A two-stage diesel scr aftertreatment system and method

Abstract

The present application belongs to the field of diesel engine post-processing system control method, and provides a diesel engine two-stage SCR post-processing system and method, the system comprising a front-stage SCR system, a high-pressure HC injection system, a DOC system, a DPF system and a rear-stage SCR system connected in sequence along an engine turbine exhaust port; the present application uses a high-pressure HC injector to provide the HC oxidant required for DPF active regeneration, and the engine does not need to perform far post-injection to provide HC, thereby avoiding HC poisoning caused by high HC passing through the front-stage SCR; the present application uses a high-pressure HC injector to effectively improve the post-processing temperature, promote the downstream SCR to quickly ignite, and ensure the NOx conversion efficiency at low temperature; the present application does not need to add a close-coupled DOC at the front end of the front-stage SCR, and the front-stage SCR can quickly ignite; the present application effectively improves the SCR control precision and ensures the NOx emission reduction efficiency.

Description

Technical Field

[0001] This invention belongs to the field of diesel engine aftertreatment system control methods, specifically relating to a two-stage SCR aftertreatment system and method for diesel engines. Background Technology

[0002] In the near-zero emission stage, the two-stage SCR aftertreatment system (ccSCR+DOC+DPF+SCR / ASC) has become the most promising low-NOx aftertreatment configuration. This configuration still uses the DPF as a particulate emission solution. During active regeneration of the DPF while parked, the engine controller needs to adjust the after-injection to increase the amount of HC in the exhaust, thereby causing the DOC to oxidize HC and release heat to raise the aftertreatment temperature to reach the soot regeneration temperature. However, because the DPF is located downstream of the cCSCR, when the active regeneration mode is activated, a large amount of HC will pass through the cCSCR with the exhaust. At low temperatures, some HC is adsorbed onto the cCSCR catalyst in liquid or solid form, covering the active sites; at high temperatures, some HC decomposes to form coke that deposits in the catalyst channels, leading to a decrease in specific surface area. Long-term accumulation will result in a permanent reduction in catalytic activity. Simultaneously, HC may react with adsorbed NH3 to generate nitrogen-containing organic compounds (such as amines), further consuming available NH3 and generating harmful byproducts. Therefore, avoiding high HC contact with the cSCR can effectively extend the service life and efficiency of the cSCR, and ensure the NOx conversion efficiency of the cSCR.

[0003] On the other hand, in the two-stage SCR aftertreatment system ccSCR+DOC+DPF+SCR / ASC configuration, the addition of tightly coupled SCR components extends the aftertreatment length. By the time the exhaust reaches the downstream SCR, significant temperature loss occurs, resulting in slower and later ignition of the downstream SCR catalyst. Therefore, the system can only rely on the tightly coupled SCR to treat NOx emissions under low-temperature conditions. Since the NO2 / NOx conversion efficiency in the SCR catalytic reduction reaction is optimal between 0.4 and 0.6, and the original exhaust NO2 / NOx ratio is approximately 0.1 to 0.2, the ccSCR cannot operate within its optimal efficiency range, failing to achieve the minimum NOx emission reduction effect. Therefore, there is an urgent need to find an aftertreatment thermal management measure that, without altering the original engine exhaust conditions, enables rapid ignition of the downstream SCR to achieve the minimum NOx emission reduction effect. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a two-stage SCR aftertreatment system and method for diesel engines, thereby resolving the issues in the prior art. The technical solution adopted by this invention is as follows:

[0005] A two-stage SCR aftertreatment system for a diesel engine includes a front-stage SCR system, a high-pressure HC injection system, a DOC system, a DPF system, and a rear-stage SCR system connected sequentially along the engine turbine exhaust port.

[0006] The pre-stage SCR system includes a pre-stage urea injector, a mixer, and pre-stage SCR components;

[0007] The high-pressure HC injection system includes an HC injector and an HC injection actuator, which are connected by a metal pipeline.

[0008] The downstream SCR system includes a downstream urea injector, a mixer, an SCR component, and an ASC component.

[0009] The front end of the pre-stage SCR system is connected to the engine turbine outlet, and the rear end is connected to the DOC system; the front end of the DOC system is connected to the pre-stage SCR system, and the rear end is connected to the DPF system; the HC injector in the high-pressure HC injection system is located on the empty exhaust pipe connecting the pre-stage SCR system and the DOC system, with sufficient empty exhaust pipe space reserved to ensure that the injected HC can mix well with the exhaust; the HC injection actuator is located on the engine block or aftertreatment mounting bracket; the front end of the DPF system is connected to the DOC system, and the rear end is connected to the subsequent SCR system; the front end of the subsequent SCR system is connected to the DPF system, and the rear end is the engine exhaust pipe.

[0010] Furthermore, it also includes four NOx sensors, namely:

[0011] The first NOx sensor is located at the inlet of the front-stage SCR system to monitor the original NOx emissions from the engine, provide the NOx concentration signal to the front-stage SCR system to calculate the urea demand and perform urea injection control.

[0012] The second NOx sensor is located at the outlet of the pre-stage SCR system. It is used to provide NOx concentration signal input for the urea injection feedback control of the pre-stage SCR system and to detect the conversion efficiency of the pre-stage SCR system.

[0013] The third NOx sensor is located at the inlet of the downstream SCR system to monitor the NOx concentration at the inlet of the downstream SCR system, provide the NOx concentration signal to the downstream SCR system to calculate the urea demand and perform urea injection control.

[0014] The fourth NOx sensor, located after the downstream SCR system, monitors the final NOx tailpipe emissions. It provides NOx concentration signal input for the urea injection feedback control of the downstream SCR system and simultaneously detects the conversion efficiency of the downstream SCR system.

[0015] All NOx sensors are connected to the engine ECU via data cables, which input signals and issue commands.

[0016] Furthermore, it also includes four temperature sensors, namely:

[0017] The first temperature sensor is located at the inlet of the upstream SCR system to provide the temperature signal required for the control of the upstream urea injector.

[0018] The second temperature sensor is located at the DOC system inlet. When a DPF active regeneration request is received, the second temperature sensor monitors the DOC system temperature, determines the DOC system conversion efficiency range, and then determines whether to perform DPF active regeneration.

[0019] The third temperature sensor is located at the DOC system outlet and the DPF system inlet. When the DPF regeneration state is entered, the third temperature sensor provides a temperature signal input to determine the regeneration temperature, and then controls the HC ejector to spray the HC required for DOC oxidation and heat release.

[0020] The fourth temperature sensor is located at the inlet of the downstream SCR system to provide the temperature signal required for the control of the downstream urea injector.

[0021] All temperature sensors are connected to the engine ECU via data cables, which input signals and issue commands.

[0022] A method for active regeneration control of DPF in a two-stage SCR aftertreatment system for a diesel engine includes the following steps:

[0023] Step 1: After the engine starts, first read the ECU data to determine whether the DPF system has entered the DPF regeneration request state;

[0024] Step 2: If the status recognition value is 0, it is not in the DPF active regeneration state, then the engine remains idle and waits for ECU commands.

[0025] If the status identification value is 1, indicating DPF active regeneration status, the DPF system initiates DPF regeneration. The ECU receives a temperature sensor signal, all urea injectors stop injecting, and then the following steps are performed:

[0026] When the DOC system inlet temperature is ≤225℃ and the engine speed is increased to 2000r / min, the ECU controls the engine after-injection to be activated, which strengthens the after-combustion of the engine, causes the exhaust temperature to rise, and increases the DOC system inlet temperature.

[0027] When the DOC system inlet temperature exceeds 225℃ and the engine speed increases to 2500 r / min, the high-pressure HC injection system starts working, raising the DPF system inlet temperature until the target regeneration temperature of 500℃ is reached, and regeneration begins. After the DPF system regeneration is started, the second and third temperature sensors input temperature signals, and the ECU provides feedback to adjust and control the HC injector injection quantity, so that the DPF system inlet temperature is maintained at the regeneration temperature. After regeneration is completed, the high-pressure HC injection system stops working, the HC injector stops injecting, the regeneration state ends, and the ECU controls the engine to return to normal mode.

[0028] A control method for HC injection thermal management of a two-stage SCR aftertreatment system for a diesel engine includes the following steps:

[0029] Step 1: After the engine starts, the ECU data is read first to determine whether the DPF system has entered the DPF regeneration request state. If the state recognition value is 1, DPF active regeneration is performed; if the state recognition value is 0, it is not in the DPF active regeneration state. Then the ECU reads the first temperature sensor signal to determine the operating status of the front-stage SCR system and further controls the HC injection actuator and HC injector to operate.

[0030] If the inlet temperature of the pre-stage SCR system is ≤200℃, the pre-stage urea injector will stop spraying to prevent ammonia leakage from the pre-stage SCR, and the downstream SCR system will not work.

[0031] If the inlet temperature of the pre-stage SCR system is >200℃, the pre-stage urea injector will inject urea at an ammonia-to-nitrogen ratio of 1 to reduce NOx, and the downstream SCR system will not work.

[0032] Step 2: ECU20 reads the signals from the fourth and second temperature sensors to determine the operating status of the downstream SCR system and further controls the actuators and HC injectors.

[0033] If the inlet temperature of the downstream SCR system is >250℃, the downstream urea injector will start injecting urea at an ammonia-to-nitrogen ratio of 1 to reduce NOx, the HC injector will not operate, and the upstream SCR urea injector will stop injecting.

[0034] If the inlet temperature of the downstream SCR system is ≤250℃, the downstream urea injector will not inject, the HC injection actuator will start working, the HC injector will enter the injection standby state, and then the DOC system inlet temperature will be judged to determine whether the DOC system has reached the DOC catalyst ignition temperature, and then step 3 will be executed:

[0035] Step 3: If the DOC system inlet temperature is ≥200℃, the downstream urea injector stops injecting and the HC injector starts injecting, increasing the downstream SCR system inlet temperature. ECU20 reads the signal from the fourth temperature sensor to monitor the downstream SCR system inlet temperature. When the downstream SCR system inlet temperature is >250℃, step 2 is executed.

[0036] Step 4: If the DOC system inlet temperature is <200℃, the downstream urea injector stops injecting, the engine speed is increased to 2000r / min, the ECU controls the engine to start the post-injection, and increases the DOC system inlet temperature. The ECU reads the signal from the second temperature sensor to monitor the DOC system inlet temperature. When the DOC system temperature is ≥200℃, step 3 is executed.

[0037] Step 5: When the ECU issues an engine shutdown command, the two-stage SCR system after-treatment system ends its working state.

[0038] Furthermore, in step 3, the injection control of the HC injector adopts a model-based injection control method, including: establishing a dynamic exhaust temperature model for the high-pressure HC injection system based on experimental data; converting the inlet temperature of the downstream SCR system into a functional model of exhaust temperature, exhaust flow rate, HC injection flow rate, engine EGR rate, and DOC conversion efficiency; predicting the thermodynamic state in the next 3-5 seconds; and inversely optimizing the HC injection quantity of the current HC injector.

[0039] A state-space model is constructed based on thermodynamic equations and historical data:

[0040] ;

[0041] in, The inlet temperature of the subsequent SCR system is a function of the time step. This refers to the exhaust flow rate; It is a function of exhaust temperature; Let HC be the jet flow rate function;

[0042] Within each control cycle, the optimal injection quantity function is solved by predicting the future temperature trajectory based on current sensor data and the exhaust temperature dynamic model.

[0043] ;

[0044] in, The inlet temperature function of the subsequent SCR system during the control cycle; Target exhaust temperature; This is the HC injection flow rate function.

[0045] The present invention has the following beneficial effects:

[0046] (1) The high-pressure HC injector is used to provide the HC oxidant required for active regeneration of DPF. The engine does not need to provide HC by remote back injection, thus avoiding HC poisoning caused by high HC passing through ccSCR.

[0047] (2) The high-pressure HC injector effectively increases the post-treatment temperature, promotes rapid combustion of downstream SCR, and ensures NOx conversion efficiency at low temperature;

[0048] (3) No ccDOC needs to be installed at the front end of the ccSCR, and the ccSCR can start up quickly;

[0049] (4) Effectively improve the SCR control accuracy and ensure NOx emission reduction efficiency. Attached Figure Description

[0050] Figure 1 This is a schematic diagram of the overall structure of a two-stage SCR aftertreatment system for diesel engines.

[0051] Figure 2 This is a flowchart of the DPF active regeneration control method;

[0052] Figure 3 This is a flowchart of the control method for HC jet thermal management;

[0053] Figure 4 This is the control logic diagram of a two-stage SCR aftertreatment system for diesel engines. Detailed Implementation

[0054] The following will be based on embodiments of the present invention. Figures 1-4 The technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Unless otherwise specified, the technical means used in the embodiments are conventional means well known to those skilled in the art.

[0055] A two-stage SCR aftertreatment system for a diesel engine includes a front-stage SCR system (front-stage tightly coupled selective catalytic converter system), a high-pressure HC injection system, a DOC system 15 (oxidation catalyst), a DPF system 14 (particulate filter), and a rear-stage SCR system (downstream selective catalytic converter system) connected sequentially along the engine turbine exhaust port.

[0056] The pre-stage SCR system includes a pre-stage urea injector 1, a mixer 17, and a pre-stage SCR component 3.

[0057] The high-pressure HC injection system includes an HC injector 4 and an HC injection actuator 19, which are connected by a metal pipe. The HC injection actuator 19 is a high-pressure common rail system or an air-assisted injector used according to the actual needs of the engine.

[0058] The downstream SCR system includes a downstream urea injector 7, a mixer 12, an SCR component 9, and an ASC component 10.

[0059] The front end of the pre-stage SCR system is connected to the engine turbine outlet, and the rear end is connected to the DOC system 15; the front end of the DOC system is connected to the pre-stage SCR system, and the rear end is connected to the DPF system 14; the HC injector 4 of the high-pressure HC injection system is arranged on the empty exhaust pipe connecting the pre-stage SCR system and the DOC system 15, and sufficient empty exhaust pipe space is reserved to ensure that the injected HC can mix well with the exhaust; the HC injection actuator 19 is arranged on the engine block or aftertreatment bracket; the front end of the DPF system 14 is connected to the DOC system 15, and the rear end is connected to the post-stage SCR system; the front end of the post-stage SCR system is connected to the DPF system 14, and the rear end is the engine exhaust pipe.

[0060] In this invention, the pre-stage SCR system is mainly used for NOx treatment when the catalyst in the post-stage SCR system has not yet reached the high-efficiency conversion range at low temperatures and the post-stage urea injector has not yet injected urea; the high-pressure HC injection system mainly provides the oxidizing components required for DOC during the active regeneration of the DPF system, and also provides the oxidizing components required for the DOC system under the thermal management state of the post-stage SCR; the post-stage SCR system is mainly used for NOx treatment when the engine exhaust temperature is high and the ammonia storage effect of the pre-stage SCR system decreases due to high temperature. All of the above systems together constitute a two-stage SCR aftertreatment system.

[0061] Furthermore, it also includes four NOx sensors, namely:

[0062] The first NOx sensor 18 is located at the inlet of the front-stage SCR system to monitor the original NOx emissions from the engine, provide the NOx concentration signal to the front-stage SCR system to calculate the urea demand and perform urea injection control.

[0063] The second NOx sensor 16 is located at the outlet of the pre-stage SCR system and is used to provide NOx concentration signal input for the urea injection feedback control of the pre-stage SCR system and to detect the conversion efficiency of the pre-stage SCR system.

[0064] The third NOx sensor 13 is located at the inlet of the downstream SCR system to monitor the NOx concentration at the inlet of the downstream SCR system, provide the NOx concentration signal to the downstream SCR system to calculate the urea demand and perform urea injection control.

[0065] The fourth NOx sensor 11 is located after the downstream SCR system to monitor the final NOx tailpipe emission. It is used to provide NOx concentration signal input for the urea injection feedback control of the downstream SCR system and to detect the conversion efficiency of the downstream SCR system.

[0066] All NOx sensors are connected to the engine ECU20 via data cables, which input signals and issue commands.

[0067] Furthermore, it also includes four temperature sensors, namely:

[0068] The first temperature sensor 2 is located at the inlet of the front-stage SCR system and provides the temperature signal required for the control of the front-stage urea injector 1.

[0069] The second temperature sensor 5 is located at the inlet of the DOC system 15. When a DPF active regeneration request is received, the second temperature sensor 5 monitors the temperature of the DOC system 15, determines the conversion efficiency status of the DOC system 15, and then determines whether to perform DPF active regeneration.

[0070] The third temperature sensor 6 is located at the outlet of the DOC system 15 and the inlet of the DPF system 14. When the DPF regeneration state is entered, the third temperature sensor 6 provides a temperature signal input to determine the regeneration temperature and then controls the HC ejector 4 to spray the HC required for the DOC oxidation and heat release.

[0071] The fourth temperature sensor 8 is located at the inlet of the downstream SCR system and provides the temperature signal required for the control of the downstream urea injector 7.

[0072] All temperature sensors are connected to the engine ECU20 via data cables, which input signals and issue commands.

[0073] A method for active regeneration control of DPF in a two-stage SCR aftertreatment system for a diesel engine includes the following steps:

[0074] Step 1: After the engine starts, first read the data from ECU20 to determine whether DPF system 14 has entered the DPF regeneration request state;

[0075] Step 2: If the status recognition value is 0, it is not in the DPF active regeneration state, then the engine will remain idle and wait for the ECU20 command.

[0076] If the status identification value is 1, indicating DPF active regeneration status, then DPF system 14 initiates DPF regeneration, ECU 20 receives the temperature sensor signal, all urea injectors stop injecting, and then the following steps are performed:

[0077] When the DOC system 15 inlet temperature is ≤225℃ and the engine speed is increased to 2000r / min, the ECU20 controls the engine after-injection to be activated, which strengthens the after-combustion of the engine, promotes the rise of exhaust temperature, and increases the DOC system 15 inlet temperature.

[0078] When the inlet temperature of DOC system 15 exceeds 225℃, the engine speed increases to 2500 r / min, and the high-pressure HC injection system starts working, raising the inlet temperature of DPF system 14 until the target regeneration temperature of 500℃, and regeneration begins. After regeneration of DPF system 14 is started, the second temperature sensor 5 and the third temperature sensor 6 input temperature signals, and ECU20 provides feedback to adjust and control the injection quantity of HC injector 4, so that the inlet temperature of DPF system 14 is maintained at the regeneration temperature. After regeneration is completed, the high-pressure HC injection system stops working, HC injector 4 stops injecting, the regeneration state ends, and ECU20 controls the engine to return to normal mode.

[0079] A control method for HC injection thermal management of a two-stage SCR aftertreatment system for a diesel engine includes the following steps:

[0080] Step 1: After the engine starts, first read the data from ECU20 to determine whether DPF system 14 has entered the DPF regeneration request state. If the state identification value is 1 (repeat step 2 in a DPF active regeneration control method for a two-stage SCR aftertreatment system of a diesel engine), enter the DPF active regeneration state and perform DPF active regeneration. If the state identification value is 0, it is not the DPF active regeneration state. Then, ECU20 reads the signal from the first temperature sensor 2 to determine the operating status of the upstream SCR system and further controls the operation of HC injection actuator 19 and HC injector 4.

[0081] If the inlet temperature of the pre-stage SCR system is ≤200℃, the pre-stage urea injector 1 will stop spraying to prevent ammonia leakage from the pre-stage SCR, and the downstream SCR system will not work.

[0082] If the inlet temperature of the pre-stage SCR system is >200℃, the pre-stage urea injector 1 will inject urea at an ammonia-to-nitrogen ratio of 1 to reduce NOx, and the downstream SCR system will not work.

[0083] Step 2: ECU20 reads the signals from the fourth temperature sensor 8 and the second temperature sensor 5 to determine the operating status of the downstream SCR system and further controls the actuator 19 and HC injector 4 to operate.

[0084] If the inlet temperature of the downstream SCR system is >250℃, the downstream urea injector 7 will start injecting urea at an ammonia-to-nitrogen ratio of 1 to reduce NOx, the HC injector 4 will not operate, and the upstream SCR urea injector 1 will stop injecting.

[0085] If the inlet temperature of the downstream SCR system is ≤250℃, the downstream urea injector 7 will not inject, the HC injection actuator 19 will start working, the HC injector 4 will enter the injection standby state, and then the inlet temperature of the DOC system 15 will be judged to determine whether the DOC system 15 has reached the DOC catalyst ignition temperature, and step 3 will be executed:

[0086] Step 3: If the inlet temperature of DOC system 15 is ≥200℃, the downstream urea injector 7 stops injecting and the HC injector 4 starts injecting, increasing the inlet temperature of the downstream SCR system. ECU20 reads the signal of the fourth temperature sensor 8 to monitor the inlet temperature of the downstream SCR system. When the inlet temperature of the downstream SCR system is >250℃, step 2 is executed.

[0087] Step 4: If the inlet temperature of DOC system 15 is <200℃, the downstream urea injector 7 stops injecting, the engine speed is increased to 2000r / min, the ECU20 controls the engine to start the post-injection, and increases the inlet temperature of DOC system 15. The ECU20 reads the signal of the second temperature sensor 5 and monitors the inlet temperature of DOC system 15. When the temperature of DOC system 15 is ≥200℃, step 3 is executed.

[0088] Step 5: When ECU20 issues an engine stop command, the two-stage SCR system after-treatment system ends its working state.

[0089] Specifically, in step 3, the injection control of HC injector 4 adopts a model-based injection control method. Based on the exhaust temperature, exhaust flow rate, HC injection flow rate, engine EGR rate, and DOC conversion efficiency data collected from experiments, a dynamic exhaust temperature model is established for the high-pressure HC injection system. The inlet temperature of the downstream SCR system is transformed into a functional model of exhaust temperature, exhaust flow rate, HC injection flow rate, engine EGR rate, and DOC conversion efficiency to predict the thermodynamic state in the next 3-5 seconds and inversely optimize the HC injection quantity of the current HC injector 4.

[0090] A state-space model is constructed based on thermodynamic equations and historical data:

[0091] ;

[0092] in, The inlet temperature of the subsequent SCR system is a function of the time step. This refers to the exhaust flow rate; It is a function of exhaust temperature; Let HC be the jet flow rate function;

[0093] Within each control cycle, based on current sensor data and model predictions of future temperature trajectories, the optimal injection quantity function is solved:

[0094] ;

[0095] in, To control the inlet temperature function of the subsequent SCR system within the control cycle, through the aforementioned Function prediction; Target exhaust temperature; Here, the injection flow rate is corrected for the HC injection flow rate function.

[0096] The prediction model is implemented in a host computer, which communicates with the ECU20 and the after-processing system via a data cable.

[0097] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Any modifications, alterations, substitutions, or variations made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for active regeneration control of DPF in a two-stage SCR aftertreatment system for a diesel engine, characterized in that, Includes the following steps: Step 1: After the engine starts, first read the ECU (20) data to determine whether the DPF system (14) has entered the DPF regeneration request state; Step 2: If the status identification value is 0, it is not the DPF active regeneration state, then the engine will remain idle and wait for the ECU (20) command. If the status identification value is 1, it indicates that the DPF is in active regeneration mode. The DPF system (14) then initiates DPF regeneration, the ECU (20) receives the temperature sensor signal, all urea injectors stop injecting, and then the following steps are performed: When the inlet temperature of the DOC system (15) is ≤225℃, the engine speed is increased to 2000r / min, and the ECU (20) controls the engine to start the afterburner, which strengthens the afterburner and promotes the exhaust temperature to rise, thereby increasing the inlet temperature of the DOC system (15). When the inlet temperature of the DOC system (15) is >225℃, the engine speed increases to 2500r / min, the high-pressure HC injection system starts working, increases the inlet temperature of the DPF system (14) until the target regeneration temperature of 500℃, and starts regeneration; after the regeneration of the DPF system (14) is started, the second temperature sensor (5) and the third temperature sensor (6) input temperature signals, and the ECU (20) feeds back to adjust and control the injection quantity of the HC injector (4) so ​​that the inlet temperature of the DPF system (14) is maintained at the regeneration temperature; after the regeneration is completed, the high-pressure HC injection system stops working, the HC injector (4) stops injecting, the regeneration state ends, and the ECU (20) controls the engine to return to normal mode.

2. A control method for HC injection thermal management of a two-stage SCR aftertreatment system for a diesel engine, characterized in that, Includes the following steps: Step 1: After the engine starts, first read the data of ECU (20) to determine whether the DPF system (14) has entered the DPF regeneration request state. If the state identification value is 1, DPF active regeneration is performed; if the state identification value is 0, it is not the DPF active regeneration state. Then ECU (20) reads the signal of the first temperature sensor (2) to determine the operating status of the front-stage SCR system and further controls the HC injection actuator (19) and HC injector (4) to operate. If the inlet temperature of the pre-stage SCR system is ≤200℃, the pre-stage urea injector (1) will stop spraying to prevent ammonia leakage from the pre-stage SCR and the subsequent SCR system will not work. If the inlet temperature of the pre-stage SCR system is >200℃, the pre-stage urea injector (1) injects urea at an ammonia-to-nitrogen ratio of 1 to reduce NOx, and the subsequent SCR system does not work. Step 2, the ECU (20) reads the signals from the fourth temperature sensor (8) and the second temperature sensor (5), determines the operating status of the downstream SCR system, and further controls the actuator (19) and HC injector (4) to operate. If the inlet temperature of the downstream SCR system is >250℃, the downstream urea injector (7) will start injecting urea at an ammonia-to-nitrogen ratio of 1 to reduce NOx, the HC injector (4) will not operate, and the upstream urea injector (1) will stop injecting. If the inlet temperature of the downstream SCR system is ≤250℃, the downstream urea injector (7) will not inject, the HC injection actuator (19) will start working, the HC injector (4) will enter the ready-to-injection state, and then the inlet temperature of the DOC system (15) will be judged. It will be judged whether the DOC system (15) has reached the ignition temperature of the DOC catalyst, and step 3 will be executed: Step 3: If the inlet temperature of the DOC system (15) is ≥200℃, the downstream urea injector (7) stops injecting and the HC injector (4) starts injecting, raising the inlet temperature of the downstream SCR system. The ECU (20) reads the signal of the fourth temperature sensor (8) and monitors the inlet temperature of the downstream SCR system. When the inlet temperature of the downstream SCR system is >250℃, step 2 is executed. Step 4: If the inlet temperature of the DOC system (15) is <200℃, the downstream urea injector (7) stops injecting, the engine speed increases to 2000r / min, the ECU (20) controls the engine to start the post-injection, and increases the inlet temperature of the DOC system (15). The ECU (20) reads the signal of the second temperature sensor 5 and monitors the inlet temperature of the DOC system (15). When the temperature of the DOC system (15) is ≥200℃, step 3 is executed. Step 5: When the ECU (20) issues an engine stop command, the two-stage SCR system after-treatment system ends its working state.

3. The control method for HC injection thermal management of a two-stage SCR aftertreatment system for a diesel engine according to claim 2, characterized in that, In step 3, the injection control of the HC injector (4) adopts a model-based injection control method, including: establishing a dynamic exhaust temperature model for the high-pressure HC injection system based on experimental data, converting the inlet temperature of the downstream SCR system into a function model of exhaust temperature, exhaust flow rate, HC injection flow rate, engine EGR rate, and DOC conversion efficiency, predicting the thermodynamic state in the next 3-5 seconds, and inversely optimizing the HC injection quantity of the current HC injector (4): A state-space model is constructed based on thermodynamic equations and historical data: in, The inlet temperature of the subsequent SCR system is a function of the time step. This refers to the exhaust flow rate; It is a function of exhaust temperature; Let HC be the jet flow rate function; Within each control cycle, the optimal injection quantity function is solved by predicting the future temperature trajectory based on current sensor data and the exhaust temperature dynamic model. in, The inlet temperature function of the subsequent SCR system during the control cycle; Target exhaust temperature; This is the HC injection flow rate function.

4. A two-stage SCR aftertreatment system for a diesel engine, characterized in that, The method of claim 1 or claim 2 includes a front-stage SCR system, a high-pressure HC injection system, a DOC system (15), a DPF system (14), and a rear-stage SCR system connected sequentially along the engine turbine exhaust port. The pre-stage SCR system includes a pre-stage urea injector (1), a first mixer (17), and a pre-stage SCR component (3). The high-pressure HC injection system includes an HC injector (4) and an HC injection actuator (19), which are connected by a metal pipe. The downstream SCR system includes a downstream urea injector (7), a second mixer (12), an SCR component (9), and an ASC component (10). The front end of the pre-stage SCR system is connected to the engine turbine outlet, and the rear end is connected to the DOC system (15); the front end of the DOC system is connected to the pre-stage SCR system, and the rear end is connected to the DPF system (14); the HC injector (4) of the high-pressure HC injection system is arranged on the blank exhaust pipe connecting the pre-stage SCR system and the DOC system (15), and sufficient blank exhaust pipe space is reserved to ensure that the injected HC can be well mixed with the exhaust; the HC injection actuator (19) is arranged on the engine block or aftertreatment mounting bracket; the front end of the DPF system (14) is connected to the DOC system (15), and the rear end is connected to the post-stage SCR system; the front end of the post-stage SCR system is connected to the DPF system (14), and the rear end is the engine exhaust pipe.

5. A two-stage SCR aftertreatment system for a diesel engine according to claim 4, characterized in that, It also includes four NOx sensors, namely: The first NOx sensor (18) is located at the inlet of the front-stage SCR system to monitor the original NOx emissions of the engine, provide the NOx concentration signal to the front-stage SCR system to calculate the urea demand and perform urea injection control. The second NOx sensor (16) is located at the outlet of the pre-stage SCR system and is used to provide NOx concentration signal input for the urea injection feedback control of the pre-stage SCR system and to detect the conversion efficiency of the pre-stage SCR system. The third NOx sensor (13) is located at the inlet of the downstream SCR system to monitor the NOx concentration at the inlet of the downstream SCR system, provide the NOx concentration signal to the downstream SCR system to calculate the urea demand and perform urea injection control. The fourth NOx sensor (11) is arranged after the downstream SCR system to monitor the final NOx tailpipe emission. It is used to provide NOx concentration signal input for the urea injection feedback control of the downstream SCR system and to detect the conversion efficiency of the downstream SCR system. All NOx sensors are connected to the engine ECU (20) via data lines, and input signals and issue commands via the data lines.

6. A two-stage SCR aftertreatment system for a diesel engine according to claim 4, characterized in that, It also includes four temperature sensors, namely: The first temperature sensor (2) is located at the inlet of the front-stage SCR system to provide the temperature signal required for the control of the front-stage urea injector (1); The second temperature sensor (5) is located at the inlet of the DOC system (15). When a DPF active regeneration request is received, the second temperature sensor (5) monitors the temperature of the DOC system (15), determines the conversion efficiency status of the DOC system (15), and then determines whether to perform DPF active regeneration. The third temperature sensor (6) is located at the outlet of the DOC system (15) and the inlet of the DPF system (14). When the DPF regeneration state is entered, the third temperature sensor (6) provides a temperature signal input to determine the regeneration temperature and then controls the HC injector (4) to inject the HC required for DOC oxidation and heat release. The fourth temperature sensor (8) is located at the inlet of the downstream SCR system to provide the temperature signal required for the control of the downstream urea injector (7); All temperature sensors are connected to the engine ECU (20) via data cables, and input signals and issue commands via the data cables.

Images