CONCENTRATION DETERMINATION DEVICE AND METHOD

Abstract

Concentration determination device of a cleaning system (301), wherein the cleaning system (301) comprises: an SCR catalyst (4) which is arranged in an exhaust pipe of an internal combustion engine to clean NOx in the exhaust gas of the internal combustion engine; a reducing agent supply unit (5; 302), which is provided and is arranged to supply urea or ammonia as a reducing agent to the SCR catalyst (4); an ammonia sensor (202) which is provided and arranged to detect an ammonia concentration of the exhaust gas flowing out of the SCR catalyst (4) as a downstream ammonia concentration; and a NOx sensor (201) which is provided and arranged to detect a NOx concentration of the exhaust gas flowing out of the SCR catalyst (4) as a downstream NOx concentration; wherein the concentration determination device is configured to determine a value of the downstream ammonia concentration based on a detection result of the ammonia sensor (202), characterized in that the concentration determination device comprises: a feed control section (S2, S30; S10, S35) which controls the reducing agent feed unit (5; 302) to supply the reducing agent to the SCR catalyst in a fuel-free state in which the supply of fuel to the combustion engine is stopped; a feed correction section (S50 to S120) which corrects the determined value of the downstream ammonia concentration based on a detection result of the NOx sensor (201) and an oxygen concentration of the exhaust gas after the feed of the reducing agent to the SCR catalyst (4) with the control of the reducing agent feed unit (5; 302) by the Feed correction section (S2, S30; S10, S35), wherein the concentration determination device converts the detection result of the ammonia sensor (202) into an ammonia concentration value according to an ammonia concentration correction formula which represents a relationship between first and second converted ammonia concentrations, wherein the first converted ammonia concentration is a value calculated according to a relationship formula between the detection result of the ammonia sensor (202) and the ammonia concentration of the exhaust gas, and the second converted ammonia concentration is a value calculated from the NOx sensor (201) detection result and the oxygen concentration of the exhaust gas, and wherein the concentration determination device determines the ammonia concentration value as the downstream ammonia concentration; and wherein the feed correction section (S2, S30; S10, S35) corrects the determined value of the downstream ammonia concentration by updating the ammonia concentration correction formula, wherein the ammonia concentration correction formula in a two-dimensional coordinate system, in which the ammonia concentration converted from the electromotive force is plotted along the x-axis and the ammonia concentration converted from the NOx output is plotted along the y-axis, represents a straight line in the two-dimensional coordinate system among several most recently stored, connects a maximum coordinate point and a minimum coordinate point with the output values ​​obtained from the gas sensor (201, 202).

Description

Scope of the invention

[0001] The present invention relates to a concentration determination device and a method for determining the concentration of ammonia emitted by an SCR catalyst. Background Invention

[0002] A conventional system for cleaning NOx from the exhaust gas of an internal combustion engine is known, in which a catalyst operating on the principle of selective catalytic reduction, hereinafter referred to as an SCR catalyst, is arranged in an exhaust pipe of the internal combustion engine, and urea is injected into the SCR catalyst as a reducing agent. This cleaning system is conventionally equipped with a NOx sensor and an ammonia sensor for detecting NOx and ammonia emissions from the SCR catalyst. It has been proposed to correct an output value of the ammonia sensor by comparing an output value of the NOx sensor with the output value of the ammonia sensor (see, for example, Patent Document 1: Published Japanese Patent Application No. JP 2014-224504A).It has also been proposed to correct an ammonia concentration measurement based on an output from an ammonia sensor and an oxygen concentration calculated on the basis of a pump current flowing through a pump cell (see patent document 2: European patent application EP 2 293 055 A1). Overview of the invention

[0003] In patent document 1, the output value of the ammonia sensor is corrected based on either the division of the NOx sensor output value by the ammonia sensor output value or the difference between the NOx sensor output value and the ammonia sensor output value. This results in low accuracy of the ammonia concentration determination after the sensor output value correction.

[0004] The present invention was made with regard to the aforementioned problem. One objective of the invention is to improve the accuracy of ammonia concentration determination after sensor output correction.

[0005] To achieve the above objective, according to a first aspect of the present invention, a concentration determination device for a cleaning system according to claim 1 is provided, wherein the cleaning system comprises: an SCR catalyst arranged in an exhaust pipe of an internal combustion engine to clean NOx in the exhaust gas of the internal combustion engine; a urea supply unit provided and arranged to supply urea as a reducing agent to the SCR catalyst; an ammonia sensor provided and arranged to detect an ammonia concentration of the exhaust gas flowing out of the SCR catalyst as a downstream ammonia concentration;and a NOx sensor which is provided and arranged to detect a NOx concentration of the exhaust gas flowing out of the SCR catalyst as a downstream NOx concentration, wherein the concentration determination device is configured to determine a value of the downstream ammonia concentration based on a detection result of the ammonia sensor.

[0006] The concentration determination device of the first aspect comprises a urea supply control section and a urea supply correction section. The urea supply control section controls the urea supply unit to deliver urea to the SCR catalyst in a fuel-free state, in which the fuel supply to the internal combustion engine is stopped. The urea supply correction section corrects the determined value of the downstream ammonia concentration based on a detection result from the NOx sensor and the oxygen concentration of the exhaust gas after the urea has been added to the SCR catalyst, by controlling the urea supply unit via the urea supply control section.

[0007] As mentioned previously, the concentration detection device of the first aspect is configured to allow the addition of urea by the urea injection unit in the fuel-free state. In the fuel-free state, the fuel supply to the internal combustion engine is stopped, so NOx is not present in the exhaust gas of the internal combustion engine. In this state, the NOx sensor performs a concentration detection that responds not only to NOx but also to ammonia. The detection result of the NOx sensor is therefore related to the ammonia concentration after the urea has been added to the SCR catalyst in such a quantity that urea can exit downstream in the fuel-free state.Therefore, in the first aspect, even in the case where the detection result of the ammonia sensor is not stable due to seasonal or daily fluctuations, it is possible to ensure the stability of the determined downstream ammonia concentration value by correcting the determined downstream ammonia concentration value based on the detection result of the NOx sensor.

[0008] Since the downstream ammonia concentration detection result of the NOx sensor varies depending on the oxygen concentration of the exhaust gas, the concentration determination device corrects the determined downstream ammonia concentration value based not only on the downstream NOx concentration detection result of the NOx sensor but also on the oxygen concentration of the exhaust gas. It is therefore possible to further improve the accuracy of the downstream ammonia concentration determination.

[0009] The concentration determination device of the first aspect is configured to convert the detection result of the ammonia sensor into an ammonia concentration value in accordance with an ammonia concentration correction formula and to determine the ammonia concentration value as the downstream ammonia concentration. The ammonia concentration correction formula represents a relationship between a first converted ammonia concentration, which is calculated according to a relationship formula between the direction result of the ammonia sensor and the ammonia concentration of the exhaust gas, and a second converted ammonia concentration, which is calculated from the NOx sensor detection result and the oxygen concentration of the exhaust gas.

[0010] It is therefore possible to allow the urea feed correction section to correct the determined downstream ammonia concentration value by updating the ammonia concentration correction formula.

[0011] The concentration determination device of the first aspect may further include a prevention section which, if the NOx concentration of the exhaust gas exceeds a predetermined prevention assessment concentration level, prevents the urea supply correction section from updating the ammonia concentration correction formula.

[0012] In this case, it is also possible, in a situation where a large amount of NOx is contained in the exhaust gas even in the fuel-free state, to avoid correcting the determined downstream ammonia concentration value based on the detection result of the NOx sensor, thereby further improving the accuracy of determining the downstream ammonia concentration.

[0013] To achieve the aforementioned objective, a concentration determination device for a cleaning system is also provided according to a second aspect of the present invention, wherein the cleaning system comprises: an SCR catalyst arranged in an exhaust pipe of an internal combustion engine to clean NOx in the exhaust gas of the internal combustion engine; an ammonia supply unit provided and arranged to supply ammonia to the SCR catalyst as a reducing agent; an ammonia sensor provided and arranged to detect an ammonia concentration of the exhaust gas flowing out of the SCR catalyst as a downstream ammonia concentration;and a NOx sensor which is provided and arranged to detect a NOx concentration of the exhaust gas flowing out of the SCR catalyst as a downstream NOx concentration, wherein the concentration determination device is configured to determine a value of the downstream ammonia concentration based on a detection result of the ammonia sensor.

[0014] The concentration determination device of the second aspect comprises an ammonia supply control section and an ammonia supply correction section. The ammonia supply control section controls the ammonia supply unit to supply ammonia to the SCR catalyst in a fuel-free state, in which the fuel supply to the internal combustion engine is stopped. The ammonia supply correction section corrects the determined value of the downstream ammonia concentration based on a detection result from the NOx sensor and the oxygen concentration of the exhaust gas after the ammonia has been supplied to the SCR catalyst, under the control of the ammonia supply unit by the ammonia supply control section.

[0015] As mentioned above, the concentration-measuring device of the second aspect is configured to determine and correct the downstream ammonia concentration during the ammonia feed instead of the urea feed. Therefore, it is possible to achieve the same effects with the second aspect as with the first.

[0016] In the concentration determination device according to the first and second aspects, the NOx sensor can each have the form of a limiting current gas sensor. The limiting current NOx sensor performs an oxygen pumping action on the exhaust gas introduced into it in order to maintain the oxygen concentration of the exhaust gas in the NOx sensor at a constant value. In particular, the limiting current NOx sensor has a function of detecting the oxygen concentration of the exhaust gas in accordance with the amount of oxygen pumped into or out of the NOx sensor.

[0017] By using such a limiting current gas sensor, it is possible to simplify the configuration of the cleaning system without the need to provide a separate oxygen sensor to detect the oxygen concentration of the exhaust gas.

[0018] Furthermore, the ammonia sensor and the NOx sensor in the concentration determination devices of the first and second aspects can each be combined into a single integrated gas sensor. In this case, the ammonia sensor and the NOx sensor are used to detect the downstream ammonia concentration of the exhaust gas in essentially the same range. In particular, the ammonia sensor and the NOx sensor do not differ with respect to their exhaust gas concentration detection range. By using such an integrated gas sensor, it is possible to further improve the accuracy of determining the downstream ammonia concentration.

[0019] To achieve the above-mentioned objective, according to a third aspect of the present invention, a concentration determination method according to claim 5 is provided for a cleaning system, wherein the cleaning system comprises an SCR catalyst, a urea supply unit, an ammonia sensor, and a NOx sensor, wherein the concentration determination method is provided to determine a downstream ammonia concentration based on a detection result of the ammonia sensor.

[0020] The concentration determination procedure of the third aspect comprises a urea supply control step and a urea supply correction step. In the urea supply control step, the urea supply unit is activated to supply urea to the SCR catalyst in a fuel-free state, in which the fuel supply to the combustion engine is stopped. In the urea supply correction step, the determined downstream ammonia concentration is calculated based on a detection result from the NOx sensor and the oxygen concentration of the exhaust gas after the urea has been supplied to the SCR catalyst by the urea supply control step.

[0021] Since the concentration determination method of the third aspect is carried out in the concentration determination device of the first aspect, it is possible to achieve the same effects with the third aspect as with the first aspect.

[0022] To achieve the above-mentioned objective, according to a fourth aspect of the present invention, a concentration determination method for a cleaning system is provided, wherein the cleaning system comprises an SCR catalyst, an ammonia supply unit, an ammonia sensor, and a NOx sensor, wherein the concentration determination method is provided to determine a downstream ammonia concentration based on a detection result of the ammonia sensor.

[0023] The concentration determination procedure of the fourth aspect comprises an ammonia feed control step and an ammonia feed correction step. In the ammonia feed control step, the ammonia feed unit is activated to supply ammonia to the SCR catalyst in a fuel-free state, in which the fuel supply to the combustion engine is stopped. In the ammonia feed correction step, the determined downstream ammonia concentration is adjusted based on a detection result from the NOx sensor and the oxygen concentration of the exhaust gas after the ammonia has been supplied to the SCR catalyst by the ammonia feed control step.

[0024] Since the concentration determination method of the fourth aspect is carried out in the concentration determination device of the second aspect, it is possible to achieve the same effects with the fourth aspect as with the second aspect. Brief description of the drawings Fig. Figure 1 is a schematic view of a urea SCR system 1 according to a first embodiment of the present invention. Fig. Figure 2 is a configuration diagram of an upstream NOx sensor 6 and a NOx sensor controller 7 in the urea SCR system. Fig. Figure 3 is a configuration diagram of a multi-gas sensor 8 and a multi-gas sensor controller 9 in the urea SCR system. Fig. Figure 4 is a perspective exploded view of an ammonia sensor unit 202 of the multi-gas sensor. Fig. Figure 5 is a flowchart of a concentration determination process according to the first embodiment of the present invention. Fig. Figure 6 is a flowchart of a correction factor calculation process according to the first embodiment of the present invention. Fig. 7A and Fig. Figures 7B are graphs showing changes in downstream NOx output and downstream ammonia output in the case where concentration correction factors are updated according to the first embodiment of the present invention. Fig. 8A and Fig. Figure 8B shows graphs that depict changes in downstream NOx output and downstream ammonia output in the case where concentration correction factors are not updated. Fig. Figure 9 is a flowchart of a concentration determination process according to a second embodiment of the present invention. Fig. Figure 10 is a flowchart of a concentration determination process according to a third embodiment of the present invention. Fig. Figure 11 is a schematic diagram of a cleaning system 301 according to a fourth embodiment of the present invention. Fig. Figure 12 is a flowchart of a concentration determination process according to the fourth embodiment of the present invention. Description of embodiments (First embodiment)

[0025] A first embodiment of the present invention is described below with reference to the drawings.

[0026] As in Fig. Figure 1 shows an SCR system, that is, a system for carrying out selective catalytic reduction, 1 according to the first embodiment of the present invention comprising an oxidation catalyst 2, a diesel particulate filter 3 hereinafter referred to as DPF unit, an SCR catalyst 4, an injector 5 for aqueous urea solution, an upstream NOx sensor 6, a NOx sensor control 7, a multiple gas sensor 8, a multiple gas sensor control 9 and a cleaning control 10.

[0027] The oxidation catalyst 2 is designed and arranged to receive exhaust gas from a diesel engine 51 via an exhaust pipe 52 and to convert nitrogen oxide (NO) into nitrogen dioxide (NO2) among the NOx components in the exhaust gas.

[0028] The DPF unit 3 is arranged and designed to capture the exhaust gas flowing out of the oxidation catalyst 2 via the exhaust pipe 52 and to remove particles from the exhaust gas.

[0029] The SCR catalyst 4 is arranged and designed to receive the exhaust gas flowing out of the DPF unit 3 via the exhaust pipe 52 and, during the generation of ammonia by hydrolysis of urea supplied from the upstream side, to convert NOx in the exhaust gas into nitrogen and water through the reduction effect of the generated ammonia, thereby releasing NOx-reduced exhaust gas.

[0030] The injector 5 for urea solution is located on the exhaust pipe 52 at a point between the DPF unit 3 and the SCR catalyst 4 to inject urea solution into the exhaust gas. The injected urea solution is hydrolyzed to ammonia gas under high-temperature conditions, so that the ammonia gas is used as the reducing agent for NOx reduction, as previously explained.

[0031] The upstream NOx sensor 6 is located on the exhaust pipe 52 at a position between the DPF unit 3 and the SCR catalyst 4 to detect the concentration of NOx in the exhaust gas flowing out of the DPF unit 3.

[0032] The NOx sensor controller 7 is configured to control the operation of the upstream NOx sensor 6 and to determine the concentration of NOx in the exhaust gas flowing out of the DPF unit 3 (hereinafter referred to as the "upstream NOx concentration"), based on the detection result of the upstream NOx sensor 6. Furthermore, the NOx sensor controller 7 is configured to perform data communication with the cleaning controller 10 via a communication line, so that data indicating the upstream NOx concentration can be transmitted to the cleaning controller 10.

[0033] The multi-gas sensor 8 is attached to the downstream side of the SCR catalyst 4 on the exhaust pipe 52 to detect the concentration of NOx and ammonia in the exhaust gas flowing out of the SCR catalyst 4.

[0034] The multi-gas sensor controller 9 is configured to control the operation of the multi-gas sensor 8 and to determine the concentrations of NOx, oxygen, and ammonia in the exhaust gas flowing out of the SCR catalyst 4 (hereinafter also referred to as "downstream NOx concentration," "downstream oxygen concentration," and "downstream ammonia concentration," respectively), based on the detection results of the multi-gas sensor 8. Furthermore, the multi-gas sensor controller 9 is configured to perform data communication with the cleaning controller 10 via a communication line, so that data indicating the downstream NOx concentration and data indicating the downstream ammonia concentration can be transmitted to the cleaning controller 10.

[0035] The cleaning control unit 10 comprises, as its main component, a microcomputer equipped with a CPU 21, a ROM 22, a RAM 23, and a signal input / output section 24. The urea solution injector 5, the NOx sensor control unit 7, and the multi-gas sensor control unit 8 are connected to the signal input / output section 24. As previously mentioned, the cleaning control unit 10 is configured to perform data communication with the NOx sensor control unit 7 and the multi-gas sensor control unit 9 via the communication lines. The cleaning control unit 10 is also configured to perform data communication with an electronic control unit 53 via a communication line. The electronic control unit 53 is arranged and configured to control the operation of the diesel engine 51. Hereinafter, the electronic control unit 53 is referred to as the "engine ECU" 53 (electronic control unit).

[0036] As in Fig. As shown in Figure 2, the upstream NOx sensor 6 comprises an insulating layer 113, a solid electrolyte layer 114, an insulating layer 115, a solid electrolyte layer 116, an insulating layer 117, a solid electrolyte layer 118, an insulating layer 119, and an insulating layer 120, which are stacked on top of each other in that order. In the first embodiment, the insulating layers 113, 115, 117, 119, and 120 are mainly composed of aluminum oxide; and the solid electrolyte layers 114, 116, and 118 are mainly composed of oxygen-ion-conducting zirconium oxide.

[0037] There is a first measuring chamber 121, which is bounded between the solid electrolyte layer 114 and the solid electrolyte layer 116 in the upstream NOx sensor 6. In the NOx sensor unit 101 of the upstream NOx sensor, a diffusion resistance element 122 is arranged between the solid electrolyte layer 114 and the solid electrolyte layer 116 on one side adjacent to the first measuring chamber 121, so that the exhaust gas is introduced into the first measuring chamber 121 from the outside via the diffusion resistance element 122. A diffusion resistance element 123 is arranged between the solid electrolyte layer 114 and the solid electrolyte layer 116 on the other side adjacent to the first measuring chamber 121, so that the exhaust gas is released from the first measuring chamber 121 to the outside via the diffusion resistance element 123. The diffusion resistance elements 122 and 123 are made of a porous material, such as aluminium oxide.

[0038] The upstream NOx sensor 6 has a first pump cell 130, which is formed from the solid electrolyte layer 114 and pump electrodes 131 and 132. In the first embodiment, the pump electrodes 131 and 132 are essentially made of platinum. The electrode 131 is located on a surface of the solid electrolyte layer 114 and is in contact with the first measuring chamber 121. A protective layer 133 is formed from a porous material to cover a surface of the pump electrode 131 that faces the first measuring chamber 121. The pump electrode 132 is located on a surface of the solid electrolyte layer 114 that is opposite the pump electrode 131. A portion of the insulating layer 113, corresponding in its position to the pump electrode 132 and its surroundings, has been removed.A porous material 134 is filled into this part instead of the insulating layer 113 in order to allow a gas flow (of oxygen) between the pump electrode 132 and the environment.

[0039] The upstream NOx sensor 6 also includes an oxygen concentration detection cell 140, which is formed by the solid electrolyte layer 116, a detection electrode 141, and a reference electrode 142. In the first embodiment, the detection electrode 141 and the reference electrode 142 are mainly made of platinum. The detection electrode 141 is formed on a surface of the solid electrolyte layer 116 that is in contact with the first measuring chamber 121, specifically at a position downstream of the pump electrode 131 within the first measuring chamber 121 (i.e., a position closer to the diffusion resistance element 123 than to the diffusion resistance element 122). The reference electrode 142 is arranged on a surface of the solid electrolyte layer 116 that is opposite the detection electrode 141.

[0040] In the upstream NOx sensor 6, there is a reference oxygen chamber 146, which is defined between the solid electrolyte layer 116 and the solid electrolyte layer 118 and is in contact with the reference electrode 142. The interior of the reference oxygen chamber 146 is filled with a porous material.

[0041] Furthermore, there is a second measuring chamber 148, which is limited between the solid electrolyte layer 114 and the solid electrolyte layer 118 through the insulating layer 115, the solid electrolyte layer 116 and the insulating layer 117 in the upstream NOx sensor 6, so that the exhaust gas emitted from the first measuring chamber 121 via the diffusion resistance element 123 is introduced into the second measuring chamber 148.

[0042] The upstream NOx sensor 6 has a second pump cell 150, which is formed by the solid electrolyte layer 118 and the pump electrodes 151 and 152. In the first embodiment, the pump electrodes 151 and 152 are mainly made of platinum. The pump electrode 151 is arranged on a surface of the solid electrolyte layer 118 that is in contact with the second measuring chamber 148. The pump electrode 152 is arranged on a surface of the solid electrolyte layer 118 that is opposite the reference electrode 142 with respect to the reference oxygen chamber 146.

[0043] The upstream NOx sensor 6 has a heater 160. The heater 160 is a heating resistor, which is mainly composed of platinum or the like, to generate heat when energy is supplied to it, and it is arranged between the insulating layer 119 and the insulating layer 120.

[0044] The NOx sensor control 7 comprises a control circuit module 180 and a microcomputer 190 (hereinafter referred to as "Mikon 190").

[0045] The control circuit module 180 is an analog circuit module mounted on a circuit board. Specifically, the control circuit module 180 comprises an Ip1 driver circuit 181, a Vs detection circuit 182, a reference voltage comparison circuit 183, an Icp supply circuit 184, a Vp2 system circuit 185, an Ip2 detection circuit 186, and a heating driver circuit 187.

[0046] Pump electrode 131, detection electrode 141, and pump electrode 151 are connected to a reference potential. Pump electrode 132 is connected to the Ip1 driver circuit 181. Reference electrode 142 is connected to the Vs detection circuit 182 and the Icp supply circuit 184. Pump electrode 152 is connected to the Vp2 system circuit 185 and the Ip2 detection circuit 186. Heater 160 is connected to the heater driver circuit 187.

[0047] The Ip1 driver circuit 181 generates a first pump current Ip1 between the pump electrode 131 and the pump electrode 132, and it detects the supplied first pump current Ip1.

[0048] The Vs detection circuit 182 detects a voltage Vs between the detection electrode 141 and the reference electrode 142 and outputs the detected voltage to the reference voltage comparison circuit 183.

[0049] The reference voltage comparison circuit 183 compares the output (voltage Vs) of the VS detection circuit 182 with a reference voltage (for example, 425 mV) and outputs the comparison result to the Ip1 driver circuit 181. The Ip1 driver circuit 181 controls the direction and strength of the first pump current Ip1 such that the voltage Vs equals the reference voltage, and it adjusts the oxygen concentration in the first measuring chamber 121 to a predetermined level at which NOx decomposition does not occur.

[0050] The Icp supply circuit 184 supplies a small current Icp between the detection electrode 141 and the reference electrode 142. By supplying such a small current, oxygen is pumped from the first measuring chamber 121 through the solid electrolyte layer 116 to the reference oxygen chamber 146, so that the concentration of oxygen in the reference oxygen chamber 146 is adjusted to a predetermined reference oxygen concentration.

[0051] The Vp2 system circuit 185 applies a constant voltage Vp2 (for example, 450 mV) between pump electrode 151 and pump electrode 152. By applying this constant voltage, NOx in the second measuring chamber 148 is dissociated by the catalytic activity of pump electrodes 151 and 152 of the second pump cell 150. The oxygen atoms obtained in this way flow through the solid electrolyte layer 118 between pump electrode 151 and pump electrode 152, thereby generating a second current Ip2. The Ip2 detection circuit 186 detects the generated second pump current Ip2.

[0052] The heating driver circuit 187 operates the heating element 160 by applying a positive energy supply voltage to one end of the heating resistor and a negative energy supply voltage to the other end of the heating resistor.

[0053] The microcomputer 190 has a CPU 191, a ROM 192, a RAM 193 and a signal input / output section 194.

[0054] The CPU 191 performs various processing operations to control the upstream NOx sensor 6 according to programs stored in the ROM 192. The signal input / output section 194 is connected to the Ip1 driver circuit 181, the Vs detection circuit 182, the Ip2 detection level 186, and the heater driver circuit 187.

[0055] For example, the CPU 191 controls the operation of the NOx sensor to adjust the oxygen concentration in the first measuring chamber 121 by means of the pumping action of the first pump cell 130 and to adjust the oxygen concentration in the second measuring chamber 148 to a level at which NOx detection is possible. It also determines the NOx concentration of the exhaust gas based on the value of the second pump current Ip2. Furthermore, the CPU 191 outputs a driver signal via the signal input / output unit 194 to the heating driver circuit 187, thereby controlling the heating element 160.

[0056] As in Fig. As shown in Figure 3, the multi-gas sensor 8 comprises a NOx sensor unit 201 and an ammonia sensor unit 202.

[0057] The NOx sensor unit 201 has the same structure as the upstream NOx sensor 6, except that: a solid electrolyte layer 112 is stacked on the insulating layer 113, and an insulating layer 111 is stacked on the solid electrolyte layer 112. In the first embodiment, the insulating layer 111 is mainly composed of aluminum oxide, and the solid electrolyte layer 112 is mainly composed of oxygen-ion-conducting zirconium oxide.

[0058] The ammonia sensor unit 202 has a detection electrode 211, a reference electrode 212, a selective reaction layer 213 and a diffusion layer 214.

[0059] As in Fig. As shown in Figure 4, the detection electrode 211 and the reference electrode 212 are arranged at a distance from each other on the solid electrolyte layer 112. In the first embodiment, the detection electrode 211 is made of a material containing gold as a major component, and the reference electrode 212 is made of a material containing platinum as a major component. Since the detection electrode 211 exhibits greater reactivity for ammonia than the reference electrode 212, an electromotive force is developed between the detection electrode 211 and the reference electrode 212.

[0060] The selective reaction layer 213 is mainly composed of a metal oxide to cover the detection electrode 211 and the reference electrode 212. The selective reaction layer 213 functions to combust any combustible gas component other than ammonia, so that the ammonia sensor unit 202 can detect the ammonia in the exhaust gas without being affected by the combustible gas component.

[0061] The diffusion layer 214 is formed from a porous material to cover the selective reaction layer 213. The function of the diffusion layer 214 is to adjust the diffusion rate of the exhaust gas introduced from the outside into the ammonia sensor unit 202.

[0062] The multi-gas sensor control 9 includes, as shown in Fig. Figure 3 shows a control circuit module 220 and a microcomputer 230.

[0063] The control circuit module 220 differs from the control circuit module 180 of the NOx sensor control 7 in that it additionally includes a detection circuit 121 for electromotive force. The electromotive force detection circuit 221 detects the electromotive force generated between the detection electrode 211 and the reference electrode 212 (hereinafter referred to as "electromotive ammonia force EMF") and outputs the detection result as a detection signal to the microcomputer 230.

[0064] The microcomputer 230 has a CPU 231, a ROM 232, a RAM 233, an EEPROM 234 and a signal input / output unit 235.

[0065] The CPU 231 performs various processing operations to control the multiple gas sensor 8 according to programs stored in the ROM 232. The signal input / output section 235 is connected to the Ip1 driver circuit 181, the Vs detection circuit 182, the Ip2 detection circuit 186, the heater driver circuit 187, and the electromotive force detection circuit 221.

[0066] The CPU 231 determines the NOx concentration in the same way as the CPU 191. The CPU 231 also determines the oxygen concentration based on the direction and magnitude of the flow of the first pump stream Ip1.

[0067] The CPU 231 further determines the ammonia concentration by converting the electromotive force (EMF) into an ammonia concentration value in accordance with an ammonia concentration correction formula. The ammonia concentration correction formula represents a relationship between the ammonia concentration value calculated in accordance with a relationship formula between the electromotive force (EMF) and the oxygen concentration (hereinafter referred to as "ammonia concentration converted from the electromotive force"), and the ammonia concentration value calculated from the downstream NOx concentration and the downstream oxygen concentration (hereinafter referred to as "ammonia concentration converted from the NOx output").The ammonia concentration correction formula is a linear equation that uses an offset and a gradient as coefficients and takes the ammonia concentration, converted from the electromotive force (EMF) and the oxygen concentration, as a variable. The offset and gradient of the ammonia concentration correction formula are stored as the first and second ammonia concentration correction factors in the EEPROM 234.

[0068] The CPU 231 also outputs a driver signal to the heating driver circuit 187 via the signal input / output section 235, thereby controlling the heating 160.

[0069] The microcomputer 230 of the multi-gas sensor control 9 performs a concentration determination process.

[0070] The concentration determination process is explained below. The concentration determination process begins immediately after the start-up of the microcomputer 230 of the multi-gas sensor control unit 9.

[0071] At the beginning of the concentration determination process, the CPU 231 of the microcomputer 230 sets various ammonia concentration determination parameters in a Fig. Step S2, shown in section 5, is set to predetermined initial values. Processing step S2 sets the first and second ammonia concentration correction factors mentioned above to their respective initial values, and clears the initialization flag mentioned below.

[0072] Then, in step S4, the CPU assesses whether diesel engine 51 is in a fuel-off state, in which the fuel supply is stopped. Diesel engine 51 is assessed as being in a fuel-off state if the fuel cut-off signal from cleaning control 10 is ON. If the fuel supply signal from cleaning control 10 is OFF, diesel engine 51 is assessed as not being in a fuel-off state. The fuel cut-off signal indicates whether diesel engine 51 is in a fuel-off state or not. The ON state of the fuel cut-off signal refers to the case in which the voltage level of the fuel cut-off signal is set to high during fuel-off operation of the engine.The OFF state of the fuel cut-off signal refers to the case where the voltage level of the fuel cut-off signal is set to the low level during fuel-free operation of the engine. This fuel cut-off signal is sent from the engine ECU 53 to the cleaning control 10 and then from the cleaning control 10 to the multi-gas sensor control 9.

[0073] If the diesel engine is not in the fuel-free state (NO in step S4), the CPU proceeds to step S8. If the diesel engine is in the fuel-free state (YES in step S4), the CPU proceeds with the execution of the correction factor calculation step S6 mentioned below, and then with step S8.

[0074] In step S8, the CPU determines the ammonia concentration converted from the electromotive force according to the formula relating the electromotive force (EMF) to the oxygen concentration, as explained previously. Furthermore, in step S8, the CPU converts the ammonia concentration derived from the electromotive force into the ammonia concentration derived from the NOx output according to the ammonia concentration correction formula. After completing step S8, the CPU proceeds to step S4.

[0075] The correction factor calculation process performed in step S6 is explained below.

[0076] At the start of the correction factor calculation process, the CPU 231 of the microcomputer 230 assesses whether the initialization flag has been set in step S12 or not, as shown in Fig. Figure 6 shows that the initial value of the initialization flag is set to 0. This is because the initialization flag is cleared at the beginning.

[0077] If the initialization flag is set (YES in step S12), the CPU proceeds to step S50. If the initialization flag is not set (NO in step S12), the CPU proceeds to step S20. In step S20, the CPU assesses whether the upstream NOx concentration, specified by the upstream NOx concentration data, is less than or equal to a predetermined injection assessment concentration level. In the first embodiment, the injection assessment concentration level is set to approximately 0 ppm. The upstream NOx concentration data is obtained from the communication between the multi-gas sensor controller 9 and the cleaning controller 10. If the upstream NOx concentration exceeds the injection assessment concentration level (NO in step S20), the correction factor calculation continues.If the upstream NOx concentration is less than or equal to the injection assessment concentration level (YES in step S20), the CPU proceeds to step S22. In step S22, the CPU sets the initialization flag. Then, in step S30, the CPU sends a corrective urea injection command to the cleaning controller 10. In accordance with this injection command, the cleaning controller 10 controls the urea solution injector 5 to inject a predetermined corrective injection quantity of urea solution. In step S40, the CPU sends a urea injection stop command to the cleaning controller 10. In accordance with this injection stop command, the cleaning controller 10 temporarily stops the urea injection operation of the urea solution injector 5.

[0078] In step S50, the CPU assesses whether data can be stored in RAM 233, and specifically whether the free space in RAM 233 is greater than or equal to a predetermined data storage assessment capacity. The data storage assessment capacity refers to the data capacity required to store the data described in step S60. If data cannot be stored in RAM 233 (NO in step S50), the CPU proceeds to step S80.

[0079] If data can be stored in RAM 233 (YES in step S50), the CPU proceeds to step S60. In step S60, the CPU stores the most recent values ​​of the downstream NOx output, the downstream ammonia output, and the downstream oxygen output value obtained from the multi-gas sensor 8. In the first embodiment, the value of the second pump current Ip2 is obtained as the upstream NOx output. The value of the electromotive force EMF for ammonia is obtained as the downstream ammonia output, and the value of the first pump current Ip2 is obtained as the downstream oxygen output.

[0080] In step S70, the CPU assesses, in the same way as in step S4, whether the engine is in the fuel-off state or not. If the engine is in the fuel-off state (YES in step S70), the correction factor calculation process ends. If the engine is not in the fuel-off state (NO in step S70), the CPU proceeds to step S80.

[0081] In step S80, the CPU assesses whether the NOx output difference is greater than or equal to a predetermined NOx correction assessment level. The NOx sensor output difference refers to the difference between the maximum and minimum values ​​of the majority of downstream NOx output values ​​stored in RAM 233. If the NOx output difference is less than the NOx correction assessment level (NO in step S80), the CPU proceeds to step S130.

[0082] If the NOx output difference is greater than or equal to the NOx correction assessment level (JA in step S80), the CPU proceeds to step S90. In step S90, the CPU determines the ammonia concentration values ​​corresponding to the maximum and minimum values ​​of the downstream NOx output stored in RAM 233, with respect to a predetermined ammonia concentration determination mapping. The ammonia concentration determination mapping defines a relationship between the downstream NOx output and the ammonia concentration and is stored in ROM 232. Hereinafter, the ammonia concentration value corresponding to the maximum value of the downstream NOx output is referred to as the "maximum ammonia concentration value," and the ammonia concentration value corresponding to the minimum value of the downstream NOx output is referred to as the "minimum ammonia concentration value."

[0083] In step S100, the CPU corrects the maximum and minimum ammonia concentration values ​​in accordance with the downstream oxygen concentration. Specifically, the maximum and minimum ammonia concentration values ​​are corrected as follows. First, an oxygen concentration adjustment factor for the downstream oxygen output, corresponding to the maximum value of the downstream NOx output (hereinafter referred to as the "maximum concentration adjustment factor"), is set with reference to a predetermined adjustment factor setting mapping. The adjustment factor setting mapping defines a relationship between the downstream oxygen output and the oxygen concentration adjustment factor and is stored in ROM 232.Similarly, an oxygen concentration adjustment factor for the downstream oxygen output, corresponding to the minimum value of the downstream NOx output (hereinafter referred to as the "minimum concentration adjustment factor"), is set with reference to a predetermined adjustment factor setting mapping. Then, the maximum ammonia concentration value is corrected to a corrected maximum ammonia concentration value by multiplying the maximum ammonia concentration value by the maximum concentration adjustment factor. The minimum ammonia concentration value is similarly corrected to a corrected minimum ammonia concentration value by multiplying the minimum ammonia concentration value by the minimum concentration adjustment factor.

[0084] In step S105, the CPU determines values ​​of the ammonia concentration converted from the electromotive force based on the downstream ammonia output (i.e., the electromotive ammonia force EMF) and the downstream oxygen output, which corresponds to the maximum and minimum values ​​of the downstream NOx output.

[0085] In step S110, the CPU calculates the aforementioned first and second ammonia concentration correction factors from the calculated maximum and minimum ammonia concentration values. Specifically, the first and second ammonia concentration correction factors are calculated as follows. In the two-dimensional orthogonal coordinate system, where the ammonia concentration converted from the electromotive force is plotted along the x-axis and the ammonia concentration converted from the NOx output is plotted along the y-axis, the linear equation is established to specify a straight line that defines a coordinate point corresponding to the respective corrected maximum ammonia concentration value (hereinafter referred to as the "maximum coordinate point") and a coordinate point corresponding to the corrected minimum ammonia concentration value (hereinafter referred to as the "minimum coordinate point").The offset and gradient of the linear equation are used as the first and second ammonia concentration correction factors, respectively. The maximum coordinate point refers to the point (x1, y1) in the two-dimensional orthogonal coordinate system, assuming that x1 is the ammonia concentration converted from the electromotive force, which corresponds to the corrected maximum ammonia concentration value, and y1 is the corrected maximum ammonia concentration value; and the minimum coordinate point refers to the point (x2, y2) in the two-dimensional orthogonal coordinate system, assuming that x2 is the ammonia concentration converted from the electromotive force, which corresponds to the corrected minimum ammonia concentration value, and y2 is the corrected minimum ammonia concentration value.

[0086] In step S120, the CPU replaces the first and second ammonia concentration correction factors stored in EEPROM 234 with those calculated in step S110, thereby updating the first and second ammonia concentration correction factors. The CPU then proceeds to step S130.

[0087] In step S130, the CPU clears all downstream NOx output, downstream ammonia output, and downstream oxygen output stored in RAM 233. In step S132, the CPU clears the initialization flag. In step S140, the CPU sends a urea injection restart command to cleaning controller 10. In accordance with this injection restart command, cleaning controller 10 restarts the urea injection control of injector 5 for urea solution.

[0088] The Fig. 7A, Fig. 7B, Fig. 8A and Fig. Figure 8B shows examples of changes in downstream NOx output and downstream ammonia output when urea injection is performed in the fuel-off state. In the Fig. 7A, Fig. 7B, Fig. 8A and Fig. 8B urea solution is injected through injector 5 for urea solution under the conditions that the fuel cut-off signal is switched to the ON state (see arrow FC1) and the upstream NOx concentration is less than or equal to the injection assessment concentration level, and, after injection of urea solution, the fuel cut-off signal is switched to the OFF state (see arrow FC2).

[0089] Fig. 7A refers to the case where, after the injection of urea solution, the maximum value of the downstream NOx output L1 is large and essentially equal to the maximum value of the downstream ammonia output L2. In this case, the first and second ammonia concentration correction factors are updated because the NOx output difference DN is greater than or equal to the NOx correction assessment level.

[0090] Fig. 7B refers to the case where, after the injection of urea solution, the maximum value of the downstream NOx output L1 is large and the maximum value of the downstream ammonia output L2 is smaller than the maximum value of the downstream NOx output L1. In this case, the first and second ammonia concentration correction factors are also updated because the NOx output difference DN is greater than or equal to the NOx correction assessment level.

[0091] Fig. 8A refers to the case where, after the injection of urea solution, the maximum value of the downstream NOx output L1 is small and essentially equal to the maximum value of the downstream ammonia output L2. In this case, the first and second ammonia concentration correction factors are not updated because the NOx output difference DN is smaller than the NOx correction assessment level.

[0092] Fig. 8B refers to the case where, after the injection of urea solution, the maximum value of the downstream NOx output L1 is small and the maximum value of the downstream ammonia output L2 is larger than the maximum value of the downstream NOx output L1. In this case, the first and second ammonia concentration correction factors are also not updated because the NOx output difference DN is smaller than the NOx correction assessment level.

[0093] As discussed previously, the multi-gas sensor controller 9 is integrated into the urea SCR system 1 with the SCR catalyst 4, the urea solution injector 5, the ammonia sensor unit 202, and the NOx sensor unit 201. The multi-gas sensor controller 9 determines the downstream ammonia concentration based on the detection result of the ammonia sensor unit 202.

[0094] The multi-gas sensor control 9 also controls the injector 5 for urea solution to supply urea to the SCR catalyst 5 in the fuel-free state in which the fuel supply to the diesel engine 51 is stopped (steps S2 and S30).

[0095] The multi-gas sensor control 9 then corrects the determined downstream ammonia concentration value by updating the first and second ammonia concentration correction factors based on the detection result of the NOx sensor unit 201 and the oxygen concentration of the exhaust gas after the supply of urea to the SCR catalyst 4 with the control of the injector 5 for urea solution (steps S50 to S120).

[0096] In this way, the multi-gas sensor control 9 is configured to allow the supply of urea through injector 5 for urea solution in the fuel-free state. In the fuel-free state, the fuel supply to the diesel engine 51 is stopped, so that NOx is not present in the exhaust gas of the diesel engine 51. Here, the NOx sensor unit 201 performs a concentration detection, which responds not only to NOx but also to ammonia. The detection result of the NOx sensor unit 201 therefore correlates with the ammonia concentration after the urea is supplied to the SCR catalyst 4 in the fuel-free state.The multi-gas sensor control 9 is therefore able to ensure the stability of the determined downstream ammonia concentration value by correcting the determined downstream ammonia concentration value based on the detection result of the NOx sensor unit 202, even in the case where the detection result of the ammonia sensor unit 202 is not stable due to seasonal or daily fluctuations.

[0097] Since the detection result of the downstream ammonia concentration by the NOx sensor unit 201 varies depending on the oxygen concentration of the exhaust gas, the multi-gas sensor control 9 corrects the determined downstream ammonia concentration value based not only on the downstream NOx concentration detection result of the NOx sensor unit 201 but also on the oxygen concentration of the exhaust gas. Therefore, the multi-gas sensor control 9 is able to further improve the accuracy of the determination of the downstream ammonia concentration.

[0098] The multi-gas sensor controller 9 is further configured to prevent the correction of the determined downstream ammonia concentration value by preventing the updating of the first and second ammonia concentration correction factors when the downstream NOx concentration exceeds the injection assessment concentration level (NO in step S20). Therefore, the multi-gas sensor controller 9 can further improve the determination accuracy of the downstream ammonia concentration by avoiding the correction of the determined downstream ammonia concentration value based on the detection result of the NOx sensor unit in a situation where a large amount of NOx is present in the exhaust gas, even in the fuel-free state.

[0099] In the first embodiment, the NOx sensor unit 201 is a limiting flow gas sensor. When exhaust gas is introduced into the first measuring chamber 121, the NOx sensor unit 201 enables the first pump cell 130 to perform an oxygen pumping action on the exhaust gas contained in the first measuring chamber 121, thereby maintaining the oxygen concentration of the exhaust gas in the first measuring chamber 121 at a constant value. The NOx sensor unit 201 has the function, in particular, of detecting the oxygen concentration of the exhaust gas in accordance with the direction and magnitude of the flow of the first pump current Ip1. By using such a limiting flow gas sensor, it is possible to simplify the configuration of the urea SCR system 1 without having to provide a separate oxygen sensor to detect the oxygen concentration of the exhaust gas.

[0100] Furthermore, in the first embodiment, the ammonia sensor unit 202 and the NOx sensor unit 201 are integrated into a multi-gas sensor 8. In this case, the ammonia sensor unit 202 and the NOx sensor unit 201 are used to detect the downstream ammonia concentration of the exhaust gas in essentially the same range. In particular, the ammonia sensor unit 202 and the NOx sensor unit 201 do not differ with respect to their range or their position for concentration detection in the direction of exhaust gas flow. By using such an integrated gas sensor, it is possible to further improve the accuracy of determining the downstream ammonia concentration.

[0101] In the first embodiment described above, the multi-gas sensor control 9 corresponds to the claimed concentration determination device; the diesel engine 51 corresponds to the claimed internal combustion engine; the SCR catalyst 4 corresponds to the claimed SCR catalyst; the injector 5 for urea solution corresponds to the claimed urea supply unit; the ammonia sensor unit 202 corresponds to the claimed ammonia sensor; the NOx sensor unit 201 corresponds to the claimed NOx sensor; and the urea SCR system 1 corresponds to the claimed cleaning system.

[0102] The processing of steps S2 and S30 corresponds to the claimed urea supply control section or step; the processing of steps S50 to S120 corresponds to the claimed urea supply correction section or step, and the processing of step S20 corresponds to the claimed prevention section.

[0103] The ammonia concentration converted from the electromotive force corresponds to the first converted ammonia concentration claimed; and the ammonia concentration converted from the NOx output corresponds to the second converted ammonia concentration claimed. (Second embodiment)

[0104] A second embodiment of the present invention will now be described with reference to the drawings. The following description focuses on the differences between the second embodiment and the first embodiment.

[0105] A urea SCR system 1 of the second embodiment is similar to that of the first embodiment, except for the processing of the correction factor calculation process.

[0106] The correction factor calculation process of the second embodiment is similar to that of the first embodiment, except that the processing of step S25 is performed instead of the processing of step S20.

[0107] As in Fig. As shown in Figure 9, the multi-gas sensor controller 9, if the initialization flag is not set (NO in step S12), assesses in step S25 whether the last determined downstream NOx concentration value is less than or equal to the injection assessment concentration level. If the downstream NOx concentration value exceeds the injection assessment concentration level (NO in step S25), the correction factor calculation process ends. If the downstream NOx concentration value is less than or equal to the injection assessment concentration level (YES in step S25), the controller proceeds to step S22.

[0108] As previously described, the multi-gas sensor controller 9 is configured to prevent the updating of the first and second ammonia concentration correction factors when the downstream NOx concentration value exceeds the injection assessment concentration level (NO in step S25). Therefore, the multi-gas sensor controller 9 can further improve the determination accuracy of the downstream ammonia concentration by avoiding the correction of the determined downstream ammonia concentration value based on the NOx sensor detection result, even in the fuel-free state, in situations where the exhaust gas contains a large amount of NOx.

[0109] In the second embodiment described above, the processing of step S25 corresponds to the claimed prevention section. (Third embodiment)

[0110] A third embodiment of the present invention will now be described with reference to the drawings. The following description focuses on the differences between the third embodiment and the first embodiment.

[0111] A urea SCR system 1 of the third embodiment is similar to that of the first embodiment, except for the processing of the concentration determination process.

[0112] The concentration determination process of the third embodiment is similar to that of the first embodiment, except that in the concentration determination process the processing of step S3 is additionally carried out.

[0113] As in Fig. As shown in Figure 10, after completion of step S2, the controller assesses in step S3 whether the motor's rotational speed is greater than or equal to a predetermined starting assessment speed level. If the motor's rotational speed is less than the starting assessment speed level (NO in step S3), the controller proceeds to step S8. If the motor's rotational speed is greater than or equal to the starting assessment speed level (YES in step S3), the controller proceeds to step S4.

[0114] Furthermore, after completing step S8, the control returns to step S3.

[0115] As previously described, the multi-gas sensor controller 9 is configured to calculate the first and second ammonia concentration correction factors when the exhaust gas flow rate in the exhaust pipe 52 is greater than or equal to a predetermined level. The multi-gas sensor controller 9 is therefore able to prevent the deterioration in the ammonia concentration detection accuracy of the upstream NOx sensor 6, which is caused by the combustion of ammonia gas due to heat within the upstream NOx sensor 6 when the exhaust gas flow rate is low. (fourth embodiment)

[0116] A fourth embodiment of the present invention will now be described with reference to the drawings. The following description focuses on the differences between the fourth embodiment and the first embodiment.

[0117] As in Fig. As shown in Figure 11, a cleaning system 301 of the fourth embodiment differs from the system of the first embodiment in that a NOx storage reduction catalyst 302 (hereinafter referred to as "NSR catalyst 302") is provided instead of the oxidation catalyst 2 and the injector 5 for urea solution.

[0118] The NSR catalyst 302 is located in the exhaust pipe 52, positioned between the diesel engine 51 and the DPF unit 3. When the exhaust gas in the exhaust pipe 52 is low in fuel with respect to the stoichiometric air-fuel ratio, the NOx in the exhaust gas is stored in the NSR catalyst 302. When the exhaust gas in the exhaust pipe 52 is high in fuel with respect to the stoichiometric air-fuel ratio, nitrogen is produced in the NSR catalyst 302 by reducing the stored NOx with HC, CO, etc. in the exhaust gas and is then released by the NSR catalyst 302.

[0119] When the exhaust gas is in the fuel-rich state, ammonia is also produced at the NSR catalyst 302, for example by reduction of O with CO and H2O, as shown in the following reaction equations, (1) and (2). CO + H2O → H2 + CO2 (1) 2NO + 3H2 + 2CO → 2NH3 + 2CO2 (2)

[0120] The cleaning system 301 of the fourth embodiment also differs from the system of the first embodiment with regard to the processing of the correction factor calculation process.

[0121] The correction factor calculation process of the fourth embodiment is similar to that of the first embodiment, except that the processing of steps S30, S40 and S140 is omitted and the processing of step S35 is additionally performed in the correction factor calculation process.

[0122] As in Fig.As shown in Figure 12, after completion of step S22 in step S35, the control unit sends a corrective ammonia generation command to the cleaning control unit 10. The corrective ammonia generation command is sent by the cleaning control unit 10 to the engine ECU 53. In accordance with this command, the engine ECU 53 executes a fuel-rich peak control. During the fuel-rich peak control, the diesel engine 5 operates in a state where the air-fuel ratio is temporarily set to fuel-rich. Therefore, during the execution of the fuel-rich peak control, the diesel engine 51 emits fuel-rich gas, which contains a large amount of unburned components and is low in oxygen. During this flow of fuel-rich exhaust gas, ammonia gas is generated at the NSR catalyst 302, as described above.

[0123] The correction coefficient calculation process ends after step 132 is completed.

[0124] As described above, the multi-gas sensor controller 9 in the cleaning system 301 is equipped with the SCR catalyst 4, the NSR catalyst 302, the ammonia sensor unit 202, and the NOx sensor unit 201. The multi-gas sensor controller 9 determines the downstream ammonia concentration based on the detection result of the ammonia sensor unit 202.

[0125] Furthermore, the multi-gas sensor control 9 controls the NSR catalyst 302 to supply ammonia to the SCR catalyst 4 in the fuel-free state in which the fuel supply to the diesel engine 51 is stopped (steps S10 and S35).

[0126] The multi-gas sensor control 9 then corrects the determined downstream ammonia concentration value by updating the first and second ammonia concentration correction factors based on the detection result of the NOx sensor unit 201 and the oxygen concentration of the exhaust gas after the addition of urea to the SCR catalyst 4 with the control of the NSR catalyst 302 (steps S50 to S120).

[0127] In this way, the multiple gas sensor control 9 is configured to allow the supply of ammonia to the NSR catalyst 302 in the fuel-free state. Therefore, the multiple gas sensor control 9 is able to ensure the stability of the determined downstream ammonia concentration value in the fourth embodiment just as it does in the first embodiment.

[0128] In the fourth embodiment described above, the NSR catalyst 302 corresponds to the claimed ammonia supply unit; the processing steps S10 and S35 correspond to the claimed ammonia supply control section or step; and the processing steps S50 to S120 correspond to the claimed ammonia supply correction section or step.

[0129] Although the present invention has been described with reference to the specific embodiments described above, the present invention is not limited to these specific embodiments. Various modifications and variations of the above embodiments are possible within the technical scope of the present invention.

[0130] In the above embodiments, the NOx sensor control 7, the multi-sensor control 9, and the cleaning control 10 are provided separately. Alternatively, however, it is also possible to use an integrated control unit instead of the separate controls 7, 9, and 10, which is equipped with the functions of these controls 7, 9, and 10.

[0131] Although the first and second ammonia concentration correction factors in the above embodiments are calculated based on two downstream NOx output values, it is alternatively possible to calculate the first and second ammonia concentration correction factors based on three or more downstream NOx output values.

[0132] In the embodiments described above, the first and second ammonia concentration correction factors are calculated by assessing that the engine is in the fuel-free state and that the upstream NOx concentration is less than or equal to the injection assessment concentration level. Alternatively, however, it is also possible to calculate the first and second ammonia concentration correction factors after a predetermined time has elapsed following the engine being placed in the fuel-free state. Description of the reference symbols 1 Urea SCR system 4 SCR catalyst 5 Injectors for urea solution 6 Upstream NOx sensor 7 NOx sensor control 8 Multi-gas sensor 9 Multi-gas sensor control 10 Cleaning control 201 NOx sensor unit 202 Ammonia sensor unit 301 Cleaning System 302 NSR Catalyst

Claims

[1] Concentration determination device of a cleaning system (301), wherein the cleaning system (301) comprises: an SCR catalyst (4) which is arranged in an exhaust pipe of an internal combustion engine to clean NOx in the exhaust gas of the internal combustion engine; a reducing agent supply unit (5; 302), which is provided and is arranged to supply urea or ammonia as a reducing agent to the SCR catalyst (4); an ammonia sensor (202) which is provided and arranged to detect an ammonia concentration of the exhaust gas flowing out of the SCR catalyst (4) as a downstream ammonia concentration; and a NOx sensor (201) which is provided and arranged to detect a NOx concentration of the exhaust gas flowing out of the SCR catalyst (4) as a downstream NOx concentration; wherein the concentration determination device is configured to determine a value of the downstream ammonia concentration based on a detection result of the ammonia sensor (202), characterized by , that the concentration determination device includes: a feed control section (S2, S30; S10, S35) which controls the reducing agent feed unit (5; 302) to supply the reducing agent to the SCR catalyst in a fuel-free state in which the supply of fuel to the combustion engine is stopped; a feed correction section (S50 to S120) which corrects the determined value of the downstream ammonia concentration based on a detection result of the NOx sensor (201) and an oxygen concentration of the exhaust gas after the feed of the reducing agent to the SCR catalyst (4) with the control of the reducing agent feed unit (5; 302) by the Feed correction section (S2, S30; S10, S35), wherein the concentration determination device converts the detection result of the ammonia sensor (202) into an ammonia concentration value according to an ammonia concentration correction formula which represents a relationship between first and second converted ammonia concentrations, wherein the first converted ammonia concentration is a value calculated according to a relationship formula between the detection result of the ammonia sensor (202) and the ammonia concentration of the exhaust gas, and the second converted ammonia concentration is a value calculated from the NOx sensor (201) detection result and the oxygen concentration of the exhaust gas, and wherein the concentration determination device determines the ammonia concentration value as the downstream ammonia concentration; and wherein the feed correction section (S2, S30; S10, S35) corrects the determined value of the downstream ammonia concentration by updating the ammonia concentration correction formula, wherein the ammonia concentration correction formula in a two-dimensional coordinate system, in which the ammonia concentration converted from the electromotive force is plotted along the x-axis and the ammonia concentration converted from the NOx output is plotted along the y-axis, represents a straight line in the two-dimensional coordinate system among several most recently stored, connects a maximum coordinate point and a minimum coordinate point with the output values ​​obtained from the gas sensor (201, 202). [2] Concentration determination device according to claim 1, further comprising: a prevention section (S20) which, if the NOx concentration of the exhaust gas exceeds a predetermined prevention assessment concentration level, prevents the feed correction section from updating the ammonia concentration correction formula. [3] Concentration determination device according to claim 1 or 2, wherein the NOx sensor (201) is a limiting current gas sensor. [4] Concentration determination device according to one of claims 1 to 3, wherein the ammonia sensor (202) and the NOx sensor (201) are combined into one sensor as an integrated gas sensor. [5] Concentration determination method for a cleaning system, wherein the cleaning system comprises: an SCR catalyst (4) which is arranged in an exhaust pipe of an internal combustion engine to clean NOx in the exhaust gas of the internal combustion engine; a reducing agent supply unit (5; 302), which is provided and is arranged to supply urea or ammonia as a reducing agent to the SCR catalyst; an ammonia sensor (202) which is provided and arranged to detect an ammonia concentration of the exhaust gas flowing out of the SCR catalyst (4) as a downstream ammonia concentration; and a NOx sensor (201) which is provided and arranged to detect a NOx concentration of the exhaust gas flowing out of the SCR catalyst (4) as a downstream NOx concentration; wherein the concentration determination method is designed to determine a value of the downstream ammonia concentration based on a detection result of the ammonia sensor (202), characterized by , that the concentration determination procedure includes: Control of the reducing agent supply unit (5; 302) to supply the reducing agent to the SCR catalyst (4) in a fuel-free state in which the supply of fuel to the internal combustion engine is stopped; and Correcting the determined value of the downstream ammonia concentration based on a detection result of the NOx sensor (201) and an oxygen concentration of the exhaust gas after the supply of the reducing agent to the SCR catalyst (4) by controlling the reducing agent supply unit (5; 302), wherein the downstream ammonia concentration is determined by converting the detection result of the ammonia sensor (202) into an ammonia concentration value according to a ammonia concentration correction formula is converted, which represents a relationship between the first and second converted ammonia concentrations, wherein the first converted ammonia concentration is a value calculated according to a relationship formula between the detection result of the ammonia sensor (202) and the ammonia concentration of the exhaust gas, and the second converted ammonia concentration is a value calculated from the NOx sensor (201) detection result and the oxygen concentration of the exhaust gas; and wherein the value of the downstream ammonia concentration determined in this way is corrected by updating the ammonia concentration correction formula, wherein the ammonia concentration correction formula in a two-dimensional coordinate system, in which the ammonia concentration converted from the electromotive force is plotted along the x-axis and the ammonia concentration converted from the NOx output is plotted along the y-axis, represents a straight line which connects a maximum coordinate point and a minimum coordinate point in the two-dimensional coordinate system among several last stored output values ​​obtained from the gas sensor (201, 202).

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