internal combustion engine

Abstract

Even if an error occurs in the detection of the intake air volume, the amount of urea water supplied to the SCR catalyst is prevented from becoming excessive or insufficient. [Solution] The SCR catalyst reduces and purifies NOx in exhaust gas using urea water as a reducing agent. An air-fuel ratio error calculation unit 105 calculates an air-fuel ratio error ΔA / F, which is the difference between an estimated air-fuel ratio AFe and an air-fuel ratio AFd, calculated based on an intake air amount Ga and a fuel injection amount Qf. A correction ratio calculation unit 107 calculates a correction ratio a, which corresponds to the proportion of contribution of injection amount variation of the fuel injection valve to the air-fuel ratio error ΔA / F. A calculation air amount calculation unit 108 corrects the intake air amount Ga based on the correction ratio a and the air-fuel ratio error ΔA / F to calculate a calculation air amount GaN. A NOx amount calculation unit 109 calculates an estimated NOx amount Nxe from the NOx concentration Nxd and the calculation air amount GaN, and a urea water supply amount calculation unit 110 calculates a urea water supply amount Qu based on the SCR temperature Tc and the estimated NOx amount Nxe.

Description

[Technical Field]

[0001] The present disclosure relates to internal combustion engines. [Background technology]

[0002] International Publication No. 2012 / 157037 (Patent Document 1) describes a method for reducing exhaust emissions by using the error between an estimated air-fuel ratio and a detected air-fuel ratio (air-fuel ratio error). The estimated air-fuel ratio is calculated based on the detected air amount detected by an intake air amount sensor and the fuel injection amount. The detected air-fuel ratio is detected by an air-fuel ratio sensor provided in the exhaust passage.

[0003] The internal combustion engine of Patent Document 1 is equipped with an exhaust gas recirculation device (EGR (Exhaust Gas Recirculation) device). In Patent Document 1, the amount of exhaust gas recirculated by the EGR device is controlled so that the air-fuel ratio error becomes zero. This prevents the amount of NOx (nitrogen oxides) emitted from the internal combustion engine from increasing due to an error in detecting the amount of intake air or a deviation (variation) in the characteristics of the fuel injection valve. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2012 / 157037 Summary of the Invention [Problem to be solved by the invention]

[0005] It is known to use a selective catalytic reduction (SCR) catalyst to reduce the amount of NOx emitted into the atmosphere from an internal combustion engine. Urea water is supplied to the exhaust passage from a urea addition valve, and ammonia (NH3) produced by decomposition of the urea water is used as a reducing agent to reduce and purify the NOx in the exhaust gas by the SCR catalyst, thereby reducing the amount of NOx emitted into the atmosphere.

[0006] The amount of urea solution supplied to the SCR catalyst is determined based on the amount of NOx emitted from the internal combustion engine (hereinafter also referred to as the "engine-output NOx amount") in order to obtain a predetermined purification rate. The engine-output NOx amount is estimated from the intake air amount detected by an intake air mass sensor and the NOx concentration detected by a NOx sensor provided in the exhaust passage. Therefore, if the characteristics of the intake air mass sensor vary due to aging or other reasons, causing an error in the detection of the intake air mass, the accuracy of estimating the engine-output NOx amount will deteriorate. If the accuracy of estimating the engine-output NOx amount deteriorates, the amount of urea solution supplied to the SCR catalyst will be either too much or too little, making it difficult to obtain the predetermined purification rate and raising concerns about an increase in the amount of NOx emitted into the atmosphere.

[0007] An object of the present disclosure is to suppress an excess or deficiency in the amount of urea water supplied to an SCR catalyst even if an error occurs in detecting the amount of intake air, and to suppress an increase in the amount of NOx released into the atmosphere. [Means for solving the problem]

[0008] 1) The internal combustion engine of the present disclosure is an internal combustion engine including an intake air amount sensor that detects the intake air amount, a fuel injection valve that supplies fuel to the combustion chamber, an air-fuel ratio sensor that detects the air-fuel ratio in the combustion chamber, a NOx sensor that detects the NOx concentration in the exhaust gas discharged from the combustion chamber, a selective reduction catalyst provided in an exhaust passage, an SCR temperature acquisition means that acquires the SCR temperature, which is the temperature of the selective reduction catalyst, a urea addition valve that supplies urea water to the selective reduction catalyst, and a control device. The control device includes a fuel supply amount calculation unit that calculates a fuel supply amount that is the amount of fuel supplied from the fuel injection valve, an estimated air-fuel ratio calculation unit that calculates an estimated air-fuel ratio from the fuel supply amount and a detected air-fuel ratio detected by the intake air amount sensor, an air-fuel ratio error calculation unit that calculates an air-fuel ratio error that is the difference between the estimated air-fuel ratio and the detected air-fuel ratio detected by the air-fuel ratio sensor, a correction ratio calculation unit that calculates a correction ratio corresponding to the rate at which the detection error of the intake air amount sensor contributes to the air-fuel ratio error or the rate at which injection amount variation of the fuel injection valve contributes to the air-fuel ratio error, based on the rotation speed of the internal combustion engine and the fuel supply amount, a calculation air amount calculation unit that corrects the detected air amount based on the air-fuel ratio error and the correction ratio, a NOx amount calculation unit that calculates an estimated NOx amount based on the NOx concentration and the calculation air amount, and a urea water supply amount calculation unit that calculates a urea water supply amount based on the estimated NOx amount and the SCR temperature. The control device controls the urea addition valve so that the amount of urea water supplied from the urea addition valve becomes the urea water supply amount. The correction ratio is set on the assumption that the air-fuel ratio error is caused only by the detection error of the intake air amount and the variation in the injection amount of the fuel injector. The calculation air amount calculation section includes a correction coefficient calculation section that calculates a correction coefficient using a correction coefficient map with the air-fuel ratio error and the correction ratio as parameters, and calculates the calculation air amount based on the correction coefficient and the detected air amount.

[0009] According to this configuration, NOx emitted from an internal combustion engine is reduced and purified by a selective catalytic reduction (SCR) catalyst. The amount of urea water supplied to the SCR catalyst is calculated in a urea water supply amount calculation unit based on the estimated NOx amount and the SCR temperature. The estimated NOx amount is calculated in a NOx amount calculation unit based on the NOx concentration and a calculation air amount. The calculation air amount is calculated in a calculation air amount calculation unit by correcting the detected air amount based on the air-fuel ratio error and a correction ratio.

[0010] The air-fuel ratio error calculation unit calculates an air-fuel ratio error, which is the difference between the estimated air-fuel ratio and the detected air-fuel ratio detected by the air-fuel ratio sensor. If a detection error occurs in the intake air mass sensor due to aging or other reasons, the estimated air-fuel ratio calculated from the detected air mass detected by the intake air mass sensor and the fuel supply amount deviates from the detected air-fuel ratio, resulting in an air-fuel ratio error. Since the air-fuel ratio error includes not only the detection error of the intake air mass sensor but also the injection amount variation (characteristic deviation) of the fuel injection valve, the correction ratio is set assuming that the air-fuel ratio error is caused only by the detection error of the intake air mass sensor and the injection amount variation of the fuel injection valve. The correction ratio calculation unit calculates a correction ratio corresponding to the proportion of the detection error of the intake air mass sensor that contributes to the air-fuel ratio error or the proportion of the injection amount variation of the fuel injection valve that contributes to the air-fuel ratio error, based on the rotational speed and the fuel supply amount. The calculation air amount calculation unit corrects the detected air amount based on the air-fuel ratio error and the correction ratio to calculate the calculation air amount.

[0011] The contribution of the detection error of the intake air mass sensor to the air-fuel ratio error and the contribution of the variation in the injection amount of the fuel injector to the air-fuel ratio error are correlated. The calculation air mass obtained by correcting the detected air mass based on the air-fuel ratio error and the correction ratio appropriately corrects the detection error of the intake air mass sensor included in the air-fuel ratio error.

[0012] The NOx amount calculation unit calculates the estimated NOx amount based on the NOx concentration and the calculation air amount. The calculation air amount is appropriately corrected for detection errors of the intake air amount sensor, which are included in the air-fuel ratio error, so the estimated NOx amount is calculated with high accuracy. The urea water supply amount calculation unit calculates the urea water supply amount based on the estimated NOx amount and the SCR temperature. The control device controls the urea addition valve so that the amount of urea water supplied from the urea addition valve becomes the urea water supply amount. Even if a detection error in the intake air amount occurs, the detection error is appropriately corrected and a highly accurate estimated NOx amount is calculated, so that an excess or deficiency in the amount of urea water supplied to the SCR catalyst can be suppressed and an increase in the amount of NOx released into the atmosphere can be suppressed.

[0013] Furthermore, the correction coefficient calculation section of the calculation air amount calculation section uses a correction coefficient map to determine the correction coefficient and calculate the calculation air amount, so that the calculation air amount calculation section can be implemented relatively easily.

[0014] 2) Preferably, when the urea water supply amount calculated by the urea water supply amount calculation unit is supplied from the urea addition valve, the amount of NOx emitted from the selective reduction catalyst may be set to a predetermined amount.

[0015] According to this configuration, the correction ratio is set so that the amount of NOx emitted from the selective reduction catalyst becomes a predetermined amount based on the urea water supply amount calculated by the urea water supply amount calculation unit. As a result, even if a detection error occurs due to either or both of a detection error of the intake air mass sensor and a variation in the injection amount of the fuel injection valve, an increase in the amount of NOx emitted into the atmosphere can be suppressed.

[0016] 3) Preferably, in the above 1 and 2, the calculation air amount calculation section may calculate the detected air amount as the calculation air amount when there is no air-fuel ratio error.

[0017] According to this configuration, when the estimated air-fuel ratio and the detected air-fuel ratio are approximately the same and there is no air-fuel ratio error, the detected air amount is calculated as the calculation air amount. When the estimated air-fuel ratio and the detected air-fuel ratio are approximately the same, it is estimated that there is no variation in the intake air amount sensor and the fuel injector, so even if the detected air amount is used as the calculation air amount, the amount of NOx released into the atmosphere will not increase.

[0018] 4) In the above items 1 to 3, the internal combustion engine preferably further includes an EGR device that recirculates a portion of the exhaust gas discharged from the combustion chamber to an intake passage. The air-fuel ratio control unit may include an EGR control unit that controls the amount of exhaust gas recirculated, and the EGR control unit may control the amount of exhaust gas recirculated so that the estimated air-fuel ratio matches the detected air-fuel ratio.

[0019] According to this configuration, the estimated air-fuel ratio is controlled by controlling the exhaust gas recirculation amount (EGR amount). Since the air-fuel ratio of the internal combustion engine is controlled by the EGR amount, the amount of NOx emitted from the internal combustion engine can be set to an appropriate value, and the amount of NOx released into the atmosphere can be appropriately suppressed. [Effects of the Invention]

[0020] According to the present disclosure, even if an error occurs in detecting the intake air amount, it is possible to prevent an excess or deficiency in the amount of urea water supplied to the SCR catalyst, and to prevent an increase in the amount of NOx released into the atmosphere. [Brief explanation of the drawings]

[0021] [Figure 1] 1 is a diagram schematically showing the overall configuration of an internal combustion engine according to an embodiment of the present invention. [Figure 2] 2 is a diagram showing an example of functional blocks configured in an ECU in the present embodiment. FIG. [Figure 3] 10A and 10B are diagrams illustrating the setting of correction ratios in the present embodiment. [Figure 4] 3 is a flowchart showing an example of engine control executed by an ECU in the present embodiment. [Figure 5] FIG. 10 is a diagram illustrating an example of a correction coefficient map. [Figure 6] FIG. 10 is a diagram for explaining the setting of a correction ratio in a modified example. DETAILED DESCRIPTION OF THE INVENTION

[0022] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are designated by the same reference numerals, and description thereof will not be repeated.

[0023] FIG. 1 is a diagram schematically showing the overall configuration of an internal combustion engine according to this embodiment. Referring to FIG. 1, engine 1 is a compression ignition internal combustion engine (diesel engine) equipped with an exhaust gas purification device 70. Engine 1 is used, for example, as a drive source for a vehicle. Engine 1 is an internal combustion engine that performs compression auto-ignition by injecting fuel from a fuel injection valve (injector) 14 into combustion chambers formed in cylinders 12 of an engine body 10. In this embodiment, engine 1 has four cylinders. An intake passage 20 of engine 1 is provided with an air cleaner 22, an intercooler 24, and a diesel throttle valve 26. Fresh air, from which foreign matter has been removed by air cleaner 22, is supercharged (compressed) by a compressor 32 of a turbocharger 30, cooled by the intercooler 24, supplied to an intake manifold 28, and supplied to each combustion chamber through an intake port.

[0024] Fuel is stored in a fuel tank 40. The fuel in the fuel tank 40 is supplied to a high-pressure fuel pump 42 by a feed pump 41, and the high-pressure fuel discharged from the high-pressure fuel pump 42 is pumped to a common rail 44 via a fuel passage 43. The high-pressure fuel stored in the common rail 44 is injected from the fuel injection valve 14 into the combustion chamber (into the cylinder).

[0025] Exhaust gases discharged from the combustion chambers are collected in an exhaust manifold 50 and released into the atmosphere via an exhaust passage 52. A portion of the exhaust gases is returned to the intake manifold 28 via an EGR (Exhaust Gas Recirculation) passage 60. An EGR cooler 62 and an EGR valve 64 are provided in the EGR passage 60.

[0026] The exhaust passage 52 is provided with the turbine 34 of the turbocharger 30, and downstream of the turbine 34, an exhaust purification device 70 is provided with an oxidation catalyst 71, a DPF (Diesel Particulate Filter) 72, a selective catalytic reduction (SCR) catalyst 73, and an oxidation catalyst 74. The oxidation catalyst 71 oxidizes and purifies CO (carbon monoxide), HC (hydrocarbons), and SOF (soluble organic fractions) contained in the exhaust gas. Furthermore, when burning and removing particulate matter trapped in the DPF 72, the oxidation catalyst 71 burns (oxidizes) the supplied HC, thereby raising the exhaust gas temperature.

[0027] An SCR catalyst 73 is arranged in the exhaust passage 52 downstream of the DPF 72. The SCR catalyst 73 is, for example, a ceramic carrier supporting copper (Cu) ion-exchanged zeolite as a catalyst, and exhibits a high NOx purification rate by using ammonia (NH3) as a reducing agent. The ammonia used as the reducing agent is produced by hydrolysis and thermal decomposition of urea water supplied to the exhaust passage 52 upstream of the SCR catalyst 73. A urea addition valve (urea water injection injector) 80 is provided in the exhaust passage upstream of the SCR catalyst 73, and urea water pumped from a urea water tank 81 by a pump 82 is injected from the urea addition valve 80 into the exhaust passage 52 upstream of the SCR catalyst 73.

[0028] An oxidation catalyst 74 is provided in the exhaust passage 52 downstream of the SCR catalyst 73, and oxidizes and purifies the ammonia discharged (slip) from the SCR catalyst 73. Note that the oxidation catalyst 74 may be omitted.

[0029] The ECU (Electronic Control Unit) 100 includes a CPU (Central Processing Unit) 100a, a memory 100b consisting of a ROM (Read Only Memory) and a RAM (Random Access Memory), an input / output port (not shown) for inputting and outputting various signals, and executes predetermined arithmetic processing based on information stored in the memory 100b and information from various sensors to control the fuel injection valve 14, the diesel throttle valve 26, the EGR valve 64, the urea addition valve 80, etc. The ECU 100 corresponds to an example of a "control device" in the present disclosure.

[0030] The various sensors input to the ECU 100 include, for example, an engine rotation speed sensor 111, an accelerator pedal sensor 112, an air flow meter 113, a NOx sensor 114, an intake air temperature sensor 115, and an intake pressure sensor 116. The engine rotation speed sensor 111 detects the rotation speed NE of the engine 1. The accelerator pedal sensor 112 detects the amount of accelerator pedal operation by the user (hereinafter also referred to as "accelerator opening") AP. The air flow meter 113 is provided in the intake passage 20 and detects the intake amount (intake air amount) Ga of the engine 1.

[0031] The NOx sensor 114 is provided in the exhaust passage 52 downstream of the DPF 72, and detects the NOx concentration Nxd in the exhaust gas discharged from the engine 1. The NOx sensor 114 is also configured to be able to detect the oxygen concentration in the exhaust gas, and functions as an air-fuel ratio sensor to detect the air-fuel ratio AFd of the exhaust gas discharged from the engine 1. The air-fuel ratio AFd is substantially the same as the air-fuel ratio in the combustion chamber. The NOx sensor 114 corresponds to an example of the "NOx sensor" and "air-fuel ratio sensor" of the present disclosure.

[0032] The intake air temperature sensor 115 detects the intake air temperature TA downstream of the intercooler 24. The intake air pressure sensor 116 detects the intake air pressure PM downstream of the diesel throttle valve 26.

[0033] The amount of urea water supplied to the SCR catalyst 73 (urea water supply amount) is controlled so that the purification rate of NOx emitted from the engine 1 becomes a target purification rate. The target NOx purification rate is determined so that the amount of NOx released into the atmosphere (flowing through the exhaust passage 52 downstream of the SCR catalyst 73) becomes equal to or less than a set value. The purification rate of the SCR catalyst 73 is determined by the temperature of the SCR catalyst 73 and the amount of urea water supplied. If the amount of urea water supplied is insufficient, the NOx purification rate decreases, and if the amount of urea water supplied is excessive, the amount of ammonia that slips from the SCR catalyst 73 increases.

[0034] The amount of urea to be supplied to obtain a target purification rate is calculated from the amount of NOx emitted from the engine 1 (engine-output NOx amount) and the temperature of the SCR catalyst. The engine-output NOx amount is calculated from the intake air amount Ga detected by the air flow meter 113 and the NOx concentration Nxd detected by the NOx sensor 114. If variations occur in the detection characteristics of the air flow meter 113 due to aging or other reasons, a detection error will occur in the intake air amount Ga. If a detection error occurs in the intake air amount Ga, the calculation accuracy of the engine-output NOx amount will deteriorate. If the calculation accuracy of the engine-output NOx amount deteriorates, the amount of urea to be supplied will be excessive or insufficient, which could result in a decrease in the NOx purification rate.

[0035] In this embodiment, by correcting the detection error of the intake air amount Ga using the air-fuel ratio AFd detected by the NOx sensor 114, even if a detection error of the intake air amount Ga occurs, an excess or deficiency in the amount of urea water supplied to the SCR catalyst 73 is suppressed, and an increase in the amount of NOx released into the atmosphere is suppressed.

[0036] 2 is a diagram showing an example of functional blocks configured in ECU 100 in this embodiment. Fuel supply amount calculation unit 101 calculates fuel injection amount Qf from accelerator opening AP and engine rotation speed NE. For example, fuel supply amount calculation unit 101 obtains fuel injection amount Qf from accelerator opening AP and engine rotation speed NE using a fuel injection amount map stored in memory 100b. ECU 100 controls fuel injection valve 14 so that fuel of fuel injection amount Qf is injected into the combustion chamber. Fuel injection amount Qf corresponds to the "fuel supply amount" in this disclosure.

[0037] The opening degree calculation unit 102 calculates the opening degree Tθ of the diesel throttle valve 26 from the fuel injection amount Qf and the engine rotation speed NE. For example, an opening degree map is stored in the memory 100b, and the opening degree calculation unit 102 obtains the opening degree Tθ from the opening degree map using the fuel injection amount Qf and the engine rotation speed NE. The ECU 100 controls the diesel throttle valve 26 so that the opening degree of the diesel throttle valve 26 becomes the opening degree Tθ.

[0038] The SCR temperature acquisition unit 103 calculates the temperature Tc of the SCR catalyst 73 (SCR temperature) from the fuel injection amount Qf and the engine rotation speed NE. For example, an SCR temperature map stored in the memory 100b is used to determine the SCR temperature Tc using the fuel injection amount Qf and the engine rotation speed NE as parameters. The SCR temperature map may be a map having different tables depending on the coolant temperature THW of the engine 1, or may be a three-dimensional map that includes the coolant temperature THW. In this case, the state of the coolant temperature THW is taken into account, so the SCR temperature Tc can be calculated with greater accuracy. Note that a temperature sensor may be provided in the SCR catalyst 73 to detect the SCR temperature Tc.

[0039] The estimated air-fuel ratio calculation unit 104 calculates an estimated air-fuel ratio AFe (AFe=Ga / Qf) from the intake air amount Ga and the fuel injection amount Qf detected by the air flow meter 113. The air flow meter 113 corresponds to the "intake air amount sensor" in this disclosure, and the intake air amount Ga corresponds to the "detected air amount" in this disclosure.

[0040] The air-fuel ratio error calculation unit 105 calculates an air-fuel ratio error ΔA / F from the air-fuel ratio AFd detected by the NOx sensor 114 and the estimated air-fuel ratio AFe. In this embodiment, the air-fuel ratio error ΔA / F is the ratio between the air-fuel ratio AFd and the estimated air-fuel ratio AFe, and is calculated as "ΔA / F = AFe / AFd." The air-fuel ratio AFd corresponds to the "detected air-fuel ratio" in this disclosure. When the air-fuel ratio error ΔA / F is "1," it can be assumed that there is no detection error in the intake air amount Ga, and there is no variation in the injection amount of the fuel injection valve 14. When the air-fuel ratio error ΔA / F is greater than "1," the air-fuel ratio AFd is richer than the estimated air-fuel ratio AFe. In this case, it is estimated that the injection amount of the fuel injection valve 14 varies in an increasing direction and / or that the detected intake air amount Ga is larger than the actual intake air amount, and when the air-fuel ratio error ΔA / F is smaller than 1, the air-fuel ratio AFd is leaner than the estimated air-fuel ratio AFe. In this case, it is estimated that the injection amount of the fuel injection valve 14 varies in a decreasing direction and / or that the detected intake air amount Ga is smaller than the actual intake air amount.

[0041] The air-fuel ratio control unit 106 controls the air-fuel ratio of the engine 1 so that the air-fuel ratio AFd and the estimated air-fuel ratio AFe coincide with each other. In this embodiment, the air-fuel ratio control unit 106 includes an EGR control unit 106a, and controls the air-fuel ratio of the engine 1 mainly by the EGR control unit 106a.

[0042] The EGR control unit 106a calculates a target EGR rate TRegr from an EGR rate map stored in the memory 100b using the fuel injection amount Qf and the engine rotation speed NE as parameters. The EGR control unit 106a calculates an estimated EGR rate RegrE, which is an estimate of the actual EGR rate, using the amount of gas Gc flowing into the combustion chamber and the amount of intake air Ga. The EGR control unit 106a feedback-controls the opening degree Eo of the EGR valve 64 so that the estimated EGR rate RegrE becomes the target EGR rate TRegr.

[0043] More specifically, the EGR control unit 106a calculates the gas amount Gc based on the intake air temperature TA detected by the intake air temperature sensor 115 and the intake air pressure PM detected by the intake pressure sensor 116. The EGR control unit 106a corrects the intake air amount Ga so that the air-fuel ratio AFd and the estimated air-fuel ratio AFe coincide with each other (so that the air-fuel ratio error ΔA / F becomes 1). The EGR control unit 106a calculates an estimated EGR rate RegrE based on the gas amount Gc and the corrected intake air amount Ga. The EGR control unit 106a then feedback-controls the opening degree Eo of the EGR valve 64 so that the estimated EGR rate RegrE becomes the target EGR rate TRegr. Such a method of calculating the gas amount Gc and a method of correcting the intake air amount Ga are well known, and for example, the calculation method and correction method described in Patent Document 1 may be adopted. The opening degree Eo of the EGR valve 64 is feedback controlled so that the estimated EGR rate RegrE calculated in this way becomes the target EGR rate TRegr, and the air-fuel ratio of the engine 1 is controlled so that the air-fuel ratio AFd and the estimated air-fuel ratio AFe coincide with each other.

[0044] The correction ratio calculation unit 107 calculates a correction ratio corresponding to the rate at which the detection error of the air flow meter 113 (intake air amount Ga) contributes to the air-fuel ratio error ΔA / F or the rate at which the injection amount variation of the fuel injection valve 14 contributes to the air-fuel ratio error ΔA / F, based on the engine rotation speed NE and the fuel injection amount Qf. The injection amount variation is the difference between the amount of fuel actually injected from the fuel injection valve 14 and the fuel injection amount Qf. The air-fuel ratio error ΔA / F is caused by the detection error of the intake air amount Ga and the injection amount variation (characteristic deviation) of the fuel injection valve 14.

[0045] In this embodiment, a correction ratio a is calculated, which corresponds to the proportion of the contribution of the injection amount variation of the fuel injection valve 14 to the air-fuel ratio error ΔA / F. The correction ratio a is a value between 0 and 1. When there is no injection amount variation of the fuel injection valve 14 (when the difference between the amount of fuel actually injected from the fuel injection valve 14 and the fuel injection amount Qf is "0"), the correction ratio a becomes "0". When there is no detection error in the intake air amount Ga, the correction ratio a becomes "1". Therefore, the following equation (1) holds between the air-fuel ratio error ΔA / F and the correction ratio a.

[0046] ΔA / F=(ΔA / F) a ×(ΔA / F) (1-a) ···(1)

[0047] The correction ratio a is determined in advance by experiment, and a map (correction ratio map) using the engine rotation speed NE and the fuel injection amount Qf as parameters is stored in the memory 100b. The method for setting the correction ratio a will be described later.

[0048] The calculation air amount calculation unit 108 corrects the intake air amount Ga based on the air-fuel ratio error ΔA / F and the correction ratio a, and calculates the calculation air amount GaN. In this embodiment, the calculation air amount calculation unit 108 includes a correction coefficient calculation unit 108a. The correction coefficient calculation unit 108a calculates a correction coefficient K based on the air-fuel ratio error ΔA / F and the correction ratio a. The correction coefficient K may be calculated, for example, by the following equation (2).

[0049] K=1 / (ΔA / F) (1-a) ···(2)

[0050] The calculation air amount calculation unit 108 calculates the calculation air amount GaN by multiplying the intake air amount Ga by the correction coefficient K (GaN=Ga×K). A correction coefficient map using the air-fuel ratio error ΔA / F and the correction ratio a as parameters is stored in the memory 100b, and the correction coefficient calculation unit 108a may use this correction coefficient map to determine the correction coefficient K from the air-fuel ratio error ΔA / F and the correction ratio a. When the air-fuel ratio error ΔA / F is "1" (ΔA / F=1), it is estimated that there is no detection error in the intake air amount Ga and no variation in the injection amount of the fuel injector 14, so the correction coefficient K is calculated as "1". As a result, when the air-fuel ratio error ΔA / F is "1", the intake air amount Ga is calculated as the calculation air amount GaN (GaN=Ga).

[0051] The NOx amount calculation unit 109 calculates the estimated NOx amount Nxe based on the NOx concentration Nxd and the calculation air amount GaN. The estimated NOx amount Nxe is an estimated value of the engine-output NOx amount. The calculation air amount GaN is a value obtained by correcting a detection error in the intake air amount Ga contained in the air-fuel ratio error ΔA / F. The NOx amount calculation unit 109 calculates the estimated NOx amount Nxe using the calculation air amount GaN, so that the estimated NOx amount Nxe can be calculated with high accuracy even if there is a detection error in the intake air amount Ga.

[0052] The urea water supply amount calculation unit 110 calculates the urea water supply amount Qu based on the estimated NOx amount Nxe and the SCR temperature Tc. For example, a urea water supply amount map is stored in the memory 100b, and the urea water supply amount calculation unit 110 obtains the urea water supply amount Qu from the urea water supply amount map using the estimated NOx amount Nxe and the SCR temperature Tc. The control device 100 controls the urea addition valve 80 so that the amount of urea water supplied from the urea addition valve 80 becomes the urea water supply amount Qu.

[0053] In this embodiment, the correction ratio a may be set, for example, as follows. First, the engine 1 is operated at a predetermined engine speed NE and fuel injection amount Qf using an air flow meter 113 that has no detection error in the intake air amount Ga and fuel injection valves 14 that have no fuel injection amount variation. This operating state of the engine speed NE and fuel injection amount Qf is also referred to as a set operating state. In this set operating state, the amount of NOx released into the atmosphere (flowing through the exhaust passage 52 downstream of the SCR catalyst 73) with the correction coefficient K set to "1" is obtained as the reference released NOx amount NOxb.

[0054] Next, the engine 1 is operated under set operating conditions using the air flow meter 113 with no detection error of the intake air amount Ga and the fuel injection valve 14 with variation in the fuel injection amount. Then, the value of the correction ratio a is sequentially changed, and the amount of NOx released into the atmosphere is acquired as a first NOx release amount NOX1. The sequentially changed value of the correction ratio a may be any value including "0" and "1," and may be in 0.1 increments, such as "0," "0.1," "0.2," "0.3," ... "1," or may be sequentially changed in 0.2 increments. Then, the standard release NOx amount NOxb is subtracted from the sequentially acquired first NOx release amount NOX1 to calculate a variation NOx amount (first variation NOx amount NV1) (NV1 = NOX1 - NOXb). The acquired first variation NOx amount NV1 is plotted on a graph with the correction ratio a on the horizontal axis and the variation NOx amount on the vertical axis.

[0055] Next, the engine 1 is operated under set operating conditions using the air flow meter 113 with a detection error of the intake air amount Ga and the fuel injection valve 14 with no variation in the fuel injection amount. Then, the value of the correction ratio a is sequentially changed, and the amount of NOx released into the atmosphere is acquired as the second NOx release amount NOX2. The value of the correction ratio a that is sequentially changed may be the same as when acquiring the first NOx release amount NOX1 described above. Then, the variation NOx amount (second variation NOx amount NV2) is calculated by subtracting the reference release NOx amount NOxb from the sequentially acquired second NOx release amount NOX2 (NV2 = NOX2 - NOXb). Then, the acquired second variation NOx amount NV2 is plotted on a graph with the correction ratio a on the horizontal axis and the variation NOx amount on the vertical axis.

[0056] FIG. 3 is a diagram for explaining the setting of the correction ratio a. In FIG. 3A, the horizontal axis represents the correction ratio a, and the vertical axis represents the variation NOx amount. In FIG. 3A, the open triangles (△) are plots of the first variation NOx amount NV1, and the open squares (□) are plots of the second variation NOx amount NV2. The first variation NOx amount NV1 is the variation NOx amount when using a fuel injection valve 14 with no detection error in the intake air amount Ga and with variation in the fuel injection amount. In this case, the detection error in the intake air amount Ga (air flow meter 113) does not affect the air-fuel ratio error ΔA / F, and as the correction ratio a approaches 1 (the degree to which the intake air amount Ga is corrected becomes smaller), the excess or deficiency of the urea water supply amount Qu is suppressed, and therefore the first variation NOx amount NV1 becomes a smaller value. The second variation NOx amount NV2 is the variation NOx amount when using an air flow meter 113 that has a detection error in the intake air amount Ga. As the correction ratio a approaches 0, the detection error in the intake air amount Ga is more appropriately corrected and the excess or deficiency of the urea water supply amount Qu is suppressed, so the second variation NOx amount NV2 becomes a smaller value.

[0057] In FIG. 3A, line L1 is a line connecting the plotted first variation NOx amounts NV1, and line L2 is a line connecting the plotted second variation NOx amounts NV2. The value of the correction ratio a at the point where lines L1 and L2 intersect (intersection) (denoted as "S" in FIG. 3A) is set to the value of the correction ratio a in this set operating state. The air-fuel ratio AFd detected by the NOx sensor 114 may contain a detection error. However, the effect of the air-fuel ratio error ΔA / F resulting from the detection error of the air-fuel ratio AFd on the increase in the amount of NOx released into the atmosphere (the amount of NOx flowing through the exhaust passage 52 downstream of the SCR catalyst 73) is extremely small compared to the detection error of the intake air amount Ga and the injection amount variation of the fuel injection valve 14. Therefore, by setting the value of the correction ratio a at the intersection of lines L1 and L2 as the correction ratio a in this set state, the correction ratio a is set so as to reduce the amount of NOx emitted from the SCR catalyst 73 when urea-water is injected (supplied) at the urea-water supply amount Qu from the urea addition valve 80, even if the air-fuel ratio error ΔA / F is caused by either or both of a detection error in the intake air amount Ga and a variation in the injection amount of the fuel injection valve 14. The amount of NOx emitted into the atmosphere at the intersection of lines L1 and L2 corresponds to the "predetermined amount" in the present disclosure in the phrase "when the air-fuel ratio error is caused by a detection error in the intake air amount Ga and a variation in the injection amount of the fuel injection valve, the amount of NOx emitted from the selective reduction catalyst becomes a predetermined amount when the urea-water supply amount is supplied from the urea addition valve." As a result, in this set operating state, even if there is a detection error in the intake air amount Ga or if there is injection amount variation in the fuel injection valve 14, the urea water supply amount Qu supplied to the SCR catalyst 73 can be suitably prevented from becoming excessive or insufficient, and an increase in the amount of NOx released into the atmosphere can be suppressed.

[0058] FIG. 3(B) is a correction ratio map stored in memory 100b. This correction ratio map is created by sequentially changing the combination of engine speed NE and fuel injection amount Qf, operating engine 1 in each set operating state, and determining the correction ratio a for each set operating state, and then mapping it. In the region corresponding to the set operating state in FIG. 3(A), the above-mentioned value "S" is set as the correction ratio a. The correction ratio map set in this manner is stored in memory 100b, and correction ratio calculation unit 107 determines the correction ratio a from the correction ratio map using engine speed NE and fuel injection amount Qf as parameters. Note that for an operating state (combination of engine speed NE and fuel injection amount Qf) that is not in the correction ratio map, the correction ratio a may be calculated by interpolation.

[0059] 4 is a flowchart showing an example of engine control executed by ECU 100 in this embodiment. This flowchart is repeatedly processed at predetermined intervals while engine 1 is operating. In step (hereinafter, step will be abbreviated as "S") 10, a fuel injection amount Qf is calculated based on accelerator opening AP and engine rotation speed NE using a fuel injection amount map stored in memory 100b. As is well known, when the fuel injection timing arrives, fuel in the fuel injection amount Qf is injected from fuel injection valve 14 into the combustion chamber.

[0060] In S11, an opening degree Tθ of the diesel throttle valve 26 is calculated from the fuel injection amount Qf and the engine rotation speed NE using an opening degree map stored in memory 100b. The diesel throttle valve 26 is controlled so that the opening degree becomes the opening degree Tθ. In the following S12, an SCR temperature Tc is calculated from the SCR temperature map stored in memory 100b using the fuel injection amount Qf and the engine rotation speed NE as parameters.

[0061] In S13, an estimated air-fuel ratio AFe (AFe=Ga / Qf) is calculated from the intake air amount Ga and the fuel injection amount Qf. In S14, an air-fuel ratio error ΔA / F is calculated from the air-fuel ratio AFd detected by the NOx sensor 114 and the estimated air-fuel ratio AFe (ΔA / F=AFe / AFd).

[0062] In the next step S20, EGR control is executed. The EGR control may be, for example, the EGR control described in Patent Document 1. First, a target EGR rate TRegr is calculated from an EGR rate map calculated based on the fuel injection amount Qf and the engine rotation speed NE. The amount of gas Gc flowing into the combustion chamber is calculated based on the intake air temperature TA detected by the intake air temperature sensor 115 and the intake pressure PM detected by the intake pressure sensor 116. The intake air amount Ga is corrected so that the air-fuel ratio AFd and the estimated air-fuel ratio AFe coincide with each other (so that the air-fuel ratio error ΔA / F becomes 1), and an estimated EGR rate RegrE is calculated from the corrected intake air amount Ga and the gas amount Gc. Then, the opening degree Eo of the EGR valve 64 is feedback-controlled so that the estimated EGR rate RegrE coincides with the target EGR rate TRegr. This controls the estimated air-fuel ratio AFe to coincide with the air-fuel ratio AFd.

[0063] In S16, the correction ratio a is calculated from the engine rotation speed NE and the fuel injection amount Qf using a correction ratio map (FIG. 3(B)). In S17, the correction coefficient K is calculated from the air-fuel ratio error ΔA / F and the correction ratio a. FIG. 5 is a diagram showing an example of the correction coefficient map. The correction coefficient map is a two-dimensional map with the air-fuel ratio error ΔA / F and the correction ratio a as parameters, and is set based on the above equation (2). In S17, this correction coefficient map is used to find the correction coefficient K from the air-fuel ratio error ΔA / F and the correction ratio a. In a region not included in the correction coefficient map (a combination of the air-fuel ratio error ΔA / F and the correction ratio a), the correction coefficient K may be calculated by interpolation. Note that in the correction coefficient map, in a region where the air-fuel ratio error ΔA / F is "1", the correction coefficient K is set to "1". The correction coefficient K may also be calculated using the above equation (2) without using a map.

[0064] In S17, the calculation air amount GaN is calculated by multiplying the intake air amount Ga by a correction coefficient K (GaN = Ga x K). In S18, the estimated NOx amount Nxe is calculated from the NOx concentration Nxd and the calculation air amount GaN. The estimated NOx amount Nxe is an estimated value of the engine-output NOx amount. In S19, the urea water supply amount Qu is calculated from the estimated NOx amount Nxe and the SCR temperature Tc using a urea water supply amount map stored in the memory 100b. Urea water is injected from the urea addition valve 80 into the exhaust passage 52 according to the urea water supply amount Qu.

[0065] According to this embodiment, the calculation air amount GaN is a value obtained by correcting a detection error in the intake air amount Ga, which is included in the air-fuel ratio error ΔA / F. Since the estimated NOx amount Nxe is calculated using the calculation air amount GaN, even if there is a detection error in the intake air amount Ga, the estimated NOx amount Nxe, which is the amount of NOx output from the engine, can be calculated with high accuracy. This makes it possible to accurately calculate the urea-water supply amount Qu, and even if there is a detection error in the intake air amount Ga, it is possible to prevent an excess or deficiency in the amount of urea-water supplied to the SCR catalyst 73 and to prevent an increase in the amount of NOx released into the atmosphere.

[0066] According to this embodiment, the correction ratio a correlates with the proportion of the contribution of the detection error of the intake air amount Ga to the air-fuel ratio error ΔA / F and the proportion of the contribution of the injection amount variation of the fuel injection valve 14 to the air-fuel ratio error ΔA / F. In this embodiment, the intersection of line L1 (a line connecting the plots (dots) of the first variation NOx amount NV1) and line L2 (a line connecting the plots of the second variation NOx amount NV2) shown in FIG. 3A is set as the correction ratio a. Therefore, even if the air-fuel ratio error ΔA / F is caused by either or both of the detection error of the intake air amount Ga and the injection amount variation of the fuel injection valve 14 in the set operating state, the correction ratio a is set so that the amount of NOx emitted from the SCR catalyst 73 is reduced when the urea addition valve 80 injects (supplies) the urea water supply amount Qu. This makes it possible to suitably prevent the urea water supply amount Qu supplied to the SCR catalyst 73 from becoming excessive or insufficient, and to suppress an increase in the amount of NOx released into the atmosphere.

[0067] In this embodiment, the opening degree Eo of the EGR valve 64 is feedback-controlled so that the estimated EGR rate RegrE becomes the target EGR rate TRegr, and the estimated air-fuel ratio AFe is controlled to match the air-fuel ratio AFd. Since the engine 1 can be operated at the air-fuel ratio AFd with a small amount of engine-output NOx, and the amount of engine-output NOx can be reduced by EGR, the amount of NOx released into the atmosphere can be more suitably suppressed.

[0068] (Variation) In the above embodiment, the correction ratio used is the correction ratio a, which corresponds to the rate at which the injection amount variation of the fuel injection valve 14 contributes to the air-fuel ratio error ΔA / F. However, the correction ratio b, which corresponds to the rate at which the detection error of the intake air amount Ga (air flow meter 113) contributes to the air-fuel ratio error ΔA / F, may also be used. In this case, when there is no detection error in the intake air amount Ga, the correction ratio b is "0." When there is no injection amount variation of the fuel injection valve 14, the correction ratio b is "1." Therefore, the following equation (3) holds between the air-fuel ratio error ΔA / F and the correction ratio b. ΔA / F=(ΔA / F) (1-b) ×(ΔA / F) b ···(3)

[0069] When the correction ratio b is used, the correction coefficient K is calculated by the following equation (4). K=1 / (ΔA / F) b ···(4)

[0070] The setting of the correction ratio b is the same as the setting of the correction ratio a. FIG. 6 is a diagram for explaining the setting of the correction ratio b in the modified example. In FIG. 6, the horizontal axis represents the correction ratio b, and the vertical axis represents the variation NOx amount. The open triangles (△) are plots of the first variation NOx amount NV1 obtained by sequentially changing the correction ratio b. The open squares (□) are plots of the second variation NOx amount NV2 obtained by sequentially changing the correction ratio b.

[0071] The first variation NOx amount NV1 is the variation NOx amount when a fuel injection valve 14 having variation in the fuel injection amount but no detection error in the intake air amount Ga is used. In this case, the detection error in the intake air amount Ga (air flow meter 113) does not affect the air-fuel ratio error ΔA / F, and as the correction ratio b approaches 0 (the smaller the degree to which the intake air amount Ga is corrected), excess or deficiency in the urea-water supply amount Qu is suppressed, so the first variation NOx amount NV1 becomes a smaller value. The second variation NOx amount NV2 is the variation NOx amount when an air flow meter 113 having detection error in the intake air amount Ga is used. As the correction ratio b approaches 1, the detection error in the intake air amount Ga is suitably corrected and excess or deficiency in the urea-water supply amount Qu is suppressed, so the second variation NOx amount NV2 becomes a smaller value.

[0072] As in the above embodiment, the value of the correction ratio b at the intersection of the line L1 connecting the plotted first variation NOx amounts NV1 and the line L2 connecting the plotted second variation NOx amounts NV2 ("T" in FIG. 6) is set as the value of the correction ratio b in this set operating state. Also, as in the above embodiment, the combination of the engine rotation speed NE and the fuel injection amount Qf is changed sequentially, the engine 1 is operated in each set operating state, the correction ratio b is obtained for each set operating state, and a correction ratio map is set.

[0073] In this modification, the correction ratio calculation unit 107 calculates the correction ratio b based on the engine speed NE and the fuel injection amount Qf. The calculation air amount calculation unit 108 corrects the intake air amount Ga based on the air-fuel ratio error ΔA / F and the correction ratio b, and calculates the calculation air amount GaN. The correction coefficient calculation unit 108a may calculate the correction coefficient K using the above equation (4), and the calculation air amount calculation unit 108 may calculate the calculation air amount GaN by multiplying the intake air amount Ga by the correction coefficient K. As in the above embodiment, the correction coefficient K may be determined using a correction coefficient map using the air-fuel ratio error ΔA / F and the correction ratio b as parameters. Note that, as in the above embodiment, when the air-fuel ratio error ΔA / F is "1", the intake air amount Ga is calculated as the calculation air amount GaN (GaN = Ga).

[0074] In this modified example, even if the air-fuel ratio error ΔA / F is caused by either or both of a detection error in the intake air amount Ga and a variation in the injection amount of the fuel injection valve 14, the correction ratio b is set so as to reduce the amount of NOx emitted from the SCR catalyst 73 when the urea-water supply amount Qu is injected (supplied) from the urea addition valve 80. As a result, even if there is a detection error in the intake air amount Ga or even if there is a variation in the injection amount of the fuel injection valve 14, it is possible to suitably prevent the urea-water supply amount Qu supplied to the SCR catalyst 73 from becoming excessive or insufficient, and to suppress an increase in the amount of NOx released into the atmosphere.

[0075] In the above embodiment, the air-fuel ratio control is performed by EGR control so that the air-fuel ratio AFd and the estimated air-fuel ratio AFe coincide with each other. However, instead of or in addition to the EGR control, the air-fuel ratio control may be performed by correcting the opening degree Tθ of the diesel throttle valve 26 calculated by the opening degree calculation unit 102 so that the air-fuel ratio AFd and the estimated air-fuel ratio AFe coincide with each other.

[0076] In the above embodiment, the NOx concentration Nxd and the air-fuel ratio AFd are detected by the NOx sensor 114, but an air-fuel ratio sensor may also be provided to detect the air-fuel ratio AFd.

[0077] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of the present disclosure is defined by the claims, not by the description of the above embodiments, and is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0078] 1 engine, 10 engine body, 12 cylinder, 14 fuel injection valve, 20 intake passage, 22 air cleaner, 24 intercooler, 26 diesel throttle valve, 28 intake manifold, 30 turbocharger, 32 compressor, 34 turbine, 40 fuel tank, 41 feed pump, 42 high-pressure fuel pump, 43 fuel passage, 44 common rail, 50 exhaust manifold, 52 exhaust passage, 60 EGR passage, 62 EGR cooler, 64 EGR valve, 70 exhaust purification device, 71 oxidation catalyst, 72 DPF, 73 selective catalytic reduction catalyst (SCR catalyst), 74 oxidation catalyst, 80 urea addition valve, 81 urea water tank, 82 pump, 100 ECU, 100a CPU, 100b memory, 101 fuel supply amount calculation unit, 102 opening calculation unit, 103 SCR temperature acquisition unit, 104 estimated air-fuel ratio calculation unit, 105 air-fuel ratio error calculation unit, 106 air-fuel ratio control unit, 106a EGR control unit, 107 correction ratio calculation unit, 108 calculation air amount calculation unit, 108a correction coefficient calculation unit, 109 NOx amount calculation unit, 110 urea water supply amount calculation unit, 111 engine rotation speed sensor, 112 accelerator pedal sensor, 113 air flow meter, 114 NOx sensor, 115 intake air temperature sensor, 116 intake pressure sensor.

Claims

1. an intake air amount sensor that detects an intake air amount; a fuel injection valve for supplying fuel to a combustion chamber; an air-fuel ratio sensor for detecting an air-fuel ratio in the combustion chamber; a NOx sensor for detecting the NOx concentration in exhaust gas discharged from the combustion chamber; a selective reduction catalyst provided in the exhaust passage; a urea addition valve for supplying urea water to the selective reduction catalyst; An internal combustion engine comprising: The control device a fuel supply amount calculation unit that calculates a fuel supply amount, which is the amount of fuel supplied from the fuel injection valve; an estimated air-fuel ratio calculation unit that calculates an estimated air-fuel ratio from the detected air amount detected by the intake air amount sensor and the fuel supply amount; an air-fuel ratio control unit that controls the air-fuel ratio so that the detected air-fuel ratio detected by the air-fuel ratio sensor coincides with the estimated air-fuel ratio; an air-fuel ratio error calculation unit that calculates an air-fuel ratio error, which is the error between the estimated air-fuel ratio and the detected air-fuel ratio; a correction ratio calculation unit that calculates a correction ratio corresponding to a rate at which a detection error of the intake air mass sensor contributes to the air-fuel ratio error or a rate at which an injection amount variation of the fuel injection valve contributes to the air-fuel ratio error, based on the rotation speed of the internal combustion engine and the fuel supply amount; a calculation air amount calculation unit that corrects the detected air amount based on the air-fuel ratio error and the correction ratio and calculates a calculation air amount; a NOx amount calculation unit that calculates an estimated NOx amount based on the NOx concentration and the calculation air amount; a urea water supply amount calculation unit that calculates a urea water supply amount based on the temperature of the selective reduction catalyst and the estimated NOx amount, controlling the urea addition valve so that the amount of urea water supplied from the urea addition valve becomes the urea water supply amount; the correction ratio is set on the assumption that the air-fuel ratio error is caused only by a detection error of the intake air amount and a variation in the injection amount of the fuel injector, The calculation air amount calculation unit a correction coefficient calculation unit that calculates a correction coefficient using a correction coefficient map with the air-fuel ratio error and the correction ratio as parameters; The calculation air amount is calculated based on the correction coefficient and the detected air amount.

2. 2. The internal combustion engine according to claim 1, wherein the correction ratio is set so that the amount of NOx emitted from the selective reduction catalyst becomes a predetermined amount when the urea water supply amount is supplied from the urea addition valve.

3. 2. The internal combustion engine according to claim 1, wherein the calculation air amount calculation section calculates the detected air amount as the calculation air amount when there is no air-fuel ratio error.

4. an EGR device that recirculates a portion of the exhaust gas discharged from the combustion chamber to an intake passage, the air-fuel ratio control unit includes an EGR control unit that controls an exhaust gas recirculation amount, 4. The internal combustion engine according to claim 1, wherein the EGR control unit controls the exhaust gas recirculation amount so that the estimated air-fuel ratio coincides with the detected air-fuel ratio.

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