Hydrogen engine control device

Abstract

To suppress urea deposition in a selective catalyst reduction device.SOLUTION: A control device 30 controls a hydrogen engine 10 which includes an SCR device 23 arranged in an exhaust passage 13, and a urea addition valve 20 for adding urea to exhaust gas flowing in an upstream side portion of the SCR device 23 in the exhaust passage 13. When a urea deposition amount of the SCR device 23 is equal to or more than a prescribed value, the control device 30 executes reduction processing for reducing a urea addition amount of the urea addition valve 20 as compared with a case where the urea deposition amount is less than the prescribed value. The control device 30 further executes lean restriction processing for controlling an air-fuel ratio of an air-fuel mixture burned in the hydrogen engine 10 to be equal to or less than a prescribed lean restriction value during reduction of the urea addition amount by the reduction processing.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a control device for a hydrogen engine. [Background technology]

[0002] One of the engine exhaust purification systems is the urea SCR (Selective Catalytic Reduction) system, which is equipped with a urea addition device that adds urea water to the exhaust gas and an SCR device that reduces NOx (nitrogen oxides) in the exhaust gas through selective catalytic reduction.

[0003] Patent Document 1 describes a control device for a diesel engine equipped with a urea SCR system. When the amount of urea accumulated in the SCR device exceeds a predetermined value, the control device performs a calcination control to calcine the accumulated urea by raising the temperature of the exhaust gas. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-320854 Summary of the Invention [Problem to be solved by the invention]

[0005] It is possible to apply the above-mentioned urea SCR system to a hydrogen engine. Hydrogen engines generally have lower exhaust temperatures than diesel engines. When the exhaust temperature is low, it is more difficult for the SCR device to heat up. The lower the temperature of the SCR device, the more easily urea accumulates in the device. Therefore, if a urea SCR system is used in a hydrogen engine, it is possible that firing control will need to be performed more frequently. [Means for solving the problem]

[0006] A control device for a hydrogen engine that solves the above problem is a device for controlling a hydrogen engine that has a selective catalytic reduction device installed in an exhaust passage and a urea addition device that adds urea to exhaust gas flowing in a portion of the exhaust passage upstream of the selective catalytic reduction device, and when the amount of urea deposited in the selective catalytic reduction device is equal to or greater than a predetermined value, performs a reduction process that reduces the amount of urea added by the urea addition device compared to when the amount of urea deposited is less than the predetermined value, and a lean limit process that sets the air-fuel ratio of the mixture burned in the hydrogen engine to a predetermined lean limit value or less while the urea addition amount is being reduced by the reduction process, and is configured so that the lean limit value is a value that is richer than the upper limit value of the control range of the air-fuel ratio when the urea addition amount is not being reduced by the reduction process. [Effects of the Invention]

[0007] The above-described hydrogen engine control device can suppress urea deposition in the selective catalytic reduction device. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a diagram showing a schematic configuration of an embodiment of a control device for a hydrogen engine; [Figure 2] 4 is a flowchart of a urea deposition suppression control routine executed by the control device. [Figure 3] FIG. 3(a) is a time chart showing the transition of the estimated SCR temperature, FIG. 3(b) is a time chart showing the transition of the target air-fuel ratio, FIG. 3(c) is a time chart showing the transition of the ignition timing, and FIG. 3(d) is a time chart showing the transition of the urea accumulation amount during the firing control by the above-mentioned control device. DETAILED DESCRIPTION OF THE INVENTION

[0009] An embodiment of a control device for a hydrogen engine will be described in detail below with reference to Figures 1 to 3. The control device of this embodiment is configured as a device that controls a hydrogen engine mounted on a vehicle.

[0010] <Configuration of Hydrogen Engine 10> First, the configuration of a hydrogen engine 10 to which the control device of this embodiment is applied will be described with reference to Figure 1. The hydrogen engine 10 includes an intake passage 11, a combustion chamber 12, and an exhaust passage 13.

[0011] A throttle valve 14 is installed in the intake passage 11. The throttle valve 14 is a valve that adjusts the flow rate of intake air in the intake passage 11. Intake air is introduced into the combustion chamber 12 through the intake passage 11. An injector 15 and an ignition device 16 are installed in the combustion chamber 12. The injector 15 injects hydrogen gas to form a mixture of intake air and hydrogen gas in the combustion chamber 12. The ignition device 16 ignites the mixture in the combustion chamber 12 by spark discharge. Exhaust gas produced by combustion of the mixture in the combustion chamber 12 is discharged into the exhaust passage 13.

[0012] A urea addition valve 20 is installed in the exhaust passage 13. A urea water tank 22 is connected to the urea addition valve 20 via a urea pump 21. The urea pump 21 delivers urea water stored in the urea water tank 22 to the urea addition valve 20. The urea addition valve 20 adds the urea water delivered from the urea pump 21 to the exhaust gas in the exhaust passage 13. In this embodiment, the urea addition valve 20, the urea pump 21, and the urea water tank 22 constitute a urea addition device.

[0013] An SCR (selective catalytic reduction) device 23 is installed in the exhaust passage 13 downstream of the urea addition valve 20. The urea water added to the exhaust gas by the urea addition valve 20 is hydrolyzed into ammonia. The SCR device 23 purifies the NOx in the exhaust gas by reducing it with ammonia.

[0014] <Configuration of the control device 30> Next, the configuration of the control device 30 for the hydrogen engine 10 will be described with reference to Figure 1. The control device 30 is configured as an electronic control module comprising a processing circuit 31 and a storage device 32. The storage device 32 stores programs and data for controlling the hydrogen engine 10. The processing circuit 31 reads and executes programs from the storage device 32, thereby carrying out various processes for controlling the hydrogen engine 10.

[0015] The control device 30 receives detection signals from various sensors for detecting the operating conditions of the hydrogen engine 10. The various sensors include an air flow meter 33, a NOx sensor 34, an exhaust temperature sensor 35, a crank angle sensor 36, and an accelerator pedal sensor 37. The air flow meter 33 is a sensor that detects the intake air flow rate GA. The NOx sensor 34 is a sensor that detects the NOx concentration in the exhaust gas flowing into the SCR device 23. The exhaust temperature sensor 35 is a sensor that detects the temperature of the exhaust gas flowing out from the SCR device 23. The crank angle sensor 36 is a sensor that detects the crank angle of the hydrogen engine 10. The accelerator pedal sensor 37 is a sensor that detects the amount of depression of the accelerator pedal.

[0016] The control device 30 determines the operation variables of the hydrogen engine 10 based on the detection signals of these sensors. Then, based on the determined operation variables, the control device 30 drives the actuators of the hydrogen engine 10 to control the operating state of the hydrogen engine 10. The operation variables determined by the control device 30 include the opening of the throttle valve 14, the amount of hydrogen gas injected by the injector 15, the ignition timing of the ignition device 16, and the amount of urea added by the urea addition valve 20.

[0017] <Air-fuel ratio control of hydrogen engine 10> Next, we will explain the air-fuel ratio control of the hydrogen engine 10 executed by the control device 30. When controlling the air-fuel ratio, the control device 30 determines the required torque of the hydrogen engine 10 based on the accelerator pedal depression amount, engine rotation speed, etc. The engine rotation speed is the rotation speed of the crankshaft of the hydrogen engine 10. The control device 30 calculates the engine rotation speed based on the detection result of the crank angle sensor 36.

[0018] Next, the control device 30 determines the target air-fuel ratio λt based on the required torque and engine rotation speed. More specifically, the control device 30 determines the target air-fuel ratio λt as the lean limit value of the air-fuel ratio range that does not cause combustion instability or emissions deterioration at the operating point of the hydrogen engine 10 determined by the required torque and engine rotation speed.

[0019] Next, the control device 30 commands the injector 15 to inject an amount of hydrogen gas that will obtain a torque equal to the required torque. Then, the control device 30 controls the opening of the throttle valve 14 so that the air-fuel ratio of the mixture burned in the combustion chamber 12 becomes equal to the target air-fuel ratio λt.

[0020] <Urea addition control for hydrogen engine 10> Next, the urea addition control of the hydrogen engine 10 executed by the control device 30 will be described. The control device 30 estimates the internal temperature of the SCR device 23 based on the detection results of the exhaust gas temperature sensor 35, etc. In the following description, the internal temperature of the SCR device 23 estimated by the control device 30 will be referred to as the estimated SCR temperature. The control device 30 adds urea water from the urea addition valve 20 on the condition that the estimated SCR temperature is equal to or higher than the addition start temperature. When the hydrogen engine 10 is started, the lower limit of the internal temperature of the SCR device 23 at which the SCR device 23 becomes active and can purify NOx is set as the addition start temperature.

[0021] While the urea solution is being added, the control device 30 calculates the amount of NOx that flows into the SCR device 23 based on the detection result of the NOx sensor 34, etc. The control device 30 calculates the amount of urea solution to be added that is necessary to reduce and purify the amount of NOx that flows in. Then, the control device 30 drives the urea addition valve 20 to add the calculated amount of urea solution.

[0022] <Urea accumulation suppression control> Urea gradually accumulates in the SCR device 23 of such a hydrogen engine 10. When the amount of urea accumulated in the SCR device 23 exceeds a certain amount, the SCR device 23 is unable to fully demonstrate its ability to purify NOx. The control device 30 of this embodiment performs urea accumulation suppression control to suppress urea accumulation in the SCR device 23.

[0023] 2 shows a flowchart of a urea deposition suppression control routine executed by the control device 30. After the hydrogen engine 10 is started, the control device 30 starts the processing of this routine.

[0024] When this routine starts, the control device 30 first calculates the amount of urea deposited in the SCR device 23 in step S100. When calculating the amount of urea deposited, the control device 30 calculates the amount of urea that will newly deposit in the SCR device 23 based on the amount of urea water added by the urea addition valve 20. The control device 30 also calculates the amount of urea that will be thermally decomposed in the SCR device 23. Next, the control device 30 calculates the increase in the amount of urea deposited in the SCR device 23 by subtracting the amount of urea that will be thermally decomposed from the amount of urea that will newly deposit. The control device 30 then calculates the amount of urea deposited in the SCR device 23 by adding the calculated increase to the previous value of the amount of urea deposited.

[0025] In the next step S110, the control device 30 determines whether the amount of urea accumulation is equal to or greater than a predetermined value X. If the amount of urea accumulation is less than the predetermined value X (NO), the control device 30 sets a predetermined first temperature T1 as the value of the addition start temperature in step S120. The first temperature T1 is set to the lower limit of the temperature range of the SCR device 23 at which NOx can be effectively purified. Then, the control device 30 waits for the predetermined control period to elapse, and then resumes processing of this routine from step S100.

[0026] On the other hand, if the urea accumulation amount is equal to or greater than the predetermined value X (YES), the control device 30 sets a predetermined second temperature T2 higher than the first temperature T1 as the value of the addition start temperature in step S130. Next, the control device 30 determines in step S140 whether the estimated SCR temperature is equal to or greater than the first temperature T1 and less than the second temperature T2.

[0027] If the estimated SCR temperature is equal to or higher than the first temperature T1 and lower than the second temperature T2 (S140: YES), the control device 30 sets the predetermined lean limit value as the value of the target air-fuel ratio λt in step S150. Then, the control device 30 resumes the processing of this routine from step S100 after the above control period has elapsed.

[0028] As described above, the control device 30 normally determines the target air-fuel ratio λt based on the required torque and engine rotation speed. The lean limit value is set to a value on the richer side than the upper limit value of the setting range of the target air-fuel ratio λt based on the required torque and engine rotation speed.

[0029] On the other hand, if the estimated SCR temperature is less than the first temperature T1 or equal to or greater than the second temperature T2 (S1440: NO), the control device 30 proceeds to step S160. In step S160, the control device 30 determines whether the estimated SCR temperature is equal to or greater than a predetermined third temperature T3. The third temperature T3 is set to a value higher than the second temperature T2. If the estimated SCR temperature is less than the third temperature T3 (S160: NO), the control device 30 resumes processing of this routine from step S100 after the above control period has elapsed.

[0030] On the other hand, if the estimated SCR temperature is equal to or higher than the third temperature T3 (S160: YES), in step S170, the control device 30 executes burning control to burn the urea deposited in the SCR device 23. After the burning control ends, the control device 30 resumes the processing of this routine from step S100.

[0031] <Baking control> Next, the details of the firing control will be explained with reference to Fig. 3. Fig. 3 shows the transitions of each parameter of the hydrogen engine 10 during, before, and after the execution of the firing control. That is, Fig. 3(a) shows the transitions of the estimated SCR temperature, Fig. 3(b) shows the transitions of the target air-fuel ratio λt, Fig. 3(c) shows the transitions of the ignition timing, and Fig. 3(d) shows the transitions of the urea accumulation amount.

[0032] In the case of FIG. 3, at time t1, the estimated SCR temperature rises to the third temperature T3. The control device 30 starts the firing control at this time t1. The control device 30 calculates the duration of the firing control required to burn the accumulated urea based on the amount of urea accumulated in the SCR device 23 at the start of the firing control. In response to the start of the firing control, the control device 30 changes the target air-fuel ratio λt to the rich side, as shown in FIG. 3(b). Furthermore, in response to the start of the firing control, the control device 30 retards the ignition timing, as shown in FIG. 3(c).

[0033] By enriching the target air-fuel ratio λt and retarding the ignition timing, the temperature of the exhaust gas discharged from the combustion chamber 12 to the exhaust passage 13 increases. This causes the internal temperature of the SCR device 23 to also increase.

[0034] Thereafter, at time t2, when the internal temperature of the SCR device 23 rises to the calcination temperature T4 required for calcining the urea, calcination of the urea accumulated in the SCR device 23 begins. At time t3, when the above-mentioned duration has elapsed since time t1 and the amount of urea accumulated in the SCR device 23 becomes "0," the control device 30 terminates the calcination control by canceling the enrichment of the target air-fuel ratio λt and the retardation of the ignition timing.

[0035] <Actions and Effects of the Embodiment> The operation and effects of this embodiment will be described. The control device 30 performs urea solution addition through the urea addition valve 20 on the condition that the estimated SCR temperature is equal to or higher than the addition start temperature. When the amount of urea deposited in the SCR device 23 is less than a predetermined value X, the control device 30 sets the addition start temperature to a first temperature T1. When the amount of urea deposited in the SCR device 23 is equal to or higher than the predetermined value X, the control device 30 sets the addition start temperature to a second temperature T2, which is higher than the first temperature T1. Therefore, when the amount of urea deposited is less than the predetermined value X, urea solution addition through the urea addition valve 20 is performed if the estimated SCR temperature is equal to or higher than the first temperature T1. On the other hand, when the amount of urea deposited is equal to or higher than the predetermined value X, urea solution addition through the urea addition valve 20 is not performed if the estimated SCR temperature is less than the second temperature T2, even if the estimated SCR temperature is equal to or higher than the first temperature T1. In this way, when the amount of urea accumulated in the SCR device 23 is equal to or greater than the predetermined value X, the control device 30 performs a reduction process to reduce the amount of urea added by the urea addition valve 20 to 0 when the estimated SCR temperature is equal to or greater than the first temperature T1 and less than the second temperature T2.

[0036] Furthermore, when the urea deposition amount is equal to or greater than the predetermined value X, the control device 30 sets the lean limit value as the value of the target air-fuel ratio λt when the estimated SCR temperature is equal to or greater than the first temperature T1 and less than the second temperature T2. As a result, the air-fuel ratio of the mixture burned in the hydrogen engine 10 is fixed to the lean limit value. On the other hand, as described above, when the urea deposition amount is equal to or greater than the predetermined value X, the control device 30 reduces the urea addition amount through the reduction process when the estimated SCR temperature is equal to or greater than the first temperature T1 and less than the second temperature T2. In this way, the control device 30 of this embodiment performs the lean limit process to set the air-fuel ratio to equal to or less than the predetermined lean limit value while the urea addition amount is being reduced through the reduction process. The lean limit value is set to a value richer than the upper limit value of the control range of the air-fuel ratio when the urea addition amount is not being reduced through the reduction process.

[0037] When the temperature of the SCR device 23 is low, urea tends to accumulate in the SCR device 23. In the amount reduction process, the control device 30 raises the urea addition start temperature from the first temperature T1 to the second temperature T2 when the urea accumulation amount is equal to or greater than a predetermined value X. As a result, when the urea accumulation amount is equal to or greater than the predetermined value X, urea addition is not performed when the estimated SCR temperature is equal to or less than the second temperature T2. Therefore, the progress of urea accumulation in the SCR device 23 is suppressed when the urea accumulation amount is equal to or greater than the predetermined value X.

[0038] In the range of the estimated SCR temperature equal to or greater than the first temperature T1 and less than the second temperature T2, urea addition is performed when the urea accumulation amount is less than a predetermined value X, but is not performed when the urea accumulation amount is equal to or greater than the predetermined value X. If urea addition is not performed, the SCR device 23 will no longer be able to purify NOx in the exhaust gas using urea-derived ammonia as a reducing agent. Meanwhile, the amount of NOx generated by combustion in the hydrogen engine 10 increases as the air-fuel ratio becomes leaner. In response to this, the control device 30 performs a lean limiting process while the urea addition is being reduced through the reduction process. This prevents combustion in the hydrogen engine 10 at an air-fuel ratio leaner than the lean limit value, thereby suppressing the generation of NOx. As a result, the increase in NOx released into the outside air due to the suspension of urea addition to suppress urea accumulation is suppressed.

[0039] According to the control device 30 for the hydrogen engine 10 of this embodiment, the following effects can be achieved. (1) By performing the above-described reduction process and lean limiting process, the control device 30 can suppress the progress of urea accumulation in the SCR device 23 while suppressing the release of NOx into the outside air.

[0040] (2) The control device 30 performs the amount reduction process by setting the addition start temperature to a first temperature T1 when the amount of urea accumulation is less than a predetermined value X, or to a second temperature T2 higher than the first temperature T1 when the amount of urea accumulation is equal to or greater than the predetermined value X. Since the amount reduction process prohibits urea addition at low temperatures when urea accumulation is likely to progress, the progress of urea accumulation in the SCR device 23 can be effectively suppressed.

[0041] (3) When the amount of urea accumulation is equal to or greater than the predetermined value X and the estimated SCR temperature is equal to or greater than the third temperature T3, the control device 30 executes calcination control to raise the temperature of the SCR device 23 to or above the temperature T4 required for calcining urea. This prevents excessive urea accumulation in the SCR device 23.

[0042] (4) When the air-fuel ratio is enriched and the ignition timing is retarded by the firing control, the combustion efficiency decreases, and the fuel consumption of the hydrogen engine 10 increases. Meanwhile, the execution time of the firing control increases as the temperature of the SCR device 23 at the start of the firing control decreases. In contrast, the control device 30 of this embodiment executes the firing control on the condition that the estimated SCR temperature is equal to or higher than the third temperature T3. This prevents the firing control from being prolonged. Furthermore, the increase in fuel consumption due to the execution of the firing control is also suppressed.

[0043] (5) Some diesel engines have a DPF (Diesel Particulate Filter) installed in the exhaust passage to capture particulate matter (PM) in the exhaust. In such diesel engines, filter regeneration control is performed to combust and purify PM accumulated in the DPF. During filter regeneration control, urea accumulated in the SCR device is calcined. Therefore, urea accumulation in the SCR device can be suppressed without separately performing calcination control. In contrast, in the case of a hydrogen engine 10, it is necessary to perform calcination control to calcinate urea accumulated in the SCR device 23. In the control device 30 of this embodiment, urea accumulation in the SCR device 23 is suppressed by the amount reduction process and the lean limiting process. Therefore, it is possible to reduce the frequency and duration of calcination control.

[0044] (6) The control device 30 controls the combustion by changing the air-fuel ratio to the rich side and retarding the ignition timing, thereby increasing the temperature of the SCR device 23 and burning the accumulated urea.

[0045] (Other embodiments) This embodiment can be modified as follows: This embodiment and the following modifications can be combined and implemented within the scope of technical compatibility.

[0046] <About baking control> In the above embodiment, firing control is performed by enriching the air-fuel ratio and retarding the ignition timing. However, firing control may be performed by raising the temperature of the SCR device 23 above the urea firing temperature using another method. For example, firing control may be performed by only enriching the air-fuel ratio or retarding the ignition timing.

[0047] In the above embodiment, the firing control is executed when the amount of urea accumulated in the SCR device 23 is equal to or greater than the predetermined value X and the estimated SCR temperature is equal to or greater than the third temperature T3. However, the execution conditions for the firing control may be changed to other conditions.

[0048] Execution of calcination control is not essential. If the temperature of the SCR device 23 rises above the temperature at which urea can be calcined during normal operation or due to other control, excessive accumulation of urea in the SCR device 23 can be avoided even if calcination control is not executed.

[0049] <About lean limit processing> In the above embodiment, the target air-fuel ratio λt is fixed to the lean limit value in the lean limit processing. However, in the lean limit processing, the target air-fuel ratio λt may be variably set within a range of air-fuel ratios richer than the lean limit value.

[0050] In the above embodiment, the execution time of the lean limiting process was determined based on the amount of urea accumulated in the SCR device 23 at the start of the lean limiting process. However, the execution time of the lean limiting process may be determined by other methods. For example, the amount of urea accumulated in the SCR device 23 may be calculated during the lean limiting process. Then, the lean limiting process may be executed until the amount of urea accumulated becomes equal to or less than a predetermined value.

[0051] <About weight reduction processing> In the above embodiment, the reduction process was a process of raising the addition start temperature from the first temperature T1 to the second temperature T2. However, when the urea accumulation amount in the SCR device 23 is equal to or greater than the predetermined value X, any other process may be performed as the reduction process, as long as it reduces the amount of urea added by the urea addition device compared to when the amount of urea accumulation in the SCR device 23 is less than the predetermined value X. In either case, reducing the amount of urea added reduces the NOx purification capability of the SCR device 23. Therefore, it is desirable to perform lean limiting process while the urea addition amount is being reduced by the reduction process, thereby reducing the amount of NOx generated by combustion in order to suppress NOx release into the outside air.

[0052] (Additional notes) [Appendix 1] A control device for a hydrogen engine comprising a selective catalytic reduction device installed in an exhaust passage, and a urea addition device that adds urea to exhaust gas flowing in a portion of the exhaust passage upstream of the selective catalytic reduction device, wherein when the amount of urea deposited in the selective catalytic reduction device is equal to or greater than a predetermined value, a reduction process is performed to reduce the amount of urea added by the urea addition device compared to when the amount of urea deposited is less than the predetermined value, and a lean limit process is performed to keep the air-fuel ratio of the mixture burned in the hydrogen engine at or below a predetermined lean limit value while the amount of urea added is being reduced by the reduction process, and the lean limit value is a value on the richer side of the upper limit value of the control range of the air-fuel ratio when the amount of urea added is not being reduced by the reduction process.

[0053] [Appendix 2] The control device is configured to add urea to the exhaust gas by the urea addition device on the condition that the temperature of the selective catalytic reduction device is equal to or higher than an addition start temperature, and the amount reduction process is a process of setting a predetermined first temperature as the addition start temperature when the urea accumulation amount is less than the predetermined value, and setting a second temperature higher than the first temperature as the addition start temperature when the urea accumulation amount is equal to or higher than the predetermined value. A control device for a hydrogen engine described in [Appendix 1].

[0054] [Appendix 3] The control device for a hydrogen engine according to [Appendix 1] or [Appendix 2], wherein the lean limiting process is a process for fixing the air-fuel ratio to the lean limit value. [Appendix 4] A control device for a hydrogen engine described in any one of [Appendix 1] to [Appendix 3], wherein, when the amount of urea accumulation is equal to or greater than the predetermined value and the temperature of the selective catalytic reduction device is equal to or greater than a predetermined temperature, calcination control is executed to raise the temperature of the selective catalytic reduction device to a temperature required for calcining the urea or higher.

[0055] [Appendix 5] The control device for a hydrogen engine according to [Appendix 4], wherein the firing control is a control that changes the air-fuel ratio to the rich side and retards the ignition timing. [Explanation of symbols]

[0056] 10 Hydrogen engine 11 Intake passage 12 Combustion chamber 13 Exhaust passage 14 Throttle valve 15 injectors 16 Ignition system 20 Urea addition valve (urea addition device) 21 Urea pump (urea addition device) 22 Urea water tank (urea addition device) 23 SCR device (selective catalytic reduction device) 30 Control device 31 Processing circuit 32 Storage device 33 Air flow meter 34 NOx sensor 35 Exhaust gas temperature sensor 36 Crank angle sensor 37 Accelerator pedal sensor

Claims

1. A control device for a hydrogen engine including a selective catalytic reduction device installed in an exhaust passage, and a urea addition device that adds urea to exhaust gas flowing in a portion of the exhaust passage upstream of the selective catalytic reduction device, a reduction process for reducing the amount of urea added by the urea addition device when the amount of urea deposited in the selective catalytic reduction device is equal to or greater than a predetermined value, compared to when the amount of urea deposited is less than the predetermined value; a lean limiting process for limiting the air-fuel ratio of the mixture combusted in the hydrogen engine to a predetermined lean limit value or less while the urea addition amount is being reduced by the reduction process; and the lean limit value is a value on the rich side of the upper limit value of the control range of the air-fuel ratio when the urea addition amount is not being reduced by the reduction process. Hydrogen engine control device.

2. the control device is configured to add urea to the exhaust gas by the urea addition device on condition that the temperature of the selective catalytic reduction device is equal to or higher than an addition start temperature, The amount reduction process is a process of setting a predetermined first temperature as the addition start temperature when the urea accumulation amount is less than the predetermined value, and setting a second temperature higher than the first temperature as the addition start temperature when the urea accumulation amount is equal to or greater than the predetermined value. The control device for a hydrogen engine according to claim 1.

3. 2. The hydrogen engine control device according to claim 1, wherein the lean limiting process fixes the air-fuel ratio to the lean limit value.

4. 2. The control device for a hydrogen engine according to claim 1, wherein, when the amount of urea accumulation is equal to or greater than the predetermined value and the temperature of the selective catalytic reduction device is equal to or greater than a predetermined temperature, calcination control is executed to raise the temperature of the selective catalytic reduction device to a temperature equal to or greater than a temperature required for calcining the urea.

5. 5. A control device for a hydrogen engine according to claim 4, wherein the firing control is a control for changing the air-fuel ratio to the rich side and retarding the ignition timing.

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