JP6725320B2 - Concentration calculating device and concentration calculating method - Google Patents

Description

The present invention relates to a concentration calculation device and a concentration calculation method for calculating the concentration of ammonia discharged from a selective reduction catalyst.

Conventionally, in order to purify NOx contained in exhaust gas discharged from an internal combustion engine, an SCR (Selective Catalytic Reduction) catalyst is installed in an exhaust pipe of the internal combustion engine, and urea is injected as a reducing agent to the SCR catalyst. Existed system. In such a system, the output value of the ammonia sensor is corrected by comparing the output value of the NOx sensor that detects NOx flowing out from the SCR catalyst with the output value of the ammonia sensor that detects ammonia flowing out from the SCR catalyst. There is a known technique (for example, see Patent Document 1).

JP, 2014-224504, A

However, in the technique disclosed in Patent Document 1, the output value of the NOx sensor is divided by the output value of the ammonia sensor, or the difference value between the output value of the NOx sensor and the output value of the ammonia sensor is used for the ammonia sensor. Since the output value is corrected, there is a problem that the detection accuracy of the ammonia concentration after correction is low.

The present invention has been made in view of these problems, and an object thereof is to improve the detection accuracy of the corrected ammonia concentration.

A first invention made to achieve the above object is to calculate a downstream ammonia concentration based on a detection result of an ammonia sensor in a purification system including a selective reduction catalyst, a urea supply device, an ammonia sensor, and a NOx sensor. It is a concentration calculation device that does. The selective reduction catalyst is installed in the exhaust pipe of the internal combustion engine in order to purify NOx contained in the exhaust gas discharged from the internal combustion engine. The urea supply device supplies urea as a reducing agent to the selective reduction catalyst. The ammonia sensor detects the downstream ammonia concentration, which is the concentration of ammonia contained in the exhaust gas discharged from the selective reduction catalyst. The NOx sensor detects the downstream NOx concentration, which is the concentration of NOx contained in the exhaust gas discharged from the selective reduction catalyst.

The concentration calculation device of the first aspect of the invention includes a urea supply control means and a urea supply correction means. The urea supply control means controls the urea supply device so as to supply urea to the selective reduction catalyst in the fuel cut state where the fuel supply to the internal combustion engine is stopped. The urea supply correction means, based on the detection result of the NOx sensor after the urea is supplied to the selective reduction catalyst by the control of the urea supply device by the urea supply control means, and the concentration of oxygen contained in the exhaust gas, the downstream ammonia. Correct the density calculation result.

The concentration calculating device of the first aspect of the invention configured as described above causes the urea supplying device to supply urea in the fuel cut state. In the fuel cut state, the fuel supply to the internal combustion engine is stopped, so the exhaust gas discharged from the internal combustion engine does not contain NOx. The NOx sensor reacts not only with NOx but also with ammonia to detect the concentration. That is, in the fuel cut state, the detection result of the NOx sensor after urea is supplied to the selective reduction catalyst to the extent that ammonia leaks to the downstream shows a value having a correlation with the concentration of ammonia. Therefore, the concentration calculating device according to the first aspect of the invention corrects the calculated value of the ammonia concentration based on the detection result of the NOx sensor even when the detection result of the ammonia sensor is not stable due to a change over time or a daily change. As a result, the stability of the calculation result of the downstream ammonia concentration can be ensured.

Since the detection result of the downstream ammonia concentration by the NOx sensor changes according to the concentration of oxygen contained in the exhaust gas, the concentration calculating device according to the first aspect of the invention is not limited to the detection result of the NOx sensor that detects the downstream NOx concentration. Instead, the calculation result of the downstream ammonia concentration is corrected based on the concentration of oxygen contained in the exhaust gas. Thereby, the concentration calculating device of the first aspect of the present invention can improve the accuracy of the calculation result of the downstream ammonia concentration.

Further, the concentration calculating device according to the first aspect of the present invention includes the first converted ammonia concentration calculated using the relational expression showing the correspondence between the detection result of the ammonia sensor and the ammonia concentration, the detection result of the NOx sensor, and the exhaust gas. The downstream ammonia concentration is calculated by converting the detection result of the ammonia sensor into the ammonia concentration using an ammonia concentration correction formula that shows the correspondence relationship between the oxygen concentration and the second converted ammonia concentration. It may be done. In this case, the urea supply correction means of the concentration calculation device of the first aspect of the invention may correct the calculation result of the downstream ammonia concentration by updating the ammonia concentration correction formula.

Further, the concentration calculating device according to the first aspect of the invention includes a prohibiting unit that prohibits the urea supply correcting unit from updating the ammonia concentration correction formula when the concentration of NOx contained in the exhaust gas exceeds a preset prohibition determination concentration. It may be provided.

As a result, the concentration calculating device of the first aspect of the present invention corrects the calculation result of the downstream ammonia concentration based on the detection result of the NOx sensor in the situation where the exhaust gas contains a large amount of NOx even in the fuel cut state. This can be avoided, and the accuracy of the calculation result of the downstream ammonia concentration can be further improved.

A second invention made to achieve the above object is to calculate a downstream ammonia concentration based on a detection result of an ammonia sensor in a purification system including a selective reduction catalyst, an ammonia supply unit, an ammonia sensor, and a NOx sensor. It is a concentration calculation device that does. The ammonia supply unit supplies ammonia as a reducing agent to the selective reduction catalyst.

Then, the concentration calculating device of the second aspect of the invention includes an ammonia supply control means and an ammonia supply correction means. The ammonia supply control means controls the ammonia supply unit so as to supply ammonia to the selective reduction catalyst when the fuel cut state where the fuel supply to the internal combustion engine is stopped. The ammonia supply correction means, based on the detection result of the NOx sensor after the ammonia is supplied to the selective reduction catalyst by the control of the ammonia supply part by the ammonia supply control means and the concentration of oxygen contained in the exhaust gas, the downstream ammonia. Correct the density calculation result.

The concentration calculating device of the second invention corrects the calculation result of the downstream ammonia concentration by supplying ammonia instead of urea, and can obtain the same effect as the concentration calculating device of the first invention.

Further, in the concentration calculating device of the first invention and the second invention, the NOx sensor may be a limiting current type gas sensor. The limiting current type NOx sensor pumps in or pumps out oxygen contained in the exhaust gas introduced into the NOx sensor, and keeps the concentration of oxygen contained in the exhaust gas introduced into the NOx sensor constant. That is, the limiting current type NOx sensor can detect the concentration of oxygen contained in the exhaust gas based on the amount of oxygen pumped in or the amount of oxygen pumped out.

As a result, the concentration calculating device according to the first invention and the second invention does not need to separately install an oxygen sensor for detecting the concentration of oxygen contained in the exhaust gas in the purification system, and simplifies the configuration of the purification system. You can

Further, in the concentration calculating device of the first invention and the second invention, the ammonia sensor and the NOx sensor may be configured as an integrated gas sensor. As a result, the ammonia sensor and the NOx sensor can detect the downstream ammonia concentration in the substantially same region of the exhaust gas. That is, it is possible to prevent the ammonia sensor and the NOx sensor from having different exhaust gas regions for concentration detection. Thereby, the concentration calculating device of the first invention and the second invention can further improve the accuracy of the calculation result of the downstream ammonia concentration.

A third invention made to achieve the above object is to calculate a downstream ammonia concentration based on a detection result of an ammonia sensor in a purification system including a selective reduction catalyst, a urea supply device, an ammonia sensor, and a NOx sensor. This is a method for calculating the concentration.

The concentration calculation method of the third aspect of the invention includes a urea supply control procedure and a urea supply correction procedure. The urea supply control procedure controls the urea supply device so as to supply urea to the selective reduction catalyst when the fuel cut state is that the fuel supply to the internal combustion engine is stopped. The urea supply correction procedure is performed based on the detection result of the NOx sensor after the urea is supplied to the selective reduction catalyst by the control of the urea supply apparatus by the urea supply control procedure and the concentration of oxygen contained in the exhaust gas. Correct the density calculation result.

The concentration calculating method of the third invention is a method executed by the concentration calculating device of the first invention, and by executing the method, the same effect as that of the concentration calculating device of the first invention can be obtained.
A fourth invention made to achieve the above object is to calculate a downstream ammonia concentration based on a detection result of an ammonia sensor in a purification system including a selective reduction catalyst, an ammonia supply unit, an ammonia sensor, and a NOx sensor. This is a method for calculating the concentration.

The concentration calculating method of the fourth aspect of the invention includes an ammonia supply control procedure and an ammonia supply correction procedure. The ammonia supply control procedure controls the ammonia supply unit so as to supply ammonia to the selective reduction catalyst in the fuel cut state where the fuel supply to the internal combustion engine is stopped. The ammonia supply correction procedure is based on the detection result of the NOx sensor after the ammonia is supplied to the selective reduction catalyst by the control of the ammonia supply unit by the ammonia supply control procedure and the concentration of oxygen contained in the exhaust gas. Correct the density calculation result.

The concentration calculating method of the fourth invention is a method executed by the concentration calculating device of the second invention, and by executing the method, the same effect as that of the concentration calculating device of the second invention can be obtained.

It is a figure which shows schematic structure of the urea SCR system 1. It is a figure which shows schematic structure of the upstream NOx sensor 6 and the NOx sensor control apparatus 7. It is a figure which shows schematic structure of the multi gas sensor 8 and the multi gas sensor control apparatus 9. It is a development view showing a schematic configuration of an ammonia detection unit 202. It is a flow chart which shows density calculation processing of a 1st embodiment. It is a flow chart which shows correction coefficient calculation processing of a 1st embodiment. It is a graph which shows a change of a downstream NOx output value and a downstream ammonia output value in the situation where an ammonia correction coefficient is updated. 6 is a graph showing changes in the downstream NOx output value and the downstream ammonia output value in a situation where the ammonia correction coefficient is not updated. It is a flow chart which shows correction coefficient calculation processing of a 2nd embodiment. It is a flowchart which shows the density calculation process of 3rd Embodiment. It is a figure which shows schematic structure of the purification system 301. It is a flowchart which shows the density calculation process of 4th Embodiment.

(First embodiment)
Hereinafter, a first embodiment of the present invention will be described with reference to the drawings.
As shown in FIG. 1, a urea SCR (Selective Catalytic Reduction) system 1 of an embodiment to which the present invention is applied includes an oxidation catalyst 2, a DPF (Diesel Particulate Filter) 3, an SCR catalyst 4, a urea water injector 5, an upstream NOx. The sensor 6, the NOx sensor control device 7, the multi-gas sensor 8, the multi-gas sensor control device 9, and the purification control device 10 are provided.

The oxidation catalyst 2 takes in exhaust gas discharged from the diesel engine 51 via the exhaust pipe 52 of the diesel engine 51, oxidizes NOx nitrogen oxide (NO) in the taken-in exhaust gas, and oxidizes nitrogen dioxide (NO ₂ ) Is generated.

The DPF 3 takes in the exhaust gas discharged from the oxidation catalyst 2 via the exhaust pipe 52 and removes the particulate matter in the taken-in exhaust gas.
The SCR catalyst 4 hydrolyzes urea supplied from the upstream side into ammonia, takes in exhaust gas discharged from the DPF 3 through the exhaust pipe 52, and reduces NOx in the taken-in exhaust gas with this ammonia. And converts NOx into nitrogen gas and water vapor. As a result, the SCR catalyst 4 discharges the exhaust gas in which NOx has been reduced.

The urea water injector 5 is installed between the DPF 3 and the SCR catalyst 4 in the exhaust pipe 52 and injects urea water into the exhaust gas. The injected urea water is hydrolyzed at high temperature, and ammonia gas is generated thereby. This ammonia gas is used as a reducing agent for NOx reduction.

The upstream NOx sensor 6 is installed in the exhaust pipe 52 between the DPF 3 and the SCR catalyst 4, and detects the NOx concentration in the exhaust gas discharged from the DPF 3.
The NOx sensor control device 7 controls the upstream NOx sensor 6 and calculates the NOx concentration (hereinafter, also referred to as upstream NOx concentration) in the exhaust gas discharged from the DPF 3 based on the detection result of the upstream NOx sensor 6. .. The NOx sensor control device 7 is configured to be able to send and receive data to and from the purification control device 10 via a communication line, and sends upstream NOx concentration information indicating the upstream NOx concentration to the purification control device 10. To do.

The multi-gas sensor 8 is installed downstream of the SCR catalyst 4 in the exhaust pipe 52 and detects the NOx concentration and the downstream ammonia concentration in the exhaust gas discharged from the SCR catalyst 4.
The multi-gas sensor control device 9 controls the multi-gas sensor 8, and based on the detection result of the multi-gas sensor 8, the NOx concentration (hereinafter also referred to as the downstream NOx concentration) in the exhaust gas discharged from the SCR catalyst 4, and the oxygen concentration. A concentration (hereinafter, also referred to as a downstream oxygen concentration) and an ammonia concentration (hereinafter, also referred to as a downstream ammonia concentration) are calculated. The multi-gas sensor control device 9 is configured to be able to transmit and receive data to and from the purification control device 10 via a communication line, and indicates downstream NOx concentration information indicating downstream NOx concentration and downstream ammonia concentration. The downstream ammonia concentration information is transmitted to the purification control device 10.

The purification control device 10 mainly includes a microcomputer including a CPU 21, a ROM 22, a RAM 23, a signal input/output unit 24, and the like. The urea water injector 5, the NOx sensor control device 7, and the multi-gas sensor control device 9 are connected to the signal input/output unit 24.

The purification control device 10 is configured to be able to send and receive data to and from the NOx sensor control device 7 and the multi-gas sensor control device 9 via a communication line. Furthermore, the purification control device 10 is configured to be able to transmit and receive data to and from the electronic control device 53 that controls the diesel engine 51 via a communication line. Hereinafter, the electronic control unit 53 will be referred to as an engine ECU (Electronic Control Unit) 53.

As shown in FIG. 2, the upstream NOx sensor 6 includes 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 sequentially stacked. Is configured. The insulating layers 113, 115, 117, 119 and 120 are formed mainly of alumina. The solid electrolyte layers 114, 116, 118 are mainly formed of zirconia having oxygen ion conductivity.

The upstream NOx sensor 6 includes a first measurement chamber 121 formed between the solid electrolyte layer 114 and the solid electrolyte layer 116. The NOx detection unit 101 is arranged from the outside to the inside of the first measurement chamber 121 via the diffusion resistor 122 arranged between the solid electrolyte layer 114 and the solid electrolyte layer 116 so as to be adjacent to the first measurement chamber 121. Introduce exhaust gas into. The upstream NOx sensor 6 is arranged between the solid electrolyte layer 114 and the solid electrolyte layer 116 so as to be adjacent to the first measurement chamber 121 on the side opposite to the diffusion resistor 122 across the first measurement chamber 121. Exhaust gas is discharged to the outside of the first measurement chamber 121 via the diffusion resistor 123. The diffusion resistors 122 and 123 are made of a porous material such as alumina.

The upstream NOx sensor 6 includes a first pumping cell 130. The first pumping cell 130 includes a solid electrolyte layer 114 and pumping electrodes 131 and 132. The pumping electrodes 131 and 132 are formed mainly of platinum. The pumping electrode 131 is arranged on the surface of the solid electrolyte layer 114 that contacts the first measurement chamber 121. The surface of the pumping electrode 131 on the first measurement chamber 121 side is covered with a protective layer 133 made of a porous body. The pumping electrode 132 is arranged on the surface of the solid electrolyte layer 114 on the side opposite to the pumping electrode 131 with the solid electrolyte layer 114 interposed therebetween. The insulating layer 113 in the region where the pumping electrode 132 is arranged and the region around it is removed, and the porous body 134 is filled in place of the insulating layer 113. The porous body 134 allows gas (oxygen) to flow in and out between the pumping electrode 132 and the outside.

The upstream NOx sensor 6 includes an oxygen concentration detection cell 140. The oxygen concentration detection cell 140 includes the solid electrolyte layer 116, a detection electrode 141, and a reference electrode 142. The detection electrode 141 and the reference electrode 142 are mainly formed of platinum. The detection electrode 141 is located on the downstream side of the pumping electrode 131 in the first measurement chamber 121 (that is, on the side closer to the diffusion resistor 123 than the diffusion resistor 122), and the first measurement in the solid electrolyte layer 116 is performed. It is placed on the surface that contacts the chamber 121. The reference electrode 142 is arranged on the surface of the solid electrolyte layer 116 on the side opposite to the detection electrode 141 with the solid electrolyte layer 116 interposed therebetween.

The upstream NOx sensor 6 includes a reference oxygen chamber 146. The reference oxygen chamber 146 is formed between the solid electrolyte layer 116 and the solid electrolyte layer 118 so as to be in contact with the reference electrode 142. The inside of the reference oxygen chamber 146 is filled with a porous body.

The upstream NOx sensor 6 includes a second measurement chamber 148. The second measurement chamber 148 is formed between the solid electrolyte layer 114 and the solid electrolyte layer 118, penetrating the insulating layer 115, the solid electrolyte layer 116, and the insulating layer 117. The upstream NOx sensor 6 introduces the exhaust gas discharged from the first measurement chamber 121 via the diffusion resistor 123 into the second measurement chamber 148.

The upstream NOx sensor 6 includes a second pumping cell 150. The second pumping cell 150 includes a solid electrolyte layer 118 and pumping electrodes 151 and 152. The pumping electrodes 151 and 152 are formed mainly of platinum. The pumping electrode 151 is arranged on the surface of the solid electrolyte layer 118 that contacts the second measurement chamber 148. The pumping electrode 152 is arranged on the surface of the solid electrolyte layer 118 on the side opposite to the reference electrode 142 with the reference oxygen chamber 146 interposed therebetween.

The upstream NOx sensor 6 includes a heater 160. The heater 160 is formed mainly of platinum, is a heating resistor that generates heat when energized, and is disposed between the insulating layer 119 and the insulating layer 120.

The NOx sensor control device 7 includes a control circuit 180 and a microcomputer 190 (hereinafter, referred to as a microcomputer 190).
The control circuit 180 is an analog circuit arranged on the circuit board. The control circuit 180 includes an Ip1 drive circuit 181, a Vs detection circuit 182, a reference voltage comparison circuit 183, an Icp supply circuit 184, a Vp2 application circuit 185, an Ip2 detection circuit 186, and a heater drive circuit 187.

Then, the pumping electrode 131, the detection electrode 141, and the pumping electrode 151 are connected to the reference potential. The pumping electrode 132 is connected to the Ip1 drive circuit 181. The reference electrode 142 is connected to the Vs detection circuit 182 and the Icp supply circuit 184. The pumping electrode 152 is connected to the Vp2 applying circuit 185 and the Ip2 detecting circuit 186. The heater 160 is connected to the heater driving circuit 187.

The Ip1 drive circuit 181 supplies the first pumping current Ip1 between the pumping electrodes 131 and 132 and detects the supplied first pumping current Ip1.

The Vs detection circuit 182 detects the voltage Vs between the detection electrode 141 and the reference electrode 142, and outputs the detection result to the reference voltage comparison circuit 183.
The reference voltage comparison circuit 183 compares the reference voltage (for example, 425 mV) with the output (voltage Vs) of the Vs detection circuit 182, and outputs the comparison result to the Ip1 drive circuit 181. Then, the Ip1 drive circuit 181 controls the direction in which the first pumping current Ip1 flows and the magnitude of the first pumping current Ip1 so that the voltage Vs becomes equal to the reference voltage, and also determines the oxygen concentration in the first measurement chamber 121. , NOx is adjusted to a predetermined value such that NOx is not decomposed.

The Icp supply circuit 184 causes a weak current Icp to flow between the detection electrode 141 and the reference electrode 142. As a result, oxygen is sent from the first measurement chamber 121 to the reference oxygen chamber 146 via the solid electrolyte layer 116, so that the reference oxygen chamber 146 is set to a predetermined reference oxygen concentration.

The Vp2 applying circuit 185 applies a constant voltage Vp2 (for example, 450 mV) between the pumping electrode 151 and the pumping electrode 152. As a result, in the second measurement chamber 148, NOx is dissociated (reduced) by the catalytic action of the pumping electrodes 151 and 152 that form the second pumping cell 150. Oxygen ions obtained by this dissociation move in the solid electrolyte layer 118 between the pumping electrode 151 and the pumping electrode 152, so that the second pumping current Ip2 flows. The Ip2 detection circuit 186 detects the second pumping current Ip2.

The heater driving circuit 187 drives the heater 160 by applying a positive voltage for energizing the heater to one end of the heater 160 that is a heating resistor and applying a negative voltage for energizing the heater to the other end of the heater 160. ..

The microcomputer 190 includes a CPU 191, a ROM 192, a RAM 193, and a signal input/output unit 194.
The CPU 191 executes a process for controlling the upstream NOx sensor 6 based on the program stored in the ROM 192. The signal input/output unit 194 is connected to the Ip1 drive circuit 181, the Vs detection circuit 182, the Ip2 detection circuit 186, and the heater drive circuit 187.

The CPU 191 adjusts the oxygen concentration of the first measurement chamber 121 by the pumping operation of the first pumping cell 130, sets the oxygen concentration of the second measurement chamber 148 to a concentration for NOx detection capable of NOx detection, and then performs the second pumping. The NOx concentration is calculated based on the current value of the current Ip2. Further, the CPU 191 controls the heater 160 by outputting a drive signal to the heater drive circuit 187 via the signal input/output unit 194.

As shown in FIG. 3, the multi-gas sensor 8 includes a NOx detection unit 201 and an ammonia detection unit 202.
The NOx detection unit 201 is the same as the upstream NOx sensor 6 except that the solid electrolyte layer 112 is laminated on the insulating layer 113 of the upstream NOx sensor 6, and the insulating layer 111 is further laminated on the solid electrolyte layer 112. The insulating layer 111 is formed mainly of alumina. The solid electrolyte layer 112 is formed mainly of zirconia having oxygen ion conductivity.

The ammonia detection unit 202 includes a detection electrode 211, a reference electrode 212, a selective reaction layer 213, and a diffusion layer 214.
The detection electrode 211 and the reference electrode 212 are arranged on the solid electrolyte layer 112 so as to be separated from each other, as shown in FIG. The detection electrode 211 is formed of a material whose main component is gold. The reference electrode 212 is formed of a material whose main component is platinum. Since the detection electrode 211 has a higher reactivity with ammonia than the reference electrode 212, an electromotive force is generated between the detection electrode 211 and the reference electrode 212.

The selective reaction layer 213 is formed with a metal oxide as a main component, and is arranged so as to cover the detection electrode 211 and the reference electrode 212. The selective reaction layer 213 has a function of burning a combustible gas component other than ammonia. That is, the ammonia detection unit 202 can detect ammonia in the exhaust gas without being affected by the combustible gas component by the selective reaction layer 213.

The diffusion layer 214 is made of a porous material and is arranged so as to cover the selective reaction layer 213. The diffusion layer 214 is configured to be able to adjust the diffusion rate of the exhaust gas that flows into the ammonia detection unit 202 from the outside.

As shown in FIG. 3, the multi-gas sensor control device 9 includes a control circuit 220 and a microcomputer 230.
The control circuit 220 differs from the control circuit 180 of the NOx sensor control device 7 in that an electromotive force detection circuit 221 is added. The electromotive force detection circuit 221 detects an electromotive force (hereinafter referred to as an ammonia electromotive force EMF) between the detection electrode 211 and the reference electrode 212, and outputs a signal indicating the detection result to the microcomputer 230.

The microcomputer 230 includes a CPU 231, a ROM 232, a RAM 233, an EEPROM 234, and a signal input/output unit 235.
The CPU 231 executes processing for controlling the multi-gas sensor 8 based on the program stored in the ROM 232. The signal input/output unit 235 is connected to the Ip1 drive circuit 181, the Vs detection circuit 182, the Ip2 detection circuit 186, the heater drive circuit 187, and the electromotive force detection circuit 221.

The CPU 231 calculates the NOx concentration similarly to the CPU 191. Further, the CPU 231 calculates the oxygen concentration based on the energizing direction and the current value of the first pumping current Ip1.
Further, the CPU 231 causes the ammonia concentration obtained from the relational expression between the ammonia electromotive force EMF and the oxygen concentration (hereinafter referred to as electromotive force converted ammonia concentration), and the ammonia concentration obtained from the downstream NOx output value and the downstream oxygen concentration (hereinafter referred to as NOx). The ammonia concentration is calculated by converting the ammonia electromotive force EMF into the ammonia concentration using an ammonia concentration correction formula showing a correspondence relationship with the output value conversion ammonia concentration). The ammonia concentration correction formula is a linear formula in which the offset value and the slope are used as coefficients, and the electromotive force conversion ammonia concentration obtained from the ammonia electromotive force EMF and oxygen concentration is used as a variable. The offset value and the slope of the ammonia concentration correction formula are stored in the EEPROM 234 as the first ammonia correction coefficient and the second ammonia correction coefficient, respectively.

Further, the CPU 231 controls the heater 160 by outputting a drive signal to the heater drive circuit 187 via the signal input/output unit 235.
Further, the microcomputer 230 of the multi-gas sensor control device 9 executes concentration calculation processing.

Here, the procedure of the density calculation process will be described. This concentration calculation process is started immediately after the microcomputer 230 of the multi-gas sensor control device 9 is activated.
When this concentration calculation processing is executed, the CPU 231 of the microcomputer 230 first sets various parameters provided for calculating the ammonia concentration to preset initial values in S2, as shown in FIG. .. In the process of S2, the first ammonia correction coefficient and the second ammonia correction coefficient described above are set to initial values, and the initial flag described below is cleared.

Then, in S4, it is determined whether or not the fuel cut state in which the fuel supply to the diesel engine 51 is stopped is performed. Specifically, when the fuel cut signal received from the purification control device 10 is in the on state, it is determined that it is in the fuel cut state, and when the fuel cut signal is in the off state, it is determined that it is not in the fuel cut state. .. The fuel cut signal indicates whether or not the fuel cut state is set. The voltage of the fuel cut signal becomes high level in the fuel cut state. In this case, the fuel cut signal is said to be on. The voltage of the fuel cut signal becomes low level when the fuel cut state is not set. In this case, the fuel cut signal is in the off state. The fuel cut signal is transmitted from the engine ECU 53 to the purification control device 10, and is further transmitted from the purification control device 10 to the multi-gas sensor control device 9.

If the fuel cut state is not set (S4: NO), the process proceeds to S8. On the other hand, if it is in the fuel cut state (S4: YES), a correction coefficient calculation process described later is executed in S6, and the process proceeds to S8.

Then, in S8, as described above, the electromotive force conversion ammonia concentration is calculated from the relational expression between the ammonia electromotive force EMF and the oxygen concentration. Further, in S8, the ammonia concentration correction formula is used to convert the electromotive force converted ammonia concentration into the NOx output value converted ammonia concentration, and this NOx output value converted ammonia concentration is calculated as the ammonia concentration. When the process of S8 ends, the process proceeds to S4.

Next, the procedure of the correction coefficient calculation process executed in S6 will be described.
When this correction coefficient calculation process is executed, the CPU 231 of the microcomputer 230 first determines in S12 whether or not the initial flag is set, as shown in FIG. The initial flag is set to 0 as an initial value. That is, the initial state of the initial flag is a cleared state.

Here, if the initial flag is set (S12: YES), the process proceeds to S50. On the other hand, when the initial flag is not set (S12: NO), it is determined in S20 whether the upstream NOx concentration indicated by the upstream NOx concentration information is equal to or lower than the preset injection determination concentration. The injection determination concentration is set to a value near 0 ppm. The multi-gas sensor control device 9 acquires upstream NOx concentration information through communication with the purification control device 10. Here, when the upstream NOx concentration exceeds the injection determination concentration (S20: NO), the correction coefficient calculation process ends. On the other hand, when the upstream NOx concentration is equal to or lower than the injection determination concentration (S20: YES), the initial flag is set in S22. Then, in S30, the correction urea injection command is transmitted to the purification control device 10. As a result, the purification control device 10 causes the urea water injector 5 to inject the urea water of the correction injection amount set in advance for correction. Further, in S40, the urea injection stop command is transmitted to the purification control device 10. Accordingly, the purification control device 10 temporarily stops the control of the urea water injection of the urea water injector 5.

Then, in S50, it is determined whether data can be stored in the RAM 233. Specifically, it is determined whether or not there is a free space in the RAM 233 that is equal to or larger than the storage determination data amount preset as the data amount required for the data storage executed in S60. If data cannot be stored in the RAM 233 (S50: NO), the process proceeds to S80.

On the other hand, when the data can be stored in the RAM 233 (S50: YES), the latest downstream NOx output value, downstream ammonia output value, and downstream oxygen output value acquired from the multi-gas sensor 8 are stored in the RAM 233 in S60. .. The downstream NOx output value is the value of the second pumping current Ip2. The downstream ammonia output value is the value of the ammonia electromotive force EMF. The downstream oxygen output value is the value of the first pumping current Ip1.

Then, in S70, similarly to S4, it is determined whether or not the fuel cut state is set. Here, when it is not in the fuel cut state (S70: NO ), the correction coefficient calculation process ends. On the other hand, if a fuel cut state (S70: YES), the process proceeds to S80.

Then, in S80, it is determined whether or not the NOx output difference is equal to or greater than a preset corrected NOx determination value. The NOx output difference is the difference between the maximum value and the minimum value of the plurality of downstream NOx output values stored in the RAM 233. Here, when the NOx output difference is less than the corrected NOx determination value (S80: NO), the process proceeds to S130.

On the other hand, when the NOx output difference is equal to or greater than the corrected NOx determination value (S80: YES), the ammonia corresponding to the maximum value and the minimum value of the plurality of downstream NOx output values stored in the RAM 233 is determined in S90. Calculate the concentration. Specifically, the ammonia concentration corresponding to the maximum value and the minimum value in the downstream NOx output value is calculated by referring to the ammonia concentration calculation map in which the correspondence between the downstream NOx output value and the ammonia concentration is preset. To do. The ammonia concentration calculation map is stored in the ROM 232. Hereinafter, the ammonia concentration corresponding to the maximum downstream NOx output value is referred to as the maximum ammonia concentration. Further, the ammonia concentration corresponding to the minimum downstream NOx output value is referred to as the minimum ammonia concentration.

Next, in S100, the maximum ammonia concentration and the minimum ammonia concentration are corrected according to the downstream oxygen concentration. Specifically, first, by referring to the adjustment coefficient setting map in which the correspondence relationship between the downstream oxygen output value and the oxygen concentration adjustment coefficient is set in advance, the downstream oxygen output value corresponding to the maximum downstream NOx output value An oxygen concentration adjustment coefficient (hereinafter referred to as maximum adjustment coefficient) is extracted from the adjustment coefficient setting map. The adjustment coefficient setting map is stored in the ROM 232. Similarly, by referring to the adjustment coefficient setting map, the oxygen concentration adjustment coefficient (hereinafter referred to as the minimum adjustment coefficient) at the downstream oxygen output value corresponding to the minimum downstream NOx output value is extracted from the adjustment coefficient setting map. .. Then, a multiplication value obtained by multiplying the maximum ammonia concentration by the maximum adjustment coefficient is calculated as the corrected maximum ammonia concentration. Similarly, a multiplication value obtained by multiplying the minimum ammonia concentration by the minimum adjustment coefficient is calculated as the corrected minimum ammonia concentration.

Further, in S105, the electromotive force conversion ammonia concentration is calculated from the downstream ammonia output value (that is, the ammonia electromotive force EMF) and the downstream oxygen output value corresponding to the maximum value and the minimum value of the downstream NOx output value.

Then, in S110, the above-described first ammonia correction coefficient and second ammonia correction coefficient are calculated using the corrected maximum ammonia concentration and the corrected minimum ammonia concentration. Specifically, in a two-dimensional orthogonal coordinate system in which the electromotive force-converted ammonia concentration is on the X-axis and the NOx output value-converted ammonia concentration is on the Y-axis, the coordinate point corresponds to the corrected maximum ammonia concentration (hereinafter referred to as the maximum coordinate point). And an offset value and a slope of a linear expression showing a straight line connecting the coordinate point corresponding to the corrected minimum ammonia concentration (hereinafter, referred to as the minimum coordinate point) are set as the first ammonia correction coefficient and the second ammonia correction coefficient, respectively. The maximum coordinate point is located at (x1, y1) in the two-dimensional Cartesian coordinate system when the electromotive force conversion ammonia concentration corresponding to the corrected maximum ammonia concentration is expressed as x1 and the corrected maximum ammonia concentration is expressed as y1. It is a point. The minimum coordinate point is located at (x2, y2) in the two-dimensional orthogonal coordinate system when the electromotive force conversion ammonia concentration corresponding to the corrected minimum ammonia concentration is expressed as x2 and the corrected minimum ammonia concentration is expressed as y2. It is a point.

Then, in S120, the first ammonia correction coefficient and the second ammonia correction coefficient calculated in S110 are overwritten in the EEPROM 234 to update the first ammonia correction coefficient and the second ammonia correction coefficient, and the process proceeds to S130.

Then, in S130, all the downstream NOx output values, the downstream ammonia output values, and the downstream oxygen output values stored in the RAM 233 are erased. In S132, the initial flag is cleared. Then, in S140, the urea injection restart command is transmitted to the purification control device 10, and the process proceeds to S10. As a result, the purification control device 10 restarts the control of the urea water injection by the urea water injector 5.

Next, specific examples of changes in the downstream NOx output value and the downstream ammonia output value when urea water injection is performed in the fuel cut state are shown in FIGS. 7(a) and (b) and FIGS. Shown in.
7(a), 7(b) and 8(a), 8(b), when the fuel cut signal is in the ON state (see arrow FC1) and the upstream NOx concentration is equal to or lower than the injection determination concentration, urea The situation is shown in which urea water is injected by the water injector 5 and then the fuel cut signal is turned off (see arrow FC2).

FIG. 7A shows a situation in which the maximum value of the downstream NOx output value L1 after the urea water injection is high, and the maximum value of the downstream NOx output value L1 and the maximum value of the downstream ammonia output value L2 substantially match. Showing. In this case, since the NOx output difference DN is equal to or larger than the corrected NOx determination value, the first and second ammonia correction coefficients are updated.

FIG. 7B shows a situation where the maximum value of the downstream NOx output value L1 after the urea water injection is high and the maximum value of the downstream ammonia output value L2 is lower than the maximum value of the downstream NOx output value L1. .. In this case, since the NOx output difference DN is equal to or larger than the corrected NOx determination value, the first and second ammonia correction coefficients are updated.

FIG. 8A shows a situation in which the maximum value of the downstream NOx output value L1 after the urea water injection is low, and the maximum value of the downstream NOx output value L1 and the maximum value of the downstream ammonia output value L2 substantially match. Showing. In this case, since the NOx output difference DN is less than the corrected NOx determination value, the first and second ammonia correction coefficients are not updated.

FIG. 8B shows a situation in which the maximum value of the downstream NOx output value L1 after the urea water injection is low and the maximum value of the downstream ammonia output value L2 is higher than the maximum value of the downstream NOx output value L1. .. In this case, since the NOx output difference DN is less than the corrected NOx determination value, the first and second ammonia correction coefficients are not updated.

In the urea SCR system 1 including the SCR catalyst 4, the urea water injector 5, the ammonia detection unit 202, and the NOx detection unit 201, the multi-gas sensor control device 9 configured in this way detects the detection result of the ammonia detection unit 202. The downstream ammonia concentration is calculated based on

Then, the multi-gas sensor control device 9 controls the urea water injector 5 so as to supply urea to the SCR catalyst 4 in the fuel cut state where the fuel supply to the diesel engine 51 is stopped (S2, S30). ..

Further, the multi-gas sensor control device 9 detects the detection result of the NOx detection unit 201 after urea is supplied to the SCR catalyst 4 by the control of the urea water injector 5 by the multi-gas sensor control device 9 and the concentration of oxygen contained in the exhaust gas. Based on, the first and second ammonia correction coefficients are updated to correct the calculation result of the downstream ammonia concentration (S50 to S120).

In this way, the multi-gas sensor control device 9 causes the urea water injector 5 to supply urea in the fuel cut state. In the fuel cut state, since the fuel supply to the diesel engine 51 is stopped, NOx is not included in the exhaust gas discharged from the diesel engine 51. Then, the NOx detection unit 201 detects not only NOx but also ammonia to detect the concentration. That is, the detection result of the NOx detection unit 201 after the urea is supplied to the SCR catalyst 4 in the fuel cut state shows a value having a correlation with the concentration of ammonia. Therefore, even if the detection result of the ammonia detection unit 202 is not stable due to a change over time or a daily change, the multi-gas sensor control device 9 corrects it based on the detection result of the NOx detection unit 201, and It is possible to ensure the stability of the calculation result of the ammonia concentration.

Since the detection result of the downstream ammonia concentration by the NOx detection unit 201 changes according to the concentration of oxygen contained in the exhaust gas, the multi-gas sensor control device 9 detects the detection result of the NOx detection unit 201 that detects the downstream NOx concentration. Not only that, the calculation result of the downstream ammonia concentration is corrected based on the concentration of oxygen contained in the exhaust gas. Thereby, the multi-gas sensor control device 9 can improve the accuracy of the calculation result of the downstream ammonia concentration.

Further, when the upstream NOx concentration exceeds the injection determination concentration (S20: NO), the multi-gas sensor control device 9 prohibits the updating of the first and second ammonia correction coefficients to correct the calculation result of the downstream ammonia concentration. Is prohibited (S20). As a result, the multi-gas sensor control device 9 avoids correcting the calculation result of the downstream ammonia concentration based on the detection result of the NOx sensor in a situation where the exhaust gas contains a large amount of NOx even in the fuel cut state. Therefore, the accuracy of the calculation result of the downstream ammonia concentration can be further improved.

Further, the NOx detection unit 201 is a limiting current type gas sensor. The NOx detection unit 201 pumps in or pumps out oxygen contained in the exhaust gas introduced into the first measurement chamber 121 by the first pumping cell 130, and oxygen contained in the exhaust gas introduced into the first measurement chamber 121. The concentration of is kept constant. That is, the NOx detection unit 201 can detect the concentration of oxygen contained in the exhaust gas based on the energization direction and the current value of the first pumping current Ip1. Accordingly, the multi-gas sensor control device 9 does not need to separately install an oxygen sensor that detects the concentration of oxygen contained in the exhaust gas in the urea SCR system 1, and can simplify the configuration of the urea SCR system 1. ..

Further, the ammonia detecting unit 202 and the NOx detecting unit 201 are configured as an integrated multi-gas sensor 8. As a result, in the multi-gas sensor control device 9, the ammonia detector 202 and the NOx detector 201 can detect the downstream ammonia concentration in the substantially same region of the exhaust gas. That is, it is possible to prevent the ammonia detection unit 202 and the NOx detection unit 201 from having different exhaust gas regions for concentration detection. Thereby, the multi-gas sensor control device 9 can further improve the accuracy of the calculation result of the downstream ammonia concentration.

In the embodiment described above, the multi-gas sensor control device 9 is the concentration calculation device of the present invention, the diesel engine 51 is the internal combustion engine of the present invention, the SCR catalyst 4 is the selective reduction catalyst of the present invention, and the urea water injector 5 is the urea of the present invention. The supply device, the ammonia detection unit 202 is the ammonia sensor of the present invention, the NOx detection unit 201 is the NOx sensor of the present invention, and the urea SCR system 1 is the purification system of the present invention.

Further, the processing of S2 and S30 is the urea supply control means and the urea supply control procedure of the present invention, the processing of S50 to S120 is the urea supply correction means and the urea supply correction procedure of the present invention, and the processing of S20 is the prohibition means of the present invention. is there.

The electromotive force conversion ammonia concentration is the first conversion ammonia concentration in the present invention, and the NOx output value conversion ammonia concentration is the second conversion ammonia concentration in the present invention.
(Second embodiment)
A second embodiment of the present invention will be described below with reference to the drawings. In the second embodiment, parts different from the first embodiment will be described.

The urea SCR system 1 of the second embodiment is the same as that of the first embodiment except that the correction coefficient calculation process is changed.
The correction coefficient calculation process of the second embodiment is the same as that of the first embodiment except that the process of S25 is executed instead of the process of S20.

That is, as shown in FIG. 9, when the initial flag is not set (S12: NO), whether the latest downstream NOx concentration calculated by the multi-gas sensor control device 9 is equal to or lower than the injection determination concentration in S25. To judge. Here, when the downstream NOx concentration exceeds the injection determination concentration (S25: NO), the correction coefficient calculation process ends. On the other hand, when the downstream NOx concentration is equal to or lower than the injection determination concentration (S25:YES), the process proceeds to S22.

When the downstream NOx concentration exceeds the injection determination concentration (S25: NO), the multi-gas sensor control device 9 configured in this way prohibits the first and second ammonia correction coefficients from being updated, thereby reducing the downstream ammonia concentration. The correction of the calculation result of is prohibited (S25). Thereby, the multi-gas sensor control device 9 avoids correcting the calculation result of the downstream ammonia concentration based on the detection result of the NOx sensor in a situation where the exhaust gas contains a large amount of NOx even in the fuel cut state. Therefore, the accuracy of the calculation result of the downstream ammonia concentration can be further improved.

In the embodiment described above, the processing of S25 is the prohibition means in the present invention.
(Third Embodiment)
The third embodiment of the present invention will be described below with reference to the drawings. In the third embodiment, parts different from the first embodiment will be described.

The urea SCR system 1 of the third embodiment is the same as that of the first embodiment except that the concentration calculation process is changed.
The density calculation process of the third embodiment is the same as the first embodiment except that the process of S3 is added.

That is, as shown in FIG. 10, when the process of S2 ends, it is determined in S3 whether or not the engine speed is equal to or higher than a preset start determination speed. If the engine speed is less than the start determination speed (S3: NO), the process proceeds to S8. On the other hand, when the engine speed is equal to or higher than the start determination speed (S5: YES), the process proceeds to S4.

When the process of S8 ends, the process proceeds to S3.
The multi-gas sensor control device 9 configured as described above can calculate the first and second ammonia correction coefficients when the flow velocity of the exhaust gas in the exhaust pipe 52 is equal to or higher than a predetermined value. As a result, the multi-gas sensor control device 9 burns the ammonia gas introduced into the upstream NOx sensor 6 with the heat of the upstream NOx sensor 6 when the flow velocity of the exhaust gas is small, and the ammonia concentration of the upstream NOx sensor 6 is reduced. It is possible to prevent the detection accuracy from decreasing.

(Fourth Embodiment)
A fourth embodiment of the present invention will be described below with reference to the drawings.
As shown in FIG. 11, the purification system 301 of the fourth embodiment is that the oxidation catalyst 2 and the urea water injector 5 are omitted, and that the NOx storage reduction catalyst 302 (hereinafter, NSR catalyst 302) is added. Different from the first embodiment.

The NSR catalyst 302 is installed in the exhaust pipe 52 between the diesel engine 51 and the DPF 3. The NSR catalyst 302 stores NOx in the exhaust gas when the exhaust gas in the exhaust pipe 52 is in a lean state in which the fuel is leaner than the stoichiometric air-fuel ratio. Then, when the exhaust gas in the exhaust pipe 52 is in a rich state in which the fuel is in excess of the stoichiometric air-fuel ratio, the NOx stored in the NSR catalyst 302 is reduced by HC, CO, etc. in the exhaust gas to generate nitrogen. And nitrogen is discharged from the NSR catalyst 302.

When the exhaust gas is in a rich state, ammonia is also generated in the NSR catalyst 302. For example, as shown in the following reaction formulas (1) and (2), ammonia is generated by reducing NO with CO and H ₂ O.

CO+H ₂ O → H ₂ +CO ₂ (1)
2NO+3H ₂ +2CO → 2NH ₃ +2CO ₂ (2)
Further, the purification system 301 of the fourth embodiment is different from the first embodiment in that the correction coefficient calculation process is changed.

The correction coefficient calculation process of the fourth embodiment is the same as that of the first embodiment except that the processes of S30, S40, and S140 are omitted and the process of S35 is added.
That is, as shown in FIG. 12, when the process of S22 ends, in S35, a correction ammonia generation command is transmitted to the purification control device 10, and the process proceeds to S50. Then, the purification control device 10 is configured to send a correction ammonia generation command to the engine ECU 53. As a result, the engine ECU 53 executes rich spike control. The rich spike control is a control for operating the diesel engine 51 with the air-fuel ratio temporarily made rich. By executing the rich spike control, the rich gas containing a large amount of unburned components and lacking oxygen is discharged from the diesel engine 51. As a result, ammonia is produced in the NSR catalyst 302.

When the process of S132 ends, the correction coefficient calculation process ends.
The multi-gas sensor control device 9 configured as above is based on the detection result of the ammonia detection unit 202 in the purification system 301 including the SCR catalyst 4, the NSR catalyst 302, the ammonia detection unit 202, and the NOx detection unit 201. Then, the downstream ammonia concentration is calculated.

Then, the multi-gas sensor control device 9 controls the NSR catalyst 302 so as to supply ammonia to the SCR catalyst 4 in the fuel cut state where the fuel supply to the diesel engine 51 is stopped (S10, S35).

Further, the multi-gas sensor control device 9 determines the detection result of the NOx detection unit 201 after the ammonia is supplied to the SCR catalyst 4 by the control of the NSR catalyst 302 by the multi-gas sensor control device 9 and the concentration of oxygen contained in the exhaust gas. Based on this, the calculation result of the downstream ammonia concentration is corrected by updating the first and second ammonia correction coefficients (S50 to S120).

In this way, the multi-gas sensor control device 9 causes the NSR catalyst 302 to supply ammonia in the fuel cut state. Therefore, the multi-gas sensor control device 9 can secure the stability of the calculation result of the downstream ammonia concentration, as in the first embodiment.

In the embodiment described above, the NSR catalyst 302 is the ammonia supply unit in the present invention, the processing of S10 and S35 is the ammonia supply control means and the ammonia supply control procedure in the present invention, and the processing of S50 to S120 is the ammonia supply correction means in the present invention. And the ammonia supply correction procedure.

Although one embodiment of the present invention has been described above, the present invention is not limited to the above embodiment, and various forms can be adopted as long as they are within the technical scope of the present invention.
For example, in the above embodiment, the NOx sensor control device 7, the multi-gas sensor control device 9, and the purification control device 10 are shown. However, instead of the control devices 7, 9 and 10, an integrated control device having the functions of the control devices 7, 9 and 10 may be provided.

Further, in the above embodiment, the first and second ammonia correction coefficients are calculated using two downstream NOx output values, but three or more downstream NOx output values may be used.
In the above embodiment, the first and second ammonia correction coefficients are calculated when the fuel cut state is set and the upstream NOx concentration is equal to or lower than the injection determination concentration. However, the first and second ammonia correction coefficients may be calculated after a certain time has elapsed since the fuel cut state was entered.

DESCRIPTION OF SYMBOLS 1... Urea SCR system 1, 4... SCR catalyst, 5... Urea water injector, 6... Upstream NOx sensor, 7... NOx sensor control device, 8... Multi gas sensor, 9... Multi gas sensor control device, 10... Purification control device, 201 ...NOx detection unit, 202...Ammonia detection unit, 301...Purification system, 302...NSR catalyst

Claims (5)

A selective reduction catalyst installed in an exhaust pipe of the internal combustion engine to purify NOx contained in exhaust gas discharged from the internal combustion engine; and a urea supply device for supplying urea to the selective reduction catalyst as a reducing agent. An ammonia sensor that detects a downstream ammonia concentration that is a concentration of ammonia contained in the exhaust gas discharged from the selective reduction catalyst, and a downstream NOx concentration that is a concentration of NOx contained in the exhaust gas discharged from the selective reduction catalyst. In a purification system including a NOx sensor for detecting, a concentration calculation device for calculating the downstream ammonia concentration based on a detection result of the ammonia sensor,
In a fuel cut state where fuel supply to the internal combustion engine is stopped, urea supply control means for controlling the urea supply device to supply urea to the selective reduction catalyst,
The detection result of the NOx sensor after urea is supplied to the selective reduction catalyst by the control of the urea supply device by the urea supply control unit, and the concentration of oxygen contained in the exhaust gas discharged from the selective reduction catalyst. And a urea supply correction means for correcting the calculation result of the downstream ammonia concentration ,
The concentration calculating device calculates a first converted ammonia concentration using a relational expression indicating a correspondence relationship between a detection result of the ammonia sensor and a concentration of oxygen contained in the exhaust gas, and a detection result of the NOx sensor. By converting the detection result of the ammonia sensor into an ammonia concentration by using an ammonia concentration correction formula showing the correspondence relationship between the concentration of oxygen contained in the exhaust gas and the second converted ammonia concentration, the downstream Configured to calculate the ammonia concentration,
The concentration calculation device, wherein the urea supply correction means corrects the calculation result of the downstream ammonia concentration by updating the ammonia concentration correction formula . Claims wherein when the concentration of NOx contained in the exhaust gas exceeds a preset prohibition determination concentration, characterized in that it comprises a prohibiting means for prohibiting the updating by the ammonia concentration correction equation the urea feed correction means Item 2. The concentration calculation device according to item 1 . The NOx sensor, the concentration calculating apparatus according to claim 1 or claim 2, characterized in that a limiting current type gas sensor. The concentration calculation device according to any one of claims 1 to 3 , wherein the ammonia sensor and the NOx sensor are configured as an integrated gas sensor. A selective reduction catalyst installed in an exhaust pipe of the internal combustion engine to purify NOx contained in exhaust gas discharged from the internal combustion engine; and a urea supply device for supplying urea to the selective reduction catalyst as a reducing agent. An ammonia sensor that detects a downstream ammonia concentration that is a concentration of ammonia contained in the exhaust gas discharged from the selective reduction catalyst, and a downstream NOx concentration that is a concentration of NOx contained in the exhaust gas discharged from the selective reduction catalyst. In a purification system including a NOx sensor for detecting, a concentration calculation method for calculating the downstream ammonia concentration based on a detection result of the ammonia sensor,
In a fuel cut state in which fuel supply to the internal combustion engine is stopped, a urea supply control procedure for controlling the urea supply device to supply urea to the selective reduction catalyst,
The detection result of the NOx sensor after urea is supplied to the selective reduction catalyst by the control of the urea supply device according to the urea supply control procedure, and the concentration of oxygen contained in the exhaust gas discharged from the selective reduction catalyst. based on the bets, and a urea supply correction procedure for correcting the calculation result of the downstream ammonia concentration,
The concentration calculating method includes a first converted ammonia concentration calculated using a relational expression indicating a correspondence relationship between a detection result of the ammonia sensor and a concentration of oxygen contained in the exhaust gas, and a detection result of the NOx sensor. By converting the detection result of the ammonia sensor into an ammonia concentration by using an ammonia concentration correction formula showing the correspondence relationship between the concentration of oxygen contained in the exhaust gas and the second converted ammonia concentration, the downstream Calculate the ammonia concentration,
The concentration calculation method, wherein the urea supply correction procedure corrects the calculation result of the downstream ammonia concentration by updating the ammonia concentration correction formula .