Control device and urea solution injection method

Abstract

The present invention provides a control device that can reduce the amount of nitrogen oxides passing through an exhaust gas purification system. [Solution] The exhaust pipe 14 discharges the exhaust gas emitted by the hydrogen engine 11 to the outside of the vehicle. The exhaust gas purification device 12 reduces nitrogen oxides contained in the exhaust gas. The nitrogen oxide sensor 15 acquires a measured value that reflects the concentration of nitrogen oxides contained in the exhaust gas and outputs an output value which is the measured value minus a value that increases with the amount of water contained in the exhaust gas. The injector 13 injects urea water into the exhaust gas purification device 12. If the output value is above a predetermined value, the control device 10 injects an amount of urea water determined based on the output value and the exhaust gas flow rate into the injector 13. If the output value is below a predetermined value, the control device 10 injects an amount of urea water determined based on the predetermined value and the exhaust gas flow rate into the injector 13.

Description

Technical Field

[0001] The present invention relates to a control device and a method for injecting aqueous urea solution.

Background Art

[0002] Patent Document 1 describes an exhaust system. The exhaust system includes an exhaust purification device, an injector, and a nitrogen oxide (NOx) sensor. The exhaust purification device includes a catalyst on which ammonia is adsorbed. The exhaust purification device uses ammonia to reduce NOx in the exhaust discharged from the engine. The injector adds urea as a reducing agent to the exhaust purification device. The nitrogen oxide sensor measures the concentration of NOx in the exhaust flowing upstream of the exhaust purification device.

[0003] The exhaust system determines the amount of aqueous urea solution that the injector injects toward the exhaust purification device based on the measurement value of the nitrogen oxide sensor. Then, the exhaust system causes the injector to inject the determined amount of aqueous urea solution.

[0004] The nitrogen oxide sensor also reacts with moisture in the exhaust in addition to NOx in the exhaust. Therefore, the measurement value of the nitrogen oxide sensor may be larger than the value reflecting the concentration of NOx contained in the exhaust.

[0005] The exhaust system corrects the measurement value of the nitrogen oxide sensor according to the amount of moisture in the exhaust, and then determines the amount of aqueous urea solution that the injector injects.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] One possible method for the exhaust system to correct the measured value is to subtract an arbitrary value from the actual measured value obtained by the nitrogen oxide sensor, and if the measured value becomes zero or less, treat the measured value as zero. In this case, the exhaust system should subtract a larger value from the measured value as the amount of moisture in the exhaust increases.

[0008] When an engine uses hydrogen gas as fuel, the amount of moisture in the exhaust increases. Therefore, when an engine uses hydrogen gas as fuel, the value that the exhaust system subtracts from the measured value also increases.

[0009] The degree to which a nitrogen oxide sensor reacts to moisture in the exhaust gas depends not only on the amount of moisture in the exhaust gas, but also on the exhaust gas flow rate and the temperature of the nitrogen oxide sensor. Therefore, in environments where the exhaust gas flow rate and the temperature of the nitrogen oxide sensor are such that the nitrogen oxide sensor is less likely to react to moisture, but the amount of moisture in the exhaust gas is high and the value subtracted from the measurement is large, the corrected measurement value may become zero even though NOx is present in the exhaust gas. In this case, the exhaust system does not inject urea solution into the injector, so NOx passes through the exhaust gas purification device. [Means for solving the problem]

[0010] The control device for solving the above problems controls an injector in the exhaust system by acquiring the output value of a nitrogen oxide sensor. The exhaust system includes an exhaust pipe that discharges exhaust gas emitted by a hydrogen engine, which uses hydrogen as fuel, to the outside of the vehicle. The exhaust system includes an exhaust purification device installed in the middle of the exhaust pipe that reduces nitrogen oxides contained in the exhaust gas. The exhaust system includes a nitrogen oxide sensor located upstream of the exhaust purification device in the exhaust pipe that acquires a measured value reflecting the concentration of nitrogen oxides contained in the exhaust gas and outputs an output value which is the measured value minus a value that increases with the amount of water contained in the exhaust gas. The exhaust system includes an injector located downstream of the nitrogen oxide sensor and upstream of the exhaust purification device in the exhaust pipe that injects urea water into the exhaust purification device. When the output value is greater than or equal to a predetermined value, the control device injects an amount of urea water determined based on the output value and the exhaust gas flow rate into the injector. If the output value falls below the default value, this control device injects an amount of urea solution determined based on the default value and the flow rate into the injector.

[0011] A method for injecting urea solution to solve the above problems is a method for injecting urea solution in an exhaust system. The exhaust system includes an exhaust pipe that discharges exhaust gas emitted by a hydrogen engine, which uses hydrogen as fuel, to the outside of the vehicle. The exhaust system includes an exhaust purification device installed in the middle of the exhaust pipe that reduces nitrogen oxides contained in the exhaust gas. The exhaust system includes a nitrogen oxide sensor disposed upstream of the exhaust purification device in the exhaust pipe that acquires a measured value reflecting the concentration of nitrogen oxides contained in the exhaust gas and outputs an output value which is the measured value minus a value that increases with the amount of water contained in the exhaust gas. The exhaust system includes an injector disposed downstream of the nitrogen oxide sensor and upstream of the exhaust purification device in the exhaust pipe that injects urea solution into the exhaust purification device. The exhaust system includes a control device that acquires the output value of the nitrogen oxide sensor and controls the injector. This urea solution injection method includes the steps of the control device determining the amount of urea solution to be injected by the injector based on the output value and the exhaust flow rate when the output value is equal to or greater than a predetermined value, and determining the amount of urea solution to be injected by the injector based on the predetermined value and the flow rate when the output value is less than a predetermined value. This urea solution injection method includes the step of the injector injecting the amount of urea solution determined by the control device into the exhaust gas purification device. [Effects of the Invention]

[0012] The above-described control device and urea solution injection method can reduce the amount of nitrogen oxides passing through the exhaust gas purification device. [Brief explanation of the drawing]

[0013] [Figure 1] Figure 1 is a schematic diagram showing the configuration of an exhaust system equipped with a control device according to one embodiment. [Figure 2] Figure 2 is a schematic diagram showing the arrangement of cells in a nitrogen oxide sensor. [Figure 3] Figure 3 is a schematic diagram showing the internal structure of the nitrogen oxide sensor. [Figure 4]Figure 4 is a flowchart showing the series of processes performed by the control device in Figure 1. [Figure 5] Figure 5 is a table showing how the control device in Figure 1 determines the amount of urea solution injected by the injector. [Figure 6] Figure 6 is a graph showing how the control device in Figure 1 corrects the output value of the nitrogen oxide sensor, where (a) shows the change between the output value of the nitrogen oxide sensor and the default value, (b) shows the change in the amount of urea added, and (c) shows the change in the nitrogen oxide concentration after purification. [Modes for carrying out the invention]

[0014] An embodiment of the control device will be described below with reference to Figures 1 to 6. <Configuration of exhaust system 100> As shown in Figure 1, the exhaust system 100 is applied to a vehicle equipped with a hydrogen engine 11. The hydrogen engine 11 uses hydrogen as fuel.

[0015] As shown in Figure 1, the exhaust system 100 includes an exhaust pipe 14. The exhaust from the hydrogen engine 11 is discharged outside the vehicle through the exhaust pipe 14. As shown in Figure 1, the exhaust system 100 is equipped with an exhaust gas purification device 12. The exhaust gas purification device 12 is installed in the middle of the exhaust pipe 14. The exhaust gas purification device 12 is a selective catalytic reduction (SCR) that reduces nitrogen oxides (NOx) in the exhaust gas using ammonia (NH3) as a reducing agent. NH3 produced by the hydrolysis of urea is adsorbed onto the exhaust gas purification device 12.

[0016] As shown in Figure 1, the exhaust system 100 is equipped with a nitrogen oxide sensor 15. The nitrogen oxide sensor 15 is located upstream of the exhaust gas purification device 12 in the exhaust pipe 14. The nitrogen oxide sensor 15 measures the concentration of NOx contained in the exhaust gas.

[0017] As shown in FIG. 1, the exhaust system 100 includes an injector 13. The injector 13 is disposed downstream of the nitrogen oxide sensor 15 and upstream of the exhaust purification device 12 in the exhaust pipe 14. The injector 13 adds urea to the exhaust purification device 12 by injecting urea water toward the exhaust purification device 12.

[0018] As shown in FIG. 1, the exhaust system 100 includes a urea water tank 16. The urea water tank 16 is connected to the injector 13. The urea water tank 16 stores the urea water injected by the injector 13.

[0019] As shown in FIG. 1, the exhaust system 100 includes a control device 10. The control device 10 controls the injection of urea water by the injector 13. <Configuration and Function of Nitrogen Oxide Sensor 15> Hereinafter, the configuration and function of the nitrogen oxide sensor 15 will be described with reference to FIGS. 2 and 3.

[0020] The nitrogen oxide sensor 15 includes a plurality of cells for measuring the concentration of NOx contained in the exhaust. FIG. 2 shows the arrangement of the cells in the nitrogen oxide sensor 15. As shown in FIG. 2, the nitrogen oxide sensor 15 includes a pump cell 17, a sensor cell 18, and a monitor cell 19. The solid arrows in FIG. 2 indicate the direction in which the exhaust flows into the nitrogen oxide sensor 15.

[0021] FIG. 3 shows the internal structure of the nitrogen oxide sensor 15 when viewed from the position A in FIG. 2 in the direction of the dotted arrow in FIG. 2. As shown in FIG. 3, the nitrogen oxide sensor 15 is separated into a first gas chamber 23 and a second gas chamber 24. In FIG. 3, the solid arrow indicates the direction in which the exhaust flows into the nitrogen oxide sensor 15. As shown in FIG. 3, the exhaust flowing into the nitrogen oxide sensor 15 passes through the first gas chamber 23.

[0022] As shown in Figure 3, the nitrogen oxide sensor 15 is equipped with a heater 22. As will be described later, each cell in the nitrogen oxide sensor 15 reacts with components in the exhaust gas on its electrodes. The heater 22 is provided to raise the temperature of the nitrogen oxide sensor 15 when its temperature is low, thereby facilitating the reaction on the electrodes.

[0023] As shown in Figures 2 and 3, the exhaust gas flowing into the nitrogen oxide sensor 15 first flows into the pump cell 17 among several cells. As shown in Figure 3, in the pump cell 17, two electrodes, the pump electrode 25 and the reference electrode 21, are installed so as to sandwich the solid electrolyte 20. The solid electrolyte 20 is made of a material that allows oxygen ions to pass through.

[0024] The pump electrode 25 is composed of a material that is highly reactive with oxygen and low reactive with NOx. For example, the pump electrode 25 is mainly composed of platinum and gold. When a voltage is applied to the pump electrode 25, the oxygen in the exhaust gas is reduced to oxygen ions on the pump electrode 25. The dotted arrows in Figure 3 indicate the direction in which the oxygen ions move. The oxygen ions generated in the pump cell 17 pass through the solid electrolyte 20 and are released into the second gas chamber 24 from the reference electrode 21.

[0025] Thus, the pump cell 17 has the function of removing oxygen from the exhaust gas. Furthermore, the current flowing through the pump cell 17 reflects the oxygen concentration in the exhaust gas. Therefore, the nitrogen oxide sensor 15 can determine the air-fuel ratio based on the current value flowing through the pump cell 17.

[0026] As shown in Figure 2, the sensor cell 18 and the monitor cell 19 are installed side by side so that the exhaust gas that has passed through the pump cell 17 flows into them. In Figure 2, the sensor cell 18 is located in front of the monitor cell 19 when viewed from position A.

[0027] The sensor cell 18 and monitor cell 19, like the pump cell 17, are installed with two electrodes sandwiching the solid electrolyte 20. In Figure 3, the sensor cell 18 is shown as seen from position A in Figure 2, with the monitor cell 19 located behind the sensor cell 18.

[0028] In the monitor cell 19, two electrodes, a monitor electrode 27 and a reference electrode 21, are positioned so as to sandwich the solid electrolyte 20. The monitor electrode 27, like the pump electrode 25, is composed of a material that is highly reactive with oxygen and low reactive with NOx. For example, the monitor electrode 27 is mainly composed of platinum and gold.

[0029] Oxygen that could not be removed in the pump cell 17 flows into the sensor cell 18 and the monitor cell 19. When a voltage is applied to the monitor electrode 27, the oxygen in the exhaust is reduced to oxygen ions on the monitor electrode 27. The oxygen ions generated in the monitor cell 19 pass through the solid electrolyte 20 and are released into the second gas chamber 24 from the reference electrode 21.

[0030] The current flowing through the monitor cell 19 reflects the oxygen concentration in the exhaust gas that has passed through the monitor cell 19. Therefore, the nitrogen oxide sensor 15 can determine the oxygen concentration in the exhaust gas that has passed through the pump cell 17 based on the current value flowing through the monitor cell 19.

[0031] In the sensor cell 18, two electrodes, a sensor electrode 26 and a reference electrode 21, are positioned so as to sandwich the solid electrolyte 20. The sensor electrode 26 is made of a material that is highly reactive with NOx. For example, the monitor electrode 27 is mainly composed of platinum and rhodium.

[0032] When a voltage is applied to the sensor electrode 26, NOx in the exhaust gas is reduced on the sensor electrode 26, generating oxygen ions. The oxygen ions generated in the sensor cell 18 pass through the solid electrolyte 20 and are released into the second gas chamber 24 from the reference electrode 21.

[0033] The current flowing through the sensor cell 18 reflects the concentration of NOx in the exhaust gas. Therefore, the nitrogen oxide sensor 15 can determine the concentration of NOx in the exhaust gas based on the current value flowing through the sensor cell 18.

[0034] At the sensor electrode 26, in addition to NOx, oxygen that has passed through the pump cell 17 also reacts with the sensor electrode 26 to generate oxygen ions. Therefore, the nitrogen oxide sensor 15 can measure the concentration of NOx in the exhaust gas by calculating the difference between the current value of the sensor cell 18 and the current value of the monitor cell 19. In this way, the nitrogen oxide sensor 15 can obtain a measurement value that reflects the concentration of NOx contained in the exhaust gas.

[0035] <Output method of output values ​​from nitrogen oxide sensor 15> The sensor electrode 26 reacts not only with NOx and oxygen, but also with moisture in the exhaust. The moisture that reacts with the sensor electrode 26 generates oxygen ions. Therefore, the measured value in the nitrogen oxide sensor 15 becomes higher than the actual concentration of NOx contained in the exhaust.

[0036] The nitrogen oxide sensor 15 outputs an output value based on the acquired measurement values. The output value is a value obtained by correcting the measurement values ​​to take into account the moisture content in the exhaust gas. The amount of water in the exhaust can be estimated from the air-fuel ratio. The nitrogen oxide sensor 15 stores, for example, a map showing the relationship between the previously measured air-fuel ratio and the amount of water in the exhaust of the hydrogen engine 11.

[0037] The nitrogen oxide sensor 15 measures the air-fuel ratio based on the current value of the pump cell 17. Subsequently, the nitrogen oxide sensor 15 estimates the amount of moisture in the exhaust gas based on the acquired air-fuel ratio.

[0038] The nitrogen oxide sensor 15 estimates the amount of moisture in the exhaust gas, and then outputs an output value by correcting the measured value according to the estimated amount of moisture. Specifically, the nitrogen oxide sensor 15 outputs a value obtained by subtracting an arbitrary value from the measured value, and treats the output value as zero if it is zero or less. In this case, the nitrogen oxide sensor 15 increases the value subtracted from the measured value according to the estimated amount of moisture.

[0039] <Functions of the control device 10> As shown in Figure 1, the control device 10 is communicatively connected to the injector 13 and the nitrogen oxide sensor 15. The control device 10 acquires an output value from the nitrogen oxide sensor 15. Based on the acquired output value, the control device 10 determines the amount of urea solution to be injected by the injector 13, and then injects the determined amount of urea solution into the injector 13.

[0040] As mentioned earlier, the output value is calculated by subtracting a larger value from the measured value as the amount of moisture in the exhaust increases. Since the hydrogen engine 11 uses hydrogen as fuel, its exhaust contains a lot of moisture. Therefore, when the output value is output, the value that the nitrogen oxide sensor 15 subtracts from the measured value tends to be large.

[0041] On the other hand, the degree to which moisture reacts with the sensor electrode 26 in the sensor cell 18 is also affected by the temperature of the nitrogen oxide sensor 15 and the exhaust gas flow rate. The lower the temperature of the nitrogen oxide sensor 15, the less moisture reacts with the sensor electrode 26. The slower the exhaust gas flow rate, the less moisture reacts with the sensor electrode 26. Therefore, depending on the temperature of the nitrogen oxide sensor 15 and the exhaust gas flow rate, the value subtracted when outputting the power output may become excessive. Furthermore, in vehicles using the hydrogen engine 11, the value that the nitrogen oxide sensor 15 subtracts from the measured value tends to be large, so the value subtracted when outputting the power output tends to become excessive.

[0042] Furthermore, as mentioned earlier, when the temperature of the nitrogen oxide sensor 15 is low, the heater 22 operates to ensure that the electrode reacts sufficiently with the components in the exhaust. Since the exhaust from the hydrogen engine 11 contains a lot of moisture, the exhaust temperature tends to be low. As a result, the heater 22 operates more frequently, which makes it more susceptible to deterioration over time. When the heater 22 deteriorates over time, the reactivity between the electrode and the components in the exhaust decreases, which in turn reduces the reactivity between NOx in the sensor cell 18 and the sensor electrode 26.

[0043] Thus, in vehicles using the hydrogen engine 11, the output value measured by the nitrogen oxide sensor 15 tends to be low. Therefore, even though NOx is present in the exhaust, the output value after correction by subtracting the value from the measured value may be zero or less. The control device 10 takes these circumstances into consideration and determines the amount of urea solution injected by the injector 13 while correcting the output value according to the conditions in the vehicle.

[0044] <Processing methods by the control device 10> Figure 4 shows a series of processes performed by the control device 10. The control device 10 performs the series of processes shown in Figure 4 at regular intervals while the hydrogen engine 11 is operating.

[0045] In step S11, the control device 10 checks the status of the vehicle. At this time, the control device 10 checks whether the fuel cut is in effect. The control device 10 also checks whether the nitrogen oxide sensor 15 is active. The control device 10 checks the operating status of the hydrogen engine 11, including the intake air volume of the hydrogen engine 11. Having checked the status of the vehicle, the control device 10 proceeds to step S12.

[0046] In step S12, the control device 10 determines the amount of urea solution injected by the injector 13. For the exhaust gas purification device 12 to sufficiently reduce NOx in the exhaust gas, it is desirable that the more NOx contained in the exhaust gas, the more urea solution is injected and the more urea is added to the exhaust gas purification device 12. The control device 10 determines the amount of urea solution injected by the injector 13 based on the concentration of NOx in the exhaust gas and the flow rate of the exhaust gas.

[0047] The control device 10 estimates the exhaust flow rate from the intake air volume in the vehicle, for example. The control device 10 changes the type of value used as the NOx concentration in the exhaust according to the vehicle conditions. Figure 5 shows the process in which the control device 10 determines the amount of urea solution injected by the injector 13 during step S12.

[0048] When the nitrogen oxide sensor 15 is not in an active state, the measured and output values ​​do not accurately reflect the NOx concentration in the exhaust gas. As shown in Figure 5, when the nitrogen oxide sensor 15 is inactive, the control device 10 determines the amount of urea solution to be injected based on the estimated NOx concentration and the exhaust gas flow rate.

[0049] The NOx concentration in the exhaust can be estimated from the operating state of the hydrogen engine 11. The operating state of the hydrogen engine 11 refers to, for example, the magnitude of the torque output by the hydrogen engine 11 and the rotational speed of the hydrogen engine 11. The control device 10 outputs the estimated NOx concentration by referring to, for example, a map created in advance that shows the relationship between the operating state of the hydrogen engine 11 and the NOx concentration in the exhaust. In addition to the intake volume, the operating state of the hydrogen engine 11 also includes, for example, the rotational speed of the output shaft of the hydrogen engine 11, the valve timing of the intake and exhaust valves, the ignition timing, the opening degree of the EGR valve, and the exhaust temperature.

[0050] As shown in Figure 5, when the nitrogen oxide sensor 15 is active and fuel cut is not in progress, the control device 10 determines the amount of urea solution to be injected based on the output value obtained by correcting the output value from the nitrogen oxide sensor 15 so that the default value is the lower limit, and the exhaust flow rate.

[0051] As mentioned above, in vehicles using the hydrogen engine 11, there may be cases where the output value is zero or less, even though NOx is present in the exhaust. The control device 10 corrects the output value so that a default value greater than zero becomes the lower limit of the output value when the nitrogen oxide sensor 15 is active and fuel cut is not in progress.

[0052] Figure 6 shows the changes in the output value of the nitrogen oxide sensor 15, the amount of urea added to the exhaust gas purification device 12, and the concentration of NOx contained in the exhaust gas purified by the exhaust gas purification device 12. In Figure 6, values ​​less than zero ppm are output as output values, but the control device 10 treats output values ​​less than zero ppm as zero ppm.

[0053] In Figure 6(a), the trend of output values ​​that have not been corrected by the control device 10 is shown by a dashed line, and the trend of output values ​​used by the control device 10 to determine the amount of urea solution injected is shown by a solid line.

[0054] In Figure 6, the periods shown for R1 and R2 represent the times when the nitrogen oxide sensor 15 is active and the fuel cut-off is not active. In Figure 6(a), the default value is 3 ppm. The default value is determined based on the concentration of NOx in the exhaust gas when the vehicle is idling.

[0055] During the R1 period in Figure 6(a), the output value remains below the default value. As shown in Figure 6(a), the control device 10 corrects the output value as if it were the default value when the output value is below the default value. In other words, when the nitrogen oxide sensor 15 is active and the fuel cut is not in effect, if the output value falls below the default value, the control device 10 determines the amount of urea solution injected based on the default value and the exhaust gas flow rate.

[0056] During the R2 period in Figure 6(a), the output value remains above the default value. As shown in Figure 6, the control device 10 does not correct the output value when it is above the default value. In other words, when the nitrogen oxide sensor 15 is active and the fuel cut-off is not in effect, the control device 10 determines the amount of urea solution injected based on the output value and the exhaust flow rate when the output value is above the default value.

[0057] As shown in Figure 5, when the nitrogen oxide sensor 15 is active and fuel cut is in progress, the control device 10 determines the amount of urea solution injected based on the output value from the nitrogen oxide sensor 15 and the exhaust gas flow rate. Figure 6 shows the duration of fuel cut. As shown in Figure 6(a), during the fuel cut period, the control device 10 does not apply a correction to the output value to set the default value as the lower limit. As described above, in this case, the control device 10 treats output values ​​less than zero ppm as zero ppm.

[0058] Having determined the amount of urea solution to be injected in the manner described above, the control device 10 proceeds to step S13. In step S13, the control device 10 injects the amount of urea solution determined in step S12 into the injector 13. After that, the control device 10 completes the series of processes shown in Figure 4.

[0059] Thus, even if the output value falls below zero, the control device 10 determines the amount of urea solution injected based on the output value, which has been corrected to set the default value as the lower limit, provided certain conditions are met.

[0060] Figure 6(b) shows the change in the amount of urea added when the urea solution injection amount is determined based on the output value before correction by the control device 10, shown by a dashed line. Figure 6(b) shows the change in the amount of urea added when the urea solution injection amount is determined based on the output value after correction by the control device 10, shown by a solid line. As shown in Figure 6(b), the control device 10 can add urea to the injector 13 by correcting the output value, even during periods when urea cannot be added.

[0061] Figure 6(c) shows, with a dashed line, the change in NOx concentration in the exhaust after passing through the exhaust purification device 12 when the amount of urea solution injected is determined based on the output value before correction by the control device 10. Figure 6(c) also shows, with a solid line, the change in NOx concentration in the exhaust after passing through the exhaust purification device 12 when the amount of urea solution injected is determined based on the output value corrected by the control device 10. As shown in Figure 6(c), if the output value is not corrected, urea is not added even if NOx is present in the exhaust. However, if the output value is corrected, urea is added, thus reducing the emission of NOx outside the vehicle.

[0062] <Operation of this embodiment> The control device 10 sets a certain lower limit on the output value of the nitrogen oxide sensor 15, so that even if the output value of the nitrogen oxide sensor 15 should be zero, it will still inject urea solution into the injector 13.

[0063] <Effects of this embodiment> (1) The control device 10 can reduce the amount of NOx that passes through the exhaust gas purification device 12.

[0064] (2) During fuel cut-off, the control device 10 injects an amount of urea solution into the injector 13 determined based on the output value and the flow rate. During fuel cut-off, the hydrogen engine 11 does not emit NOx. Therefore, during fuel cut-off, it is desirable that the injector 13 does not inject urea solution when the output value of the nitrogen oxide sensor 15 becomes zero. During fuel cut-off, the control device 10 determines the amount of urea solution to be injected by the injector 13 based on an output value that does not have a lower limit, rather than an output value that has a lower limit. This allows the control device 10 to prevent the injector 13 from injecting urea solution when the hydrogen engine 11 does not generate NOx.

[0065] (3) The default value is determined based on the concentration of NOx in the exhaust when the vehicle is idling. If the default value is high, there is a higher possibility that the injector 13 will inject an excessive amount of urea solution relative to the concentration of NOx in the exhaust. The control device 10 uses a default value determined based on the concentration of NOx emitted by the hydrogen engine 11 when the vehicle is idling. This allows the control device 10 to prevent the injector 13 from injecting an excessive amount of urea solution.

[0066] (4) If the nitrogen oxide sensor 15 is inactive, the control device 10 estimates the NOx concentration in the exhaust based on the operating status of the hydrogen engine 11. The control device 10 injects an amount of urea solution determined based on the estimated NOx concentration and flow rate into the injector 13.

[0067] When the nitrogen oxide sensor 15 is not in an active state, the measured and output values ​​do not accurately reflect the NOx concentration in the exhaust gas. When the nitrogen oxide sensor 15 is not in an active state, the control device 10 does not use the measured and output values, but instead determines the amount of urea solution to be injected by the injector 13 based on the NOx concentration estimated by the control device 10. This allows the control device 10 to determine the amount of urea solution to be injected by the injector 13 with greater precision.

[0068] (5) The exhaust system 100 comprises an exhaust pipe 14, an exhaust purification device 12, a nitrogen oxide sensor 15, an injector 13, and a control device 10. The exhaust pipe 14 discharges exhaust gas emitted by the hydrogen engine 11, which uses hydrogen as fuel, to the outside of the vehicle. The exhaust purification device 12 is installed in the middle of the exhaust pipe 14 and reduces NOx contained in the exhaust gas. The nitrogen oxide sensor 15 is located upstream of the exhaust purification device 12 in the exhaust pipe 14. The nitrogen oxide sensor 15 acquires a measured value that reflects the concentration of NOx contained in the exhaust gas and outputs an output value which is the measured value minus a value that increases with the amount of water contained in the exhaust gas. The injector 13 is located downstream of the nitrogen oxide sensor 15 and upstream of the exhaust purification device 12 in the exhaust pipe 14 and injects urea water into the exhaust purification device 12. The control device 10 acquires the output value of the nitrogen oxide sensor 15 and controls the injector 13. The urea solution injection method includes the step of the control device 10 determining the amount of urea solution to be injected by the injector 13 based on the output value and the exhaust flow rate when the output value is greater than or equal to a predetermined value, and determining the amount of urea solution to be injected by the injector 13 based on the predetermined value and the exhaust flow rate when the output value is less than a predetermined value (step S12). The urea solution injection method also includes the step of the injector 13 injecting the amount of urea solution determined by the control device 10 into the exhaust purification device 12 (step S13).

[0069] The urea solution injection method involves setting a certain lower limit on the output value of the nitrogen oxide sensor 15, so that even when the output value of the nitrogen oxide sensor 15 would normally be zero, urea solution is injected into the injector 13. As a result, the urea solution injection method can reduce the amount of NOx passing through the exhaust gas purification device 12.

[0070] <Example of changes> The above embodiment can be implemented with the following modifications. The above embodiment and the following modifications can be combined with each other to the extent that they do not contradict each other technically.

[0071] In the above embodiment, the control device 10 determines the amount of urea solution to inject into the injector 13 based on the output value, which has not been corrected to a predetermined lower limit, and the exhaust flow rate, while the nitrogen oxide sensor 15 is active and fuel cut is in effect.

[0072] Even when the nitrogen oxide sensor 15 is active and fuel cut is in progress, the control device 10 may inject an amount of urea solution into the injector 13 determined based on the output value, which has been corrected to have a predetermined value as its lower limit, and the flow rate.

[0073] Furthermore, the control device 10 may adopt a configuration in which, when the nitrogen oxide sensor 15 is active and fuel cut is in effect, it uniformly refrains from injecting urea solution into the injector 13, regardless of the output value and flow rate.

[0074] The default value does not necessarily have to be determined based on the concentration of NOx in the exhaust when the vehicle is idling. For example, the default value may be arbitrarily determined by the designer of the control device 10.

[0075] In the above embodiment, the control device 10 determines the amount of urea solution based on the estimated NOx concentration and the exhaust flow rate when the nitrogen oxide sensor 15 is inactive. Alternatively, the control device 10 may determine the amount of urea solution based on the output value and the exhaust flow rate even when the nitrogen oxide sensor 15 is inactive. [Explanation of Symbols]

[0076] 10...Control device, 11...Hydrogen engine, 12...Exhaust gas purification device, 13...Injector, 14...Exhaust pipe, 15...Nitrogen oxide sensor, 16...Urea water tank, 17...Pump cell, 18...Sensor cell, 19...Monitor cell, 20...Solid electrolyte, 21...Reference electrode, 22...Heater, 23...First gas chamber, 24...Second gas chamber, 25...Pump electrode, 26...Sensor electrode, 27...Monitor electrode, 100...Exhaust system

Claims

1. An exhaust pipe that discharges exhaust gases from a hydrogen engine, which uses hydrogen as fuel, to the outside of the vehicle, An exhaust gas purification device installed in the middle of the exhaust pipe, which reduces nitrogen oxides contained in the exhaust gas, A nitrogen oxide sensor is disposed upstream of the exhaust gas purification device in the exhaust pipe, acquires a measurement value that reflects the concentration of nitrogen oxides contained in the exhaust gas, and outputs an output value which is obtained by subtracting a value that increases as the amount of moisture in the exhaust gas increases from the measurement value. An injector is provided in the exhaust pipe, downstream of the nitrogen oxide sensor and upstream of the exhaust gas purification device, which injects urea solution into the exhaust gas purification device. An exhaust system comprising the above, wherein the control device acquires the output value of the nitrogen oxide sensor and controls the injector, If the output value is equal to or greater than a predetermined value, the amount of urea solution determined based on the output value and the exhaust flow rate is injected into the injector. If the output value falls below the default value, the amount of urea solution determined based on the default value and the flow rate is injected into the injector. Control device.

2. During fuel cut-off, an amount of urea solution determined based on the output value and the flow rate is injected into the injector. The control device according to claim 1.

3. The aforementioned default value is determined based on the concentration of nitrogen oxides in the exhaust gas when the vehicle is idling. The control device according to claim 1.

4. If the nitrogen oxide sensor is inactive, the concentration of nitrogen oxides in the exhaust is estimated based on the operating status of the hydrogen engine. The amount of urea solution determined based on the estimated nitrogen oxide concentration and the flow rate is injected into the injector. The control device according to claim 1.

5. An exhaust pipe that discharges exhaust gases from a hydrogen engine, which uses hydrogen as fuel, to the outside of the vehicle, An exhaust gas purification device installed in the middle of the exhaust pipe, which reduces nitrogen oxides contained in the exhaust gas, A nitrogen oxide sensor is disposed upstream of the exhaust gas purification device in the exhaust pipe, acquires a measurement value that reflects the concentration of nitrogen oxides contained in the exhaust gas, and outputs an output value which is obtained by subtracting a value that increases as the amount of moisture in the exhaust gas increases from the measurement value. An injector is provided in the exhaust pipe, downstream of the nitrogen oxide sensor and upstream of the exhaust gas purification device, which injects urea solution into the exhaust gas purification device. A control device that acquires the output value of the nitrogen oxide sensor and controls the injector, A method for injecting urea solution in an exhaust system equipped with the following: The control device determines the amount of urea solution to be injected by the injector based on the output value and the exhaust flow rate when the output value is equal to or greater than a predetermined value, and determines the amount of urea solution to be injected by the injector based on the predetermined value and the flow rate when the output value is less than a predetermined value. The steps include: the injector injecting the amount of urea solution determined by the control device into the exhaust gas purification device; including Method for spraying urea solution.

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