhaust system to the intake system of an internal combustion engine, and an EGR valve (exhaust gas recirculation control valve) that controls the amount of exhaust gas recirculation, and comprises the steps of: determining a basic target EGR valve opening area based on a required exhaust gas recirculation rate; measuring a differential pressure across the EGR valve and calculating an actual EGR valve opening area based on this differential pressure across the EGR valve and the amount of exhaust gas recirculated at that time; determining an opening area ratio between this actual EGR valve opening area and the basic target EGR valve opening area; correcting the basic target EGR valve opening area using this opening area ratio to determine a final target EGR valve opening area; and controlling the opening degree of the EGR valve in accordance with this final target EGR valve opening area.

[0007] That is, basically, the basic target EGR valve opening area is calculated based on the required exhaust gas recirculation rate in a so-called feedforward manner without depending on the actual measured value of the differential pressure across the exhaust gas.

[0008] Meanwhile, in parallel with the calculation of this basic target EGR valve opening area, the differential pressure across the EGR valve is measured, and an actual EGR valve opening area is calculated based on this measured differential pressure. This actual EGR valve opening area corresponds to the EGR valve opening area required to obtain the exhaust gas recirculation amount at that time under the measured differential pressure across the EGR valve. The basic target EGR valve opening area is corrected using the opening area ratio between this actual EGR valve opening area and the basic target EGR valve opening area to calculate the final target EGR valve opening area.

[0009] In other words, the basic target EGR valve opening area is feedback corrected based on the actually measured differential pressure across the exhaust gas, improving the control accuracy of the exhaust gas recirculation amount. Furthermore, since the basic target EGR valve opening area is basically calculated in a feedforward manner, stable control is possible without being affected by the pulsating components of the exhaust gas and intake air.

[0010] While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments and is not to be construed as limiting the invention.

[0011] An embodiment of the present invention will now be described in detail with reference to the accompanying drawings.

[0012] 1 shows an exhaust gas recirculation system for an internal combustion engine 1 equipped with a turbocharger 3 as one embodiment of the present invention. This internal combustion engine 1 is, for example, a four-stroke, spark-ignition gasoline engine. An exhaust turbine 4 of the turbocharger 3 is disposed in its exhaust passage 6, and an upstream catalytic converter 7 and a downstream catalytic converter 8, each using, for example, a three-way catalyst, are disposed downstream of the exhaust turbine 4. An exhaust silencer (not shown) is disposed further downstream of the exhaust passage 6, and the exhaust passage 6 is opened to the outside via the exhaust silencer. The exhaust turbine 4 is equipped with a known bypass passage 9 and a wastegate valve 10 for controlling boost pressure.

[0013] The compressor 5 of the turbocharger 3 is disposed in the intake passage 11 of the internal combustion engine 1, and an electronically controlled throttle valve 12 for controlling the amount of intake air is disposed downstream of the compressor 5. Note that an intercooler (not shown) for cooling the supercharged intake air may be provided between the compressor 5 and the throttle valve 12.

[0014] An exhaust gas recirculation passage 21 for recirculating a portion of the exhaust gas to the intake system is provided between the exhaust passage 6 and the intake passage 11. A base end 21a of the exhaust gas recirculation passage 21 branches off from the exhaust passage 6 downstream of the exhaust turbine 4, more specifically, between the upstream catalytic converter 7 and the downstream catalytic converter 8. A tip end 21b of the exhaust gas recirculation passage 21 is connected to the intake passage 11 at a position upstream of the compressor 5.

[0015] That is, the illustrated exhaust gas recirculation system is a so-called low-pressure exhaust gas recirculation system in which exhaust gas that has passed through the exhaust turbine 4 and has become low-pressure is recirculated to the intake system.

[0016] An EGR valve (exhaust gas recirculation control valve) 22, the opening of which is variably controlled by the engine controller 2, is disposed in the exhaust gas recirculation passage 21. The EGR valve 22 may be of any type as long as it can control the flow rate of exhaust gas. In one embodiment, however, a so-called duty control solenoid valve is used, which adjusts the flow rate by variably controlling the on-duty of a drive pulse signal (i.e., PWM control). Furthermore, an EGR gas cooler 23, which cools the recirculated exhaust gas, is disposed at a position closer to the exhaust passage 6 than the EGR valve 22 (i.e., upstream in terms of the exhaust flow).

[0017] In a preferred embodiment, a vacuum generating valve 24 with an adjustable opening for generating the differential pressure required for exhaust gas recirculation is disposed upstream of the connection of the intake passage 11 with the exhaust gas recirculation passage 21. The opening of this vacuum generating valve 24 is controlled by the engine controller 2 in accordance with the operating conditions of the internal combustion engine 1 (in other words, the target exhaust gas recirculation rate (EGR rate)). However, this vacuum generating valve 24 is not an essential part of the present invention, and a detailed description thereof will be omitted.

[0018] The exhaust gas recirculation passage 21 is provided with a differential pressure sensor 25 that detects a pressure difference ΔP across the EGR valve 22. The differential pressure sensor 25 is, for example, of a type that detects the pressure difference between two pressure chambers, with the pressure upstream of the EGR valve 22 (on the EGR gas cooler 23 side) being introduced into one pressure chamber and the pressure downstream of the EGR valve 22 (on the intake passage 11 side) being introduced into the other pressure chamber. The differential pressure sensor 25 may be of a type that measures the pressures of the two pressure chambers individually and outputs the pressure difference between the two. Alternatively, two pressure sensors that measure the upstream pressure and the downstream pressure individually may be used instead of the differential pressure sensor 25, and the differential pressure ΔP across the EGR valve 22 may be calculated from the respective detected pressure values.

[0019] In addition to the detection signal from the differential pressure sensor 25, detection signals from various sensors such as an air flow meter 31 that detects the intake air amount, an air-fuel ratio sensor 32 that detects the exhaust air-fuel ratio, a crank angle sensor 33 that detects the engine rotation speed, a water temperature sensor 34 that detects the coolant temperature, a boost pressure sensor 35 that detects the boost pressure, an accelerator opening sensor 36 that detects the amount of depression of the accelerator pedal, an atmospheric pressure sensor 37 that detects the atmospheric pressure, and an outside air temperature sensor 38 that detects the outside air temperature are input to the engine controller 2. Based on these detection signals and requests from other controllers in the vehicle, the engine controller 2 optimally controls the fuel injection amount and injection timing, ignition timing, the opening of the throttle valve 12, the boost pressure, the EGR rate, etc.

[0020] 2 is a functional block diagram showing an embodiment of EGR valve control, which will be described below. Each of the functional blocks shown in the diagram is realized as a function of the engine controller 2.

[0021] As shown in FIG. 2 , the EGR valve control of one embodiment includes a target EGR rate setting unit 51, a target EGR valve passing flow rate calculation unit 52, an EGR valve upstream pressure estimation unit 53, an EGR valve downstream pressure estimation unit 54, an EGR valve gas temperature estimation unit 55, a basic target EGR valve opening area calculation unit 56, an actual EGR valve opening area calculation unit 57, an opening area ratio calculation unit 58, a limiter 59, a steady state determination unit 60, a final target EGR valve opening area calculation unit 61, and an EGR valve control unit 62.

[0022] A target EGR rate setting unit 51 sets a target EGR rate #EGR using, for example, a map based on the operating conditions (mainly the load and rotation speed) of the internal combustion engine 1. A target EGR valve passage flow rate calculation unit 52 calculates a target EGR valve passage flow rate (mass flow rate of exhaust gas passing through the EGR valve 22) #Qegr required to achieve the target EGR rate #EGR.

[0023] An EGR valve upstream pressure estimating unit 53 estimates the pressure upstream (inlet side) of the EGR valve 22 by estimating the exhaust pressure and integrating the pressure losses of various components, and outputs an estimated EGR valve upstream pressure P1. Similarly, an EGR valve downstream pressure estimating unit 54 estimates the pressure downstream (outlet side) of the EGR valve 22 by integrating the pressure losses of various components, using atmospheric pressure as a base point, and outputs an estimated EGR valve downstream pressure P2. The pressure difference across the EGR valve 22 is estimated from the estimated EGR valve upstream pressure P1 and the estimated EGR valve downstream pressure P2.

[0024] The EGR valve gas temperature estimation unit 55 estimates the exhaust temperature according to the operating conditions, heat exchange in the EGR gas cooler 23, temperature drop in the pipe, etc., to estimate the temperature of the exhaust passing through the EGR valve 22, and outputs the estimated EGR valve gas temperature Tegr.

[0025] Based on these target EGR valve passing flow rate #Qegr, estimated EGR valve upstream pressure P1, estimated EGR valve downstream pressure P2, and estimated EGR valve gas temperature Tegr, the basic target EGR valve opening area calculation unit 56 treats the EGR valve 22 as a type of orifice and calculates the opening area of ​​the EGR valve 22 required to obtain the target EGR valve passing flow rate #Qegr, and outputs this as the basic target EGR valve opening area #Aegr0.

[0026] The actual EGR valve opening area calculation unit 57 receives the upstream / downstream differential pressure ΔP output by the differential pressure sensor 25, the target EGR valve passage flow rate #Qegr, and the estimated EGR valve gas temperature Tegr as inputs, and calculates the opening area of ​​the EGR valve 22 required to achieve the target EGR valve passage flow rate #Qegr, treating the EGR valve 22 as a type of orifice based on these. The calculated opening area is then output as the actual EGR valve opening area rAegr. Note that the signal output by the differential pressure sensor 25 vibrates relatively greatly due to pulsating components of the exhaust and intake air, so the signal is subjected to appropriate filtering before being input to the actual EGR valve opening area calculation unit 57 as the upstream / downstream differential pressure ΔP.

[0027] The actual EGR valve opening area calculation unit 57 basically performs the same calculation as the basic target EGR valve opening area calculation unit 56, but instead of the estimated EGR valve upstream pressure P1 and the estimated EGR valve downstream pressure P2 input to the basic target EGR valve opening area calculation unit 56, the EGR valve opening area is determined based on the front-to-rear differential pressure ΔP actually measured by the differential pressure sensor 25. Therefore, the actual EGR valve opening area rAegr can be said to be the EGR valve opening area required to achieve the target EGR valve passing flow rate #Qegr under the actually measured front-to-rear differential pressure ΔP.

[0028] The opening area ratio calculation unit 58 calculates an opening area ratio Ra between the actual EGR valve opening area rAegr calculated by the actual EGR valve opening area calculation unit 57 and the basic target EGR valve opening area #Aegr0 calculated by the basic target EGR valve opening area calculation unit 56. For example, in one embodiment, the opening area ratio calculation unit 58 outputs the ratio of the actual EGR valve opening area rAegr when the basic target EGR valve opening area #Aegr0 is set to "1" as the "opening area ratio Ra" to the final target EGR valve opening area calculation unit 61. That is, in one embodiment, "Ra = rAegr / #Aegr0". Here, a limiter 59 is provided for the output value of the opening area ratio calculation unit 58, and the opening area ratio Ra is limited to a range of predetermined upper and lower limits. For example, the opening area ratio Ra is limited to a range of about several percent with "1" as the center.

[0029] The steady state determination unit 60 determines whether the internal combustion engine 1 is in a steady state. For example, it determines that the internal combustion engine 1 is in a steady state when the rate of change of the load and the rotation speed is small, and outputs a steady state determination signal S. In a preferred embodiment, the basic target EGR valve opening area #Aegr0 is corrected only when the internal combustion engine 1 is in a steady state.

[0030] A final target EGR valve opening area calculation unit 61 corrects the basic target EGR valve opening area #Aegr0 using the opening area ratio Ra output by the opening area ratio calculation unit 58 and limited by a limiter 59, and outputs the corrected value as the final target EGR valve opening area #Aegr to an EGR valve control unit 62. This correction is performed on the condition that the steady state determination unit 60 determines that the engine is in a steady state. If the engine is not in a steady state, the basic target EGR valve opening area #Aegr0 is output as is as the final target EGR valve opening area #Aegr to the EGR valve control unit 62. For example, in one embodiment, the final target EGR valve opening area #Aegr is calculated by multiplying the basic target EGR valve opening area #Aegr0 by the opening area ratio Ra.

[0031] The final target EGR valve opening area #Aegr calculated in this manner is a value that literally corresponds to the "area" of the orifice, and is therefore converted into the opening degree of the EGR valve 22 in the EGR valve control unit 62, and further converted into the on-duty in the PWM control of the EGR valve 22.

[0032] 3 is a flowchart showing the process flow of the EGR valve control. The process shown in this flowchart is repeatedly executed by the engine controller 2.

[0033] First, in step 1, a target EGR rate #EGR is set based on the operating conditions of the internal combustion engine 1 at that time, and in step 2, a target EGR valve passing flow rate #Qegr required to achieve the target EGR rate #EGR is calculated.

[0034] Next, in step 3, the estimated EGR valve upstream pressure P1 and the estimated EGR valve downstream pressure P2 are estimated, and in step 4, the estimated EGR valve gas temperature Tegr is estimated. Note that these pressure and temperature estimations are always performed in another routine (not shown), and the values ​​thereof are read.

[0035] In step 5, as described above, the basic target EGR valve opening area #Aegr0 is calculated using the target EGR valve passing flow rate #Qegr, the estimated EGR valve upstream pressure P1, the estimated EGR valve downstream pressure P2, and the estimated EGR valve gas temperature Tegr.

[0036] Substantially in parallel with the calculation of the basic target EGR valve opening area #Aegr0, in step 6, the actual EGR valve opening area rAegr is calculated using the front-to-rear differential pressure ΔP measured by the differential pressure sensor 25.

[0037] Then, in step 7, the ratio Ra of the opening areas of the two is calculated.

[0038] In step 8, it is determined whether the internal combustion engine 1 is in a steady state. If it is in the steady state, the process proceeds to step 9, where the basic target EGR valve opening area #Aegr0 is corrected using the opening area ratio Ra to set the final target EGR valve opening area #Aegr. Then, in step 10, the EGR valve 22 is driven in accordance with the corrected final target EGR valve opening area #Aegr.

[0039] On the other hand, if the engine is not in a steady state, the process proceeds from step 8 to step 11, where the basic target EGR valve opening area #Aegr0 is set as the final target EGR valve opening area #Aegr without any correction. Then, in step 10, the EGR valve 22 is driven in accordance with this final target EGR valve opening area #Aegr.

[0040] As described above, in the above embodiment, the EGR valve opening area is basically controlled in a feedforward manner using the estimated EGR valve upstream pressure P1, the estimated EGR valve downstream pressure P2, and the estimated EGR valve gas temperature Tegr, so that stable exhaust flow control is achieved without being affected by exhaust and intake pulsation components. Furthermore, when the internal combustion engine 1 is in a steady state, the actual EGR valve opening area rAegr obtained based on the upstream-to-rear differential pressure ΔP actually detected by the differential pressure sensor 25 is used as a kind of feedback signal to correct the basic target EGR valve opening area #Aegr0, thereby correcting variations in the feedforward control system and improving the control accuracy of the exhaust flow rate. Therefore, it is possible to increase the target EGR rate close to the combustion limit.

[0041] In addition, during unsteady, i.e., transient, conditions, it is preferable not to perform correction based on the actual EGR valve opening area rAegr, i.e., the opening area ratio Ra, from the viewpoint of control delay, etc. In the above embodiment, the opening area ratio Ra, which serves as a type of correction coefficient, is limited within an appropriate range by the limiter 59, so that overcorrection and destabilization of flow control due to disturbances, etc., can be avoided.

[0042] Although one embodiment of the present invention has been described above, the present invention is not limited to the above embodiment and various modifications are possible.

Claims

1. A control method for an exhaust gas recirculation device equipped with an exhaust gas recirculation passage that recirculates a portion of the exhaust gas from the exhaust system of an internal combustion engine to the intake system, and an EGR valve that controls the amount of exhaust gas recirculation, comprising the steps of: determining a basic target EGR valve opening area based on a required exhaust gas recirculation rate; measuring a differential pressure before and after the EGR valve, and calculating an actual EGR valve opening area based on this differential pressure before and after and the amount of exhaust gas recirculated at that time; determining an opening area ratio between this actual EGR valve opening area and a basic target EGR valve opening area; correcting the basic target EGR valve opening area using this opening area ratio to determine a final target EGR valve opening area; and controlling the opening degree of the EGR valve in accordance with this final target EGR valve opening area.

2. The method for controlling an exhaust gas recirculation system according to claim 1, further comprising estimating a differential pressure across the EGR valve, and determining the basic target EGR valve opening area using the estimated differential pressure across the EGR valve.

3. A control method for an exhaust gas recirculation device as described in claim 1, which determines whether the state is steady or transient, and if the state is steady, corrects the basic target EGR valve opening area using the opening area ratio, and if the state is transient, uses the basic target EGR valve opening area as the final target EGR valve opening area.

4. The method for controlling an exhaust gas recirculation system according to claim 1, wherein the measurement of the differential pressure across the exhaust gas recirculation control valve is carried out using a differential pressure sensor.

5. A control method for an exhaust gas recirculation device as described in claim 1, further comprising the steps of: determining a target EGR valve passing flow rate based on a required exhaust gas recirculation rate; determining the basic target EGR valve opening area based on this target EGR valve passing flow rate; and when calculating the actual EGR valve opening area, regarding the target EGR valve passing flow rate as the exhaust gas recirculation flow rate at that time.

6. A control method for an exhaust gas recirculation device as set forth in claim 1, further comprising estimating a gas temperature of gas passing through the EGR valve, and correcting the basic target EGR valve opening area and the actual EGR valve opening area based on the gas temperature.

7. A control method for an exhaust gas recirculation system according to claim 1, wherein the exhaust gas recirculation system is a low-pressure exhaust gas recirculation system for recirculating exhaust gas from the downstream side of a turbine to the upstream side of a compressor in a spark ignition internal combustion engine equipped with a turbocharger.

8. A control device for an exhaust gas recirculation device equipped with an exhaust gas recirculation passage for recirculating a portion of the exhaust gas from the exhaust system of an internal combustion engine to an intake system, and an EGR valve for controlling the amount of exhaust gas recirculation, comprising: a basic target EGR valve opening area calculation unit for determining a basic target EGR valve opening area based on a required exhaust gas recirculation rate; a differential pressure sensor for measuring a differential pressure before and after the EGR valve; an actual EGR valve opening area calculation unit for calculating an actual EGR valve opening area based on the differential pressure before and after the exhaust gas recirculation amount at that time; a steady state determination unit for determining whether the state is steady or transient; and an EGR valve control unit for controlling the opening degree of the EGR valve based on the actual EGR valve opening area if the state is steady, and controlling the opening degree of the EGR valve based on the basic target EGR valve opening area if the state is transient.

9. The control device for an exhaust gas recirculation device according to claim 8, wherein the actual EGR valve opening area is limited so that the ratio of the actual EGR valve opening area to the basic target EGR valve opening area falls within a predetermined range with "1" as the center.

10. A control device for an exhaust gas recirculation device as described in claim 8, further comprising: an opening area ratio calculation unit which calculates an opening area ratio between an actual EGR valve opening area and a basic target EGR valve opening area; a limiter which limits the opening area ratio within a predetermined upper and lower limit range; and a final target EGR valve opening area calculation unit which corrects the basic target EGR valve opening area using the opening area ratio limited by the limiter to calculate a final target EGR valve opening area, wherein the EGR valve control unit controls the opening degree of the EGR valve by taking the final target EGR valve opening area as the actual EGR valve opening area when in a steady state.

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